Properties, Classification and Applied Aspects of Nerve Fibers

Introduction

• Nerve fibers are highly excitable structures that conduct impulses rapidly without fatigue and obey the all-or-none principle. Their properties vary with diameter and velocity, while injury leads to degeneration, altered sensitivity, and regeneration supported by neurotrophic factors.

Properties Of Nerve Fibers

• Excitability is the ability to respond to stimuli and generate impulses.
• Conductivity is the ability to transmit impulses along the fiber.
• Nerve fibers show unfatigability during repeated activity.
• They exhibit a refractory period after activation.
• They follow the all-or-none law.
• They show summation and accommodation under specific conditions.

Excitability

• Excitability is the ability of nerve fibers to respond to internal or external stimuli. It results from changes in ionic balance across the cell membrane.
• Nerve fibers are highly excitable and respond to mechanical, thermal, chemical, and electrical stimuli.
• Electrical stimuli are preferred experimentally due to precise control of intensity and frequency.
• Excitation is indicated by generation of a depolarization wave or action potential.

Factors Affecting Excitability

• The strength and duration of a stimulus determine the response.
• Decreased extracellular calcium increases excitability by lowering threshold.
• Increased extracellular calcium stabilizes the membrane and reduces excitability.

Conductivity

• Conductivity is the ability to transmit action potentials along the nerve fiber.
• The impulse travels from the point of origin to the axon terminal without loss.

Orthodromic and Antidromic Conduction

• Normally, impulses travel from the cell body toward the axon terminal, called orthodromic conduction.
• Conduction in the reverse direction is called antidromic conduction.
• Antidromic conduction may occur in special situations such as axon reflexes.

Summation

• A single subthreshold stimulus does not produce an action potential.
• Repeated subthreshold stimuli can combine to reach threshold.
• This property is called summation and leads to excitation.

Accommodation

• Accommodation is a decrease in excitability during prolonged stimulation. It results from a gradual rise in threshold.
• Nerve endings show a similar process called adaptation.
• Accommodation reduces impulse transmission during continuous stimulation.

Unfatigability

• Nerve fibers are unfatigable and can function continuously.
• They primarily conduct impulses rather than performing mechanical work.
• Energy consumption is minimal during impulse transmission.
• These properties ensure efficient and reliable nerve function and signal transmission.

Classification Of Nerve Fibers

• Nerve fibers are classified based on function, myelination, diameter, and conduction velocity.
• Functionally, they are divided into motor, sensory, and secretomotor fibers.
• Structurally, they are classified as myelinated and unmyelinated fibers.
• The most widely used system is the Erlanger–Gasser classification.

Erlanger-Gasser Classification

• This classification is based on fiber diameter and conduction velocity.
• Nerve fibers are grouped into Type A, Type B, and Type C.

Type A Nerve Fibers

• Type A fibers are large, myelinated, and conduct impulses rapidly.
• Conduction velocity ranges from 70 to 120 meters per second.
• Diameter ranges from 12 to 20 micrometers.
• Subtypes include:

1. A alpha fibers supply skeletal muscles and carry proprioceptive signals.
2. A beta fibers transmit touch and pressure sensations.
3. A gamma fibers supply muscle spindles.
4. A delta fibers carry fast pain and temperature sensations.

Type B Nerve Fibers

• Type B fibers are myelinated but smaller in diameter.
• They are mainly preganglionic autonomic fibers.
• Diameter is less than 3 micrometers.
• Conduction velocity ranges from 3 to 15 meters per second.

Type C Nerve Fibers

• Type C fibers are unmyelinated and conduct impulses slowly.
• Diameter ranges from 0.3 to 1.3 micrometers.
• Conduction velocity ranges from 0.5 to 2 meters per second.
• They include sensory fibers and postganglionic sympathetic fibers.
• This classification helps in understanding functional properties and clinical behavior of nerve fibers.

