Chapter 10, Section 4
Graded and Action Potentials
Changes in Membrane Potential Resting Membrane Potential (RMP) for a neuron = -70mV • Membrane potential of a cell at rest
Environmental stimuli cause changes in 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. A membrane potential of -100mV is hyperpolarized If membrane potential becomes less negative, it has depolarized e.g. A membrane potential of -60mV is depolarized
Local Potential Changes Graded Potentials • Local changes in membrane potential (usually occurs at dendrites) • Magnitude of response is proportional to stimulus • Graded potentials summate (add together) • Graded potentials generate action potentials
If a graded potential reaches threshold stimulus (-55mV), it results in an action potential
Summation of Graded Potentials Summation of graded potentials my occur by: 1. Spatial Summation – stimulating multiple dendrites 2. Temporal Summation – Stimulating a dendrite at a high frequency 3. Combined – stimulating multiple dendrites at a high frequency Graded Potentials are summed together at the Axon Hillock “Trigger Zone” • If summation of graded potentials reaches threshold stimulus (-55mV), an action potential is initiated at the axon hillock.
Figure 10.15. (a) 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.
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
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Action Potential
Figure 10.16(a) At rest, the membrane is polarized (RMP = -70mV). Sodium is mostly outside the cell and potassium is within the cell.
Figure 10.16(b) When a stimulus reaches threshold stimulus (-55mV), voltage-gated Na+ channels open. With Na+ channels open, sodium rapidly diffuses into the cell, depolarizing the membrane up to +30mV.
Action Potential
Figure 10.16(c) When the membrane reaches +30mV, voltage-gated K+ channels open an 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.
Action Potential Propagation Once initiated an action potential is propagated along the entire axon at full strength. It does not weaken. Figure 10.18 An action potential in one region, depolarizes the adjacent region to threshold stimulus (-55mV). Once the adjacent region reaches threshold stimulus, 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 All-or-none response • Action potentials occur completely, or they do not occur at all. • An action potential occurs whenever a stimulus of threshold intensity or above is applied to a neuron. • Greater stimulation does not produce a stronger impulse (although a greater stimulation will produce more impulses per second)
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 • 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