Human Nervous System

The human nervous system consists of central nervous system (CNS) and peripheral nervous system (PNS). The CNS has central location. They lie in the midline of the body. The PNS, which is further divided into the somatic division and the autonomic division, includes all the cranial nerves and spinal nerves. The two systems work together and are connected to one another.

 

Central Nervous System or CNS

It consists of brain and spinal cord, and both are hollow. The brain and spinal cord are covered with three protective membranes called meninges (singular: meninx).

Human Brain 

Brain

It is enclosed within the cranium. The three meninges are, duramatter (next to the bone of cranium), arachnoid matter (middle membrane), and pia matter (next to the nervous tissue). Between the arachnoid arid pia matter there is a fluid, the cerebro-spinai fluid, which helps to cushion the brain from shock. The brain is divided into three parts, forebrain, midbrain, and hindbrain.

 

Forebrain

It consists of cerebrum, thalamus and hypothalamus. Cerebrum is the largest part of the human brain. The surface of the cerebrum is called cerebral cortex containing 10 billion neurons. The cerebrum consists of two layers. The first layer is composed of gray matter. Beneath the cerebral cortex is the thick white matter, of the cerebrum, which constitutes the second layer. Cerebral cortex has many folds or convolutions forming ridges or gyri (singular, gyrus) which are separated by grooves. A shallow groove is called a sulcus (plural, sulci) and a deep groove is called a fissure. The cortex covers and overlaps most of the other brain area. The cerebrum is differentiated into gray and white matter. Cerebrum is divided into two cerebral hemispheres. Two hemispheres are separated by longitudinal fissure. At the base of the longitudinal fissure, there is a tough band of axon called corpus callosum. It connects two hemispheres. Each hemisphere contains four surface lobes, which corresponds roughly with the bones of the cranium, frontal, parietal, temporal and occipital lobe. Their cavity is called lateral ventricle.

 

Functions of Cerebrum

The regions of the cerebral cortex that perform specific functions have been identified. The cortex can be divided into three areas:

MOTOR AREAS

SENSORY AREAS

ASSOCIATION AREAS

 

1- Motor Areas

The primary motor areas of the cerebral cortex lie in the frontal lobe. Most of the nerve fibers of this area cross over from one side of the brain to the other within the brain stem. As a result, the motor area of the right cerebral hemisphere generally controls skeletal muscles on the left side of the body and vice versa. In addition to the primary motor areas, certain other regions of the frontal lobe are involved with motor functions. It coordinates the complex muscular actions of the mouth, tongue and larynx, which make the speech possible. Another area controls the voluntary movement of the eyes and eyelids. Another region just in front of the primary motor area controls the muscular movements of the hands and fingers that make skills such as writing.

2- Sensory Areas

These areas occur in several lobes of the cerebrum function in interpreting impulses that arrived from various sensory receptors (see figure). The center of the right cerebral hemisphere interprets impulses originating from the left side of the brain and vice versa.

3- Association Areas

These areas occupy the anterior portions of the frontal lobes and are widespread in the lateral portions of the parietal, temporal and occipital lobes. Association areas function in the analysis and interpretation of sensory experiences and involved with memory, reasoning, judgment and feelings.

According to research work of Nobel Prize winner Roger Sperry, our two cerebral hemispheres look alike superficially. Their primary motor and sensory areas also function in the same way, however, the left and right association function so differently that we could almost say we have two brains in one, the left hemisphere house our language centers. It also has association areas for logic and mathematical abilities. In contrast, the right hemisphere lack language, logic, and math centers, but has association areas that underline our Imagination, spatial perceptions, artistic and emotional abilities.

Thalamus

Thalamus

It is below the cerebrum and is covered by the cerebral hemisphere. The thalamus consists of gray mater located in the sides of the Ventricles, which Is the cavity of Thalamus, It receives all sensory Impulses (except sense of smell) and channels them to limbic system and to appropriate regions of the cortex for Interpretation.

Hypothalamus 

Hypothalamus

On the ventral side of the thalamus is the hypothalamus, which forms the floor of the third ventricle. It maintains homeostasis or the constancy of the internal environment, and contains centers for regulating hunger, sleep, thirst, body temperature, water balance, and blood pressure, menstrual cycle and the sleep wake cycle. The hypothalamus also controls the pituitary gland and thereby serves as a link between the nervous and endocrine system.

Limbic System

The limbic system involves portions of both the subconscious and conscious brain. It lies just beneath the cerebral cortex and contains neural pathways that connect portions of the frontal lobes, the temporal lobes, the thalamus and the hypothalamus. It Is a two partial rings formed by portions of cerebral cortex around thalamus and hypothalamus. One cerebral structure is amygdala, or amygdaloid (a -mig' da-loyd; meaning almond shaped) nucleus, which is a cluster of neurons producing sensation of pleasure, punishment or sexual arousal when stimulated. It also involves in the feeling of fear and rage. The other cerebral structure is hippocampus (hip-o-kampus; meaning shaped like a seahorse). It interacts closely with another part of the cerebral cortex, (the prefrontal cortex), which is involved in complex learning, reasoning and personality. Limbic system is essential to both short term and long-term memory.

