Human Biology

Researchers from the Massachusetts Institute of Technology (MIT) report in the Journal of Neuroscience that a chemical receptor in the brain associated with learning and memory might also play a vital role in fetal development of the respiratory system. The study proposes a possible connection to Sudden Infant Death Syndrome (SIDS) and preemie problems.
Dr. Chi-Sang Poon, a Principal Research Scientist in the Harvard-MIT Division of Health Sciences and Technology (HST) and first author of the paper, with co-authors  Zhongren Zhou, a former HST postdoctoral fellow, and Jean Champagnat of the Centre National de la Recherche Scientifique in France, states that it would be sensible for pregnant women to avoid prolonged exposure to substances that stimulate this receptor. Such substances include alcohol, PCP (angle dust) and come common anesthetic and analgesic drugs such as ketamine. Their work was supported by the National Institutes of Health, the Office of Naval Research, and the Human Frontier Science Program.

Dr. Poon noted that mutant mice lacking the receptor could not breathe or suck well. These symptoms are common in premature babies and are risk factors for Sudden Infant Death Syndrome. Their study assumes a possible correlation between abnormal activity receptors and problems in newborns, although further studies have yet to clarify this theory.
A surprise finding in the study and research, the lack of this receptor also led to high amounts of long term synaptic depression (LTD), which is an activity linked to learning and memory. They found that the increased LTD was found in the brainstem, and area in the brain not usually linked with high level functions.
The brainstem coordinates lower behaviors like breathing and other vital functions, Dr. Poon explains. The fact that there was found to be learning activity in the brainstem supports his argument that there is more intelligence going on subconsciously. The NMDA receptor has been of interest to scientists studying learning and memory. The N-methyl-D-aspartate (NMDA) receptor is the key to the communication of a chemical signal between two nerve cells. That process, repeated between many cells is how a signal is propagated through the brain.
The study, however, had an unexpected hitch – the animals died soon after birth. This was later solved by restricting the NMDA knockout to a specific area of the brain rather than throughout the organ. The team, though, interested in why the first animals were dying, explored a number of reasons. They eventually found the reason, that the lack of NMDA receptors during the prenatal development led to fatal respiratory distress. Normal newborn mice treated with drugs that block NMDA receptor activity did not have any respiratory problems.
Dr. Poon says that this is the first indication that prenatal development of specific regions in the brain controlling vital functions is very dependent on NMDA receptor activity. A lack of NMDA receptor activity in the fetus could affect newborns’ breathing after birth. Also, the unexpected increase in LTD in the brainstems of the mutant mice shows that learning and memory at a subconscious level could profoundly influence our vital functions. READ ARTICLE

Here is an article that talks about how cells store and release energy. It deviates from the traditional use of chemical storehouse cells because this was observed in sperm cells of horse shoe crabs.

Fertilization relies on sperm cells for most, if not all, sexually reproductive animals. They need to break into the protective surrounding of an egg cell. Biochemical reactions in cells are guided by enzymes, specialized proteins that speed up chemical reactions. Sperm Cells of horse shoe crabs however, differed in this process. It utilizes molecular harpoons, like that of barbed spear used in hunting, to penetrate through the egg’s membrane.

As scientist have observed, this harpoon consists of two proteins intertwined in a bundle of filaments. What they can’t decipher however, is the source of energy of this harpoon, so as to enable it to fire the spear in that certain level. Using a high power telescope microscope however, scientists have imaged this spear down to one billionth of a meter. This allowed them to discover how it works.

The research team figured out the exact structure of this protein strand, consisting of two molecules. These two molecules were actin and scruin. Actin is the protein involved in cell movement, which is present in muscle tissue where it plays a role in contraction. Scruin however, holds the microfilaments of the core process in a tensioned pattern so that the process is looped. These two molecules are tightly intertwined with each other, developing a slightly unbalanced, almost disfigured, helix. Yet, with this irregularity unfolds a secret that unveils the secret power of the sperm cell. The minor distortion in the helix brought about the internal tension to develop in the filament. As the tension is enclosed inside the sperm cell at the base of the nucleus, it is then patterned to the spring inside of a jack-in-the-box with the cover tightly locked. The latch then opens and the loop harpoons the egg, as a natural signal triggers the mechanism.

