Neuro 4 action potentials Flashcards
In which cells are action potential made
Excitatory cells - muscle, neurone and some endocrine
What does AP In nerves allow
the transmission of information reliably and quickly over long distances
Give Example os some of the role AP plays
They play a central role in cell-to-cell communication and can be used to activate intracellular processes
Eg – muscle cells, an action potential is the first of a series of events leading to contraction.
Eg – beta cells of the pancreas, they provoke release of insulin
What is permeability of ions in membrane dependant on
Conformation of ion channel
Open - depolarisation
Inactivated - sustained depolarisation
Closed - depolarisation/hyperpolarisation
If there are changes in membrane potential during an AP, what is it not caused by
Ion pumps, and the resting membrane potential is not caused by Na+/K+ ATPase
Where is AP made
axon
Phase 1 of AP
Resting membrane permeability
High perm to Na+ small perm to K+
Caused by potassium moving out the cell
Membrane potential nearer equilibrium potential for K+ than that for Na+
Phase 2 of AP
Stimulus
depolarises membrane potential, moving to more positive value until reach threshold - -55mV
Phase 3 of AP
Upstroke
Starts at threshold potential
Permeability of Na increases because the voltage-gated Na+ channels open quickly
Na+ ions enter the cell down their electrochemical gradient
Perm of K+ decreases as the voltage-gated K+ channels start to open slowly
K+ ions leave the cell down their electrochemical gradient
Less than Na+ entering
Membrane potential moves toward the Na+ equilibrium potential
Phase 4 of AP
Repolarisation
Permeability of Na+ decreases as channels are inactivated so Na+ entry stops
Permeability of K+ increases as more channels open and remain open
Membrane potential is closer to equilibrium potential of K+
What happens at start of repolarisation
Absolute refractory period
In Na+ the inactivation gate is closes, but activation gate is open, so AP cannot be made, regardless of how bit it is. Also causes more potassium channels to open
What happens later in repolarisation
Absolute refractory period
In Na+ the inactivation gate and activation gate is closed
Phase 5 of AP
After hyperpolarisation
At rest voltage gated K+ channels remain open, However as membrane equilibrium get closer to K`+ equilibrium potential, some voltage gated Na+ channels close.
Then Membrane potential returns to the resting potential
What happens During after-hyperpolarization
Inactivation gate is open
Stronger than normal stimulus required to trigger an action potential
Describe the Regenerative relationship between PNa and membrane potential
Once threshold is reached the cycle continues via positive feedback behaviour
Cycle continues until the voltage-gated Na+ channels inactivate
Membrane remains in a refractory state until the voltage-gated Na+ channels recover from inactivation
Describe ion movement in an AP
Only a very small number of ions cross the membrane and change the membrane potential so very small concentration changes lead to large changes in potential
Ion pumps are NOT directly involved in the ion movements during the Action Potential T/F
T
How is the electrochemical equilibrium is restored following the action potential
It is restored by K+ and Na+ ions moving through non voltage gated channels. Some ions are exchanged through pumps but this is a relatively slow process (seconds vs milliseconds)
Describe passive propagation
Only resting K+ channels are open.
Axon size and myelination alters propagation distance and velocity
Describe hoe an Active propagation moves along axon
Used for action potentials, local currents depolarise adjacent area of the axon; previous section refractory so cannot go backwards
How is conduction velocity increased
Increased axon diameter and myelination
How is conduction velocity decreased
cold, anoxia, compression and drugs (some anaesthetics)
