Neurophysiology questions,
with the lesson behind each one.

1. Core

   What is cerebrospinal fluid, and where is it found?

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   Cerebrospinal fluid is a clear, near-acellular, low-protein transcellular fluid that fills the ventricular system and the subarachnoid space, so it surrounds and suspends both the brain and the spinal cord.

   It occupies the two lateral ventricles, the third ventricle, the cerebral aqueduct, the fourth ventricle and the central canal of the spinal cord, and then the cranial and spinal subarachnoid space including the basal cisterns. Total volume in an adult is about 150 mL, roughly half of it inside the ventricles and half in the subarachnoid space.

   What CSF is, and where it is found →

2. Core

   How much CSF is there, how fast is it made, and how often is it turned over?

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   About 150 mL in total, produced at roughly 0.35 mL per minute, which is about 500 mL per day. So the whole volume turns over about three to four times every twenty-four hours.

   Texts differ a little at the edges — some quote 100 to 150 mL, and daily production between 450 and 550 mL — but 150 mL, 0.35 mL per minute and 500 mL a day is internally consistent and is the safest set to quote, because the three numbers agree with each other.

   CSF production at the choroid plexus →

3. Core

   Where is CSF produced, and by what mechanism?

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   Mostly by the choroid plexus in the lateral, third and fourth ventricles. The majority — texts say between about half and three-quarters — comes from the plexus, and the remainder from cerebral capillary endothelium and from brain metabolic water reaching the ventricles.

   The mechanism is two-stage: ultrafiltration of plasma across a fenestrated choroidal capillary into the stroma, then active secretion across the choroid plexus epithelium. The epithelium loads ions across its basolateral surface and secretes them across the apical surface into the ventricle, generating an osmotic gradient that water follows.

   The proof that it is secretion and not simple filtration is the composition: CSF is not an ultrafiltrate of plasma. Potassium, calcium and glucose are lower than plasma and chloride and magnesium are higher, which passive filtration alone cannot produce.

   CSF production at the choroid plexus →

4. Core

   What is the blood–CSF barrier, and how does it differ from the blood–brain barrier?

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   The blood–CSF barrier is the choroid plexus epithelium, sealed by tight junctions near the apical ends of its lateral membranes. It is the barrier, not the capillary — the choroidal capillary is deliberately fenestrated so that plasma water and small solutes can reach the stroma freely.

   The blood–brain barrier is different tissue in a different place: the cerebral capillary endothelium itself, which has tight junctions, no fenestrations, a high mitochondrial density, a thick basement membrane and astrocyte foot processes applied to it.

   The distinction matters in the circumventricular organs. There the blood–brain barrier is deficient and the capillaries are fenestrated, but the blood–CSF barrier remains intact — so a substance can reach those structures without reaching the CSF and the neurons beyond it.

   CSF production at the choroid plexus →

5. Core

   How does CSF differ from plasma, and why do those differences matter?

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   Sodium and osmolality are essentially the same as plasma. What differs is that CSF has much less protein — of the order of 0.2 to 0.4 g/L against about 70 g/L — and almost no cells, normally nought to five per cubic millimetre.

   Then: potassium is lower, around 2.9 against 4.5 mmol/L. Glucose is lower, about two-thirds of the plasma value. Chloride is higher, and magnesium is higher. Calcium is lower. The pH is lower, about 7.32 against 7.40, and the CSF carbon dioxide tension is higher than arterial.

   Why it matters: low, tightly regulated potassium and calcium keep neuronal excitability stable independent of plasma swings. Almost no protein means almost no oncotic pressure, which is what makes absorption at the arachnoid villi a pressure-driven bulk flow, and it means very little non-bicarbonate buffering — so CSF pH follows arterial carbon dioxide closely and quickly, which is the basis of central chemoreception.

   CSF composition →

6. Core

   What does CSF actually do?

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   Six things, and I would give each a mechanism rather than just a name.

   Buoyancy. The brain weighs about 1400 g in air but has an effective weight under about 50 g suspended in CSF, because the fluid’s specific gravity is close to the brain’s. That reduces its inertia and the traction on its vessels and nerve roots.

   Mechanical protection. A fluid cushion between brain and the ridged skull base, damping acceleration and deceleration.

   A stable ionic and chemical environment for neurons, maintained by active transport rather than by whatever plasma happens to be doing.

   A volume buffer for intracranial pressure — CSF displacement into the spinal subarachnoid space is the first compensation for any expanding intracranial mass.

   Chemical signalling and acid–base signalling: neuropeptides are carried between regions, and because CSF buffers weakly, its pH tracks arterial carbon dioxide and drives the central chemoreceptors.

   Waste clearance. The brain has no conventional lymphatics within its parenchyma; CSF entering periarterial spaces, crossing the parenchyma and leaving by perivenous routes — the glymphatic pathway — performs that role, and is more active during sleep and general anaesthesia.

   Functions of CSF →

7. Core

   Trace the circulation of CSF from where it is made to where it is absorbed.

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   Lateral ventricles, then through the interventricular foramina — the foramina of Monro into the third ventricle. From the third ventricle through the cerebral aqueduct, the aqueduct of Sylvius, into the fourth ventricle.

   It leaves the fourth ventricle through the median aperture, the foramen of Magendie, and the paired lateral apertures, the foramina of Luschka, into the cisterna magna and the basal cisterns.

   From there it flows over the cerebral hemispheres in the cranial subarachnoid space, and caudally into the spinal subarachnoid space. It is absorbed through the arachnoid villi and granulations into the dural venous sinuses, principally the superior sagittal sinus.

   Movement is driven by the pressure gradient from production to absorption, helped by ciliary action of the ependyma and by arterial pulsation and respiratory oscillation.

   CSF circulation and absorption →

8. Core

   How is CSF absorbed, and what determines the rate?

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   Through arachnoid villi and granulations projecting into the dural venous sinuses. About 85 to 90 per cent through intracranial villi, mostly of the sagittal and sigmoid sinuses, and 10 to 15 per cent through spinal arachnoid villi around the nerve roots.

   They behave as one-way, pressure-sensitive valves. Bulk flow begins once CSF pressure exceeds dural venous sinus pressure by roughly 1.5 mmHg. Below about 5 to 7 mmHg absorption is negligible; above that it increases essentially linearly with CSF pressure.

   Because CSF protein is so low, its oncotic pressure is effectively zero, so this really is a hydrostatic bulk flow rather than a Starling balance. That is why absorption is pressure-dependent while production is not — and why a rise in dural venous sinus pressure, from jugular obstruction or sinus thrombosis, immediately impairs absorption.

   CSF circulation and absorption →

9. Core

   Distinguish communicating from obstructive hydrocephalus.

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   Hydrocephalus is abnormal resistance to the circulation of CSF, or impaired absorption of it, so that production outstrips the rate at which CSF can get past the problem.

   In obstructive, or non-communicating, hydrocephalus the block is inside the ventricular system, before CSF reaches the subarachnoid space — at a foramen of Monro, the cerebral aqueduct, or the outlets of the fourth ventricle. Ventricles proximal to the block dilate; those distal to it do not.

   In communicating hydrocephalus CSF still reaches the subarachnoid space, but absorption fails — arachnoid granulations blocked by blood after subarachnoid haemorrhage, or by chronic meningitis. The whole ventricular system tends to dilate.

   There is a third, rare category: overproduction, with no obstruction at all, from a choroid plexus papilloma.

   Hydrocephalus →

10. Applied

    A child has congenital aqueduct stenosis. Which parts of the ventricular system enlarge, and which do not?

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    Both lateral ventricles and the third ventricle enlarge. The fourth ventricle does not, and neither does the subarachnoid space.

    The reasoning is purely anatomical: everything upstream of the block accumulates. The aqueduct connects the third to the fourth ventricle, so the lateral ventricles and third ventricle are proximal and dilate; the fourth ventricle is distal and stays normal or small.

    This is why the site of obstruction can be read off the imaging. A block at one foramen of Monro dilates only that lateral ventricle. A block at the fourth-ventricle outlets dilates all four.

    Hydrocephalus →

11. Stretch

    Are the arachnoid villi the only route by which CSF leaves the cranium?

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    For the examination, the arachnoid villi and granulations draining into the dural venous sinuses are the answer, and I would give that first.

