Neurons are specialized cells that carry information through the nervous system. For AQA Psychology, you need to know how sensory, relay, and motor neurons are built and how each supports behavior.

What neurons do

Neurons are specialized cells built to receive, carry, and pass on information. Their structure is not accidental: each part of the cell helps it move impulses in a particular direction. In A-Level Psychology, three main neuron types are usually identified: sensory neurons, relay neurons, and motor neurons. They differ in shape, location, and role, but all contribute to rapid coordination of responses.

A labeled reflex arc showing the flow of information from a stimulus along a sensory neuron into the spinal cord, through an interneuron (relay neuron), and out via a motor neuron to a muscle (effector). This reinforces the core AQA idea that relay neurons in the CNS link sensory input to motor output in a fast, coordinated pathway. Source

Most neurons share some basic features:

A labeled diagram of a prototypical neuron highlighting dendrites (input region), the cell body/soma, the axon (conduction pathway), and axon terminals (output region), with myelin shown as insulation along the axon. This provides a clear visual anchor for the direction of information flow and the structural terms used throughout the sensory, relay, and motor neuron descriptions. Source

• Dendrites receive input from other cells or receptors.

• The cell body contains the nucleus and keeps the neuron alive.

• The axon carries the impulse away from the cell body.

• The axon terminals pass the signal to the next neuron, muscle, or gland.

The exact arrangement of these parts changes depending on the job the neuron performs.

Sensory neurons

Sensory neurons carry information from receptors to the central nervous system. They are activated when a stimulus such as light, pressure, temperature, or sound is detected.

Structure

In a typical sensory neuron, the cell body is located partway along the neuron, rather than at one end. One long dendron carries the impulse from the receptor toward the cell body, and a short axon carries it from the cell body into the CNS. This makes sensory neurons look different from motor neurons in diagrams. Many sensory neurons are also myelinated, meaning they have a fatty insulating covering that helps impulses travel more quickly.

Function

The function of a sensory neuron is to detect change in the internal or external environment and send that information to the CNS for processing. Its structure suits this role:

• the long dendron allows information to travel from a receptor over some distance

• myelin helps increase the speed of transmission

• the ending of the axon allows the message to be passed to neurons in the CNS

Sensory neurons are therefore the main route by which stimulation enters the nervous system.

Relay neurons

Relay neurons are found within the CNS and link other neurons together. Their main function is to pass information between sensory and motor neurons, although they can also connect different neurons in more complex pathways.

Structure

Relay neurons usually have short dendrites and a short axon. Because they often only have to transmit information across a small distance inside the brain or spinal cord, they do not need the long dendron of a sensory neuron or the long axon of a motor neuron. Their branching structure allows them to receive input from several neurons and send output onward.

Function

The function of a relay neuron is not simply to carry on a message unchanged. It helps organize neural pathways by selecting, combining, and passing information to the appropriate neuron. In exam answers, it is important to state that relay neurons act as the link between sensory input and motor output. Their short connections are well suited to rapid communication across networks in the CNS.

Motor neurons

Motor neurons carry impulses from the CNS to effectors, producing a response such as muscle contraction or gland activity.

Structure

A motor neuron has a cell body at one end with many short dendrites spreading from it. These dendrites receive input from other neurons. From the cell body, a single long axon extends to the effector. The axon may be covered by a myelin sheath, which helps the impulse move quickly along the neuron. At the far end, axon terminals branch out so the motor neuron can communicate with the muscle or gland it controls.

Function

The function of a motor neuron is to carry the final command that produces action. Its structure supports this role:

• many dendrites allow input from relay neurons or other cells

• a long axon allows the impulse to travel from the CNS to a distant effector

• branching terminals help one motor neuron influence many muscle fibers or parts of a gland

Motor neurons therefore convert neural processing into an observable response.

Comparing the three neuron types

The main differences are easiest to remember by linking structure to function.

• Sensory neurons: from receptor to CNS; typically have a long dendron, cell body along the length, and short axon

• Relay neurons: found within the CNS; usually have short dendrites and a short axon because they connect nearby neurons

• Motor neurons: from CNS to effector; usually have many short dendrites, a cell body at one end, and one long axon

A common exam mistake is to describe only what each neuron does. A stronger answer explains how the neuron’s shape helps it do that job.