• Structures known as?synapses?are found at the?junctions between cells in the nervous system?e.g.
  • In the sense organs there are synapses between?sensory receptor cells?and?sensory neurones
  • In muscles there are synapses between?motor neurones?and?muscle fibres
• The structure of a synapse includes the following features
  • A gap between the neurones known as the?synaptic cleft
  • The neurone before the synapse is known as the?presynaptic neurone?and has a rounded end known as the?synaptic knob
  • The neurone after the synapse is known as the?postsynaptic neurone
  • Nerve impulses are passed across the synaptic cleft by the?diffusion of chemicals?known as?neurotransmitters?e.g. acetylcholine
    • Neurotransmitters are contained within?vesicles?in the synaptic knob

Synapses are the junctions between neurones e.g. between a sensory neurone and a relay neurone

Synaptic transmission

• Electrical impulses cannot ‘jump’ across the synaptic cleft
• When an action potential arrives at the end of the axon of the?presynaptic neurone?the?membrane?becomes?depolarised, causing?voltage gated calcium ion channels?to open
• Calcium ions?diffuse into the synaptic knob via?calcium ion channels?in the membrane
• The calcium ions cause vesicles in the synaptic knob to move towards the presynaptic membrane where they fuse with it and?release chemical messengers?called?neurotransmitters?into the synaptic cleft by?exocytosis
  • A common neurotransmitter is?acetylcholine, or?ACh
• The neurotransmitters?diffuse?across the?synaptic cleft?and?bind with receptor molecules?on the?postsynaptic membrane; this causes associated?sodium ion channels?on the postsynaptic membrane to open, allowing?sodium ions?to diffuse into the postsynaptic cell
• If enough neurotransmitter molecules bind with receptors on the postsynaptic membrane then an?action potential?is generated, which then travels down the?axon?of the?postsynaptic neurone
  • Whether or not an action potential is generated depends on whether or not?threshold potential?is reached, which in turn depends on the?number of action potentials arriving at the presynaptic knob
    • Many action potentials will cause more neurotransmitter to be released by exocytosis
    • A large amount of neurotransmitter will cause many sodium ion channels to open
    • Many sodium ion channels opening will allow a large influx of sodium ions, increasing the likelihood of threshold being reached
• The neurotransmitters are then?broken down?to prevent continued stimulation of the postsynaptic neurone
  • The enzyme that breaks down acetylcholine is?acetylcholinesterase

Impulses are transmitted across the synaptic cleft by the diffusion of neurotransmitters such as acetylcholine

Additional roles of synapses

• Synapses enable
  • Unidirectionality?of impulse transmission
    • Synapses ensure the?one-way transmission?of impulses
    • Impulses can only pass in?one direction?at synapses because?neurotransmitter is released on one side?and its?receptors are on the other;?chemical transmission cannot occur in the opposite direction
  • Divergence?of nerve impulses
    • One neurone can connect to?several other neurones?at a synapse, allowing nerve signals to be sent in several directions from a single presynaptic neurone
  • Amplification?of nerve signals by?summation
    • When an impulse arrives at a synapse it?does not always?cause an impulse to be generated in the next neurone; a?single impulse?that arrives at a synaptic knob may be?insufficient?to generate an action potential in the post-synaptic neurone
      • Only a small amount of acetylcholine may release into the synaptic cleft
      • A small number of sodium ion channels are opened in the postsynaptic axon membrane
      • An insufficient number of sodium ions pass through the membrane
      • The threshold potential is not reached
    • The effect of multiple impulses can be added together to overcome this in a process known as?summation
    • Summation can be achieved by
      • Several presynaptic neurones?converging to meet a single postsynaptic neurone
        • This is known as?synaptic convergence
      • Many action potentials?arriving at a postsynaptic knob in quick succession