As theorized by the researchers, actin normally operates by chemical means; however this process doesn’t entail chemical substance. It plainly involves mechanics, where motions of objects and their responses to forces is concerned.

The essential part of this study is the ever-present protein in all species, including humans is actin. It is involved in various activities like splitting up of cells and tightening of muscles.The actin filaments can accumulate energy like this, then any type of filament or polymer contained in a cell can also imitate this energy storage ability.

Scientists are now open to possibilities that they could fabricate tools and apparatus on the nanoscale, which is beyond the fundamentals of cell biology. This is possible in designing computers and other medical devices, which are potential products of the union of biotechnology and other forms of engineering. Researchers are now looking into various molecular ways that could power up these super machines. It opened a broader prospective that there are a lot of ways of powering machines which were not discovered before. READ ARTICLE

The stomach, in human biology, is the portion of the ali­mentary canal that is richly supplied with blood vessels. Both the quantity of blood delivered to the stomach and the richness of the intramural gastric vascular anastomotic net­work are impressive. The large majority of the gastric blood supply is from the celiac axis via four named arteries. The left and right gastric arteries form an anastomotic arcade along the lesser curvature, and the right and left gastroepiploic arteries form an ar­cade along the greater gastric curvature. The consistently largest artery to the stomach in human biology is the left gastric artery. This artery usually arises directly from the celiac trunk and divides into an ascending and de­scending branch along the lesser gastric curvature. Approximately 15% of the time, the left gastric artery supplies an aberrant ves­sel, which travels in the gastrohepatic ligament (lesser omentum) to the left side of the liver. Rarely, this is the only arterial blood supply to this part of the liver, and inadvertent ligation may lead to clinically significant hepatic ischemia that could lad to hepatic injury or failure. The second largest artery to the stomach is usually the right gastroepiploic artery, which arises fairly consistently from the gastroduodenal artery behind the first portion of the duodenum. The left gastroepiploic artery arises from the splenic artery, and together with the right gastroepiploic artery, forms the rich gastroepiploic arcade along the greater curvature. The right gastric artery usually arises from the hepatic artery near the py­lorus and hepatoduodenal ligament, and runs proximally along the distal stomach. In the fundus along the proximal greater curvature, the short gastric arteries and veins arise from the splenic circulation. There also may be vascular branches to the proximal stomach from the phrenic circulation.

In human biology, the veins draining the stomach generally parallel the arteries. The left gastric or coronary vein and right gastric veins usually drain into the portal vein, though occasionally the coronary vein drains into the splenic vein. The right gastroepiploic vein drains into the superior mesenteric vein near the inferior border of the pancreatic neck, and the left gastroepiploic vein drains into the splenic vein.

The richness of the gastric blood supply and the extensive­ness of the anastomotic connections have some important clini­cal implications in human biology including: (1) erosion of a peptic ulcer or gastric cancer into a large perigastric vessel sometimes causes life threatening hemorrhage; (2) because of the rich venous intercon­nections, a distal splenorenal shunt, which connects the distal end of the divided splenic vein to the side of the left renal vein, can ef­fectively decompress esophagogastric varices in patients with portal hypertension; and (3) if necessary, at least two of the four named gastric arteries may be occluded or ligated with impunity. This is done routinely when the stomach is mobilized and pedicellated on the right gastric and right gastroepiploic vessels to reach into the neck as an esophageal replacement.

There are four distinct layers of the gastric wall in human biology: mucosa, sub­mucosa, muscularis propria, and serosa. The inner layer of the stomach is the mucosa, which is lined with columnar epithe­lial cells of various types. Beneath the basement membrane of the epithelial cells is the lamina propria, which contains connective tis­sue, blood vessels, nerve fibers, and inflammatory cells. Beneath the lamina propria is a thin muscle layer called the muscularis mucosa. The epithelium, lamina propria, and muscularis mucosa constitute the mucosa. The epithelium of the gastric mucosa is columnar glandular. A scanning electron micrograph shows a smooth mucosal carpet punctuated by the openings of the gastric glands. The gastric glands, when viewed under a microscope, are lined with different types of epithelial cells, depending upon their location in the stomach. There are also endocrine cells present in the gastric glands that can be seen under a microscope. Progenitor cells at the base of the glands differentiate and replenish sloughed cells on a regular basis. Throughout the stomach, the carpet consists primarily of mucus-secreting surface epithelial cells that extend down into the gland pits for variable distances. These cells also secrete bicarbon­ate and play an important role in protecting the stomach from injury due to acid, pepsin, and/or ingested irritants. In fact, all epithelial cells of the stomach (except the endocrine cells) contain carbonic anhydrase and are capable of producing bicarbonate.

