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Membrane Potential- 3

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Local circuit of current flow from the depolarized areas of the membrane to the adjacent resting membrane areas. ... Neuron/ Nerve Cell It is the basic structural and ... – PowerPoint PPT presentation

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Title: Membrane Potential- 3


1
Membrane Potential- 3
  • 11/5/10

2
Calcium Ions.
  • The membranes of almost all cells of the body
    have a calcium pump
  • Calcium serves along with sodium in some cells to
    cause most of the action potential.
  • Calcium pump calcium ions from the interior to
    the exterior of the cell membrane (or into the
    endoplasmic reticulum of the cell).

3
  • This creates a Calcium gradient on inside of the
    membrane. This leaves an inside to be more
    negative as compared to the outside.

4
Voltage - Gated Calcium Channels.
  • These channels are slightly permeable to sodium
    ions as well as to calcium ions.
  • Opening of the theses channels lead to both
    calcium and sodium ions flow to the interior of
    the fiber. Therefore, these channels are also
    called Ca - Na channels.

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  • Calcium Channels
  • slow to become activated (longer time)
  • called Slow channels.
  • Sodium Channels
  • They get activated in short span of time.
  • called Fast channels.

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Deficit of Calcium Ions.
  • The concentration of calcium ions in the
    extracellular fluid also has a profound effect on
    the voltage level at which the sodium channels
    become activated.

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  • Deficit of calcium ions
  • Sodium channels become activated (opened) by very
    little increase of the membrane potential from
    its normal.
  • Therefore, the nerve fiber becomes highly
    excitable sometimes discharging repetitively than
    remaining in the resting state.

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  • Tetany
  • Calcium ion concentration 50 below the normal
    levels is sufficient for spontaneous discharge
    to take place. peripheral nerves.
  • Leads to muscle "tetany" which can be lethal
    because of tetanic contraction of the respiratory
    muscles.

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MOA
  • Ca-ions bind to the exterior surfaces of the
    sodium channel protein molecule.
  • The positive charges of these calcium ions in
    turn alter the electrical state of the channel
    protein
  • altering the voltage level required to open the
    sodium gate.

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  • Any event causes enough initial rise in the
    membrane potential from -90 millivolts toward
    the zero level
  • The rising voltage itself causes many
    voltage-gated sodium channels to open.
  • This allows rapid inflow of sodium ions, which
    causes a further rise in the membrane potential.

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  • More voltage-gated sodium channels open and more
    stream of sodium ions move to interior of the
    fiber.
  • This is positive-feedback vicious cycle
  • Once the feedback is strong enough, continues
    until all the voltage-gated sodium channels have
    become activated (opened).

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  • Within another fraction of a millisecond, the
    rising membrane potential causes closure of the
    sodium channels
  • And wide opening of potassium channels
  • Action potential soon terminates.

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Threshold for Initiation of the Action Potential.
  • An action potential will not occur until the
    initial rise in membrane potential is great
    enough to create the vicious cycle
  • This occurs when the number of Na ions entering
    the fiber becomes greater than the number of K
    ions leaving the fiber.

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Propagation of the Action Potential
  • Action potential as it occurs at one spot on the
    membrane.
  • Action potential elicited at anyone point on an
    excitable membrane usually excites adjacent
    portions of the membrane
  • Resulting in propagation of the action potential
    along the membrane.

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Sequence Of Events
  • Nerve fiber excited in its midportion
  • Mid portion suddenly develops increased
    permeability to sodium.
  • Local circuit of current flow from the
    depolarized areas of the membrane to the adjacent
    resting membrane areas.

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Nerve or Muscle Impulse
  • This transmission of the depolarization process
    along a nerve or muscle fiber is called a Nerve
    or Muscle Impulse.

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Direction Of Action Potential
  • Action potential travels in all directions away
    from the stimulus even along all branches of a
    nerve fiber until the entire membrane has become
    depolarized.

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All-or-Nothing Principle.
  • When onset of action potential takes place at
    any point on the membrane of a normal fiber
  • Wave of depolarization travels over the entire
    membrane if conditions are right.

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  • In all normal excitable tissues when action
    potential reaches a point on the membrane at
    which it does not generate sufficient voltage to
    stimulate the next area of the membrane, spread
    of depolarization stops.

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Safety Factor For Propagation.
  • For continued propagation of an impulse
  • Ratio of action potential to threshold for
    excitation must at all times be greater than 1.
  • This greater than 1 requirement is called the
    safety factor for propagation.

