Action Potential SDL Flashcards
What is the job of the neuronal AP?
to initiate release of neurotransmitters at the synapse
What has to be reached in order for an AP to initiate and propagate along the axon?
threshold
What are the 5 phases of an AP?
rising phase, overshoot, peak, falling phase, and after hyperpolarization
Which phase of the AP lasts the longest?
the after hyper polarization phase
What is the resting membrane potential?
-70 mV
What causes the rising phase of the AP?
influx of Na+ ions
What is the threshold (in mV)?
about 15 mV above the resting MP (-55 mV)
What happens at threshold?
some voltage-gated Na+ selective channels open (activate), increasing the permeability of the membrane to Na+
What does influx of Na+ do to the membrane?
depolarizes the membrane, causing more Na+ channels to open (positive feedback condition)
The fast voltage-gated Na+ channels possess what two gates?
an activation/deactivation gate and an inactivation/deinactivation gate
An increase in K+ permeability begins when?
during the rising phase of the AP (it increases more slowly than that for Na+ and continues after Na+ channel inactivation)
The efflux of K+ (caused by the positive MP) results in what phase and consequence?
the falling phase resulting in repolarization, it overshoots the resting potential causing the after hyperpolarization
What is the equilibrium potential of K+?
-90 mV
What is the equilibrium potential for Na+?
+60 mV
What toxin blocks voltage-gated Na+ channels?
Tetrodotoxin (found in puffer fish)
What ions block voltage-gated K+ channels?
Tetraethylammonium (TEA)
Why are axons and dendrites considered poor conductors?
their high internal resistance (ions moving in a water environment) rapidly dissipates the energy of an ionic current
What happens to potentials, depolarizing or hyper polarizing, that do not reach threshold?
they are passively conducted along the axon and die out within a few millimeters
Longer distance transmission of APs requires a system which _____ the AP along the axon.
regenerates
At threshold, voltage-gated Na+ channels open and Na+ rushes in causing ____ _____?
eddy currents
How is the AP propagated?
entering Na+ ions are attracted to nearby negative regions, this depolarizes the nearby membrane to threshold generating an AP, the Na+ ions entering from this AP evoke more eddy currents which depolarize to threshold the adjacent membrane
In what kind of fibers is conduction slow because the AP is generated sequentially on every piece of the membrane?
unmyelinated fibers
Why is propagation of APs in myelinated fibers faster?
because the eddy currents skip the internal segment and depolarize only the membrane at the node where the voltage-gated Na+ channels are congregated
What is saltatory conduction?
(saltare = to leap), the AP skips from node to node
Where are the voltage-gated K+ channels in myelinated nerve fibers?
they are sequestered under the myelin, so they normally have no part in the AP
In the case of myelinated nerve fibers, repolarization of the AP is due to what?
K+ leakage channels that are not actively opened (thus no after-hyperpolarization)
What is the benefit of not having after-hyperpolarization?
an axon can conduct AP’s at a faster rate
What is the danger in demyelinating diseases, such as MS?
As the AP in the myelinated part of the nerve fiber enters the unmyelinated region, its velocity greatly decreases or the AP ceases because it doesn’t have enough energy to continue
What is an absolute refractory period?
during the spike part of the AP, the membrane cannot initiate another AP
What is the relative refractory period?
during the early part of the after-depolarization, only some of the inactivated Na+ channels are reset and all K+ channels are still open, so another AP can be initiated if the stimulus is increased in strength
The after hyperpolarization phase corresponds to what?
the subnormal period
What is happening during the subnormal period?
K+ channels are still open and the membrane potential is near the equilibrium potential for K+, but another AP can be elicited by a stimulus if it is large enough to shift the MP to threshold (voltage-gated Na+ channels are reset by this time)
The normal conduction direction for the AP, away from the soma
orthodromic conduction
conduction of the AP toward the soma
antidromic conduction
What is the benefit of refractory periods?
they insure the forward progress of the AP
Where is the AP normally initiated?
the axon hillock
What site has the largest concentration of voltage-gated Na+ channels?
the axon hillock
What will happen if a nerve fiber is electrically stimulated somewhere along its length?
APs will conduct in both directions but they cannot reverse directions
What are electrotonic potentials?
changes in the membrane potential which occur in a small region of the cytoplasmic membrane
the amplitude of the response is proportional to what?
the amplitude of the stimulus
All responses, whether de- or hyperpolarizing, that do not move the membrane potential to threshold are called what? they do not propagate and dissipate quickly.
electrotonic potentials
More positive charges on one side of the cytoplasmic membrane attract more ____ charges on the other side, hence it behaves like an electrical capacitor
negative
The change in membrane potential can be either positive (depolarizing) or negative (hyperpolarizing) without what?
without actual flow of ions across the membrane
Actual current across the membrane only occurs when…?
when the permeability (due to opening of channels) of the membrane changes
If the cathode (negative pole) of an electrical source, such as a battery, is placed near a nerve fiber and a brief stimulus is applied, the nerve fiber adjacent to the cathode will be _____.
depolarized (+) (this is due to the attraction of positive ions toward the membrane internally depolarizing it)
Electrical stimulation of nerve fibers in a peripheral nerve produces a _____ AP, the summation of many APs in individual axons.
compound AP
Since larger diameter fibers have faster conduction velocities, when do their peaks arrive at the recording electrode?
they arrive first (alpha type), followed by peaks of slower conducting groups of axons
The largest diameter fibers have the ____ threshold for electrical stimulation, and the smallest fibers have the ____ electrical threshold.
lowest; highest
An unhealthy nerve fiber has slowed conduction velocity and is said to be affected by a form of _____.
neuropathy
If the MP remains above threshold, the neuron is considered to be in ______ _____. this occurs due to hypoxia or anoxia.
depolarization block
Does the soma of a neuron or a nerve fiber tolerate anoxia for longer periods of time?
nerve fiber
If the MP is maintained below the resting potential level, the neuron is considered to be in ______ _____.
hyperpolarization block
What do local anesthetics block?
voltage-gated Na+ channels
What do electrical currents do when you cut a nerve fiber?
electrical currents flow across a gap in the membrane
