The Nervous System

NERVOUS SYSTEM

System of specialized cells (neurons, or nerve cells) that conduct stimuli from a sensory receptor through a neuron network to the site (e.g., a gland or muscle) where the response occurs.

In humans, it consists of the central and peripheral nervous systems, the former consisting of the brain and spinal cord and the latter of the nerves, which carry impulses to and from the central nervous system. The cranial nerves handle head and neck sensory and motor activities, except the vagus nerve, which conducts signals to visceral organs. Each spinal nerve is attached to the spinal cord by a sensory and a motor root. These exit between the vertebrae and merge to form a large mixed nerve, which branches to supply a defined area of the body. Disorders include amyotrophic lateral sclerosis, chorea, epilepsy, myasthenia gravis, neural tube defect, parkinsonism, and poliomyelitis. Effects of disorders range from transient tics and minor personality changes to major personality disruptions, seizures, paralysis, and death.

AUTONOMIC NERVOUS SYSTEM

Part of the nervous system that is not under conscious control and that regulates the internal organs.

It includes the sympathetic, parasympathetic, and enteric nervous systems. The first, which connects the internal organs to the brain via spinal nerves, responds to stress by increasing heart rate and blood flow to the muscles and decreasing blood flow to the skin. The second comprises the cranial nerves and the lower spinal nerves, which increase digestive secretions and slow the heartbeat. Both have sensory fibres that send feedback on the condition of internal organs to the central nervous system, information that helps maintain homeostasis. The third division, embedded in the walls of the stomach and intestines, controls digestive movement and secretions.

AUTONOMIC NERVOUS SYSTEM

Pathways of the autonomic nervous system. Nerve impulses begin in motor neurons in the brain or spinal cord. Each motor neuron connects with a second motor neuron outside the central nervous system to carry the impulse to the glands and smooth muscles. In the sympathetic nervous system, these second motor neurons are found in ganglia (masses of neurons) along either side of the spinal cord. In the parasympathetic nervous system, ganglia are located closer to, or sometimes within, the target organs. Some ganglia form large clusters called plexuses. Preganglionic fibres of the sympathetic system emerge along the thoracic (T) and first three lumbar (L) segments of the spinal cord. Fibres of the parasympathetic system originating in the brainstem arise from the third, seventh, ninth, and tenth cranial nerves; other parasympathetic fibres emerge from the second, third, and fourth sacral (S) segments of the spinal cord.

THE BRAINConcentration of nerve tissue in the front or upper end of a body.

It handles sensory information, controls motion, is vital to instinctive acts, and in higher vertebrates is the centre of learning. Vertebrate brains consist of the hindbrain (rhombencephalon), midbrain (mesencephalon), and forebrain (prosencephalon). The hindbrain comprises the medulla oblongata and the pons, which connects the spinal cord with higher brain levels and transfers information from the cerebral cortex to the cerebellum. The midbrain, a major sensory integration centre in other vertebrates, serves primarily to link the hindbrain and forebrain in mammals. Large nerve bundles connect the cerebellum to the medulla, pons, and midbrain. In the forebrain the two cerebral hemispheres are connected by a thick bundle of nerve fibres (corpus callosum) and are divided by two deep grooves into four lobes (frontal, parietal, temporal, and occipital). The cerebrum, the largest part of the human brain, is involved with its more complex functions. Motor and sensory nerve fibres from each hemisphere cross over in the medulla to control the opposite side of the body.

Side view of the brain showing its major structures. The large cerebrum is divided into two halves, or hemispheres, connected by the corpus callosum, a band of nerve fibres. Two grooves divide the hemispheres into four lobes: frontal, temporal, parietal, and occipital. Many nerve cells are found in the convoluted cerebrum's outer surface, or cerebral cortex, which controls sensory and motor activities. The thalamus relays incoming sensory impulses from the spinal cord to the cortex. The hypothalamus's many functions include control of breathing, blood flow, temperature regulation, and emotions. The pituitary gland is attached to and regulated by the hypothalamus. The midbrain relays signals between the forebrain and hindbrain. The cerebellum, along with the cerebrum, plays a role in voluntary movement as well as balance. The pons serves as a relay point linking the medulla oblongata, midbrain, cerebellum, and cerebrum. The medulla, lying between the pons and the spinal cord and continuous with both, plays a role in essential involuntary regulatory and reflexive responses (including breathing, swallowing, and heartbeat) and relays signals between the spinal cord and other brain regions

PARTS OF BRAIN

1. Cerebrum : Largest part of the brain.

The two cerebral hemispheres consist of an inner core of myelinated nerve fibres, the white matter, and a heavily convoluted outer cortex of gray matter (see cerebral cortex). Nerve fibres in the white matter connect functional areas of the cortex in the same hemispheres, connect functional areas of the cortex in opposite hemispheres, and connect the cerebral cortex to lower centres (e.g., the spinal cord). A front-to-back fissure divides the cerebrum's two hemispheres. Each hemisphere controls the opposite side of the body. The corpus callosum, a thick band of white matter, connects them, allowing integration of sensory data and responses from both sides of the body. Other important cerebral structures include the hypothalamus and the thalamus.

