Action potential mechanisms Flashcards
A potential difference exists across the membrane of all cells. What is this called?
The resting membrane potential
In the resting membrane potential what is the ICF like in respect to ECF?
- ICF is negative with respect to the ECF
There are equal numbers of +ve and -ve charges in the ECF and ICF. How come the ICF is negative with respect to the ECF?
- Because ion/charge distribution is ‘polarised’
Are negative charges on the inside or outside of the plasma membrane of a cell?
- On the inside
Are positive charges on the inside or outside of the plasma membrane of a cell?
Outside
Is energy required to keep a cell at the resting membrane potential?
Yes
Are there more Na+ ions in the ECF or the ICF?
The ECF
Are there more K+ ions in the ECF or the ICF?
ICF
Does sodium want to diffuse into or out of the cell?
Wants to diffuse into the cell
Does potassium want to diffuse into or out of the ce ll?
Out of the cell
When the cell is at resting potential is the membrane permeable to Na+ ions?
- No, it is impermeable
When the cell is at resting potential is the membrane permeable to K+ ions?
It is permeable
What does diffusion of K+ out of the cell leave an excess of inside the cell?
- Leaves an excess of -ve charge inside the cell
- This potential gradient arising from diffusion is the ‘membrane potential’
How does a resting membrane potential arise?
From the separation of charges on either side of the membrane
What happens to the small amount of sodium That manages to leak into the cell?
It is expelled by the Na+/K+ pump
Although the RMP is due mainly to diffusion of K+ from cell interior the Na+/K+ pump also contributes. How does it do this? (2 points)
- Contributes by exchanging unequal numbers of Na+ and K+
- The pump moves 3 Na+ outwards and 2 K+ inwards
How many Na+ ions does the Na+/K+ pump move outwards?
3 ions
How many K+ ions does the Na+/K+ pump move inwards?
2 ions
The Na+/K+ pump is ‘electrogenic’, what does this mean?
It produces a change in the electrical potential of a cell
Is ATP required for the Na+/K+ pump to work?
Yes
What is the rising phase of an action potential due to?
Due to a SODIUM INFLUX
What is the falling phase of an action potential due to?
- Due to POTASSIUM EFLUX
What are ion channels made from?
Membrane proteins (more specifically transmembrane)
What is an ion channel?
An aqueous channel through the membrane
What are to 2 types of ‘gated opening’ ion channels?
- Ligand
- Voltage
Can a channel have more than one gated opening?
Yes, Sodium ion channels have a 2 gate system
What does it mean by ion channels being selective?
Only a specific ion can pass through that channel
What are the 2 gates in the voltage gated sodium channel?
- M-gate
- H-gate
When the voltage-gated sodium channel is initially closed. which gate is open and which gate is closed?
- m-gate closed
- h-gate open
When the voltage-gated sodium channel is opem. which gate is open and which gate is closed?
- M-gate open
- h-gate open
When the voltage-gated sodium channel is closed and in the refractory period. which gate is open and which gate is closed?
m-gate open
h-gate closed
Is an action potential an all or none response?
Yes
What happens at the ‘threshold’ of the action potential?
- Voltage-gated Na+ channels open
- Na+ diffuse in -> further depolarisation
- Positive feedback involved here
What happens at the peak of an action potential?
- Na+ channels close; voltage-gates K+ channels open
- K+ diffuse out -> repolarisation
- Then return to the resting membrane potential
When is the M-gate in the Na+ ion channel involved?
- In the first phase through the action potential
When is the H-gate in the Na+ ion channel involved?
In the second phase through the refractory period
Where are the M and H-gates on the Na+ ion channel located?
Located on the intracellular portion of the channel
this is important if you are going to block the sodium channel - have to do this from the inside of the cell
After an AP is initiated, the neuron cannot generate another AP until the first one has ended. What is this period of excitability called?
- The refractory period
What is the refractory period due to?
- 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
What are the consequences of a refractory period? (4 points)
- Limits the maximum firing frequency of action potentials in axons
- Ensures unidirectional propagation of action potentials
- Prevents summation of action potentials
- Prevents summation of contractions in cardiac muscle - the cardiac AP lasts as long as the ventricular contraction
What does an AP in one section of an axon set up?
Longitudonal direction flow
- This depolarises adjacent ‘resting’ parts of the axons
What happens in AP propagation?
- An AP in one section of an 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 the axon is activated
- AP’s travel along the axon as waves of depolarisation
How does an AP travel along the length of an axon?
- Current flows in ICF and ECF from +ve to -ve regions
- This current flow alters the membrane potential in adjacent regions, and the action potential ‘creeps’ along the axon
What does the speed of AP propagation increase with?
- Increases with axon diameter
- Large axons conduct impulses more rapidly than small ones
What speeds up the process of AP conduction along an axon?
The presence of the myelin sheath
Why do axons that are myelinated use more energy?
As the axon will have thousands of Schwann cells - large increas ein the number of cells that need energy
What does myelin consist of?
Many layers of cell membranes wrapped round the axon
Which type of cells lays down myelin?
Glial cells
Myelin forms an insulating layer, what does this do?
Reduces leakage of current from axon
The myelin sheath is interrupted at intervals. What are these intervals called?
Nodes of Ranvier
What happens at a node of Ranvier?
The axon membrane is exposed to the ECF, and ion flow can occur
What is saltatory conduction?
- In myelinated nerve, the passive currents spread further along the axon
- There are fewer regeneration steps per unit length of axon
- Thus, the AP propagates more rapidly that in unmyelinated axons (speeds up the propagation)
Peripheral nerves contain many axons. These axons are different in several respects. What are these differences? (2 points)
Size:
- Axon diameter & conduction velocity
Function:
- Sensory, motor
Are A-beta fibres myelinated or unmyelinated?
Myelinated
Are A-delta fibres myelinated or unmyelinated?
Myelinated
Are C fibres myelinated or unmyelinated?
- Unmyelinated
What is the function of A-beta fibres?
Mechanoreceptors
What are the functions of A-delta fibres? (4 points)
- Mechanoreceptors
- Thermoreceptors (cold)
- Nociceptors
- Chemoreceptors (taste)
What are the functions of C fibres? (3 points)
- Mechanoreceptors
- Thermoreceptors (hot and cold)
- Nociceptors
