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The Nervous System of Humans

The nervous system of all vertebrates including man is divisible into three parts:
  1. The central nervous system,
  2. The peripheral nervous system, and
  3. The autonomic nervous system.
Central Nervous System
The central nervous system (CNS) includes
  1. Forebrain,
  2. Midbrain and
  3. Hindbrain.

The Brain

The brain is situated in a bony case called the cranium which protects it from external injuries. The brain is covered over with three membranous sheets of connective tissue. These are: the innermost membrane which surrounds the brain, called piamater, the middle one called arachnoid layer, and the outermost one called the duramater.The piamater is very thin and highly vascular and forms a close fitting covering to the brain and the spinal cord. The arachnoid layer is traversed by a network of fibres and the duramater is a thick tough, protective layer lining the cranial cavity just below the skull and the vertebral column. These membranes are called the meninges. The space between the meninges is filled with fluid called the cerebrospinal fluid which protects the brain from mechanical injuries and shocks.

The brain is divided into three parts, namely
  1. forebrain,
  2. midbrain and
  3. hindbrain.
  1. Fore Brain
    There are two important constituents of the fore brain: Cerebrum and diencephalon. The cerebrum makes up about two-thirds of the human brain. It consists of an outer gray matter and an inner white matter. The gray matter shows many convolutions. The white matter consists of nerve fibres and nerve tissue called basal ganglia. A deep longitudinal fissure divides the cerebrum into the hemispheres which are connected together by a horizontal sheet of nerve fibres called corpus callosum. Two club shaped small structures called olfactory lobes are at the anterior end of the cerebral hemispheres. These structures which are concerned with the sense of smell, are visible only in a ventral view of the brain. The cerebral hemispheres control numerous functions of the body, such as thinking, perception of smell, sight, hearing, ability of speech and movements of various parts of the body. The cavities of the cerebral hemispheres are called lateral ventricles.

Longitudinal Section of Human Brain. Inset: Location of Grey and White Matter

Dorsal View of Human Brain

It is the narrow region of the fore brain lying behind and below the cerebral hemispheres. Its cavity is known as the third ventricle. The roof of the third ventricle, dorsally, is formed by the anterior choroid plexus which is thickened and folded piamater. Anterior choroids plexus is highly vascular. From the dorsal surface of the diencephalon arises the pineal body. The sides of the diencephalon are thickened to form the optic thalami. On the ventral side of the diencephalon lies a funnel-shaped depression called the infundibulum. A sac like oval body called the hypophysis is attached to the infundibulum. The hypophysis and infundibulum together form the pituitary body which is a major endocrine gland. In front of the pituitary body lies the optic chiasma; formed by the crossing of the optic nerves.

The diencephalon is an important part of the brain controlling the sensory and automatic adjustments. The hypothalamus and pituitary together form the major integrating centre for nervous and hormonal control mechanisms in the body.
  1. MidBrain
    It consists of two parts: Corpora quadrigemina and Cerebral peduncles. The dorsal and lateral sides of the mid brain are thickened to form two pairs of optic lobes which collectively constitute the corpora quadrigemina. The optic lobes enclose very narrow optic ventricles. The corpora quadrigemina serve as centers for certain visual and auditory reflexes. The cerebral peduncles are bundles of fibres connecting the cerebral cortex with other parts of the brain and the spinal cord.
  2. HindBrain
    It consists of two parts: Cerebellum, and Medulla Oblongata.
It is a large, solid and complex structure. Its surface is much folded and forms the dorsal side of the hind brain. The cerebellum consists of a large central lobe called the vernius, two lateral lobes and two floccular lobes. The cerebellum regulates and coordinates muscle contraction.

Medulla Oblongata
It is the posterior most part of the brain and continues as the spinal cord. It is dorsoventrally flattened and is somewhat triangular. Its cavity is called fourth ventricle. The fourth ventricle is roofed by the posterior choroid plexus. On the antero-ventral side of the medulla oblongata lies a transverse band of nerve tissue called pons varoli. It connects the two sides of the cerebellum. Posteriorly the fourth ventricles communicate with the central canal of the spinal cord. The walls of the medulla are thick containing many nerve centres. These works as reflex centers for regulating respiration, heart rate, dilation and constriction of blood vessels, swallowing and vomiting.
The ventricles of the brain are filled with cerebrospinal fluid which is secreted by the anterior and posterior choroid plexus. The fluid is nutritive and shock absorbing in function.

