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The Nervous System
The brain relays messages by way of the
spinal cord through nerve fibers. Nerves
radiate to every part of the body to
provide connections for input and output
data.
The nervous system is divided into the
central nervous system which includes
the brain and spinal cord, and the
peripheral nervous system which is
composed of the cranial nerves, spinal
nerves, autonomic nerves and ganglia
Nervous system cells are called neurons.
All neurons have a cell body (soma) one
axon and one or more dendrites. The
cell body has a nucleus which is
responsible for maintaining the life of
the cell. The dendrites extend like tiny
trees conducting nerve impulses toward
the cell body The axon is a single
process that extends out from the cell
body and ends in a fine spreading branch
called a terminal twig
The dendrites and axons are also called
nerve fibers. Bundles of these fibers
found together are called nerves. There
are several types of nerve fibers. Some
are myelinated with a white fatty
material called the myelin sheath. The
myelin sheath is interrupted along the
length of the fiber at regularly spaced
intervals called nodes of Ranvier
Some fibers have only a thin
layer of myelin and are called
non-myelinated. These fibers are
found especially in the
autonomic nervous system.
The nerve cells and filaments
are held together and supported
by a specialized type of tissue
called neuroglia
The neuroglial cells form a dense
network between neurons. They
are divided into four main types
*astrocytes
*microglia
*oligodendroglia
*Schwann cells
Neurons are divided into sensory,
motor and connector neurons
Normally impulses pass in only one
direction.
Sensory neurons conduct impulses
from the sensory organs to the
spinal cord.
Motor neurons conduct impulses
from the brain and spinal cord to
muscles and glands.
The point at which an impulse is
transmitted is called the synapse.
There is no physical contact
between the neurons at the
synapse. The electrical impulse
causes chemical release.
The neurotransmitter chemical is
released to activate other
impulses in the dendrites of
connecting neurons
Video
NEURONS AND
NEUROTRANSMITTERS
The Central Nervous System:
The central nervous system
includes the brain and the spinal
cord. It is also called the
cerebrospinal system.
The CNS contains both white and
gray matter.
White: myelinated fibers
Gray: masses of nerve bodies
The Meninges:
Three membranes that envelop the
central nervous system that are
composed of white fibrous connective
tissue.
They separate the brain and spinal cord
from the body cavities
*the dura mater (outermost layer)
* the arachnoid (middle layer)
*the pia mater (innermost layer)
The space between the dura
mater and the arachnoid is
known as the subdural space.
Between the arachnoid and pia
mater is the subarachnoid space
The brain
The brain contains about 100
billion neurons. The canine brain is
more immature than the human
brain at birth but maturation of
cerebral function proceeds at a
higher rate. The divisions of the
brain are:
*the fore brain
*the midbrain
*the hindbrain
The Forebrain
The cerebrum is the largest part of
the brain and is divided into two
hemispheres. The outer surface is
made up of gray matter. As the gray
matter increases in size each
hemisphere is thrown into folds
called gyri. The gyri are separated
by furrows called sulci and deeper
furrows called fissures
The cerebral cortex is separated into
the frontal, temporal, parietal and
occipital.
Frontal: voluntary movement
Parietal: sensations
Temporal: awareness and auditory
Occipital: visual perception and
visual memory
The left hemisphere controls the
right side of the body. In humans
the left hemisphere is usually
dominant and involves language,
logic, analytic thinking ordering
of events and symbols.
The right hemisphere is linked to
imagination, creativity, spatial
and depth perception.
The diencephalon is the part of the
forebrain that contains the thalamus,
epithalamus and hypothalamus.
Thalamus: plays a role in integrating
sensations
Epithalamus: olfactory correlations and
circadian rhythms.
Hypothalamus: controls body temp,
sleep, and behavior for eating and
drinking.
The Midbrain
This contains auditory, visual
and muscle control centers. It is
also involved with body posture
and equilibrium
The Hindbrain
Composed of the cerebellum, the
pons and the medulla oblongata.
The cerebellum fine tunes motor
activity and muscle tone.
The pons serve as a bridge to
connect the cerebrum, cerebellum
and medulla oblongata.
The medulla oblongata controls
respiration and circulation
The Limbic System
this is the center for emotional
activity and behavior. The term
limus means border.
The brain contains four large
fluid filled cavities called
ventricles. The cerebrospinal
fluid is a thin transparent watery
fluid that serves as a protective
cushion and provides some
nutrients. It is produced by a
network of capillaries called the
choroid plexus.
The Spinal Cord
This is an extension of the brain.
Sensations are received by the
sensory nerves and are relayed to
the spinal cord where they are
transferred to the brain or to
motor nerves. If the sensation is
transferred to a motor nerve it
travels to a muscle or gland and
produces an action
The spinal cord is enclosed in the
vertebral column. Like the brain
it has a pia mater, arachnoid
mater and dura mater an it is
bathed in cerebrospinal fluid. It
is made of an inner core of gray
matter and an outer core of white
matter. In cross section the gray
matter resembles a butterfly.
The outer white matter contains
the tracts. The ascending tract
conducts afferent (sensory)
nerve impulses and the
descending tract conducts the
efferent (motor) impulses.
The Peripheral Nervous System
This provides a means of
communication where stimuli
are transmitted from receptor
organs to the central nervous
system and visa versa. The
peripheral nervous system
includes all of the nerves and
ganglia located outside the brain
and spinal cord.
The Cranial Nerves
The first segment of the
peripheral nervous system
consists of the 12 pairs of cranial
nerves. The are numbered in
Roman Numerals.
Fin
Acoustic
And
The Spinal Nerves:
These nerves arise from the spinal cord
and emerge from the vertebrae. After
leaving the spinal cord the nerves are
named after their corresponding
vertebrae. A spinal nerve has a dorsal
and ventral root. The dorsal root carries
afferent (sensory) impulses and the
ventral root carries efferent (motor)
impulses.
The spinal nerves generally
supply fibers to the region of the
body in the region where they
emerge from the spinal cord. In
some areas they merge and form
a plexus. The spinal nerves
extend beyond the level of the
spinal cord and is called the
cauda equina
Autonomic Nervous System
This is an element of the
peripheral nervous system. It
functions automatically and is
composed of the sympathetic
and parasympathetic.
Sympathetic
The nerve cells of origin are
located in the thoracic and
lumbar segments of the spinal
cord
Parasympathetic
This system originates in the
brain stem
The sympathetic nerves are
involved in flight or fight and
the parasympathetic nerves are
involved with restful situations
Examples of the opposition of
these groups are:
S: dilates pupils
PS: constricts pupils
S: dilates the bronchial tubes
PS: constricts the bronchial
tubes
S: increases heart rate
PS: decreases heart rate