    But no, they are not the only route. There is additional outflow along meningeal and perineural lymphatic pathways — along cranial and spinal nerve sheaths, and across the cribriform plate to cervical lymphatics — and in animal work that route carries a substantial share in health, with the villi becoming relatively more important as CSF pressure rises.

    The safe way to present it is: the classical pressure-dependent arachnoid-villus pathway is the mark-earning answer, and lymphatic outflow is current context that explains why the classical model alone does not account for all measured outflow.

    CSF circulation and absorption →

12. Core

    Define intracranial pressure and give its normal value.

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    Intracranial pressure is the pressure generated within the rigid cranium by its contents — brain, blood and CSF — measured relative to atmospheric pressure. In practice it is the hydrostatic pressure of the CSF and brain parenchyma.

    Normal in a supine adult at rest is 5 to 15 mmHg, which is about 7 to 20 cmH₂O. Some texts narrow that to 8 to 12 mmHg; 5 to 15 is the value to quote.

    Two qualifications worth adding unprompted. It is not a single steady number — it varies through the cardiac and respiratory cycles. And it is posture-dependent: quoting a value without saying supine is incomplete, because head-up posture lowers it and head-down raises it.

    Defining and measuring ICP →

13. Applied

    How would you measure intracranial pressure?

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    It cannot be estimated reliably from clinical signs; it has to be measured invasively.

    The external ventricular drain is the reference standard: a catheter in the lateral ventricle, which also lets you drain CSF therapeutically and sample it. Its drawbacks are that it must be clamped to read a pressure, it can be difficult to place if the ventricles are small or shifted, it blocks, and infection risk runs up to about five per cent.

    An intraparenchymal fibre-optic probe is easier to insert, works when the ventricles are compressed, and has a lower infection rate, at almost the same accuracy. Its limitation is that it is zeroed at insertion and cannot be re-zeroed in vivo, and it reports the pressure where it sits rather than a global value.

    Subarachnoid and subdural probes are easier and safer but much less accurate, and are now largely obsolete.

    Defining and measuring ICP →

14. Core

    State and explain the Monro–Kellie doctrine.

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    The adult cranium is a rigid container of fixed total volume. It holds brain tissue, blood and CSF. Therefore an increase in the volume of any one of them must be matched by a decrease in one or both of the others, or intracranial pressure rises.

    Conventional shares: brain about 80 per cent, roughly 1400 g; CSF about 10 per cent, about 150 mL; and blood about 10 per cent, about 150 mL. Different texts give slightly different percentages, but the principle is not in dispute.

    The important consequence is that only two of those compartments are actually displaceable — CSF and venous blood. Brain tissue and arterial blood are not sacrificed without harm. So compensatory reserve is limited, and it is finite.

    It also explains why the same added volume produces completely different pressures in different patients: what matters is not the volume added but how much reserve was left before it was added.

    The Monro–Kellie doctrine →

15. Core

    Define intracranial compliance and intracranial elastance.

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    Compliance is change in volume per unit change in pressure, ΔV divided by ΔP, in millilitres per millimetre of mercury. Elastance is the reciprocal — change in pressure per unit change in volume, ΔP over ΔV, in millimetres of mercury per millilitre.

    They are reciprocals, so as compensatory reserve is used up, compliance falls and elastance rises. The two always move in opposite directions.

    One point worth making, because it is a genuine trap: the curve everyone calls the “compliance curve” is drawn with pressure on the y-axis and volume on the x-axis, so its slope is ΔP over ΔV. That is elastance, not compliance. Several major texts say so explicitly, and it is safest to label it the intracranial volume-pressure or elastance curve.

    Compliance, elastance and the volume-pressure curve →

16. Core

    Draw the intracranial volume-pressure curve and talk me through it.

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    Vertical axis intracranial pressure in mmHg, 0 to 60. Horizontal axis volume of an expanding intracranial mass — I would leave that axis without numbers, because the published figures do.

    The curve starts flat at about 10 mmHg. That flat limb is compensation: CSF is displaced into the spinal subarachnoid space and its absorption rises, and venous blood is displaced into the internal jugular veins, so volume is added without pressure rising.

    Then there is a knee — the point of decompensation — where that reserve is exhausted. Beyond it the curve rises steeply and almost exponentially, so a small further volume produces a large pressure rise.

    I would mark 20 mmHg, above which focal ischaemia occurs, and about 45 mmHg, above which ischaemia becomes global. And I would say aloud that compliance is falling and elastance rising as you move right along the curve — that is the whole point of drawing it.

    Compliance, elastance and the volume-pressure curve →

17. Core

    An intracranial mass is expanding. What compensates, and in what order?

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    CSF first. CSF is translocated out of the cranium into the spinal subarachnoid space, and because absorption at the arachnoid villi is pressure-dependent, absorption also increases. This is the first and the most important compensatory mechanism.

    Venous blood second. The dural venous sinuses are compressed and venous blood is displaced into the internal jugular veins, reducing intracranial blood volume. This reserve is smaller.

    After that, the options are bad ones: a fall in arterial blood volume as perfusion pressure drops, and displacement of brain tissue itself between compartments or through the foramen magnum.

    One thing I would not say is that CSF production falls as a compensatory mechanism. Production is essentially independent of intracranial pressure; it only falls late, when cerebral perfusion pressure drops below about 70 mmHg and choroid plexus blood flow falls — and that is a consequence of decompensation, not a compensation for it.

    Compensation and decompensation →

18. Applied

    Take me through decompensation, once that reserve has run out.

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    Compliance is now low and elastance high, so each further increment of volume raises ICP steeply.

    Rising ICP reduces cerebral perfusion pressure, since CPP is mean arterial pressure minus ICP. Once CPP falls below about 50 mmHg the cerebral arterioles are already maximally dilated and autoregulation fails; cerebral blood flow becomes pressure-passive and falls below 50 mL per 100 g per minute, and ischaemia begins.

    Ischaemia causes cytotoxic oedema and further vasodilatation, which adds volume — a positive feedback loop, so ICP rises faster still.

    Then the late systemic response: brainstem ischaemia triggers an intense sympathetic outflow causing systemic hypertension to restore perfusion, with reflex baroreceptor bradycardia and an abnormal respiratory pattern — Cushing’s triad. Finally brain tissue herniates, most dangerously the cerebellar tonsils through the foramen magnum, compressing the brainstem.

    The point I would emphasise is that Cushing is late. It is not a compensatory mechanism for raised ICP in the way CSF displacement is; it is a last-ditch attempt to preserve brainstem perfusion once decompensation is already established.

    Compensation and decompensation →

19. Applied

    Explain the Cushing response. Why bradycardia, when the sympathetic outflow is increased?

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    The Cushing response is a late physiological response to a severely raised ICP. When CPP falls far enough that autoregulation has failed and the brainstem becomes ischaemic, the vasomotor area drives a massive increase in sympathetic outflow, producing intense systemic arteriolar vasoconstriction and systemic hypertension. That raises mean arterial pressure, which raises CPP, and restores brainstem perfusion.

    The bradycardia is reflex, not direct. The rise in arterial pressure is sensed by the arterial baroreceptors, and the baroreflex responds with vagally mediated slowing of the heart. So the hypertension is the primary event and the bradycardia is secondary to it — that is exactly the point candidates most often get the wrong way round.

    The third component is an abnormal respiratory pattern from brainstem compression. Together, hypertension, bradycardia and irregular respiration are Cushing’s triad, and they signal that herniation is imminent.

    Compensation and decompensation →

20. Core

    Describe the normal intracranial pressure waveform.

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    It is a modified arterial pressure wave transmitted from the large cerebral vessels through the CSF, so one cycle lasts one cardiac cycle and it lags slightly behind the arterial trace.

    There are three components. A pulse wave with three peaks; a respiratory component, where the baseline moves with the respiratory cycle; and slow waves, which are changes in the baseline itself.

    Within one pulse there are three peaks of decreasing amplitude: P1, the percussion wave, which is the transmitted arterial pressure wave; P2, the tidal wave, thought to be the arterial wave reflected off brain parenchyma, normally about 80 per cent of P1; and P3, the dicrotic wave, following the dicrotic notch and related to venous pressure.

    If I were drawing it I would label pressure in mmHg on the vertical axis and time in seconds on the horizontal, and mark 20 mmHg as the upper limit of normal.