Table 24.1: Classification of nerve fibers.

Fiber Type	Diameter (μm)	Velocity (m/s)	Primary Function
Aα fibers	12–20	70–120	Somatic motor control and proprioception
Aβ fibers	5–12	30–70	Transmission of touch and pressure
Aγ fibers	3–6	15–30	Motor supply to muscle spindles
Aδ fibers	2–5	12–30	Fast pain, cold, and touch sensations
B fibers	< 3	3–15	Preganglionic autonomic conduction
C fibers (sensory)	0.4–1.2	0.5–2	Slow somatic sensations
C fibers (sympathetic)	0.3–1.3	0.7–2.3	Postganglionic autonomic activity

Numerical Classification

• Sensory nerve fibers are classified as Type Ia, Ib, II, III, and IV.
• This system correlates with fiber diameter, conduction velocity, and functional properties.

Table 24.2: Numerical classification of sensory fibers.

Type	Source / Receptor Origin	Equivalent Fiber Group
Type Ia	Primary muscle spindle afferents from annulospiral endings	A alpha fibers
Type Ib	Afferents from Golgi tendon organs	A alpha fibers
Type II	Secondary spindle endings and touch–pressure receptors	A beta fibers
Type III	Receptors for fast pain and cold	A delta fibers
Type IV	Receptors for slow pain and temperature	C fibers

Classification by Susceptibility to Various Agents (Hypoxia, Pressure and Local Anesthetics)

• Hypoxia: Type B fibers are most sensitive, whereas Type C fibers are least affected.
• Pressure: Type A fibers are highly susceptible due to larger diameter, while Type C fibers are resistant.
• Local anesthetics: Type C fibers are most sensitive, whereas Type A fibers are least affected.

Applied Aspects

Demyelinating Diseases

Multiple Sclerosis

• Multiple sclerosis is characterized by patchy destruction of myelin in the central nervous system, which slows or blocks impulse conduction.
• Commonly affected pathways include the pyramidal tracts, cerebellar pathways, medial longitudinal fasciculus, optic nerve, and posterior columns.
• Clinical manifestations depend on lesion location and extent.
• Sensory disturbances include reduced sensation, abnormal sensations, or neuropathic pain.
• Motor features include muscle weakness, fatigue, and increased muscle tone leading to spasticity.
• Visual involvement often presents as optic neuritis or double vision.
• Impaired coordination results in ataxia and gait instability.

Demyelinating Form of GBS

• This condition involves immune-mediated destruction of peripheral nerve myelin.
• Loss of myelin leads to markedly reduced nerve conduction velocity and conduction block.
• Patients typically present with progressive, symmetrical weakness and reduced reflexes.

Nerve Injury

• After nerve transection, the distal segment undergoes Wallerian degeneration due to loss of connection with the cell body.
• The proximal segment forms a growth cone and attempts regeneration toward the distal segment.
• The neuron detects injury through interruption of retrograde transport from axon terminals.

Grading of Nerve Injury

• Nerve injuries are classified into five degrees based on severity and structural disruption.
• First-degree injury involves transient conduction block due to mild compression and local ischemia.
• Recovery is complete and occurs within hours to weeks without structural damage.
• Second-degree injury shows axonal damage with preserved endoneurial tube, allowing guided regeneration.
• Functional recovery is delayed but usually complete due to intact supporting structures.
• Third-degree injury includes disruption of axons and endoneurium, leading to incomplete and disorganized regeneration.
• Fourth-degree injury involves severe damage with loss of fascicular architecture, preventing effective regeneration.
• Fifth-degree injury represents complete nerve transection with early degenerative changes.
• Surgical intervention is often required for higher-degree injuries to restore continuity and function.

