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Both cortex and medulla have a branching network of interconnected epithelial cells that play important roles in lymphocyte development discussed later; their roles differ in the two regions cholesterol test kit nz buy abana overnight, so they are separately described as cortical epithelial cells and medullary epithelial cells cholesterol levels england generic 60pills abana free shipping. After developing in the cortex, T cells migrate to the medulla, where they spend another 3 weeks. Epithelial cells of the thymus secrete several signaling molecules that promote the development and action of T cells both locally (as paracrines) and systemically (as hormones); these include thymosin, thymopoietin, thymulin, interleukins, and interferon. If the thymus is removed from newborn mammals, they waste away and never develop immunity. Other lymphatic organs also seem to depend on thymosins or T cells and develop poorly in thymectomized animals. Sinusoid Capillary Adipose cell Artery Endothelial cells Sinusoid Platelets and blood cell entering circulation Megakaryocyte Lymph Nodes Lymph nodes (fig. They serve two functions: to cleanse the lymph and to act as a site of T and B cell activation. A lymph node is an elongated or bean-shaped structure, usually less than 3 cm long, often with an indentation called the hilum on one side. It is enclosed in a fibrous capsule with trabeculae that partially divide the interior of the node into compartments. Between the capsule and parenchyma is a narrow, relatively clear space called the subcapsular sinus, which contains reticular fibers, macrophages, and dendritic cells. Deep to this, the gland consists mainly of a stroma of reticular connective tissue and a parenchyma of lymphocytes and antigen-presenting cells. The formed elements of blood squeeze through the endothelial cells into the sinuses, which converge on a central longitudinal vein at the lower left. When the lymph node is fighting a pathogen, these nodules acquire light-staining germinal centers where B cells multiply and differentiate into plasma cells. The medulla consists largely of a branching network of medullary cords composed of lymphocytes, plasma cells, macrophages, reticular cells, and reticular fibers. The cortex and medulla also contain lymph-filled sinuses continuous with the subcapsular sinus. Lymph flows from these vessels into the subcapsular sinus, percolates slowly through the sinuses of the cortex and medulla, and leaves the node through one to three efferent lymphatic vessels that emerge from the hilum. No other lymphatic organs have afferent lymphatic vessels; lymph nodes are the only organs that filter lymph as it flows along its course. The lymph node is a bottleneck that slows down lymph flow and allows time for cleansing it of foreign matter. Macrophages and reticular cells of the sinuses remove about 99% of the impurities before the lymph leaves the node. On its way to the bloodstream, lymph flows through one lymph node after another and thus becomes quite thoroughly cleansed of impurities. Arteries follow the medullary cords and give rise to capillary beds in the medulla and cortex. In the deep cortex near the junction with the medulla, lymphocytes emigrate from the bloodstream into the parenchyma of the node. Lymph nodes are widespread but especially concentrated in the following locations (see fig. Popliteal lymph nodes occur at the back of the knee and receive lymph from the leg proper. Physicians routinely palpate the accessible lymph nodes of the cervical, axillary, and inguinal regions for swelling. Tonsils the tonsils are patches of lymphatic tissue located at the entrance to the pharynx, where they guard against ingested and inhaled pathogens. Each is covered by an epithelium and has deep pits called tonsillar crypts lined by lymphatic nodules (fig.

Blood from the central veins ultimately converges on a few hepatic veins that exit the superior surface of the liver and empty into the nearby inferior vena cava hdl cholesterol in quail eggs discount 60 pills abana fast delivery. The Gallbladder and Bile the gallbladder is a pear-shaped sac on the underside of the liver that serves to store and concentrate bile cholesterol test why fast before purchase abana 60pills without prescription. It is about 10 cm long and internally lined by a highly folded mucosa with a simple columnar epithelium. Its head (fundus) usually projects slightly beyond the inferior margin of the liver. Its neck (cervix) leads into the cystic duct, which leads in turn to the bile duct. Bile is a light yellow-green color when secreted by the liver, but becomes a deep, intense green when concentrated in the gallbladder. It is a watery solution of minerals, cholesterol, neutral fats, phospholipids, bile pigments, bile acids, and lipidtransport vesicles called micelles (explained in section 25. The principal bile pigment is bilirubin, derived from the decomposition of hemoglobin. The liver is omitted to show more clearly the gallbladder, which adheres to its inferior surface, and the hepatic ducts, which emerge from the liver tissue. About half of this is reabsorbed in the small intestine and excreted by the kidneys. Urobilinogen remaining in the intestine is converted to stercobilin, from which feces get their brown color. In the absence of bile secretion, the feces are grayish white and marked with streaks of undigested fat (acholic feces). Bile acids, micelles, and lecithin, a phospholipid, aid in fat digestion and absorption, as discussed later. When these waste products become excessively concentrated, they may form gallstones (see Deeper Insight 25. Bile gets into the gallbladder by first filling the bile duct, then overflowing into the gallbladder. Between meals, the gallbladder absorbs water and electrolytes from the bile and concentrates it by a factor of 5 to 20 times. About 80% of the bile acids are reabsorbed in the ileum, the last portion of the small intestine, and returned to the liver, where the hepatocytes absorb and resecrete them. This route of secretion, reabsorption, and resecretion, called the enterohepatic circulation, reuses the bile acids two or more times during the digestion of an average meal. The liver synthesizes new bile acids from cholesterol