    The ICP waveform →

21. Applied

    Your patient’s ICP trace now shows P2 taller than P1. What does that tell you?

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    That intracranial compliance has fallen. P2 amplitude varies inversely with compliance, so when P2 exceeds P1 the compensatory reserve is largely spent — even if the mean ICP is still acceptable.

    The mechanism is that P2 is the arterial wave reflected back off the brain parenchyma. A stiff, poorly compliant intracranial space reflects more of it, so the tidal wave grows.

    I would expect two other changes alongside it: the whole waveform becomes more rounded, losing its distinct peaks, and the baseline rises.

    Clinically it is a warning, because it says the patient is now on the steep part of the volume-pressure curve — so any further volume, from a cough, from hypercapnia, from head-down positioning, will produce a much larger pressure rise than the same insult would have produced an hour earlier.

    The ICP waveform →

22. Stretch

    What are Lundberg waves?

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    Slow changes in the baseline of the ICP trace, as opposed to the pulse waveform riding on it. They are usually pathological.

    A waves, or plateau waves, are a steep rise in baseline to above 50 mmHg sustained for several minutes — quoted as two to twenty minutes — then an abrupt fall. They indicate a marked reduction in intracranial compliance and are always pathological.

    B waves are sharply peaked rhythmic oscillations at about 0.5 to 2 per minute, with the pressure rising by 20 to 30 mmHg and falling back. They indicate an unstable ICP.

    C waves are smaller oscillations at about four to eight per minute, related to changes in systemic vasomotor tone, and can be a normal finding.

    In practice ICP monitoring is used mostly to read the mean value and calculate CPP; the slow waves are an additional signal rather than the main purpose.

    The ICP waveform →

23. Core

    A ventilated head-injured patient has a blood pressure of 130 over 70 mmHg and an ICP of 28 mmHg. Calculate the cerebral perfusion pressure and interpret it.

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    Equation: CPP = MAP − ICP.

    Mean arterial pressure = diastolic + one-third of the pulse pressure = 70 + (60 ÷ 3) = 70 + 20 = 90 mmHg.

    CPP = 90 − 28 = 62 mmHg.

    Sanity check: that is below a normal CPP of around 80 mmHg but still above the lower limit of autoregulation, usually quoted as 50 mmHg. Units are mmHg throughout, and the answer is plausible.

    Interpretation: the blood pressure looks unremarkable, but the CPP is already significantly reduced purely because of the ICP, and it is at the bottom of the usual 60 to 70 mmHg target after traumatic brain injury. A further rise in ICP of 12 mmHg, or a fall in MAP of 12 mmHg, would take this patient below the autoregulatory threshold.

    Cerebral perfusion pressure →

24. Applied

    Is CPP always mean arterial pressure minus intracranial pressure?

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    Not always. CPP is the pressure gradient across the cerebral circulation, so it is mean arterial pressure minus the downstream pressure, and the downstream pressure is whichever is higher — intracranial pressure or central venous pressure.

    Normally ICP exceeds CVP, so CPP = MAP − ICP is right. But if central venous pressure is high — high PEEP, jugular obstruction, right heart failure, straining — then CVP becomes the effective downstream pressure and using ICP alone overestimates perfusion.

    There is a second, equivalent way to define it that is worth having: CPP = cerebral blood flow × cerebral vascular resistance. That form is more useful when the question is about what changes flow rather than what changes pressure.

    Cerebral perfusion pressure →

25. Core

    Why does hypercapnia raise intracranial pressure?

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    Carbon dioxide diffuses freely and rapidly across the blood–brain barrier. It lowers the pH of the perivascular extracellular fluid, and that fall in extracellular pH — not carbon dioxide itself — relaxes cerebral arteriolar smooth muscle.

    Vasodilatation reduces cerebral vascular resistance, so cerebral blood flow rises, and with it cerebral blood volume. Cerebral blood volume is one of the three intracranial compartments, so by the Monro–Kellie doctrine, if the other two cannot give way, ICP rises.

    The relationship is essentially linear across the physiological range — cerebral blood flow changes by about one to two millilitres per 100 g per minute for every millimetre of mercury change in arterial carbon dioxide tension. It flattens below about 25 mmHg, where the vessels are maximally constricted, and above about 75 to 80 mmHg, where they are maximally dilated.

    The step candidates skip is the middle one: the chain is carbon dioxide → extracellular pH → vasodilatation → cerebral blood volume → ICP. Naming flow without naming volume misses the link to pressure.

    Physiological determinants of ICP →

26. Core

    What about hypoxaemia — does that raise ICP too?

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    Yes, but with a threshold rather than a linear relationship, and that difference is the answer.

    Between an arterial oxygen tension of about 60 and 300 mmHg — that is roughly 8 kPa upwards — oxygen has almost no effect on cerebral blood flow. Below about 60 mmHg, or 8 kPa, where haemoglobin saturation starts to fall steeply, there is abrupt cerebral vasodilatation and cerebral blood flow rises sharply.

    The same chain then follows as for carbon dioxide: vasodilatation, increased cerebral blood volume, increased ICP.

    Practically that is why hypoxaemia is not merely a delivery problem in a head injury — it is directly an ICP problem, and why the effects of hypoxaemia and hypercapnia together are worse than either alone.

    Physiological determinants of ICP →

27. Applied

    How does obstructed cerebral venous drainage raise ICP?

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    Two mechanisms, and both matter.

    First, cerebral venous blood volume rises. The dural venous sinuses have no valves and are not collapsible, so drainage depends entirely on the pressure gradient between the sinuses and the right atrium. Anything that raises the downstream pressure engorges the venous compartment — and that added volume raises ICP directly.

    Second, CSF absorption falls. Absorption at the arachnoid villi is driven by the gradient between CSF pressure and dural venous sinus pressure. Raise the sinus pressure and the gradient narrows, so CSF accumulates as well.

    So it hits two of the three intracranial compartments at once. That is why it is such an efficient way to raise ICP, and why relieving it is the very first thing to check in a tight brain.

    Physiological determinants of ICP →

28. Applied

    You are anaesthetising a patient with a raised ICP. What do positioning, coughing and airway pressure do?

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    All three act through the same final pathway — cerebral venous outflow.

    Position. Head-up tilt, conventionally about 30 degrees, improves venous drainage and lowers ICP. Extreme neck rotation or flexion, a tight cervical collar or tight endotracheal tube ties kink or compress the internal jugular veins and raise it. Keeping the head neutral is as important as the tilt.

    Coughing and straining raise intrathoracic pressure sharply, which raises central venous and therefore dural sinus pressure, and ICP with it. Even in a normal brain, coughing, straining and sneezing transiently take ICP to around 50 mmHg. In a patient on the steep part of the volume-pressure curve that is dangerous, which is why neuromuscular blockade and adequate depth are used at laryngoscopy and during craniotomy.

    Airway pressure. Raised mean airway pressure — high PEEP, airway obstruction, bronchospasm, a kinked tube, tension pneumothorax, gas trapping — reduces the venous pressure gradient out of the head. PEEP should be the minimum that achieves adequate oxygenation, but not so low that the patient becomes hypoxaemic, because hypoxaemia raises ICP by its own route.

    Anaesthetic and clinical application →

29. Applied

    How does hyperventilation reduce ICP, and why is it only a temporary measure?

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    Hyperventilation lowers arterial carbon dioxide tension, which raises perivascular pH, which constricts cerebral arterioles. That reduces cerebral blood volume, and by the Monro–Kellie doctrine a smaller blood compartment means a lower ICP. It works within minutes.

    It is temporary for two reasons. First, the effect is not sustained: CSF pH returns toward normal over about six to eight hours as bicarbonate is extruded from the CSF, so cerebral blood flow drifts back up despite the maintained hypocapnia.

    Second, it is not free. Vasoconstriction reduces cerebral blood flow, and in a recently injured brain — where flow is already low and autoregulation impaired — that can cause ischaemia. The alkalosis also shifts the oxyhaemoglobin dissociation curve to the left, reducing oxygen offloading.

    So it is a rescue manoeuvre for a critical or uncertain ICP, not a maintenance strategy. Usual targets are a carbon dioxide tension of 4.5 to 5.0 kPa, going lower only when necessary, and one major text sets a floor around 23 to 25 mmHg.

