Neurobiology and behaviour

■ Specialised cells called neurones, such as the motor neurone and others, are assembled to form the nervous system.

The mammalian nervous system consists of a brain and spinal cord (central nervous system, CNS) and nerves serving the tissues and organs of the body (peripheral nervous system, PNS).

The nervous system links receptors (e.g. sense organs) to effectors (muscles or glands). An impulse or action potential is a temporary reversal of the electrical potential difference that is maintained across the membrane of the nerve fibres. Conduction of an action potential is extremely fast.

Action potentials are transmitted between neurones across tiny gaps at synapses. Transmission here is chemical, involving diffusion of a specific transmitter substance. 

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Define sensitivity, stimulus, response and reflex

Sensitivity is the ability to detect change and to respond to it. Changes that are detected and lead to a response are called stimuli (singular, stimulus).

A stimulus is a change in the environment (internal or external) that is detected by a receptor, and elicits a response. 

A response is the activity of a cell or organism in terms of movement, hormone secretion or enzyme production, for example, as a result of a stimulus.

A reflex is a rapid, unconscious response.     

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Explain the role of receptors, sensory neurons, re

The response of the body to a stimulus involves a receptor organ, neurones of the nervous system, and an effector organ arranged in a functional unit called a reflex arc. In the reflex arc, an impulse generated in a receptor is transmitted via neurones to the effector which brings about a response, in the sequence:


receptor sensory neurone CNS motor neurone effector


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Note down the three types of neurone found in a re

Each neurone has a cell body and a number of extensions, the nerve fibres.

  • motor neurones

These have many fine dendrites (meaning ‘little trees’) which bring impulses towards the cell body, and a single long nerve fibre called an axon which carries impulses away from the cell body. The structure of a motor neurone was introduced on page 211.

  • sensory neurones

These have a single long nerve fibre, a dendron, which brings impulses towards the cell body, and a single long axon which carries impulses away.

  • relay neurones (also known as interneurones)

These have numerous, short nerve fibres. 

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