In the cardia, the gastric glands are branched and secrete pri­marily mucus and bicarbonate, but not much acid. In the fun­dus and body, the glands are more tubular and the pits are deep. In human biology, parietal and chief cells are common in these glands. Histamine-secreting enterochromaffin-like (ECL) cells and somatostatin-secreting D cells are also found. Parietal cells se­crete acid and intrinsic factor into the gastric lumen, and bicarbon­ate into the intercellular space. They have a characteristic ultra structural appearance with secretory canaliculi (deep invaginations of the surface membrane), and cytoplasmic tubulovesicles contain­ing the acid-producing apparatus H+/K+-ATPase (proton pump). There are numerous mitochondria that can be seen under a microscope. When the parietal cell is stimulated, the cytoplasmic tubulovesicles fuse with the mem­brane of the secretory canaliculus; when acid production ceases the process is reversed. Arguably, the only truly essential substance pro­duced by the stomach is produced by the parietal cell (i.e., intrinsic factor). Parietal cells tend to occupy the midportion of the gastric glands found in the corpus of the stomach.

Chief cells also called zymogenic cells secrete pepsinogen, which is activated at a pH below 2.5. They tend to be clustered toward the base of the gastric glands and have a low columnar shape. Ultrastructurally, chief cells have the characteristics of protein-synthesizing cells: basal granular endoplasmic reticulum, supranuclear Golgi apparatus, and apical zymogen granules. When stimulated, the chief cells produce two immunologi­cally distinct proenzyme forms of pepsinogen: pepsinogen I and II. These proenzymes are activated in an acidic luminal environment. Chief cells also produce lipase in human biology.

In the antrum, the gastric glands are again more branched and shallow, parietal cells are rare, and gastrin-secreting G cells and somatostatin-secreting D cells are present. A variety of hormone­-secreting cells are present in various proportions throughout the gastric mucosa. Histologic analysis suggests that in the normal stomach, 13% of the epithelial cells are oxyntic cells, 44% are chief (zymogenic cells), 40% are mucous cells, and 3% are endocrine cells. In general terms, the antrum produces gastrin but not acid, and the proximal stomach produces acid but not gas­trin; however, it is important to recognize two facts: (1) the border between the corpus and antrum migrates proximally with age (especially on the lesser curvature side of the stomach), and (2) there are a few parietal cells in the antrum.

Deep to the mucosa is the submucosa, which is rich in branch­ing blood vessels, lymphatics, collagen, various inflammatory cells, autonomic nerve fibers, and ganglion cells of Meissner’s autonomic submucosal plexus. The collagen-rich submucosa gives strength to gastrointestinal anastomoses. The mucosa and submucosa are folded into the grossly visible gastric folds, which tend to flatten out as the stomach becomes distended.

Below the submucosa is the thick muscularis propria, which is also referred to as the muscularis externa, which consists of an incomplete Inner oblique layer, a complete middle circular layer that is continuous with the esophageal circular muscle and the circular muscle of the pylorus, and a complete outer longitudinal layer that is continuous with the longitudinal layer of the esophagus and duodenum. Within the muscularis propria is the rich network of autonomic ganglia and nerves that make up Auerbach’s myenteric plexus.

The outer layer of the stomach is the serosa, also known as the visceral peritoneum in human biology. This layer provides significant tensile strength to gastric anastomoses. When tumors originating in the mucosa penetrate and breach the serosa, microscopic or gross peritoneal metastases are common, presumably from shedding of tumor cells which would not have occurred if the serosa had not been penetrated. In this way, the serosa may be thought of as a sort of outer envelope of the stomach.

