Nerves and muscles are excitable tissues that respond to electrical, chemical, or mechanical stimuli. The action potential in nerves occurs in four stages: resting stage with a polarized negative membrane potential; depolarization where sodium channels open allowing sodium ions to enter rapidly; overshoot where some fibers see the potential become briefly positive; and repolarization where potassium channels open returning the membrane to the negative resting potential. Voltage-gated sodium channels have activation and inactivation gates that open and close the channel during an action potential. The all-or-none principle states that when a nerve is stimulated, it will either respond at maximum strength or not at all.

1. By
FATIMA SUNDUS

2. Nerves and muscles are called excitable tissues because they respond to:
a) Chemical.
b) Mechanical.
c) Electrical stimuli.

5. Resting Stage.
 The resting stage is the resting membrane potential
before the action potential begins. The membrane is
said to be “polarized” during this stage because of the
–90 millivolts negative .Membrane potential that is
present

6. Depolarization Stage.
 At this time, the membrane suddenly becomes very
permeable to sodium ions, allowing tremendous
numbers of positively charged sodium ions to diffuse
to the interior of the axon. The normal “polarized”
state of –90 millivolts is immediately neutralized by
the inflowing positively charged sodium ions, with the
potential rising rapidly in the positive direction. This is
called depolarization.

7. Overshoot
 In large nerve fibers ,the great excess of positive
sodium ions moving to the inside causes the
membrane potential to actually “overshoot” beyond
the zero level and to become somewhat positive.
 In some smaller fibers, as well as in many central
nervous system neurons, the potential merely
approaches the zero level and does not overshoot to
the positive state

8. Repolarization Stage.
 the sodium channels begin to close and the
potassium channels open more than normal. Then,
rapid diffusion of potassium ions to the exterior re-
establishes the normal negative resting membrane
potential. This is called repolarization of the
membrane.

9. Voltage-Gated Sodium Channel—Activation
and Inactivation of the Channel
This channel has two gates—one near the outside of the
channel called the activation gate, and another near
the inside called the inactivation gate.

10. Voltage-Gated Sodium and
Potassium Channels

11. ALL-OR-NONE PRINCIPLE
When nerve is stimulated by a stimulus it gives maximum response or
does not give response at all: