Imbalances in Brain Chemicals Leading to Diseases — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

Neurones communicate across synapses by releasing neurotransmitters, which bind to receptors on the postsynaptic membrane.

Parkinson's disease is a progressive condition in which neurones in a part of the midbrain, the substantia nigra, die. These neurones release dopamine in the basal ganglia, which help control voluntary movement. With less dopamine, signals that start and smooth movement are reduced. Symptoms include tremor, muscle rigidity, slow movement and difficulty starting to move.

Treatment commonly uses L-dopa, a precursor of dopamine that crosses the blood-brain barrier and is converted to dopamine in the brain. Dopamine itself cannot cross the barrier. Other drugs act as dopamine agonists or inhibit the enzyme that breaks down dopamine. They relieve symptoms but do not cure the disease, and over time they become less effective.

Depression is a mood disorder with persistent low mood and loss of interest.

The link is not simple

• SSRIs raise serotonin quickly but mood improves only after weeks, and not all patients respond.
• Genes, environment and life events also influence depression, so serotonin level is a risk factor, not the single cause.

This topic links to synaptic transmission, the effect of drugs on synapses and personalised medicine, where a patient's genes affect the response to a drug.

Other brain diseases also show chemical changes.

In schizophrenia, overactivity of dopamine pathways is thought to contribute to hallucinations and delusions.

These examples teach an important point about scientific reasoning.

Nerve cells in the brain can also be affected by too much transmitter. Excess glutamate can overstimulate and damage neurones, which is thought to contribute to damage after a stroke. This shows that balance, rather than simply more or less, is what matters in a healthy brain.