    One further trap: after a sustained period of hyperventilation, abruptly returning the carbon dioxide to normal produces a CSF acidosis, because the bicarbonate has already gone — so cerebral blood flow and ICP rebound upwards. Come back slowly.

    Anaesthetic and clinical application →

30. Applied

    Compare the effects of intravenous and volatile anaesthetic agents on ICP.

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    Intravenous agents — propofol, thiopentone, etomidate, the benzodiazepines — reduce cerebral metabolic rate, and because flow–metabolism coupling is preserved, cerebral blood flow falls in parallel. Less flow means less cerebral blood volume, so ICP falls or is unchanged. Autoregulation and carbon dioxide responsiveness are generally preserved.

    Ketamine is the exception: it increases cerebral metabolic rate and cerebral blood flow.

    Volatile agents are the interesting case because they uncouple flow from metabolism. They reduce metabolic rate, which would lower flow, but they are also direct cerebral vasodilators, which raises it. Which effect wins is dose-dependent: around half MAC the metabolic effect dominates and flow falls; at about 1 MAC the two roughly balance; above that, vasodilatation dominates and flow, blood volume and ICP rise.

    Vasodilating potency runs roughly halothane, then enflurane, desflurane, isoflurane, sevoflurane — sevoflurane being the least. Nitrous oxide is also a cerebral vasodilator and raises metabolic rate.

    The practical conclusion: volatile agents below 1 MAC as part of a balanced technique are acceptable for most neurosurgery, but where compliance is exhausted or the field is persistently tight, a predominantly intravenous technique is the safer choice.

    Anaesthetic and clinical application →

31. Stretch

    Do anaesthetic agents affect CSF secretion or absorption?

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    Yes, although the evidence is entirely from animal work and has not been examined in humans, so I would say that in the same breath.

    Of the volatiles, halothane decreases CSF secretion, isoflurane has no effect, and enflurane and desflurane increase it. For absorption, halothane and enflurane reduce it, desflurane leaves it unchanged, and isoflurane increases it.

    The combination that matters is increased production with decreased absorption, because that is the one that adds CSF volume over time. That combination occurs with enflurane, which is one theoretical reason to avoid it in a long closed-cranium procedure in a patient with poor intracranial compliance.

    For most agents and most cases the effect is far too slow to matter compared with what the same drug does to cerebral blood volume — which is the effect that acts within a minute.

    Anaesthetic and clinical application →

32. Stretch

    Which matters more for ICP — cerebral blood flow or cerebral blood volume?

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    Cerebral blood volume. ICP is set by volume inside a fixed box, so it is the volume of the blood compartment, not the flow through it, that appears in the Monro–Kellie equation.

    Flow matters because the two usually move together — vasodilatation raises both. But the magnitude of the change differs: a 50 per cent rise in cerebral blood flow produces only around a 20 per cent rise in cerebral blood volume, and normal cerebral blood volume is only about 5 mL per 100 g of brain.

    They can also move in opposite directions, which is the point worth making. During cerebral ischaemia, or as mean arterial pressure falls, the vessels dilate to maintain flow — so cerebral blood volume rises while flow is falling. Conversely, as pressure rises within the autoregulatory range the vessels constrict and cerebral blood volume actually falls.

    That is why hypotension is not automatically protective in a patient with a raised ICP, and why an examiner asking about ICP wants to hear the word volume.

    Physiological determinants of ICP →

33. Applied

    Your patient’s blood pressure is normal. Can their brain still be underperfused?

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    Yes, easily — because perfusion depends on the gradient, not the inflow pressure alone.

    Take a mean arterial pressure of 85 mmHg, which nobody would treat. If ICP is 10 mmHg, CPP is 75 and the brain is fine. If ICP is 40 mmHg, CPP is 45 — below the lower limit of autoregulation, so cerebral blood flow is now pressure-passive and falling, and the brain is ischaemic at an entirely normal blood pressure.

    That is the argument for measuring ICP rather than inferring it. Without an ICP value you cannot calculate CPP, and blood pressure alone will not tell you.

    It also explains why permissive hypotension is dangerous in a head injury, and why a single episode of systolic pressure below 90 mmHg worsens outcome.

    Cerebral perfusion pressure →

34. Stretch

    Does CSF production fall when intracranial pressure rises?

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    Essentially no, and this is worth being precise about because it is commonly given as a compensatory mechanism and is not one.

    Production is close to pressure-independent. The rate of formation stays roughly constant as intraventricular pressure rises. It is absorption that is pressure-dependent, increasing linearly with CSF pressure — so the compensation for a rising ICP is CSF displacement plus increased absorption, not reduced production.

    Production only falls late, and indirectly: once ICP has risen far enough to drop cerebral perfusion pressure below about 70 mmHg, choroid plexus blood flow falls and with it CSF formation. That is a marker of decompensation, not a defence against it.

    Pharmacologically, carbonic anhydrase inhibition does reduce CSF formation, since bicarbonate is generated inside the choroid epithelial cell from carbon dioxide and water by carbonic anhydrase. But it is far too slow to be useful for acute intracranial pressure control in theatre.

    Compensation and decompensation →

35. Stretch

    Does the Monro–Kellie doctrine still hold once the skull is open?

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    Not in its strict form, because the doctrine depends on the container being rigid and closed. Once the cranium and dura are open, added volume can escape through the craniotomy rather than raising pressure.

    But the physiology does not stop mattering — it changes what it costs you. Instead of a rising pressure you get a bulging, tight surgical field, poor operating conditions, brain herniating through the bone defect, and mechanical injury to the herniated tissue.

    So the same manipulations apply, for the same reasons: head-up posture and unobstructed venous drainage, control of carbon dioxide, avoiding vasodilators and preventing coughing. The surgeon judges the result by looking at the field rather than by reading a number.

    The same logic explains decompressive craniectomy: it converts a closed rigid box into an open one, so the volume-pressure curve no longer has a steep limb.

    The Monro–Kellie doctrine →

36. Stretch

    A ventilated patient with a traumatic brain injury has an ICP that has climbed from 15 to 32 mmHg over an hour. Reason through it physiologically.

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    I would work through the three compartments in the order I can act on them fastest.

    Venous first, because it is passive and often the cause. Is the head neutral and up? Is there a tight collar or tight tube ties compressing the jugulars? Is airway pressure high — obstruction, bronchospasm, a kinked tube, pneumothorax, excess PEEP? Is the patient coughing or straining against the ventilator?

    Then arterial. What is the carbon dioxide tension, and the oxygen tension? Is the patient febrile or fitting, both of which raise metabolic rate and therefore flow and volume? Are any vasodilators running — nitrous oxide, a volatile above 1 MAC, nitroprusside, a calcium channel blocker?

    Then CSF and the cellular compartment. Is there a ventricular drain that is clamped or blocked? And critically — is this a new mass lesion? A rise of this size over an hour in a head-injured patient means re-imaging, because an expanding haematoma is a surgical problem, not a physiological one.

    Throughout, I would calculate CPP rather than watch ICP alone, and support mean arterial pressure to keep CPP in the usual 60 to 70 mmHg target range, because the injury that actually kills neurons here is secondary ischaemia.

    Anaesthetic and clinical application →

37. Classify the nervous system for me.

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    There are two independent classifications. Structurally, the nervous system is central — the brain and spinal cord — and peripheral, which is the twelve pairs of cranial nerves, the thirty-one pairs of spinal nerves, and their ganglia and plexuses.

    Functionally, it divides into somatic and autonomic. The somatic system carries conscious sensation and voluntary motor output to skeletal muscle, with a single neuron running from the central nervous system to the effector. The autonomic system is involuntary and always has two neurons in series with a ganglion between them, and it divides again into sympathetic and parasympathetic — and, if you accept it as a third division, enteric.

    The two classifications cross-cut each other. The sympathetic nervous system, for instance, is functionally autonomic but structurally has both central components, in the lateral horn of the cord, and peripheral ones in the sympathetic chain.

    Likely follow-ups

    • Where does the enteric nervous system fit, and why is it sometimes treated separately?
    • Give me one structure that belongs to both the central and the peripheral nervous system by different parts of the same cell.

