Properties and Classification of Nerve Fibres

Nerve fiber, also known as an axon, is a long, thin projection found on nerve cells (neurons) responsible for nerve impulses’ conduction through the nerve cell body. Nerve fibers show properties including conductivity, velocity, refractive period, summation, and accommodation. They can sense stimuli and carry electrical impulses. Nerve fibers are classified based on conduction velocity, size, and presence or absence of myelin sheath into groups A, B, and C.

In this article, we will study in detail Nerve Fibres, their structural properties, the Conduction of nerve impulses, the Numerical classification of nerve fibers, and nerve fibers in invertebrates.

What are Nerve Fibers?

Nerve fibers (Axon) are a long extension of the cellular body, of various sizes. It is covered by a protected sheath known as neurolemma. The Nodes of Ranvier are the lengthwise gap between the myelin sheaths, and the side branches of axons are known as collaterals. The swollen end is composed of neurotransmitters that conduct nerve impulse transmission. The nerve and nerve fibres are the parts of the nervous system.

The nervous system is formed by the network of the brain and nerves. Nerves carry signals from the brain to different parts of the body. These are responsible for various sensations and impulses. The nervous system controls involuntary activities e.g. breathing and digesting food.

Structural Properties of Nerve Fibers

The structural properties of nerve fibers show anatomical and physiological characteristics. The anatomical properties help in understanding the structure of nerve fibers. The physiological properties help in understanding the functional mechanism of nerve fibers.

Anatomy of Nerve Fibers

• The three-layered bilipid membrane of the nerve fiber regulates the transmission of signals. Their primary role is to Transmit the information from neurons to Schwann cells.
• Cytoplasm found in nerve fibres is known as axoplasm. It comprises cell organelles like mitochondria, microtubules, and microfilaments.
• Telodendria or the end portion of Axon terminals, establishes a connection between two neurons to form synaptic junctions.
• Telodendria is also commonly known as the termination point of nerve fibres. The enlarged ends of telodendria are known as axon terminals.
• Synaptic gaps are spaces that are formed between the axon terminals of one neuron and the dendrite of another neuron but are not connected.
• A single nerve fiber with its multiple branches transmits signals to different parts of the brain forming multiple synaptic ends.
• The nerve fibres vary in length and diameter from millimeters to metres.
• An insulated fatty layer called myelin sheath is composed of oligodendrocytes and Schwann cells, that envelope the nerve fibres. Certain nerve fibres lack myelination.
• An axon hillock is the emerging point of nerve fiber from the nerve cells’ bodies that aids the differentiation of nerve fiber from other parts of the neuron, it also initiates action potentials.
• Short, unmyelinated segments known as nodes of Ranvier are scattered along nerve fibres, and their main function is to generate action potentials.

Physiology of Nerve Fibres

Physiology of nerve fibres is as follows:

• The excitement of nerve fibres: Nerve fibres have exceptionally excitable structures that may produce electrical impulses in response to various stimuli.
• Conduction of impulse: Synaptic connections enable the electrical impulses produced by nerve fibres to traverse across the entire fiber length and to various neurons, muscles, and glands.
• Defiant Period: During this time, nerve fibres can conduct a single action potential at a time. This is because the fibres excitability reduces during conduction, and no new electrical impulse can be produced. Even with constant stimulation, the nerve fibres do not become tired.
• The reaction of nerve fibres: A nerve fibre interprets an impulse in one of two ways fully or partially. An action potential is produced when a stimulus is administered up to a threshold level; the action potential is unaffected by the stimulus’s increased strength. Applying a stimulus below the threshold prevents the production of an action potential. On the contrary, an action potential is produced when several sub-threshold stimuli are delivered quickly one after the other.

Conduction of Nerve Impulse

During the resting Phase, the nerve fibres have a +ve charge on the outer surface i.e. polarised state. The polarization is due to presence of Na⁺ ions on the outer surface. On stimulation, the axon terminal permits the inward movement of Na⁺ ions which causes depolarization. The area is known as the excited region. The depolarised region stimulates the neighboring region as a result it depolarises.