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Plateau in Some Action Potentials
  • Sometimes excited membrane do not repolarize
    immediately after depolarization.
  • Instead, the potential remains on a plateau near
    the peak of the spike potential for many
    milliseconds, and then repolarization begins.

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Imp.Features Of Plateau
  • Plateau greatly prolongs the period of
    depolarization.
  • This type of action potential with plateau is
    seen in heart muscle fibers.

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Cause Of Plateau
  • In heart muscle, two types of channels cause
    depolarization
  • Voltage-activated sodium channels (fast
    channels).
  • Voltage-activated calcium-sodium channels (slow
    channels).

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  • Opening of fast channels causes the spike portion
    of the action potential.
  • The slow, prolonged opening of the slow
    calcium-sodium channels mainly allows calcium
    ions to enter the fiber.
  • This is largely responsible for the plateau
    portion of the action potential.

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  • Voltage gated potassium channels are partly
    responsible for the plateau as they open slowly,
    often not opening very much until the end of the
    plateau.
  • This delays the return of the membrane potential
    toward its normal negative value of -80 to -90
    millivolts.

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Repetitive self- Induced Discharges
  • Seen In
  • Heart
  • Smooth muscle
  • Neurons of the central nervous system.
  • Other excitable tissues can discharge
    repetitively if the threshold for stimulation of
    the tissue cells is reduced low enough.

39
Use Of Rhythmical discharges
  • Rhythmical beat of the heart.
  • Rhythmical peristalsis of the intestines.
  • Neuronal events as the rhythmical control of
    breathing.

40
Hyperpolarization
  • Toward the end of each action potential
  • Membrane becomes excessively permeable to
    potassium ions.
  • There is excessive outflow of potassium ions
  • Tremendous numbers of positive charges are on the
    outside of the membrane

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  • Leaving inside the fiber considerably more
    negativity than normal.
  • It continues for nearly a second after the
    preceding action potential is over.
  • Membrane potential reaches nearer to the
    potassium Nernst potential.
  • It is called hyperpolarization ,
    self-re-excitation will not occur at this time.

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  • Membrane potential again increases up to the
    threshold for excitation.
  • Suddenly, a new action potential results, and the
    process occurs again and again.

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Neuron/ Nerve Cell
  • It is the basic structural and functional unit of
    the nervous system. In human nervous system there
    are about 1 trillion of these cells.
  • These neurons are highly differeniated and
    specialized excitable cells.

48
Components of the Neurons
  • Neuron is composed of
  • Cell body.
  • Two processes
  • Dendrites.
  • Axon.

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Myelinated And Unmyelinated Fibers
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  • An average nerve trunk contains about twice as
    many unmyelinated fibers then, myelinated fibers.
  • Large fibers are myelinated
  • Small fibers are unmyelinated
  • Structure Of A Typical Myelinated Fiber

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  • Axon is the central core of the fiber.
  • The axon is filled in its center with axoplasm,
    which is a viscid intracellular fluid.
  • Axon is surrounded by a myelin sheath that is
    often much thicker than the axon itself.

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  • The membrane of the axon conducts the action
    potential.
  • Every 1 to 3 millimeters along the length of the
    myelin sheath is a Node of ranvier.
  • The myelin sheath is deposited around the axon by
    Schwann cells in the following manner

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  • The membrane of the Schwann cell first envelops
    the axon. Then the Schwann cell rotates around
    the axon many times.
  • Multiple layers of Schwann cell membrane
    containing the lipid substance Sphingomyelin. It
    is an excellent electrical insulator that
    decreases ion flow through the membrane.

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  • Between each two successive Schwann cells along
    the axon, a small uninsulated area only 2 to 3
    micrometers in length remains where ions still
    can flow with ease.
  • This area is called the Node of Ranvier.

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Saltatory Conduction
68
  • Even though almost no ions can flow through the
    thick myelin sheaths of myelinated nerves.
  • Flow with ease takes place through the Nodes of
    Ranvier.
  • Therefore, action potentials occur from node to
    node. This is called saltatory conduction

69
  • Flow Of Electrical Current
  • It flows through the surrounding extracellular
    fluid, outside the myelin sheath as well as
    through the axoplasm inside the axon from node to
    node, exciting successive nodes one after
    another.

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Importance Of Saltatory Conduction
72
  • Velocity of transmission of nerve impulse is
    faster.
  • Conserves energy
  • The excellent insulation by the myelin sheath and
    the decrease memb. Capacitance allow the
    repolarization to occur with little transfer of
    ions.

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  • End Of Todays Lecture!!!
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