Properties of Nerve Fibers
EXCITABILITY
Excitability is defined as the physiochemical change that occurs in a tissue when stimulus is applied. Stimulus is defined as an external agent, which produces excitability in the tissues. Different types of stimulus, qualities of stimulus and strength-duration curve are explained. Chronaxie is an important parameter to determine the condition of nerve fiber. Clinically, the damage of nerve fiber is determined by measuring the chronaxie. It is measured by chronaxie meter. Nerve fibers have a low threshold for excitation than the other cells.
Response Due to
Stimulation of Nerve Fiber
When a
nerve fiber is stimulated, based on the strength of stimulus, two types of
response develop:
1. Action
potential or nerve impulse
Action
potential develops in a nerve fiber when it is stimulated by a stimulus with
adequate strength. Adequate strength of stimulus, necessary for producing the action potential in a nerve fiber is known as threshold or minimal stimulus. Action potential
is propagated.
2. Electrotonic
potential or local potential When the stimulus
with subliminal strength is applied,
only
electrotonic potential develops and the action potential does not develop.
Electrotonic potential is nonpropagated.
Cathelectrotonic and
Anelectrotonic Potentials
While
recording electrical potential in a nerve fiber, two electrodes, namely cathode and anode are used. The potential change that
is produced at cathode is called cathelectrotonic potential. The potential that
is developed at anode is known as anelectrotonic potential. Only the
cathelectrotonic potential can be transformed into electrotonic potential or
action potential.
ACTION POTENTIAL OR
NERVE IMPULSE
Action potential
in a nerve fiber is similar to that in a muscle, except for some minor
differences Resting membrane potential in the nerve fiber is –70 mV. The firing
level is at –55 mV. Depolarization
ends at
+35 mV . Usually, the action potential starts in the initial segment of nerve
fiber.
Properties of Action
Potential
ELECTROTONIC
POTENTIAL OR LOCAL POTENTIAL
Electrotonic
potential or local potential is a non-propagated local
response that develops in the nerve fiber when
a subliminal stimulus is applied. Subliminal or subthreshold stimulus does not
produce action
potential.
But, it alters the resting membrane potential and produces slight
depolarization for about 7 mV. This slight depolarized
state is called electrotonic potential. Firing level is reached only if
depolarization occurs up to 15 mV. Then only action potential can develop. Electrotonic
potential is a graded potential
Properties of
Electrotonic Potential
1.
Electrotonic potential is non-propagated
2. It does
not obey all-or-none law. If the intensity of
the stimulus is increased gradually every time,
there is
increase in the amplitude till the firing level is reached, i.e. at 15 mV.
VOLTAGE CLAMPING
The term ‘voltage
clamping’ refers to an experimental method that uses electrodes to alter and
control the
membrane
potential. Voltage clamp technique is a modified patch
clamp technique applied
to nerve
fibers. It is used to measure the ionic current across the membrane of nerve
fiber by fixing the
membrane
potential at a desired voltage.
Principle of Voltage
Clamping
Normally,
the voltage-gated ion channels open and close in response to positive or
negative charge within the cell. In order to understand the movement of ions across
the membrane (ion flux), it would be necessary to eliminate the other variable,
i.e. the differences in the membrane potential. It is because of two reasons:
1. Both
the ion flux and membrane potential are interrelated
2.
Differences in membrane potential would lead to differences in ion flux.
So the
membrane potential is fixed (clamped) at a specific level by using voltage
clamp. It allows study of
the ion
flux through ionic channels at specific membrane potentials.
Equipment for Voltage
Clamping
Voltage
clamp equipment has three units:
1.
Recording amplifier
2. Current
generator
3.
Feedback amplifier.
1.
Recording amplifier measures the voltage of membrane potential. Two recording
electrodes
namely,
the extracelluar electrode and intracellular
electrode are connected to this amplifier.
Extracellular electrode is placed on the outer
surface of the nerve membrane and the
intracellular electrode is inserted into the nerve fiber.
2. Current
generator or signal generator is used to
control the resting membrane potential of the
nerve
fiber. The current signals generated by this instrument are passed into the
nerve fiber through a
current
electrode.
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