Neurones and synapses

Information travels through the nervous system along specialised cells called neurones. The signal moving along a neurone is an electrical impulse. A nervous pathway usually contains several neurones linked together, so an impulse can pass from one part of the body to another.

In a simple pathway, three types of neurone carry information in different directions:

• A sensory neurone carries impulses from a receptor towards the central nervous system. A receptor detects a change in the surroundings or inside the body.
• A relay neurone passes impulses from one neurone to another within the central nervous system, which consists of the brain and spinal cord.
• A motor neurone carries impulses from the central nervous system to an effector. An effector, such as a muscle or gland, produces a response.

For example, an impulse may travel from a receptor along a sensory neurone, pass to a relay neurone in the central nervous system, and then pass to a motor neurone leading to an effector.

Synapses between neurones

A synapse is a junction between two neurones. Neurones in a pathway are linked at synapses, allowing information to pass from one neurone to the next. The neurones are separate cells, rather than one continuous cell running through the whole pathway.

Recognising neurones in diagrams

The position of a neurone in a pathway and the direction in which it carries impulses are useful clues to its type. The shape of its cell body can also help:

• Sensory neurone: connects a receptor to the central nervous system. Its cell body is often shown along one side of its long fibre.
• Relay neurone: lies entirely within the central nervous system and connects other neurones. It is often shown as a relatively short neurone.
• Motor neurone: runs from the central nervous system to an effector. Its cell body is usually shown at one end, within the central nervous system, with a long fibre extending towards the effector.

Diagrams can vary, so use the neurone’s connections as well as its shape to identify it.

Reflex arcs and reflex actions

A reflex action is a rapid, automatic response to a stimulus, such as withdrawing a hand from a hot surface. It happens without a conscious decision, allowing the body to respond quickly.

A reflex connects a detected stimulus to an action by an effector. An effector may be a muscle, which contracts, or a gland, which releases a substance.

The simple reflex arc

A reflex arc is the pathway followed during a reflex action. Signals called nerve impulses pass along neurones in the following sequence:

1. A receptor detects a stimulus: a change in the internal or external environment.
2. A sensory neurone carries an impulse from the receptor towards the central nervous system (CNS), which consists of the brain and spinal cord.
3. A relay neurone passes the impulse within the CNS.
4. A motor neurone carries the impulse away from the CNS to an effector.
5. The effector produces the response.

For example, when a hand touches a hot surface, a receptor detects the stimulus and impulses pass through the reflex arc. The motor neurone carries an impulse to a muscle, which contracts and pulls the hand away. The reflex arc therefore coordinates the stimulus with a quick response.