Synapses

Question 1

state what it is meant by the key term - synapse

Answer 1

a synapse is the anatomically specialised junction between two neurones

Question 2

in essence, what does activity at a synapse do ?

Answer 2

activity at a synapse increases or decreases the likelihood that the post-synaptic neurone will fire action potentials by producing a brief graded potential

Question 3

what are the two types of synapses ?

Answer 3

excitatory and inhibitory synapses

Question 4

state what it is meant by the key term - excitatory synapse

Answer 4

the membrane potential off the post-synaptic knob is brought closer to threshold potential (i.e. - depolarisation)

Question 5

state what it is meant by the key term - inhibitory synapse

Answer 5

the membrane potential of the post-synaptic knob is either driven further from threshold potential (I/e/ - hyperpolarisation) or stabilised at resting potential

Question 6

state what it is meant by the key term - convergence

Answer 6

hundreds of synapses from Manu different pre-synaptic neurones can affect a single post-synaptic knob

Question 7

state what it is meant by the key term - divergence

Answer 7

a single pre-synaptic neurone can effect multiple post-synaptic knobs

Question 8

what does convergence allow for ?

Answer 8

convergence allows for information from many different sources to influence a cells activity

Question 9

what does divergence allow for ?

Answer 9

allows one information source to affect multiple pathways

Question 10

finish the sentence:

in an electrical synapse…

Answer 10

in an electrical synapse, the electrical activity of the pre-synaptic neurone affects the electrical activity of the post-synaptic knob via a gap junction

Question 11

finish the sentence:

in chemical synapses…

Answer 11

in chemical synapses, neurotransmitters release the signal

Question 12

how and where are neurotransmitters stored ?

Answer 12

neurotransmitters are stored in vesicles within the phospholipid bilayer –> docked on the pre-synaptic membrane in ‘active zones’

Question 13

what are the two classes of receptors ?

Answer 13

1. ionotropic receptors (ion channels)

2. metabotropic receptors (G-protein / second messenger signalling)

Question 14

what is the synaptic delay, and why ?

Answer 14

0.2 ms, due to the sequence of events

Question 15

signal termination at a synapse occurs by what 3 mechanisms ?

Answer 15

1. diffusion of neurotransmitter from the cleft
2. degradation of the transmitter by enzymes
3. re-uptake into the pre-synaptic knob for re-use

Question 16

state the first 5 steps of the synapse mechanism process

Answer 16

1. AP arrives at pre-synaptic knob
2. causes voltage gated calcium ion channels to open
   (Ca2+ diffuses in)
3. Ca2+ increases fluidity of vesicles containing ACh and they fuse with the membrane
4. ACh is released and diffuses across synaptic cleft
5. ACh binds to receptor on post-synaptic knob

Question 17

state the second 6 steps of the synapse mechanism process

Answer 17

6. causes SOME voltage-gated Na+ channels to open (Na+ diffuses in)
7. positive feedback mechanism causes more voltage gated Na+ channels to open (provided AP threshold isn met)
8. an AP is triggered in the post-synaptic neurone
9. acetylcholinesterase hydrolyses ACh and terminates the signal
10. ACh products are re-absorbed into the pre-synaptic knob and are recycled

Question 18

state 2 facts about ‘excitatory chemical synapses’

Answer 18

1. generates an EPSP (a graded potential)

2. brings membrane potential closer to the threshold potential

Question 19

state 2 facts about ‘inhibitory chemical synapses’

Answer 19

1. generates an IPSP

2. makes membrane potential more negative, making it harder to reach threshold potential

Question 20

what is the value of a single AP ?

Answer 20

0.5 mV

need 15 mV to reach threshold potential

Question 21

state what it is meant by the key term - temporal summation

Answer 21

a second potential arrives after the first has died down a bit, the synaptic potentials add together, resulting in greater depolarisation

Question 22

state what it is meant by the key term - spatial summation

Answer 22

two potentials arrive from two different neurones resulting in summation and greater depolarisation

Question 23

state 3 ‘pre-synaptic factors’ that determine synaptic strength

Answer 23

1. availability of neurotransmitter
2. axon terminal membrane potential
3. activation of membrane receptors on the pre-synaptic terminal
4. axon terminal Ca2+
5. certain drugs and diseases, which act via the above mechanisms (1-4)

Question 24

state 3 ‘post-synaptic factors’ that determine synaptic strength

Answer 24

1. immediate past history for electrical state of post-synaptic membrane
2. effects of other neurotransmitters or neuromodulators acting on the post-synaptic neurone
3. up/down regulation and desensitisation of receptors
4. certain drugs or diseases

Question 25

state 3 ‘general factors’ that determine synaptic strength

Answer 25

1. surface area of synaptic contact
2. enzyme destruction of neurotransmitters
3. geometry of diffusion path
4. neurotransmitter re-uptake