synaptic transmission

Question 1

synpase

Answer 1

the connector between:

receptor - neuron

neuron - neuron

neuron - muscle (neuro-muscular junction)

Question 2

synaptic transmission

Answer 2

• action potential travels down the axon and arrives at the synaptic knob
• triggers Ca2+ inrush
• this causes vesicles containing neurotransmitter to move out and fuse with the pre-synaptic membrane releasing neurotransmitter by exocytosis
• neurotransmitter diffuses across synaptic cleft and binds to receptors on the postsynaptic membrane
• this causes Na+ channels to open in the post-synaptic membrane, Na+ diffuses in causing local depolarisation
• if enough Na+ enters, causing enough depolarisation to reach a threshold level (-55 mVish)
• many more Na+ ion channels open, Na+ inrush takes the post-synaptic neuron to +40mV causing an action potential

Question 3

recovery

Answer 3

synaptic transmission needs to stop, so that it can happen again in response to the arrival of another action potential

• there are enzymes in the synaptic cleft that hydrolyse the neurotransmitter (eg cholinesterase with acetylcholine)
• the breakdown products are reabsorbed into the synaptic knob and used to resynthesise neurotransmitter
• in some synapses, the neurotransmitter is removed by reabsorption / active transport back into the knob

Question 4

temporal summation

Answer 4

one synaptic signal is not enough to trigger an post-synaptic action potential

several signals, in close succession are

more Na+ inrush occurs before complete recovery

Question 5

spatial summation

Answer 5

multiple synapses, all triggering Na+ inrush at the same time can take depolarisation to the threshold level, where one synapse wouldnt

Question 6

inhibitory synapses

Answer 6

at an inhibitory synapse, neurotransmitter trigger an effect that counters the depolarisation caused by an excitatory synapse

Question 7

drugs - increasing synaptic signalling

Answer 7

drug is similar to shape of neurotransmitter (neurotransmitter mimic)

drug inhibits the enzyme that is supposed to break down neurotransmitter

Question 8

drugs - decreases synaptic signalling

Answer 8

drug is similar enough to neurotransmitter to block receptor sites

Question 9

what is the function of synapses

Answer 9

electrical impulse cannot travel over junction between neurons

neurotransmitters send impulses between neurons / from neurons to effectors

new impulses can be initiated in several different neurons for multiple simultaneous responses

Question 10

describe the structure of a synapse

Answer 10

presynaptic neuron ends with synaptic knob - contains lots of mitochondria, endoplasmic reticulum and vesicles of neurotransmitter

Question 11

how do neurotransmitters cross the synaptic cleft

Answer 11

via simple diffusion

Question 12

explain why synaptic transmission is unidirectional

Answer 12

only presynaptic neuron contains vesicles of neurotransmitter and only postsynaptic membrane has complementary receptors

so impulse always travels pre to psot

Question 13

what are cholinergic synapses

Answer 13

use acetylcholine as primary neurotransmitter

excitatory or inhibitory

Question 14

what happens to acetylcholine from the synaptic cleft

Answer 14

• hydrolysis into acetyl and choline by acetylcholinesterase
• acetyl and choline diffuse back into presynaptic membrane
• ATP is used to reform acetylcholine for storage in vesicles

Question 15

importance of acetylcholinesterase

Answer 15

prevents the overstimulation of skeletal muscle cells

enables acetyl and choline to be recycled

Question 16

what happens in an inhibitory synapse

Answer 16

neurotransmitter binds to and opens calcium channels on postsynaptic membrane and triggers potassium channels to open

calcium ions move in and potassium ions move out via facilitated diffusion

• more negative
• hyperpolarisation

Question 17

describe the structure of a neuromuscular junction

Answer 17

synaptic cleft between a presynaptic neuron and a skeletal muscle cell

Question 18

how might drugs affect synaptic transmission

Answer 18

inhibit release of neurotransmitter

decrease permeability of postsynaptic membrane to ions

hyperpolarise postsynaptic membrane

increase:
(inhibit acetylcholinesterase)
(mimic shape of neurotransmitter)