The Nervous System

The nervous system consists of the brain and spinal cord, which make up the central nervous system (CNS), as well as all other neurones in the body. The purpose of this system is to allow the body to react to stimuli, which can be defined as a change in the environment.


Stimuli are detected by 5 sense organs. These organs are:


  • Eyes, which detect light using light receptors.
  • The skin, which can detect changes in temperature and pressure.
  • The nose, that can detect chemicals in the air using smell receptors.
  • The tongue, which is also sensitive to chemical stimuli using taste receptors.
  • And finally the ears, which are sensitive to sound and changes in balance using sound receptors.


Once a stimulus is detected, an electrical impulse is fired from the receptors through the nervous system to an effector. Effectors are parts of the body, usually muscles or glands, that change in response to a stimulus.


Reflexes are an automatic response to stimuli designed to minimize damage to the body. The difference between detecting a stimulus normally and a reflex is that the electrical impulse only travels through the spinal cord in a reflex arc. This is to avoid the conscious parts of the brain which would slow down the impulse reaching the effector, otherwise, more damage would be inflicted on the body.


The steps to the reflex arc, using a hot pan as the example, are:


  1. Receptors in the skin detect the hot temperature.
  2. The skin fires an electrical impulse down a neuron called the sensory neuron.
  3. This electrical impulse then travels down the relay neuron in the spinal cord from the sensory neuron.
  4. Then the impulse travels from the relay neuron to the motor neuron.
  5. The impulse finally travels down to the effector. In this case, the effector would be a muscle which would contract to cause the hand to move away from the hot pan.


The 3 different neurones I have just talked about are not all connected. The connections between neurones are called synapses. When an impulse reaches the end of a neurone, it causes the neurone to release chemicals that diffuse across the synapse. The next neurone detects these chemicals which cause this neurone to fire off a new impulse.

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