to replace the quantity lost in the feces. Cholelithiasis, the formation of gallstones, is most common in obese women over the age of 40 and usually results from excess cholesterol. Gallstones cause excruciating pain when they obstruct the bile ducts or when the gallbladder or bile ducts contract. When they block the flow of bile into the duodenum, they cause jaundice (yellowing of the skin due to bile pigment accumulation), poor fat digestion, and impaired absorption of fat-soluble vitamins. Reobstruction can be prevented by inserting a stent (tube) into the bile duct, which keeps it distended and allows gallstones to pass while they are still small. They become fully active, however, only upon exposure to bile or ions in the intestinal lumen. It has a globose head encircled by the duodenum, a midportion called the body, and a blunt, tapered tail on the left. Its endocrine part is the pancreatic islets, which secrete insulin and glucagon (see section 17. About 99% of the pancreas is exocrine tissue, which secretes 1,200 to 1,500 mL of pancreatic juice per day. Pancreatic islets are relatively concentrated in the tail of the pancreas, whereas the head is more exocrine. Over 90% of pancreatic cancers arise from the ducts of the exocrine portion (ductal carcinomas), so cancer is most common in the head of the gland. The acini open into a system of branched ducts that eventually converge on the main pancreatic duct. This duct runs lengthwise through the middle of the gland and joins the bile duct at the hepatopancreatic ampulla.

Both the somatic and autonomic nervous systems typically exhibit exaggerated reflexes cholesterol esterification definition order abana cheap online, a state called hyperreflexia or the mass reflex reaction cholesterol eyelid abana 60pills without a prescription. Stimuli such as a full bladder or cutaneous touch can trigger an extreme cardiovascular reaction. The systolic blood pressure, normally about 120 mm Hg, jumps to as high as 300 mm Hg. Pressure receptors in the major arteries sense this rise in blood pressure and activate a reflex that slows the heart, sometimes to a rate as low as 30 or 40 beats/minute (bradycardia), compared with a normal rate of 70 to 80. They may recover these functions later and become capable of ejaculating and fathering children, but without sexual sensation. The flaccid paralysis of spinal shock later changes to spastic paralysis as spinal reflexes are regained but lack inhibitory control from the brain. Spastic paralysis typically starts with chronic flexion of the hips and knees (flexor spasms) and progresses to a state in which the limbs become straight and rigid (extensor spasms). Three forms of muscle paralysis are paraplegia, a paralysis of both lower limbs resulting from spinal cord lesions at levels T1 to L1; quadriplegia, the paralysis of all four limbs resulting from lesions above level C5; and hemiplegia, paralysis of one side of the body, usually resulting not from spinal cord injuries but from a stroke or other brain lesion. Spinal cord lesions from C5 to C7 can Treatment the first priority in treating a spinal injury patient is to immobilize the spine to prevent further trauma. Given within 3 hours of the trauma, it reduces injury to cell membranes and inhibits inflammation and apoptosis. After these immediate requirements are met, reduction (repair) of the fracture is important. Treatment strategies for spinal cord injuries are a vibrant field of contemporary medical research. Some current interests are the use of antioxidants to reduce free radical damage, and the implantation of embryonic stem cells, which has produced significant (but not perfect) recovery from spinal cord lesions in rats. Public hopes have often been raised by promising studies reported in the scientific literature and news media, only to be dashed by the inability of other laboratories to repeat and confirm the results. The locations and distinctions between upper and lower motor neurons in the descending tracts 15. Decussation and its implications for cerebral function in relation to sensation and motor control of the lower body, and for the effects of a stroke 16. What it means to say that the origin and destination of a tract, or any two body parts, are ipsilateral or contralateral a spinal nerve plexus; which of these five features occur in each of the five plexuses 11. Nerves that arise from each plexus and the body regions or structures to which each nerve provides sensory innervation, motor innervation, or both 12. Dermatomes and why they are relevant to the clinical diagnosis of nerve disorders 13. Skeletal landmarks that mark the extent of the adult spinal cord, and what occupies the vertebral canal inferior to the spinal cord 3. Names and structures of the three spinal meninges, in order from superficial to deep, and the relationships of the epidural and subarachnoid spaces to the meninges 7. The two types of ligaments that arise from the pia mater; where they are found and what purpose they serve 8. Organization of spinal gray and white matter as seen in cross sections of the cord; how gray and white matter differ in composition; and why they are called gray and white matter 9. The position of the posterior and anterior horns of the gray matter; where lateral horns are also found, and the functions of all three 10. The anatomical basis for dividing white matter into three columns on each side of the cord, and for dividing each column into tracts 11. Four defining criteria of a reflex; how somatic reflexes differ from other types; and the flaw in calling somatic reflexes spinal reflexes 2. Stretch reflexes; one or more examples; the purpose they serve in everyday function; the mechanism of a stretch reflex; and an anatomical reason why stretch reflexes are often quicker than other types of somatic reflexes 6. Reciprocal inhibition and why it is important that it often accompany a stretch reflex 7. Flexor reflexes; a common purpose that they serve; and why it is beneficial for flexor reflexes to employ polysynaptic reflex arcs 8. Crossed extension reflexes and why it is important for this type of reflex to accompany a withdrawal reflex 9. Structure of a nerve, especially the relationship of the endoneurium, perineurium, and epineurium to nerve fibers and fascicles 2.

