Download Nervous tissues (NS)

Physiology
Lect( 3 )
Asst.Lect/Shaimaa Hussein
Nervous tissues (NS)
The nervous system is the body s control center and
communications network. In humans, the nervous system serves
three broad functions:
1- Sensory functions: it senses changes within the body and in
the outside environment.
2- Integrative function:it interprets the changes.
3- Motor function: it responds to the interpretation by initiating
action in the form of muscular contractions or glandular
secretions.
Neurology: the branch of medical science deals with normal
functioning and disorders of the NS.
The general organization of the nervous system
1- Structural organization:
2- Functional organization:
Cells of Nervous System
Nervous tissues consists of two types of cells:
1- The neurons which conduct impulses and make up the
impulse conducting portion of the brain, spinal cord and nerves.
2- The neuroglial cells which perform other functions.
Neurons: consist of a cell body (soma), an axon, and usually
several dendrites. In general, the axon conducts impulses away
from the cell body, while the dendrites conduct impulse toward
the cell body.
The cell body (soma) contains most of the cytoplasm and many
of the organells usually found in cells (mitochondria, colgi
apparatus, nucleus and nucleolus). The cell body also contains
Nissl granules(a complex of endoplasmic reticulum and
ribosome that serves as the site of protein synthesis for the
neuron). Neurofibrils are found in the cell body near the axon
hillock (where the axon joins the cell body).
The axon of a neuron is a long ,this process extending from the
hillock. In most neurons it extends in only one direction from
the cell body. Most axons are myelinated(that is they are
surrounded by an insulating substance called myelin). ‫بشكل يششك ي‬
‫غالفيالسلك‬
Dendrites are shorter processes than axon in most neurons.
They connect directly with the cell body. Dendrites are not
myelinated.
Schwan cells: (sometimes considered a kind of neuroglial cells)
are found wrapped around the axons of myelinated neurons of
the PNS. Schwan cells are required to produce a myelin sheath
on a singl axon. The myelin sheath has numerous small
constrictions called node of Ranvier. These nodes represents
minutes spaces between adjacent schwan cells.
Classification of Neurons
On the basis of their structure, neurons can also be classified
into three main types:

Unipolar Neurons.
Sensory neurons have only a single process or
fibre which divides close to the cell body into two main
branches (axon and dendrite). Because of their structure
they are often referred to as unipolar neurons.

Multipolar Neurons.
Motor neurons, which have numerous cell processes (an
axon and many dendrites) are often referred to
as multipolar neurons. Interneurons are also multipolar.

