Homeostasis, the nervous system and reflexes
Your body keeps its internal conditions steady even when the outside changes, and the nervous system is one of the ways it does so. It carries electrical impulses from the parts that detect a change to the parts that respond.
Part 1 of 3: Learn it
In short
- Homeostasis keeps internal conditions, such as body temperature and blood glucose, at the right level.
- Stimulus, receptor, sensory neurone, CNS, motor neurone, effector, response.
- Reflexes are automatic and rapid, and they skip the conscious part of the brain.
Where this is in your specification
Spec points: AQA 4.5.1 and 4.5.2.1 (Combined Trilogy 4.5.1 and 4.5.2), Edexcel Topic 2, OCR B3.1
| Board | Topic: Homeostasis and response |
|---|---|
| AQA 8461 | 4.5 (8461); 4.5 (8464) |
| Edexcel 1BI0 | Topic 7 |
| OCR J247 | B3 |
| Eduqas C400 | 4.1 to 4.4 |
| Cambridge IGCSE 0610 | 13.1, 14.1 to 14.5 |
| National 5 C807 75 | Multicellular organisms: 2 Control and communication |
Homeostasis
Cells work best in steady conditions: enzymes need the right temperature, and cells need the right amount of water and glucose. Control systems have receptors that detect changes, a coordination centre (the brain, spinal cord or pancreas) that processes the information, and effectors (muscles or glands) that bring about a response.
The route of an impulse
- A stimulus (a change) is detected by a receptor, such as light receptors in the eye.
- A sensory neurone carries the impulse to the central nervous system (the brain and spinal cord).
- The CNS coordinates a response and sends an impulse along a motor neurone.
- The effector responds: a muscle contracts or a gland releases a hormone.
Reflex arcs and synapses
A reflex, such as pulling your hand off a hot pan, passes through a relay neurone in the spinal cord without waiting for the conscious brain, so it is very fast and protects the body from harm.
Where two neurones meet there is a tiny gap called a synapse. The impulse cannot jump it: a chemical is released, diffuses across the gap and starts an impulse in the next neurone.
The reaction time practical
One person holds a ruler with its zero end between the other person's open finger and thumb, then drops it without warning. The catcher grabs it as fast as possible, and the distance it fell is read off. A conversion table (or the equation t = √(2d ÷ g)) turns that distance into a time. Repeat several times and take a mean.
What are the two parts of the central nervous system?
Show the answer
Your brain, together with your spinal cord.
Part 2 of 3: See it worked
Worked examples
Example 1
A ruler falls 0.18 m before it is caught. Use t = √(2d ÷ g), with g = 9.8 m/s², to find the reaction time.
- 2d = 0.36
- 0.36 ÷ 9.8 = 0.0367
- √0.0367 = 0.19
Answer: About 0.19 s.
Example 2
Five catches gave reaction times of 0.21, 0.19, 0.24, 0.20 and 0.21 s. Find the mean.
- Total: 0.21 + 0.19 + 0.24 + 0.20 + 0.21 = 1.05
- 1.05 ÷ 5
Answer: 0.21 s.
Common mistakes
- Saying the brain decides in a reflex. The conscious brain is not involved.
- Saying impulses travel across synapses as electricity. A chemical diffuses across the gap.
- Putting the motor and sensory neurones the wrong way round.
- Calling a muscle a receptor. Muscles and glands are effectors.
Give two examples of effectors.
Show the answer
Muscles and glands.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
What are the two parts of the central nervous system?
Your brain, together with your spinal cord.
Give two examples of effectors.
Muscles and glands.
Why are reflexes useful?
They are fast and automatic, so they protect the body from harm, such as burns.
Jobs that use this
- Paramedic (se abre en otra pestaña)
- Physiotherapist (se abre en otra pestaña)
- GP (se abre en otra pestaña)
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Estos apuntes están en inglés porque siguen los programas de examen del Reino Unido.
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses