Human Nervous System plays very important role in human body.The nervous system is responsible for sending, receiving, and interpreting information from all parts of the body.It consists of the brain, spinal cord, and a complex network of neurons.It also plays an important role in the changes that occur outside or inside the body. The human body consists of organs, each of which has a specific function. So that organs can work well together.To respond to stimuli, there are three features that must be owned by the nervous system, namely:
Nervous tissue is composed of cells which are called neurons.A neuron consists of a cell body with a large central nucleus and many conducting processes which are of two kinds: (a) dendrites—short, slight tendrils which spread round the cell in all directions, collecting or transmitting impulses from the processes of adjacent cells; (b) axons—which are longer processes and become nerve fibres. Many axons are invested with a fatty sheath or medulla enclosed in a thin covering, the neurilemma: this medullary sheath probably ‘ insulates ‘ the nerve, preventing leakage of the impulses passing along it. These medullated nerve fibres are found in voluntary muscles and all sensitive parts of the body. Axons having no sheaths—non-medullated nerve fibres—are found in involuntary muscles and in the walls of internal organs.
Axons, or nerve fibres, end in fine branches which connect them with various organs, or arborize around the dendrites of other cells. The places of linkage of one neuron with another are called synapses.Cells and fibres of the nervous system are bound together by connective tissue called neuroglia
Functions of Neurons.
Nerves are classified according to the function they perform.A nerve which carries sensations from the body to the cord and brain is known as a sensory or efferent nerve.A nerve which carries impulses from the brain or cord to the body is called a motor or efferent nerve.Nerves containing both motor and sensory fibres, thus carrying impulses in both directions, are termed mixed nerve trunks.
Many neurons lie wholly within the central nervous system, i.e. brain and spinal cord, and as their function is the connecting up of various other systems of neurons they are called connector or association neurons.
Grey and white Matter.
Areas of the brain and spinal cord in which cells are massed together have a pinkish-grey appearance, hence their material is known as grey matter. The matter of the rest of the brain and cord, consisting of medullated axons, has a white appearance, and is called white matter. For purposes of description the nervous system is divided into (i) the cerebrospinal system; (2) the autonomic or sympathetic system. Both systems are continuous.
The cerebro-spinal system.
This includes spinal cord, brain, and peripheral nerves.The brain lies in the cranial cavity, and weighs about three pounds: it may be divided into the cerebrum, the cerebellum, the pons varolii, and the medulla oblongata.The cerebrum, or great brain, consists of two hemispheres the upper parts of which are separated by the great longitudinal fissure and the lower parts united by a nervous band, the corpus callosum.The outer layer cortex of the cerebrum is composed of grey matter which is arranged in folds convolutions, with furrow fissures between; this arrangement increases the extent of grey matter enormously. The greater the brainpower of an individual the more convoluted is the cerebral cortex. Below the grey matter of the cerebrum is situated white matter, and at the base there are masses of grey matter, known as the basal ganglia. The anterior mass is called the corpus striatum—striped body; the posterior is the optic thalamus.
Lobes of the cerebrum.
Each hemisphere is divided into lobes, four of which correspond roughly to the bones covering them, i.e. fronto, tempero-sphenoidal, parietal, occipital. A fifth lobe, the island of Reil, lies deep in the Sylvian fissure which divides the tempero-sphenoidal and parietal lobes. The fissure of Rolando divides the frontal and parietal lobes: this may be called the most important fissure; motor fibres to the body start just in front of iin the motor area, and sensory fibres end just behind it in the sensory area.
Fibres descending from the motor area converge to form a tract known as the internal capsule which runs downwards between the corpus striatum and the thalamus, and through the pons to the medulla, where the fibres cross to the opposite side and then descend in the lateral columns of the spinal cord. By these fibres motor-impulses from the brain pass to the nerves supplying the voluntary muscles of the opposite side of the body. Impressions received in the sensory areas are from sense organs situated on the opposite side of the body.
Functions of the cerebrum.
The cerebrum is the seat of consciousness, and contains the nerve centres which govern all mental activity, reason, intelligence, memory, emotion; it is the instigator of all movement and interpreter of all sensations.The cerebellum, or lesser brain, lies below and to the back of the cerebrum: its surface is covered by grey matter, and it contains white matter: the cortex is regularly furrowed, and the folds, when cut, present a leaf-like appearance Functions. These are chiefly concerned with balance, and the coordination of movement on the same side of the body.
