Nervous System

Spinal Nerves

Thirty-one pairs of spinal nerves originate from the spinal cord. They are all mixed nerves, and they provide a two-way communication system between the spinal cord and parts of the arms, legs, neck and trunk of the body. Although spinal nerves do not have individual names, they are grouped according to the level from which they stem, and each nerve is numbered in sequence. Hence, there are eight pairs of "cervical nerves" (numbered C1 - C8), twelve pairs of "thoracic nerves" (T1 - T12), five pairs of "lumbar nerves" (L1 - L5), five pairs of "sacral nerves" (S1 - S5), and one pair of "coccygeal nerves". The nerves coming from the upper part of the spinal cord pass outward nearly horizontally, while those from the lower regions descend at sharp angles. This is derived from the consequence of growth. In early life, the spinal cord extends the entire length of the vertebral column, but with age, the column grows faster than the cord. As a result, the adult spinal cord ends at the level between the first and second lumbar vertebrae, so the lumbar, sacral, and coccygeal nerves descend to their exits beyond the end of the cord.

Brain

The brain is a jelly-like substance, which in adults weighs about three pounds. It is divided into three parts: the brain stem, which is an extension of the spinal cord, the forebrain (which consists mainly of the cerebruim) and the cerebellum. The forebrain and cerebellum are divided into two hemispheres which are linked by a thick band of nerve fibers and these hemispheres have areas, called "lobes," which perform specific functions. The brain's surface lies in rather ugly, wrinkled folds. Traditionally referred to as one's "gray matter," it does, indeed, contain gray nerve cell bodies which surround a smaller mass of white nerve fibers. The brain, like the heart, is protected by a buffer zone. This, in the form of fluid, may be the source of "water on the brain," but it is very necessary to our survival. Only these pools of fluid and the skull protect the brain from the bumps and grinds of daily living which would damage this fragile organ. With them, we are able to think, reason, love, forgive, create and remember, as well as to survive through automatic processes such as breathing and digesting, and we have reflexes which signal in case of "fight or flight" emergencies. Just think of it!

Sciatic Nerves

The sciatic nerves are branches of the lumbar nerves and are the largest and longest nerves in the body. They descend into the buttock and into the thighs, where they divide into the "tibial" and "common peroneal" nerves. The many branches of these nerves supply nerve impulses to and from the muscles and skin in the hip joints and thighs, the lower legs, feet and most of the skin below the knee.

Common Plantar Digital Nerve

The three common digital nerves, stemming from the medial plantar nerve, pass between the divisions of the plantar aponeurosis; each splits into the two "proper digital nerves." Those of the first common digital supply the adjoining side of the great and second toes; those of the second, the adjoining side of the second and third toes; those of the third, the sides of the third and fourth toes.

The Ear

The ear is an organ for hearing and balance. It consists of three parts: the outer ear, the middle ear, and the inner ear. The outer and middle ear mostly collect and transmit sound. The inner ear analyzes sound waves and contains an apparatus that maintains the body's balance. The outer ear is the part which is visible and is made of folds of skin and cartilage. It leads into the ear canal, which is about one inch long in adults and is closed at the inner end by the eardrum. The eardrum is a thin, fibrous, circular membrane covered with a thin layer of skin. It vibrates in response to changes in the air pressure that constitute sound. The eardrum separates the outer ear from the middle ear. The middle ear is a small cavity which conducts sound to the inner ear by means of three tiny, linked, movable bones called "ossicles." These are the smallest bones in the human body and are named for their shape. The hammer (malleus) joins the inside of the eardrum. The anvil (incus) has a broad joint with the hammer and a very delicate joint to the stirrup (stapes). The base of the stirrup fills the oval window which leads to the inner ear. The inner ear is a very delicate series of structures deep within the bones of the skull. It consists of a maze of winding passages, called the "labyrinth". The front (see cochlea) is a tube resembling a snail's shell and is concerned with hearing. The rear part is concerned with balance. "Keeping one's ear to the ground" means to keep up on current trends. The phrase dates back to early 20th century politicians and comes from frontier lore of both pioneers and Indians, who listened for the sound of approaching hoofbeats.

The Eye

The eyes are the two organs of sight. They are located in the front upper part of the skull and consist of structures that focus an image onto the retina at the back of the eye which is a network of nerves that convert this image into electrical impulses to be recorded in a region of the brain. The eyeball lies in pads of fat within the orbit, a bony socket that provides protection from injury. Each eyeball is moved by six delicate muscles which are activated and coordinated by nerves in the brain stem. The eyeball has a tough, outer coat called the "sclera," or white part of the eye. The front, circular part is the "cornea" and is transparent. The cornea is the main lens of the eye and performs most of the focusing. Behind the cornea is a shallow chamber full of watery fluid, at the back of which is the "iris" (colored part) with the "pupil" (center). The pupil is black and its diameter is changed by light intensity to control the amount of light which enters the eye. Immediately behind the iris, and in contact with it is the crystalline lens, which contracts to alter its shape and allow focusing power. Behind the lens is the main cavity of the eye, filled with a clear gel. On the inside of the back of the eye is the retina, a structure of nerve tissue on which the image formed by the cornea and the crystalline lens forms. The retina needs a constant supply of oxygen and sugar, and the need is supplied by a thin network of branching blood vessels which lie just under it called the choroid plexus. The eyeball is sealed off from the outside by a flexible membrane called the "conjunctiva," which is attached to the skin at the corners of the eye and forms the inner lining of the lids and contains many tiny tear-secreting and mucus-forming glands that protect the eyes from damage due to dryness. A "black eye" is really not black at all. A "shiner" is a mixture of purple, red and yellow hues caused by hundreds of tiny, broken blood vessels.

