Properties and Classification of Nerve Fibres

Key Highlights

• The four types of nerve fibres that mediate sensory, motor, and touch sensation include alpha Aα, beta Aβ, gamma Aγ and delta Aδ.
• Fibres with a myelin sheath are known as myelinated fibres, whereas unmyelinated fibres lack a myelin sheath.
• The important properties of nerve fibres include conductivity, refractive period, accommodation and summation.
• These fibres are classified based on velocity, diameter and central nervous system.
• Damage to nerve fibers may occur from trauma, inflammation, infection, or reduced cerebral blood supply. 

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The nerve fiber is a slender, long projection of a nerve cell or neuron that normally transports electrical impulses called action potentials away from the nerve cell body in vertebrates. 

• It is divided into three types, namely group A nerve fibres, group B nerve fibres, and group C nerve fibres. 
• The myelin sheath, the protective covering of these fibers, can also be damaged by aging, disease, or exposure to certain toxins.

Key Terms: Nerve Fibres, Axons, Neurons, Nerve Impulse, Myelin Sheath, Nerve Cell, Properties and Classification of Nerve Fibres, Central Nervous System, Glands

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What are Nerve fibres?

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Nerve fibres are thin extensions of nerve cells covered by a protected sheath called neurolemma. 

• It is used for the transmission of information to different muscles, neurons, and glands.
• Nerve fibres, also known as axons, travel outside the cell body to carry nerve impulses. 
• These fibres are part of cytoplasmic protrusions from the cell body and dendrite.
• The electrical impulse would travel from the periphery to the cell body and from the cell body to the spinal cord using another branch of the same nerve fibres.
• It is part of the nervous system, which is composed of neurotransmitters that help in nerve impulse transmission. 
• Nerve fibre injury can occur in the early stages of neurodegenerative disorders such as motor neuron disease (MND), Alzheimer's disease, and Parkinson's disease.

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Structural Properties of Nerve Fibres

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The structural properties of nerve fibres are divided into anatomical and physiological properties.

Anatomy of Nerve Fibres

The anatomical and functional components of the peripheral nervous system (PNS) explain the structure of nerve fibres. The PNS is made up of collections of unique, specialised cells known as neurons. 

• A nerve cell is composed of three parts: the cell body (soma), the axon (a single, long branch), and dendrites.
• Axoplasm, also called cytoplasm, is made up of microfilaments, microtubules, mitochondria, and other cell organelles.
• Several mitochondria are present in the axon terminals that participate in the secretion of neurotransmitters.
• Telodendria forms the termination point of nerve fibres.
• A coating rich in lipids and proteins called the myelin sheath surrounds some fibres, helping to speed up the conduction of electrical impulses. 
• The myelin sheaths, which are called nodes of ranvier, help generate various cells with gaps between fibres. 
• The mechanism by which the nerve fibre becomes myelinated is known as myelinogenesis. 
• Schwann cells contribute to myelinogenesis in the peripheral nervous system.
• The diameter of the nerve fibre ranges from one micrometre to up to 20 µm.
• Axon hillock refers to the area generated from the nerve fibres, which extend from the nerve cells.
• Oligodendroglial cells are responsible for myelinogenesis in the central nervous system. 
• The longest nerve fibre in the human body is the sciatic nerve, which originates from the spinal cord and runs to the big toes on both feet.

Physiology of Nerve Fibres

The physiology of nerve fibres are as follows:

Excitability

These nerve fibres can perceive stimuli and switch to an active state as their membranes are polarised. Neural excitability is mostly determined by sodium membrane, which can be affected by drugs or the ionic composition of extracellular fluids. 

Conductivity

Electrical signals transmitted by nerve cells are collectively referred to as nerve conduction.

Refractory Period

Nerve fibres are capable of conducting one potential action at a time, which means the time of excitability can be reduced during conduction, and as a result , new electrical impulses cannot be generated.

All or None Law

The law states that a nerve will either convey an impulse down its length or not at all. When a stimulus reaches a threshold level, an action potential is generated, which remains unaffected by the strength of the stimulus. Insufficiency of nerve impulses is associated with neurological diseases such as multiple sclerosis, seizures, etc.

