(06) AP: Generation and Transmission

Question 1

what causes the potential inside a neuron to change?

Answer 1

membrane permeability / ion conc change

Question 2

define hyperpolarisation

Answer 2

potential becomes MORE NEGATIVE
moves closer to Ek and away from E(Na)

Question 3

define depolarisation

Answer 3

potential becomes LESS negative
away from E(K), closer to E(Na)

Question 4

define action potential

Answer 4

brief fluctuation in membrane pot caused by a transient opening of V-gated ion channels which spreads like a wave along an axon

occur after membrane pot reaches THRESHOLD (approx. -55mV)

Question 5

Name the three stages of APs

Answer 5

1. Fast depolarisation
2. Repolarisation
3. After-hyperpolarisation (AHP) - below the original line of resting membrane pot

Question 6

what is the absolute refractory period?

Answer 6

fast depolarisation and most of the repolarisation
during this, CANNOT generate new AP

Question 7

what is the relative refractory period?

Answer 7

end of repolarisation and AHP
can generate new AP, but a stronger stimulus required

Question 8

describe the mechanisms at the stimulus stage

Answer 8

not V-gated - mechanical/ligand
increased permeability to cations
gradually more +vs MP

(before threshold reached)

Question 9

what happens to Na+ channels when MP reaches threshold?

Answer 9

sudden activation of C-gated Na+ channels, so P(Na) increases
PK/PNa = 1:20, so MP overshoot toward E(Na+)
= fast depolarisation stage

Question 10

gate mechanisms during repolarisation period

Answer 10

opening of Na+ channels short, quickly inactivate
followed by transient opening of V-gated K+ channels –> repolarisation
K+ flows OUT of cell hence return to -ve MP

Question 11

gate mechanisms during AHP

Answer 11

MP shift toward E(K+) bc V-gated K+ channels still open
so PK:PNa = 100:1

Question 12

When threshold reached and Na channels open, Na+ move into the cell along…

Answer 12

BOTH the conc and electrical gradient

Question 13

why does the influx of Na into the cell slow down and stop (near the end of the fast depolarisation)?

Answer 13

1. inside pot becomes +ve, so attracts Na+ less
2. Na+ channels inactivate

Question 14

describe the activation and inactivation of V-gated Na+ channels

Answer 14

two gates: activation (V-dependent) and inactivation (time dependent)
Activation gate open, influx of Na –> depolarisation
after a few milliseconds, inactivation gate (the ball) shuts - This stops the flow of Na regardless of the state of the activation gate!
RMP reestablished, inactivation gate releases but the activation gate is now closed

Question 15

why are action potentials described as “all or nothing” events?

Answer 15

once threshold reached, will fully happen
amplitude does NOT depend on stimulus

Question 16

approximate amplitude of APs in neurons

Answer 16

about 100mV
(from -70 to +30mV)

Question 17

how are APs evoked by a battery / external source?

Answer 17

causes local hyper/depolarisation
two paths - outside the axon from anode (+) –> cathode (-) = path of least resistance but doesn’t change anything in neuron
across membrane inside axon –> changes RMP.
if threshold reaches, initiates AP

Question 18

How are APs generated in CNS neurons?

Answer 18

Depolarisation to threshold evoked by excitatory post-synaptic potentials, passively travels from dendrites

APs first generated in axon initial segment (the part closest to cell body), has the lowest threshold = “trigger zone”
has highest density of V-gated channels

AP travels in one direction down axon

Question 19

describe unmyelinated and myelinated axons

Answer 19

unmyelinated - small diameter (about 1 um), slow continuous transmission of AP
myelinated - larger diameter (5-10um), fast saltatory transmission

Question 20

name the two stages of AP transmission

Answer 20

1. Passive spread
2. Generation of AP

Question 21

describe the mechanism and range of passive spread of current in an axon

Answer 21

subthreshold
separation of charges - area of local depolarisation inside cell in axoplasm –> +ve charges move passively along membrane to where there are negative charges on both sides of axolemma

SHORT distances (1mm), dissipates quickly!
because +ve charges leak along leak channels

Question 22

describe AP generation in an unmyelinated axon

Answer 22

threshold reached, AP occurs
influx of +ve, wants to flow to adjacent region –>depolarisation –> threshold –> adj V-Na+ channels open –> generate another AP, in both directions

Question 23

Speed of AP transmission in unmyelinated vs myelinated cells

Answer 23

1m/s unmyelinated bc AP must be repeatedly regenerated
20-100 m/s in myelinated

Question 24

what myelinates axons? CNS and PNS

Answer 24

glia cells to form the myelin sheath
in CNS = oligodendrocytes
in PNS = Schwann cells

Question 25

function of myelination

Answer 25

INSULATES the axon, stops dissipation of passive current until each node of ranvier
increases conduction velocity

Question 26

define saltatory conduction

Answer 26

conduction occurs in steps - bc myelination is discontinuous
AP regenerated only at nodes of ranvier

Question 27

why aren’t all axons myelinated?

Answer 27

bc it takes up more SPACE
even though it is more efficient (requires less current loss, less time, less energy to maintain gradients)

Question 28

why do APs only flow in one direction if passive conduction goes both ways?

Answer 28

Absolute refractory period - 1-2ms
previous gates already stimulated

Question 29

describe the process by which AP are generated in sensory neurons

Answer 29

stimulus acts on receptors in sensory neurons (eg. mechanical stim acting on muscle spindles) - does NOT immediated evoke APs
1. GRADED depolarisation (small stim = small depol) in sensory endings (=receptor potential)
2. spreads passively to nearby trigger zone
3. threshold –> APs generated, spread along axon toward CNS.

Question 30

describe the analogue-digital conversion of signals in the generation of AP in sensory neurons

Answer 30

analogue = amplitude proportional to strength of signal
digital = frequency proportional to strength

info abt strength of stimulus encoded in amp of receptor potential and freq of APs

Question 31

what happens when the optic nerve becomes inflammed resulting in demyelination?

Answer 31

transmission of AP slower, dissipate
GRADUAL blurriness / blindness, bc more than one axon (a nerve = many neurons)
= optic neuritis