B1.2 Nerves and Hormones

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The Nervous System

The nervous system enables humans to react to their surroundings and coordinate their behaviour.

  • Any change in your surroundings e.g. temperature, visual, sound etc. is potentially a detectable stimulus to one of you sensory organs e.g. skin, eyes, ears etc. The stimulus might be chemical, light, pain, position, pressure, sound, temperature, touch etc.

  • You have five different sense organs ears, eyes, nose, skin and tongue which contain receptors (groups of cells) that are sensitive to particular stimuli.

  • In the receptor cells the stimulus input is converted into an electrical signal - an electrical impulse which is sent to the brain.

  • The reflex actions that can happen by virtue of our central nervous system help prevent injury from various sources in potentially dangerous situations.


Receptors - Specialised cells that detect stimuli.
Neurone - A specialised nerve cell.
Sensory neurone -This carries the nerve impulse from the receptor to the central nervous system. 
Relay neurones - Connects a motor and sensory neurone in the central nervous system and is involved in a reflex arc.
Synapse - A junction between two neurones.
Motor neurone - Carries the nerve impulse from the central nervous system to an effector.
Effectors - Carry out the response and are either muscles or glands.
Reflex - An automatic response to stimuli that does not involve conscious thought.
Stimulus - Changes in environment.
Central Nervous System - The brain and the spinal cord.
Nerve - A bundle of nerves
Nerve Impulse - An electrical message that passes along a neurone.

Many processes within the body are coordinated by chemical substances called hormones. Hormones are secreted by glands and are usually transported to their target organs by the bloodstream.


Light receptor cells, like most animal cells, have a nucleus, cytoplasm and cell membrane.

  • Light receptor cells in the eyes that are sensitive to light; the light energy creates electrical signals that are sent to the brain for 'processing'.

  • Sound receptors in the ears that are sensitive to sound vibrations in the air

    • There are also balance receptors in the ears that are sensitive to changes in position and enable us to keep our balance.

  • The receptors on the tongue are sensitive to chemicals and enable us to taste, and therefore detect, a wide variety of different foods (bitter, salty, sour, sweet chemical stimuli etc.) or anything else in contact with the tongue - good or bad!

  • The receptors in the nose are also sensitive to chemicals and enable us to smell all sorts of different things which may be a pleasant or unpleasant experience.

Reflex Actions

The pathway of a response

Stimulus --> Receptors -(Sensory Neurone)-> CNS -(Motor Neurone)-> Effector --> Response

The Reflex Arc

  • A stimulus detected by receptors (receptor cells) causes impulses from a receptor to pass along a sensory neurone (nerve cell) to the central nervous system.

  • The relay neurone transmits the nerve impulse across the spinal cord
  • The impulse is passed to the motor neuron, which carries the impulse to the effector organ.
  • The effector is either a muscle or a gland, a muscle responds…


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