3 and 4 - Action Potential Flashcards
In an action potential, what is the depolarization phase?
Where the membrane potential is becoming increasingly less negative and continues until it reaches a peak membrane potential
In an action potential, what is the overshoot?
The overshoot is a variable point above a 0 mV membrane potential that is the peak of the action potential
In an action potential, what is the repolarization phase?
The part of the action potential where the membrane potential is becoming more and more negative and continues until it reaches its minimum membrane potential
In an action potential, what is the hyperpolarization phase?
The most negative point that the membrane potential reaches during an action potential, which is lower than the resting membrane potential
What causes a hyperpolarization phase?
A prolonged opening of the voltage-gated K+ channels
Describe a cell at rest before or after an action potential
This is the time where the cell maintains the resting membrane potential and the cell is polarized with the cell holding a negative charge within the cell
What else do we call the depolarization phase?
The rising phase
What else do we call the repolarization phase?
The falling phase
What else do we call the point of hyperpolarization?
The undershoot
What is happening on a molecular level during depolarization?
- Sodium channels are open (“activation gate”)
- Na+ rushes into the cell
- The cell becomes depolarized and therefore a less negative membrane potential is reached
What is happening on a molecular level during the overshoot?
- Na+ channels inactivate an close (inactivation gate)
- Na+ is no longer rushing into the cell
- Increasing depolarization stops and a peak is reached
What is happening on a molecular level during repolarization?
- K+ channels are open
- K+ rushes out of the cell
- The cell becomes polarized and therefore more negative membrane potential is reached
What is happening on a molecular level during hyperpolarization?
- If the K+ channels stay open, the membrane potential will undershoot (reach a level that is more negative than the resting membrane potential)
- This will bring the membrane potential near the K+ Nernst potential
What is happening on a molecular level while the cell is at rest?
The K+ channels close and the cell goes back to its resting membrane potential
What role does the conductance of an ion have in an action potential?
The conductance of an ion is related to the permeability of the membrane to the ion
This means that changing either the conductance or the permeability of the ion has a similar effect on the cell’s membrane potential
What are the conductance changes during an action potential in terms of the K+ channel?
- The K+ channel is activated by strong depolarization (conduction INCREASES)
- Inactivation of the K+ channel is very slow
- This is responsible for re-polarization
What are the three phases that experience a change in the conductance of an Na+ channel during an action potential?
Activation, inactivation and resting state
What happens in terms of conductance change in the Na+ channel during the activation phase of an action potential?
- Activation gate is open
- Na+ can readily pass through the channel
- INCREASED conductance
What happens in terms of conductance change in the Na+ channel during the inactivation phase of an action potential?
- Inactivation gate closes
- This closes the Na+ channel
- The inactivation state occurs
- DECREASED conductance
What happens in terms of conductance change in the Na+ channel during the resting state of an action potential?
- Activation gate is closed
- LOW conductance
- Inactivation gate is open
What is the threshold for an action potential?
There are two possible meanings:
- Sufficient depolarization to trigger an action potential
- The value of membrane potential at which the inward flow of Na+ exceeds the passive outward flow of K+, so the cell membrane enters a positive feedback cycle which causes rapid depolarization
What is the value of the threshold for an action potential to occur?
This value varies dependent upon the external calcium concentration
When the external Ca++ concentration is low, a __________ depolarization of the cell will allow for the threshold to be reached and the activation of an action potential to occur
Smaller
What happens when there is a higher concentration of Ca++ in the extracellular space?
More depolarization of the cell is required in order for the cell to reach the threshold for an action potential
Why does low extracellular Ca++ make it easier for an action potential to propagate?
The probability that a Na+ channel will be opened at any voltage is increased
This means that the resting membrane potential is CLOSER to the threshold, and therefore LESS depolarization is needed to reach it
Why does a high extracellular Ca++ make it harder for an action potential to propagate?
The probability that a Na+ channel will be opened at a certain voltage is decreased
This means that the resting membrane potential is FURTHER from the threshold and therefore MORE depolarization is needed to reach it
What clinical symptom may result from hypocalcemia?
Spontaneous muscle twitching
What are caropedal spasms?
Spasms or cramps in the hands or feet that can happen in patients with hypocalcemia
What are laryngospasms?
Spasms of the larynx or voice box that can also occur in patients with hypocalcemia - can lead to respiratory failure
What clinical disease can lead to calcium reduction?
