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what exists across the membrane of all cells

A potential difference exists across the membrane of all cells
resting membrane potential
in the range 20-90mV


is the ICF negative or positive with respect to the ECF



what are the charges like in the ICF and the ECF

equal numbers of positive and negative charges in the ECF and ICF
ion / charge distribution is polarised

At the very membrane side - there are only negative charges around the inside and only positive charges around the outside
This is created (it is not natural)
= resting membrane potential
Just because it is called resting does not mean it happens without the involvement of energy


what is the composition of Na+, K+ and Cl- ions in the ECF and ICF

Ion ECF (mM) ICF (mM)
Na+ 145 15
K+ 4 150
Cl- 110 10


what are the diffusion gradients

Na wants into the cell
K wants out of the cell
= driving force


what is the diffusion potential

resting membrane is impermeable to Na+

resting membrane is very permeable to K+ (there is a channel that allows K to pass through - K helps to create the arrangement of the membrane potential)

diffusion of K+ leaves excess negative charge inside the cell
this potential gradient arising from diffusion is the resting membrane potential


what does the RMP arise from

The RMP arises from the separation of charges on either side of the membrane
the RMP is due mainly to the diffusion of K+ from cell interior through K+ channels

the small amount of Na+ that leaks into the cell is expelled from the Na+/K+ pump


what else causes the RMP apart from the diffusion of K+ from cell interior

- the Na+/K+ pump contributes by exchanging unequal numbers of Na+ and K+
the pump moves 3 Na+ outwards and 2 K+ inwards
the Na+/K+ pump is electrogenic

Na enters the cell through diffusion but leaves the cell by active transport (the pump)

K leaves the cell through diffusion but returns to the cell by active transport (the pump)
ATP is converted to ADP


what is action potential

the process of bringing from the RMP to an inverted arrangement and back again

rising phase = Na influx - voltage gated Na channels

falling phase = K efflux - voltage gated K channels


what are ion channels

- membrane proteins (also termed transmembrane protein), usually 4 domains
- aqueous channel through membrane
- gated opening
> ligand
> voltage (needs to reach a certain voltage before they will open or close)
- ion selective / specific


what type of gated opening does Na+, K+ and Ca++ have?

- Na+ = voltage gated
> has more than 1 gate
> m-gate and h-gate

- K+ = voltage gated

- Ca++ = ligand gated
> something connects to the channel and causes it to open this way


explain voltage-gated sodium channel

- when the channel is closed initially
> m-gate is closed
> h-gate open

- channel opens
> m-gate opens
> h-gate open
> there is an influx of Na

- channel closed (refractory)
> m-gate open
> h-gate closes
> happens once the action potential has over shot and the reverse now happens


explain voltage-gated potassium channel

just open gate opening and closing

- channel close

- channel open
> potassium leaves the cell


how does action potential begin

a stimulus is applied and causes depolarisation
the membrane potential moves towards the threshold (-55mV)
> m gate closed
> h gate open
(assume RMP is -70mV)


what happens when the membrane potential reaches the threshold

threshold = -55mV
> Na+ channels start opening
(m gate opens)
> Na+ influx (enters the cell)
> more depolarisation and more channels are recruited to cause a greater level of depolarisation
> K+ channels remain closed


what happens when all the Na+ channels are open

there is maximum Na+ influx and the membrane protein over shoots 0mV
Na+ channel still open
K+ channel still closed


what happens when the membrane potential reaches +35mV

> Na+ channels shut - inactivation / h-gate closes
> K+ channels open = efflux of K+ begins
> the reverse of the process that has been happening occurs
- stops movement of Na
- starts movement of K
- start repolarising the cell


what happens during K+ efflux of AP

AP down stroke = recovery phase
Na channels shut = refractory period
K channels open and K efflux continues


what happens when the membrane potential returns to the resting level

kinda like everything resets
> ion channels return to resting state
> excitability restored


what is a big factor in the refractory period



give a summary of action potential

• AP is all or none; amplitude is independent of stimulus

• At 'threshold'
○ As it reaches -55mV - triggered
○ Voltage-gated Na+ channels open
○ Na+ diffuse in; = further depolarisation
○ Positive feedback involved here

• 'peak'
○ Na+ channels close; voltage gated K+ channels open;
○ K+ diffuse out = repolarisation

• Return to resting membrane potential


where are the gates of the Na channels found

inside the cell
This is important because if we are going to block the sodium channel then it needs to be blocked from inside the channel
Important with LA


what is the refractory period

After an AP is initiated, the neuron cannot generate another AP until the first one has ended
The period of inexcitability is called the refractory period
It is due to the inactivation of voltage-gated sodium channels
The inactivation ('h') gates are shut, and so Na+ cannot diffuse into the neuron

Action potentials cannot add together; they are all or none events
If we didn’t have the refractory period then the AP could move in one direction and then in another direction


what are the consequences of the refractory period

• Limits maximum firing frequency of action potentials in axons

• Ensures unidirectional propagation of action potentials

• Prevents summation of APs

• Prevents summation of contraction in cardiac muscle
The cardiac AP lasts as long as the ventricular contraction


what is 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 (Have the AP crawling)


why can the AP only crawl in one direction

due to the refractory period


how does current flow in ECF and ICF

from positive to negative regions
this current flow alters the membrane potential in adjacent region and the AP creeps along the axon
in this way the AP travels along the length of the axon


what increases the speed of AP propagation

- large axon diameter
> large axons conduct impulses more rapidly than small ones
> rapid conduction is achieved only with very large axons

- myelination (speeds up process of conduction)


what do we need a fast process of conduction for and what doesn't need a fast process of conduction

> posture fine tuning

doesn't need fast
> bowel movements


why do myelinated axons use more energy

they have schwann cells that need to be fed and maintained as well as he axon themselves