Anatomy 12.8-12.9

Neurofibril node

Action potential occurs only here, which is where the axon's voltage-gated channels are concentrated. After Na+ enters at a node it starts a rapid positive current down the inside of the axon's myelinated region

Graded potentials

are relatively small, short-lived changes in the resting membrane potential that are caused by the movement of small amounts of ion across the plasma membrane.

Action potential

involves two processes: depolarization and repolarization

Depolarization

refers to the gain of positive charge within a neuron that occurs to such an extent to change the plasma membrane potential from negative to positive. This reversal of polarity is due to the opening of voltage-gated Na+ channels and the subsequent movement of Na+ into the cell.

Repolarization

return of polarity from positive back to negative, its due to the opening of voltage-gated K+ channels and the subsequent movement of K+ out of the cell.

Hyperpolarization

the change in the membrane potential in the negative direction (e.g., −70mV to −71mV), which is caused by the opening of either chemically gated K+ channels to allow K+ to exit the neuron or chemically gated Cl− channels for Cl− to enter the neuron

Depolarization

the opening of chemically gated cation channels allows more Na+(positively charged ion) to enter the neuron (than K+ to exit), causing the inside of the neuron to become more positive (e.g., −70mV to −69mV).

Absolute refractory period

time after an action potential onset when no amount of stimulus, no matter how strong, can initiate a second action potential. During this time, the voltage-gated Na+ channels are first opened, and then the voltage-gated Na+ channels are closed in the inactivation state. Ensures that the action potential moves along the axon in only one direction toward the synaptic knobs.

Myelinated regions

Current of region becomes weaker with distance but is still strong enough to open voltage gated channels of next node. Full action potentials occur at the nodes repeatedly. The saltatory is much faster and the myelinated cells use less ATP to maintain RMP

1. Unstimulated axon has a resting membrane potential of -70mv 2. Graded potentials reach the initial segment and are added together (-70mv-55) 3. Depolarization occurs 4. Repolarization 5. Hyperpolarization 6. Voltage-Gated K+ channels are closed, and the plasma membrane has returned to resting conditions by activity of Na+/K+ pumps (-80mv -70mv)

Depolarization/Repolarzation and Its Propagation Steps?

At RMP, voltage-gated channels are closed. As Na+ enters from adjacent region, voltage-gated Na+ channels open. Na+ enters the axon causing the membrane to have a positive potential. Na+ channels close becoming inactive (unable to open) for a time

Describe Depolarization steps in action potential?

Depolarization slowly opens K+ channels, and K+ diffuses out, causing negative membrane potential. K+ channels stay open for a longer time, so K+ exit makes cell more negative than RMP. K+ channels eventually close and RMP is reestablished

Describe repolarization steps in action potential?

Cl-, Cl negative, more negative

Fill in the blank: If ___ channels open, ___ diffuses in and membrane becomes ___

Na+, Na+, less negative

Fill in the blank: If ___ channels open, ___ diffuses in and membrane becomes ____

K+, K+, more negative

Fill in the blank: __ channels open, __diffuses out and membrane becomes ___

Postsynaptic potentials

Graded potentials that occur in postsynaptic neurons

Action Potential

Has axon, with voltage gated channels, that are positive then negative with a relatively large change that causes temporary reversal of polarity. The degree of voltage change generally does not vary, with a duration that is self-propagating along the axon. The distance is the length of the axon and it has the same intensity because voltage-gated channels continue to open in sequence.

Graded Potential

Has dendrites and cell body, with chemically gated channels, has positive or negative voltage change with a relatively small change. The degree of voltage change is dependent upon magnitude of the stimulus, with a duration 1 msec to a few msec. The distance is relatively short, and the intensity decreases with distance.

Refractory period

Period of time after start of action potential when it is impossible or difficult to fire another action potential

the absolute refractory period and the relative refractory period

The refractory period has two phases that are?

Threshold membrane potential

The sensitivity of voltage-gated channels to open in response to a minimum voltage change in membrane potential is the determining factor if an action potential is initiated. When this brink is reached, the voltage-gated channels are stimulated to open, which will initiate the generation of an action potential that will be propagated along the axon.

Saltatory conduction

Transmission of an action potential along the plasma membrane of a myelinated axon.

Relative refractory period

occurs immediately after the absolute refractory period. Another action potential may now be initiated in an axon only if the stimulation of the plasma membrane is greater than the stimulus normally needed to generate an action potential. At this time, voltage-gated Na+channels have returned to their resting state, but the neuron is hyperpolarized due to the slightly extended time that voltage-gated K+ channels remain open during repolarization

Continuous conduction

occurs in unmyelinated axons and the charge opens voltage-gated channels, which allows charge to enter, which spreads to adjacent region and opens more channels, sequentially

Nerve signal

once initiated, action potentials are propagated along an axon to the synaptic knob as both voltage-gated Na+ channels and voltage-gated K+ channels open sequentially along the length of the axolemma