Homeostasis, the nervous system and reflexes – GCSE Combined Science Biology

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Your body is always sensing and reacting. It notices a hot pan, a loud noise or a ball coming towards you. Then it responds. This topic is about how that works.

It matters because your cells only work well if conditions inside you stay steady. The nervous system helps to keep them steady and keeps you safe.

By the end you will be able to explain homeostasis and follow a nerve message from start to finish. You will describe a reflex and a simple reaction time test.

Keeping things steady

Homeostasis means keeping conditions inside your body steady. Examples are your temperature, your blood glucose and the water in your body. Enzymes and cells only work well when these stay in a narrow range.

Every control system has three parts. Receptors detect a change, called a stimulus. A coordination centre, such as the brain, spinal cord or pancreas, decides what to do. Effectors, which are muscles or glands, carry out the response.

Think of a heating system. The thermostat detects the temperature, and the control box decides. The heater then switches on or off.

From stimulus to response

The nervous system carries messages as electrical impulses. A neurone is a nerve cell that carries these impulses. The central nervous system, or CNS, is the brain and spinal cord.

A receptor, for example in your skin or eye, detects a stimulus. A sensory neurone carries the impulse to the CNS. The CNS decides what to do, and a motor neurone carries the impulse out to an effector.

The effector then produces the response. A muscle contracts or a gland releases a hormone. Remember sensory means sensing, and motor means movement.

Synapses

Neurones do not quite touch each other. There is a tiny gap between two neurones called a synapse.

When an impulse reaches the end of a neurone, the neurone releases chemicals. The chemicals diffuse across the gap. Diffuse means they spread out from where there are lots. They bind to receptors on the next neurone and start a new impulse.

Think of passing a note across a gap by throwing a paper ball, not by holding hands. The chemicals only travel one way, so impulses at a synapse go in one direction only.

Reflex actions

A reflex is a fast, automatic response. You do not think about it first. Reflexes protect you, like pulling your hand away from something hot.

A reflex is quick because it uses a short route. The route is called a reflex arc. The sensory neurone passes the impulse to a relay neurone in the spinal cord. The relay neurone passes it straight to the motor neurone. The conscious part of the brain is not involved.

Think of a shortcut across the playground instead of going round by the school office.

Practical: reaction time

Reaction time is the time between a stimulus and a response. We can test it with the ruler drop test.

One person holds a ruler upright with the zero at the other person's thumb and finger. They drop it without warning. The other person catches it as fast as they can. We read the distance the ruler fell.

A shorter distance means a faster reaction. Repeat the test several times. To compare, for example caffeine against no caffeine, keep the same person, hand and ruler.

Using the results

To find the mean, add the readings and divide by how many there are. A student catches the ruler at 12 cm, 14 cm and 13 cm.

Add them up to get 39. Divide by 3 to get a mean of 13 cm. Suppose the same student catches it at 9 cm after a caffeine drink. That is a shorter distance, so the reaction is faster.

One test is not enough to draw a firm conclusion. Repeat the test and use more people to be surer.

Recap

Homeostasis keeps conditions steady, using receptors, a coordination centre and effectors.

The pathway is stimulus, receptor, sensory neurone, CNS, motor neurone, effector, response.

A synapse is a gap crossed by chemicals, in one direction.

A reflex is fast and automatic, and goes through the spinal cord.