How does the structure of a sensory neurone differ from a motor neurone?

A sensory neurone has its cell body located on a side branch in the middle of the axon (with a long dendron), whereas a motor neurone has its cell body at one end within the CNS, featuring many dendrites.

What is the difference between saltatory conduction and continuous conduction?

Saltatory conduction occurs in myelinated neurones where the impulse 'jumps' between nodes of Ranvier, making it much faster. Continuous conduction occurs in unmyelinated neurones where depolarisation must happen along the entire length of the axon.

How does the nervous system distinguish between a light touch and a heavy pressure?

It uses the frequency of action potentials. A stronger stimulus (heavy pressure) generates a higher frequency of impulses per second, while the size (amplitude) of each individual action potential remains the same.

What is a common mistake regarding the movement of sodium ions during depolarisation?

Students often think sodium is pumped in; however, sodium ions actually move into the axon via facilitated diffusion through voltage-gated channels, moving down their concentration gradient created earlier by the pump.

Why is it incorrect to say that the sodium-potassium pump stops during an action potential?

The pump operates continuously to maintain the underlying concentration gradients. The rapid voltage changes of an action potential are caused by the sudden opening of gated channels, which temporarily overwhelm the pump's activity.

What error occurs when describing the charge of the resting potential?

Students may forget to specify that the $-70$ mV is relative to the outside. The inside is negative compared to the outside, which is essential for the polarity required for signaling.

Define the 'Nodes of Ranvier'.

These are uninsulated gaps in the myelin sheath along an axon where the cell membrane is exposed. They contain a high density of voltage-gated ion channels, allowing action potentials to occur only at these points.

What is the 'Refractory Period'?

It is a short period following an action potential during which the neurone membrane cannot be depolarised again. This is caused by the temporary inactivation of sodium channels and ensures impulses are discrete and unidirectional.

Define 'Threshold Potential'.

The critical voltage level (typically $-55$ mV) to which a membrane potential must be depolarised to initiate an action potential. If the stimulus is too weak to reach this level, no impulse is generated.

Why is the resting potential negative inside the axon?

It is negative because the sodium-potassium pump removes more positive ions ($3 Na^+$) than it brings in ($2 K^+$), and the membrane is more permeable to $K^+$, allowing them to diffuse out more easily.

Neurones: Structure & Function | Pearson Edexcel A-Level Biology

1. Definition & Core Concepts

Neurones are the fundamental units of the nervous system, specialized to carry electrical signals known as nerve impulses. Unlike simple electrical wires, these impulses are momentary reversals in electrical potential across the cell membrane.
A nerve is not a single cell but a bundle of many neurones grouped together to provide a pathway for information between the central nervous system (CNS) and the rest of the body.
All neurones share three primary structural components: a cell body containing the nucleus, a long fiber called an axon for signal conduction, and axon terminals that allow communication with other cells.

2. Types of Neurones

Sensory Neurones: These carry impulses from receptors to the CNS. They are characterized by a cell body located on a side branch of the axon and a long dendron that carries the impulse toward the cell body.
Relay Neurones: Found entirely within the CNS, these act as intermediaries connecting sensory and motor neurones. They typically have short axons and highly branched dendrites to receive multiple inputs.
Motor Neurones: These transmit impulses from the CNS to effectors like muscles or glands. They feature a large cell body located at one end of the neurone with many dendrites and a long axon extending to the effector.

3. The Resting Potential

The resting potential is the stable negative charge of approximately $-70$ mV inside an axon relative to the outside when no impulse is being transmitted. This state of being 'polarized' is essential for the neurone to be ready to fire.
This potential is maintained by sodium-potassium pumps, which use ATP to actively transport three sodium ions ( $Na^+$ ) out of the cell for every two potassium ions ( $K^+$ ) pumped in.
Because the membrane is more permeable to $K^+$ than $Na^+$ , potassium ions diffuse back out through open channels faster than sodium can enter, resulting in a net loss of positive charge from the interior.

Graph of an action potential showing voltage changes over time, including depolarisation to +30mV and repolarisation back toward -70mV.

4. Action Potential & Propagation

An action potential is triggered when a stimulus causes the membrane potential to reach a threshold (usually $-55$ mV), leading to the rapid opening of voltage-gated sodium channels. This causes $Na^+$ to rush in, depolarizing the membrane to $+30$ mV.
Following the peak, sodium channels close and voltage-gated potassium channels open, allowing $K^+$ to exit the cell. This repolarisation restores the negative charge, often briefly overshooting to a state called hyperpolarisation.
The refractory period is the brief recovery time after an impulse where the membrane cannot be stimulated again. This ensures that impulses are discrete events and can only travel in one direction along the axon.

5. Myelination & Saltatory Conduction

Many neurones are insulated by a myelin sheath, a fatty layer formed by Schwann cells. This insulation prevents the leakage of ions and increases the electrical resistance of the axon membrane.
Depolarisation can only occur at the nodes of Ranvier, which are small uninsulated gaps between Schwann cells. This forces the electrical impulse to 'jump' from node to node.
This process, known as saltatory conduction, significantly increases the speed of impulse transmission compared to unmyelinated neurones, where the wave of depolarisation must travel the entire length of the membrane.

6. Key Distinctions

Feature	Myelinated Neurone	Non-myelinated Neurone
Conduction Speed	Very fast (up to 120 m/s)	Slower (approx. 1 m/s)
Mechanism	Saltatory (jumping) conduction	Continuous wave of depolarisation
Energy Efficiency	Higher (less ion pumping needed)	Lower (more ion pumping required)

All-or-Nothing Principle: An action potential is either generated fully or not at all. A stronger stimulus does not create a 'larger' impulse; instead, it increases the frequency of impulses sent to the brain.

7. Exam Strategy & Tips

Ion Directionality: Always double-check the direction of ion movement. During resting potential, $Na^+$ is pumped OUT and $K^+$ is pumped IN. During depolarisation, $Na^+$ diffuses IN through voltage-gated channels.
Structural Identification: In diagrams, look for the cell body position to identify the neurone type. If the cell body is in the middle of the fiber, it is a sensory neurone; if it is at the end with dendrites, it is a motor neurone.
Threshold Importance: Remember that if the stimulus does not reach the $-55$ mV threshold, no action potential occurs. This is a common point of failure in student explanations of the all-or-nothing principle.

Neurones: Structure & Function

Summary

1. Definition & Core Concepts

2. Types of Neurones

3. The Resting Potential

4. Action Potential & Propagation

5. Myelination & Saltatory Conduction

6. Key Distinctions

7. Exam Strategy & Tips

Neurones: Structure & Function

Summary

1. Definition & Core Concepts

2. Types of Neurones

3. The Resting Potential

4. Action Potential & Propagation

5. Myelination & Saltatory Conduction

6. Key Distinctions

7. Exam Strategy & Tips