Action Potentials Flashcards
What is the threshold potential
the level the membrane potential must be depolarised to initiate an action potential
All or nothing - size of the action potential is independent of the size of the stimulus over the threshold potential
-55mV
What occurs a threshold potential
Voltage gated fast sodium channels open, allowing free movement of Na into cell.
Voltage Gated fast Na Channels
At Rest:
- Activation gate: closed
- Inactivation gate: open
- Channel: Closed.
TP:
- Activation gate: open
- Inactivation gate: open
- Channel: open
= Free flow of Na into cell.
Rapidly after activation:
- Activation gate: open
- Inactivation gate: closes
- Channel: open but impermeable to Na.
Repolarization (below -50mV)
- channels reactivated
The membrane now moves to the RMP of Na as it is temporarily permeable to Na
- This is also more positive.
Reversal of this is caused by the opening of VSK channels.
Describe the phases of the action potential
- Membrane depolarisation beyond the threshold potential.
- Opening of VG NA channels at -50mv
- Massive influx of Na ions into cell, driving RMP towards the equilibrium potential of Na (positive).
- Inactivation of VG NA channels, stopping flow of Na into cell.
- Opening of VSK channels, causing outflow of K from cell
- RMP returns to equilibrium potential of K, more positive.
Propagation of action potential
The action potential moves along the axon in a wave of changing membrane permeability and potential. This is driven by both electric and ionic drives.
In most neutrons the action potential is initiated in the axon hillock which contains a high density of Fast Na channels.
Propogation is always unidirectional because the region behind is in refractoriness.
Refractory Period
The period of time during which it is impossible or hard to evoke another action potential.
Absolute refractory period:
- from threshold level to 1/3 repolarization.
- impossible to evoke another AP.
Relative refractory period
- From 1/3 through repolarisation to start or after depolarisation
- AP can only be elicited with greater than normal stimuli.
Saltatory Conduction
In some large diameter nerves the action potential is not continuous along the length of the fibre.
In myelinated axons, the AP jumps from one node of Ranvier to next with current sink at active node
Jumping from node to node= saltatory conduction = up to 50x quicker.
Node of Ranvier
Spaces inbetween myelination performed by Schwann cells.
Unmyelinated spaces on nerve axon.
High concentration of VGNa channels
Allow for faster conduction, as AP jumps from node to node using current sink
Current Sink
A nerve cell at rest is negative inside and positive outside.
During AP this polarity is reversed.
This sucks an internal positive charge from infront and behind the spot that is depolarising.
This is called the current sink
this decreases the polarity of the membrane ahead of AP
this initiates depolarisation to threshold
Nerve Fibres in order of conduction velocity
Aa = 60-120ms
- 12-20mm in diameter
- sk motor fibres, positioning
Ab= 40-70ms
- 5-12mm diameter
- sensory fibres, touch, pressure
Ay = 15-30ms
- 3-6mm in diameter
- Motor to m spindles.
Ad= 10-30ms
- 2-5mm diameter
- Sensory: pain, temp, touch
B= 3-15m/s
- <3mm in diameter
- Autonomic preganglionic fibres
C= 0.5-2.4m/s
- 0.3-1.3 mm in diameter
- Sensory: pain, temp, mechano
What is the difference between AP in Nerve and Ventricle muscle
Ventricular AP longer in duration, distinct plateau phase during which depolarisation is maintained
RMP:
- Cardiac muscle: -85 to -95
- Nerve : -70 to - 80
Phase 0 of An AP in Ventricular Muscle
Phase 0
-Rapid depolarisation towards threshold potential.
- Depolarisation occurs due to rise in sodium permeability
- Na influx is via fast VGNA channels.
- VGNA channels open in response to AP, Sensitive to tetrodotoxin.
- brief overshoot as Na channels are self inactivating.
- K conductance decreases.
OVERALL: increase in membrane potential from -90 to +30.
Channels: Fast VG NA Channels
Movement: Sodium influx
Phase 1 in ventricular muscle AP
Partial repolarisation
- Produced by rapid reduction in sodium permeability.
- Na Channels close
- Ca open - Ca starts to flow in
- K channels open and K starts to flow out
Overall:
- Sharp reduction in membrane potential from +30 to +10
Channels:
- Fast VG Na Channels inactivated
- Transient K channels open
Movement:
- Na influx reduced
- K efflux.
Phase 2 in cardiac m AP
Plateau Phase
Depolarisation is maintained.
- Unique slow calcium channels open. (L type Ca Channel).
- Ca influx maintains depolarisation and promotes cardiac muscle contraction.
- Na Channel closing continues which reduces Na influx and repolarisation.
- K channel permeability decreased which prevents return to RMP
- Balance between inward and outward currents - plateau
Channels:
- L Type Ca Channels open
- Outward Rectifier K Channels open
Movement:
- Ca influx
- Some K Efflux
- Na nil
Phase 3 of Cardiac M AP
Repolarization
Channels:
- L type Ca Channels Close
- Outward rectifier K channels open
- Rapid efflux of K responsible for returning the cell to RMP.
- Ion channels and electrogenic pumps return membrane to RMP.
Movement:
- Ca returns to normal (minimal)
- Na returns to normal (minimal)
- K efflux (repolarisation)
