Section 4, chapter 10

Graded and Action Potentials

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Changes in the membrane potential

Resting Membrane Potential (RMP) for a neuron = -70mV• Membrane potential of a cell at rest

Environmental stimuli cause changes in the membrane potential by opening gated ion channels• Ligand-gated ion channels• Voltage-gated ion channels• Other-gated ion channels

(respond to mechanical, temperature, or other stimulus)

If membrane potential becomes more negative, it has hyperpolarized e.g. -100mV is hyperpolarized

If membrane potential becomes less negative, it has depolarizede.g. -60mV is depolarized

Local Potential Changes

Graded Potentials or local potentials are localized changes in the membrane potential (usually occurs at dendrites)

Graded = the magnitude of response is proportional to stimulus

Graded potentials summate (add together)

if summation of graded potentials reaches a threshold potential, they will generate an action potential.

If the neuron depolarizes to threshold potential (-55 mV), it results in an action potential

Summation of Graded Potentials

graded potentials may summate by:

1. Spatial Summation – if multiple dendrites of a neuron are stimulated the depolarization of the dendrites are added together.

2. Temporal Summation – If a dendrite is stimulated at a high frequency it results in a greater depolarization.

Graded Potentials are summated together at the Axon Hillock “Trigger Zone”• If summation of graded potentials reaches threshold stimulus (-55 mV), an action

potential is initiated at the axon hillock.

Figure 10.15. (a) Subthreshold depolarization will not result in an action potential.

(b) Summation of graded potentials may reach threshold stimulus, initiating an action potential at the trigger zone. The action potential begins when voltage-gated Na+ channels open at the trigger zone.

There are 3 phases of an Action Potential

1. Depolarization Phase• Voltage-gated Na+ channels open at

-55mV (threshold stimulus)• Na+ diffuses into cell

2. Repolarization Phase• Voltage-gated K+ channels open at

+30mV• K+ rushes out of the cell repolarizing

the membrane• Na+ channels close

3. Hyperpolarization Phase• The slower voltage-gated K+ channels

remain open briefly, resulting in a slight hyperpolarization (-90mV). 3

2

1

Figure 10.17. An oscilloscope records and action potential

3 phases of an Action Potential

(a) At rest, the membrane is polarized (RMP = -70mV). Sodium is mostly outside the cell and potassium is within the cell.

(b) When the membrane depolarizes to threshold (-55mV), voltage-gated Na+ channels open. Sodium rapidly diffuses into the cell, depolarizing the membrane up to +30mV.

(c) As the membrane depolarizes to +30mV, voltage-gated K+ channels open and quickly repolarize the membrane. Sodium channels also close at this point.

Following an action potential, Na+/K+ pumps work to actively reestablish the Na+ and K+ concentration gradients.

3 phases of an Action Potential

Once initiated an action potential is propagated along the entire axon at full strength. It does not weaken.

3 phases of an Action Potential

Propagation of the Action Potential

An action potential in one region, depolarizes the adjacent region to threshold stimulus (-55mV).

Once the adjacent region reaches threshold potential, it triggers another action potential.

The second action potential causes depolarization in its adjacent region, triggering yet another action potential.

This sequence continues all the way to the end of the axon at full strength.

All-Or-None Response

A stimulus stronger than threshold does not produce a stronger impulse (although a greater stimulation does produce more impulses per second)

If the axon hillock depolarizes to threshold potential it will initiate an action potential fully and completely.

eg The action potential occurs fully and completely or not at all.

A stimulus below threshold is subthreshold and does not generate an action potential.

Refractory Period

Refractory Period: For a brief period following an action potential, a threshold stimulus will not trigger another action potential.

Absolute Refractory Period• no new action potentials can be produced• Occurs while the membrane is changing in sodium permeability• Occurs between the depolarization and repolarization phases

Relative Refractory Period• Action potential can be generated with a high intensity stimulus• Occurs while membrane is reestablishing its resting membrane potential• Lasts from the hyperpolarization phase, until RMP is reestablished

End of Chapter 10, Section 4

Refractory Period