Degenerative Changes (Wallerian Degeneration)

• Wallerian degeneration refers to progressive structural breakdown in the distal segment of a severed axon. It occurs after loss of continuity between the axon and its cell body.
• Synaptic transmission at the distal terminals ceases within a few hours after injury.
• The axon initially undergoes swelling, and nerve terminals retract from the postsynaptic target.
• Over several weeks, the distal axon and its terminal branches undergo complete degeneration.
• In myelinated fibers, the myelin sheath fragments into bead-like segments due to lipid breakdown.
• Myelin-forming cells remain viable despite structural disintegration.
• Cellular debris is removed by phagocytic cells, including macrophages and supporting glial cells.
• Clearance of debris is faster in the peripheral nervous system than in the central nervous system.
• The distal segment retains the ability to conduct impulses for up to three days after injury.
• Between the third and fifth day, conduction progressively declines.
• After approximately five days, action potential generation becomes impossible in the distal segment.
• The neuron also shows proximal changes termed retrograde degeneration.
• The cell body becomes swollen and assumes a rounded appearance.
• The nucleus enlarges and shifts toward an eccentric position.
• Rough endoplasmic reticulum redistributes toward the periphery of the cell body.
• Ribosomes become disorganized, reflecting altered protein synthesis.
• Chromatolysis occurs, characterized by dissolution of Nissl substance and reduced staining affinity.
• These coordinated changes prepare the neuron for potential regeneration if structural continuity is restored.

Regenerative Changes

• Following injury, the neuron initiates repair by increasing synthesis of structural proteins within the cell body.
• This response, called the axonal reaction, leads to distension of rough endoplasmic reticulum cisterns.
• Chromatolysis is reversible if the neuron survives and reconnects with its target.

Axonal Changes

• The proximal axon forms multiple sprouts in an attempt to re-establish continuity.
• Each sprout elongates by developing a growth cone at its advancing tip.
• Surviving Schwann cells proliferate and align to form guiding pathways for regrowth.
• Among several sprouts, usually one successfully traverses the pathway and reinnervates the original target.
• Axonal regeneration occurs at an approximate rate of one to four millimeters per day.
• Schwann cells subsequently form a new myelin sheath around the regenerated axon.

Somatic Changes

• The neuronal cell body gradually returns to its normal size and shape.
• Nissl substance reappears as protein synthesis stabilizes.
• Cellular organelles reorganize and regain normal distribution.
• The nucleus shifts back to a central position within the soma.

Changes in Target Structure

• Denervation reduces neurotransmitter release at the synaptic terminal.
• Reduced neurotransmitter levels cause upregulation of receptors on the target tissue.
• After reinnervation, the target exhibits an exaggerated response, termed denervation hypersensitivity.

Factors Influencing Regeneration

• Successful nerve regeneration depends on injury severity, condition of the cell body, site of injury, and availability of growth-promoting factors.

Severity of Injury

• A small gap between proximal and distal segments favors accurate axonal regrowth.
• When the gap exceeds about three millimeters, regenerating sprouts may intermix and form a disorganized mass.
• This swelling, called a neuroma, prevents effective reconnection with the target.
• Neuromas involving sensory fibers may produce localized pain on stimulation.

Condition of Soma

• Survival of the neuron is essential for regeneration to occur.
• Injury close to the soma leads to extensive loss of cytoplasm and membrane.
• Severe proximal damage often results in neuronal death rather than recovery.
• If the soma remains viable, synthetic activity increases to support regrowth.

Location of Injury

• Regeneration is more successful in the peripheral nervous system than in the central nervous system.
• In peripheral nerves, Schwann cells create continuous pathways that guide axonal sprouts.
• In the central nervous system, supporting cells do not provide effective guidance channels.
• Glial scar formation further obstructs the advance of the growth cone.
• As a result, functional recovery in central pathways is limited.

Neurotrophins

• Neurotrophic factors promote survival and directional growth of regenerating axons.
• Adequate levels enhance elongation of sprouts and improve target reinnervation.
• These factors also reduce disorganized sprouting and improve functional outcomes.