In human biology, the stomach is a remarkable organ with important digestive, nutri­tional, and endocrine functions. Its biological function is to store and facilitate the diges­tion and absorption of ingested food and helps regulate appetite. Treatable diseases of the stomach are common and the stomach is accessible and relatively forgiving; thus it is a favorite therapeutic target. In order to provide intelligent diagnosis and treatment, the physician and surgeon must understand human biology, gastric anatomy, physiol­ogy, and pathophysiology. This includes a sound understanding of the mechanical, secretory, and endocrine processes through which the stomach accomplishes its important biological functions. It also includes a familiarity with the common benign and malignant gastric disorders of clinical significance, especially peptic ulcer and gastric cancer.

ANATOMY

Embryology

The stomach arises as a dilatation in the tubular embryonic foregut during the fifth week of gestation. It assumes its nor­mal asymmetric shape and position by the end of the seventh week through descent, rotation, and progressive dilation, with dispropor­tionate elongation of the greater curvature. It is likely that there is a congenital predisposition to some unusual benign gastric problems such as diverticulum or massive hiatal hernia with abnormal gastric rotation and fixation.

Gross Anatomy

Anatomic Relationships and Gross Morphology

The stomach is recognized in human biology as the asymmetrical, pear shaped, most proximal abdominal organ of the digestive tract. The part of the stomach attached to the esophagus is called the cardia. Just proximal to the cardia at the gastroeso­phageal (GE) junction is the anatomically indistinct but physiologically demonstrable lower esophageal sphincter. At the distal end, the pyloric sphincter connects the stomach to the proximal duode­num. The stomach is relatively fixed at these points, but the large midportion is quite mobile.

The superior-most part of the stomach is the distensible floppy fundus, bounded superiorly by the diaphragm and laterally by the Teen. The angle of His is where the fundus meets the left side of the GE junction. Generally the inferior extent of the fundus is considered to be the horizontal plane of the GE junction, where the body or corpus of the stomach begins. The body of the stomach contains most of the parietal cells, some of which are also present in the cardia and fundus. The body is bounded on the right by the relatively straight lesser curvature and on the left by the more curved greater curvature. At the angularis incisura, the lesser curvature turns rather abruptly to the right, marking the anatomic beginning of the antrum, which comprises the distal 25 to 30% of the stomach.

The organs that commonly surround the stomach are the liver, colon, spleen, pancreas, and occasionally the kidney. The left lateral segment of the liver usually covers a large part of the anterior stomach. Inferiorly, the stomach is attached to the transverse colon by the gastrocolic omentum. The lesser curvature is tethered to the liver by the hepatogastric ligament also referred to as the lesser omentum. Posterior to the stomach is the lesser omental bursa and the pancreas.

Muscles provide the mass and shape of the human body and the joints afford flexibility.
There are three types of muscles in the human body- the skeletal muscles which are used to produce movements of the body, the smooth muscles which are found in the small and large intestines, urinary bladder, stomach and uterus, and the cardiac muscles which are found in the heart.

The skeletal muscles are also called voluntary muscles and are made up of striped muscle fibers. The muscles have two or more attachments. The attachment, that moves the least, is the origin, and that which moves the most is the insertion. The ends of the muscles are attached to bones, cartilage, or ligaments by cords of fibrous tissue called tendons. Muscles that run obliquely to the line of pull are referred to as pennate muscles resemb- ling a feather. Every muscle is in a partial state of contraction, while resting. This is called the muscle tone. Since muscle fibers are either fully contracted or relaxed, there being no intermediate stage, it follows that a few muscle fibers within the muscle are fully contracted all the time. To bring about this state and to avoid fatigue, different groups of motor units, and thus different groups of muscle fibers, are brought into action at different times. This is accomplished by the asynchronous discharge of nervous impulses in the motor neurons in the anterior gray horn of the spinal cord.

