CHAPTER 11 GRADED VS ACTION POTENTIALS

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REFRACTORY PERIOD

A time during which a neuron cannot respond to another stimulus.

ACTION POTENTIAL location of event

Axon hillock and axon; also called a *nerve impulse*

GRADED POTENTIAL repolarization

Voltage independent; occurs when stimulus is no longer present

GRADED POTENTIAL location of event

cell body and dendrites, typically

ACTION POTENTIAL peak membrane potential

+30 to +50 mV

DEPOLARIZATION

A decrease in membrane potential. The inside of the membrane becomes *less negative* (moves closer to zero) than the resting potential. [-70mV to -65 mV is a depolarization] Also includes events that reverse and move above zero to become positive.

GRADED POTENTIAL positive feed back cycle

Absent

ACTION POTENTIAL amplitude

Always the same size (all-or-none); does not decay with distance. [change in voltage from -70 mV to +30 mV or a total amplitude of about 100 mV]

HYPERPOLARIZATION

An increase in membrane potential - the inside of the membrane becomes *more negative* (moves further from zero) than the resting potential [-70mV to -75mV]

GRADED POTENTIAL stimulus for ion channels

Chemical (neurotransmitter) or sensory stimulus (light, pressure, temperature)

diameter, myelination

Conduction velocity (rate of impulse propogation) depends on two things: the axon's _____________ and the degree of ________________.

SALTATORY CONDUCTION

Current passes through a myelinated axon only at the nodes of Ranvier. Voltage-gated sodium chanels concentrated at those nodes. Action potentials triggered only at the nodes and jump from one node to the next. Much faster than unmyelinated axon conduction.

GRADED POTENTIAL peak membrane potential EPSP

Depolarizes; moves toward 0 mV

ACTION POTENTIAL summation

Does not occur; all-or-none phenomenon

ABSOLUTE REFRACTORY PERIOD

Ensures each action potential is a separate, all-or-none event and enforces one-way transmission of the action potential.

GRADED POTENTIAL function [postsynaptic (EPSP) potential]

Excitatory --short-distance signaling; depolarization that spreads to axon hillock; moves membrane potential toward threshold for generating an action potential

RECEPTOR POTENTIAL

Generator potential -- when the receptor of a sensory neuron is excited by some form of energy (heat, light, etc) the graded potential is called this.

GRADED POTENTIAL peak membrane potential IPSP

Hyperpolarizes; moves toward -90 mV

Inhibitory Postsynaptic Potentials (IPSPs)

Hyperpolarizing changes in potential (caused by binding of neurotransmitters at inhibitory synapses) reduce a postsynaptic neurons' ability to generate an action potential.

DEPOLARIZATION

Increases the probability of producing nerve impulses

GRADED POTENTIAL function [postsynaptic (IPSP) potential]

Inhibitory -- short-distance signaling; hyperpolarization that spreads to axon hillock; moves membrane potential away from threshold of generating an AP

Excitatory Postsynaptic Potentials (EPSPs)

Local graded depolarization events that occur at excitatory postsynaptic membranes

ACTION POTENTIAL distance traveled

Long distance - from trigger zone at axon hillock through entire length of axon (a few mm to over a meter). APs do not decay with distance.

ACTION POTENTIAL function

Long-distance signaling; constitutes the nerve impulse

GRADED POTENTIAL

Magnitude varies with strength of stimulus. Stronger stimulus=more voltage changes=further current flow

CONTINUOUS CONDUCTION

Occurs on unmyelinated axons only. Action potentials are generated at sites immediately adjacent to each other. Conduction is relatively slow.

GRADED POTENTIAL initial effect of stimulus IPSP

Opens chemically gated Potassium or Chlorine channels

GRADED POTENTIAL initial effect of stimulus EPSP

Opens chemically gated channels that allow simultaneous Sodium and Potassium fluxes

ACTION POTENTIAL initial effect of stimulus

Opens voltage-gated channels; first opens Sodium channels, then Potassium channels

ACTION POTENTIAL positive feed back cycle

Present

HYPERPOLARIZATION

Reduces the probability of producing nerve impulses

GRADED POTENTIAL distance traveled

Short distance, typically within cell body to axon hillock (0.1-1.0 mm) Because current is lost through the "leaky" plasma membrane, voltage declines with distance from the stimulus; the voltage is *decremental.*

Frequency of action potentals

Stimulus intensity is measured by number of impulses per second, or _____________, rather than by increases in the strength (amplitude) of the individual action potentials.

GRADED POTENTIAL summation

Stimulus responses can summate to increae amplitude of graded potential. Temporal (increased frequency of stimuli) Spatial (stimuli from multiple sources)

More often

Strong stimuli generate nerve impulses *more often or less often* in a given time interval than do weak stimuli?

Propogation of a nerve impulse

This means that the action potential is regenerated anew at each membrane patch, and every subsequent action potentail is identical to the one that was initially generated.

Function of an EPSP

To help trigger an action potential distally at the axon hillock of the postsynaptic neuron

PROPAGATING

Transmitting the action potentials

GRADED POTENTIAL amplitude

Various sizes (graded); decays with distance

ACTION POTENTIAL stimulus for ion channels

Voltage (depolarization, triggered by graded potential reaching threshold)

ACTION POTENTIAL repolarization

Voltage regulated; occurs when Sodium channels inactivate and Potassium channels open

The neuron cannot respond to another stimulus no matter how strong it is.

What happens when a patch o neuron membrane is generating an AP and its voltage-gated sodium channels are open?

THEY ARE CELLS WITH EXCITABLE MEMBRANES

What is special about neurons and muscle cells that they can generate action potentials?

RELATIVE REFRACTORY PERIOD

What is the interval following the absolute refractory period when most sodium channels have returned to their resting state and some potassium channels are still open, but repolarization is occurring.

POSTSYNAPTIC POTENTIAL

When the stimulus is a neurotransmitter released by another neuron, the graded potential is called this.


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