Human Nervous System (Edexcel IGCSE Biology: Double Science)

• The human nervous system consists of:
  • Central nervous system (CNS) – the brain and spinal cord
  • Peripheral nervous system (PNS) – all of the nerves in the body
• Information is sent through the nervous system as electrical impulses – these are electrical signals that pass along nerve cells known as neurones
  • A bundle of neurones is known as a nerve
• The nerves spread out from the central nervous system to all other regions of the body and importantly, to all of the sense organs
  • The CNS, therefore, acts as a central coordinating centre for the impulses that come in from (or are sent out to) any part of the body

The human nervous system is comprised of the CNS and the PNS

• The pathway through the nervous system can be summarised as follows:

stimulus → sensory neurone → relay neurone → motor neurone → effector → response

1. A stimulus is received by a sensory (receptor) neurone
   • Most receptors are specialised to detect particular stimuli
   • When a receptor is stimulated, it produces electrical impulses
2. These impulses then travel along a sensory neurone to the central nervous system (the coordinator is either the brain or the spinal cord)
3. In the CNS, the impulses are passed on to a relay neurone
4. The relay neurone links to a motor neurone, along which the impulses travel until they reach the effector
5. The effector is what carries out the response (the effector may be a muscle or gland)

From stimulus to response: an example of a nerve pathway showing how an electrical impulse travels through sensory, relay and motor neurones