Explain how pre-synaptic neurons can affect post-synaptic transmissions of impulses [7]

Firstly a nerve impulse coming into the pre synaptic neuron would cause calcium ions to diffuse through channels in the membrane. The calcium influx then causes vesicles containing neurotransmitters to fuse with the pre synaptic membrane, this process is called exocytosis. The neurotransmitters are then released into the synaptic cleft and diffuse across it to bind with the post synaptic neuron. This is called an excitatory synapse. The binding then cause’s ion channels to open and sodium ions will then diffuse through this channel down the concentration gradient. This then initiates the action potential which begins to move down the post-synaptic neuron because it has been depolarized. 

Opposite from an excitatory synapse is an inhibitory synapse. This is when the release of neurotransmitters into the cleft inhibits an action potential being generated in the post-synaptic neuron. This means that instead of the neurotransmitters triggering the opening of ion channels to let sodium out it will instead allow chlorine ions to enter the neurone or potassium to leave. This will then make the post-synaptic neurone more negative (hyperpolarised) and therefore less likely to initiate an action potential.


Mark scheme

a)      Pre synaptic neuron can be excitory of inhibitory

b)      Chlorinergenic neurons release acetylcholine

c)       Found in neuromuscular junctions/in autonomic nervous system/most junctions in voluntary nervous system

d)      Adrenic nerons release noradrenaline

e)      Found in sympathetic pathways (in brain)

f)       Both types of neurons can be excitory

g)      Neurotransmitters (NT) bind to receptors on post synaptic membrane

h)      Triggers opening of Na+ gates/channels/Na+ moves across membrane

i)        Causes depolarization

j)        NT’s are degraded/destroyed or recycled e.g. acetyl choline esterase breaks down acetyl choline

k)      Other inhibitory NT’s e.g. GABA (cocaine, alcohol), Dopamine

l)        Inhibitory NT’s less permeable to Na+/cause Cl- ions to diffuse in

m)    Hyper-polarization

n)      By K+ diffusing out