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.
Neuroglia: four types of neuroglia are found primarily in the
CNS.
1-Ependymal cells: from a sheet that lines the ventricles of the
brain (spaces that form and circulate cerebrospinal fluid) and the
central canal of the spinal cord.
2- Astrocytes: star shaped cells with numerous processes. They
are of two types:
a- protoplasmic astrocytes: are found in the gray matter of the
CNS.
b- fibrous astrocytes: are found in the white matter of the CNS.
It functions is a- create a supporting network for neurous and
blood vessels.
b- help to transport nutrients from blood to neurons.
3- Oligodendrocytes: resemble astrocytes in some ways, but
processes are fewer and shorter. It have supporting function by :
a- forming semirigid connective tissue rows between neurous in
brain and spinal cord.
b- produce a phospholipid myelin sheath around axons of
neurous of CNS.
4- Microglia: small cells with few processes derived from
monocytes (also called brain macrophage). They are thought to
protect nerve cells against infection. These cells are phagocytic
cells migrate to the site of an injury in the CNS and destroy
microorganisms and cellular debris.
Nerves: are bundles of nerve fibers in the PNS
Tracts: are bundles of nerve fibers in the CNS
Nerve: contain mostly myelinated and few unmylinated axons,
surrounded by several C.T. sheath most nerves excepts for some
cranial nerves (nerve that branch directly from brain), contain
both sensory and motor fibers. A connective tissue sheath (the
endoneurium), surrounds each fibers. Another C.T.sheath(the
perineurium) binds groups of fibers together into fascicles. Yet
another C.T. sheath (the epineurium) covers the whole nerve
(including its several fascicles. Blood and lymph vessels are
often located within the C.T. between fascicles of large nerves.
Nerve fibers in tracts of the CNS are mostly myelinated (they
acquired their myelin from the activities of oligodendrocytes
rather than from Sch. cells). The myelin that surrounds the fibers
of tracts gives the tracts a whitish color (they are recognized as
white matter of the CNS). Cell bodies and dendrites lack myelin
and these areas are recognized as gray matter.
Ganglia and Nuclei :
The cell bodies of the fibers of both nerves and tracts are usually
aggregated in large groups to the side of the main pathway of
the fibers. In the peripheral nervous system these aggregations
are called ganglia. Within the CNS they are called nuclei (but
they are not to be confused cellular nuclei). Each nucleus in the brain of
many cell bodies, each having its own cellular nucleus.
The Synapse:
After a signal has traveled the length of a neuron transmission of
the signal to the next neuron in the neural pathway occurs, Such
transmission takes place across, asynapse aspecilized junction
between the axon terminal of one neroun and the dendrite(or cell
body or axon) of the next neroun. Transmission across asynapse
is accomplished by a chemical substance called a
neurotransmitter. The neuron whose axon release the
neurotransmitter is the presynaptic neuron, the neuron that
receives the neurotransmitter is the postsynaptic neuron.
Atypical synapse consist of the presynaptic knob of an axon
separated from the postsynaptic region of adendrite or nerve cell
body by the synaptic cleft.
Excitability: the ability to respond to a stimulus and initiate and
conduct an electrical impulse. In living cells potential
differences are created by charged particles (ions) in and out the
cells. Each cell contains intracellular fluid and is surrounded by
extracellular fluid. Both of these fluids contain ions, but the
concentration of the various ions in each of the fluids differ. The
concentration of (Na+) in extracellular fluids is abound (20)
times greater than that in intracellular. The concentration of (K+)
in intracellular fluids is about (25) times greater than that in
extracellular fluids. Inracellular fluids also contain large
numbers of negatively charged protein molecules. Differences in
the concentrations of charged particles create a potential
difference across the cell membrane, and the cell is said to be
polarized. In resting, polarized neurons, the potential difference
about (-70mv) is called the resting potential. Thus, the inside of
the membrane is more negative than the outside by 70mv.
In resting, cell potassium ions tend to diffuse out of the cell
more easily than sodium ions diffuse in. Because large protein
molecules and negatively chargedions cannot easily diffuse
across the cell membrane, the inside of a cell tends to become
more negative than the outside. These events create the negative
potential difference . Furthermore, as Na+ leak slowly into the
cell, they are actively transported out again, and as K+ leak out
of the cell, they are actively transported back into the cell by the
action of Na-K pump. This combination of active transport and passive
diffusion of ions maintains the potential difference across the membrane.
Initiation and Conduction of Nerve impulse:
The property of excitability:
1- means the ability of neurons to respond to stimulus and to
conduct an impulse.
2- make it possible for the NS to receive information and use it
to regulate function.
The processes of initiation and conduction of an impulse only a
neuron occure as follows:
1-When a neroun is stimulated, the movment of ions across its
membrane cerates a change in the potential differences.
2-When a resting neroun is stimulated a small number of Na +
move into the cell creating an action potential, which move
along the membrane as an impulse.
3-Following a wave of depolarization is a wave of
repalarization,which prepares the neroun to receive another
stimulus.
*Factors that affect the generation and conduction of an impulse
include:
1-strength of stimulus.
2-summation.
3-the all or non principle.
4-refractoriness.
5-saltatory conduction.
6-myelination.
7-axon diameter.
*Multiple synapses on the same neroun may:
1-create excilatory postsynaptic potential (EPSPs) and facilitate
transmission.
2-create inhibitory postsynaptic potentials(IPSPs) and inhibit
transmission.
Spinal Cord:
-The spinal cord extends from the base of the brain to the second
lumbar vertebra and give rise to thirty-one paris of spinal nerves.
-In cross section the spinal cord displays:
1-a butterfly-shaped area of gray matter which contains large
numbers of neroun cell bodies and synapses.
2-an outer area of white matter which contains the myelinated
spinal tracts.
- The properties of the spinal tracts are summarized in a special
table.