The pons varolii is a bridge of nerve fibres, intermixed with grey matter which unites the cerebellar hemispheres to each other, and to the medulla.The medulla oblongata is a cone-shaped body with its base directed upwards: it lies on the occipital bone, and its apex passes through the foramen magnum, becoming continuous with the spinal cord. The white matter of the medulla is disposed outside, the grey matter is contained within. On its upper surface the tissue of the medulla is thin and covers a cavity—the fourth ventricle—on the floor of which are grouped important nerve centres which govern respiration, the heart, and blood vessels: hence it is known as the ‘ noeud vital/ or- vital knot:, here the least injury is fatal.
The Most Important Elements Of Human Nervous System
Functions of the Medulla.
It Contains tracts of fibres passing between brain and spinal cord, some of which cross here. It also contains the centres controlling vital functions, the chief being the cardiac and respiratory centres and centres for control of blood pressure, swallowing, and vomiting.
Ventricles of the brain. The brain contains cavities— the ventricles. The two lateral ventricles are situated one in either hemisphere under the corpus callosum: these communicate by small openings with the third ventricle, and this by a slender canal with the fourth ventricle, which is itself the expanded continuation of the central canal of the spinal cord. Three small openings in the roof of the fourth ventricle communicate with the subarachnoid space: thus all these spaces are continuous.
The meninges. The brain is covered and protected by three membranes.
The dura mater is a tough membrane lining the skull: projections from it form partitions between various parts of the brain. The portion dividing the hemispheres is known as the falx cerebri, and between the cerebrum and cerebellum lies the tentorium cerebelli. The dura mater also forms channels—venous sinuses—in which the venous blood returning from the brain flows to the jugular vein.
The arachnoid (like a spider’s web) is a filmy membrane lying next to the dura mater, and between it and the pia mater.
The pia mater is a delicate membrane lying nearest to the brain, and dipping into all the fissures: it carries a network of capillaries and lymph vessels.
The cerebro-spinal fluid is a clear, colourless, watery fluid, specific gravity 1005-1007: it contains traces of protein and other organic substances. It is secreted by a glandular substance in the ventricles, caU.ed the choroid gland, and is contained in the ventricles, central canal of the spinal cord, and subarachnoid space (i.e. between arachnoid and pia mater), and is finally carried away by the venous sinuses. The cerebro-spinal fluid is present as a thin layer round brain, spinal cord, and nerves as far as the sacrum: it forms a kind of water bed in which the delicate nerve fibres float, and by means of it the effects of shocks and jars are lessened before they reach the nerve centres.
The spinal cord:
It is contained in the spinal canal and is continuous with the medulla. In the infant it extends from the skull to the sacrum, in the adult it terminates at the first lumbar vertebra, its cone-shaped ending tapering to a slender filament, the filum terminate, which is continued throughout the canal, and is surrounded by nerves, which, hanging from the lower parts of the cord, resemble a horse’s tail, hence they are called the cauda equina. The cord is about eighteen inches long and half an inch thick. It is swollen in the cervical and lumbar regions, making the cervical and lumbar enlargements where nerves are given off to the extremities.Anterior and posterior fissures running from without inwards almost divide the cord into lateral halves. The anterior fissure is a distinct gap lined with pia mater: the posterior fissure shows no gap, but a connective tissue partition.
Grey and white matter of cord.
Like the brain, the spinal cord is composed of grey and white matter. The grey matter is arranged in two crescent-like masses, lying one in each half of the cord and joined by a narrow bridge of the same substance, the grey commissure, in the middle of which runs the central canal. The extremities of the crescents are called anterior and posterior horns: the horns represent columns of grey matter.
White matter surrounds the grey, and consists of fibres, forming the tracts or paths of the cord, for conducting messages to and fro in the nervous system. Each lateral half is divided into anterior, lateral, and posterior tracts. The fibres in the anterior, and part of the lateral tracts, are connected with the cells of the anterior horn: they conduct motor impulses: the fibres in the posterior, and part of the lateral tracts, are connected with the posterior horn and conduct sensory impressions.
A spinal nerve is composed of motor and sensory fibres, and is connected with the cord by two roots—an anterior root running from the motor tracts and cells, and a posterior root running to the sensory tracts and cells: on the posterior root is a swelling, the posterior root ganglion. Motor and sensory roots unite to form a mixed nerve which leaves the spinal canal through an intervertebral foramen. After its emergence from the foramen the nerve again divides into anterior and posterior primary divisions. Branches from the anterior primary divisions form plexuses which give off nerves to the extremities.Branches from the posterior primary divisions supply the skin and muscles of the back.
Functions of the spinal cord.