Muscle tone basically is dependent on the integrity of a simple monosynaptic reflex arc composed of two neurons in the nervous system. The nervous impulses travel in the afferent neurons that enter the spinal cord. There they synapse with the motor neurons situated in the anterior gray horn, which in turn, send impulses down their axons to the muscle fibers. Should the afferent or efferent pathways are cut the muscle would immediately lose its tone and become flaccid. A flaccid muscle on palpation feels like a mass of dough and has completely lost its resilience. It quickly atrophies and loses its volume.

Muscle movement is accomplished by bringing into action increasing numbers of motor units and at the same time reducing the activity of the motor units of muscles that will oppose or antagonize the movement. It is important to understand that all movements are the result of the coordinated action of many muscles. A muscle may work in any of four ways- as a prime mover, as an antagonist, as a fixator, and as a synergist.

The Smooth Muscles

In the tubes of the body, smooth muscles provide the motive power for propelling the contents of the lumen. It milks the contents onward, and also helps in mixing the contents for digestion.

The Cardiac Muscles

The cardiac muscles are made up of striated muscle fibers that branch and unite with each other. It is found in the myocardium of the heart. Its fibers tend to be arranged in whorls and spirals, and have the property of spontaneous and rhythmical contraction. Specialized cardiac muscle fibers form the conducting system of the heart. The cardiac muscle is supplied by autonomic nerve fibers that terminate in the nodes of the conducting system and in the myocardium.

In studying the human body, there are some basic anatomical structures to be learned that will greatly help the medical student. The basic anatomical structures are- the skin, the fasciae, the muscle, the joints, the ligaments, the bursae, the synovial sheath, the blood vessels, the lymphatic system, the nervous system, the mucous and serous membrane, the bones, and the cartilages.

The skin is divided into two distinct parts-the epidermis, which is the superficial part, and, the dermis, which is the deeper part. The epidermis is made up of flattened stratified epithelium seen at the surface. In other parts it is so thick like that of the soles and palms of the hands. The dermis is composed of dense connective tissues containing many blood vessels, lymphatic vessels, and nerves. The dermis is thinner in women than in men.

The appendages of the skin are the following- the nails, hair, follicles, sebaceous glands, and sweat glands. The nails are keratinized plates on the dorsal surfaces of the tips of the fingers and toes. Hairs grow out of follicles, which are invaginations of the epidermis into the dermis. A band of smooth muscle, the arrector pili, connects the undersurface of the follicle to the superficial part of the dermis. The muscle is innervated by sympathetic nerve fibers, and its contraction causes the hair to move into a more vertical position. The pull of the muscle causes dimpling of the skin surface giving the so-called gooseflesh.

The sebaceous glands pour their secretion, the sebum, into the shafts of the hairs as they pass up through the necks of the follicles. The sebum is an oily material that helps to preserve the flexibility of the emerging hair. It also oils the epidermis around the hair follicle.

The sweat glands are long, spiral and tubular glands distributed over the surface of the body, except the lips, nail beds, gland penis and clitoris. They extend through the full thickness of the dermis and their extremities may lie in the superficial fascia. The sweat glands are therefore the most deeply penetrating structures of all the epidermal appendages.

The fascia is the membranous tissue that lies between the skin and muscles, and between muscles and bones. It is divided into two types- the superficial and the deep fascia. The superficial fascia or subcutaneous tissue is a mixture of loose areolar and adipose tissues that unites the dermis of the skin to the underlying deep fascia. It contains numerous bundles of collagen fibers that hold the skin firmly to the deeper structures. The deep fascia is a membranous layer of connective tissue that invests the muscles and other deep structures. In the neck it forms well-defined layers, which may play an important role in determining the path taken by pathogenic organisms during the spread of infection. In the thorax and abdomen, it is merely a thin film of areolar tissue covering the muscles and aponeuroses. In the limbs it forms a definite sheath around the muscles and other structures, holding them in place. Fibrous septa extend from the deep surface of the membrane, between the groups of muscles, and in many places divide up the interior of the limbs into compartments. In the region of the joints, the deep fascia may be considerably thickened to form a restraining bands called retinacula.

The site where two or more bones come together whether or not there is movement between them. Joints are classified according to the tissues that lie between the bones as fibrous joints, cartilaginous joints, and synovial joints.