Facial Nerves

The facial nerve is the seventh cranial nerve. It begins in parts of the brain stem and branches into the face, neck, salivary glands and outer ear. The facial nerve performs both motor and sensory functions. Branching up from the brain stem, it divides into smaller nerves that reach into the face, neck, salivary glands and the outer ear. These branches control the muscles of the neck, the facial expressions, and the muscles of the forehead. They also stimulate secretions of the lower jaw and those salivary glands which are in the front of the mouth. Along with this, they report taste sensations from the front two-thirds of the tongue and carry sensations from the outer ear. Although a spontaneous grin and a deliberate smile both use lip and cheek muscles, each involves a different neural pathway from the brain.

Femoral Nerves

Femoral nerves are branches which stem from the lumbar nerves and divide into many smaller branches to supply motor impulses to the muscles of the thighs and legs; they receive sensory impulses from the skin of the thighs and the lower legs.

Hair Follicles

Hair is present on all skin surfaces except the palms, soles, lips, nipples, and various parts of the external reproductive organs; however, it is not always well developed. For example, it is very fine on the forehead and the inside surface of the arm. Each hair develops from a group of epidermal cells at the base of a tube-like depression called a "hair follicle." This follicle extends from the surface into the dermis and may pass into the subcutaneous layer. The cells at its base receive nourishment from dermal blood vessels that occur in a projection of connective tissue, called the derma papilla, at the base of the follicle. As these epidermal cells divide and grow, older cells are pushed toward the surface. The cells that move upward and away from the nutrient supply then die. Their remains constitute the shaft of a developing hair. In other words, a hair is composed of dead epidermal cells. A bundle of smooth muscle cells, forming the "arrector pili muscle," is attached to each hair follicle. This muscle is positioned so that the hair within the follicle stands on end when the muscle contracts. If a person is emotionally upset or very cold, nerve impulses may stimulate the arrector pili muscles to contract, causing gooseflesh or "goose bumps." Each hair follicle also has one or more sebaceous glands associated with it.

Lateral Plantar Nerve

The lateral plantar nerve supplies the skin of the fifth and lateral half of the fourth toes, as well as most of the deep muscles of the foot; its distribution can be compared to the ulnar nerve in the hand. It passes along the outside of the foot, dividing into a superficial and a deep branch.

Medial Nerves

Medial nerves are branches which stem from the cervical nerves, which supply impulses to the muscles of the forearms, and to the muscles and skin of the hands.

Medial Plantar Nerve

The medial plantar nerve, the larger of the two terminal branches of the tibial nerve (which stem from the sciatic nerve), accompanies the medial plantar artery. From its origin under the "flexor retinaculum" it passes deep into the muscles of the toes, and becomes the "proper plantar digital nerve" to the great toe. It finally divides opposite the bases of the toes into the three "common digital nerves."

Musculocutaneous Nerve

The musculocutaneous nerve in the arm is formed by splitting the brachial plexus at the inside border of the pectoral muscles into two branches. These branches continue to split as they run down through the upper arm, forearm, and into the wrist and hand. In the leg, the "superficial peroneal nerve" (also called musculocutaneous nerve) passes between the peronei and the extensor digitorum longus, pierces the deep tissues of the lower third of the leg, and divides into a "medial" and "intermediate dorsal cutaneous nerve."

Nasal Passages

Smell is often considered to be the least important of all the senses, but it may be one of the oldest, and probably acts on the subconscious more than the other senses. There is little doubt that scents have important roles in human behavior. The body is provided with glands to produce specific odors, many of which appear to be associated with sexual attraction and excitement, and others that have considerable significance as well. The bond between a baby and its mother is thought to be tightened by a form of "scent imprinting." In it, a baby which is suckling at the mother's breast pushes his or her face into a bank of scent organs that surround the nipple. A further sign of the importance of the sense of smell is the way it becomes a major source of information when other senses are not working, especially sight. Only a small part of the nose and nasal cavity is taken up by the organs of smell; the rest of it is mainly concerned with processing the airflow on its way through to the lungs. The walls of the nasal cavity, and particularly the flaplike middle and inferior conchae, are coated with respiratory mucous membranes which incorporate a vast number of tiny hairlike cells which act to move waves of mucus toward the throat. Dust, bacteria, and chemical particles which are inhaled from the air are trapped by the mucus, carried back and swallowed; they are then taken care of by gastric juices to nullify any potential harm. The sense organs themselves are made up of two yellowish-gray patches of tissue, called the olfactory membranes, each about the size of a postage stamp. They are located in a pair of clefts just under the bridge of the nose and at the top of the nasal cavity. The reasons for the coloration are not completely clear, but it seems to be necessary for the membrane to work. During normal breathing, most of the air flows through the nose, with only a small part reaching the olfactory clefts, but this is enough to get a response to a new smell. When a person "sniffs the air" to detect smells, the air moves through the nose much faster, increasing the flow that makes its way to the olfactory clefts and so carrying more odor to those sensors. If you "follow your nose," you are taking a route that lies straight ahead and is obvious (as the nose on your face), or else you are going ahead without a plan, that is, following wherever instinct leads.