Hypoparathyroidism (low thyroid activity)
It can lead to a reduction in serum calcium and therefore muscle twitching, etc.
What is hypocalcemic tetany?
Twitching or muscle spasms caused by hypocalcemia
What is latent tetany?
Tetany that is not yet severe enough to show the more severe signs such as twitching or muscle spasms
Harder to diagnose or notice that there is a problem
How can you determine if someone has latent tetany?
Trousseau sign or Chvostek’s sign
How do you evoke Trousseau’s sign?
BP cuff for 3 min
Occlude blood flow
Spasms will occur
Hand and wrist will assume curled position
How do you evoke Chvostek’s sign?
Stimulate facial nerve
Tap at angle of jaw
Facial muscles on same side of face will contract momentarily
Typically a twitch of the nose or lips
What are Trousseau sign or Chvostek’s sign caused by?
Latent tetany from hypocalcemia that is not severe enough to elicit more severe symptoms
What clinical symptoms can arise from hypercalcemia?
- A decrease in neuromuscular irritability
- Fatigue
- Lathargy
- Muscle weakness
- Deminished reflexes
- Mental confusion
- Coma (very high concentration)
What clinical disease can cause elevated serum calcium concentrations?
Hyperparathyroidism (high thyroid activity)
What is an absolute refractory period?
A period of time that is roughly the same amount of time as the action potential spike in which an action potential CANNOT fire
What accounts for the inability to fire an action potential during the absolute refractory period?
The phenomenon of Na+ channel inactivation…
A stimulus cannot open a sufficient number of Na+ channels to cause a second spike because too many Na+ channels are in the inactivated period (high conductance)
K+ channels are closed (low conductance)
What is a relative refractory period?
A period of time after the absolute refractory period during which you would need a stronger than normal stimulus in order to elicit an action potential
What accounts for the increased stimulus needed to fire an action potential during the relative refractory period?
The delayed K+ channels opening and closing..
Although there is a maintained K+ conductance due to the slow closing of the K+ channels, K+ ions leaving the cell oppose the depolarizing effect of opening Na+ channels, making it more difficult to reach threshold
What factors control the speed of the action potential conduction?
- The diameter of the axon
- Myelination
What is the relationship between the axon diameter and the speed of the action potential conductance?
As the diameter increase, the speed of conduction of the action potential increases
Where do voltage-gated Na+ channels exist? Myelinated or unmyelinated axons?
ONLY myelinated axons
Voltage-gated Na+ channels exist at the nodes of Ranvier, which only exist on myelinated axons
Which type of axon conducts an action potential more rapidly? Myelinated or unmyelinated axons?
Myelinated nerves conduct faster
Why do myelinated nerves conduct action potentials faster?
- Lower capacitance than unmyelinated nerves
- Action potentials are ONLY propagated at nodes of Ranvier because the voltage-regulated Na+ channels only exist at the nodes of Ranvier (don’t waste time generating an actionpotential at a membrane covered by myelin)
Which type of axon conducts an action potential more efficiently? Myelinated or unmyelinated axons?
Mylinated axons
- Less membrane has to generate action potential and therefore less Na+ has to flow into the cell
- It is less work for the Na+ K+ pump
What is saltatory conduction
Conduction that “jumps” from one node to the next done on myelinated axons
How does saltatory conduction occur?
An action potential at one node causes the current to flow between the active node and the next node
Once the next node reaches threshold, a new action potential is generated
How can demyelination slow or block an action potential conduction?
Demyelination will slow conduction by
- Increasing the capacitance of the axon
- Requiring more depolarization to propagate the action potential along the axon
What is a local potential?
A shift in the membrane potential in a localized area of the cell that does not spread long distances and gets smaller over time and space
The amplitude of a local potential is the same exact size as the stimulus, it does not “grow”
How is an action potential different from a local potential?
Action potentials…
- Cover long distances, not local areas
- Does not get smaller as it spreads, stays the same size
- Amplitude is much greater than stimulus because the stimulus is just used to reach threshold
- The all-or-none principle is used… You either have an action potential or you don’t - there is no proportional stimulus
What cell type is responsible for myelination of the peripheral nervous system?
Schwann cells
What cell type is responsible for myelination of the central nervous system?
Oligodendrocytes
What part of the nervous system is affected by Guillain-Barre syndrome?
Peripheral nervous system
What part of the nervous system is affected by multiple sclerosis?
Central nervous system
