LA symposium - 1 action potentials mechanisms Flashcards
What cells have membrane potentials
all
what is more negative ICF or ECF
ICF is negative with respect to the ECF
There are equal numbers of +ve and -ve charges in ECF and ICF, so why is ICF negative with respect to ECF
ion/charge distribution is ‘polarised’
What are the rough concentrations of ions (Na, K and Cl) in ECF and ICF
- Na+ (ECF = 145mM, ICF = 15mM)
- K+ (ECF = 4mM, ICF = 150mM)
- Cl- (ECF=110mM, ICF=10mM)
if the ion concentrations of Na and K are such that Na wants to come in and K wants to come out why don’t they?
the membrane doesn’t let them because they are not lipophilic
Is the resting membrane potential permeable to Na
no
is the resting membrane potential permeable to K
yes
what ion’s diffusion gradient results in an excess -ve charge inside the cell (the resting membrane potential)
the diffusion of potassium from cell interior through potassium channels
what happens to the small amount of Na that leaks into the cell (at resting membrane potential)
it is expelled by the Na/K pump
What creates the resting membrane potential
- mainly diffusion of K from cell interior
2. also Na/K pump exchanging unequal numbers of Na and K
How does the Na/K pump work
3 Na outwards 2 K inwards ADP --> ATP (active transport) The Na/K pump is electrogenic
What is the action potential threshold
-55mV
What is the resting membrane potential value
-70mV
what is the rising phase of the action potential due to
Na influx via voltage gated Na channels
what is the highest value the action potential reaches
+35mV
what is the falling phase of the action potential due to
K efflux via voltage gated K channels
What ion channel has more than 1 gate
sodium
what are the gates in the sodium channel called
- M gate
- H gate
When the voltage gated sodium channel is closed (not in refactory period), are m and h gates open or closed
m gate - closed
h gate - open
When the voltage gated sodium channel is closed during the refactory period, are the m and h gates open or closed
m gate - open
h gate - closed
when is the h gate open only
…closed???
during the refactory period
what happens at 35mV to the voltage gated sodium channel
h gate closes first, then m gate
how many gates does the voltage gated potassium channel have
1
Describe the stages of an action potential
- Stimulus applied
- depolarisation
- MP moves towards ‘threshold’
- gated ion channels closed - MP reaches ‘threshold’ (-55mV)
- Na channels start opening
- Na influx
- more depolarisation
- K channels remain closed - all Na channels open
- max Na influx
- MP overshoots 0mV
- When MP reaches +35mV
- Na channels shut (h gate closes)
- K channels open
- K efflux begins - AP down stroke
- recovery phase
- Na channels shut (-> refactory period)
- K channels open
- K efflux continues - MP returns to ‘resting’ level
- ion channels return to resting state
- excitability restored
What would happen if there was no refactory period
- signal could move in direction not allowed
- wouldn’t have control
Summarize action potentials
- all or nothing events
- amplitude independent of stimulus
- At ‘threshold’:
voltage-gated Na+ channels open, Na diffuse in, further depolarisation, positive feedback involved here - at ‘peak’
Na channels close, voltage gated K channels open, K diffuse out, repolarisation - return to resting membrane potential
What is the refactory period
- after an AP is initiated, the neuron cannot generate another AP until the first one has ended
- this period of inexcitability is called the refactory period
what is responsible for the refactory period
- inactivation of voltage gated sodium channels
- the inactivation (h) gates shut, and so Na cannot diffuse into the neuron
what are the consequences of refractory period
- limits max firing frequency of ap’s in axons
- ensures unidirectional propagation of action potentials
- prevents summation of action potentials
- prevents summation of contractions in cardiac muscle - the cardiac AP lasts as long as the ventricular contraction
if you were going to block sodium channels where would you need to block them from and why
from the inside of the cell as gates are positioned intracellularly
Describe action potential propagation
- an AP in one section of axon sets up longitudinal current flow
- this depolarises adjacent ‘resting’ parts of the axon
- the AP is regenerated further along the axon
- more current flows, and the next region of axon is activated
- action potentials travel along the axon as waves of depolarisation
in what direction does action potential propagation move along the length of the axon
- current flows in ICF and ECF from +ve to -ve regions
- this current flow alters the membrane potential in adjacent regions, and the AP ‘creeps’ along the axon
(in this way the AP travels along the length of the axon)
Does the speed of AP propagation increase or decrease with the axon diameter
increase
Do large or small axons conduct impulses more rapidly than the other
large is more rapid
what greatly increases the speed of AP propagation
myelin
What does myelin do to AP propagation
increases speed
how does myelin arrange on an axon
many layers of cell membranes wrapped round the axon
what cells lay myelin down
glial (schwann) cells
how does myelin speed up AP propagation
forms an insulating layer, reducing leakage of current from axon
what are the intervals which interupt myelin
nodes of ranvier
what happens at the nodes of ranvier
the axon membrane is exposed to the ECF, and ion flow can occur
describe saltatory conduction
- in myelinated nerve, the passive currents spread further along the axon
- there are fewer regeneration steps per unit length of axon
- thus, the AP propagates more rapidly than in unmyelinated axons
apart from axons, what do we have high concentrations of in peripheral nerves
- lipid
- fat pads
- blood vessels
- mix of myelinated and unmyelinated axons
how do axons within peripheral nerves differ
Size
- axon diameter
- conduction velocity
Function
- sensory
- motor
What axon types are prevent in ‘cutaneous’ nerves and what are their functions
A beta (myelinated) - mechanoreceptors
A delta (myelinated)
- mechanoreceptors
- thermoreceptors (cold)
- nocieptors
- chemoreceptors (taste)
C (unmyelinated)
- mechanoreceptors
- thermoreceptors (hot (and cold?))
- nocieptors
Of the mental and canine nerve, which has less myelination and why
less myelination in canine nerve
- as nerve enters the tooth they loose myelination
- has an affect on how LA works
Mental is near tooth
Canine nerve inside pulp
explain the last slide of the lecture
find out what he said
