Action Potentials Flashcards
Depolarisation definition?
Membrane potential becomes LESS negative (even positive)
Hyperpolarisation definition?
Membrane potential becomes MORE negative
What does increasing membrane potential do?
Make it more negative
Factors causing change in Vm?
Depolarisation/hyperpolarisation depends on:
Direction of ion movement - influx (into cell) or efflux (out of cell)
Charge carried by ions (+ve or -ve)
Methods of ion movement across membranes?
Via ion channels/transporters
Passive movement of an ion through an ion channel is driven by its ELECTROCHEMICAL gradient
How does Na+ move through the membrane?
Response to opening of cell membrane sodium-selective channels Na+ flows INWARDLY as conc. gradient and electrical gradient are inward
Equilibrium potential of Na+?
ENa = +60 mV
What is the membrane potential and how does it compare to ENa?
Vm = -80 mV
This is negative compared to ENa (+60 mV)
Driving force of a sodium ion?
Driving force for Na+ influx: Vm - ENa
When negative, inward movement of Na+ occurs
How is the current carried by an ion calculated?
INa = gNa (driving force or Vm - ENa) or IK = gK (driving force or Vm - EK)
How does K+ move through the membrane?
Response to opening of cell membrane potassium-selective channels
K+ flows OUTWARDLY as conc. gradient is outward and electrical gradient is inward; conc. gradient energy exceeds energy of electrical gradient
Equilibrium potential of potassium compared to membrane potential?
EK = -90 mV; more negative than the membrane potential (-80 mV)
Driving force of a potassium ion?
Driving force for K+ efflux: Vm - EK, when positive, outward movement of K+ occurs
Effect of Na+/ K+ channel opening on Vm?
Na+ channel opening - Vm driven towards ENa
K+ channel opening - Vm driven towards EK
Describe ion channels and structure
Protein complexes spanning lipid bilayer to form a central pathway, allowing rapid flow of SELECTED ions
Have 4 modules
Regulation of ions channels?
Most are regulated and exist in:
Open (o)
Closed (c)
…and additional conformations cycling between these
Opening of ion channels?
Opened (“gated”) by:
Membrane voltage - voltage-gated ion channels
Chemical substances - ligand-gated ion channels
Physical stimuli - like mechanical/thermal
Ion channels responsible for depolarisation and hyperpolarisation?
Voltage-activated Na+ channels - depolarising
Voltage-activated K+ channels - hyperpolarising
Steps of a neurone action potential?
Resting potential (for neurones, 60-70 mV)
Threshold - depolarisation to threshold which opens channels
Upstroke - rapid depolarisation moves Vm close to ENa, due to opening of voltage-activated Na+ channels
Downstroke - rapid hyperpolarisation moves Vm close to EK, due to opening of voltage-activated K+ channels and inactivation of voltage-activated Na+ channels
Undershoot - after action potential and is more negative than resting membrane potential, due to delayed closure of voltage-activated K+ channels
Action potential properties?
Brief electrical signals in which polarity of nerve cell membrane is MOMENTARILY ( approx 2ms) REVERSED
“All or none” - action potentials generated when threshold is reached
Describe “All of None” nature of action potentials
Cannot have half an action potential
Even if size of depolarising stimulus is large, action potential has a fixed amplitude
What occurs at rest?
K+ channels are leaky
Describe similarities between voltage-activated Na+ and K+ channels
Highly selective for respective cations
Activated by membrane depolarisation - Na+ channels rapidly and K+ channels with a slight DELAY
Difference between voltage-activated Na+ and K+ channels?
Activation of Na+ channels is self-reinforcing - opening of a few channels causes more channels to open, causing more depolarisation, etc (POSITIVE FEEDBACK)
Activation of K+ channels is self-limiting - outward movement of K+ causes repolarisation, turning off the stimulus for opening (NEGATIVE FEEDBACK)
Na+ channels during maintained depolarisation? Reversal?
Enter a non-conducting, inactivated state, even though they initially open due to depolarisation
Repolarisation causes the channel to enter a closed state in readiness for opening, to generate another action potential
Why is maintained depolarisation of Na+ channels important?
Na+ channels will be inactivated, which contributes to the repolarising phase of action potential and is responsible for REFRACTORY PERIOD
Repolarisation definition?
Returning the membrane potential to normal resting value
Absolute refractory period definition?
No stimulus, however strong, can elicit a second action potential immediately after generation of an action potential, as all Na+ channels are inactivated
Relative refractory period definition?
A stronger than normal stimulus may elicit a second action potential (mixed population of inactivated and closed channels)
When do absolute and relative refractory periods occur?
During downstroke and undershoot respectively
Briefly, how is impulse propagation achieved with unmyelinated and myelinated axons?
Unmyelinated axons - passive spread of current
Myelinated axons - saltatory conduction
Why do passive signals not spread far from origin?
Nerve cell membrane is “leaky”, not a perfect insulator, so passive signals do not spread far (current is lost from the membrane along the axon)
How can passive electrical signals be conducted over large distances without decaying?
Via action potentials
Passive current spread increase causes?
Decrease ri - axial resistance (increase axon diamater)
Increase rm - membrane resistance (by adding insulating material like myelin)
Describe myelin and creation
Insulating material created by SCHWANN cell in PNS and by OLIGODENDROCYTES in CNS
Conduction in myelinated axons is much faster than in non-myelinated axons of similar diameter
Node of Ranvier definition?
Points of neurones at which myelin sheath disappears
What is saltatory conduction?
Action potential jumps from one node of ranvier to next
