Homeostasis
- Homeostasis is the regulation of the internal conditions of a cell or organism to keep them at the best levels for function, whatever the changes inside or outside the body.
- In humans it controls blood glucose concentration, body temperature and water levels.
- Control systems have receptors (which detect stimuli), coordination centres such as the brain, spinal cord and pancreas (which process the information) and effectors, muscles or glands (which bring about responses).
The nervous system
- The nervous system lets humans react to their surroundings and coordinate their behaviour.
- Information travels as electrical impulses along neurones.
- The central nervous system (CNS) is the brain and spinal cord.
- Stimulus → receptor → coordinator → effector → response.
Neurones and synapses
- Sensory neurones carry impulses from receptors to the CNS. Relay neurones carry impulses within the CNS. Motor neurones carry impulses from the CNS to effectors.
- A synapse is a tiny gap between two neurones. When an impulse arrives, a chemical is released that diffuses across the gap and starts an impulse in the next neurone.
Reflexes
- Reflex actions are automatic and rapid; they do not involve the conscious part of the brain. They protect the body from harm.
- Reflex arc: receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector.
- Example: touching a hot object makes a muscle contract to pull the hand away.
Required practical: reaction time
- One person drops a ruler between the other person's finger and thumb without warning; they catch it as fast as possible.
- Record the distance the ruler falls; a shorter distance means a faster reaction. Convert to reaction time using a table.
- Repeat several times and find the mean. Test the effect of a factor such as caffeine, keeping everything else the same.
Key terms
- Homeostasis
- Keeping the internal conditions of the body at the best levels for function.
- Stimulus
- A change in the environment that is detected by a receptor.
- Receptor
- A cell that detects a stimulus.
- Effector
- A muscle or gland that brings about a response.
- Central nervous system
- The brain and spinal cord.
- Sensory neurone
- A neurone that carries impulses from receptors to the CNS.
- Relay neurone
- A neurone that carries impulses within the CNS.
- Motor neurone
- A neurone that carries impulses from the CNS to effectors.
- Synapse
- The gap between two neurones, crossed by a chemical.
- Reflex action
- A fast, automatic response that does not involve the conscious brain.