Properties and Classification of Nerve Fibres

Table of Contents

What are Nerve Fibres?

A nerve fibre, also called an axon, is a long and slender projection of nerve cells (or neurons) that carry electrical impulses away from the nerve cell body. A neuron typically has one nerve fibre emanating from its cell body that transmits impulses to other neurons, muscles or glands.

Dysfunctioning of the nerve fibre can cause major acquired and inherited neurological disorders that affect both the central nervous system and the peripheral nervous system.

Anatomical Properties of Nerve Fibres

• The nerve fibres are covered by a cell membrane called axolemma. It is a three layered bilipid membrane that specifies what goes in and out of the nerve cells. Its prime responsibility is to relay signals from the neurons to Schwann cells.
• The cytoplasm of the nerve fibres is known as axoplasm. It has a high concentration of microtubules, microfilaments and mitochondria.
• Telodendria are the end branches of a nerve fibre.
• The telodendria has swollen ends called axon terminals that join one neuron to another forming a synaptic junction.
• A single nerve fibre with its multiple branches innervates multiple parts of the brain and generates multiple synaptic terminals.
• The nerve fibres can be as small as a few millimetres or can extend upto one metre long. The longest nerve fibre of the human body is the sciatic nerve that extends from the spinal cord to the big toes of both feet.
• The diameter of the nerve fibre is also variable ranging from about one micrometre to upto 20 µm.
• The nerve fibres are covered by a layer of insulating fatty substance called myelin that is composed of Schwann cells and oligodendrocytes. Some of the nerve fibres are unmyelinated as well.
• The area from where the nerve fibres extend from the body of the nerve cells is called axon hillock.
• The axon initial segment (AIS) is a separate functional microdomain of the nerve fibres that separates nerve fibres from the rest of the neuron and also helps in initiating action potentials.
• Nodes of Ranvier are short unmyelinated segments interspersed in the nerve fibre that serve as the site of action potential generation.

Physiological Properties of Nerve Fibres

• Excitability: The nerve fibres are highly excitable structures that respond to several stimuli and can also generate electrical impulses.
• Conductivity: The electrical impulses generated in the nerve fibres are propagated along its entire length and to different neurons, muscles and glands by synaptic connections.
• Refractory Period: The nerve fibres can conduct one action potential at once, i.e., the excitability of the fibres is less during conduction and hence a new electrical impulse cannot be generated.
• The nerve fibres are not fatigued even when they are stimulated continuously.
• All or None Response: A nerve fibre translates either all of the impulse or none at all. If a stimulus is applied upto a threshold level, an action potential will be generated but increasing the strength of the stimulus will not affect the action potential.
• Summation: If a sub-threshold stimulus is applied, it cannot generate an action potential. However, when multiple sub-threshold stimuli are applied in rapid succession, an action potential is generated.

Classification of Nerve Fibres

On the Basis of Conducting Velocity and Diameter Relation

Erlanger and Gasser, two American physiologists classified nerve fibres on the basis of the mutual relation between the diameter of a nerve fibre and its nerve conduction velocity. They then grouped the nerve fibres into three major groups: group A, group B and group C.

• Group A Nerve Fibres: Group A nerve fibres are heavily myelinated nerve fibres that are further subdivided into four types: alpha Aα; beta Aβ; gamma Aγ; and delta Aδ. The fibres with larger diameters and more myelination tend to transmit the impulses at a faster rate.
• Group B Nerve Fibres: Group B nerve fibres are less myelinated than group A, but more myelinated than group C nerve fibres. They include visceral nerves such as the vagus nerve.
• Group C Nerve Fibres: Group C nerve fibres are unmyelinated fibres that usually have a smaller diameter and low conduction velocity.

On the Basis of Presence of Myelin Sheath

There are both myelinated and unmyelinated nerve fibres in the nervous system. Both the types of nerve fibres differ in their relative composition. Let us look at them.

• Myelinated Nerve Fibres: Myelinated nerve fibres are covered by a layer of insulating sheet called myelin sheath. In the peripheral nervous system, the myelin sheath is formed by the Schwann cells whereas in the central nervous system, the myelin sheaths are formed by the oligodendrocytes.
• Non Myelinated Nerve Fibres: Nonmyelinated nerve fibres are covered by cytoplasm of Schwann cells but the myelin is not secreted in such cases. They are commonly found in the autonomic nervous system.

On the Basis of Functional Relation to the Central Nervous System

On the basis of functional relation to the central nervous system, the nerve fibres are divided into afferent and efferent fibres. Let us look at them separately.

• Afferent Nerve Fibres: The peripheral nerve fibres receive impulses from different receptors of the body and transmits them to the central nervous system, these types of fibres are called afferent nerve fibres. These fibres are pseudounipolar in nature.
• Efferent Nerve Fibres: The fibres that carry nerve impulses away from the central nervous system to other effector organs such as glands and muscles are called efferent nerve fibres. Morphologically, they are multipolar in nature.

Numerical Classification of the Nerve Fibres

Later research in this region led to the discovery of another two groups of fibres that were sensory fibres. An alternative numerical classification of the sensory nerve fibres was given which is tabulated below:

Nerve Fibres in Invertebrates

Nerve fibres or axons are well studied in invertebrates as well. The longfin inshore squid is known to have the longest axon. The largest axon is found in the giant squid. The conduction velocity of some pelagic Penaeid shrimps has been found to be highest at 210 m/s.

Keep exploring BYJU’S Biology for more information.

Also see:

• An Overview of Neuron, Nerves and Nervous System
• Conduction and Transmission Of Nerve Impulses