synaptic transmission

synapse features

unidirectionality:
APs can only pass across a synapse from a pre- to post-synaptic neurone. neurotransmitter only released from the end of the pre-synaptic neurone. receptors only located on post-synaptic neurone membrane.

summation:
- low frequency AP can lead to insufficient neurotransmitter release, and therefore no AP in the postsynaptic neurone
- threshold more likely to be reached in the postsynaptic neurone, this enables an AP to be generated and neurone depolarises
- spatial summation + temporal summation

inhibition:
some synapses make it less likely that a new AP will be made on the postsynaptic neurone

spatial summation

neurotransmitter accumulates due to the convergence of two or more neurones, this makes it likely that threshold can be exceeded in the post synaptic neurone

temporal summation

neurotransmitter accumulates due to several APs arriving in quick succession from one neurone, this makes it likely that threshold can be exceeded in the post synaptic neurone

transmission across a cholinergic synapse

1. an AP arriving at the end of presynaptic neurone causes Ca2+ to diffuse in

2. synaptic vesicles containing acetylcholine fuse with pre-synaptic membrane and release acetyl choline into the synaptic cleft by exocytosis

3. acetyl choline diffuses to the post synaptic neurone and binds to its receptors opening these sodium ion channels

4. sodium ions diffuse into postsynaptic neurone causing depolarisation and if threshold is exceeded, AP generated

5. acetyl choline is broken down by acetylcholinesterase into choline and ethanoic acid

6. products diffuse back to pre-synaptic neurone and are reabsorbed

7. they are reformed into acetylcholine using the energy from ATP hydrolysis

convergence

nerve impulses from different receptors reacting to different stimuli

can contribute to a single response

divergence

one creates a number of different simultaneous responses

transmission across synapse

1. presynaptic neurone releases neurotransmitter that binds to chloride ion channel on the postsynaptic membrane → chloride ion channels open, Cl- diffuses into postsynaptic neurone

2. binding of neurotransmitter causes opening of nearby K+ channels

3. K+ diffuse out of postsynaptic neurone into synapse → inside of postsynaptic neurone becomes more negative

4. membrane potential decreases to -80mV (hyperpolarisaiton) → this makes it less likely that a new AP will be created, as a larger influx of Na+ is needed to produce one

inhibitory synapse

inhibitory neurotransmitter binding causes: Cl- enter and K+ leave

causes hyperpolarisation of neurone less likely that a new AP will be created because it will need more Na+ entering to depolarise the neurone

neuromuscular junctions

connecting motor neurones with skeletal muscles

excitatory synapse - e.g. cholinergic synapses

acetylcholine binds to postsynaptic membranes and stimulates the production of nerve impulses

depolarising the neurone

opens Na+ ion channels

inhibitory synapse - GABA is another neurotransmitter

binds to postsynaptic membranes and causes hyperpolarisation of neurone (it becomes more negative)

need more Na+ entering, so neurone less likely to depolarise

describe how calcium ions are involved in synaptic transmission.

nerve impulse causes calcium ions to enter presynaptic membrane.

calcium ions entry causes fusion of vesicles with presynaptic membrane.

explain how the release of acetylcholine at an excitatory synapse reduces the membrane potential of the postsynaptic membrane.

binds to receptor and opens sodium ion channels.

sodium ions enter and make membrane potential depolarised.