NAS WK 1 - Action Potentials

Question 1

NAME COMMON NEURO-CONDITIONS

Answer 1

stroke, dementia, epilepsy, migraine

Question 2

HOW NEURONES ARE LINKED

Answer 2

• terminals of one neurone adjacent to dendrite of other
• neurotransmitter (in synaptic vesicles) released into synaptic cleft to diffuse to dendrite (each dendrite has lots of synapses)

Question 3

TYPES OF ION CHANNELS IN MEMBRANE

Answer 3

• non gated (set Em of resting membrane)

- gated (generate AP)

Question 4

HOW -65MV MEMBRANE POTENTIAL MAINTAINED

Answer 4

• non-gated ion channels are more permeable to K+ efflux than NA+ influx so potential is negative
• Em remains constant though, NA+ influx must be equal to K+ efflux (as still -65) so NA+ driving force is larger than K+ but membrane is more permeable to K+ so balances out

Question 5

WHY CONC GRADIENT IN MEMBRANE CONSTANT

Answer 5

changes in conc. are extremely small as number of ions moving is so tiny

Question 6

ELECTROCHEMICAL FORCE

Answer 6

net force resulting from chemical & electrical influences

Question 7

TRUE ROLE OF NA+/K+ PUMP

Answer 7

• doesn’t actually maintain the Em (-65mV) it just maintains conc. Gradients by 3NA+ out & 2K+ in (opp. to normal as opposes to even the conc.)but the actual Em is only maintained by movement of ions through the non-gated ion channels not the pump
• They don’t control Em they are only there because over long time periods the constant efflux of K+ would lead to vast changes in conc. So these are just there to stop that happening.

Question 8

ION CHANNEL CONDUCTANCE (G)

Answer 8

• Gk = conductivity of K+ channel
• G(Na) = conductivity of NA+ channel
• G is proportional to number of open ion channels so change in conduction for ion = change in membrane potential

Question 9

SUMMARY OF AP

Answer 9

Summary of AP

1. Initial stimulus (depolarisation) opens the v-gated NA+ channels (increase in G(NA)) THEREFORE NA+ entry= further depolarisation & this further depolarisation leads to even higher G(NA) so membrane potential approaches NA equilibrium potential (as permeability of NA channel during AP = 1000x permeability at rest)
2. NA+ channels inactivate (lower Na conductivity) so NA+ influx starts
3. V-gated K+ channels open (higher Gk) & K+ leaves so repolarisation BUT the Gk remains high after membrane potential returns to rest THEREFORE Em approaches potassium equilibrium potential (hyperpolarisation)
4. V-gated K+ channels close (lower Gk) so leak channels return the Em to resting value

Question 10

EQUILIBRIUM POTENTIAL

Answer 10

ion is in equilibrium when chemical gradient & electrical forces are in balance

Question 11

ABSOLUTE REFRACTORY PERIOD

Answer 11

• no more AP formed by stimulus (stops neurone releasing too many AP consistently)
• most Na+ channels inactivated but many K+ remain open so Na+ influx < K+ efflux (no depolarisation)

Question 12

RELATIVE REFRACTORY PERIOD

Answer 12

• AP only form if stimulus is very strong (higher threshold needed as more Na+ channels need to open)
• Na+ recovering from inactivation & some K+ still open so it is possible but unlikely for Na+ > K+ (depolarisation)

Question 13

AP MOVEMENT ALONG UNMYELINATED AXON (ELECTROTONIC SPREAD)

Answer 13

IN AXON, AP starts at one end then spreads along membrane as depolarises one end so opens Na+ channel which leads to adjacent Na+ opening so more depolarisation & this just repeats along axon

Question 14

AP MOVEMENT ALONG MYELINATED AXON (SALTATORY CONDUCTION)

Answer 14

quicker as AP jumps between nodes of ranvier (gaps between myelin sheaths) so no need to depolarise whole axon (just jumps from + to - in nodes of ranvier)

Question 15

WHAT FACTORS AFFECT SPEED OF CONDUCTION

Answer 15

myelination, larger axon diameter & higher temperature all increase speed of AP