Graded Potentials

local changes in the membrane potential that occur in varying degrees of magnitude of strength

Example: Going from -70 mV to -50 mV is a 20 mV ____ ____

graded potentials

The initial triggering event affects what two aspects of a graded potential?

size, duration

The stronger the triggering event, the smaller or larger the graded potential?

larger

The longer the duration of the triggering event, the shorter or longer the duration of the graded potential?

longer

True or false: A graded potential can spread in either direction from its activation site, but it decreases either way as it moves away from that activation site.

true

(Think of it like a glass of water and a drop of food coloring: the location where the food coloring drops into the water is the point of entry, or initial active area, but as it spreads out, the color gets lighter and lighter, just like a graded potential gets weaker and weaker as ions move out from that point of entry)

rapid change in the membrane potential that spreads rapidly along the nerve or muscle fiber membrane

begins with a sudden change in the normal (-) resting membrane potential to a (+) potential and ends with an equally rapid change back to the normal resting membrane potential

action potential

What does it mean that an action potential is "all or none"?

once you trigger it, it's going to happen all the way through

(it's going to do the whole thing once it reaches a threshold, full depolarization and immediate repolarization and so on)

stage of action potential in which the membrane is polarized at its resting membrane potential (-70 mV)

resting

stage of action potential in which the membrane becomes permeable to sodium ions via voltage-gated channels, and the polarized membrane potential is neutralized by the sudden influx of sodium down its concentration gradient

depolarization

In the depolarization stage of action potential, as Na+ ions come in, they dissipate laterally, which causes what to happen?

triggers more voltage-gated sodium channels to open in adjacent regions and let more sodium in

True or false: Different cells have different depolarization threshold points, but once the cell reaches its threshold point, it WILL carry out an action potential.

true

How is the depolarization stage of action potential a positive feedback cycle?

when depolarization triggers Na channels to open, the resulting influx of Na further decreases the membrane potential which leads to even more depolarization

stage of action potential in which sodium stops coming in and potassium starts to flow in through its voltage-gated channels, resulting in the membrane potential returning to negative

repolarization

stage of action potential in which the membrane potential dips lower than resting potential at the end of the depolarization due to the continued outflow of potassium before its channels close

(after) hyperpolarization

Specifically what kind of channels are responsible for depolarization and repolarization in the action potential sequence?

voltage gated

How many gates does a voltage-gated sodium channel have?

2

(activation and inactivation)

How many gates does a voltage-gated potassium channel have?

1

(activation only)

When the threshold is reached to trigger an action potential, what three things are also triggered simultaneously with the sodium and potassium channels?

rapid opening of sodium activation gate, slow closing of sodium inactivation gate, delayed opening of potassium activation gate

(this allows time for the sodium channel to open and close before the potassium channel opens and allows repolarization to occur)

Describe the activation and/or inactivation gate status for the sodium and potassium channels before anything ever happens with action potential.

Na activation gate is closed, Na inactivation gate is open, K activation gate is closed

Describe the activation and/or inactivation gate status for the sodium and potassium channels when the depolarizing triggering event happens.

Na activation gate opens, Na inactivation gate is open, K activation gate is closed

Describe the activation and/or inactivation gate status for the sodium and potassium channels when depolarization switches to repolarization.

Na activation gate is open, Na inactivation gate closes, K activation gate opens

Describe the activation and/or inactivation gate status for the sodium and potassium channels when repolarization leads into hyperpolarization.

Na activation gate closes, Na inactivation gate opens, K activation gate is open

Describe the activation and/or inactivation gate status for the sodium and potassium channels when hyperpolarization switches back to resting potential.

Na activation gate is closed, Na inactivation gate is open, K activation gate closes

toxin that can block sodium channels by extracellular application

tetrodotoxin

toxin that can block potassium channels by intracellular application

tetraethylammonium ion

At rest, which ion has higher conductance: sodium or potassium?

potassium

At the onset of action potential (depolarization), which ion has higher conductance: sodium or potassium?

sodium

At repolarization, which ion has higher conductance: sodium or potassium?

potassium

At what point is the gate of the voltage-gated K+ channel triggered to open?

