Show the model answerAttempt it first — that is what makes it stick
(a) Excitable tissue and its significance 3 marks
What earns the marks3 marks
| Definition | Cells that generate AND conduct an all-or-none impulse in response to a stimulus |
|---|---|
| What is required | Resting potential, voltage-gated channels, Na⁺/K⁺-ATPase |
| Which tissues | Nerve, skeletal, cardiac and smooth muscle, some secretory cells |
| Significance — five headings | Nervous function, muscle contraction, homeostasis, sensation, secretion |
Commonly lost: Most gave only a superficial definition, and some thought excitable tissues were pacemaker cells. The physiological significance was frequently left out altogether.
- Requires three things: a maintained resting membrane potential; voltage-gated ion channels that respond to a change in it; and the Na⁺/K⁺-ATPase to restore the gradients afterwards.
- The tissues: nerve, skeletal muscle, cardiac muscle, smooth muscle, and certain secretory cells including the pancreatic β cell and the adrenal chromaffin cell.
| Heading | What excitability provides |
|---|---|
| Nervous system function | All rapid communication between periphery and central nervous system, without decrement over distance |
| Muscle contraction | The action potential triggers excitation-contraction coupling in all three muscle types |
| Maintenance of homeostasis | Every reflex arc, from baroreceptor to stretch reflex, depends on it |
| Sensation and response to environmental stimuli | Converts a graded receptor potential into a propagated frequency code |
| Cell communication and secretion | Depolarisation-triggered Ca²⁺ entry drives exocytosis at nerve terminals and in endocrine cells |
(b) Development and propagation 5 marks
What earns the marks5 marks · split evenly
| Draw the trace | Axes: membrane potential (mV) against time (ms). Mark −70, −55, 0, +30 |
|---|---|
| Development | Threshold, depolarisation, peak, repolarisation, after-hyperpolarisation |
| The ions | Na⁺ in, then K⁺ out — not calcium |
| All-or-none and refractoriness | Coding by frequency and recruitment; absolute and relative periods |
| Propagation | Local circuit → adjacent depolarisation → continuous conduction → unidirectional |
| Myelination | Saltatory conduction at the nodes of Ranvier |
Commonly lost: Many described the cardiac action potential when the question said nerve, and some named calcium as the depolarising ion instead of sodium.
Development
Resting membrane potential is about −70 mV, set principally by the potassium gradient and maintained by the Na⁺/K⁺-ATPase. A stimulus produces a graded, decremental local (electrotonic) potential; if this reaches threshold at about −55 mV, an action potential follows.
| Phase | Channel event | Ion movement | Membrane potential |
|---|---|---|---|
| Depolarisation | Voltage-gated Na⁺ activation (m) gates open | Na⁺ in, down its electrochemical gradient | −55 mV toward +30 mV. Positive feedback — which is why it is all-or-none |
| Peak | Slower Na⁺ inactivation (h) gates close; voltage-gated K⁺ channels now conducting | Na⁺ entry terminated | About +30 mV, approaching but never reaching E_Na of about +60 mV |
| Repolarisation | K⁺ channels open (delayed negative feedback) | K⁺ out | Returns toward rest |
| After-hyperpolarisation | K⁺ channels close slowly, so K⁺ permeability stays above resting | Continued K⁺ efflux | Transiently below −70 mV, toward E_K of about −95 mV |
- All-or-none: above threshold, amplitude is independent of stimulus strength. Intensity is coded by frequency of firing and by the number of fibres recruited.
- Absolute refractory period — Na⁺ channels inactivated, spanning almost the whole spike. No stimulus of any strength will fire the cell.
- Relative refractory period — covers the after-hyperpolarisation; a larger than normal stimulus can still fire it.
Propagation
- At the active site the membrane polarity is reversed — inside positive, while adjacent membrane is still inside-negative.
- This creates a local circuit current: positive charge flows along the axoplasm into the adjacent resting region and returns along the outside.
- That current depolarises the adjacent membrane to threshold, and a new, identical action potential is generated there — continuous conduction along the axon.
- Propagation is unidirectional, because the membrane behind is in its absolute refractory period and cannot be re-excited — this is what prevents retrograde flow.
In myelinated fibres, myelin raises membrane resistance and lowers capacitance, so current cannot leak across the internode. The action potential is regenerated only at the nodes of Ranvier, where voltage-gated sodium channels are concentrated, and appears to jump from node to node — saltatory conduction. It is both faster and less metabolically costly, because sodium entry and its extrusion occur only at the nodes.
Commonly lost: There was a notable lack of understanding of propagation, and some candidates did not address it at all. It is named in the question — give it half the marks of part (b).
(c) Determinants of conduction velocity 2 marks
What earns the marks2 marks
| Axon diameter | Larger is faster |
|---|---|
| Myelination | Saltatory conduction |
| Internodal distance | Longer internodes, fewer regenerations |
| Temperature | Cooling slows gating |
| Extracellular ions | K⁺ sets the distance to threshold; Ca²⁺ sets the threshold |
| Pathology and drugs | Demyelination, ischaemia, compression, local anaesthetics |
Commonly lost: Many incorrectly stated that smaller axons conduct faster. Larger is faster. And some listed the types of nerve fibre instead of what determines velocity — which is a different question.
| Determinant | Direction | Mechanism |
|---|---|---|
| Axon diameter | Larger = faster | Lower longitudinal axoplasmic resistance, so a longer length constant |
| Myelination | Myelinated = much faster | Raises membrane resistance, lowers capacitance, giving saltatory conduction. A myelinated fibre far outpaces an unmyelinated one of the same diameter |
| Internodal distance | Longer = faster, to a limit | Fewer regeneration events per unit length |
| Temperature | Cooling = slower | Slows channel gating kinetics; severe cooling blocks conduction |
| Extracellular ion concentrations | Variable | Extracellular K⁺ alters the resting potential and hence the distance to threshold; low ionised Ca²⁺ lowers the effective threshold |
| Pathology and pharmacology | Slower or abolished | Demyelination, ischaemia, chronic compression, local anaesthetic sodium channel blockade |