Structure of the Synapse

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Structure of the Synapse
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The Structure and function of the synapse

• WALT
• That synapses occur between neurones
• How the nervous impulse is transmitted across the
  synapse
• That synapses use different neurotransmitters

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Definitions

• presynaptic neuron before the synapse
• postsynaptic neuron behind the synapse with
  receptor molecules
• synaptic cleft a narrow gap about 20 nm wide

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Synapse

• When a nerve impulse arrives at the synaptic knob
  it alters the permeability of the presynaptic
  membrane to Ca, which therefore enters
• Synaptic vesicle fuse with membrane discharge
  its transmitter substance

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Synapse

• The transmitter diffuses across synaptic cleft
  fuses with receptor molecules
• This alters permeability of postsynaptic
  membrane to Na
• If excitatory postsynaptic potential (EPSP) EPSP
  by summation exceeds threshold
• Action potential results

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Synapses

• Transmitter substance (acetylcholine) is
  hydrolysed by the enzyme acetylcholinesterase on
  the postsynaptic membrane.
• Its breakdown can be reused to synthesize
  acetylcholine again at the synaptic knob, with
  energy from mitochondria.

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Synapse
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Synapse
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Neurotransmitters

• Acetylcholine - released by all motor neurones,
  activating skeletal muscles involved in the
  parasympathetic nervous system (relaxing
  responses)                      - cholinergic
  synapses
• Noradrenaline - involved in the sympathetic
  nervous system ('fight or flight'
  responses)                       - adrenergic
  synapses

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Functions of the synapse

• 1. Transmit information between neurons
• 2. Pass impulses in one direction only
• 3. Act as junctions
• 4. Filter out low level stimuli
• 5. Allow adaptation to intense stimulation

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Task

• Draw and annotate the transmission of an impulse
  across the synapse
• What is meant by adaptation?
• Where else do synapses occur?
• What name is given to those synapses that use
  acetylcholine as neurotransmitter?
• Questions 9 and 10 page 61
• Explain summation

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Types of Synapse

• Excitatory ion channel synapses -neuroreceptors
  are Na channels.  When Na channels open, local
  depolarisaition occurs, if threshold is reached
  then action potential is initated
• Inhibitory ion channels - neuroreceptors are Cl-
  channels.  When Cl- channels open,
  hyperpolarisation occurs, making action potential
  less likely
• Non channel synapses - neuroreceptors are
  membrane-bound enzymes.  When activated, they
  catalyse the 'messenger chemical', which in turn
  can affect the sensitivity of the ion channel
  receptors in the cell

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Types of Synapse

• Neuromuscular junctions - synapses formed between
  motor neurones and muscle cells.  Always use the
  neurotransmitter acetylcholine, and are always
  excitatory
• Electrical synapses - the membranes of the two
  cells actually touch and they chare proteins. 
  The action potential can pass directly from one
  membrane to the next