Brain chemicals and ill health

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Brain chemicals and ill healthSpec 8.16

In short

Naturally occurring brain chemicals, such as the neurotransmitters dopamine and serotonin, must be in balance. Loss of dopamine-producing neurones leads to Parkinson’s disease, and low serotonin activity at synapses is linked to depression. Knowing how these synapses work has allowed new drugs to be designed to correct the imbalance.

Imbalances of brain chemicals
	Parkinson’s disease	Depression
Chemical	Dopamine	Serotonin
Imbalance	Neurones that make dopamine, in a region of the brain controlling movement, die, so dopamine is deficient	Low serotonin activity at synapses is associated with low mood
Symptoms	Tremor, muscle rigidity, slow movement, poor balance	Persistent low mood, loss of interest, changes in sleep and appetite
Treatment	L-DOPA, which is converted to dopamine in the brain; drugs that act like dopamine (agonists) or reduce its breakdown	Drugs that block the reuptake of serotonin (selective serotonin reuptake inhibitors, SSRIs), so serotonin stays in the synaptic cleft for longer

Understanding how synapses work makes it possible to design new drugs that act at a particular target: a receptor, an enzyme that breaks down a transmitter or the protein that takes a transmitter back up. A drug must reach the brain, so it needs to cross the blood–brain barrier, and it must be tested for effectiveness and side effects in clinical trials before it can be used.

Common mistake:

The cause of depression is complex and an imbalance of serotonin is only one factor linked to it. Say ‘associated with’ or ‘contributes to’, not ‘is caused by’.

Quick check

Which region of the brain coordinates movement and balance?
Show answer
The cerebellum.
Which region of the brain controls heart rate and breathing rate?
Show answer
The medulla oblongata.
What does fMRI show that MRI alone does not?
Show answer
Which areas of the brain are active, by detecting changes in blood oxygen.
Which neurotransmitter is lacking in Parkinson’s disease?
Show answer
Dopamine.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between nervous and hormonal coordination?

Nervous coordination uses electrical impulses along neurones to produce fast, short-lived responses in specific effectors. Hormonal coordination uses chemicals released into the blood, which act more slowly and last longer on any target cells with receptors. The hypothalamus links the two systems by controlling the pituitary gland.

What do the different parts of the brain do?

The cerebral hemispheres control conscious thought, memory and voluntary movement. The hypothalamus regulates temperature, water balance and the pituitary gland. The pituitary secretes hormones. The cerebellum coordinates movement and balance, and the medulla oblongata controls involuntary actions such as heart rate and breathing rate.

What is the difference between MRI and fMRI?

MRI uses a magnetic field and radio waves to make detailed images of brain structure. fMRI uses the same scanner to detect changes in blood oxygen level in the brain over time. Active neurones use more oxygen, so fMRI shows which parts of the brain are working during a task.

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