The nervous system

• Animals use their senses to respond to changes in external environment e.g. running away after spotting a predator
• They also respond to changes in their internal environment to maintain appropriate conditions for metabolism (homeostasis)
• Any change in the internal or external environment is called a stimulus
• The central nervous system (CNS) is made up of the brain and spinal cord only
• Neurones are specialised cells which transfer information as electrical impulses. Neurones are found in bundles of hundreds or thousands known as nerves
• When receptors in a sense organ detect a stimulus, they send electrical impulses along sensory neurones to the CNS
• The CNS then sends electrical impulses to an effector along a motor neurone. The effector responds accordingly. The job of the CNS is to coordinate a response
• The neurones transmit information using extremely fast electrical impulses and is therefore able to bring about very rapid responses
• The neurones transmit information using extremely fast electrical impulses and is therefore able to bring about very rapid responses
• Receptors are groups of cells in the sense organs (eyes, ears, nose, tongue and skin) that detect stimuli. E.g. rod and cone cells in the eye detect changes in light
• Receptors communicate with effectors via the nervous system, the hormonal system or both. Effectors bring about a response to stimuli
• There are two types of effectors which respond in a different way:

  • Muscles which contract

  • Glands which secrete hormones
• There are three types of neurones: sensory neurones, relay neurones and motor neurones

  • These neurones have slightly different structures but all have a cell body, a neurone fibre

  and neurone endings to connect to other neurones, muscles or glands

  • A sensory neurone’s cell body is in the middle with a fibre on either side

  • A motor neurone has only one neurone fibre