Midbrain

It is reduced in humans and acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum, and it also has reflex centers for visual, auditory and tactile (touch) responses. Midbrain contains reticular formation, which is a relay center connecting hindbrain with the forebrain. Reticular formation is very important in screening the input information, before they reach higher brain center.

Hindbrain

The hindbrain consists of:

(a) Cerebellum (b) Medulla Oblongata (c) Pons

Cerebellum

It is a bulb like structure extending upwards from the pons and is situated under the cerebrum. It is the second largest part of the brain. It consists of two lateral hemispheres. It also shows folds. Its interior is made of neuron cell body.

Cerebellum controls equilibrium i.e. body position and co-ordination of the actions of individual muscles to produce complex activities such as walking, running, riding bicycles, doing delicate work with hand. The cerebellum is also involved in learning and memory storage for behavior, (it is best developed in birds and helps in complex activity of flight.)

Medulla Oblongata

It forms a swelling below the pons at the base of the brain along the spinal cord. Medulla consists mostly of ascending and descending tracts of white matter, with some gray matter in the interior. Medulla controls the automatic functions of the body, such as heartbeat, blood pressure, contraction and dilation of blood vessels, respiration, sweating, swallowing.

Pons

Pons is a small lobe like structure lying just above the medulla. It is composed of largely white matter. It acts as a bridge between the cerebellum, medulla and cerebrum. Pons appears to influence transitions between sleep and wakefulness and the rate of pattern of breathing.

Brain Stem

It maintains life support system, consists of the medulla oblongata, pons and midbrain.

Ventricles

Within the cerebral hemispheres and brain stem is a series of interconnected cavities called ventricles. These spaces are continuous with the central canal of the spinal cord and like it; they are filled with cerebrospinal fluid. The largest ventricles are the lateral ventricles (first and second ventricles) which extend into the cerebral hemisphere. A narrow space that constitutes the third ventricle is located in the midline of the brain, beneath the corpus callosum. The fourth ventricle is located in the brain stem just in front of the cerebellum.

Spinal Cord

An oval shaped hollow cylinder continues with medulla oblongata and runs below the skull down the neural canal of the backbone. The spinal cord in man ends at the third lumber vertebrae. Below this pointy the nerve travel down the neural canal rather like a horsetail.

Cross section the spinal cord shows an inner gray matter containing a central canal. The outer portion is composed of white matter. As in the other part of the nervous system, the gray matter consists of neuron cell bodies and non-myelinated parts of the fibers. The white matter is made up of bundles of myelinated nerve fibers.

Spinal Cord

Function: The Spinal cord carries out the reflex actions, conduct impulses to and from the brain and helps in better function of brain.

Peripheral Nervous System

The peripheral nervous system is made up of nerves. Nerves are the bundle of fibers i.e. axons, or dendrites bounded by connective tissues. The cell bodies of neurons are found in CNS. Ganglia (sing. ganglion) are collection of cell bodies of neurons within the PNS. Only the fibers of neurons (axon or dendrites) are found in the nerves of the PNS.

Cranial Nerves

Human have 12 pairs of cerebral or cranial nerves arising from the brain. Some are sensory, some are motor and some are mixed nerves that contains both type of fibers. Cranial nerves are largely concerned with head, neck and facial regions of the body, the vagus nerves have branches to the pharynx and larynx and to most of the internal organs.

Cranial Nerves

Spinal Nerves

Humans have 31 pairs of spinal nerves. Each spinal nerve emerges from the spinal cord by two short branches or roots, which lie within the vertebral column... The dorsal root contains fibers of sensory neuron and the ventral root contains the fibers of motor neurons. The two roots join just before a spinal nerve leaves the vertebral column. Therefore, all spinal nerves are mixed nerves. Each spinal nerve serves the particular region of the body in which it is located.

Spinal Nerves

Somatic Nervous System

The somatic nervous system includes the mixed nerves that control the voluntary movement of the musculoskeletal system and the exterior sense organs including those in the skin.

Autonomic Nervous System

The motor neurons that control the internal organs function automatically and usually without need for conscious intervention. The autonomic system is divided into (a) sympathetic (b) parasympathetic system. Both of these systems (1) function automatically and usually subconsciously In an Involuntary manner (2) Innervate (to supply nerve impulse) all internal organs (3) utilize two neurons and one ganglion for each impulse. The first of these two neurons has a cell body within the CNS and a preganglionic fiber.

The second neuron has a cell body within the ganglion and a post ganglion fiber.

Sympathetic Nervous System

Most preganglionic fibers of the sympathetic system arise from the middle or thoracic lumbar portion of the spinal cord and almost immediately terminate in ganglia that lie near the cord; the sympathetic system is specialty important during emergencies and is associated with fight or flight, as active muscle requires a ready supply of glucose and oxygen.

The system accelerates heartbeat, dilates bronchi, and inhibits the digestive tract. The neurotransmitter released by the postganglionic axon is primarily norepinephrine.

Parasympathetic Nervous System

A few cranial nerves, including the vagus nerve, together with fibers that arise from the sacral (bottom) portion of the spinal cord, form the parasympathetic nervous system. This system promotes all the internal responses that are associated with relaxed state; for example, it causes the pupil of the eye to contract, promotes digestion of food and retards heartbeat. The neurotransmitter utilized by parasympathetic system is acetylcholine.