Brain Disease (Edexcel International A Level Biology): Revision Note

Brain Disease

• Neurotransmitters are chemicals that transmit nerve impulses across synapses 

• Some disorders and diseases are linked to an imbalance of neurotransmitters in the brain

• Two examples are

  • Parkinson's disease

  • Depression

Parkinson's disease

• Parkinson's disease is a brain disorder that affects the co-ordination of movement, caused by the loss of neurones in some parts of the brain

• Symptoms include

  • A tremor to the specific parts of the body

  • Slow movement

  • Stiff and inflexible muscles

  • Difficulties with balance

  • Changes to speech

• The lost neurones normally produce the neurotransmitter dopamine

  • Dopamine is involved in muscle control

• Individuals that suffer from Parkinson's disease produce insufficient amounts of dopamine due to the loss of dopamine-producing neurones

  • Less dopamine is released into the synaptic cleft meaning less is able to bind with receptors on the postsynaptic membrane 

  • Fewer sodium channels on the membrane are opened so depolarisation of the postsynaptic neurone does not occur

  • This leads to fewer action potentials which creates the symptoms such as tremors and slow movement

• Different types of drug can be used to treat this disease

  • Dopamine agonists

    • Produce the same effect as dopamine by binding to and activating the dopamine receptors on the postsynaptic membrane

  • Dopamine precursors 

    • These are chemicals that can be converted into dopamine in the neurones

    • E.g. L-dopa

  • Enzyme inhibitors

    • Monoamine oxidase B (MAOB) inhibitors inhibit the activity of enzymes that would normally break down dopamine in the synaptic cleft, raising levels of dopamine present in the brain

• Research into other treatments for Parkinson's disease is currently ongoing, with some promising future possibilities in the areas of

  • Gene therapy

    • This would involve the addition of genes to the affected cells in the brain to either increase dopamine production or prevent the destruction of dopamine-producing cells

  • Stem cell therapy

    • Stem cells could be used to replace the lost dopamine-producing cells in the brain

Depression

• Low levels of the neurotransmitter serotonin have been linked to depression

  • Serotonin transmits nerve impulses through the areas of the brain that control mood

  • Low levels of serotonin increase episodes of depression

• Other brain chemicals linked to depression include noradrenaline and dopamine

• Some drugs that have been developed for the treatment of depression, known as antidepressants, work by increasing the levels of relevant neurotransmitters in the brain

  • SSRIs (selective serotonin reuptake inhibitors) are a class of antidepressant that prevent the uptake of serotonin at synapses; this increases the overall levels of serotonin in the brain

  • TCAs (tricyclic antidepressants) increase levels of both serotonin and noradrenaline in the brain

  • MAOB inhibitors inhibit enzymes that would otherwise break down neurotransmitters in the synaptic clefts in the brain