Nervous tissue is the main component of the two parts of the nervous system; the brain
and spinal cord of the central nervous system, and the branching peripheral nerves of the
peripheral nervous system, which regulates and controls bodily functions and activity.
It is composed of neurons, or nerve cells, which receive and transmit impulses, and neuroglia,
also known as glial cells or more commonly as just glia, which assist the propagation
of the nerve impulse as well as providing nutrients to the neuron.
Nervous tissue is made up of different types of nerve cells, all of which having an axon,
the long stem-like part of the cell that sends action potential signals to the next cell.
Functions of the nervous system are sensory input, integration, control of muscles and
glands, homeostasis, and mental activity.
Structure
All living cells have the ability to react to stimuli.
Nervous tissue is specialized to react to stimuli and to conduct impulses to various
organs in the body that bring about a response to the stimulus.
Neurons are easily stimulated and transmit impulses very rapidly.
A nerve is made up of many nerve cell fibers bound together by connective tissue.
A sheath of dense connective tissue, the epineurium, surrounds the nerve.
This sheath penetrates the nerve to form the perineurium, which surrounds bundles of nerve
fibers . The cell body is enclosed by a cell membrane
and has a central nucleus.
Granules called Nissl bodies are found in the cytoplasm of the cell body.
Within the cell body, extremely fine neurofibrils extend from the dendrites into the axon.
The axon is surrounded by the myelin sheath, which forms a whitish, non-cellular, fatty
layer around the axon.
Outside the myelin sheath is a cellular layer called the neurilemma or sheath of Schwann
cells.
The myelin sheath together with the neurilemma is also known as the medullary sheath.
This medullary sheath is interrupted at intervals by the nodes of Ranvier.
Classification Neurons are classified both structurally and
functionally.
Neurons are grouped structurally according to the number of processes extending from
their soma.
Three major neuron groups make up this classification: Multipolar neurons
These are the most common neuron type in humans and the major neuron type in the central nervous
system.
Bipolar neurons Bipolar neurons are spindle-shaped, with a
dendrite at one end and an axon at the other.
An example can be found in the light-sensitive retina of the eye.
They also rapidly grow.
Unipolar neurons Sensory neurons have only a single process
or fibre, which branches close to the cell body into an axon and a dendrite.
A variation of this type is the unipolar brush cell.
Layers of connective tissue in the nerve A nerve contains two types of tissue: nerve
fiber and connective tissue.
Dendrites and axons with Schwann cells and myelin sheath are surrounded by connective
tissue.
A nerve fiber in the peripheral nervous system consists of an axon or long dendrite, myelin
sheath and their Schwann cells.
Peripheral sensory fibers contain long dendrites, but peripheral motor fibers have long axons.
Long dendrites of sensory fibers have structural properties as motor axons.
The three layers of connective tissue surrounding each nerve are:
Endoneurium.
Each nerve axon, or fiber is surrounded by the endoneurium, which is also called the
endoneurial tube, channel or sheath.
This is a thin, delicate, protective layer of connective tissue.
Perineurium.
Each nerve fascicle containing one or more nerve fibers, is enclosed by the perineurium,
a connective tissue having a lamellar arrangement in seven or eight concentric layers.
This plays a very important role in the protection and support of the nerve fibers and also serves
to prevent the passage of large molecules from the epineurium into a fascicle.
Epineurium.
The epineurium is the outermost layer of dense connective tissue enclosing the nerve.
Function Nerve cells are functionally made to meet
each other at a junction known as a synapse, where the terminal branches of an axon and
the dendrites of another neuron lie close to each other but normally without direct
contact.
Information is transmitted across the gap by chemical secretions called neurotransmitters.
It causes activation in the post-synaptic cell.
All cells possess the ability to respond to stimuli.
The messages carried by the nervous system are electrical signals called impulses.
Clinical significance Tumours
Neoplasms in nervous tissue include: Gliomas
Gliomatosis cerebri, Oligoastrocytoma, Choroid plexus papilloma, Ependymoma, Astrocytoma,
Dysembryoplastic neuroepithelial tumour, Oligodendroglioma, Medulloblastoma, Primitive neuroectodermal
tumor Neuroepitheliomatous tumors
Ganglioneuroma, Neuroblastoma, Atypical teratoid rhabdoid tumor, Retinoblastoma, Esthesioneuroblastoma
Nerve sheath tumors Neurofibroma, Schwannoma, Neurinoma, Acoustic
neuroma, Neuroma See also
This article uses anatomical terminology; for an overview, see anatomical terminology.
Epineurium Perineurium
Endoneurium Nerve fascicle
Connective tissue in the peripheral nervous system
References