action potential

saltatory conduction

"jumping' of current across long distances to nodes of ranvier down the axon.

Sequence of events in neuromuscular transmission

1) action potential moves down motoneuron via local currents until the presynaptic terminal is depolarized--causing voltage-gated Ca2+ channels in teh presynaptic membranes to open 2) Ca2+ channels open and Ca2+ flows into the terminal down its electrochemical gradient 3) Ca2+ exocytosis of ACh vesicles 4) Ach diffuses across synaptic cleft to postsynaptic membrane/motor end plate

What conditions will make the length constant highest?

1) diameter of the neuron is long 2) the membrane resistance is high 3) the internal resistance is low

Mechanisms that increase conduction along a nerve

1) increasing nerve diameter 2) myellination

Three basic characteristics of all action potentials

1) stereotypical size and shape 2) propagation (once the action potential starts--it moves throughout the cell) 3) all-or-none response

What is the difference between a graded voltage change and an action potential?

A graded voltage change degrades with distance (because of diffusion) and thus the change in membrane potential decreases with distance. An action potential is self-sustaining because it involves voltage-gated channels, which are opened/triggered

What causes the absolute refractory period?

Closure of the inactivation gates of the sodium channel in response to depolarization. The inactivation gates are in the closed position until the cell is repolarized back to the resting membrane potential

All-or-none response

If an action potential is polarized beyond its threshold potential, an action potential will occur. If the membrane is not depolarized, no action potential can occur

time constant

Indicates how quickly a cell membrane depolarizes in response to inward current, or hyperpolarizes in response to outward current. amount of time it takes following the injection of current for the potential to change to 63% of its final value Time constant= Rm* Cm Rm=membrane resistance Cm=membrane capacitance

can normal action potentials be produced during refractory periods?

No

Can action potentials summate?

No, because the depolarization/change in membrane potential that causes the action potential is solely dependent on changes in sodium concentration. If the sodium channels are already open in the wake of the action potential, no stimulus will be able to further increase the concentration of sodium, and thus no change in charge will occur.

Why does an action potential initiated occurring during the refractory period require a greater inward current than an action potential initiated during resting potential?

The membrane potential is more negative, and more current is needed to bring the membrane to threshold for the next action potential to be initiated.

What causes the relative refractory period (why does the membrane potential fall below the original -70mV to become hyperpolarized?

There is a higher potassium conductance than is present at rest. The membrane potential is close to the potassium equilibrium potential

What is the difference in the action potentials caused by excitatory stimuli that creates a membrane potential of -50 mV and a membrane potential of - 40 mV?

There is no difference in action potential. The threshold potential is near -50 mV. Once that threshold has been reached, the action potential produced is identical.

Can an action potential be elicited during the relative refractory period?

Yes, but only if a greater than usual depolarizing (inward) current is applied.

Do all normal action potentials for a given cell type look the same? Do action potentials for different cell types look the same?

Yes. Each normal action potential for a given cell type 1) looks identical 2) depolarizes to the same potential 3) repolarizes back to the same resting potential

action potential

a phenomenon of excitable cells, such as nerve and muscle, and consists of a rapid depolarization (upstroke) followed by repolarization of the membrane potential. Action potentials are the basic mechanism for transmission of information in the nervous system and in all types of muscle.

Why does the time constant of a neuron increase when the membrane capacitance increases?

because the injected current must first discharge the the membrane's capacitance before depolarizing the cell

What is the timeframe for the relative refractory period

begins at the end of the absolute refractory period and overlaps primarily with the period of hyperpolarizing afterpotential

nodes of ranvier

breaks in the myelin sheath necessary for current flow

activation gate

charge-regulated gate of the sodium channel, allowing sodium to diffuse into the cell if the nerve cell membrane threshold rops to -60 mV

Upstroke of the action potential

depolarization to threshold causes the activation gate to open quickly.

