Brain Disease (Edexcel International A Level (IAL) 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 inside brain cells after it is taken back up from 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

  • MAOIs (monoamine oxidase inhibitors) inhibit enzymes that would otherwise break down neurotransmitters such as serotonin in the brain