Meanwhile, Na+ ions actively transport to the outside membrane and cause polarization of the depolarised area. This sequential activity reaches the terminal end of the axon which stimulates the synaptic vesicle to release neurotransmitters. these neurotransmitters then reach the dendrite of the next neuron and in this way conduction of the nerve impulse take place.

Classification of Nerve Fibres

Nerve fibers, which are extensions of nerve cells or neurons, can be classified based on various criteria, including myelination, diameter, and conduction speed. It help in understanding the functions and properties of nerve fibers, which are essential for the communication that occurs within the nervous system. The most common classifications based on myelination, diameter, and conduction speed are:

Based on Myelination

Based on myelination, nerve fibres are classified into 2 types:

• Myelinated Nerve Fibres: These fibres have a myelin cover, which is a layer of protective insulation that surrounds the nerve. Myelination increases the speed of nerve impulses.
• Unmyelinated Nerve Fibres: These fibres lack a myelin cover. They conduct nerve impulses more slowly compared to myelinated fibres and are typically found in the autonomic nervous system and parts of the central nervous system.

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

Based on diameter and conduction speed, nerve fibres are classified into 3 types:

• Type A Fibres: These are the largest and fastest conducting fibres, divided into Aα, Aβ, Aγ, and Aδ fibres. They include motor neurons and sensory fibers such as those for touch.
• Type B Fibres: These are myelinated fibres with an intermediate diameter and conduction speed. They are involved in the transmission of signals related to the autonomic nervous system.
• Type C Fibres: These are the smallest and slowest conducting fibers, and they are unmyelinated. They carry pain and temperature sensations in the autonomic nervous system.

Based on Function

Based on its function, nerve fibres are classified into 2 types:

• Sensory Nerve Fibres: These fibres carry sensory information from the body to the central nervous system. Also called Afferent nerve fibres and they are pseudounipolar in nature.
• Motor Nerve Fibres: These fibres transmit motor information from the central nervous system to muscles and glands. Also called efferent nerve fibres and they are multipolar in nature.

Numerical Classification of Nerve Fibers

The nerve fibres were previously into 3 groups based on their fiber type. Modern research has added two groups in the previous classification. The five groups are as follows:

Group	Fibre type	Sensory Receptor
Ia	Aα	Muscle spindle
Ib	Aα	Golgi tendon corpuscle
II	Aβ	Muscle spindle, touch, pressure
III	Aδ	Touch, pain, cold
IV	C	Temperature, pain and other receptors

Nerve Fibres in Invertebrates

The nerve fibres in invertebrates show two types of characteristics. The lower invertebrates have simple nerve fibers. The higher invertebrates show giant nerve fibers because of its large diameter. In invertebrates, the nerve fibres have the longest axon. The giant nerve fibers are capable of rapid conduction of the nerve impulse due to the large diameter and this causes extremely rapid movement in the invertebrates. The giant fibers are found in earthworms, insects and the largest axon is found in the giant squid which is highly developed. In some pelagic penaeid shrimps, the conduction velocity is highest at 210 m/s.

FAQs – Properties and Classification of Nerve Fibres

1. What are the Classifications of Nerve Fibres?

The Nerve fibres are classified into five types as follows: Based on Conduction of impulse and Diameter, Presence of Myelin Sheath, Functional relation to the central nervous system, Conduction of impulse, Type of neurotransmitter released.

2. What is an Action Potential?

A signal generated by a rapid rise and fall of voltage across a cellular membrane is termed as action potential.

3. What is the Characteristic of Nerve Fiber?

The nerve fibre shows characteristics like excitability to impulse, conductivity of the nerve signal, refractory period, response or no response, and summation of an action potential.

4. What is the Structure of a Nerve Fibre?

The nerve fibers has three layers of lipid membrane which controls the signal transmission. The cytoplasm in nerve fibers is called axoplasm. It contains mitochondria, microtubules, and microfilaments. The end part of axon terminals is called telodendria.

5. What are the Four Types of Nerve Fibres?

Group A nerve fibers are differentiated into four types: alpha (Aα), beta (Aβ), gamma (Aγ), and delta (Aδ).