Homeostasis and the nervous system

Biology · Homeostasis and response. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Homeostasis keeps conditions inside the body steady, so that enzymes and cells work at their best. Blood glucose concentration, body temperature and water level are all controlled automatically, by nerves or by hormones.

Every control system has three parts. Receptors detect a stimulus (a change). A coordination centre, such as the brain, spinal cord or pancreas, receives and processes the information. Effectors, which are muscles or glands, bring about the response.

In the nervous system, information travels as electrical impulses along neurones. A reflex is automatic and rapid because the conscious part of the brain is not involved. This protects the body from harm.

Rules to remember

Homeostasis: the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.
stimulus → receptor → coordinator → effector → response
Central nervous system (CNS) = brain and spinal cord.
Reflex arc: receptor → sensory neurone → relay neurone (in the CNS) → motor neurone → effector.
Sensory neurones carry impulses from receptors to the CNS. Motor neurones carry impulses from the CNS to effectors.
Synapse: the gap between two neurones. A chemical diffuses across it and starts an impulse in the next neurone.
Mean reaction time = sum of the times ÷ number of times. Leave out any anomalous result first.

Common mistake

Naming the receptor as the effector. The effector is the part that carries out the response (a muscle contracting or a gland secreting), not the part that detects the stimulus.

Worked example

A bright light is shone into a person's eye and the pupil quickly gets smaller. Name the receptor, the coordinator and the effector.

The stimulus is the bright light. It is detected by light receptor cells in the retina.
Sensory neurones carry impulses to the brain, which coordinates the response without conscious thought.
Motor neurones carry impulses to the muscles of the iris, which contract and make the pupil smaller.

Answer: Receptor: cells in the retina. Coordinator: the brain. Effector: the muscles of the iris.

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