3.6.2.2 synaptic transmission

Question 1

Describe transmission across cholinergic synapse

Answer 1

1. Depolarisation of presynaptic membrane causes opening of. voltage gated Ca 2+ channels
   ca 2+ diffuses into presynaptic knob
   2.causes vesicles containing ACh to move and fuse with presynaptic membrane
   releasing ACh into the synaptic cleft by exocytosis
2. ACh diffuses across synaptic cleft to bind to specific receptors on post synaptic membrane
3. Causes Na+ channels to open
   Na+ diffuses into into post synaptic knob causing depolarisation
   if threshold is met, action potential in initiated.

Question 2

what happens to acetylcholine after synaptic transmission

Answer 2

it is hydrolysed by acetylcholinesterase
products are re-absorbed by the presynaptic neurone
to stop overstimulation, if not removed it would keep binding to receptors causing depolarisation.

Question 3

explain how synapses result in unidirectional nerve impulses

Answer 3

neurotransmitters are only made in and released by pre synaptic neurone
receptors only on post synaptic membrane

Question 4

explain summation by synapses

Answer 4

addition of a number of impulses converging on a single post synaptic neurone
causing rapid buildup of neurotransmitter
so threshold more likely to be reached to generate an action potential
(rod cells)

Question 5

describe spacial summation

Answer 5

many presynaptic neurones share one synaptic cleft / post synaptic neurone
collectively release sufficient neurotransmitter to reach threshold to trigger action potential

Question 6

describe temporal summation

Answer 6

one presynaptic neurone releases neurotransmitter many times over a short period
sufficient neurotransmitter to reach threshold to trigger an action potential

Question 7

describe inhibition by inhibitory synapses

Answer 7

inhibitory neurotransmitters hyperpolarise postsynaptic membrane as
cl- ion channels open Cl- diffuses in
k+ ion channels open K+ diffuses out
inside of axon has a more negative charge relative to outside/ below resting potential
so more Na+ required to enter for depolarisation
reduces likelihood of threshold being met and therefore action potential being formed at post synaptic membrane

Question 8

describe the structure of a neuromuscular junction

Answer 8

there are receptors on muscle fibre sarcolemma instead of postsynaptic membrane
muscle fibres forms clefts to store enzyme acetylcholinesterase to break down neurotransmitter

Question 9

compare transmission across cholinergic synapses and neuromuscular junctions

Answer 9

• both transmission is unidirectional
  -cholinergic : neurone to neurone, neurotransmitters can be excitatory or inhibitory, action potential may be initiated in postsynaptic neurone
  -neuromuscular junction- motor neurone to muscle, always excitatory, action potential propagates along sarcolemma down T Tubules

Question 10

effect of drugs on synapse

Answer 10

stimulate th NS
-similar shape to neurotransmitter
-stimulate the release of more neurotransmitter
-inhibit enzyme that breaks down neurotransmitter, Na+ continues to enter
Inhibit NS
-inhibit release of neurotransmitter, prevent opening of Calcium ion channels
Block receptors