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.
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
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.