It consists of the paths which conduct impulses to and from the brain.It is an important centre for reflex action.
Impulses from the brain travel by the motor path: this consists of two sections: (a) an upper motor neuron, comprising a cell in the motor (pre-Rolandic) area of the cortex, its dendrons, and its axon which passes down through the internal capsule, and the pons, to the medulla, where usually it crosses to the opposite side and descends through the cord to end in an arborization round a motor cell in the anterior horn; (b) a lower motor neuron comprising the motor cell in the anterior horn, its dendrons and its axon, which passes down in a peripheral nerve to the muscle and ends in the muscle fibre.
At least three neurons form the sensory nerve path. The lower or peripheral neuron has a cell in the ganglion of the posterior nerve root which divides into two limbs: one, corresponding to an elongated dendron, communicates with the end organ, e.g. skin; the other, the axon, passes up in the posterior columns of the cord to arborize with the cell in the medulla. From here the nerve impulse is relayed by a second central neuron to the optic thalamus. From the optic thalamus a third neuron conveys the impulse through the internal capsule to the cortical sensory centres in the post-Rolandic area of the cerebrum.
A reflex has been described as a ‘ motor response to a sensory stimulus.’ The essential parts concerned are:
sensory (receptor) organ to receive a stimulus
sensory neuron to convey the stimulus to the central nervous system synapse of the sensory neuron with motor neuron to carry the impulse to a muscle or gland (effector organ) which carries out the reaction.
The above structures constitute a reflex arc in its simplest form, but in man there is always, in addition, at least one connector neuron in the circuit.A reflex act takes place independently of the will, and many are unconsciously performed.
Some examples of reflexes:
The flexion reflex. If the sole of the foot is pricked, the knee and ankle joints bend, removing the part from danger: this reaction occurs even in sleep and is protective.
The corneal reflex. If the eyeball is touched, the eyelids close.
The knee jerk. If the patella tendon is tapped, the leg kicks.
Many complex actions which are automatically performed, such as walking, are classed as reflexes, and are built up of a number of simple reflexes.
The peripheral nerves.
These are the bundles of axons, bound together by connective tissue, that emerge from the brain and spinal cord.The cranial nerves. Twelve pairs of nerves are given off from the brain. Some of them are entirely sensory, others entirely motor, and, again, others are mixed.
Olfactory.special nerve of smell (sensory).
Optic—special nerve of sight (sensory).
Oculo-motor—motor to muscles of eyeball.
Trochlear—motor to muscles of eyeball.
Trigeminal—mixed. Sensory branches to the face and head. Motor branch to the muscles of mastication.
Abducens—motor to muscles of eyeball.
Facial—motor to muscles of facial expression.
Auditory (sensory)—special nerve of hearing and balance.
Glosso-pharyngeal (mixed)—special nerve of taste. Sensory to the tongue. Motor to the pharyngeal muscles.
Spinal accessory motor to stemo-mastoid and trapezius muscles.
Hypoglossal—motor to tongue muscles.
The fifth nerve, trigeminal or trifacial, has a ganglion at the root (Gasserian ganglion) which lies in the middle cranial fossa: from it three divisions are given off:
a) The ophthalmic nerve, which passes through the orbit to the eye, forehead, and nose (sensory
(b) The maxillary nerve, which goes to the upper jaw, teeth, and upper lip (sensory).
(c) The mandibular nerve, which sends sensory fibres to the lower jaw, teeth, skin, and mucous membrane, and a motor branch to the muscles of mastication.
The seventh or facial nerve leaves the cranial cavity with the auditory nerve. It winds through the middle ear and leaves the skull through the stylomastoid foramen, and passing forwards through the ear breaks into a number of branches to the muscles of facial expression.
The tenth or vagus or -pneumogastric nerve leaves the skull through the jugular foramen and passes down through the neck, chest, and abdomen to terminate in branches to the stomach. In its course branches are given off to the larynx, pharynx, lungs, and heart.
Spinal nerves. There are thirty-one pairs of spinal nerves. They leave the canal at the intervertebral foramina and are named accordingly:
- Cervical 8
- Thoracic 12
- Lumbar 5
- Sacral 5
- Coccygeal 1
Cervical plexus. This is formed by the first four cervical nerves. It is situated in the neck under the sterno- mastoid muscle, and gives off numerous branches to the skin and muscles of the neck and shoulders.
The phrenic nerves arise here, and passing through the thorax to the diaphragm receive involuntary fibres on their way.
