Download Myelination

Histology:2nd stage
2015-2016
Dr. Raja Ali
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Nerve Tissue & the Nervous system:
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 SchmidtLantermann 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) .
Fig.(1 ):Myelin in the Peripheral and Central Nervous Systems
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.
Fig.(2): Stages of myelin formation in PNS.
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.
Fig.(3): Peripheral nerve.
Fig.(4): Optic nerve.
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):
Table (1)::Differences between sensory and motor ganglia.
Fig.(5): Spinal ganglion.
Fig.(6): Sympathetic ganglion.
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):
Table (2): Distinguishing features of astrocytes, oligodendrocytes and
microglia
 There are two type of neuroglia in the PNS:
Shwann cells &stellate cells
Figurc (1). Glial cells of the CNS and PNS
 The principal functions of neuroglia are summarized in table (3)
Table (3): Principal functions of neuroglial cells.
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.