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  • Synapses
    • Synapses and Neurotransmitters
      • A synapse is the junction between a neurone and another neurone, or between a neurone and an effector cell.
        • The tiny gap between the cells at a synapse is called the synaptic cleft.
          • The presynaptic neurone has a swelling called a synaptic knob. This contains synaptic vesicles filled with chemicals called neurotransmitters.
      • When an action potential reaches the end of a neurone it causes neurotransmitters to be released into the synaptic cleft.
        • They diffuse across to the postsynaptic membrane and bind to specific receptors.
          • When neurotransmitters bind to receptors they might trigger an action potential, cause muscle contraction or cause hormone secretion.
      • Neurotransmitters are removed from the cleft so the response doesn't keep happening.
      • Cholinergic Synapses
        • A cholinergic synapse uses the neurotransmitter acetylcholine (ACh) which binds to receptors called cholinergic receptors.
    • Synaptic Transmission
      • Arrival of an action potential
        • An action potential arrives at the synaptic knob of the presynaptic neurone.
        • The action potential stimulates voltage-gated calcium ion channels in the presynaptic neurone to open.
        • Calcium ions diffuse into the synaptic knob. (They're pumped out afterwards by active transport)
      • Fusion of the vesicles
        • The influx of calcium ions into the synaptic knob causes he synaptic vesicles to move to the presynaptic membrane.
        • They then fuse with the presynaptic membrane. The vesicles release ACh into the synaptic cleft by exocytosis.
      • Diffusion of ACh
        • ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane.
        • This causes sodium ion channels in the postsynaptic neurone to open.
        • The influx of sodium ions into the postsynaptic neurone causes depolarisation.
        • An action potential on the postsynaptic membrane is generated if the threshold is reached.
        • ACh is removed from the synaptic cleft so the response doesn't keep happening. It's broken down by an enzyme called acetylcholinesterase and the products are re-absorbed by the presynaptic neurone and used to make more ACh.
    • Disruption of synaptic transmission
      • Because synapses uses chemical communication they can be affected by chemicals like drugs, toxins or poisons.
      • Some chemicals block receptors so they can't be activated by neurotransmitters. This means fewer transmitters can be activated. e.g. nicotine.
      • Some chemicals inhibit the enzyme that breaks down neurotransmitters. This means there are more neurotransmitters in the synaptic cleft to bind to receptors. e.g. curare.
      • Some chemicals inhibit the release of neurotransmitters from the presynaptic neurone so fewer receptors are activated. e.g. opioids.
    • Roles of synapses
      • Divergence and convergence
        • When one neruone connects to many neurones information can be dispersed to different parts of the body. This is called synaptic divergence.
        • When many neurones connect to one neurone information can be amplified. This is called synaptic convergence.
      • Summation
        • If a stimulus is weak, only a small amount of neurotransmitter will be released from a neurone into the synaptic cleft.
        • This might not be enough to excite the postsynaptic membrane to the threshold level and stimulate an action potential.
        • Summation is where the effect of neurotransmitters released from many neurones is added together.
        • SPATIAL SUMMATION- where two or more presynaptic neurones release their neurotransmitters at the same time onto the same postsynaptic neurone.
          • The small amount of neurotransmitter released from each of these neurones can be enough altogether to reach the threshold in the postsynaptic neuroneand trigger an action potential.
          • TEMPORAL SUMMATION- is where two or more nerve impulses arrive in quick succession from the same presynaptic neurone. This makes an action potential more likely because more neurotransmitters is released into the synaptic cleft.


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