    Neurophysiology 1: Organisation of the Nervous System →

38. What do astrocytes do?

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    Six things, and they are all homeostatic rather than signalling. First, they induce and maintain the blood-brain barrier — the tight junctions are endothelial, but the astrocyte end-feet are what tell the endothelium to form them. Second, they buffer extracellular potassium, taking up the potassium released by repetitive firing so that excitability does not drift. Third, they take up glutamate and GABA from the cleft, terminating transmission and preventing excitotoxicity. Fourth, they supply neurons with lactate as a metabolic substrate. Fifth, they contribute to neurovascular coupling — glutamate acting on astrocytic metabotropic receptors generates vasoactive arachidonic acid metabolites. Sixth, they form the glial scar after injury.

    The first thing to fail in ischaemia is glutamate uptake, because it is sodium-coupled and therefore depends on the sodium gradient the ATPase maintains. Once the ATP fails, the transporter can even run backwards, which converts the astrocyte from a glutamate sink into a glutamate source — the beginning of the excitotoxic cascade.

    Likely follow-ups

    • Which of those functions fails first in ischaemia?
    • Why does the brain have no lymphatics, and what does it use instead?

    Neurophysiology 1: Organisation of the Nervous System →

39. Why can a nerve not be tetanised the way a muscle can?

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    Because the absolute refractory period of a nerve spans almost the whole action potential — around one millisecond — so successive action potentials cannot fuse. Each one is a discrete, identical event. In skeletal muscle the action potential is short relative to the contraction it triggers, so mechanical responses can summate and fuse into tetanus even though the electrical events do not.

    The maximum frequency is therefore set by the absolute refractory period: roughly one impulse per millisecond gives a ceiling around 1000 Hz in theory, though real fibres rarely exceed a few hundred hertz because the relative refractory period raises the threshold as well.

    A stronger stimulus is signalled two ways: the individual fibre fires at a higher frequency, and more fibres are recruited — including higher-threshold ones. That is the basis of the compound action potential, whose amplitude increases with stimulus strength up to a maximal stimulus and no further.

    Likely follow-ups

    • What sets the maximum firing frequency of a nerve fibre?
    • How does a nerve signal a stronger stimulus if the action potential is all-or-none?

    Neurophysiology 2: Neuronal Excitability and the Nerve Action Potential →

40. A patient with a working spinal block tells you they can feel you touching them. What do you say, and why?

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    I would reassure them that touch and pressure sensation are expected to persist and that this does not mean the block has failed — different fibre types are blocked at different rates, and the large myelinated Aβ fibres carrying touch and pressure are among the last to be affected. I would confirm the block by testing a modality carried by fibres that are blocked.

    Cold sensation is the most sensitive test, because it is carried by small unmyelinated C fibres and thinly myelinated Aδ fibres, which are blocked early; pinprick, also Aδ, is the next most useful. Loss of cold to ice or ethyl chloride gives the highest, and therefore the earliest, indication of block level.

    The sympathetic level is higher because the preganglionic sympathetic fibres are small B fibres, thinly myelinated, and are blocked by a lower concentration of local anaesthetic than the sensory fibres. As the drug is diluted by cerebrospinal fluid with distance from the injection site, there is a zone at the top of the spread where the concentration is enough to block B fibres but not Aδ — usually quoted as two or more segments.

    Likely follow-ups

    • Which modality would you test to confirm the block is adequate?
    • Why is the sympathetic block level higher than the sensory level?

    Neurophysiology 2: Neuronal Excitability and the Nerve Action Potential →

41. Classify synapses for me, and give me one physiological advantage of each type.

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    Synapses are fundamentally either electrical or chemical, and chemical synapses divide again by receptor type into ionotropic and metabotropic.

    Electrical synapses are gap junctions formed by connexons, with a gap of about three nanometres and direct cytoplasmic continuity. Their advantage is speed and synchrony: there is essentially no synaptic delay, and because they are bidirectional they couple whole populations of cells so that they behave as one functional unit.

    Ionotropic chemical synapses use a ligand-gated ion channel — the receptor is the channel. Their advantage is speed with control: the response takes a millisecond or so, but unlike an electrical synapse it can be excitatory or inhibitory, it can be amplified, and it can be modulated.

    Metabotropic chemical synapses use a G-protein-coupled receptor and a second-messenger cascade. Their advantage is amplification and duration: one bound receptor activates many G proteins, so a small signal produces a large and prolonged response, and the cascade can reach the nucleus and alter gene expression. That is what makes plasticity possible.

    Electrical synapses are uncommon in the mammalian central nervous system — they occur at dendrodendritic contacts where synchronisation matters, and in the retina — but they are the rule in cardiac muscle and in single-unit smooth muscle, where the whole tissue must contract as a syncytium.

    Likely follow-ups

    • Where in the human body would I find an electrical synapse?
    • Why does the neuromuscular junction use an ionotropic receptor rather than a metabotropic one?

    Neurophysiology 3: Synaptic Transmission →

42. An opioid acts both presynaptically and postsynaptically in the dorsal horn. Describe both, and say why the combination is more effective than either alone.

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    The µ receptor is a G_i/o-coupled metabotropic receptor, so both actions begin with inhibition of adenylyl cyclase and direct G-protein effects on ion channels.

    Presynaptically, on the central terminal of the C fibre, the activated G protein closes voltage-gated calcium channels. Less calcium enters when the action potential arrives, so fewer vesicles fuse and less substance P and glutamate are released into the cleft.

    Postsynaptically, on the second-order neuron, the G protein opens potassium channels. Potassium efflux hyperpolarises the cell and moves it away from threshold — a classic IPSP.

    The combination is more effective than either alone because the two act at different points in series: less transmitter arrives, and the cell that receives it is less able to respond. That is also the physiological argument for combining a spinal opioid with a local anaesthetic, which acts on a third mechanism entirely.

    Likely follow-ups

    • Which G protein does the µ receptor couple to?
    • What is the ionic basis of the postsynaptic action?

    Neurophysiology 3: Synaptic Transmission →

43. Draw the circle of Willis and label it.

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    Draw the ring with anterior at the top. Start in the midline with the anterior communicating artery, and take an anterior cerebral artery down and laterally from each end of it to the termination of the internal carotid artery. From each carotid termination take the middle cerebral artery straight out laterally, and bring the internal carotid itself up from below. From each carotid run a posterior communicating artery backwards to meet the posterior cerebral artery of that side. Bring the two posterior cerebral arteries to the midline tip of the basilar artery, and bring the basilar up from the union of the two vertebral arteries. That is nine named vessels and a closed ring.

    Then the branches that complete it: the cerebellar vessels — posterior inferior cerebellar from each vertebral, anterior inferior cerebellar and superior cerebellar from the basilar — and the statement that the carotids supply about two-thirds of cerebral blood flow and the vertebrals about one-third.

    Likely follow-ups

    • Which vessels make up the anterior circulation and which the posterior?
    • Why is the circle not the protection against carotid occlusion it appears to be?
    • Where do berry aneurysms most commonly arise, and why there?

    Neurophysiology 4: The Meninges and the Cerebral Circulation →

44. Your patient is positioned sitting for a posterior fossa craniotomy. Take me through the anatomy that makes this position risky.

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    The risk follows from three anatomical facts in sequence. First, the operative site is above the level of the right atrium, so the hydrostatic pressure in the cerebral veins and dural sinuses at that height is subatmospheric. Second, the posterior fossa exposure involves the transverse and sigmoid sinuses and the diploic veins of the occipital bone. Third, and decisively, the dural sinuses are channels between the two layers of dura and are held open by them, so unlike a peripheral vein they do not collapse when the pressure within them falls. An opened sinus at that height therefore entrains air rather than bleeding, and it continues to do so for as long as the gradient exists.

    The consequences follow from where the air then goes: into the right heart, where it impairs output and can cause an air lock; into the pulmonary circulation, raising dead space and lowering end-tidal carbon dioxide; and, if a patent foramen ovale is present, across into the systemic circulation as a paradoxical embolus, which is why that anatomical variant matters so much in this position specifically.

    Likely follow-ups

    • How would you detect venous air embolism, and in what order of sensitivity?
    • What is the significance of a probe patent foramen ovale in this patient?

    Neurophysiology 4: The Meninges and the Cerebral Circulation →

45. Why does the brain receive such a disproportionate share of the cardiac output?

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    Because flow follows metabolic demand, and the brain’s metabolic rate per unit mass is very high. It is about 2% of body weight but takes 12 to 15% of the cardiac output and about 20% of total body oxygen consumption — roughly 50 mL of oxygen a minute for the whole brain.