Inward current

flow of positive charges into the cell down its concentration gradient. Cause of depolarization.

outward current

flow of positive charges out of the cell, down their concentration gradient. Cause of hyperpolarization

Outward K+ current

flow of potassium out of the cell.

synaptic cleft

gap between the pre-synaptic cell membrane and the postsynaptic cell membrane

Accomodation

inability to fire an action potential when the threshold potential is passed, due to a slow or elongated depolarization. 1) Consider that the sodium ion channels prefer to be in their closed state (spontaneous and thermodynamically favorable). The slow or elongated depolarization (rather than sudden signal) causes a constant cathodal current through the membrane and allows the membrane channel to equilibrate with only minimal sodium entering the cell 2) by having a sudden increase in voltage above the threshold, there is no chance to equilibrate and the sodium channels must be in their open state, thus allowing for the action potential to occur

length constant

indicates how far a depolarizing current will spread across a nerve (length from the injected current at which the current is 63% of its original value) L=sqrt(Rm/Ri)

inward sodium current

influx of positive charge due to influx of sodium ions. Cuases the membrane potential to become less negative

nicotinic receptors

ligand-gated ion channel that allows positive ions to move upon bonding of Ach to the receptor

myellin

lipid insulator of nerve actions that increases membrane resistance and decreases membrane capacitance

What is the mechanism of myellination?

lipid insulators increase membrane resistance and decreases membrane capacitance. 1) The increased membrane resistance forces current to flow along the path of least resistance of the axon interior rather than across the high resistance path of the axonal membrane 2) Decreased membrane capacitance produces a decrease intime constant and thus at breaks in the myelin sheath, the axonal membrane depolarizes faster in response to inward current

Membrane resistance

measures membrane's resistance to charge flow. When membrane is high, current does not readily flow across the membrane, making it difficult to change the membrane potential and increasing the time constant/time it takes to transmit an action potential

resting potential

membrane potential at rest (-70 mV) caused by the diffusion of potassium ions out of the cell. -sodium conductance is low

hyperpolarizing after-potential (undershoot)

membrane potential that is more negative than resting potential, occur after repolarization, the potassium conductance is higher than at rest and the membrane potential is driven even closer to the potassium equilibrium

Threshold potential

membrane potential that triggers an action potential.

membrane capacitance

membrane's ability to store charge

local conduction

method by which action potentials work, which triggers voltage gated channels to open and allow sodium channels to oepn

Tetraethylammonium (TEA)

molecule that blocks voltage-gated potassium channels, causing an outward potassium current and repolarization

refractory period

period during which another normal action potential cannot be elicited in an excitable cell (absolute or relative)

overshoot

portion of the action potential where the membrane potential is positive, occurring right as the action potential takes place.

undershoot/ hyperpolarizing aftershoot

portion of the action potential, following re-polarization, where the membrane potential is actually more negative than it is at rest.

Depolarization

process of making the membrane potential less negative. As noted, the usual resting membrane potential of excitable cells is oriented with the cell interior negative. Depolarization makes the interior of the cell less negative, or it may even cause the cell interior to become positive.

hyperpolarization

process of making the membrane potential more negative.

synapse

site where information is transmitted from one cell to another through chemical or electrical means.

What is the status of the activation gate and inactivation gate at the peak of the potential

slow inactivation gate finally responds and closes, causing the channel itself to close. Thus, repolarization begins as the cell starts pumping sodium out and potassium in

Conduction velocity

speed at which action potentials are conducted along a nerve or muscle fiber

what is the timeframe of the absolute refractory period

the absolute refractory period overlaps with almost the entire duration of the action potential.

What is status of the activation gate of the sodium nerve at rest?

the activation gate is closed. The inactivation gate is open (because the membrane potential is hyperpolarized). Sodium cannot move through the channel

During the upstroke of the action potential, is the inactivation gate open or closed? Is the activation gate open?

the inactivation gate is still open because it responds to depolarization more slowly than the activation gate. The inactivation gate is open.

upstroke of the action potential

the membrane potential becomes less negative due to an inward current, usually the result of current spread from action potentials at neighboring sites, causing depolarization and rapid opening of activation gates of the sodium channel

repolarization of the action potential

upstroke is terminated and membrane potential repolarizes to hyperpolarizing afterpotential, before eventually restoring the resting potential (-70 mV) 1) inactivation gates on sodium channel close in response to depolarization 2) potassium channels open in response to depolarization

internal resistance

value inversely related to the ease of current flow in the cytoplasm of the nerve cell -inversely proportional to the cross sectional area