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

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

VideoNEURONS 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 fibersGray: 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 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

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

sensationsEpithalamus: olfactory correlations and

circadian rhythms.Hypothalamus: controls body temp, sleep, and behavior for eating and

drinking.

The MidbrainThis contains auditory, visual

and muscle control centers. It is also involved with body posture

and equilibrium

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

The Limbic Systemthis 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 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

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 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.

The Cranial NervesThe first segment of the

peripheral nervous system consists of the 12 pairs of cranial

nerves. The are numbered in Roman Numerals.

Acoustic

FinAnd

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 SystemThis is an element of the

peripheral nervous system. It functions automatically and is composed of the sympathetic

and parasympathetic.

SympatheticThe nerve cells of origin are located in the thoracic and

lumbar segments of the spinal cord

ParasympatheticThis 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 pupilsPS: constricts pupils

S: dilates the bronchial tubesPS: constricts the bronchial

tubesS: increases heart rate

PS: decreases heart rate