Neurons and the nerve impulse

MDCAT Biology · Biology · Coordination and control. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A neuron has a cell body, dendrites that carry impulses towards the cell body, and an axon that carries them away. Sensory neurons run from receptors to the central nervous system, motor neurons run from it to effectors, and relay neurons (interneurons) link the two inside it.

At rest the inside of the axon is negative, about −70 mV. This is the resting potential. When a stimulus reaches threshold, sodium channels open and Na⁺ rushes in, so the inside becomes positive (depolarisation). Potassium channels then open and K⁺ flows out (repolarisation). This brief reversal is the action potential, and it travels along the axon.

At a synapse the impulse crosses a gap as a chemical, the neurotransmitter.

Rules to remember

Resting potential: sodium–potassium pump moves 3 Na⁺ out and 2 K⁺ in; the membrane leaks K⁺ out far more easily than Na⁺ in; large negative ions stay inside
Depolarisation = Na⁺ diffuses in. Repolarisation = K⁺ diffuses out. The pump then restores the original ion balance
All-or-none: below threshold there is no impulse; above it every impulse has the same size. A stronger stimulus gives more impulses per second, not bigger ones
Myelin sheath (Schwann cells) insulates the axon; the impulse jumps between nodes of Ranvier (saltatory conduction), up to about 100 m s⁻¹
Synapse: impulse arrives → Ca²⁺ enters the synaptic knob → vesicles release neurotransmitter → it diffuses across the cleft → binds receptors on the postsynaptic membrane → new impulse
Acetylcholine is broken down by acetylcholinesterase. A synapse passes impulses in one direction only
Sensory neuron cell bodies lie in the dorsal root ganglion; Nissl's granules are groups of ribosomes in the cell body

Common mistake

Writing that the sodium–potassium pump causes the action potential. The pump sets up the gradients; the action potential is ions diffusing through channels, Na⁺ in and then K⁺ out.

Worked example

A poison blocks the voltage-gated sodium channels of an axon but leaves all other channels and the pump working. What happens when the axon is stimulated above threshold?

The resting potential stays normal, because the pump and K⁺ leak still work.
Depolarisation needs Na⁺ to rush in through voltage-gated sodium channels.
These channels are blocked, so the inside cannot become positive.

Answer: No action potential is produced, so no impulse travels along the axon.

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