The human brain and imagingEdexcel International A Level Biology: Revision notes

• Cerebral hemispheres: the two large folded halves at the top and front of the brain. They control conscious thought, memory, language, interpretation of sensory information and voluntary movement.
• Cerebellum: at the back, below the hemispheres. It coordinates movement, balance and posture.
• Medulla oblongata: at the base of the brain, joining the spinal cord. It controls breathing rate and heart rate and other involuntary actions.
• Hypothalamus: below the hemispheres. It monitors and regulates body temperature and the water potential of the blood, and controls the pituitary gland.
• Pituitary gland: attached to the hypothalamus. It secretes hormones, including ADH.

The hypothalamus detects changes in the blood, such as a fall in water potential, and stimulates the pituitary gland to release hormones. For example, when the blood is too concentrated, the pituitary releases ADH. ADH makes the kidney collecting ducts more permeable to water, so more water is reabsorbed and less is lost in urine.

Damage to the pituitary can therefore cause the production of large volumes of dilute urine.

• CT scan: uses X-rays from many angles to build images of structure. Quick, so good for bleeding, strokes and fractures. Uses ionising radiation.
• MRI scan: uses a strong magnetic field and radio waves to give very detailed images of soft tissue structure, such as tumours. No ionising radiation, but not for people with some metal implants.
• fMRI scan: an MRI scan that detects changes in blood flow and oxygen use, showing which regions are active. Used in research into brain function.
• PET scan: a radioactive tracer (for example radioactive glucose) is injected. Active areas take up more tracer, and gamma rays are detected. Shows function, but involves ionising radiation and is expensive.

In Parkinson's disease, dopamine-secreting neurones degenerate. Less dopamine is released at synapses controlling movement, so there is tremor, stiffness and slow movement.

PET with a radioactive tracer taken up by dopamine neurones shows reduced uptake in patients. MRI or CT can rule out other causes. L-DOPA is converted into dopamine in the brain and reduces the symptoms.

Low activity of serotonin at synapses is associated with depression. Some antidepressants inhibit the reuptake of serotonin into the presynaptic knob, so it stays in the cleft longer and stimulates the postsynaptic neurone more.

The link is a correlation, and not all patients respond, so other factors are involved. New drugs must be tested in clinical trials for effectiveness and safety, and scans can compare brain activity before and after treatment.