L2: Action Potentials Flashcards
What primarily determines membrane potential?
- The ionic concentration gradients and permeability to those ions, primarily determined by potassium ion leak channels
- If the membrane is not permeable to a particular ion, that ion does not affect the membrane potential
Draw a graph for the membrane potential of K+, Na+ and RMP, and a change is Na+ across the membrane?
What happens to the membrane potential when sodium channels open?
It depolarises, or becomes more positive, due to sodium influx
What is the level of depolarisation proportional to?
The strength of the stimulus applied
What is the threshold potential for triggering an action potential?
Approximately -55 millivolts, where sodium influx surpasses potassium outflow
What is depolarisation?
The process by which the membrane potential becomes less negative, moving toward the equilibrium potential of sodium due to sodium influx
Describe the “all-or-nothing” principle in action potentials
Once the threshold is reached, an action potential occurs. If it isn’t reached, no action potential will form
What occurs when the membrane reaches threshold potential?
Voltage-gated sodium channels open rapidly, allowing Na+ ions to enter and further depolarise the membrane, triggering an action potential
What is the threshold?
The point at which Na+ entry is greater than K+ efflux
What happens when a stimulus opens chloride channels?
Chloride ions enter the cell, causing hyperpolarisation, which moves the membrane potential further from the threshold, potentially inhibiting an action potential
Describe the rising phase of an action potential by numbers
1) RMP
2) Stimulus, voltage-gated sodium channels open and the membrane potential starts to become more positive due to sodium influx into the cell
3) Membrane depolarises more, reaching the threshold - VGNa and VGK channels activated to open (but VGK channels are slower to open)
4) VGNa channels open and there is Na+ influx causing depolarisation (approach the equilibrium potential of Na at +60mV) - the peak of the action potential is +30mV - does not reach equilibrium potential of Na as VGK channels are open
Describe the repolarisation phase of the action potential by numbers
5) Peak of action potential - VGNa channels inactivate - the VGK channels open stopping Na influx and more potassium efflux happens
6) VGK channels fully open, K+ efflux and repolarisation of the membrane begins
7) VGK channels slow to close and stay open for longer and membrane potential becomes more negative that original RMP - hyperpolarisation occurs
8) Return to resting potential
What are te two types of VGNa channels involved in Na+ movement?
Activation and inactivates gates
What are the roles of activation and inactivation gates in sodium channels during an action potential?
- The activation gate opens to allow Na+ influx during depolarisation
- The inactivation gate closes to stop Na+ influx and depolarisation, ensuring controlled action potential propagation and allows for repolarisation, the membrane potential becomes more negative
What are the VGNa channel gates like at RMP?
- Activation gate is closed
- Inactivation gate on cytoplasmic side is open
- Membrane potential -70mV
What are the VGNa channel gates like at depolarisation?
- Activation gate open
- Both gates are now open
- Membrane potential more positive
What is the delay on the inactivation gate closing?
Approximately 0.5ms
How do voltage-gated sodium channels reset?
They reset when the membrane potential returns to a negative level, making them ready to open for another action potential
Why can’t action potentials overlap or occur back-to-back without a reset?
Due to the refractory periods, where channels need to reset to ensure directional flow and prevent overlapping signals
What is conductance?
Refers to the ease of ion flow through the channel in the plasma membrane; varies with the state of the channel provided the channels are present and open
What is the purpose of the refractory periods in neurons?
They ensure that each action potential is a discrete event and that signals move in one direction along the axon
What are the two types of refractory periods?
- Absolute
- Relative
What is the absolute refractory period?
- The period during which no new action potential can be generated because VGNa channels are inactivated
- During repolarisation phase
What is the relative refractory period?
- The period following the absolute refractory phase where a stronger-than-normal stimulus can generate another action potential
- VGNa channels are starting to reset and membrane potential more negative
- VGK channels open and K+ efflux
- During hyperpolarisation phase
Why are refractory periods important?
- Sets frequency action potentials are generated
- Limits rate signals can be transmitted
- Ensures one way travel of action potential
Describe how the action potential moves along the axon
It initiates at the axon hillock and propagates as each segment of membrane depolarizes, followed by inactivation of sodium channels, ensuring one-way flow
How does an action potential propagate along an axon?
The depolarisation spreads along the axon, opening sodium channels ahead, while inactivated channels behind prevent backward propagation
What is the role of the axon hillock in action potential initiation?
It integrates incoming signals; if the combined stimulus is strong enough to reach the threshold, an action potential is generated
How does myelination affect the conduction of an action potential?
It speeds up conduction by insulating axons and allowing the action potential to “jump” between nodes of Ranvier
What is saltatory conduction?
A form of rapid action potential transmission in myelinated neurons, where the impulse “jumps” from one node of Ranvier to the next
What is the significance of nodes of Ranvier in myelinated axons?
They contain concentrated sodium channels, allowing rapid action potential propagation between nodes (saltatory conduction)
How does the nervous system increase action potential conduction speed?
By using myelinated axons and concentrating sodium channels at nodes of Ranvier for saltatory conduction
How does a stronger stimulus affect action potential frequency?
A stronger stimulus can increase the frequency of action potentials, though each individual action potential remains the same size
What role do glial cells play in action potential conduction?
They form the myelin sheath, insulating axons and facilitating faster action potential transmission
What happens to action potential conduction in the absence of myelin?
Conduction speed is significantly reduced, as continuous sodium channel activation along the axon is required, slowing the signal
What can affect the action potential?
Affected by drugs/toxins that target ion channels
