Axons and axonal transmission

Question 1

what are the types of synapse?

Answer 1

excitatory, inhibitory and modulatory

Question 2

what is the purpose of a modulatory synapse?

Answer 2

to dampen or amplify the synaptic response altering it’s effectiveness

Question 3

how do modulatory synapses work?

Answer 3

they affect the presynaptic neurons metabollic pathways this can happen over minutes/hours/days

• DNA transcription
• alteration of enzyme activity
• protein synthesis

Question 4

what are three neuro-modulators

Answer 4

serotonin, dopamine and nor-adrenaline

Question 5

describe the process of neurotransmitter release.

Answer 5

• vesicles containing neurotransmitters are held close to the presynaptic membrane
• depolaristion reaches presynaptic membrane causing the opening of Ca2+ voltage gated channels
• Ca2+ enters the cell and causes vesicles to be released from Ca2+ sensitive proteins attaching them to the cytoskeleton
• vesicles fuse to the presynaptic membrane causing exocytosis of neurotransmitter

Question 6

what happens once neurotransmitter is exocytosed?

Answer 6

• diffuses across the synaptic cleft
• activates specific transmitter gated channels
• causes depolarisation only if the synapse is excitatory

Question 7

what happens at an ihibitory synapse?

Answer 7

neurotransmitters causes Cl- & K+ channels to open either causes an inhibitory post synaptic or stabilisation of the post synaptic membrane by Cl- ions causing hyperpolaristion making it harder for an action potential to be generated

Question 8

what is resting potential of an axon? what causes it?

Answer 8

• 70mV generated by the action of Na+/K+ pump
• 3 Na+ are pumped out and 2 K+ are pumped in
• K+ then leaves axon by leakage channels causing an negative charge on the inside

Question 9

what causes an axon to depolarise?

Answer 9

• stimulation by either a neurotransmitter or a sensory stimuli causes sodium channels to open partially and slightly depolarise
• if threshold potential is reached Na+ channels fully open and depolarisation occurs

Question 10

what does an axon depolarise to ? what mediates this?

Answer 10

• depolarises to +40mV

- influx of Na+ through sodium voltage gated channels

Question 11

what causes axon repolarisation?

Answer 11

+40mV charge causes voltage gated K+ channels to open and K+ moves out

Question 12

what are the two parts of the refractory period? what are they?

Answer 12

absolute refractory period - a new action potential cannot be generated
relative refractory period - the membrane is still hyperpolarised but if the stimuli is stronger an action potential can be generated.