MAVO Year 4 · Coordination and control · Section 10.2

Neurons and nerves

What are neurons?

The nervous system contains large numbers of neurons, or nerve cells, specialised for communication. A neuron has a cell body containing its nucleus and extensions called nerve processes. Dendrites generally receive signals, while an axon carries impulses towards other cells. Some axons extend for a metre or more.

Three types of neuron

• Sensory neurons carry information from receptors towards the central nervous system. Many have a long process passing from a receptor towards the spinal cord, with the cell body situated along the route.
• Motor neurons carry impulses from the central nervous system towards effectors such as muscles. Their long axons carry signals away from the cell body.
• Relay neurons, or interneurons, connect neurons within the central nervous system, for example linking sensory and motor pathways.

What is a nerve?

A nerve contains bundles of nerve fibres surrounded by supportive connective tissue. Many axons have a fatty myelin sheath, which provides electrical insulation and speeds impulse conduction. Not every nerve fibre is myelinated.

• Sensory nerves carry sensory fibres towards the central nervous system.
• Motor nerves carry fibres towards effectors.
• Mixed nerves contain both sensory and motor fibres. Many peripheral nerves are mixed.

The different fibres transmit their own signals; they do not act as one shared electrical wire.

Routes to and from the brain

Much sensory information from the trunk and limbs reaches the brain through the spinal cord. Commands to muscles in these regions also travel through spinal pathways. Cranial nerves serve many structures in the head and neck, with most connecting to the brainstem rather than entering through the spinal cord. Some cranial nerves also serve organs elsewhere in the body.