Neural signalling

Biology · C Interaction and interdependence. 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 is a cell that carries electrical impulses. Dendrites are short branched fibres that bring signals to the cell body. The axon is one long fibre that carries impulses away from it.

At rest the inside of the neuron is negative compared with the outside, at about −70 mV. This is the resting potential. An impulse is an action potential: a brief reversal of this voltage that travels along the axon.

Neurons meet at synapses, where there is a tiny gap. The signal crosses the gap as a chemical called a neurotransmitter, which is released by one neuron and binds to receptors on the next.

Rules to remember

Sodium–potassium pump: uses ATP to pump 3 Na⁺ out and 2 K⁺ in, so more positive charge leaves than enters.
Depolarization: Na⁺ channels open and Na⁺ diffuses in; the inside rises to about +30 mV.
Repolarization: K⁺ channels open and K⁺ diffuses out; the inside becomes negative again.
Speed is higher in myelinated fibres and in fibres of larger diameter.
speed of conduction = distance ÷ time (m s⁻¹); 1 ms = 0.001 s.
At a synapse: impulse arrives, Ca²⁺ diffuses into the presynaptic neuron, vesicles fuse with the membrane and release neurotransmitter by exocytosis.
Acetylcholine diffuses across the cleft and binds to receptors; Na⁺ channels open and the postsynaptic membrane depolarizes (excitatory postsynaptic potential).

Common mistake

Students say the pump causes the action potential. The pump only sets up the resting potential. The action potential is caused by ions diffusing through channels: Na⁺ in, then K⁺ out.

Worked example

An impulse travels 1.2 m along a myelinated axon in 15 ms. Calculate the speed of conduction.

Change the time to seconds: 15 ms = 0.015 s.
Speed = distance ÷ time = 1.2 ÷ 0.015 = 80.

Answer: 80 m s⁻¹.

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C2.2 Neural Signalling

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