Year 1 · Senses, behaviour and coordination · Section 5.4
The nervous system
The structure of the nervous system
The nervous system has two main parts:
- Central nervous system: The brain and spinal cord.
- Peripheral nerves: Connect the central nervous system to the rest of the body.
Cranial nerves connect directly to the brain and mainly serve the head and neck, although some extend farther. Spinal nerves connect to the spinal cord and serve the trunk and limbs. The spine protects the spinal cord.
How the nervous system works
Sensory receptors detect stimuli and initiate signals. Nerve impulses travel towards the brain, where information is processed. This allows conscious perception and voluntary responses.
- Vision and smell receptors send signals towards the brain.
- The brain can send signals towards muscles and glands.
- Glands produce and release substances, such as saliva.
- Muscles contract, for example when you reach for an object.
The nervous system therefore helps control muscles and glands.
Nerve cells
A nerve cell, or neuron, has a cell body containing the nucleus and long extensions that carry signals towards or away from it.
Neurons pass signals to other neurons or to muscle and gland cells. Many cell bodies lie in the central nervous system; others occur in clusters outside it.
Nerves
Nerves are bundles of nerve fibres surrounded by protective connective tissue. Many individual fibres also have an insulating layer that speeds signal transmission.
Thousands of fibres can carry different impulses at the same time. Some fibres are about a metre long, connecting distant body regions with the central nervous system.
Routes taken by impulses
Signals can follow different routes:
- Voluntary response: Sensory signals reach the brain, you decide how to respond, and motor signals travel to muscles.
- Reflex: A rapid, automatic response organised by a reflex pathway, without waiting for a conscious decision.
In a withdrawal reflex, sensory signals travel to the spinal cord. Relay neurons pass signals to motor neurons, which activate muscles. Signals also reach the brain, allowing you to become aware of what happened.
The pathway is called a reflex arc. Some reflexes, such as the pupil reflex and blinking reflex, are coordinated by circuits in the brainstem rather than the spinal cord. Reflexes help protect the body from harm.
Glossary
- Voluntary response
- A response involving conscious decision-making in the brain.
- Cell body
- The part of a neuron containing the nucleus.
- Central nervous system
- The brain and spinal cord, which process information and coordinate responses.
- Gland
- An organ or tissue producing and releasing substances such as saliva, sweat or sebum.
- Reflex
- A rapid, automatic response to a stimulus, often protective.
- Reflex arc
- The pathway followed by signals during a reflex, from a receptor through nerve cells to an effector such as a muscle.
- Spinal cord
- The part of the central nervous system carrying signals between brain and body and coordinating many reflexes.
- Relay neurons
- Neurons in the central nervous system that pass signals between other neurons.
- Extensions
- Long processes of neurons carrying signals towards or away from the cell body.
- Nerve
- A bundle of nerve fibres carrying signals between body regions and the central nervous system.
- Neurons
- Cells specialised to receive and transmit signals, consisting of a cell body and extensions.