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.

Acoustic Fin 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