    The demand is high because maintaining ionic gradients across a very large membrane surface is expensive: the majority of neuronal ATP goes to the sodium-potassium ATPase, restoring the gradients that every action potential and every synaptic potential dissipates.

    Grey matter has roughly four times the flow of white matter because it contains the cell bodies, dendrites and synapses — that is where the electrophysiological work is done. White matter is myelinated axon, which conducts saltatorily and is metabolically cheap by comparison.

    Likely follow-ups

    • How much oxygen does the whole brain consume per minute?
    • Why is grey matter flow so much higher than white matter flow?

    Neurophysiology 5: Cerebral Metabolism →

46. A patient has a return of spontaneous circulation four minutes after cardiac arrest. What has been happening in the brain, and what is your priority now?

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    During the arrest, cerebral blood flow fell to zero. ATP was exhausted within seconds, the sodium-potassium ATPase failed, and the membrane depolarised. Calcium entered, massive glutamate release began an excitotoxic loop, and calcium-activated phospholipases, proteases and nitric oxide synthase started to damage membranes, cytoskeleton and DNA. The cells swelled. Four minutes at normothermia is at the edge of what is recoverable, and some neurons will already be committed to necrosis while others will die by apoptosis over the following days.

    My priority now is to prevent a secondary insult being added to the primary one, by keeping supply above demand. That means a cerebral perfusion pressure above 60 mmHg — in practice a mean arterial pressure above 80 mmHg, using a vasopressor if needed — adequate arterial oxygen content and oxygen delivery, normocapnia, normoglycaemia, and active avoidance of hyperthermia, because cerebral metabolic rate rises 6 to 7% for every degree.

    The penumbra is tissue receiving roughly 6 to 15 mL per 100 g per minute: not enough to support electrical activity, so the electroencephalogram is flat there, but enough to prevent membrane failure for a time. It is viable and recoverable, and it is what the perfusion targets are protecting.

    If autoregulation remains intact, the curve is shifted to the right — the lower limit rises, so a blood pressure that was previously adequate may no longer be. That is the reason for the higher pressure target, and it is a different statement from “autoregulation fails”.

    Likely follow-ups

    • What is the ischaemic penumbra?
    • If autoregulation remains intact after the arrest, what has happened to the curve?

    Neurophysiology 5: Cerebral Metabolism →

47. Two anticholinergic drugs, atropine and glycopyrrolate. Why does one cause confusion and the other not?

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    Because of ionisation. Atropine is a tertiary amine: at physiological pH a substantial fraction is unionised and lipid-soluble, so it crosses the blood-brain barrier by simple diffusion through the endothelial cell membrane and antagonises central muscarinic receptors. That produces sedation, confusion and, at the extreme, central anticholinergic syndrome.

    Glycopyrrolate is a quaternary ammonium compound: it carries a permanent positive charge, so there is no unionised fraction, no lipid route, and no central penetration. Its antimuscarinic effects are confined to the periphery.

    In an elderly patient I would pair neostigmine with glycopyrrolate rather than atropine, both for that reason and because glycopyrrolate’s slower onset matches neostigmine’s better than atropine’s does, giving a less abrupt tachycardia.

    The neostigmine-physostigmine pair makes the same point from the other direction: neostigmine is quaternary and stays peripheral, physostigmine is tertiary and enters the brain, which is exactly why physostigmine is the treatment for central anticholinergic syndrome.

    Likely follow-ups

    • Which would you choose with neostigmine in an elderly patient, and why?
    • Name another drug pair that differs only in whether it crosses the barrier.

    Neurophysiology 6: The Blood-Brain Barrier →

48. Draw me the autoregulation curve and talk me through it.

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    I would draw cerebral blood flow in mL per 100 g per minute on the vertical axis, from 0 to 100, with 50 marked, against mean arterial pressure in mmHg on the horizontal axis, from 0 to 200. The curve rises steeply to the lower limit at about 70 mmHg, crosses a broad plateau near 50 with a gentle upward slope, and rises steeply again above the upper limit at about 150 mmHg.

    On the plateau, flow is held relatively constant by a myogenic mechanism: rising pressure depolarises arteriolar smooth muscle, calcium enters, the muscle contracts, cerebral vascular resistance rises, and flow barely changes. Below the lower limit the capacity to dilate is exhausted and flow falls with pressure; above the upper limit the capacity to constrict is exhausted, flow rises passively, and the barrier is at risk.

    In chronic hypertension the traditional teaching is that the whole curve shifts right, both limits moving up, so a mean arterial pressure that is normal for a normotensive patient may lie below this patient’s lower limit. More recent studies find autoregulation preserved in hypertensive patients, so I would limit an acute fall in mean arterial pressure to about 30-35% of their baseline.

    After a head injury autoregulation is frequently defective, so flow follows pressure more directly: hypotension produces ischaemia in brain regions whose flow is already low, and a hypertensive surge raises cerebral blood flow, cerebral blood volume and intracranial pressure.

    Likely follow-ups

    • What happens to that curve in a chronically hypertensive patient?
    • Why is a patient with a head injury at risk at both ends of the pressure range?

    Neurophysiology 8: Cerebral Blood Flow →

49. At skin incision your patient's blood pressure rises to 160/105. What happens to cerebral blood flow, and why?

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    In an intact brain, cerebral blood flow is essentially unchanged, or rises only modestly. The mean arterial pressure is about 123 mmHg. That raises cerebral perfusion pressure, but it stays within the autoregulatory range of about 70 to 150 mmHg, so the arterioles constrict, cerebral vascular resistance rises, and flow is held close to 50 mL per 100 g per minute.

    The mechanism is myogenic autoregulation: arteriolar smooth muscle depolarising and contracting as transmural pressure rises. Sympathetic activation from the noxious stimulus also acts on the cerebral vessels, limiting the rise in downstream pressure and flow and protecting the blood-brain barrier against the surge.

    A useful secondary point is that cerebral blood volume falls slightly, since the vessels have constricted, so in an intact brain a pressor response of this size should not raise intracranial pressure.

    After a head injury the answer changes. Autoregulation is frequently defective, so flow becomes more pressure-passive: it rises with the blood pressure, cerebral blood volume rises with it, and intracranial pressure rises. That is why obtunding the pressor response matters in this population.

    Likely follow-ups

    • Which mechanism is responsible?
    • Would your answer differ if this patient had a head injury?

    Neurophysiology 8: Cerebral Blood Flow →

50. A patient has weakness and loss of vibration sense in the right leg, and loss of pinprick in the left leg. Where is the lesion, and why?

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    This is a right-sided hemisection — Brown-Séquard syndrome — and the side is given by the motor and dorsal column findings, not by the pain findings.

    The corticospinal tract has already decussated in the medullary pyramids, so below the lesion it is supplying the same side it runs on: a right cord lesion gives right-sided weakness. The dorsal columns have not yet crossed — they ascend ipsilaterally to the medulla — so a right cord lesion abolishes vibration and proprioception on the right. The spinothalamic tract, by contrast, crossed in the cord within one or two segments of entry, so the fibres running in the right cord are carrying information from the left side of the body: hence contralateral loss of pain and temperature.

    The pinprick level begins one or two segments below the lesion precisely because of that delayed crossing — fibres entering at the level of the lesion have not yet reached the midline and are spared.

    At the level of the lesion itself I would expect a narrow band of ipsilateral loss of all modalities, from destruction of the dorsal horn and entering root fibres, and lower motor neuron signs in that myotome from anterior horn cell damage — flaccidity and wasting, in contrast to the upper motor neuron pattern below.

    Likely follow-ups

    • Why does the pinprick level start a segment or two below the lesion?
    • What would you expect at the level of the lesion itself?

    Neurophysiology 11: The Spinal Cord and Its Tracts →

51. Why does a lesion of one dorsal column cause ipsilateral loss, while a lesion of one spinothalamic tract causes contralateral loss?

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    Entirely because of where each system decussates relative to the lesion.

    The dorsal column first-order neuron enters the cord and ascends without synapsing and without crossing, all the way to the gracile and cuneate nuclei in the medulla. Only there does the second-order neuron cross, as the internal arcuate fibres forming the medial lemniscus. So at any point in the cord, the dorsal column fibres are still carrying information from the same side — and a cord lesion produces ipsilateral loss.

