The nervous system and the reflex arc
The nervous system carries electrical messages, called nerve impulses, along cells called neurones. It is fast and precise, which is exactly what you want when you touch something hot. The central nervous system (the brain and spinal cord) processes information, while nerves carry signals to and from the rest of the body.
Some responses are so important for survival that they skip conscious thought entirely. These are reflexes, and the pathway an impulse follows is the reflex arc. Because the impulse does not wait for the brain to decide, the response is extremely quick and it protects you from harm.
Put the stages of the reflex arc in order when you touch a hot pan and pull your hand away.
Receptor and effector are opposites
A receptor detects a change; an effector (a muscle or gland) does something about it. Getting these two the right way round earns easy marks and keeps the whole arc in order.
The endocrine system and hormones
The endocrine system is the body's second messaging system. Instead of electrical impulses it uses chemical messengers called hormones, released by glands straight into the blood. The blood carries them everywhere, but each hormone only affects certain target organs. Hormonal messages are slower to start than nervous ones, but they last much longer.
- Hormone
- A chemical messenger made by an endocrine gland and carried in the blood to a target organ, where it causes a response.
| Feature | Nervous system | Endocrine system |
|---|---|---|
| Message carried by | Electrical impulses in neurones | Hormones in the blood |
| Speed | Very fast | Slower |
| How long it lasts | Short | Long |
| Area affected | Precise, one target | Can be widespread |
Key examples worth knowing: the pancreas releases insulin and glucagon to control blood glucose, and adrenaline from the adrenal glands prepares the body for action.
Homeostasis and negative feedback
Homeostasis is keeping the conditions inside your body steady, such as temperature, water and blood glucose, despite changes outside. It matters because enzymes, which run every reaction in your cells, only work well within a narrow range of conditions.
- Homeostasis
- The maintenance of a constant internal environment (temperature, water content, blood glucose and so on) within narrow limits.
The body keeps things steady using negative feedback. The idea is simple: if a level rises too high, the body acts to bring it back down; if it falls too low, the body acts to bring it back up. The correction always opposes the change, which is why it is called "negative".
Where this is assessed
Describing a negative feedback loop as receptor, coordination, effector, correction is Criterion A (knowing and understanding). Discussing how conditions like diabetes affect people and how treatments help is Criterion D (reflecting on the impacts of science).
Controlling blood glucose and body temperature
Blood glucose is controlled by two hormones from the pancreas working in opposite directions:
| Situation | Hormone released | Effect |
|---|---|---|
| Blood glucose too high (after a meal) | Insulin | Liver and muscles take in glucose and store it as glycogen, so blood glucose falls. |
| Blood glucose too low (after exercise) | Glucagon | Liver breaks glycogen back down into glucose and releases it, so blood glucose rises. |
Because insulin lowers glucose and glucagon raises it, the level is nudged back to normal from either direction, a clear negative feedback loop. In type 1 diabetes the pancreas cannot make enough insulin, so blood glucose can rise dangerously high and is often controlled with insulin injections.
Body temperature is controlled the same way, aiming for about 37 degrees Celsius:
| If you are too... | The body responds by... |
|---|---|
| Hot | Sweating (evaporation cools the skin), vasodilation (blood vessels near the skin widen to lose heat). |
| Cold | Shivering (muscle movement releases heat from respiration), vasoconstriction (vessels narrow to keep heat in), hairs stand up to trap air. |
Check yourself
1. List the parts of the reflex arc in order, from stimulus to response. +
Stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response. The reflex is fast because the impulse passes through the spinal cord without waiting for the brain.
2. Give two differences between how the nervous and endocrine systems send messages. +
The nervous system uses electrical impulses along neurones and its effects are fast but short-lived. The endocrine system uses hormones carried in the blood and its effects are slower but longer-lasting.
3. Blood glucose rises after a meal. Explain how negative feedback brings it back down. +
The high glucose is detected and the pancreas releases insulin. Insulin makes the liver and muscles take up glucose and store it as glycogen, so blood glucose falls back to normal. The response opposes the rise, which is negative feedback.
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