Nerve impulse transmission

Where it happens

Nerve impulse transmission takes place along the axon of a neurone, the long, thin extension that carries signals away from the cell body. Most long axons are wrapped in a fatty myelin sheath, formed by Schwann cells, with small gaps called nodes of Ranvier between them.

A neurone has three regions. The cell body contains the nucleus and most of the cytoplasm; the dendrites are short, branched extensions that receive signals and carry them toward the cell body; and the axon carries impulses away from the cell body toward the axon terminals.

In a sensory neurone the cell body sits to one side of a long fibre, while in a motor neurone the cell body is at one end with a cluster of dendrites around it. The impulse travels in one direction only, because the synapse at the axon terminal releases neurotransmitter from one side only.

Inputs and outputs

• Input: a stimulus at a receptor, or neurotransmitter arriving at a dendrite, that is strong enough to reach the threshold.
• Input: sodium ions in the tissue fluid outside the axon, which flow inward through sodium channels when the membrane is stimulated.
• Input: potassium ions inside the axon, which flow outward through potassium channels to restore the resting state.
• Input: ATP from mitochondria in the axon, used by the sodium-potassium pump to maintain the resting ion gradients.
• Output: a wave of depolarisation (the action potential) that travels along the axon to the axon terminal.
• Output: release of neurotransmitter at the synapse, which passes the signal to the next neurone or to an effector.
• Output: the ion gradients being restored behind the impulse, so the same stretch of axon can carry the next impulse.

The steps

1. At rest, the axon membrane is polarised: the sodium-potassium pump uses ATP to move sodium ions out and potassium ions in, so the outside is positively charged relative to the inside (the resting potential).
2. A stimulus at the threshold strength opens sodium channels at that point, and sodium ions rush into the axon down their concentration gradient.
3. The inside of the membrane at that point becomes positive relative to the outside, this reversal is depolarisation, and it forms the action potential.
4. The local reversal of charge sets up small currents that open sodium channels in the neighbouring part of the membrane, so the depolarisation moves along the axon as a wave.
5. Behind the impulse, sodium channels close and potassium channels open; potassium ions flow out of the axon, restoring the negative inside and positive outside (repolarisation).
6. For a brief moment after repolarisation the membrane cannot be stimulated again (the refractory period), which stops the impulse from travelling backward and keeps it moving in one direction.
7. The sodium-potassium pump then restores the original ion distribution using ATP, so the membrane is ready for the next impulse.
8. In a myelinated axon, ions can cross the membrane only at the nodes of Ranvier, so the impulse jumps from node to node (saltatory conduction), which is far faster than continuous conduction along an unmyelinated axon.

Why it matters and how it is controlled

Nerve impulse transmission allows information from receptors to reach the brain or spinal cord, and allows instructions to reach effectors such as muscles and glands, very quickly. The myelin sheath speeds this up further, which is important for rapid responses such as reflexes.

Transmission is controlled by the all-or-nothing principle: a stimulus below the threshold produces no impulse, and any stimulus at or above the threshold produces an impulse of the same size. A stronger stimulus does not make a bigger impulse; it makes impulses more frequent.

Speed depends on three factors, the presence of a myelin sheath, the diameter of the axon (wider axons conduct faster) and temperature. Diseases in which the myelin sheath is damaged, such as multiple sclerosis, slow or block transmission and show why the sheath matters.

Some substances change transmission: local anaesthetics block sodium channels so that impulses cannot start, and certain toxins keep channels open so that muscles stay contracted.

How it is examined

You may be asked to describe how depolarisation and the return to resting charge move along an axon, to explain why a myelinated neurone transmits impulses faster than an unmyelinated one, or to interpret a graph of electrical charge across the axon membrane over time.

A typical graph question plots the electrical potential across the axon membrane against time in milliseconds. You should be able to label the resting potential, the rising phase caused by sodium ions entering, the peak, and the falling phase caused by potassium ions leaving, and to explain each in terms of ion movement.

Comparison questions ask for two differences between transmission along an axon and transmission across a synapse: the axon uses an electrical wave and is fast, while the synapse uses a chemical neurotransmitter, is slower and can only pass the signal in one direction.

Common misconceptions

Worked exam-style question

Question. Two axons were stimulated in a laboratory and the time taken for an impulse to travel a fixed distance was recorded. Axon P is myelinated and the impulse took 0.5 milliseconds; axon Q is unmyelinated and the impulse took 5.0 milliseconds.

(a) Explain, in terms of ion movement, what happens at the point on axon Q where the stimulus is applied. (b) Explain why the impulse along axon P is faster.

(c) When the stimulus on axon Q was doubled in strength, the size of each impulse stayed the same. Explain this observation.

Model answer. (a) The stimulus opens sodium channels, so sodium ions diffuse into the axon. The inside of the membrane becomes positive relative to the outside, which is called depolarisation; this local change triggers the same change in the neighbouring part of the membrane, so the impulse moves along the axon.

Potassium ions then leave the axon to restore the resting potential. (b) Axon P has a myelin sheath that insulates the membrane, so ions can only cross at the nodes of Ranvier.

The impulse jumps from node to node (saltatory conduction) instead of depolarising every part of the membrane, so it travels the distance in less time. (c) Nerve impulses obey the all-or-nothing principle: once the threshold is reached, an impulse of fixed size is produced.

A stronger stimulus increases the frequency of impulses, not their size.

Source:SRC-DSKP-EN

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