Properties Of Nerve Fibers Notes

Understanding the properties of nerve fibers and how they work is crucial for comprehending the overall functionality of the nervous system. In this article, we will explore the properties of nerve fibers, their classification, and the physiological mechanisms underlying their functions.

Excitability of Nerve Fibers

• Excitability is defined as the physiochemical change that occurs in a tissue when a stimulus is applied. The stimulus is defined as an external agent, which produces excitability in the tissues. The different types of stimulus, the qualities of stimulus, and the strength duration curve are explained.
• The chronaxie is an important parameter to determine the condition of nerve fibers. Clinically, the damage of the 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. When the nerve fiber is stimulated, two types of response occur based on the strength of the stimulus:

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1. Action potential or nerve impulse: Action potential develops in a nerve fiber when it is stimulated by a stimulus with adequate strength. The 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 response: When the stimulus with subliminal strength is applied, only electrotonic. potential develops and the action potential does not develop. The electrotonic potential is nan-propagated.

• Cathelectrotonic and Anelectrotonic Potentials: Cathelectrotonic potential is the potential change that is produced at the cathode. The anelectrotonic potential is the potential that is developed at anode. Only the cath electrotonic potential can be transformed into electrotonic potential or action potential.

Action Potential Or Nerve Impulse

• The action potential in a nerve fiber is similar to that in a muscle, except for some minor differences. The action potential in a skeletal muscle fiber is described.
• The 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 the nerve fiber.
• Properties of Action Potential: Properties of action potential are given.

Electrotonic Potential Or Local Response

• Electrotonic potential is a nonpropagated local potential that develops in the nerve fiber when a subliminal stimulus is applied. The subliminal or subthreshold stimulus does not produce an 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 or local response (phenomenon). The 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
  • The electrotonic potential is nonpropagated
  • 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. The 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 the study of the ion flux through ionic channels at specific membrane potentials.

Equipment for Voltage Clamping: Voltage damp equipment has three units

1. Recording amplifier
2. Current generator
3. Feedback amplifier.

 

1. The recording amplifier measures the voltage of membrane potential. Two recording electrodes namely, the exlraceiluar 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. The 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.
3. The feedback amplifier receives feedback inputs from recording amplifier and current generator and accordingly modifies the current signals that are sent into the nerve fiber. Thus, by voltage clamping, it is possible to maintain the constant membrane potential at a desired voltage.

Nerve Fibers Used for Voltage Clamping: Earlier, the voltage clamp tests were done on the giant axon of the squid Loligo, whose size facilitates such tests. Then the investigations were done on the neurons of small mammals. Nowadays, the tests are done on the human nerve fibers obtained from surgical procedures.

Conductivity of Nerve Fibers

• Conductivity is the ability of nerve fibers to transmit the impulse from the area of stimulation to the other areas. The action potential is transmitted through the nerve fiber as nerve impulse. Normally in the body, the action potential is transmitted through the nerve fiber in only one direction.
• However, in experimental conditions when, the nerve is stimulated, the action potential travels through the nerve fiber in either direction.

Mechanism Of Conduction Of Action Potential: The depolarization occurs first at the site of stimulation in the nerve fiber. It causes depolarization of the neighboring areas. Like this, depolarization travels throughout the nerve fiber. Depolarization is followed by repolarization.

Conduction Through Myelinated Nerve Fiber – Saltatory Conduction:

• Saltatory conduction is the form of conduction of nerve impulse in which, the impulse jumps from one node to another. Conduction of impulse through a myelinated nerve fiber is about 50 times faster than through a nonmyelinated fiber.
• It is because the action potential jumps from one node to another node of Ranvier instead of travelling through the entire nerve fiber.
• Mechanism of Saltatory Conduction: The myelin sheath is not permeable to ions. So, the entry of sodium from the extracellular fluid into nerve fiber occurs only in the node of Ranvier, where the myelin sheath is absent. It causes depolarization in the node, and not in the internode. Thus, the depolarization occurs at successive nodes. So, the action potential jumps from one node to another. Hence, it is called saltatory conduction (saltare = jumping).

Refractory Period

The refractory period is the period at which the nerve does not give any response to a stimulus.

Types Of Refractory Period: Refractory period is of two types

1. Absolute Refractory Period: Absolute refractory period is the period during which the nerve does not show any response at all, whatever may be the strength of stimulus.
2. Relative Refractory Period:
   • It is the period, during which the nerve fiber shows response, if the strength of the stimulus is increased to maximum.
   • The absolute refractory period corresponds to the period from the time when the firing level is reached till the time when 1/3 of repolarization is completed. The relative refractory period extends through rest of the repolarization period.

Relative Refractory Period Summation

• When one subliminal stimulus is applied, it does not produce any response in the nerve fiber because, the subliminal stimulus is very weak.
• However, if two or more subliminal stimuli are applied within a short interval of about 0.5 msec, the response is produced, it is because the subliminal stimuli are summed up together to become strong enough to produce the response. This phenomenon is known as summation.

Relative Refractory Period Adaptation

• While stimulating a nerve fiber continuously, the excitability of the nerve fiber is greater in the beginning.
• Later the response decreases slowly and finally, the nerve fiber does not show any response at all. This phenomenon is known as adaptation or accommodation.
• The causes for adaptation are:
  1. When a nerve fiber is stimulated continuously, depolarization occurs continuously
  2. The continuous depolarization inactivates the sodium pump and increases the efflux of potassium ions.

Relative Refractory Period Infatigability

A nerve fiber cannot be fatigued, even if it is stimulated continuously for a long time. The reason for this is the nerve fiber can conduct only one action potential at a time. At that time, it is completely refractory and does not conduct another action potential.

All Or None Law: All or none law states that when a nerve is stimulated by s stimulus it gives maximum response or does not respond at all.