Edexcel A (SNAB) A Level Biology:复习笔记8.2.6 Imbalances in Brain Chemicals Leading to Diseases

Imbalances in Brain Chemicals Leading to Diseases

  • 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 drugs 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


  • 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