(06) AP: Generation and Transmission Flashcards
what causes the potential inside a neuron to change?
membrane permeability / ion conc change
define hyperpolarisation
potential becomes MORE NEGATIVE
moves closer to Ek and away from E(Na)
define depolarisation
potential becomes LESS negative
away from E(K), closer to E(Na)
define action potential
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)
Name the three stages of APs
- Fast depolarisation
- Repolarisation
- After-hyperpolarisation (AHP) - below the original line of resting membrane pot
what is the absolute refractory period?
fast depolarisation and most of the repolarisation
during this, CANNOT generate new AP
what is the relative refractory period?
end of repolarisation and AHP
can generate new AP, but a stronger stimulus required
describe the mechanisms at the stimulus stage
not V-gated - mechanical/ligand
increased permeability to cations
gradually more +vs MP
(before threshold reached)
what happens to Na+ channels when MP reaches threshold?
sudden activation of C-gated Na+ channels, so P(Na) increases
PK/PNa = 1:20, so MP overshoot toward E(Na+)
= fast depolarisation stage
gate mechanisms during repolarisation period
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
gate mechanisms during AHP
MP shift toward E(K+) bc V-gated K+ channels still open
so PK:PNa = 100:1
When threshold reached and Na channels open, Na+ move into the cell along…
BOTH the conc and electrical gradient
why does the influx of Na into the cell slow down and stop (near the end of the fast depolarisation)?
- inside pot becomes +ve, so attracts Na+ less
- Na+ channels inactivate
describe the activation and inactivation of V-gated Na+ channels
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
why are action potentials described as “all or nothing” events?
once threshold reached, will fully happen
amplitude does NOT depend on stimulus
approximate amplitude of APs in neurons
about 100mV
(from -70 to +30mV)
how are APs evoked by a battery / external source?
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
How are APs generated in CNS neurons?
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
describe unmyelinated and myelinated axons
unmyelinated - small diameter (about 1 um), slow continuous transmission of AP
myelinated - larger diameter (5-10um), fast saltatory transmission
name the two stages of AP transmission
- Passive spread
- Generation of AP
describe the mechanism and range of passive spread of current in an axon
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
describe AP generation in an unmyelinated axon
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
Speed of AP transmission in unmyelinated vs myelinated cells
1m/s unmyelinated bc AP must be repeatedly regenerated
20-100 m/s in myelinated
what myelinates axons? CNS and PNS
glia cells to form the myelin sheath
in CNS = oligodendrocytes
in PNS = Schwann cells
function of myelination
INSULATES the axon, stops dissipation of passive current until each node of ranvier
increases conduction velocity
define saltatory conduction
conduction occurs in steps - bc myelination is discontinuous
AP regenerated only at nodes of ranvier
why aren’t all axons myelinated?
bc it takes up more SPACE
even though it is more efficient (requires less current loss, less time, less energy to maintain gradients)
why do APs only flow in one direction if passive conduction goes both ways?
Absolute refractory period - 1-2ms
previous gates already stimulated
describe the process by which AP are generated in sensory neurons
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.
describe the analogue-digital conversion of signals in the generation of AP in sensory neurons
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
what happens when the optic nerve becomes inflammed resulting in demyelination?
transmission of AP slower, dissipate
GRADUAL blurriness / blindness, bc more than one axon (a nerve = many neurons)
= optic neuritis
