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Physiology I
Section 2
Excitable Cells

Action Potential

Suggested Reading:  Guyton Chapter 5

Key Words

Voltage-Gated Sodium and Potassium Channels:  Channels necessary in causing both depolarization and repolarization of the nerve membrane during the action potential.
Na+ channels open and Na+ rush into ICF which creates depolarization.
K+ channels open.  K+ rushes out which equals repolarization.

Depolarization:  Happens when the membrane suddenly becomes permeable to Na+ ions, allowing huge amounts of these + ions to flow into the interior of the axon.  The normal polarized state of -90mV is lost with the potential rising rapidly in the + direction.

(Polarization):  Condition in which ions of opposite charges are separated by a semi-permeable membrane.

Repolarization:  State in which the Na+ channels open more than they normally do.  Then rapid diffusion of K+ ion to the exterior re-establishes the norms (negative resting membrane potential)

Hyperpolarization:  Caused by the excessive outflow of K+ ions during repolarization which tremendous #'s of + charges carried to the outside of the membrane, creating inside the fiber considerably more - than would otherwise occur.  This continues for a short time after the action potential is over, thus drawing the membrane potential nearer to the K+ Nernst Potential.

"All-Or-None" Response:  Occasionally the action potential will reach a point on the membrane at which it does not generate sufficient voltage to stimulate the next area of the membrane.  When this occurs, the spread of depolarization stops.  Guyton pg. 66

Local Response:  

Absolute Refractory Period:  The period during which a second action potential can not be elicited.  Happens due to closing of the Na+ or Ca++

Relative Refractory Period:  Happens after the absolute refractory period.  During this time, stronger than normal stimuli can excite the fiber.  Guyton, pg 70

Auto-Rhythmicity:  Repetitive self-induced discharges that occur in the (pg 66-67) heart, most smooth muscle and in many of the neurons in the CNS.

Continuous vs. Saltatory Conduction:  Continues:  Happens in a naked axon at rate of 5 m/sec.  Depolarization along the length of the axon depends on the adjacent membrane being at resting state and ready to be excited.   Saltatory (pg. 68):  The nerve impulse jumps down the fiber.  Electrical current flows though the surrounding ECF' outside the myelin sheath as well as the axoplasma from node to node...exciting successive nodes one after another.  Rate 100 m/sec (length of a football field).

Myelinated Axon:  Large nerve fiber surrounded by a myelin sheath (insulation).  Sheath interrupted by nodes of Ranvier.  Myelin made up of Schwann cells.  Guyton pg 68

Node of Ranvier:  A small uninsulated area 2-3 micrometers in length.  Located at the juncture between 2 successive Schwann cells.  Ion can flow with ease between the ECF and the axon.

Learning Objectives

Compare and contrast sodium and potassium channels in nerve cell membranes, including a description of the behavior of the various types of "gates for each channel:  
a):  Potassium-Sodium Leak Channel-  This channel is 100X's as permeable to K+ than Na+ (Passive leakage).  Very small amount of Na+ leaks through.
b):  Voltage gated Na+ channel-  has 2 gates.  The activation gate is near the outside of the channel and the  inactivation gate is nearest the inside.  When the membrane is "resting" at -90 mV the activation gate is closed, the inactivation gate is open.  When the membrane potential becomes less negative (rising from -90 towards zero = depolarization) it finally reaches a voltage, usually somewhere between -70 and -50 mV that causes a conformational change in the activation gate, flipping it to the open position = Activated state.  Now Na+ ions can POUR into the channel increasing the permeability of the membrane to Na+ as much as 500-5000 fold.  The same increase in voltage that causes the opening to the activation gate closes the inactivation gate.  The inactivation gate closes a few 10,000ths of a second after the activation gate opens.  (The conformational change that closes the inactivation gate is a slow process whereas the change that opens the activation gate is a rapid process.)  The inactivation gate won't reopen until the membrane potential returns either to OR nearly to the original resting membrane potential level.
c):  Voltage gated K+ channel-  When the membrane potential rises from -90 toward zero, this voltage change causes a slow conformational opening of the gate and allows increased K+ diffusion outward.  They open just at the same time the Na+ channels are beginning to close SO reduced Na+ entry and increased K+ exit greatly speed repolarization leading to full recovery within a few 10,000ths of a second.

Identify and describe the sequence of events and probable underlying mechanisms that characterize the generation of an action potential:  See previous objective.  In brief:  A particular force raises the potential of the membrane above -90mV, the Voltage gated Na+ channels open and Na+ rushes in in huge amounts = depolarization.  Read class notes and Guyton pages 61-62

Discuss the role of positive feedback in the process of nerve cell depolarization:  Any event that causes enough of an initial rise in the membrane potential from -90mV up towards the zero level will case Na+ channels to start opening = rapid influx of Na+ ions, which causes still further rise of the membrane potential thus opening more gates (voltage gated Na+ channels) and allowing more streaming of Na+ ions to the interior of the fiber.  This process is a positive feedback vicious circle that, once the feedback is strong enough, will continue until all the voltage gated Na+ channels have opened.  Guyton page 65

Describe what is meant by the absolute and relative refractory periods, and explain the changes in height of the action potentials obtained during the relative refractory period:  Absolute refractory period- period during which a second action potential cannot be elicited, even with a strong stimulus.  Relative refractory period-  During this time, stronger than normal stimuli can excite the fiber.  See Guyton page 77 for complete description of "refractory period."

Describe the mechanisms and characteristics of action potential propagation along myelinated and unmyelinated nerve fibers:  Basically any factor that causes Na+ ions to begin to diffuse inward through the membrane in sufficient #'s will set off the automatic regenerative opening of the Na+ channels.  
In Unmyelinated nerve fibers, an action potential is initiated at one point on the fiber and spreads down and around the fiber in both directions until it reaches the end of the fiber (see class notes).  Each adjacent area of excitatory tissue must depolarize for impulse transmission to the move along the fibers length.
In Myelinated fibers the impulse conduction is said to be Saltatory, that is it jumps down the fiber.  Action potential can occur only at the Nodes of Ranvier.  Electrical current flows through the surrounding extracellular fluids outside the myelin sheath as well as through the axoplasma from node to node, exciting successive nodes one after another.  Guyton page 68

Identify and discuss the mechanisms underlying intrinsic rhythmicity in excitable cells:  Repetitive self-induced discharges, or rhythmicity, occur normally in the heart, most smooth muscle and in many of the central nervous systems neurons.  For spontaneous rhythmicity to occur, the membrane must be permeable enough to Na+ ions (or to Ca+ ions and Na+ ions through the slow Ca+ channels) to allow automatic membrane depolarization.  In these type of cells a resting membrane potential of only -60 to -70 mV exists.  This is not enough negative voltage to keep the Na+ and Ca+ channels closed.  SO 1):  Na+ and Ca+ ions flow in  2):  This increases membrane permeability  3):  still more ions flow in  4):  Action potential is generated  5):  Then the cell repolarizes due to an excessive out flow of K+ ions.  Guyton pages 67-68

Explain the differences in action potential wave forms among cells from the brain, skeletal muscle, smooth muscle, and cardiac muscle:  


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