2. 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 depolarized
e.g. -60mV is depolarized
3. 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
4. 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.
5. 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.
6. 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).
Figure 10.17. An oscilloscope
records and action potential
2
1
3
7. 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.
8. 3 phases of an Action Potential
(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.
9. 3 phases of an Action Potential
Once initiated an action potential is propagated along the
entire axon at full strength. It does not weaken.
10. 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.
11.
12. All-Or-None Response
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 stronger than threshold does not produce a
stronger impulse (although a greater stimulation does
produce more impulses per second)
A stimulus below threshold is subthreshold and does not
generate an action potential.
13. 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
