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Physiological Psychology, 7. Action Potential


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Objectives General LINKS Characteristics TERMS Graded VS Action QUIZ Excitability RETURN

Lesson Objectives

1. 2. 3. 4. 5. 6. 7. 8.


Describe the ways that the resting potential may be altered. What is the significance of the axon hillock? Discuss the similarities and differences between the resting and action potentials. Describe the absolute and relative refractory periods and explain how they influence conduction. Explain the similarities and differences between the graded and action potentials. Describe how the action potential effects the release of transmitter substance at the synapse. Explain the significance of spatial and temporal summation. Explain how the graded potential allows the neuron to function as a decision maker.

General Characteristics

q q q q

Results from a reversal of the resting potential. K+ and Cl- are in equilibrium: Na+ has both chemical & electrical forces acting to push into cell. Na+ is kept outside by the closed channels in the cell membrane. The action potential results because the Na+ permeability of the membrane is changed allowing Na+ to rush into the cell. (1 of 4) [2/19/2001 6:11:08 PM]

Physiological Psychology, 7. Action Potential


The resting potential may be altered in 1 of 4 ways:

1. Hyperpolarization - increase in resting potential (inhibitatory). 2. Hypopolarization - reduce the resting potential. 3. Depolarization - abolish resting potential. 4. Action potential - complete polarity reversal Graded Potentials r The action of transmitter substance from other neurons first influence the cell membrane at the dendrites and/or cell bodies. r Hypopolarization (EPSP) - Na+ entering cell along dendrites. r Hyperpolarization (IPSP) - Cl- entering cell on cell body. r Both EPSP's and IPSP's are weighted as to distance (closer to axon hillock the greater the effect). r Both decline with distance and time as they travel toward the axon hillock. Action Potentials r Axon hillock is unique in that there is a high density of votage gated Na+ channels located here. r The electrical effects of the EPSP's and IPSP's combine (add or subtract) r Temporal summation r Spatial summation r If the sum reaches threshold at the axon hillock there is a sudden inrush of Na+ (action potential) r Na+ channels are immediately opened. r Na+ ions rush in & locally cancel resting potential. r Threshold - point where membrane no longer offers resistance (area of approx. 1 cm). r Voltage inside the cell reverses from -70 to +30mv. r K+ and Cl- ions are no longer in equilibrium. r Change is brief & involves few particles. r Na+ pump's operation then works to exchange Na+ and K+ ions. Propagation of Action Potentials r In unmyelinated neurons this process is continuous along axon to terminal endings r In Myelinated neurons the voltage gated Na+ channels are located only at the nodes of Ranvier and is called saltatory conduction. Action Potentialfigure

MENU (2 of 4) [2/19/2001 6:11:08 PM]

Physiological Psychology, 7. Action Potential

Characteristics of Action Potentials

q q q q

q q


Passive - heat liberated during recovery. Ion movement is in & out, not down the axon. Cl- movement is negligible Ion movement studied by use of voltage clamp: Feedback device which can prevent depolarization of axon. Works by delivering current which would exactly equal flow of ions across membrane. Allows this required voltage to be recorded.

Graded vs Action Potentials

Graded Potentials Dendrites & Cell bodies Decreases with time and distance Amplitude function of intensity Temporal or Spatial summation Hypopolarization


Action Potentials Axons Self propagating Same amplitude All-or-none Polarity reversal

Excitability of the Neuron

Excitability: Reciprocal of the stimulation threshold. Absolute refractory period: Period when Na+ is rushing in (1 msec) & K+ out (4msec). Relative refractory period: (negative after potential) q Higher threshold q Repeated stimuli can prolong period Supernormal period: Increased excitability following relative refractory period (approx 12 msec). Subnormal period q Reduced excitability q Tetanus increases time of occurrence q Stimuli can be timed to inhibit activity during subnormal period. (3 of 4) [2/19/2001 6:11:08 PM]

Physiological Psychology, 7. Action Potential

Terms to Know

Action potential Hyperpolarization Hypopolarization Depolarization Polarization Temporal summation Graded Potentials Spatial summation Threshold Polarity reversal Absolute refractory period Relative refractory period Axon hillock


Links to Associated Areas

LINK Nerve Impulse LINK Neuron Course LINK Integrate and Fire Neuron Model (4 of 4) [2/19/2001 6:11:08 PM]


Physiological Psychology, 7. Action Potential

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