Myelinated Neuron Diagram - Structure, Functions and Myelinogenesis

Myelinated neurons are unique in that they are surrounded by a thick layer of lipoprotein, known as myelin. This layer is not continuous and is missing at regular intervals. These gaps in the myelin sheath are referred to as the nodes of Ranvier . The segment of the nerve fiber located between two nodes is called the internode .

The myelin sheath is what gives nerve fibers their white color.

Visualizing a Myelinated Neuron

The Structure of Neurons

A typical neuron consists of three parts: the nerve cell body ( soma ), axon, and dendrite. The axon and dendrite are filament-like structures of the neuron, with the axon being longer than the dendrite. The axon is responsible for transmitting impulses from the soma to the axon terminal , which then sends the impulse to another neuron.

Categorizing Nerve Fibers

The extended process of a neuron is known as the axon or nerve fiber. These nerve fibers can be categorized as follows:

1. By structure — myelinated and non-myelinated nerve fibers
2. By function — motor and sensory nerve fibers
3. By origin — spinal and cranial nerve fibers
4. By distribution — somatic and autonomic nerve fibers

The Role of the Myelin Sheath

Nerve fibers can be classified as myelinated or unmyelinated, depending on the presence or absence of the myelin sheath.

The myelin sheath is a lipoprotein layer composed of concentric layers of proteins and alternating lipids, including lecithin, cholesterol, and cerebroside. The process of forming this myelin sheath around a neuron's axon is known as ‘myelinogenesis’.

Myelinogenesis

Myelinogenesis begins during the fourth month of fetal development in the peripheral nerves and is not completed until two years after birth.

The myelin sheath is formed by Schwann cells in the neurilemma region, which is simply the cytoplasmic area surrounding the Schwann cells. Before myelinogenesis, the Schwann cells of the neurilemma are closely associated with the axolemma. This is also true for most unmyelinated neurons.

As myelinogenesis progresses, the Schwann cells wrap around and rotate in concentric layers, eventually fusing to form the myelin sheath. However, the outermost neurilemma remains within the Schwann cell. The nucleus of these Schwann cells is located between the neurilemma and the myelin sheath.

Similarly, in the central nervous system, the myelin sheath is formed by oligodendrocytes (neuroglial cells), as the neurilemma is not present in the central nervous system.

Functions of the Myelin Sheath

• The myelin sheath enables rapid conduction of impulses along the nerve fibers. In myelinated neurons, impulses jump from node to node in a process known as saltatory conduction, which increases the speed of action potential.
• The myelin sheath also has a high insulating capacity . This characteristic allows the myelin sheath to confine the nerve impulse within a single nerve fiber, preventing the stimulation of adjacent nerve fibers. This is crucial for normal sensory and motor functions.

Any damage to the myelin sheath in the nerve fiber can lead to functional deficiency. In non-myelinated neurons, the neurilemma takes the place of the myelin sheath.

Related Topics:

Human Nervous System

Difference Between Sensory and Motor Neuron

Difference between Neuron and Neuroglia

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