How a Neuron Carries a Nerve Impulse (Explained Simply)

The neuron's job

A neuron is the basic structural and functional unit of the nervous system. Its job is to carry a nerve impulse, a rapid electrical signal, from one part of the body to another, so that the body can detect a stimulus and respond to it quickly.

A typical neuron has three key regions: the dendrites, which receive an impulse; the cell body, which contains the nucleus and controls the cell; and the axon, a long fibre that carries the impulse away from the cell body toward the next neuron or an effector such as a muscle.

Speed matters here because many responses, such as pulling a hand away from something sharp or hot, need to happen within a fraction of a second to protect the body from harm. A chain of neurons, working together and passing the impulse from one to the next, is what makes such a quick, coordinated response possible.

There are three main types of neuron in the human nervous system: sensory neurons, which carry impulses from a receptor toward the central nervous system; relay neurons, which connect neurons within the brain and spinal cord; and motor neurons, which carry impulses from the central nervous system to an effector, such as a muscle. The path described below applies to how an impulse travels along any one of these neurons.

The path of an impulse

1. A stimulus, such as touch, light, or a chemical, is detected by a receptor, which generates a nerve impulse.
2. The dendrites receive the impulse and carry it toward the cell body.
3. The impulse passes through the cell body and travels along the axon, a long fibre extending away from the cell body.
4. If the axon is myelinated, the impulse jumps rapidly between gaps in the myelin sheath called nodes of Ranvier, which greatly increases the speed of transmission.
5. The impulse reaches the axon terminal, also called the synaptic knob, at the end of the axon.

Why the myelin sheath matters

Many axons are wrapped in a myelin sheath, a fatty, insulating layer produced by surrounding Schwann cells. Because the myelin sheath is not continuous, it is broken at regular intervals by small gaps called nodes of Ranvier, the impulse cannot travel smoothly along the whole length of the axon.

Instead, it jumps from node to node, a process called saltatory conduction, which is far faster than moving continuously along an unmyelinated membrane.

This is why myelinated neurons transmit impulses much faster than unmyelinated ones, and why damage to the myelin sheath slows down or disrupts nerve signalling. Not every neuron relies on speed in the same way, myelinated neurons are common where a fast response matters most, while unmyelinated neurons are more often found in pathways where the timing is less critical.

Handing the impulse to the next neuron

An electrical impulse cannot cross the tiny gap between one neuron and the next, called a synapse, on its own. When the impulse reaches the axon terminal, it triggers the release of chemical messengers called neurotransmitters into the synaptic gap.

These neurotransmitters diffuse across the gap and bind to specific receptors on the dendrite of the next neuron, which generates a brand new electrical impulse in that neuron.

Because neurotransmitters are only released from the axon terminal on one side of the synapse and received by receptors on the dendrite side of the next neuron, an impulse can only cross a synapse in one direction. This one-way property keeps signals moving reliably in the correct order along a chain of neurons, from receptor to effector, without doubling back.

Common mix-ups

What affects how fast a nerve impulse travels?

Not every neuron conducts an impulse at the same speed. Three features of the neuron and its surroundings make the biggest difference, which is why the body uses fast pathways for urgent responses and slower ones where timing is less critical.

• Myelination, a myelinated axon carries an impulse far faster than an unmyelinated one, because the impulse jumps between nodes of Ranvier instead of moving continuously along the membrane.
• Axon diameter, a wider axon offers less resistance to the flow of the impulse, so a thicker fibre conducts faster than a thin one.
• Temperature, within the body's normal range, impulses travel faster when the axon is warmer, because the movement of ions across the membrane speeds up.

How is the neuron topic examined in SPM Biology?

In the written papers, this topic is usually assessed through diagrams and precise terminology rather than long descriptions. A common structured task is a labelled diagram of a neuron, where marks are awarded for naming the dendrite, cell body, axon, myelin sheath, nodes of Ranvier and axon terminal correctly, and for stating the function of each.

Explaining why a myelinated axon conducts faster, naming saltatory conduction, and why a synapse transmits in one direction are the two explanation points that recur most.

It helps to know where the marks sit across the papers.

• Paper 1 is 40 objective questions in 1 hour 15 minutes, where a neuron question is usually a single labelling or function item.
• Paper 2 carries 100 marks over 2 hours 30 minutes, and this is where a full labelled diagram or a compare-and-explain answer on impulse speed and synaptic transmission is most likely to carry the bulk of the marks on this topic.
• Answers are marked on precise terms, 'saltatory conduction', 'nodes of Ranvier', 'neurotransmitter', 'one direction', so vague phrasing loses the mark even when the idea is roughly right.

Source:SRC-DSKP-EN

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