The Synapse

• synapses are found at the junctions between cells in the nervous system
• 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
• Structure of a synapse:

  • synaptic cleft: gap between neurones
  • a presynaptic neurone which has a synaptic knob with neurotransmitter vesicles
  • neurotransmitters (e.g. acetylcholine) diffuse across the cleft to the postsynaptic neurone - the neurotransmitters are contained within vesicles in the synaptic knob
• synapses ensure the one-way transmission of impulses as the neurotransmitter is released on one side and its receptors are on the other
• 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
• a single impulse may not release enough acetylcholine to reach the threshold in the postsynaptic neurone and open the sodium ion channels, so no action potential is generated
• Synaptic convergence

  summation (the effect of multiple impulses added together to reach a threshold potential) can be achieved by several presynaptic neurones converging to meet a single postsynaptic neurone
• a wave of depolarisation from an action potential arrives at the end of the presynaptic neurone, causing voltage-gated calcium ion channels to open, which allows calcium ions diffuse into the synaptic knob.

  the calcium ions cause vesicles in the synaptic knob to move towards the presynaptic membrane where they fuse with it and release neurotransmitters by exocytosis into the cleft.
• what is an example of a neurotransmitter?

  acetylcholine
• Post-synaptic activation:

  Neurotransmitters diffuse across the synaptic cleft and bind to receptors, opening sodium channels on the postsynaptic neurone, and allowing sodium ions to diffuse into the postsynaptic cell. If enough neurotransmitter molecules bind with receptors on the postsynaptic membrane, an action potential is generated, which travels down the axon of the postsynaptic neurone
• The generation of an action potential at a synapse is dependent on:

  • the number of action potentials arriving at the presynaptic knob
  • many action potentials will cause more neurotransmitters to be released by exocytosis
  • a large amount of neurotransmitters will cause many sodium ion channels to open - allowing a large influx of sodium ions
• After reaching an action potential, the neurotransmitters are broken down to prevent continued stimulation of the postsynaptic neurone
• what is the enzyme that breaks down acetylcholine?

  acetylcholinesterase