The Brachial plexus is formed by the four lower cervical and first thoracic nerves: it is situated in the lower part of the neck under the clavicle and in the axilla. Among the many important nerves given off from it are the following:
The circumflex [axillary) supplies the deltoid and teres minor muscles.
The musculo-cutaneous supplies biceps and flexors of the elbow and the skin on the outer side of the forearm.
The musculo-spiral (radial) winds round the back of the humerus in the triceps muscle in the musculo-spiral groove to the front of the outer side of the elbow. It gives off branches to the triceps and skin, and divides into a deep branch to the extensor and supinator muscles of the wrist, thumb, and fingers, and a superficial branch to the skin of the back of the hand and outer three and a half fingers.
The median nerve passes from the axilla to the inner side of the arm with the brachial artery to the elbow, and thence down the middle of the forearm to the palm of the hand. It is the chief nerve supply of the flexor and pronator muscles of wrist and fingers.
The ulnar nerve passes down the inner side of the arm and reaches the forearm; passing between the inner epi- condyle and the olecranon process, it supplies the inner flexor muscles of the forearm and the small, deep muscles of the hand.
The intercostal nerves are twelve pairs of thoracic nerves which supply the intercostal muscles and the skin of the chest.
The Lumbar plexus is formed by the first four lumbar nerves, it lies in the substance of the psoas muscle; its main branches to the leg are:
The anterior crural (femoral nerve) passes under Poupart’s ligament with the psoas tendon, and supplies the extensor muscles on the front of the thigh (quadriceps).
The obturator nerve passes to the thigh through the obturator foramen and supplies the adductor muscles and hip and knee joints.
The Sacral plexus is formed by the lumbosacral cord (fifth lumbar and all sacral nerves) in the hollow of the sacrum: its great branch is the great sciatic nerve, which leaves the pelvis through the great sciatic notch, and goes down the back of the thigh supplying the hamstring muscles. About halfway down the thigh the nerve
divides into the internal and external popliteal nerves. The external popliteal nerve winds round the head of the fibula and gives off branches which supply the skin and muscles of the outer side of the leg and dorsum of the foot. The internal popliteal divides behind the inner ankle into two plantar nerves which supply the skin and muscles of the sole of the foot and the toes.
The Autonomic nervous system.
This is sometimes called the Involuntary nervous system, and is that part of the nervous system which supplies involuntary muscle all over the body. It is divided into :
The Parasympathetic system (also called the Craniosacral division).
The Sympathetic (Thoracolumbar division).
10 Facts Everyone Should Know About Human Nervous System
The Parasympathetic system has its centres (a) in the medulla, and (b) in the sacral portion of the spinal cord. Fibres from the medulla are found chiefly in the vagus nerve which supplies branches to all the thoracic and abdominal organs, but also in the motor nerves to the eye, the glosso-pharyngeal, and facial nerves. Fibres from the lowest part of the spinal cord emerge from the anterior roots of the sacral nerves and run down to the large intestine and bladder.
The Sympathetic system—also called the thoracolumbar —consists of a pair of gangliated nerve cords running • down in front and on either side of the bodies of the vertebrae from the skull to the pelvis. Each ganglion is connected by fine branches (rami communicantes) with the nearest spinal nerves, and from the ganglia fibres pass to unstriped muscle all over the body—to every blood vessel, to brain and heart, to the unstriped muscle in all glands, in the sphincters, in the intestines, and in the skin: they also supply the bladder sphincter, and internal generative organs.The splanchnic nerves are the largest branches from the ganglia; they are given off in the chest, and, piercing the diaphragm, send fibres to all the upper abdominal organs.
Most of the fibres to the internal organs form three great plexuses before going to the organs themselves. These are:
The cardiac plexus, in the chest round the heart and the root of lungs.
The solar plexus, in front of the aorta and at the beginning of the celiac artery.
The hypogastric plexus, in the pelvis.
It will be seen that many organs are supplied with two sets of fibres often having opposing functions.
The parasympathetic division slows the heart (through the vagus), aids digestion by its action on the glands, produces peristalsis, and contracts the bladder.
The sympathetic system regulates blood pressure by its branches, constituting the vasomotor nerves, which contract and dilate the vessels: it controls sweating, and quickens the heart and respiration. Intimately associated with the sympathetic system are the adrenal glands, and all sympathetic activity is accompanied by increased adrenal secretion.
The involuntary system contains sensory fibres as well as motor, and these, when stimulated, may give rise to pain; the pain, however, is usually not in the affected organ itself, but on’ the surface of the body—for instance, impulses from the stomach may give rise to pain and tenderness of the skin near the ribs: this is known as referred pain.