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Nervous system
The nervous system is the part of an animal's body that
coordinates its actions and transmits signals to and from
different parts of its body.
The nervous system consists of two main parts, the central
nervous system (CNS) and the peripheral nervous system
(PNS).
1-The central nervous system includes the brain and spinal
cord.
2-The peripheral nervous system (PNS) consists mainly
of nerves, which are enclosed bundles of the long fibers
(axons) that connect the CNS to every other part of the body.
It includes 1- Spinal nerves, 2- Cranial nerves
Nerves coming out from the spinal cord are called spinal
nerves.
Nerves coming out directly from the brain are called cranial
nerves
Nerves that transmit signals from the brain to parts of the body
are called motor or efferent nerves,
Nerves that transmit information from the body to the CNS
are called sensory or afferent. Most nerves serve both
functions (afferent and efferent) and are called mixed nerves.
Autonomic nervous system overlaps the central and
peripheral nervous systems (the higher central autonomic
Centre is located in the Hypothalamus)
The autonomic nervous system is further subdivided into
the sympathetic and the parasympathetic nervous systems.
The sympathetic nervous system is activated in cases of
emergencies to mobilize energy, prepairing the animal
for fight-or-flight response
The parasympathetic nervous system is activated when
animals are in a relaxed state.
Autonomic nervous systems function involuntarily.
The Peripheral Nervous System includes a- somatic and bautonomic nerves. Somatic nerves mediate voluntary
movement. The autonomic nerves serve the involuntary action.
At the cellular level
The nervous system is defined by the presence of a special
type of cell, called the neuron, also known as a "nerve cell".
Neurons have special structures that allow them to send
signals rapidly and precisely to other cells. They send these
signals in the form of electrochemical waves traveling along
thin
fibers
called axons,
which
cause
chemicals
called neurotransmitters to
be
released
at
junctions
called synapses.
A cell that receives a synaptic signal from a neuron may be
excited, inhibited, or otherwise modulated.
Along with neurons, the nervous system contains other
specialized cells called glial cells (or simply glia), which
provide structural and metabolic support.
The brain is an organ of the human central nervous system. It
is located in the head, protected by the skull.
The brain includes the following parts
1- Cerebrum: has two cerebral hemispheres.
2- Brain stem: (Midbrain, Pons and Medulla)
3- Cerebellum
Cerebrum
The cerebrum (or forebrain), which makes up 75% of the
brain size and 85% by weight, is divided by a large
groove, known as the longitudinal fissure, into two
distinct hemispheres. The left and right hemispheres are
convoluted and joined by a large bundle of nerve fibres
called the corpus callosum, and also by other smaller
connections called commissures
Cerebral hemisphere has outer layer called Cortex which
is brown in colour called Grey matter containing cell
bodies
The inside of the hemisphere is whitish in colour and
called White Matter, it contains axons only
The forebrain is subdivided into two parts A- cerebral
hemispheres laterally and B- central part composed
mainly of two thalami and Hypothalamus
(diencephalon)
Each hemisphere is divided in lobes
1-Frontal lobe
2-Parietal lobe
3-Occipital lobe
4-Temporal lobe
The surface of the cerebral hemisphere is thrown into
elevations called Gyri (Gyrus) and depressions called
Sulci (Sulcus). It’s for increasing the surface area of the
brain
Lateral Sulcus
The most important sulci on cerebral cortex are
1- Lateral sulcus
2- Central sulcus
The most important Gyri on cerebral cortex are
1-Precentral gyrus: containing the Motor Centre of
the whole body
2-Post central gyrus: containing the Sensory Centre
of the whole body
The brain has cavities called Ventricles, They are called
1- Lateral ventricles, they are two
2- Third Ventricle: one ventricle
3- Fourth Ventricle: one ventricle
These ventricles are interconnected and contained
cerebrospinal fluid
The brain and spinal cord is covered by three membranes
called meninges, they are
1- Dura Mater: Extends down to the level of 2nd Sacral
vertebra
2- Arachnoid: is adherent to the dura mater and extends
down to the level of 2nd Sacral vertebra
3- Pia Mater: it ends with the spinal cord at the level of
the lower end of the 1st lumbar vertebra
The dura mater and arachnoid are adherent to each
other and lining the skull and the vertebral canal
The Pia mater clothing the brain, spinal cord and part
of the spinal nerves inside the spinal canal up till the
intervertebral foramen
Circulation of the cerebrospinal fluid (CSF)
Between the arachnoid and pia mater there is a space
called the subarachnoid space. It contains the
cerebrospinal fluid (CSF)
The CSF is secreted from the blood through arachnoid
granulation inside the ventricles and return back to the
blood through arachnoid Villi and granulation.
Functions of Cerebrospinal Fluid
1-Delivers nutrients from blood to the brain and spinal
cord
2-Protects the brain and spinal cord
3-Removes the wastes from the brain and spinal cord
4-Shock absorber that reduces trauma.
Function of the cerebrum
It controls voluntary movements, and mental activities. It
is the higher centre for sensation It is the centre for all
consciousness. It is also the higher centre for autonomic
nervous system (located in the lateral wall of the
hypothalamus)
Brain Stem
Composed of the following parts
1-Midbrain: Is involved in Visual reflexes, movements of
eye ball and dilatation of pupil. It is also involved in
auditory reflexes
2-Pons: Controls respiration
3-Medulla oblongata: Contains the centres that
regulate
Heart beat, Respiratory rate, Blood pressure,
Swallowing, Vomiting, Sneezing and cough
Cerebellum
Regulates muscular movements, Balance and body
position.
Spinal Cord
It extends from the medulla oblongata at Foramen
Magnum of the skull and ends at the level of the lower
end of the first lumbar vertebra. The spinal cord and its
meninges are located in the vertebral canal. It measures
about 45 Cm in men. The spinal cord contains spinal canal.
The spinal cord ends as a tapering end called
conus
medullaris.
From apex of conus medullaris, filum terminale, is a delicate
strand of fibrous tissue, about 20 cm in length, proceeding
downward. It is one of the modifications of pia mater.It is
attached downward to the dura mater. It gives longitudinal
support to the spinal cord
31 pairs of spinal nerves originate from the spinal cord:
8 pairs are called Cervical spinal nerves, 12 pairs are
Thoracic nerves, 5 pairs are Lumbar nerves, 5 pairs are
Sacral nerves and I pair are coccygeal nerves
Spinal cord is covered with meninges and CSF. It has
central canal.
Peripheral Nervous System
It connects the central nervous system to the body. It
includes
1- 31 pairs of spinal nerves (arise from the spinal
cord)
2- 12 pairs of cranial nerves (arise directly from the
brain)
The cranial nerves