Action Potential

Question 1

What are the two types of membrane potentials?

Answer 1

Gradee potentials and action potentials

Question 2

What are graded potentials?

Answer 2

Short distance signals, where ions flow rapidly across membrane in ion channels, causing depolarisation

Question 3

Outline the process of local current loops in graded potentials

Answer 3

Na+ ions into cell through open Na+ channels due to both concentration and electrical gradients (+ve attracted to less +ve regions); causes depolarisation at this point; once Na+ ions are inside cell, move away from depolarisation towards less +ve regions; Na+ inside flow increasingly further from open channel; Na+ outside flow consistently towards open channel; signal moves along membrane

Question 4

How do graded potentials contribute to initiation of action potentials?

Answer 4

Single graded potential too small to reach threshold, but though summation can trigger action potential

Question 5

How does summation of graded potentials occur?

Answer 5

Stimuli occur and trigger graded potentials before the previous graded potential dies away

Question 6

What are the two types of graded potentials, and their implications?

Answer 6

Excitatory and inhibitory. Excitatory causes depolarisation and is more likely to create an action potential. Inhibitory causes hyperpolarisation and means an action potential is less likely to form.

Question 7

How are graded and action potentials in their relationship to size of stimulus?

Answer 7

Graded potentials’ size is proportional to size of stimulus, whilst size of action potentials is not

Question 8

Why are graded potentials only short distance?

Answer 8

They decay as they move over distance due to leakage of charge across plasmalemma reducing current at sites further along membrane (away from open channel). Thus depolarisation is confined to a small region of the membrane

Question 9

What are action potentials?

Answer 9

Long distance signals

Question 10

How fast are action potentials?

Answer 10

1-4ms

Question 11

What is the threshold?

Answer 11

-55mV

Question 12

What is the membrane depolarised from and to?

Answer 12

-70mV to +30mV

Question 13

How are action potentials self-propagating over long distances?

Answer 13

Through positive feedback: Na+ entry causes depolarisation, which causes more Na+ v-gated channels to open (as v-gated ion channels always open when membrane is depolarised)

Question 14

What is the initial depolarisation caused by?

Answer 14

Graded potentials (entry of Na+)

Question 15

In afferent neurons, what is a graded potential called?

Answer 15

Receptor potential, as is generated by sensory receptors at peripheral nerve ends

Question 16

In efferent and inter-neurons, what is a graded potential called?

Answer 16

Synaptic potential, as is generated by synaptic input to neuron

Question 17

What are the differences between Na+ and K+ v-gated channels?

Answer 17

Na+ open and inactivate very rapidly, whilst K+ open and close slowly; Na+ has to inactivate before returning to “rest” (close) and opening again, whilst K+ simply open and shut

Question 18

How does Na+ v-gated channel go from open to open again?

Answer 18

Gate enters pore to inactivate, breaking positive feedback; repolarisation pops inactivation gate out of pore, closing the channel; Na+ channel opens with depolarisation

Question 19

What is the absolute refractory period?

Answer 19

From start of depolarisation to start of hyperpolarisation; another stimulus cannot produce an action potential; caused by Na+ v-gated channels being open or inactivated (and thus unable to open further, or at all)

Question 20

What is the purpose of the absolute refractory period?

Answer 20

To limit number of action potentials produced by membrane in a period of time; separate action potentials; determine direction of action potential, as cannot propagate in direction from which action potential came as membrane is still in refractory period

Question 21

What is the relative refractory period?

Answer 21

During hyperpolarisation; only a very large stimulus can produce an action potential; caused by some K+ v-gated channels still being open, and some Na+ channels being returned to resting state

Question 22

Which movement of ions is controlled by positive feedback?

Answer 22

Movement of Na+ in

Question 23

Which movement of ions is controlled by negative feedback?

Answer 23

Movement of K+ out

Question 24

Outline the process of an action potential

Answer 24

At rest, Na+ and K+ gated channels are closed; a graded potential causes depolarisation of the membrane, opening Na+ and K+ channels; due to the rapid opening of Na+ channels, rapid depolarisation occurs as Na+ enter down concentration and electrical gradients; depolarisation of membrane causes further Na+ channels to open; membrane almost reaches peak and Na+ channels are inactivated, breaking the positive feedback, stopping depolarisation; due to K+ v-gated channels opening slowly in reaction to the depolarisation created, repolarisation slowly occurs as K+ moves out of cell due to concentration gradient; repolarisation causes Na+ channels to return to rest, and K+ v-gated to close (as repolarisation causes v-gated channels to close); due to K+ v-gated channels closing slowly, there is an overshoot to hyperpolarisation as K+ keeps leaving the cell after resting membrane potential is returned; full closure of K+ v-gated channels allows the Na+/K+ pumps to return the membrane to resting potentials

Question 25

Why are K+ v-gated channels controlled by negative feedback?

Answer 25

K+ v-gated channels opening allows K+ to leave cell, causing repolarisation, but repolarisation causes v-gated channels to close

Question 26

What factors influence speed of an action potential?

Answer 26

Myelination, fibre diameter

Question 27

How does myelination increase speed of action potential?

Answer 27

Saltatory conduction along nodes of Ranvier; less charge leakage means action potentials do not become weaker down axon, so more charge arrives at next mode, thus pumps do less work, and so is more efficient

Question 28

How does increased diameter of fibre increase speed of action potential?

Answer 28

Less resistance to current

Question 29

How fast does an action potential travel in a unmyelinated neuron with small diameter?

Answer 29

0.5m/s

Question 30

How fast does an action potential travel in a myelinated neuron with large diameter?

Answer 30

100m/s