6.2.2 Synaptic transmission

Question 1

what is present at a synapse

Answer 1

• presynaptic terminal
• post synaptic membrane

Question 2

what is at the presynaptic terminal

Answer 2

• vesicles of neurotransmitter
• voltage-gated calcium channels
• extensive RER and Golgi apparatus for neurotransmitter synthesis and vesicle formation

Question 3

what is at the post synaptic membrane

Answer 3

• gated ion channels with neurotransmitter receptors
• voltage gated Na+ channels
• cholinergic

Question 4

what does it mean if a synapse is cholinergic

Answer 4

acetylcholinesterase is associated with the ligand-gated ion channels

Question 5

what occurs at a cholinergic synapse

Answer 5

• depolarisation of presynaptic membrane (from action potential propagation)
• voltage gated Ca2+ channels open causing an influx of Ca2+
• triggers exocytosis of neurotransmitter
• neurotransmitter diffuses across synapse and binds to receptors on post-S membrane
• causes ligand-gated Na+ channels to open
• if threshold potential is reached, voltage gated Na+ channels open and AP occurs
• acetylcholine broken down, choline and acetyl unbind from receptor and diffuse back into presynaptic terminal

Question 6

what happens to acetyl and choline when they unbind from receptors and diffuse back into synaptic terminal

Answer 6

used to synthesise acetylcholine and stored in vesicles

Question 7

what does acetylcholinesterase do

Answer 7

breaks down acetylcholine into acetyl and choline

Question 8

what is unidirectionality

Answer 8

• neurotransmitter is only synthesised and released from presynaptic membrane
• receptors are only on the post synaptic membrane
• so transmission can only occur in one direction

Question 9

what is summation

Answer 9

when a single action potential does not cause the release of sufficient neurotransmitter to reach the threshold potential so several post-synaptic combine to reach the threshold

Question 10

what are the different types of summation

Answer 10

temporal
spatial

Question 11

what is spatial summation

Answer 11

several pre-synaptic terminals release neurotransmitter to the same post-synaptic membrane

Question 12

what is temporal summation

Answer 12

a single pre-synaptic terminal receives several action potentials in a short time so release enough neurotransmitter to reach threshold potential

Question 13

what is inhibition

Answer 13

when some neurotransmitters cause hyperpolarization of the post synaptic membrane, so an action potential is not produced

Question 14

what causes inhibition

Answer 14

• influx of Cl-
• efflux of K+
• means more influx of Na+ is needed to reach threshold potential and action potential is less likely to occur

Question 15

what are the similarities of events at a cholinergic synapse and a neuromuscular junction

Answer 15

• Acetylcholine (neurotransmitter) is transported by diffusion
• Receptors, that on binding with neurotransmitter, cause an influx of sodium ions
• Use sodium-potassium pump to repolarise
• Use enzymes to break down neurotransmitter

Question 16

what are the differences between events of the cholinergic synapse and a neuromuscular junction

Answer 16

• Synapses can be excitatory or inhibitory - NM junction is always excitatory
• Synapses link neurones to neurones, or neurones to other effectors but NM junction only links motor neurone to muscles
• Acetylcholine receptors are on post-synaptic neurone in synapses, but on muscle cells in NM junctions
• Only motor neurones involved at NM junction; motor, sensory or intermediate neurones involved in cholinergic synapses