In the olfactory lobes, cerebral hemispheres, diencephalon and cerebellum, the grey matter is external and the white matter is internal. In optic lobes and medulla oblongata, the grey matter is internal and the white matter is external. The grey matter is composed of nerve cells and the white matter is composed of nerve fibres.
Functions of the Brain
  1. Olfactory lobes are concerned with the sense of smell.
  2. Cerebral hemispheres are the seat of intelligence, memory, emotion, experiences, associations and will. They also control the voluntary movements.
  3. Diencephalon controls the involuntary functions of the autonomic nervous system, metabolism of carbohydrates and fats, regulates body temperature and genital functions.
  4. Optic lobes control and sense of sight. They have also an inhibiting influence on the reflex activity of the spinal cord. The posterior optic lobes are also concerned with visual functions.
  5. Cerebellum maintains equilibrium and controls the movements of the body.
  6. Medulla oblongata controls the various organs of the body, activities such as respiration , heart beat, sound production, swallowing, glandular secretions and movements of digestive organs.
As the name implies, hypothalamus nestles at the base of the thalamus, and so of the brain. Although relatively small, just 4 grams, about 1/300 of the total brain mass is highly vascularised. It integrates and controls the visceral activities. The hypothalamus, through its connection with the brain stem, maintains homeostasis and the body's internal equilibrium, specializing in involuntary behavior control. The nuclei in it signal the body to eat, drink, get angry, keep cool, and make love and so on. Hypothalamus organizes behavior related to survival of species: fighting, feeding, fleeting and mating. It keeps body temperature at roughly 37oC by means of a complex thermostat system. It also influences respiration and heartbeat, and sends out signals to correct them when they are wrong. Through connections with the pituitary gland, the hypothalamus controls growth and sexual behavior. It also controls many more functions.

Above the hypothalamus, attached to the interior tips of both forks, is almond-shaped amygdala. This bulge of neurons is like a defense castle controlling the moods, especially anger and rage. Various regions of the amygdala play an important role in emotional behavior, such as aggression and remembering fear. The remarkable organ deals with a strange mix of signals about smells and memories. The hippocampus functions as a kind of index for recall of an event with its associated memory. The hippocampus converts information from short-term to long-term memory, essential in learning. The septum linked to the hypothalamus contains yet another emotion center for sexual arousal.

Brain stem
It includes the area of the brain between the thalamus and the spinal cord. The midbrain contains four little lobes, the corpora quadrigemina. Its principal structures are superior colliculi and inferior colliculi. The superior pair of colliculi receive sensory impulses from the eyes and muscles of the head and control visual reflexes. For example, they control and coordinate the movement of the head and eyes to fix and focus on an object. The inferior pair of colliculi receive sensory impulses from the ears and muscles of the head and control auditory reflexes, such as the movement of the head to locate and detect the source of a sound. Pons, (Latin meaning: the bridge) forms the floor of the brain stem. It serves as a neuronal link between the cerebral cortex and the cerebellum. Medulla oblongata, literally meaning oblong marrow, is the posterior most part that connects to the reticular formation that connects to the thalamus and major nerves in the spinal cord, is the gatekeeper to consciousness.

Spinal Cord

The spinal cord is the posterior continuation of the medulla oblongata of the brain. It lies in the neural canal of the vertebral column. Like the brain, it is also invested with three meninges, inner piamater, middle arachnoid layer and the outer duramater. On either side of the arachnoid layer, the arachnoid space is filled with cerebrospinal fluid which protects the spinal cord from mechanical injuries.

The spinal cord is a dorsoventrally flattened, somewhat cylindrical structure of variable diameter. The main regions of the spinal cord are shown in the figure below. Along the mid-ventral line of the spinal cord runs a shallow groove called dorsal sulcus or dorsal fissure. Along the mid-ventral line runs a deep groove called the ventral fissure. Through the axis of the spinal cord runs a narrow longitudinal canal called the central canal. Anteriorly the central canal communicates with the fourth ventricle of the brain and posteriorly it ends blindly. It is internally lined by a glandular epithelium.

Spinal cord is formed of grey matter and white matter. The grey matter surrounds the central canal. It is formed by nerve cells. In a cross section it is butterfly-shaped and is produced into two dorsal horns and two ventral horns. The white matter lies outside the grey matter. It is composed of nerve fibres.

Groups of afferent fibres enter the spinal cord on the dorsal side of the body. These groups from the dorsal roots are sensory. Afferent fibres leave the spinal cord on the front side of ventral side via the ventral roots. Soon after leaving the cord, the ventral and dorsal roots from the same level combine to form a spinal nerve.

All the neurons in the spinal cord are connector neurons. The axons and dendrites of the white matter are separated into ascending tracts which carry impulses to the brain and descending tracts which carry impulses from the brain. All the impulses are passed to and from the brain through the spinal cord. The spinal cord is also concerned with reflex actions. It relieves the brain form too much strain

There are four cavities within the brain that are called cerebral ventricles. Each of the two lateral ventricles is located in the hemisphere of the cerebrum. The third ventricle is a vertical slit at the mid line between the lateral ventricles. The fourth ventricle lies between the brain stem and the cerebellum. Extracellular fluid, continuously circulating through the subarachnoid space between the arachnoid and pia mater around the brain and spinal cord and through cavities within the brain, is called cerebrospinal fluid or CSF. It is a clear, colourless fluid, similar to blood plasma, except that it has a much lower proportion of protein and cholesterol. It has several functions, including:
  1. Protection of the delicate brain and spinal cord by providing shock-absorbing medium. It acts to cushion jolts to CNS and lessen their impact.
  2. Buoyancy to the brain because it is immersed in CSF, and so the net weight of the brain is reduced from about 1.4 kg to about 0.18 kg. Therefore, the pressure at the base is reduced.
  3. Excretion of waste products. One-way flow from CSF to the blood takes potentially harmful metabolites, drugs and other substances away from the brain.
  4. Endocrine medium for the brain to transport hormones to other areas of the brain. Hormones released into CSF can be carried to remote sites of the brain, where they may have some action.

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