The central nervous system (brain and spinal cord) acts as an integration and coordination system for the control of body systems

• The CNS processes sensory information received from the body and relays motor responses to effector organ
  

Sensory neurons transmit information to the central nervous system from receptors (via the afferent pathway)

• Receptors are specialised cells that convert environmental stimuli into electrical nerve impulses
  

• Examples include thermoreceptors (heat), baroreceptors (pressure), chemoreceptors (chemicals) and mechanoreceptors (movement)
  

Motor neurons transmit information from the central nervous system to effectors (via the efferent pathway)

• Effectors are organs (either muscles or glands) that produce a response to an environmental stimulus
  

• Muscles are stimulated to contract, while glands are stimulated to release a chemical product (e.g. hormone)
  

Stimulus-Response Model

Reflex Arcs

The spinal column is capable of the immediate processing of certain involuntary responses (reflex actions)

• Reflex actions do not involve the brain – this results in a faster response, but one that does not involve conscious thought or deliberation
  

• Reflex actions are particularly beneficial in survival situations, when quick reactions are necessary to avoid permanent damage
  

A common example of a reflex action is a pain response – which only involves three neurons 

• A pain stimulus is detected by a free sensory nerve ending in the skin acting as a pain receptor
  

• The signal is transmitted to a single interneuron (relay neuron) within the grey matter of the spinal cord
  

• A response is sent via a motor neuron to skeletal muscles, resulting in movement away from the pain stimulus