Neural Control and Coordination — the Neuron, the Nerve Impulse and the Synapse
The heart of this chapter is a single event — how a nerve impulse travels — and it is worth building slowly because everything else hangs on it. A neuron at rest is polarised: negative inside, positive outside, maintained by a sodium–potassium pump. A stimulus flips a patch to positive (depolarisation), the flip travels as a wave, and the patch behind recovers (repolarisation). Then at the synapse the electrical signal converts to a chemical one — a neurotransmitter crosses the gap — and back to electrical. Get that sequence and the numbers (resting potential, the ion that rushes in) become meaningful rather than arbitrary. Around this core sit the map of the nervous system and the structure of the neuron, both largely recall.
Generation and Conduction of the Nerve Impulse
🎯 Resting, then a wave: how the impulse actually moves
At rest the neuron's membrane holds a resting potential of about –70 mV: the inside is negative relative to the outside. This is kept by the sodium–potassium pump, which throws three Na⁺ out for every two K⁺ it lets in, leaving Na⁺ high outside and K⁺ high inside. When a stimulus arrives, sodium channels open at that spot and Na⁺ rushes in, so the inside briefly becomes positive — this reversal is depolarisation, the action potential (about +30 mV). The depolarised patch acts as a stimulus for the patch next to it, so the reversal travels along the axon as a self-propagating wave — the nerve impulse. Behind the wave, K⁺ moves out and the resting negative charge is restored — repolarisation — after which the pump quietly re-sorts the ions. Two exam facts follow directly. First, the impulse is one-way because the region just behind is briefly refractory and cannot be re-excited immediately. Second, in a myelinated axon the impulse jumps node to node (saltatory conduction), which is far faster than creeping along a bare fibre — the reason myelination matters.🧠 Sodium in, then potassium out — do not swap them
The single most common error is confusing which ion does what. Fix it with the order of events: depolarisation is sodium rushing IN (making the inside positive), and repolarisation is potassium moving OUT (restoring the inside negative). The sodium–potassium pump, running all the time in the background, is what re-establishes the original gradients afterwards, but the fast electrical events themselves are sodium-in then potassium-out. A related trap is the resting potential's sign: the inside is negative (–70 mV), not positive, at rest. And note the pump is an active process needing ATP, whereas the ion rushes during the impulse are passive flows down open channels — so "the action potential requires ATP directly" is false; the ATP cost is in the pump that resets the stage between impulses.The Synapse
- At a chemical synapse the two neurons do not touch; a gap called the synaptic cleft lies between them. The impulse cannot jump the gap electrically, so it is carried across by a chemical neurotransmitter (such as acetylcholine).
- When the impulse reaches the axon terminal, it opens calcium channels; Ca²⁺ enters and makes the synaptic vesicles fuse with the membrane and release the neurotransmitter into the cleft. It binds receptors on the next neuron and generates a fresh impulse there.
- Because the transmitter is released on only one side and received on the other, the chemical synapse enforces one-way transmission — a second reason impulses travel in a single direction. Electrical synapses (rarer) allow faster, direct current flow.
ℹ️ Why the synapse is where drugs and diseases act
The conversion from electrical to chemical and back at the synapse looks like an inefficiency — why not just keep it electrical? But the chemical step is what makes the nervous system flexible and controllable. Because transmission depends on a released chemical binding a receptor, the strength of the signal can be amplified, dampened, or blocked at the synapse, and one neuron can integrate inputs from many others before deciding to fire. It is also why the synapse is the target of so many drugs and toxins: caffeine, nicotine, many anaesthetics and nerve agents all work by altering neurotransmitter release, binding or breakdown. This is the deeper reason to learn the synapse well — it is not just a junction to memorise but the point at which the whole system becomes adjustable, and the point at which most interference, medical or chemical, takes hold.Key takeaways
- Neuron = cell body + dendrites (receive) + one axon (send); myelin sheath, broken at nodes of Ranvier, speeds conduction.
- Resting potential ≈ –70 mV (inside negative), maintained by the Na⁺–K⁺ pump (3 Na⁺ out : 2 K⁺ in).
- Depolarisation = Na⁺ rushing IN (inside becomes positive); repolarisation = K⁺ moving OUT.
- In myelinated axons the impulse jumps node to node — saltatory conduction — which is much faster.
- At a chemical synapse, Ca²⁺ entry releases a neurotransmitter across the synaptic cleft; this enforces one-way transmission.
- CNS = brain + spinal cord; PNS has afferent (sensory) and efferent (motor) fibres; autonomic = sympathetic vs parasympathetic.
- Cerebellum = balance and coordination; medulla = heartbeat and breathing; hypothalamus = homeostasis.
- The Na⁺–K⁺ pump needs ATP; the ion rushes of the action potential itself are passive flows through open channels.
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
The depolarisation phase of a nerve impulse is caused by the rapid inflow of:
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Answer: A — Sodium ions (Na⁺)
When sodium channels open, Na⁺ rushes in and makes the inside momentarily positive — that reversal is depolarisation. Potassium flows outward during repolarisation, so swapping the two ions is the classic mistake this question is built to catch.The resting membrane potential of a neuron is approximately:
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Answer: A — –70 mV, with the inside negative
At rest the inside of the membrane is about 70 mV negative to the outside, kept there by the sodium–potassium pump. The +30 mV figure is the peak of the action potential, not the resting state, so it describes a depolarised membrane instead.At a chemical synapse, the arrival of the impulse causes the release of neurotransmitter after the entry of:
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Answer: A — Calcium ions into the axon terminal
The impulse opens calcium channels at the terminal; incoming Ca²⁺ makes the vesicles fuse and release the transmitter into the cleft. Sodium entry belongs to impulse generation in the axon, not to transmitter release, so that option confuses two different steps.Saltatory conduction, in which the impulse jumps from node to node, occurs in:
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Answer: A — Myelinated axons
Myelin insulates the axon except at the nodes of Ranvier, so the impulse regenerates only at the nodes and effectively leaps between them — much faster than continuous conduction along an unmyelinated fibre. That is precisely why unmyelinated axons cannot show saltatory conduction.Control of heartbeat and breathing rate lies in the:
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Answer: A — Medulla oblongata
The medulla oblongata in the hindbrain houses the vital centres for heartbeat, breathing and blood pressure. The cerebrum handles thought and voluntary action and the cerebellum handles balance and coordination, so neither runs these automatic vital functions.The sympathetic nervous system, on activation, typically:
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Answer: A — Increases heart rate and dilates the pupils
The sympathetic division drives the "fight or flight" response — faster heart, dilated pupils, diverted blood flow. Slowing the heart and stimulating digestion is the opposing parasympathetic "rest and digest" role, which the second option describes instead.The sodium–potassium pump that maintains the resting potential is:
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Answer: A — An active transport process requiring ATP
The pump moves ions against their gradients — three Na⁺ out and two K⁺ in per cycle — which requires ATP, making it active transport. The fast ion movements of the action potential are passive flows through channels, a separate and energy-free set of events.One-way transmission of a nerve impulse across a chemical synapse is ensured because the neurotransmitter is:
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Answer: A — Released only from the pre-synaptic side and received only on the post-synaptic side
Vesicles of transmitter sit only in the pre-synaptic terminal and receptors only on the post-synaptic membrane, so the chemical can travel in just one direction. Current cannot leap the cleft in a chemical synapse, which is exactly why the chemical step is needed and why it enforces direction.