Explain the process of synaptic transmission [7]

A nerve impulse would first travel to end of pre synaptic neuron, this will then trigger a influx of calcium ions. In the pre synaptic neuron there are swollen membranous areas called terminal buttons and within these there are many vesicles filled with neurotransmitters. The calcium influx then causes synaptic vesicles containing neurotransmitters to fuse with the pre synaptic membrane by exocytosis. The neurotransmitters are then released into the synaptic cleft and diffuse across it to bind with the receptors on the post synaptic neuron. This then causes ion channels to open on neuron and sodium diffuses into postsynaptic neuron. This initiates the action potential to begin moving down the postsynaptic neuron because it’s been depolarized. The neurotransmitter is then degraded and broken into two or more fragments by specific enzymes. They’re then released from the receptor protein. The ion channel closes to sodium ions. The neurotransmitter fragments diffuse back across the synaptic gap to be reassembled into the terminal buttons of the pre synaptic neuron. 


Mark scheme

a)      Pre synaptic neurons pass a stimulus to post-synaptic

b)      Pre synaptic releases NT into cleft

c)       Process involves exocytosis

d)      Exocytosis triggered by Ca2+ into neuron (bulb)

e)      NT binds with receptor on postsynaptic cleft

f)       NT binding causes ion channels to open

g)      Ions diffuse into/out of cell

h)      Depolarization or Hyper-polarization

i)        Outcome depends on type of receptor

j)        E.g. Na+ going into postsynaptic neuron = depolarization

k)      Cl- passing into the post synaptic neuron = hyper polarization

l)        NT destroyed/deactivated by enzymes