Palmar Nerve

The palmar nerve is a division of the ulnar nerve that supplies the sides of the digits

Posterior Femoral Cutaneous Nerve

The posterior femoral cutaneous nerve (small sciatic nerve) is distributed to the skin of the perineum and the back surface of the thigh and leg. It stems from the sacrum and leaves the pelvis through the greater sciatic foramen (opening in the bone). It accompanies the inferior gluteal artery to the gluteus maximus (large muscle in the buttocks) and runs down the outer thigh and deep into the tissue at the back of the knee. It accompanies the small saphenous vein to the middle of the back of the leg.

Pudendal Nerve

The pudendal nerve arises from the sacrum and leaves the pelvis through the greater sciatic foramen (opening through the bone). It then crosses the spine of the ischium and reenters the pelvis through the lesser sciatic foramen. It splits into two terminal branches as it approaches the urogenital diaphragm (which fills the space within the pubic arch).

Saphenous Nerve

The saphenous nerve is the largest and longest branch of the femoral nerve; it supplies the skin on the inside of the leg.

Sciatic Nerves

The sciatic nerves are branches of the lumbar nerves and are the largest and longest nerves in the body. They descend into the buttock and into the thighs, where they divide into the "tibial" and "common peroneal" nerves. The many branches of these nerves supply nerve impulses to and from the muscles and skin in the hip joints and thighs, the lower legs, feet and most of the skin below the knee.

Skin

Skin is the outside covering of body tissue, which protects inner cells and organs from the outside environment. The skin is the largest organ of the body, and its cells are continuously replaced as they are lost to normal wear and tear. The skin totals between twelve and twenty square feet in area and accounts for 12%% of body weight. It is composed of three integrated layers: the epidermis, the dermis and the subcutis. The thickness of the epidermis and the dermis varies over different parts of the body. It is thickest on the palms of the hands and feet, where friction is needed for gripping, and it is thinnest on the eyelids, which must be light and flexible. The epidermis also grows into fingernails, toenails and hair. The dermis, or true skin, is thick, sturdy, rich in nerves and blood vessels and in sweat glands. It shields and repairs injured tissue. This layer consists mostly of collagen, which originates from cells called fibroblasts and is one of the strongest proteins found in nature. It gives skin durability and resilence. The subcutis, joined to the bottom of the dermis, is the deepest layer of the skin. It contains "lipocytes," which produce lipids for the subcutaneous tissue to make a fatty layer which cushions muscles, bones and inner organs against shocks, and acts as an insulator and source of energy during lean times. The skin registers sensation constantly and supports a teeming, unseen population of tiny organisms. Not only does the skin harden from use, but it molds into varied shapes, and it responds to the most delicate touch, becoming an organ of communication - sometimes more eloquent than words. So tough and durable is the skin that when a 2,000-year-old Egyptian mummy was fingerprinted, the ridges were perfectly preserved.

Sural nerve

The sural nerve (short saphenous nerve) lies with the small saphenous vein. It supplies the branches to the skin on the back of the leg and then continues as the "lateral dorsal cutaneous nerve" along the outside of the foot and little toe.

Tongue

Anchored to the floor of the mouth and slung at the rear from muscles attached to a spiky outgrowth at the base of the skull, the tongue is a strong muscle that is covered by the lingual membrane, which has special areas which detect the flavor of food. The tongue is made up of muscles covered by mucous membranes. These muscles are attached to the lower jaw and to the hyoid bone (a small, U-shaped bone, which lies deep in the muscles at the back of the tongue) above the larynx. There are very small nodules, called papillae, from the top surface of the tongue, which give it its rough texture. Between the papillae at the sides and base of the tongue are small, bulblike structures that are sensory organs, called "taste buds," which enable us to enjoy the sensations of flavor and warn us when food is unfit to eat. The muscle fibers are heavily supplied with nerves, so it can manipulate food in the mouth and place it between the teeth for chewing - without being bitten in the process. Babies have many more taste buds than an adult, and they have these almost everywhere in the mouth, including the cheeks. Nevertheless, adults enjoy more flavors than babies, who dislike bitter tastes and prefer bland food. The tongue also aids in the formation of sounds of speech and coordinates its movements to aid in swallowing. It is especially helpful when we are forced to "eat our words." Enjoy!

Ulnar Nerves

The ulnar nerves are branches stemming from the cervical nerves, which supply impulses to and from the muscles of the forearms and hands and from the skin of the hands.