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The brain

Homeostasis and response · The human nervous system · note 4 of 8

Spec 4.5.2.2
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The brainSpec 4.5.2.2

In short

The brain controls complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions. The cerebral cortex is responsible for consciousness, intelligence, memory and language. The cerebellum coordinates muscular activity and balance. The medulla controls unconscious activities such as heart rate and breathing.

The brain controls complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions.

Regions of the brain
RegionFunction
Cerebral cortexConsciousness, intelligence, memory and language
CerebellumCoordinates muscular activity and balance
MedullaControls unconscious activities such as heart rate and breathing
Side view of the human brain with the cerebral cortex, cerebellum, medulla and spinal cord labelled. (opens full size in a new tab)
Regions of the brain: the cerebral cortex, the cerebellum and the medulla.
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Investigating the brain and treating brain disorders

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Neuroscientists have mapped regions of the brain to particular functions in three main ways.

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  • Studying patients with brain damage and seeing which abilities are lost.
  • Electrically stimulating different parts of the brain and seeing what happens.
  • Using MRI scanning techniques.
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The complexity and delicacy of the brain makes investigating and treating brain disorders very difficult. It is made of billions of interconnected neurones, and its tissue is easily damaged during investigation or treatment. Any procedure on the brain or nervous system has benefits and risks that must be weighed up.

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Exam tip:

For ‘difficulties’ questions, use the words complex and delicate, and link them to the risk of damaging the brain during investigation or treatment.

Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026

Frequently asked questions

What is a reflex arc?

A reflex arc is the pathway of an automatic, rapid response. A receptor detects a stimulus, a sensory neurone carries the impulse to the CNS, a relay neurone passes it on, and a motor neurone carries it to an effector, a muscle or gland, which responds. It does not involve the conscious part of the brain.

Why are reflexes faster than normal responses?

Reflexes are faster because the reflex arc does not go through the conscious part of the brain. The impulse passes from the sensory neurone, through a relay neurone, to the motor neurone, so the response happens before you are aware of it. This protects the body, for example pulling your hand away from a hot object.

How does a nerve impulse cross the synapse?

A nerve impulse crosses a synapse using chemicals. A synapse is a gap between two neurones. When an impulse reaches the end of one neurone, chemicals diffuse across the gap and start an impulse in the next neurone. This is how an impulse passes from a sensory neurone to a relay neurone, and on to a motor neurone.

All 5 questions on The human nervous system