Shared Flashcard Set


Colden - AP Propagation
Action Potential Propagation

Additional Physiology Flashcards




List the different components of a nerve cell along with their functions
Cell body - contains the nucleus of the nerve cell

axon hilock - contains a large amount of voltage-gated Na+ channels so this is where the action potential can start

node of ranvier - a break in myelination where membrane resistance is low and action potentials can easily occur, important in propagation

myelin sheath - enables propagation to the next node of ranvier by preventing dissipation of the AP by stopping Na+ efflux

dendrite - receives the signal from a neighboring nerve cell
What are the factors that determine the rate of propagation of the action potential?
the space (length) constant

the time constant

myelinated vs unmyelinated

note that the space and time constants are not dependent on voltage but instead on physical properties of the membrane
What are the ionic principles behind action potential propagation?
the initial segment of the axon fires an action potential

at the active site positive charges flow from the initial segment to the inactive adjacent area and activate that area

the original active region repolarizes back to RMP

this repeats over and over down the nerve cell
What is time constant and how is action potential propagation velocity related?
time constant (tau) = RmCm

Rm = resistive properties of the membrane and is the inverse of the permeability

Cm = membrane capacitance that describes the ability of the membrane to store charge

The smaller the time constant, the higher the propagation velocity.

This is because the membrane has a greater ability to respond rapidly to stimulus currents.

Note the time constant refers to the time it takes for the potential change to reach 63% of its final value
What is the space (length) constant and how does it relate to the action potential propagation velocity?
The length constant (gamma) is defined as the square root of ((Rm*d)/Ri*4)).

Rm = membrane resistance
d = diameter of the axon
Ri = longitudinal resistance

The greater the length constant, the greater the propagation velocity

Note the length constant is the distance it takes for the depolarizing displacement to decay by 63% of its initial value.
How is conduction velocity related to axonal properties on unmyelinated nerve fibers?
Conduction velocity is proportional to the diameter of the axon.

The larger the diameter of the axon, the greater the speed of propagation.

Remember length constant is proportional to the diameter and a higher length constant means a higher conduction velocity.
How is conduction velocity related to axonal properties on myelinated nerve fibers?
The velocity of propagation is determined by the distance between the nodes of Ranvier.

The greater the distance between the nodes of Ranvier, the greater the velocity of action potential propagation.
Explain myelination
Myelin is a lipid insulator of nerve fibers that greatly increases membrane resistance.

Myelinating a nerve fiber causes the depolarizing current to flow down the interior of the nerve where resistance is low.
What is the importance of the nodes of ranvier?
At intervals of 1 to 2 mm there are breaks in the myelin sheath called nodes of ranvier.

at the node of ranvier the change from myelinated to unmyelinated allows the depolarizing current to flow across from a high resistance area to a low resistance area to propagate the AP.

without the breaks in myelination the action potential would simply dissipate (remember the length constant) because there wouldn't be any changes in resistance.
What is saltatory conduction?
This is where action potentials "jump" long distances between one node of ranvier to the next.

This is possible because at the nodes, membrane resistance is low so current can flow across the membrane and cause an action potential.
What axonal characteristics will give cause a faster conduction velocity?
Increased Rm - this encourages to current to flow internally

Increased diameter - this decreases the Ri

Decreased Ri - less internal resistance means more internal travel

Decreased Cm - the shorter the membrane stores the charge, the faster the current can travel

myelination - saltatory conduction is faster than normal conduction

This is because of the principle of traveling along the path of least resistance.

If the Rm is high and the Ri is low then the current will want to flow in the cytoplasm
Supporting users have an ad free experience!