Synaptic Transmission

Question 1

what type of neurones contain acetylcholine (neurotransmitter)

Answer 1

• cholinergic neurones

Question 2

describe the mechanism of synaptic transmission

Answer 2

• arrival of action potential at synaptic knob causes depolarisation of presynaptic membrane ,stimulating the calcium channels to open and calcium ions to diffuse into the synaptic knob
• calcium ions cause synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine which diffuses across the synaptic cleft
• acetylcholine attaches to specific protein receptor sites on the postsynaptic membrane stimulating the entry of sodium ions
• influx of sodium ions leads to the depolarisation of postsynaptic neurone and transmission of an impulse as the threshold level is reached
• acetylcholine is broken down in the postsynaptic membrane by the enzyme acetyl cholinesterase forming acetyl and choline which are both taken up to the synaptic knob by active transport and acetylcholine is re-synthesised
• active transport and re-synthesis require ATP yielded by mitochondria during aerobic respiration

Question 3

What causes synapses to be unidirectional?

Answer 3

• only the presynaptic neurone has vesicles to release the neurotransmitter into the synapse
• only the post-synaptic neurone has receptors for the neurotransmitter

Question 4

What is the difference between spatial and temporal summation

Answer 4

Spatial summation - two or more impulses arrive in different regions of the same neurone at the same time
Temporal summation - two or more impulses arrive in rapid succession at the same place

Question 5

How can synapses have an inhibitory effect on the postsynaptic neurone

Answer 5

• by stimulating the influx of negative (chloride) ions or by stimulating the removal of positive potassium ions
• both cause a more negative charge inside axon therefore reaching the threshold level during depolarisation is more difficult to achieve

Question 6

How can different drugs affect the speed of transmission of nerve impulses

Answer 6

• same structure and effect = bind to receptor, faster transmission
• same structure and different effect = bind to receptor and slows transmission
• prevents release of transmitter = transmission is stopped
• inhibits enzymes that break down transmitter = faster transmission (more transmitters)

Question 7

what does summation mean

Answer 7

the additive effect of several impulses causing depolarisation of the postsynaptic membrane

Question 8

describe the transmission of nerve impulse across a neuromuscular junction

Answer 8

• arrival of action potential at synaptic knob causes depolarisation of presynaptic membrane which stimulates calcium channels to open and calcium ions to diffuse into the synaptic knob
• calcium ions cause synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter (acetylcholine) which diffuses across the synaptic cleft
  -acetylcholine attaches to specific protein receptor sites on the sarcolemma stimulating the entry of sodium ions causing depolarisation
• this results in an action potential along the sarcolemma and into the muscle fibre stimulating the release of calcium ions which initiates muscle contraction
• acetylcholine is broken down in the postsynaptic membrane by the enzyme acetyl cholinesterase forming acetyl and choline which are both taken up to the synaptic knob by active transport and acetylcholine is re-synthesised
• active transport and re-synthesis require ATP yielded from mitochondria