11.5 Electrical Signals (Part 2)

Action Potentials

are the means by which neurons communicate with their effectors.

Action potentials

result from summation of graded potentials.

Action potential

is a large change in the membrane potential that propagates, without changing its magnitude, over long distances along the plasma membrane.

All-or-None Principle

Principle that action potential follows

"All" Part

If a stimulus produces a depolarizing graded potential that is large enough to reach threshold, all the permeability changes responsible for an action potential proceed without stopping and are constant in magnitude

"none" part

If a stimulus is so weak that the depolarizing graded potential does not reach threshold, few of the permeability changes occur. The membrane potential returns to its resting level after a brief period without producing an action potential

Depolarization Phase

characterized by the rapid increase in positive charge inside the neuron

Repolarization Phase

characterized by the rapid return of the membrane potential to the resting membrane potential

Repolarization Phase

This is when the inside of the cell returns to its negative state.

Afterpotential

a period of hyperpolarization that follows each action potential.

sodium-potassium pump

restores normal resting ion concentrations by transporting these ions in the opposite direction of their movement during the action potential.

Refractory Period

The time period when once an action potential is produced at a given point on the plasma membrane, that area becomes less sensitive to further stimulation.

absolute refractory period

The first part of the refractory period, during which complete insensitivity exists to another stimulus

relative refractory period

The second part of the refractory period that , follows the absolute refractory period.

relative refractory period

During this, a very strong stimulus, or a stronger-than-threshold stimulus, can initiate another action potential

action potential frequency

is the number of action potentials produced per unit of time in response to a stimulus

subthreshold stimulus

any stimulus not strong enough to produce a graded potential that reaches threshold. Therefore, no action potential is produced

threshold stimulus

produces a graded potential that is just strong enough to reach threshold and cause the production of a single action potential.

maximal stimulus

just strong enough to produce a maximum frequency of action potentials

submaximal stimulus

includes all stimuli between threshold and the maximal stimulus strength.

supramaximal stimulus

any stimulus stronger than a maximal stimulus. Because an axon's ability to produce action potentials is limited, these stimuli cannot produce a greater frequency of action potentials than a maximal stimulus.

continuous conduction

Conduction in unmyelinated axons

continuous conduction

each section of membrane along the length of the axon generates an action potential.

local current

diffusion of Na+, as a result of this, the part of the membrane immediately adjacent to the action potential depolarizes

saltatory conduction

conduction that myelinated axons employ

saltatory conduction

an action potential is conducted from one node of Ranvier to another.

Nerve fibers (axons)

classified according to their size and degree of myelination

Type A fibers

large-diameter, myelinated axons that conduct action potentials at 15-120 m/s

Type B fibers

medium-diameter, lightly myelinated axons that conduct action potentials at 3-15 m/s

type C fibers

small-diameter, unmyelinated axons that conduct action potentials at 2 m/s or less

action potential

is a larger change in the resting membrane potential that spreads over the entire surface of the cell.

Threshold

is the membrane potential at which a graded potential depolarizes the plasma membrane sufficiently to produce an action potential.

Depolarization

occurs as the inside of the membrane becomes more positive because Na+ diffuses into the cell through voltage-gated ion channels

Repolarization

is a return of the membrane potential toward the resting state. It occurs because voltage-gated Na+ channels close and Na+ diffusion into the cell slows to resting levels and because voltage-gated K+ channels continue to open and K+ diffuses out of the cell.

afterpotential

is a brief period of hyperpolarization following repolarization

absolute refractory period

is the time during an action potential when a second stimulus, no matter how strong, cannot initiate another action potential.

relative refractory period

follows the absolute refractory period and is the time during which a stronger-than-threshold stimulus can evoke another action potential

subthreshold stimulus

produces only a graded potential

threshold stimulus

causes a graded potential that reaches threshold and results in a single action potential.

submaximal stimulus

is greater than a threshold stimulus and weaker than a maximal stimulus. The action potential frequency increases as the strength of the submaximal stimulus increases.

maximal or a supramaximal stimulus

produces a maximum frequency of action potentials.

absolute refractory period

It prevents the reversal of the direction of action potential propagation