Introduction to the
Central Nervous System

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1-9. MAJOR COMPONENTS OF THE CENTRAL NERVOUS SYSTEM

a. Brain. The brain fills the cranium and weighs about three pounds in the average adult. The brain is shaped like a mushroom. The brain consists of four principal parts: the brain stem, the diencephalon, the cerebrum, and the cerebellum. The diencephalon, also known as the forebrainstem, includes the thalamus and hypothalamus.

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Figure 1-6. Principal parts of the brain.

(1) Brain matter. There are two types of matter in the brain: gray matter and white matter. Gray matter is in the active portion of the brain. Gray matter receives and stores impulses. Answering impulses originate in the brain's gray matter. Cell bodies of neurons and neuroglia are in the gray matter. White matter in the brain carries impulses to and from gray matter. White matter is composed of nerve fibers (axons).

(2) Cerebrum. The cerebrum forms the bulk of the brain and is supported on the brain stem. The cerebrum is divided into two hemispheres. Each hemisphere controls the activities of the side of the body opposite that hemisphere. Each hemisphere is further subdivided into four lobes:

(a) Frontal lobe. This lobe is responsible for voluntary motor function (origin of pyramidal motor system) and higher mental functions such as judgment and foresight, affect, and personality.

(b) Temporal lobes. These lobes are responsible for hearing, speech in the dominant hemisphere, vestibular sense, behavior, and emotion.

(c) Parietal lobe. This lobe is responsible for sensory function, sensory association areas, higher level processing of general sensory modalities, e.g., stereognosis -- recognizing the size and shape of objects by the sense of touch.

(d) Occipital lobe. This lobe is responsible for vision.

(3) Cerebellum. The cerebellum is located behind and below the cerebrum. Its functions include the following:

(a) Awareness of posture, movement, and voluntary muscle movement; for example, equilibrium.

(b) Receipt of relayed tactile, auditory, and visual input; for example, processing of information obtained by what you see and hear.

(c) Fine motor coordination; for example, writing.

(4) Midbrain. The midbrain is located above the pons, extending from the pons to the lower part of the diencephalon. The midbrain provides conduction pathways to and from higher and lower centers. The righting, postural, and audiovisual reflexes are reflex centers located in the midbrain. The righting reflex helps keep the head right-side up. Postural reflexes deal with positioning the head in relation to the trunk of the body. Visual and auditory reflexes cause you to respond by turning your head in the direction of a loud noise.

(5) Pons. The pons is located anterior and slightly superior to the cerebellum and between the midbrain and the medulla. The pons acts as a pathway to higher structures. It contains conduction pathways between the medulla and higher brain centers. It also serves to connect the two halves of the cerebellum. There is a respiratory center in the pons which prolongs inspiration (breathing in). The beginnings of some cranial nerves are in the pons.

(6) Medulla oblongata (brain stem). This part of the brain is an expanded continuation of the spinal cord. The brain stem is located between the pons and the spinal cord and is only about one inch long. Contained in the brain stem are the centers for the regulation of respirations, heartbeat, and basomotor activators. These centers are often called the vital centers because they are essential to life. Some nerves cross over at the medulla oblongata which explains why one side of the brain controls activities on the opposite side of the body.

(7) Thalamus. The thalamus is located in the walls of the third ventricle of the brain and is the area of arousal and conscious recognition of crude sensations; for example, temperature and pain. Sensory and afferent impulses go to the thalamus and are sorted and grouped there. Next, these impulses are sent to the proper area of the cerebral cortex where the impulses are interpreted. According to the Law of Specific Nerve Energies, the place at which an impulse ends in the thalamus determines the sensation to be felt. For example, if the impulse ends in the heat area of the thalamus, the individual feels heat.

(8) Hypothalamus. The hypothalamus is a small part of the diencephalon. This portion of the brain has several functions. It monitors the chemical composition of blood. The hypothalamus acts as a relay station between the cerebrum and the lower autonomic centers. It controls hormone secretion by the pituitary gland and also controls the appetite.

b. Spinal Cord. The spinal cord is a cylindrical structure which extends from the foramen magnum through the spinal foramina of the vertebral column to the upper portion of the lumbar region. Extension varies from the 12th thoracic vertebra to the 2nd lumbar vertebra. The length of the cord remains fairly constant in adults: 18 inches in males and 16 inches in females. The conus medullaris is the cone-shaped termination of the cord. This portion of the spinal cord weighs about one ounce and is approximately one and one-half inches wide. The spinal cord itself appears wider from right to left than from anterior to posterior. The size and shape, however, do vary depending on the vertebral region. For example, the spinal cord presents cervical and lumbar enlargements, which are areas of nerve origin in the upper and lower lumbar region. The spinal cord is composed of a series of 31 segments. A pair of spinal nerves comes out of each segment. The region of the spinal cord from which a pair of spinal nerves originates is called the spinal segment. Both motor and sensory nerves are located in the spinal cord.

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Figure 1-7. Spinal cord and spinal nerves.

c. Meninges. The meninges are the three membranes that envelop the brain and the spinal cord. The outermost layer is the dura mater. The middle layer is the arachnoid, and the innermost layer is the pia mater. These three spinal meninges cover the spinal nerves to the point where the spinal column goes through the intervertebral foramen. The meninges offer protection to the brain and the spinal cord by acting as a barrier against bacteria.

d. Cerebrospinal Fluid (CSF). The cerebrospinal fluid protects the brain against injury. CSF circulates through the subarachnoid space (the area between the arachnoid and pia mater), around the brain and spinal cord and through the ventricles of the brain. In addition to protecting the brain, this clear fluid nourishes the central nervous system and carries off wastes.

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