Nervous system

What the exam asks

Expect a trace, a table of ion concentrations, or a dose-response curve, with the mechanism handed to you in the stem. The commonest items are: given the equilibrium potentials and which channel just opened, say which way the voltage moves; given a change in extracellular potassium, predict the resting potential; given a drug, say which step of transmission it hit; and given a dose-response curve, classify the antagonist. The trap that catches most candidates is treating the sodium-potassium pump as the thing that holds the resting potential, and so predicting that blocking it collapses the voltage at once. It does not. Permeability sets the voltage, the pump sets the gradients, and the gradients take minutes to run down. The second trap is the mirror of the enzyme one: a lower response at one dose is consistent with every kind of antagonist, so read the plateau at the right-hand end before you classify anything.

The nervous system is examined as a signalling problem, not as anatomy. Nobody is going to ask you to name a tract. You are given a membrane, a set of ions, a channel that opens, or a drug that binds something, and asked what the voltage or the response does next.

Almost every action potential question comes off one idea. The membrane potential sits between the equilibrium potentials of the ions the membrane currently lets through, pulled towards whichever ion it is most permeable to. At rest the membrane is mostly permeable to potassium, so the voltage sits near the potassium equilibrium potential: around -70 mV, with E_K near -90 mV. Open the voltage-gated sodium channels and permeability swings to sodium, so the voltage chases E_Na near +60 mV. Shut them, open more potassium channels, and it falls back. Carry that one sentence in and the rest is bookkeeping.

The synapse is where the electrical signal is converted to a chemical one and back, and the exam likes it because every step is a step that a drug or a stem can block. Arrival, calcium entry, vesicle fusion, receptor binding, and removal from the cleft: name which step failed and the answer follows.

The pharmacology is the enzyme kinetics graph wearing different labels. An agonist is a substrate, an antagonist is an inhibitor, and the question is always whether more of the natural ligand can outcompete the blocker. If it can, the curve shifts right and the ceiling survives. If it cannot, the ceiling falls.

What to hold

• An ion's equilibrium potential is the voltage at which the electrical pull exactly cancels its concentration gradient, so there is no net flow of that ion.
• The membrane potential is a weighted average of the equilibrium potentials of the permeant ions, weighted by how permeable the membrane is to each. This is why the resting voltage sits near E_K and the action potential peak heads towards E_Na.
• The sodium-potassium pump maintains the gradients that make those equilibrium potentials what they are, and contributes a few millivolts directly. It is not what holds the resting potential moment to moment.
• Depolarisation past threshold opens voltage-gated sodium channels, and the sodium entry depolarises further, which opens more of them. That positive feedback is why the action potential is all-or-none.
• Sodium channels then inactivate and delayed potassium channels open, so permeability swings back to potassium and the membrane repolarises, often overshooting below rest.
• The absolute refractory period exists because inactivated sodium channels cannot reopen until the membrane repolarises. It caps firing frequency and forces the impulse to travel one way.
• Action potential amplitude is fixed, so stimulus intensity is coded by firing frequency and by how many fibres are recruited, never by a bigger spike.
• Conduction is faster in wider axons and in myelinated ones, because both let local current spread further before it needs regenerating.
• Transmitter release requires calcium entering the terminal through voltage-gated calcium channels. An action potential that arrives with no extracellular calcium releases nothing.
• Postsynaptic potentials are graded and they decay, so they must summate in space and time to bring the trigger zone to threshold. The all-or-none decision happens there.
• A reflex arc needs a receptor, an afferent neuron, an integrating centre, an efferent neuron, and an effector. The brain is informed afterwards, which is why the response beats the perception.
• Most organs get both sympathetic and parasympathetic input, so blocking one division reveals the tone the other was exerting all along.