Show the model answerAttempt it first — that is what makes it stick
(a) Define consciousness, and the processes that bring it about 2 marks
What earns the marks2 marks
| A clear definition | A sentence, separating arousal from awareness |
|---|---|
| Reticular formation | The brainstem substrate |
| Ascending reticular activating system | Both pathways, with their nuclei and transmitters |
| Thalamus | The gate — tonic against burst firing |
| Cortex | The content, requiring intact connectivity |
Commonly lost: Many gave a one-word definition. And asked for the processes, some described the types of sleep instead.
| Component | What it is | Anatomy | Dissociation that proves it |
|---|---|---|---|
| Arousal (level) | Wakefulness — a continuum from alertness through drowsiness to coma | Brainstem and thalamus | Vegetative state: arousal present, awareness absent |
| Awareness (content) | What is experienced — perception, memory, emotion, self-recognition | Cerebral cortex and its connections | Locked-in syndrome: both intact, motor output gone |
The processes, working upward
The ascending arousal system and its two routes to the cortex
Arousal is not generated by one nucleus but by a distributed brainstem and hypothalamic network, each component with its own transmitter. Two pathways leave it. The dorsal route runs through the thalamus, principally the intralaminar and reticular nuclei, and gates the flow of sensory information to cortex. The ventral route bypasses the thalamus through the hypothalamus and basal forebrain to reach cortex directly.
Every sedative drug you use acts somewhere on this diagram, which is the practical reason for learning it: dexmedetomidine at the locus coeruleus, antihistamines at the tuberomammillary nucleus, propofol and the benzodiazepines by enhancing GABAergic inhibition of the whole system, and orexin — whose loss causes narcolepsy — stabilising the switch between wake and sleep.
| Level | Structure | Transmitter | Contribution |
|---|---|---|---|
| Brainstem core | Reticular formation, rostral pons and midbrain | — | Receives collaterals from every ascending sensory pathway, so any strong stimulus of any modality arouses |
| Dorsal pathway | Pedunculopontine and laterodorsal tegmental nuclei, relaying through the thalamus | Acetylcholine | Thalamic route to cortex |
| Ventral pathway | Locus coeruleus / raphe / tuberomammillary / ventral tegmental area / lateral hypothalamus, via hypothalamus and basal forebrain | Noradrenaline / serotonin / histamine / dopamine / orexin | Bypasses the thalamus, reaching cortex directly |
| The gate | Thalamus; rhythm imposed by the GABAergic thalamic reticular nucleus | — | All modalities except olfaction relay here. Tonic mode transmits; hyperpolarised burst mode does not |
| Content | Cerebral cortex | — | Requires intact thalamocortical and corticocortical connectivity to integrate input into one experience |
Clinical corollary. 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 even extensive unilateral damage leaves it intact.
(b) Physiological changes in the two types of sleep 8 marks
What earns the marks8 marks · the bulk of the question
| Organising idea | Non-REM = quiet brain, movable body. REM = active brain, paralysed body |
|---|---|
| Cerebral | Metabolism and flow fall in non-REM, equal or exceed awake in REM |
| Cardiovascular | Stable fall in non-REM; unstable with sympathetic surges in REM |
| Respiratory | Regular and reduced in non-REM; irregular and diaphragm-dependent in REM |
| Muscle tone | Reduced in non-REM; atonia in REM |
| Thermoregulation | Maintained at a lower set point in non-REM; suspended in REM |
| Endocrine | Growth hormone in slow-wave sleep; cortisol nadir then morning rise |
| Other | Renal, gastrointestinal, autonomic balance |
Commonly lost: Many described sleep patterns and stages rather than the physiological changes asked for — the same mistake as in part (a). Most gave only the changes common to both types, and knowledge was superficial.
| System | Non-REM | REM |
|---|---|---|
| Cerebral | Metabolic rate and blood flow fall, by up to about 25% in slow-wave sleep. EEG synchronised — slow, high-voltage | Metabolic rate and blood flow equal or exceed the awake value. EEG desynchronised, fast, low-voltage — hence paradoxical sleep |
| Cardiovascular | Heart rate, blood pressure and cardiac output fall steadily; vagal predominance. Stable and regular | Marked variability with sympathetic surges. Greatest risk period for myocardial ischaemia and arrhythmia |
| Respiratory | Rate and tidal volume fall; minute ventilation falls 10-15%; PaCO₂ rises 0.5-1 kPa (about 3-8 mmHg). Chemoreceptor sensitivity reduced. Regular | Irregular in rate and depth. Chemoreceptor and load responses further blunted. Accessory and intercostal contribution lost — ventilation becomes diaphragm-dependent |
| Upper airway | Pharyngeal dilator tone reduced but present | Dilator tone markedly reduced or lost — obstructive events longest, desaturation deepest |
| Muscle tone | Reduced; posture maintained, movement occurs | Atonia except diaphragm and extraocular muscles. Actively produced: pontine REM-on neurons drive medullary interneurons that hyperpolarise spinal α motor neurons |
| Thermoregulation | Maintained around a lower set point. Shivering and sweating both occur | Effectively suspended — poikilothermic. Neither shivering nor sweating; core temperature drifts with ambient |
| Endocrine | Growth hormone peaks in slow-wave sleep, early in the night. Prolactin rises; cortisol at its nadir | Cortisol rises towards the morning peak; testosterone peaks |
| Renal / gastrointestinal | Urine output falls and concentrates; gastric acid secretion continues | Reduced gut motility; swallowing and salivation reduced |
| Autonomic balance | Parasympathetic predominance throughout | Unstable — phasic sympathetic bursts on a parasympathetic background |
| Other | Pupils constricted. Night terrors, sleepwalking and enuresis arise here | Penile or clitoral tumescence. Vivid dreaming, recalled if woken from it |
The architecture, in one paragraph
A normal young-adult hypnogram
Four features to be able to draw. Sleep is entered through non-REM stage 1, never directly into REM. The first descent is the deepest, with the long block of slow-wave sleep in the first third of the night. The cycle repeats at roughly 90-minute intervals, giving four to six REM periods a night. And the balance shifts across the night: slow-wave sleep dominates early and REM periods lengthen towards morning, so the last REM period is the longest and often ends in waking.
Brief arousals are normal and are drawn. In an adult, REM occupies about a quarter of total sleep time — half in the full-term neonate, and falling again in old age. This is a representative night: no source publishes a per-minute trace, so the shape encodes the stated timings rather than measured data.
- Sleep is entered through non-REM: N1 (theta), N2 (defined by sleep spindles and K complexes), N3 (slow-wave, high-voltage delta).
- Cycles with REM about every 90 minutes, four to six times a night.
- Adult proportions approximately N1 5%, N2 45%, N3 25%, REM 25%.
- Slow-wave sleep is front-loaded; REM periods lengthen towards morning.
Commonly lost: This is context, not the answer. Give it in a few lines and spend the marks on the table above.
Anaesthetic relevance, if marks remain
| Point | Why it follows from the physiology |
|---|---|
| Sleep is not anaesthesia | Sleep is reversible by sensory stimulation and homeostatically regulated, so a debt accumulates and is repaid. Anaesthesia is neither |
| OSA is worst in REM | That is where pharyngeal dilator tone is lost most completely |
| Postoperative REM rebound | REM and slow-wave sleep are suppressed on nights 1-2 by pain, opioids and the stress response, then rebound on nights 3-5 — which is why hypoxaemic and ischaemic events cluster on nights 2-4 |