2. Cerebellum

Part of the brain that integrates sensory input from the inner ear and from proprioceptors in muscle with nerve impulses from the cerebrum (see cerebral cortex), coordinating muscle responses to maintain balance and produce smooth, coordinated movements.

Located below the cerebral hemispheres and behind the upper medulla oblongata and pons, each of its two connected hemispheres has a core of white matter within a cortex of gray matter. Disorders usually produce neuromuscular disturbances, in particular ataxia.

NERVESSensory neurons relay information from sense organs, motor

neurons carry impulses to muscles and glands, and interneurons transmit impulses between sensory and motor neurons. A typical neuron consists of dendrites (fibres that receive stimuli and conduct them inward), a cell body (a nucleated body that receives input from dendrites), and an axon (a fibre that conducts the nerve impulse from the cell body outward to the axon terminals). Both axons and dendrites may be referred to as nerve fibres. Impulses are relayed by neurotransmitter chemicals released by the axon terminals across the synapses (junctions between neurons or between a neuron and an effector cell, such as a muscle cell) or, in some cases, pass directly from one neuron to the next. Large axons are insulated by a myelin sheath formed by fatty cells called Schwann cells. Bundles of fibres from neurons held together by connective tissue form nerves.

NERVE CELL

Structure of a neuron. Dendrites, usually branching fibres, receive and conduct impulses to the cell body, where inputs arriving from various dendrites are integrated. Nerve impulses are conducted along the axon. When an impulse reaches the axon terminals, neurotransmitter chemicals are released into a gap (synapse) between the neuron and a neighbouring cell, and the impulse is transmitted to an adjacent neuron or effector cell (as of a muscle or gland). Schwann cells surround large axons, forming an insulating sheath. Spaces (nodes of Ranvier) between Schwann cells serve to conduct the nerve impulse quickly along the axon.

THE SPINAL CORD

The spinal cord is cylinder of nerves, that is as thick as your finger. It starts from below the medulla and goes down to the middle of the back till the waist. It is protected by the backbone or spine. All messages travell from the brain to the spinal cord and then to different parts of the body. Message from different parts of the body enter the spinal cord and travel up to the brain

SENSE ORGANMechanism by which information is received about

one's external or internal environment.

Stimuli received by nerves, in some cases through specialized organs with receptor cells sensitive to one type of stimulus, are converted into impulses that travel to specialized areas of the brain, where they are analyzed. In addition to the “five senses”—sight, hearing, smell, taste, and touch—humans have senses of motion (kinesthetic sense), heat, cold, pressure, pain, and balance. Temperature, pressure, and pain are cutaneous (skin) senses; different points on the skin are particularly sensitive to each

1. EYES-FOR SIGHT

Organ that receives light and visual images.Non-image forming, or direction, eyes are found among worms, mollusks, cnidarians, echinoderms,

and other invertebrates; image-forming eyes are found in certain mollusks, most arthropods, and nearly all vertebrates. Arthropods are unique in possessing a compound eye, which results in their seeing a multiple image that is partially integrated in the brain. Lower vertebrates such as fish have eyes on either side of the head, allowing a maximum view of the surroundings but producing two separate fields of vision. In predatory birds and mammals, binocular vision became more important. Evolutionary changes in the placement of the eyes permitted a larger overlap of the two visual fields, resulting in the higher mammals in a parallel line of direct sight. The human eye is roughly spherical. Light passes through its transparent front and stimulates receptor cells on the retina (cones for colour vision, rods for black-and-white vision in faint light), which in turn send impulses through the optic nerve to the brain. Vision disorders include near- and farsightedness and astigmatism (correctable with eyeglasses or contact lenses), colour blindness, and night blindness. Other eye disorders (including detached retina and glaucoma) can cause visual-field defects or blindness

1. EYES-STRUCTURE

Structure of the human eye. The outer portion consists of the white protective sclera and transparent cornea, through which light enters. The middle layer includes the blood-supplying choroid and pigmented iris. Light passing into the interior through the pupil is regulated by muscles that control the pupil's size. The retina comprises the third layer and contains receptor cells (rods and cones) that transform light waves into nervous impulses. The lens, lying directly behind the iris, focuses light onto the retina. The macula lutea, in the centre of the retina, is a region of high visual acuity and colour discrimination. Nerve fibres pass out through the optic nerve to the brain's visual centre. The eye's anterior and posterior chambers contain a watery fluid that nourishes the cornea and lens. The vitreous humour helps maintain the eye's shape. A thin layer of mucous membrane (conjunctiva) protects the eye's exposed surface. External muscles, including the medial rectus and lateral rectus muscles, connect and move the eye in its socket.