The articulating surfaces of the bones are joined by fibrous tissue, and thus very little movement is possible. The degree of the movement depends on the length of the collagen fibers uniting the bones. The sutures of the vault of the skull and the inferior tibiofibular joints are examples of fibrous joints.

Cartilaginous joints may be divided into two types, the primary and secondary. The primary cartilaginous joint is one in which the bones are united by a plate or bar of hyaline cartilage. Thus the union between the epiphysis and the diaphysis of a growing bone and that between the first rib and the manubrium sterni are examples of such a joint. No movement is possible. A secondary cartilaginous joint is one in which the bones are united by a plate of fibrocartilage, and the articular sufaces of the bones are covered by a thin layer of hyaline cartilage. Examples are the intervertebral joints and the symphysis pubis. The amount of movement possible is dependent on the physical qualities of the fibrocartilage.

Synovial Joints

The articular surfaces of the bones are covered by a thin layer of hyaline cartilage separated by a joint cavity. This arrangement permits a great degree of mobility. The cavity of the joint is lined by synovial membrane which extends from the margins of one articular surface to those of the other.

The synovial membrane is protected on the outside by a tough fibrous membrane referred to as the capsule of the joint. The articular surfaces are lubricated by a viscous fluid called synovial fluid. In certain synovial joints, for example, in the knee joint, discs or wedges of fibrocartilage are interposed between the articular surfaces of the bones. These are referred to as the articular discs. Fatty pads are found in some synovial joints lying between the synovial membrane and the fibrous capsule or the bones. Examples are those found in the hips and knee joints. The degree of movement in a synovial joint is limited by the shape of the bones participating in the joint, the coming together of adjacent anatomical structures and the presence of fibrous ligaments uniting the bones.
While most ligaments lie outside the joint capsule, but in the knee, some important ligaments, like the cruciate ligaments, lie within the capsule.

Types of Synovial Joints

Synovial joints are somehow classified according to the arrangement of the articular surfaces and the types of movements possible. They are- the plane joints, the hinge joint, pivot joints, condyloid joints, ellipsoid joints, saddled joints, and the ball-and-socket joints. In the plane joints, the apposing articular surfaces are flat or almost flat, and this permits the bones to slide upon one another. Examples are sternoclavicular and acromioclavicular joints. The hinge joints resemble the hinge on a door, so that flexion and extension movements are possible. Like that of the elbow, knee, and ankle joints. In the ball-and –socket joints, the ball-shape head of the bone fits into a socket-like concavity of another, and this permits very free movements, including flexion, extension, abduction, adduction, medial rotation, lateral rotation, and circumduction. The shoulder and the hip joints are fine examples of this type of joint.

The Nervous System of man is divided into two main parts or divisions, the central nervous system consisting of the brain and spinal cord, and the peripheral nervous system consisting of the cranial and spinal nerves and their associated ganglia.

The central nervous system is composed of large numbers of nerve cells and their processes supported by specialized tissue called neuroglia. The neuron is the nerve cell and its processes. The long process of the nerve cell is called an axon or nerve fibers and the shorter ones are called the dendrites. The interior of the central nervous system is organized into gray and white matter. The gay matter consists of nerve cells and the proximal portions of their processes embedded in the neuroglia. The white matter consists of nerve fibers embedded in neuroglia.

There are twelve pairs of cranial nerves that leave the brain and pass through foramina in the skull. There are thirty one pairs of spinal nerves that leave the spinal cord and pass through the intervertebral foramina in the vertebral column. The spinal nerves are named according to the regions of the vertebral column with which they are associated: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Each spinal nerve is connected to the spinal cord by two roots, the anterior root and the posterior root. The anterior root consists of bundles of nerve fibers carrying nerve impulses away from the central nervous system. Such nerve fibers are called efferent fibers. Those efferent fibers that go to the skeletal muscles and cause them to contract are called motor fibers. Their cells of origin lie in the anterior gray horn of the spinal cord. The posterior root consists of bundles of nerve fibers that carry impulses towards the central nervous system and are called afferent fibers. Since these fibers are concerned with conveying information about sensations of touch, pain, temperature, and vibrations, they are called sensory fibers. The cell bodies of these nerve fibers are situated in a swelling on the posterior root called posterior root ganglion. At each intervertebral foramen the anterior and posterior roots unite to form a spinal nerve. Here the motor and sensory fibers become mixed together, so that a spinal nerve is made up of a mixture of motor and sensory fibers. On emerging from the foramen, the spinal nerves divide into a large anterior ramus and smaller posterior ramus.