    The spinothalamic first-order neuron synapses immediately, in the dorsal horn, and the second-order neuron crosses in the anterior white commissure within one or two segments before ascending. So the fibres running in the anterolateral cord are already carrying information from the opposite side, and a cord lesion produces contralateral loss.

    The sensory level starts one or two segments below the lesion for the same reason: fibres entering at or just below the level of the lesion have not yet reached the midline, so they are spared.

    Likely follow-ups

    • Where exactly does each one cross?
    • Why does the spinothalamic sensory level start a segment or two below the lesion?

    Neurophysiology 12: Sensory Receptors and Somatosensory Pathways →

52. How does morphine given intravenously reduce the pain signal reaching the cortex?

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    At three levels. In the brain, µ receptors in the periaqueductal grey sit on tonically active GABAergic neurons. Morphine inhibits them, which removes the brake on the PAG’s output to the rostral ventromedial medulla and locus coeruleus. Their descending fibres run in the dorsolateral funiculus and release serotonin and noradrenaline in the dorsal horn.

    In the dorsal horn, morphine also acts directly. Most opioid receptors there are presynaptic, on the central terminals of nociceptors: through Gi/Go they inhibit voltage-gated calcium channels, so less glutamate and substance P are released. Postsynaptic receptors on the projection neuron open potassium channels and hyperpolarise it. Both reduce transmission through the first synapse, and the descending serotonin and noradrenaline add to that.

    In the periphery, receptors transported down the axon to the terminals of nociceptors in inflamed tissue can be activated too. The result is less transmission at every level at which the signal can be modulated, rather than a block of one of them.

    Likely follow-ups

    • Why does naloxone reverse the analgesia produced by stimulating the periaqueductal grey?
    • Where outside the brain and spinal cord can an opioid act?

    Neurophysiology 13: Pain Physiology →

53. At incision the patient moves their arm away. Describe the neural pathway involved, from receptor to effector.

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    This is the flexor withdrawal reflex, and the answer needs both limbs — the afferent and the efferent. Describing only the pain pathway leaves out half of it.

    Afferent limb. The incision activates nociceptors — free nerve endings of Aδ and C fibres — in the skin of the forearm. The first-order neuron, with its cell body in the dorsal root ganglion, enters the cord through the lateral division of the dorsal root and synapses in the dorsal horn.

    Central connections. This is a polysynaptic reflex: interneurons in the dorsal horn and intermediate grey distribute the signal over several segments — the arm is supplied by C5 to T1, so a single-segment reflex could not withdraw it. Those interneurons excite the α motor neurons of the flexors and, through inhibitory interneurons, inhibit the α motor neurons of the extensors — reciprocal innervation.

    Efferent limb. α motor neurons in the anterior horn — laminae VIII and IX — conduct in Aα fibres to the neuromuscular junction, where acetylcholine acts on nicotinic receptors to produce an end-plate potential, a muscle action potential, and contraction of the flexors, withdrawing the limb.

    Separately and simultaneously, the second-order neuron in the dorsal horn crosses and ascends in the spinothalamic tract to the thalamus and cortex, where the pain is perceived. That ascending limb is not part of the reflex — the withdrawal happens before, and independently of, the perception, which is why a decerebrate or anaesthetised patient can still withdraw.

    If the stimulus is strong the response can be bilateral through the crossed extensor reflex: commissural interneurons cross the cord to excite the contralateral extensors, so that the opposite limb takes the weight while the stimulated one withdraws.

    Likely follow-ups

    • Which part of that pathway produces the sensation of pain, and is it the same pathway?
    • Why is the response bilateral if the stimulus is unilateral?

    Neurophysiology 16: Motor Control and the Descending Tracts →

54. Compare the muscle spindle with the Golgi tendon organ.

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    Structure it on the geometry, because every other difference follows from it.

    Position. The spindle lies in parallel with the extrafusal fibres, within the muscle belly. The Golgi tendon organ lies in series with them, at the musculotendinous junction.

    Adequate stimulus. A parallel element is stretched when the muscle lengthens, so the spindle measures length and its rate of change. A series element bears the force transmitted through it, so the Golgi tendon organ measures tension.

    Behaviour during active contraction — the discriminating question. Contraction shortens the muscle, so the parallel spindle is unloaded and its firing falls, unless γ co-activation takes up the slack. The same contraction raises tension, so the series Golgi tendon organ increases its firing. Passive stretch increases both.

    Reflex connections. Ia from the spindle is monosynaptic and excitatory to its own muscle. Ib from the Golgi tendon organ acts through an inhibitory interneuron, so it is disynaptic and inhibitory to its own muscle.

    Innervation of the receptor. Only the spindle has a motor supply — the γ efferents to the contractile poles of the intrafusal fibres. It is not an output pathway; it sets the gain of the receptor, and through alpha-gamma co-activation keeps the spindle sensitive during shortening.

    Function. The spindle is the sensor of a length servo — the basis of muscle tone, posture and the tendon reflexes. The Golgi tendon organ provides tension feedback for smooth force control, and at extremes protects against damaging load.

    Likely follow-ups

    • What happens to the discharge of each during active contraction of the muscle?
    • What is the function of the gamma efferent supply?

    Neurophysiology 17: Spinal Reflexes and Proprioceptors →

55. A patient becomes hypotensive and bradycardic ten minutes after a spinal for caesarean section. Explain the physiology.

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    Two mechanisms, and a good answer separates them.

    Hypotension is sympathetic denervation. Local anaesthetic blocks the preganglionic sympathetic fibres arising from T1 to L2. These are small myelinated B fibres, and are more readily blocked than the larger Aδ and Aβ fibres, so the sympathetic block extends two to six segments higher than the sensory level — the differential block. The result is arteriolar dilatation, reducing systemic vascular resistance, and — more important — venodilatation of the capacitance vessels, which hold about 70% of blood volume. Venous return and therefore preload fall, so by the Frank-Starling relationship stroke volume and cardiac output fall. Aortocaval compression by the gravid uterus compounds the fall in venous return.

    Bradycardia has two explanations. First, if the block reaches T1 to T4 it denervates the cardiac accelerator fibres, leaving vagal tone unopposed. Second, and independent of block height, the fall in venous return reduces stretch of the right atrium and great veins, so the Bainbridge reflex is withdrawn; profound emptying of the ventricle can also trigger the Bezold-Jarisch reflex — mechanoreceptors in the underfilled left ventricle producing bradycardia, vasodilatation and, at the extreme, asystole.

    So the normal compensatory tachycardia is unavailable: the efferent limb of the baroreceptor reflex is pharmacologically blocked, and the reflexes that remain drive the heart rate the wrong way.

    Management follows the physiology — left uterine displacement to relieve aortocaval compression, fluid to restore venous return, and a vasopressor. Phenylephrine, a pure α1 agonist, restores resistance and venous return, though its reflex bradycardia may need treating; ephedrine has β1 activity and is preferred when the heart rate is already low. Atropine or glycopyrronium treats the vagal component directly.

    Likely follow-ups

    • Why is the bradycardia paradoxical, given that hypotension normally causes tachycardia?
    • Why does the block extend higher for sympathetic fibres than for sensory ones?

    Neurophysiology 18: The Autonomic Nervous System →

56. Define consciousness, and describe the neural structures responsible for maintaining it.

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    Definition first, and in two parts. Consciousness is the state of awareness of oneself and of one’s environment together with the ability to respond to it. It comprises arousal, the level of wakefulness, a brainstem and thalamic function; and awareness, the content of experience, a cortical function. Awareness requires arousal, but arousal can exist without awareness — which is the vegetative state.

    Structures, working upward. The ascending reticular activating system arises from the reticular formation of the rostral pons and midbrain and projects to the cortex by two routes. The dorsal cholinergic pathway runs through the thalamus from the pedunculopontine and laterodorsal tegmental nuclei. The ventral monoaminergic pathway bypasses the thalamus through the hypothalamus and basal forebrain and comprises the locus coeruleus (noradrenaline), raphe nuclei (serotonin), tuberomammillary nucleus (histamine), ventral tegmental area (dopamine) and the orexinergic neurons of the lateral hypothalamus.