2. EARS-TO LISTENOrgan of hearing and balance.The outer ear directs sound vibrations through the auditory canal to the

eardrum, which is stretched across the end of the auditory canal and which transmits sound vibrations to the middle ear. There a chain of three tiny bones conducts the vibrations to the inner ear. Fluid inside the cochlea of the inner ear stimulates sensory hairs; these in turn initiate the nerve impulses that travel along the auditory nerve to the brain. The inner ear is also an organ of balance: the sensation of dizziness that is felt after spinning is caused when fluid inside the inner ear's semicircular canals continues to move and stimulate sensory hairs after the body has come to rest. The eustachian tube connects the middle ear with the nasal passages; that connection allows the common cold to spread from the nasal passages to the middle ear, especially in infants and small children. The most common cause of hearing loss is otosclerosis, a surgically correctable disease in which one of the bones of the middle ear loses its capacity to vibrate.

STRUCTURES OF THE HUMAN EAR

Structures of the human ear. The cartilaginous auricle and the auditory canal of the outer ear direct sound waves to the middle ear. The eardrum, stretched across the end of the canal, vibrates as sound waves reach it. Vibrations are transmitted via three small bones (hammer, anvil, stirrup) to the membranous oval window, which links the middle ear to the inner ear. The cochlea is a coiled, fluid-filled tube lined with sensory hairs. Vibrations in the oval window cause movement of the cochlear fluid, stimulating the hairs to initiate impulses that travel along a branch of the auditory nerve to the brain. The eustachian tube, running from the middle ear to the nasopharynx, equalizes pressure between the middle and outer ear. The fluid-filled semicircular canals play a role in balance, as hairs in the canals respond to movement-induced changes in the fluid by initiating impulses that travel to the brain.

3. NOSE-TO SMEEL

Prominent structure between and below the eyes.

With the complex nasal cavity behind it, it functions for breathing and smelling. Behind the front section (vestibule), which includes the nostrils, it is divided vertically by three convoluted ridges (conchae) into air passages. In the highest one, the olfactory region, a small segment of mucous membrane lining contains neurons covered by a moisture layer, in which microscopic particles in inhaled air dissolve and stimulate the neurons. The rest of the cavity warms and moistens inhaled air and filters particles and bacteria out of it. Sinus cavities in the bone on both sides of the nose drain into the air passages. During swallowing, the soft palate closes off the back of the nose against food.

4. TOUNGE- TO TASTEMuscular organ on the floor of the mouth.

It is important in motions of eating, drinking, and swallowing, and its complex movements shape the sounds of speech. Its top surface consists of thousands of raised projections (papillae). The receptors of taste (taste buds) are embedded in the papillae and are sensitive to four basic flavours: sweet, salty, sour, and bitter. More specific flavours are influenced by the sense of smell. The tongue's appearance (e.g., coated or red) can give clues to disease elsewhere. Disorders of the tongue include cancer (often caused by smokeless tobacco), leukoplakia (white patches), fungal infection, and congenital disorders. Different animals use the tongue to serve varied functions; for example, frogs have an elongated tongue adapted to capturing prey, the snake's tongue collects and transfers odours to a specialized sensory structure to help locate prey, and cats use their tongues for grooming and cleaning.

Tounge for Taste

Special sense for perceiving and distinguishing the sweet, sour, bitter, or salty quality of a dissolved substance, mediated by taste buds on the tongue.

More than 9,000 taste buds on the tongue are responsible for the chemoreception of taste. Some taste buds are also found on the roof of the mouth and throat.

A. Taste centres on the tongue's surface. Taste buds on the tip of the tongue are most sensitive to sweet tastes, those on the sides to sour, those at the back to bitter, and those on the tip and sides to salty. B. Close-up of a papilla showing location of the taste buds. C. Structure of a taste bud. Each is composed of narrow modified epithelial taste cells with specialized hairs (microvilli) that project into a pore opening onto the tongue's surface and of broader supporting cells. Impulses from the taste cells are carried by nerve pathways to the brain.

5. SKIN-TO FEELSurface covering of the body that protects it and receives external sensory

stimuli, consisting of an epidermis over a thicker dermis.

The epidermis contains cells involved in immune defenses, sensory receptors, pigment cells, and keratin-producing cells. The last harden and migrate to the surface to form a dead, relatively dry outer layer of horny tissue that constantly sloughs away. The dermis contains sensory nerves and blood vessels within connective tissue. Collagen and elastin fibres give skin its tough, elastic quality. Cells scattered through it produce its components and take part in immune and other skin responses. A fat layer under the dermis provides nutritional storage, cushioning, and insulation. Skin disorders range from dermatitis and acne to skin cancer. Changes in skin colour (e.g., jaundice) or texture may be clues to systemic disorders

SKINA section through the skin. The tough, dead cells of the outer epidermal surface (corneal layer) serve as a physical barrier and are continually replaced by cells produced in the basal layer. The thick supportive layer of dermis contains nerve endings, blood vessels, sweat glands, hair follicles, and oil glands. The hair follicle encloses the root of the hair. Oil glands associated with hair follicles secrete an oily substance (sebum) which lubricates the skin surface. The watery secretions of the tubular sweat glands are released onto the skin's surface through small pores. A layer of fat cells lies below the dermis.

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Name : MUKUL

Class : X ‘B’

K.V.S sec-14 Gurgaon

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