A. when the threshold potential is reached

B. at the peak of the action potential

C. during the depolarization phase of the action potential

D. during the repolarization phase of the action potential

A

(even though it doesn't actually open until the peak of action potential, since its opening is delayed)

time after an action potential when another action potential cannot start

refractory period

refers to a refractory period in which the sodium activation gate is open; sodium inactivation gate is closing/closed already; and the membrane potential is more positive than the resting potential

**occurs at the peak of the action potential; NO new action potential can occur

absolute

refers to a refractory period in which the sodium channel inactivation gates are open; sodium channel activation gates are closed; voltage gated potassium channels are still open; and the membrane is hyperpolarized

**occurs in the "bowl" shaped phase of the action potential; you could get another action potential triggered, but it would have to have a stronger initiator

relative

Why are the absolute and relative refractory periods important in action potential?

allow for unidirectional propagation of action potential

(if an action potential is going towards the right down an axon, you don't want another one to start going left down an axon)

events that trigger some depolarization, but not enough to bring the membrane to threshold to trigger a full-on action potential

acute subthreshold potentials

What recharges the ion gradients after action potential has occurred and the gates are closed?

Hint: Responsible for the long-term maintenance of the Na+/K+ gradient!

sodium potassium pump

(the more frequent the action potentials, the more the pump is used)

general term/specific area(s) of the neuron where incoming signals are received

input zone/cell body and dendrites

True or false: Action potentials may be triggered in the cell body and dendrites.

false

(but we can have some graded potentials)

general term/specific area(s) of the neuron where action potential is initiated

trigger zone/axon hillock

general term/specific area(s) of the neuron where the action potential is conducted and moves along

conducting zone/axon

general term/specific area(s) of the neuron where vesicles containing neurotransmitters are released

output zone/axon terminal

What cells myelinate axons in the CNS?

oligodendrocytes

What cells myelinate axons in the PNS?

Schwann cells

gaps in the myelin sheath to which voltage-gated sodium and potassium channels are confined

nodes of Ranvier

type of conduction in which conduction occurs along the entire circumference of the membrane; there is an active area at the peak of action potential, and depolarization spreads into adjacent inactive areas, making them become active and reach threshold

**keeps occurring in subsequent adjacent regions down the axon as previous areas go back to resting membrane potential

contiguous

type of conduction in which voltage-gated channels are in the nodes of Ranvier and conduction occurs as the action potential "jumps" from node to node

**an active node triggers depolarization in an adjacent inactive node, and then when THAT one reaches threshold depolarization, the action potential hops over there, and so on down the axon

saltatory

Which type of action potential conduction is faster: contiguous or saltatory?

saltatory

Which type of action potential conduction occurs in unmyelinated axons: contiguous or saltatory?

contiguous

Which type of action potential conduction occurs in myelinated axons: contiguous or saltatory?

saltatory

How does saltatory conduction conserve energy?

sodium potassium pump works less to move ions across the whole membrane

What two factors affect the speed of action potential conduction?

myelination, diameter of myelin sheath

Does myelination increase or decrease the speed of an action potential propagation?

increase

(which is why saltatory conduction is faster than contiguous conduction)

Do smaller or larger diameter axons conduct faster action potentials?

larger

True or false: The action potential varies depending on whether it's barely above threshold or way over threshold.

false

(ALL OR NONE)

What determines the strength of a signal from an action potential?

frequency of action potentials

(MORE action potentials = STRONGER stimulus)

Do more action potentials signify a weaker or stronger stimulus?

stronger

(for example, "warm" and "hot" both generate signals above threshold, but the intensity of hot or cold is determined by its higher frequency of action potential generation)

Which of the following ensures the one-way propagation of action potentials away from the activation site on a neuron?

A. myelination

B. after hyperpolarization

C. refractory period

D. Na+/K+ ATPase

C

Which of the following is responsible for the falling phase of an action potential?

A. opening of Na+ gates

B. Na/K pump restoring the ions to their original locations

C. greatly increased permeability to Na+

D. Na+ efflux

E. none of these

E

Saltatory conduction...

A. occurs in unmyelinated nerve fibers

B. is slower than contiguous conduction because myelin acts as an insulator to slow the impulse

C. involves the impulse jumping from one node of Ranvier to the adjacent node

D. refers to the action potential spreading from one Schwann cell to the adjacent Schwann cell

E. occurs along dendrites and axons of certain neurons

C

If a neuron were experimentally stimulated at both ends simultaneously, then...

A. the action potentials would stop as they met in the middle

B. the action potentials would pass in the middle and travel to the opposite ends

C. the action potentials would meet in the middle and then be propagated back to their starting positions

D. summation would occur when the action potentials met in the middle, resulting in two larger action potentials traveling in opposite directions

E. the strongest action potential would override the weaker action potential

A

Opening either a chemically gated Na+ channel or a chemically gated K+ channel during the resting potential would cause...

A. an impulse to be propagated

B. a graded potential

C. an action potential

D. the membrane's potential to become more negative

E. the threshold voltage to be reached

B