Science

The nervous system

The nervous system allows the body to respond to changes in the environment. This is a process usually coordinated by the brain. Reflex actions are extra-rapid responses to stimulistimuli: things that set off a reaction in the nervous system - for example, light, heat, sound, gravity, smell, taste, or temperature. The singular is stimulus, and this process also involves the nervous system, but bypasses the brain.

Receptors and effectors

Receptors

Receptors are groups of specialised cells. They can detect changes in the environment, which are called stimuli, and turn them into electrical impulses. Receptors are often located in the sense organs, such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus.

Receptors

sense organsreceptors sensitive to
A baby holding its mother's finger

Skin

touch, pressure, pain and temperature
A womens tongue

Tongue

chemicals in food

Nose

chemicals in the air

Eyes

light

Ears

sound and position of the head

The central nervous system - CNS - in humans consists of the brain and spinal cord. When a receptor is stimulated, it sends a signal along the nerve cells - neurones - to the brain. The brain then co-ordinates the response.

Effectors

An effector is any part of the body that produces the response. Here are some examples of effectors:

  • a muscle contracting to move the arm
  • a muscle squeezing saliva from the salivary gland
  • a gland releasing a hormonehormone: chemical messengers produced in glands and carried by the blood to specific organs in the body into the blood

Neurones

Neurones are nerve cells. They carry information as tiny electrical signals. There are three different types of neurones, each with a slightly different function.

  1. Sensory neurones carry signals from receptorsreceptors: organs which recognize and respond to stimuli to the spinal cord and brain.
  2. Relay neurones carry messages from one part of the CNSCNS: Central Nervous System (brain and spinal cord). to another.
  3. Motor neurones carry signals from the CNSCNS: Central Nervous System (brain and spinal cord). to effectors.

The diagram below shows a typical neurone - in this case, a motor neurone. It has tiny branches at each end and a long fibre carries the signals.

a neurone has a 'head' at one end where the nucleus, cytoplasm, cell membrane and dendrite are. The axon is tail-like, with nerve endings at the end which look like branches.

A motor neurone

Synapses

Where two neurones meet, there is a tiny gap called a synapse. Signals cross this gap using chemicals. One neurone releases the chemical into the gap. The chemical diffuses across the gap and makes the next neurone transmit an electrical signal.

Reflex actions

When a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-ordinates the response. But sometimes a very quick response is needed, one that does not need the involvement of the brain. This is a reflex action.

Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. The animation below allows you to step through each stage of the reflex arc.

This is what happens:

  1. receptor detects a stimulus - change in the environment
  2. sensory neurone sends signal to relay neurone
  3. motor neurone sends signal to effector
  4. effector produces a response

The way the iris in our eye adjusts the size of the pupil in response to bright or dim light is also a reflex action.

In bright light:

  • Radial muscles of the iris relax.
  • Circular muscles of the iris contract.
  • Less light enters the eye through the contracted pupil.

In dim light:

  • Radial muscles of the iris contract.
  • Circular muscles of the iris relax.
  • More light enters the eye through the dilated pupil.

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