Summation

In summation, when a subthreshold stimulus is applied, it will not generate an action potential. However, when we apply multiple subthreshold stimuli, an action potential is generated in rapid succession.

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Conduction of Nerve Impulse

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When cells are in the resting phase then nerve fibres will create a positive polarised charge on the outer surface of the periphery. 

• The polarization of cells is due to the presence of sodium ions found in the nervous system. 
• When stimulation is initiated, the sodium ion will move towards the axon terminal for depolarisation. 
• Sodium ion is transportated to the outer membrane which causes polarization of the depolarised area. 
• Synaptic vesicles will release neurotransmitters that will move to the dendrite of the next neuron, thus initiating the conduction of the nerve impulse.

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Classification of Nerve Fibres

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Nerve fibres are classified on the basis of factors such as myelination, diameter, and conduction speed.

Based on Myelination

Nerve fibres are divided into two categories on the basis of presence of Myelin Sheath

Myelinated Nerve Fibres

Myelinated nerve fibres refer to the neurons in the somatic nervous system responsible for controlling the body's voluntary skeletal muscle movement. 

• A fatty insulating layer known as the myelin sheath surrounds these axons. 
• In the peripheral nervous system, they are formed by the Schwann cells.
• However in the case of the central nervous system, the myelin sheaths are formed by the oligodendrocytes.

Unmyelinated Nerve Fibres

Unmyelinated nerve fibres refer to the neurons in the autonomic nervous system, which control the involuntary contraction of smooth muscles. It includes the heart, blood arteries, and intestines.

Based on Diameter and Conduction Speed (A, B, C fibers)

Nerve fibres are categorised into three categories based on the basis of conducting velocity and diameter relation:

Type A Fibres

These fibres are further classified as Aα (A Alpha), Aβ (A Beta), Aγ (A Gamma), and Aδ (A Delta). Type A fibres belong to the category of the largest and fastest conducting fibres. They consist of touch-sensitive nerves and motor neurons.

Type B Fibres

Myelinated fibres having an intermediate diameter and speed are referred to as Type B fibres. They play an essential part in the autonomic nervous system's signal transduction. These fibres also include visceral nerves such as the vagus nerve.

Type C Fibres

Type C fibres are the slowest fibres in terms of speed. They use the autonomic nervous system to transmit sensations such as temperature and pain feelings.

The tabulation of nerve fibres on the basis of conduction velocity and diameter are as follows:

Based on Function with Central Nervous System 

The primary function of nerve fibre includes transmitting signals from neurons to muscles and glands. An axon is a neuron's direct link to another neuron. It helps in establishing connections with three different kinds of neurons, which are as follows:

Sensory Neurons

Sensory neurons are the nerve cells that transmit information to our five senses: touch, smell, hearing, sight, and touch-related feelings. They are also known as afferent nerve fibres, which are pseudounipolar in nature.

Motor Neurons

Motor neurons control glandular and muscular activity. They are also responsible for controlling an individual's movement, both voluntary and involuntary. They are also known as efferent nerve fibres, which are multipolar in nature.

Interneurons

Interneurons are responsible for creating an arrangement of neurons called a neural circuit. These neurons also help establish connections between nerve cells in the same area of the spinal cord or brain. 

Classification of Nerve Fibres

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Numerical Classification of Nerve Fibers

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According to modern research, nerve fibres are classified on the basis of numbers which are as follows:

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Nerve Fibres in Invertebrates

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Axons are also found in invertebrates and are generally considered the longest axon. The nerve fibres in invertebrates showcase two properties, which are as follows:

• The lower invertebrates have simple types of nerve fibres, whereas the higher invertebrates have giant nerve fibres. 
• Due to their large diameters, the giant nerve fibers create rapid conduction of the nerve impulse which in turn creates rapid movement in the invertebrates. 
• Some examples of giant fibres include earthworms, squid and insects.
• The conduction velocity of pelagic penaeid shrimps is 210 m/s.