action potential

3.  An action potential will not occur until the initial
rise in membrane potential is sufficient enough
 This occurs when the number of Na+ ions entering
the fiber becomes greater than the number of K+
ions leaving the fiber.

5. Calcium Ions.
 with sodium in some cells to cause most of the
action potential.
 Calcium pumps, calcium ions from the interior to
the exterior of the cell membrane. The membranes
of almost all cells of the body have a calcium pump.
 Calcium serves along

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

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

8. Calcium Channels:
 Slow to become activated (longer time)
 Called Slow channels.
Sodium Channels:
 They get activated in short span of time.
 Called Fast channels.

9. 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.

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

13. Cause Of Plateau
In heart muscle, following types of channels cause
depolarization :
 Voltage-activated sodium channels (fast channels).
 Voltage-activated calcium channels (slow channels).
 Voltage-gated potassium channels.

15.  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.

18.  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.

20. 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

21.  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.

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

26. 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.

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

28.  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.

31. 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.

33.  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.

34.  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.

36.  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:

37.  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.

40.  Membrane of the axon conducts the action
potential i.e.
 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:

42.  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
contain a lipid substance Sphingomyelin. It is an
excellent electrical insulator that decreases ion
flow through the membrane.

46.  At the junction 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.

51.  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

52. 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.

56. 1. Depolarization process jumps long intervals along
the axis of the nerve fiber, this increases the velocity
of nerve transmission in myelinated fibers as much as
5 to 50 fold.
2. Conserves energy for the axon because only the nodes
depolarize, less loss of ions takes place.

57.  Requiring little metabolism for re-establishing the
sodium and potassium concentration differences across
the membrane after a series of nerve impulses.
3.In large myelinated fibers there is excellent insulation
by the myelin sheath. There is decrease in membrane
capacitance allowing repolarization to occur with very
little transfer of ions.


58. Velocity of Conduction In Nerve Fibers.
 Small unmyelinated fibers, it is about 0.25 m/sec.
 Large myelinated fibers it is as great as 100 m/ sec

61. End of the;
 lecture;