Thus cholesterol levels goals discount 60pills abana, the central nervous system can judge stimulus strength from the firing frequency of afferent neurons (fig cholesterol diet foods to avoid discount abana 60pills fast delivery. There is a limit to how often a neuron can fire, set by its absolute refractory period. As the stimulus intensity rises, these fibers fire at a higher and higher frequency, up to a certain maximum. If the stimulus intensity exceeds the capacity of these low-threshold fibers, it may recruit less sensitive, high-threshold fibers to begin firing. Still further increases in intensity cause these high-threshold fibers to fire at a higher and higher frequency. Neural Pools and Circuits So far, we have dealt with interactions involving only two or three neurons at a time. Actually, neurons function in larger ensembles called neural pools, each of which may consist of thousands of interneurons concerned with a particular body function-one to control the rhythm of your breathing, one to move your limbs rhythmically as you walk, one to regulate your sense of hunger, and another to interpret smells, for example. At this point, we explore a few ways in which neural pools collectively process information. Discharge and Facilitated Zones Information arrives at a neural pool through one or more input neurons, which branch repeatedly and synapse with numerous interneurons in the pool. Within the discharge zone of an input neuron, that neuron acting alone can make the postsynaptic cells fire (fig. But in a broader facilitated zone, it synapses with still other neurons in the pool, with fewer synapses on each of them. It can stimulate those neurons to fire only with the assistance of other input neurons; that is, it facilitates the others. It "has a vote" on what the postsynaptic cells in the facilitated zone will do, but it cannot determine the outcome by itself. Such arrangements, repeated thousands of times throughout the central nervous system, give neural pools great flexibility in integrating input from several sources and "deciding" on an appropriate output. In a reverberating circuit, neurons stimulate each other in a linear sequence from input to output neurons, but some of the neurons late in the path send axon collaterals back to neurons earlier in the path and restimulate them. As a result, every time C fires it not only stimulates Input Output output neuron D, but also restimulates A and starts the process over. Such a circuit produces a prolonged or repetitive effect that lasts until one or more neurons in the circuit fail to fire, or an inhibitory signal from another source stops Output Input one of them from firing. A reverberating circuit sends repetitious signals to your diaphragm Reverberating Parallel after-discharge and intercostal muscles, for example, to make you inhale. Sustained output from the circuit ensures that the respiratory muscles contract for the 2 seconds or so that it normally takes to fill the lungs. When the circuit stops firing, you Input Output exhale; the next time it fires, you inhale again. Arrows indicate the direction of the "storms" of neural activity that occur in epilepsy. Which of these four circuits is likely to fire the longest after a stimulus ceases Each chain has a different number of synapses, but eventually they all reconverge on one or a few output neurons. Since the chains differ in total synaptic Types of Neural Circuits delay, their signals arrive at the output neurons at different the functioning of a radio can be understood from a circuit diatimes, and the output neurons may go on firing for some gram showing its components and their connections. Unlike a reverberating circuit, the functions of a neural pool are partly determined by its neural this type has no feedback loop. Continued firing after electronic devices are constructed from a relatively limited number the stimulus stops is called after-discharge. It explains why of circuit types, a wide variety of neural functions result from the you can stare at a lamp, then close your eyes and continue to operation of four principal kinds of neural circuits (fig. In a diverging circuit, an individual neuron sends signals to tant in withdrawal reflexes, in which a brief pain produces a multiple downstream neurons, or one neural pool may send longer-lasting output to the limb muscles and causes you to output to multiple downstream neural pools. Such a circuit allows signals from one motor neuron of the brain, for example, to ultiIn relation to these circuit types, the nervous system handles informately stimulate thousands of muscle fibers. A converging circuit is the opposite of a diverging circuit- processing, neurons and neural pools relay information along a input from many nerve fibers or neural pools is funneled to pathway in a relatively simple linear fashion and can process only fewer and fewer intermediate or output pathways. For example, you can read a ple, you have a brainstem respiratory center that receives book or watch a television movie and the language recognition converging information from other parts of your brain, blood centers of your brain can process one linguistic input or the other, chemistry sensors in your arteries, and stretch receptors in but you cannot do both simultaneously.