graded potential

main way to change the membrane potential in an average cell

Increase permeability to ion

- by opening existing channels. These

channels are gated (can be voltage-gated).

Could decrease too

Channel mediated facilitated diffusion

All passive

Na+ channels open

Vm decreases, more positive, and depolarizes

Na increase

K+ channel closes

K+ decreases

Vm decreases, more positive and depolarizes

K+ channels open

K+ increase

Vm increases, more negative, and hyperpolarizes

Cl- channel opens

Vm increases, more negative, hypolarizes

Ca2+ opens

Vm decreases, more positive, depolarization

significance of changing the membrane potential?

It is how electrical signals are sent

Two types of electrical signals

Graded potential

Action potential

To continue the electrical signal:

Depolarizing Graded Potential initiates Action Potential

To stop the electrical signal:

Hyperpolarizing Graded Potential

Graded potential

• IN A NEURON, usually occurs in dendrites or

cell body

• Variable in amplitude

• Hyperpolarizing or depolarizing

• Dissipates over time/distance

• Can summate

Directly uses ligand- or mechanically-gated channels

Graded potential begins with

Depolarizing or Hyperpolarizing with a change in permeability to an ion!

Subthreshold

A graded potential starts above threshold (T) at its initiation point, but decreases in strength as it travels through the cell body. At the trigger zone it is below threshold and therefore does not initiate an action potential.

Suprathreshold

A stronger stimulus at the same point on the cell body creates a graded potential that is still above threshold by the time it reaches the trigger zone, so an action potential results.

Made by summation

Spatial summation

Summation of several subthreshold signals results in action potential

Temporal summation

When two graded potentials from one presynaptic neuron occur close together in time

No summation: Two subthreshold graded potentials will not initiate an action potential if they are far apart in time.

Summation causing action potential: If two subthreshold potentials arrive at the trigger zone within a short period of time, they may sum and initiate an action potential.

Threshold

the minimum Vm required to open voltage-gated Na+ channels

A suprathreshold graded potential at the axon hillock causes an action potential

Action potential

• IN A NEURON, usually begins at axon hillock

• Occurs only in axon

• “All-or-none”

• Has threshold

• Regenerative: occurs over and over again, down the axon

• Does not vary in amplitude: so it does not get lost in axon

• Does not summate

Mostly uses voltage gated channels

Resting membrane potential

Depolarizing stimulus

Membrane depolarizes to threshold.
Voltage-gated Na* channels open and Na* enters cell. Voltage-gated K+ channels begin to open slowly.

Rapid Na* entry depolarizes cell.

Na+ channels close and slower k+ channels open

K+ channels remain open and additional K* leaves cell, hyperpolarizing it.

Voltage-gated K* channels close less K* leaks out of the cell

Cell returns to resting ion permeability and resting membrane potential.

Signal is unidirectional because of

refractory period: once region has action potential go into it it will no longer have action potential going thru that region again

Signal spread by

Local current flow: change in membrane potential changes charge. in adjacent region so copy of signal can persist