    The thalamus is the gate: all sensory modalities except olfaction relay through it, and its relay neurons switch between a tonic mode that transmits faithfully and a hyperpolarised burst mode that does not. The cortex, with intact corticocortical and thalamocortical connectivity, supplies the content.

    Anaesthetic action, if asked. Both from below and from above: most agents potentiate GABA_A transmission, inhibiting the arousal nuclei and potentiating the sleep-promoting ventrolateral preoptic nucleus, while also disrupting the long-range cortical connectivity that integration requires. Ketamine is the exception — NMDA antagonism producing a dissociative state with a high-frequency EEG.

    The anatomical asymmetry. The arousal system is compact and paired in the rostral brainstem, so a small bilateral lesion abolishes consciousness; the cortex is large and distributed, so unilateral damage, however extensive, leaves consciousness intact.

    Likely follow-ups

    • How do general anaesthetics abolish consciousness?
    • Why does a small bilateral brainstem lesion abolish consciousness when a large unilateral cortical lesion does not?

    Neurophysiology 19: Consciousness and Arousal →

57. What is the electroencephalogram, and what factors alter it?

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    Definition and origin. The EEG is a recording of the spontaneous electrical activity of the cerebral cortex from scalp electrodes, of the order of 10-100 µV over 0.5-50 Hz. It represents the summated excitatory and inhibitory postsynaptic potentials in the apical dendrites of cortical pyramidal cells — not action potentials, which are too brief to summate — and it is detectable only because those cells are aligned perpendicular to the cortical surface so their fields add.

    The organising principle. Amplitude depends on synchrony. A synchronised cortex gives slow, high-voltage activity; a desynchronised cortex gives fast, low-voltage activity. Frequency and amplitude therefore move in opposite directions, and the alert patient has the smallest trace.

    Factors, grouped.

    • Physiological state: arousal, sleep stage, eye opening (alpha blocking), and age — the trace is slower in children and in the elderly.
    • Anaesthetic agents: the orderly progression from beta activation through frontal alpha, theta and delta to burst suppression and isoelectricity. The exceptions are ketamine and nitrous oxide, which produce fast low-voltage activity, and sevoflurane and enflurane, which can produce epileptiform activity at high concentration with hypocapnia.
    • Cerebral perfusion: slowing below about 20 mL/100 g/min and isoelectricity below about 15, while cell death occurs only below about 6 — the gap that makes the EEG a useful ischaemia monitor.
    • Temperature: progressive slowing, isoelectric at about 18-20 °C.
    • Metabolic: hypoglycaemia, hypoxaemia, hypercapnia, hepatic and uraemic encephalopathy, hypothyroidism — all slow it.
    • Pathology: epilepsy (spike and wave), space-occupying lesions (focal slowing), encephalitis.
    • Artefact: electromyographic activity, eye movement, ECG, diathermy and mains interference.

    Likely follow-ups

    • Why does the amplitude fall when the patient becomes more alert?
    • At what cerebral blood flow does the EEG become isoelectric, and why is that number useful?

    Neurophysiology 20: The EEG and Evoked Potentials →

58. Describe the stages of sleep and the physiological changes that accompany them.

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    Structure the answer as two types, then systems. Sleep is a reversible state of reduced awareness and responsiveness, distinguished from coma and anaesthesia by being reversible with sensory stimulation and by being homeostatically regulated. It comprises non-REM sleep in three stages and REM sleep.

    Non-REM. N1 is the transition, with theta activity replacing alpha and slow rolling eye movements; N2 shows sleep spindles and K complexes and is the largest stage at around 45%; N3, slow-wave sleep, shows high-voltage delta and is the deepest. Sleep is entered through non-REM, the cycle is about 90 minutes, and slow-wave sleep is concentrated in the first third of the night.

    REM occupies about 25% of adult sleep, its periods lengthening towards morning. The EEG is desynchronised and resembles wakefulness — hence paradoxical sleep — with rapid conjugate eye movements, vivid dreaming and near-complete skeletal muscle atonia sparing the diaphragm and extraocular muscles.

    Physiological changes, contrasting the two. Cerebral metabolic rate and blood flow fall in non-REM, by up to about a quarter, and equal or exceed the awake value in REM. Cardiovascularly, non-REM is stable with a steady fall in heart rate, blood pressure and cardiac output, while REM is markedly unstable with sympathetic surges — which is why myocardial ischaemia clusters in the early morning. Respiration slows and regularises in non-REM with minute ventilation falling around 10-15% and PaCO₂ rising by about 0.5-1 kPa, while in REM it becomes irregular, chemoreceptor responses are further blunted, and accessory muscle contribution is lost so that ventilation becomes diaphragm-dependent. Muscle tone is reduced in non-REM and abolished in REM. Thermoregulation is maintained around a lower set point in non-REM but is effectively suspended in REM, which is poikilothermic. Growth hormone peaks in slow-wave sleep early in the night; cortisol is at its nadir then and rises towards morning.

    Obstructive sleep apnoea, if asked. Airway patency depends on pharyngeal dilator tone opposing inspiratory collapsing pressure. REM atonia removes that tone almost completely, so obstructive events are longest and desaturation deepest in REM — and the postoperative REM rebound on nights two to four is when these patients are at greatest risk.

    Likely follow-ups

    • Why is obstructive sleep apnoea worst in REM sleep?
    • How does anaesthesia differ from sleep?

    Neurophysiology 21: Sleep Physiology →

59. Classify memory, and describe the anatomical structures underlying each type.

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    Classify twice — that is the structure of the answer.

    By duration. Short-term or working memory lasts seconds to minutes, holds of the order of seven items, depends on ongoing reverberating neural activity with no structural change, and is a function of the prefrontal cortex. Because it requires continuing activity, it is abolished by anything that interrupts that activity — concussion, seizure, electroconvulsive therapy or anaesthesia. Long-term memory lasts hours to a lifetime, has effectively unlimited capacity, and depends on structural and biochemical change requiring protein synthesis. Conversion between them is consolidation, favoured by rehearsal, emotional salience and sleep.

    By content. Declarative or explicit memory is for facts and events and is consciously accessible; it subdivides into episodic memory for personal events and semantic memory for general knowledge. It requires the medial temporal lobe and hippocampus for encoding and consolidation, the mammillary bodies and dorsomedial thalamus as the diencephalic limb, and the neocortex for storage. Non-declarative or implicit memory is for skills, habits, priming and conditioning, is not consciously accessible, and depends on the basal ganglia for procedural learning, the cerebellum for conditioned motor responses, and the amygdala for emotional conditioning.

    The clinical proof of the classification. Bilateral hippocampal damage produces dense anterograde amnesia for declarative material while leaving remote memories and all skill learning intact — the patient can learn a new motor skill and improve at it while denying ever having practised it. That dissociation is the argument that the two systems are anatomically distinct, and it also shows that the hippocampus encodes rather than stores.

    Mechanism, if asked. Long-term potentiation — a persistent increase in synaptic strength after brief high-frequency stimulation, best characterised in the hippocampus. The NMDA receptor makes it associative: at rest its channel is blocked by magnesium, and it conducts only when glutamate is bound and the postsynaptic membrane is depolarised enough to expel that block. It is therefore a coincidence detector. The calcium that enters activates CaMKII and protein kinase C, causing insertion of AMPA receptors early and gene transcription with new spine growth late.

    The anaesthetic point. The concentration-response curves for the components of anaesthesia are separate, and amnesia occurs at concentrations well below those producing unconsciousness — which is why a patient may be briefly conscious under a light anaesthetic and yet have no explicit recall.

    Likely follow-ups

    • What is long-term potentiation, and which receptor makes it associative?
    • Why does a general anaesthetic abolish memory at a lower concentration than it abolishes consciousness?

    Neurophysiology 22: Memory, Emotion and Higher Function →

60. Why does hyperventilation lower ICP?

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    Reduced PaCO₂ raises CSF pH, constricts cerebral arterioles and lowers CBF and cerebral blood volume; the effect is temporary as CSF bicarbonate adapts.

    Physiology SAQ Paper 1 · Q3 →

61. Why can aggressive hypocapnia harm a patient with TBI?

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    Excessive vasoconstriction may reduce CBF enough to cause cerebral ischaemia.

    Physiology SAQ Paper 1 · Q3 →

Open the neurophysiology module →