Action Potential Flashcards
What is the time constant?
The amount of time it takes for the voltage to change by a certain percentage (63%) of the eventual new steady state value
t=RC
What is the length constant?
The distance between the injection site and the point where the steady state transmembrane voltage change has decayed by 63% from its peak value.
What happens to the size of the action potential as it goes down an axon?
Stay the same
What is the “all or nothing” principle?
Size of action potential does not depend on the size of the triggering stimulus
What are the six stages of a membrane depolarization, starting with the cell at rest?
- Cell at rest; polarized-cell negative inside
- Depolarization phase - sometimes called rising phase
- Overshoot - variable
- Peak of action potential
- Repolarization phase - sometimes called falling phase
- Hyperpolarization; sometimes called undershoot phase
What is the ionic basis for the depolarization phase?
Opening if Na channels, causing Na to rush in
What is the ionic basis for the repolarization phase?
Na channels close (inactivation gate)
K channels open to allow K to rush outside the cell
What is the ionic basis for the hyperpolarization phase?
K channels remain open long enough to go past the resting membrane potential
If Na/K pumps are disable, say through oubain poisoning, how long are nerve fibers still competent?
Many thousands of nerve impulses
What are the two domains of the Na/K pump?
Activation gate and inactivation gate
When the Na channel is in its resting state, what is the state of the activation gate? Inactivation gate?
Activation gate= Closed
Inactivation gate = open
What causes the activation gate to swing open?
Rise in membrane potential
What is the inactivation state of the Na/K pump?
When the inactivation gate is closed
What closes the inactivation gate?
Same change in membrane potential as the activation gate, but it closes more slowly.
True or false: The Na+ channel cannot go directly from the inactivated to the activated state; it must first go back to the resting state.
True
What causes the change in the Na/K pump from the inactivation state to the resting state?
Change in membrane potential back to the resting potential
What activates the K channels (opens them)?
Depolarization, but takes longer to open
What are the K channels responsible for?
The re-polarization of the membrane
What is the meaning of a threshold (what chemically determines this)?
The value of membrane potential at which inward flow of Na+ exceeds the passive outward flow of K
Where is the threshold of an axon usually the lowest?
At the initial segment of the axon
Where is the “initial” segment of the axon?
Axon hillock to about 20-50 μm down the axon
What happens in hypocalcemia?
The probability that a Na channel will open is greatly increased (hyper sensitivity of nerves)
What is the clinical consequence of hypocalcemia and hypoparathyroidism?
Hypoparathyroidism leads to reduction in serum calcium levels and a tendency for muscles to begin twitching spontaneously.
What are the three types of electrical signals that occur in the body?
- Receptor signals
- Synaptic potentials
- Action potentials
The cell membrane can be modeled as a simple circuit. Which part of the membrane is a capacitor, and which part is a resistor?
Capacitor = bilayer Resistor = ion channels
What happens to the permeability of Na/K as it goes from the resting state, to the depolarized state, to the repolarized state?
Resting, K»Na
Depolarizaed: K<>Na
How do local anesthetics work?
Block Na channels through various routes
What determines the voltage threshold of an action potential?
The minimal voltage needed to kick the Na channels into the fast positive feedback cycle
What are the three factors that determine the AP threshold?
Na channel
K channel
[Ca]
What happens in hypocalcemia to neurons?
Hyper sensitivity d/t lowering of threshold
What are the four clinical manefistations of hypocalcemia?
- Convulsion
- Arrhythmias
- Tetany
- Spasms and stridor
CATS
What is chvostek’s sign?
Contraction of the muscles of the eye, mouth, or nose elicited by tapping over the facial nerve in front of the ear. Seen sometimes in hypocalcemia
What is trousseau’s sign?
Muscles spasms that results after application of a BP cuff that is raised to higher than SBP
What is the physiological effect of hypercalcemia on the action potential threshold?
Raises it (the probability that a Na+ channels is open at a certain voltage is decreased )
What are the clinical manifestations of hypercalcemia?
Decrease in neuromuscular irritability
What is a condition that results in hypercalcemia?
Hyperparathyroidism
Does [Ca] change the resting potential of a membrane? Why or why not?
No, the cell membrane is not permeable to it
How does Ca change the threshold potential?
Changing the Na channel opening probability
At low [Ca] is it easier or more difficult to open Na channels?
easier
How does extracellular [Na] affect the rise time and shape of an action potential?
Lowering the extracellular [Na] reduces both the rate and rise of the action potential, and its peak amplitude
What accounts for the plateau seen in cardiac muscle cell action potential?
[Ca]
What is the refractory period?
Period of time after the absolute refractory period during which you’d need a stronger than normal stimulus to elicit a new action potential
What is the absolute refractory period? How long is it usually?
The period of time when an action potential cannot be generated regardless of conditions
Usually just the duration of the actual action potential
What is the chemical basis for the absolute refractory period?
The Na channels are stuck in an inactivated state
What is the chemical basis for the relative refractory period?
The delayed K channel opening and hyperpolarization
K+ ions leaving the cell oppose the depolarizing effect of opening Na+ channels
What sets the upper limit of firing frequency?
The relative refractory period
What creates the phenomenon of propagation?
When one patch of a neuron is depolarized, it creates a current of ion flow in/out in neighboring patches, which do likewise (but at a lesser degree)
What will happen if an action potential is set up in the middle of a neuron?
The action potential will flow in both directions
What ensure that an action potential only flows in one direction?
The axon hillock’s lower action potential threshold
How does the axon diameter affect the conduction velocity? How?
Larger = faster
Since the electrical resistance decreases faster than the capacitance increases, increasing axon diameter increases the speed of conduction.
When the diameter of an axon increases, what happens to its resistance?
decreases in proportion to the square of the radius of the axon (area of the axon)
When the diameter of an axon increases, what happens to its capacitance?
Increases in direct proportion to the radius of the axon (C = 2πr)
How does the mylenation of axons contribute to the velocity of the action potential?
Greatly decreases the axon membrane capacitance, and increases the membrane resistance
What is mylein composed of?
Lipids, cholesterol, other non-conducting proteins
What is the myelinating cells in the PNS?
Schwann cells
What is the myelinating cells in the CNS?
Oligodendrocytes
What is the effect of schwann cells?
Allows nerves to only form circuits at the nodes of ranvier, since these are the only places that Na channels are
Where can action potentials be generated in a myelinated nerve?
Only at the nodes of ranvier
Why is conduction speed increased with myleination (chemially)?
Capacitance is lower, and do not waste time generating small action potentials everywhere
Is myleinating nerves energy efficient or inefficient? Why?
Efficient, because there are fewer spots that have action potentials generated, meaning fewer Na/K pumps
What is the cause of MS?
Antibodies attacking oligodendrocytes, causing demyelination of the CNS
What is Guillain-Barre Syndrome, and what is is caused by?
Antibodies mistakenly attack the PNS Schwann cells, causing demyelination. This is usually reversible.
