Neurones: Function & Structure (SL IB Biology)

The nervous system

• 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
• It allows us to make sense of our surroundings and respond to them, and to coordinate and regulate body functions
• Information is sent through the nervous system in the form of 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 acts as a central coordinating centre for the impulses that come in from, and are sent out to, any part of the body

Central Nervous System Diagram

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

Neurones

• The following features are found in neurones:
  • Neurones have a main, long, fibre known as an axon
  • The axon is often insulated by Schwann cells which form the myelin sheath which prevents loss of nerve impulses along the axon
  • They have a cell body that contains the nucleus and other cellular structures
  • Their cell bodies and axon terminals contain many extensions called dendrites
  • These dendrites allow them to connect to many other neurones and receive impulses from them, forming a network for easy communication

Structure of a Neurone Diagram

Neurones have a characteristically elongated structure which allows them to transfer information between the central nervous system and the rest of the body

Different types of neurone

• There are three main types of neurone: sensory, relay and motor
  • Sensory neurones carry impulses from receptors to the CNS (brain or spinal cord)
  • Relay (intermediate) neurones are found entirely within the CNS and connect sensory and motor neurones
  • Motor neurones carry impulses from the CNS to effectors (muscles or glands)
• Each type of neurone has a slightly different structure
• Motor neurones have:
  • A large cell body at one end that lies within the spinal cord or brain
  • A nucleus that is always in its cell body
  • Many highly-branched dendrites extending from the cell body, providing a large surface area for the axon terminals of other neurones
  Relay neurones have:
  • Short, but highly branched, axons and dendrites
• Sensory neurones have:
  • A cell body that branches off in the middle of the cell
  • A single long dendron that carries impulses to the cell body and a single long axon that carries impulses away from the cell body

Three Types of Neurone Diagram

The three types of neurone – the red line shows the direction of impulses. Note that the axon always carried impulses away from the cell body.