Properties of Nerve Fibers

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.