The postrerior ramus passes posteriorly around the vertebral column to supply the muscles and skin of the back and the anterior ramus continues anteriorly to supply the muscles and skin over the anterolateral body wall and all the muscles and skin of the limbs. At the root of the limbs, the anterior rami join one another to form complicated nerve plexuses. At the root of the arms are the cervical and brachial plexuses, and at the root of the legs, the lumbar and sacral plexuses. The fine terminal branches of the sensory axon, called dendrites, leave the sensory organs of the skin and unite to form the axon of the sensory nerve. The axon passes up the leg in the sural nerve and then in the tibial and sciatic nerve s to the lumbosacral plexus. It then passes through the posterior root of the 1st sacral nerve to reach the cell body in the posterior root ganglion of the first sacral nerve. The central axon now enters the postrior white column of the spinal cord and passes to the nucleus gracilis in the medulla oblongata. This is a demonstration of how a touch on the little toe reaches the central nervous system.

The Autonomic Nervous System is the part of the nervous system concerned with the innervation of involuntary structures such as the heart, smooth muscles, and glands throughout the body. It is distributed throughout the central and peripheral nervous system.

The autonomic system may be divided into two parts, the sympathetic and the parasympathetic, and in both parts there are afferent and efferent nerve fibers. The activities of the sympathetic part of the autonomic system prepare the body for an emergency. It accelerates the heart rate, causes constriction of the peripheral blood vessels, and raises the blood pressure. The sympathetic part of the autonomic system brings about a redistribution of the blood so that it leaves the areas of the skin and intestines and becomes available to the brain, heart, and skeletal muscles. At the same time, it inhibits peristalsis of the intestinal tract and closes the sphincters. The activities of the parasympathetic part of the autonomic system aim at conserving and restoring energy. They slow the heart rate, increase peristalsis of the intestines and glandular activity, and open the sphincters.

The gray matter of the spinal cord, from the first thoracic segment to the second lumbar segment, possesses a lateral horn, or column, in which are located the cell bodies of the sympathetic connector neurons. The myelinated axons of these cells leave the spinal cord in the anterior nerve roots and then pass via the white rami communicates to the paravertebral ganglia of the sympathetic trunk. The connector cell fibers are called the preganglionic as they pass to a peripheral ganglion. Once the preganglionic fibers reach the ganglia in the sympathetic trunk, they pass to the following destinations- they may terminate in the ganglion they have entered by synapsing with the excitor cell in the ganglion. A synapse is the site where two neurons come into close proximity but not into anatomical continuity. The gap between the two neurons is bridged by a neurotransmitter substance, acetylcholine. The axons of the excitor neurons leave the ganglion and are non-myelinated. These postganglionic nerve fibers now pass to the thoracic spinal nerves as gray rami communicate and are distributed in the branches of the spinal nerves to supply the smooth muscles in the walls of blood vessels, the sweat glands, and the arrector pili muscles of the skin.

Another destination, the nerve fibers entering the ganglia of the sympathetic trunk high up in the thorax, may travel up in the sympathetic trunk to the ganglia in the cervical region, where they synapse with excitor cells. The postganglionic nerve fibers leave the sympathetic trunk as gray rami communicate and most of them join the cervical spinal nerves. Many of the preganglionic fibers entering the lower thoracic and upper two lumbar and sacral regions travel down to the ganglia in the lower lumbar and sacral regions, where they synapse with excitor cells. The postganglionic fibers leave the sympathetic trunk as gray rami communicate that join the lumbar, sacral, and coccygeal spinal nerves.

Another destination of the preganglionic fibers is that they pass through the ganglia on the thoracic part of the sympathetic trunk without synapsing. These myelinated fibers form the splanchnic nerves of which there are three types- the greater splanchnic nerve, the lesser splanchnic nerve, and the lowest splanchnic nerve.