nervous system

).Nervous Tissue (system-4
Is divided into
1- The central nervous system (CNS),
consisting of the brain and the spinal cord
2-The peripheral nervous system (PNS) ,
composed of nerve fibers and nerve cells
which called nerve ganglia or neurons .

. Neurons
Are anatomic and functional units consist of
three parts :-
1- Dendrites , multiple
elongated processes
specialized in receiving
stimuli from the
environment .
2- Cell body, perikaryon the
trophic center.
3- The axon , single process
specialized in generating or
conducting nerve impulses
to other cells .


Neurons are classified on the basis of the
number of processes extending from the
cell body to :-
1-Multipolar, neurons have an axon and two
or more dendrites
2-bipolar , neurons have one axon and one
dendrites
3-Unipolar, (actually pseudounipolar) have
one process the axon , that divides close to
cell body to two long processes.

Synapses
Are specialized junctions between neurons that
facilitate transmission of impulses from one neuron to
another.
Synapses between neurons may be classified
morphologically as :
1-Axodendritic, occuring between axons and dendrites.
2-Axosomatic, occuring between axons and cell body
3-Axoaxonic , occuring between axons and axons.
4-Dendrodendritic, occuring between dendrites and
dendrites.

Myelinated and Unmyelinated Nerves
Nerves are composed of bundles of nerve fibers held
. together by perineurium and connective tissue
Each nerve fiber consist of an
axon that is surrounded by a
cellular investment called the
neurilemma or the sheath of
Schwann .
In addition , the fiber may be
myelinated or unmyelinated
which do not have myelin , if
present, ediately around the
axon then called myelinated
nerves.The myelin sheath is
segmented and this site is
called the node of Ranvier .
Myelinated and Unmyelinated Nerves
• A myelin sheath is a protective layer that develops
over nerves in the central nervous and peripheral
nervous systems. Myelin sheaths are composed of
fatty tissues and proteins.
• In the central nervous system, these sheaths are
produced by oligodendrocytes, which can lay a
myelin sheath over a number of axons.
• In the peripheral nervous system, myelin sheaths
are produced by Schwann cells, which place a
myelin sheath over a single axon.


Spinal cord
In cross section, the spinal cord is organized into
two discrete parts exhibits a butterfly - shaped
grayish-tan inner substance, the gray matter
surrounding the central canal ,
and a whitish peripheral substance , the white
matter which contains only myelinated and
unmyelinated axons traveling to and from
other parts of the spinal cord and to and from
the brain .
The gray matter contains neuronal cell bodies
and their dendrites, along with axons and
neuralgia

Nerve ending
1-Pacinian corpuscles,are large ovoid structures
found in the deeper dermis and hypodermis also
it found in the pancreas .
2-Meissner ,s corpuscles , are touch receptors that
found in the papillary layer of hairless skin
3-Ruffini endings , are the simplest
mechanoreceptors ,they have an elongated
fusiform shape .
4-Motor end plates , are the physiologic contact
between nerve and muscle.