Nerve Tissue & the Nervous system

By:

Dr. Raja Ali

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Nerve Tissue & the Nervous system II:

Myelinated and Unmyelinated Axons:

In the PNS, all axons are enveloped by Schwann cells which provide both

structural and metabolic support.

Many axons with small diameter invaginate into one Schwann cell longitudinally

and are simply surrounded by the cytoplasm of Schwann cells. They are called

unmyelinated nerve fibres.

Other axons, especially the ones with larger diameter, invaginate into the

Schwann cell and are wrapped by concentric layers of the Schwann cell plasma

membrane forming myelin sheath. These axons are called myelinated nerve

fibres (Fig.1a ).

There are gaps (areas of axon not covered by myelin) along the length

of myelin sheath at regular intervals called nodes of Ranvier.

In large myelinated axons, the nerve impulse jumps from node to node

resulting in faster conduction (salutatory conduction).

The segment of myelin between two nodes of Ranvier is called

internode.

The myelin of one internode is formed by a single Schwann cell.

The myelin sheath shows cone-shaped clefts called Schmidt-

Lantermann clefts. They are areas of remnants of cytoplasm of

Schwann cells present within the myelin sheath.

In the PNS, the myelin sheath of an individual axon is provided by

many Schwann cells lying along the length of the axon.

In the CNS, the myelin sheath is formed by processes of oligodendrocytes

Fig.(1b)

Myelination:

Myelination begins with the invagination of the axon into the Schwann

cell. The invaginated axon is suspended from the periphery of the cell by

a fold of fused plasma membrane called mesaxon (Fig.2a).

As myelination proceeds the Schwann cell and mesaxon rotates itself

around the axon several times resulting in enveloping the axon in

concentric layers of Schwann cell cytoplasm and plasma membrane

alternately (Fig.2b&2c).

With further rotation cytoplasm between the concentric layers of plasma

membrane is squeezed out and the opposing inner surfaces of the

plasma membrane fuse with each other forming myelin sheath.

Thus myelin sheath is actually composed of many layers of modified

cell membrane of Schwann cell.

Peripheral Nerve:

Each peripheral nerve (spinal or cranial) is made of bundles (fascicles) of

nerve fibres (axons) which may be myelinated and/or unmyelinated.

The bundles are held together by connective tissue which provides

structural support as well as nutritional support by carrying blood

vessels to nerve fibres.

The connective tissue framework is well appreciated in cross

section of a nerve (Fig. 3&4 ), where following structures can be

observed:

Epineurium: Dense connective tissue sheath surrounding the

entire nerve.

Perineurium: A sleeve of fl attened specialised epithelial cells

surrounding the bundles of nerve fi bres.

Endoneurium: Loose connective tissue composed of reticular

fibers supporting individual nerve fibers.

In the case of optic nerve, it is surrounded by meninges of brain

Ganglia:

Ganglia are oval bodies made of aggregation of cell bodies of neurons

outside the CNS.

They serve as relay centres in the neuronal pathway.

They are usually covered by a dense connective tissue capsule known as

epineurium.

The cell bodies of the neurons are enveloped by a layer of cuboidal cells

called satellite cells.

Two types of ganglia can be distinguished on the basis of morphology and

function; sensory and motor ganglia (Fig.5&6 ). Their distinguishing

features are enumerated in table (1):

Neuroglia (in CNS)

There are six kinds of glial cells

Neuroglia are highly branched cells that support the neurons by

occupying the spaces between them.

Providing both structural and metabolic support.

There are four principal types of neuroglia in the CNS; namely

astrocytes, oligodendrocytes, microglia and ependymal cells.

Of the four types, ependymal cells form a specialized simple low

columnar epithelium which lines the ventricles of brain and central

canal of spinal cord. The epithelium lacks a basement membrane.

The characteristic features and functions of the other three neuroglial

cells are presented in Table (2):

Glial Cell Type Location Main Functions

Oligodendrocyte Central nervous system Myelin production, electric

insulation.

Neurolemmocyte Peripheral nerves Myelin production, electric

insulation.

Astrocyte Central nervous system Blood-brain barrier,

metabolic exchanges.

Ependymal cells Central nervous system Lining cavities of central

nervous system.

Microgl ia Central nervous system lmmune-related activity

Satellite Cells Peripheral nerves Supportive role.