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The Nervous SystemThe 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
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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
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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
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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
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The neuroglial cells form a dense network between neurons. They are divided into four main types
*astrocytes*microglia
*oligodendroglia*Schwann cells
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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.
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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
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VideoNEURONS AND
NEUROTRANSMITTERS
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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 fibersGray: masses of nerve bodies
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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)
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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
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The brain
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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
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The ForebrainThe 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
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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
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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.
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The diencephalon is the part of the forebrain that contains the thalamus,
epithalamus and hypothalamus.Thalamus: plays a role in integrating
sensationsEpithalamus: olfactory correlations and
circadian rhythms.Hypothalamus: controls body temp, sleep, and behavior for eating and
drinking.
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The MidbrainThis contains auditory, visual
and muscle control centers. It is also involved with body posture
and equilibrium
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The HindbrainComposed 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
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The Limbic Systemthis is the center for emotional
activity and behavior. The term limus means border.
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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.
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The Spinal CordThis 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
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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.
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The outer white matter contains the tracts. The ascending tract
conducts afferent (sensory) nerve impulses and the
descending tract conducts the efferent (motor) impulses.
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The Peripheral Nervous SystemThis 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.
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The Cranial NervesThe first segment of the
peripheral nervous system consists of the 12 pairs of cranial
nerves. The are numbered in Roman Numerals.
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Acoustic
FinAnd
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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.
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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
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Autonomic Nervous SystemThis is an element of the
peripheral nervous system. It functions automatically and is composed of the sympathetic
and parasympathetic.
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SympatheticThe nerve cells of origin are located in the thoracic and
lumbar segments of the spinal cord
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ParasympatheticThis system originates in the
brain stem
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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:
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S: dilates pupilsPS: constricts pupils
S: dilates the bronchial tubesPS: constricts the bronchial
tubesS: increases heart rate
PS: decreases heart rate