Coordination and Control

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The Nervous System

The nervous system enables humans to react to their surroundings and coordinate their behaviour.

Cells called receptors detect stimuli (changes in the environment).  These include:

  • receptors in the eyes which are sensitive to light
  • receptors in the ears which are sensitive to sound
  • receptors in the ears which are sensitive to changes in position and enable us to keep our balance
  • receptors on the tongue and in the nose which are sensitive to chemicals and enable us to taste and to smell
  • receptors in the skin that are sensitive to touch, pressure, pain and to temperature changes.

Information from receptors passes along cells called neurones (nerve cells) to the brain. These are found in bundles of hundreds or thousands of neurones known as nerves.  This information is processed by the brain which coordinates any response.

Reflex Actions

Sometimes your response to a stimulus is so rapid that you do not have time to think about it.  This is called a reflex action.  Relex actions help you to avoid danger or harm. For example, if you touch something hot you rapidly move your arm away from the hot object.  If something comes close to your eyes, you blink.  Shining a light into somebody's eyes causes the pupils to get smaller.

Reflex actions often involve sensory, relay and motor neurones.  How they are linked together is shown in the diagram below.

Impulses from a receptor pass along a sensory neurone to the central nervous system.  The central nervous system is made up of the brain and the spinal cord.

At a junction (called a synapse) between a sensory neurone and a relay neurone in the central nervous system, a chemical is released which causes an impulse to be sent along a relay neurone.

A chemical is then released at the synapse between a relay neurone and a motor neurone in the central nervous system, causing impulses to be sent along a motor neurone to the organ (the effector) which brings about the response.  The effector is either a muscle or a gland.  Glands respond by releasing chemical substances (e.g. adrenaline).

For example if you touch a hot object with your hand, a pain receptor in your skin is stimulated.  A nerve impulse passes along a sensory neurone to the central nervous system (in this case the spinal cord).  The message crosses a synapse to a relay neurone and then crosses a second synapse to a motor neurone.  The motor neurone carries the impulse to a muscle in the arm (the effector).  The muscle will then contract to move your hand away from the hot object.  This action is called the response. This chain of events can be summarised as follows:

Stimulus → Receptor → Coordinator → Effector → Response

The coordinator is the central nervous system (brain or spinal cord).

Can you correctly label the Reflex Arc?