By M. Musan. Louisiana Tech University. 2018.
An artificial lens may be im- • Fixation buy v-gel 30 gm amex herbals and vitamins, the training of the eyes on a stationary object planted in its place buy v-gel 30gm line wholesale herbs, or eyeglasses may be used to replace • Tracking movements, used to follow the course of a the refractive power of the lens. The peak spectral sensitivity ward to fix on near objects for the red-sensitive pigment is 560 nm; for the green-sen- • Nystagmus, a series of slow and saccadic movements sitive pigment, it is about 530 nm; and for the blue-sensi- (part of a vestibular reflex) that serves to keep the retinal tive pigment, it is about 420 nm. At wavelengths away from the optimum, the receives a slightly different image of the same object. Be- property, binocular vision, along with information about cause of the interplay between light intensity and wave- the different positions of the two eyes, allows stereoscopic length, a retina with only one class of cones would not be vision and its associated depth perception, abilities that are able to detect colors unambiguously. Many ab- of the three pigments in each cone removes this uncer- normalities of eye movement are types of strabismus tainty. Colorblind individuals, who have a genetic lack of (“squinting”), in which the two eyes do not work together one or more of the pigments or lack an associated trans- properly. Other defects include diplopia (double vision), duction mechanism, cannot distinguish between the af- when the convergence mechanisms are impaired, and am- blyopia, when one eye assumes improper dominance over the other. Failure to correct this latter condition can lead to loss of visual function in the subordinate eye. The retina is a multi- B layered structure containing the photoreceptor cells and a complex web of several types of nerve cells (Fig. There are 10 layers in the retina, but this discussion em- ploys a simpler four-layer scheme: pigment epithelium, photoreceptor layer, neural network layer, and ganglion cell layer. These four layers are discussed in order, begin- ning with the deepest layer (pigment epithelium) and mov- ing toward the layer nearest to the inner surface of the eye C (ganglion cell layer). Note that this is the direction in which visual signal processing takes place, but it is opposite to the path taken by the light entering the retina. This opaque material, which also extends between portions of individual rods and cones, prevents the scattering of stray light, thereby greatly sharpening the resolving power of the retina. Its presence ensures that a tiny spot of light (or a tiny portion of an image) will excite only those receptors on which it falls directly. People with albinism lack this pig- ment and have blurred vision that cannot be corrected ef- D fectively with external lenses. The pigment epithelial cells also phagocytose bits of cell membrane that are constantly shed from the outer segments of the photoreceptors. Because of the eye’s mode of embryologic development, the photore- E ceptor cells occupy a deep layer of the retina, and light must pass through several overlying layers to reach them. The cones are responsible for photopic (daytime) vision, which is in color (chromatic), and the rods are responsible for scotopic FIGURE 4. C, Photore- are basically similar, although they have important struc- ceptor layer. CHAPTER 4 Sensory Physiology 75 retinal is isomerized back to the 11-cis form, and the rhodopsin is reconstituted. All of these reactions take place in the highly folded membranes comprising the outer seg- ment of the rod cell. Outer segment The biochemical process of visual signal transduction is (with disk-shaped shown in Figure 4. The coupling of the light-induced re- lamellae) actions and the electrical response involves the activation of transducin, a G protein; the associated exchange of GTP for GDP activates a phosphodiesterase. This, in turn, cat- alyzes the breakdown of cyclic GMP (cGMP) to 5’-GMP. Inner segment When cellular cGMP levels are high (as in the dark), mem- (with cell organelles) brane sodium channels are kept open, and the cell is rela- tively depolarized. Under these conditions, there is a tonic release of neurotransmitter from the synaptic body of the rod cell. A decrease in the level of cGMP as a result of light- Nucleus induced reactions causes the cell to close its sodium chan- nels and hyperpolarize, thus, reducing the release of neuro- transmitter; this change is the signal that is further processed by the nerve cells of the retina to form the final Synaptic body response in the optic nerve. An active sodium pump main- Cone Rod Photoreceptors of the human retina. Loss of a single color system produces GTP dichromatic vision and lack of two of the systems causes GDP + + TR RH* monochromatic vision. If all three are lacking, vision is GC Active + monochromatic and depends only on the rods. The lamellae near the tip are reg- ularly shed and replaced with new membrane synthesized at the opposite end of the outer segment.
Widening of the pre- erosive discovertebral “Romanus” lesions in ankylosing dental space by more than 3 mm in adults and 5 mm in spondylitis demonstrated by contrast enhanced Gd-DTPA children during flexion may be demonstrated by radi- magnetic resonance imaging buy 30 gm v-gel with mastercard equine herbals. Skeletal Radiol 29:27-33 IDKD 2005 Degenerative Diseases of the Spine D generic v-gel 30 gm online zip herbals. McCall2 1 Department of Radiology, University Hospital, Zurich, Switzerland 2 Department of Diagnostic Imaging, The Robert Jones & Agnes Hunt Hospital, Shropshire, UK Introduction cytes and has ground substance. Therefore, the inner ring has high signal intensity on T2-weighted images. Degenerative disease of the spine, in particular low back The second component of the intervertbral disk is the pain (LBP), is one of the most common causes of work dis- nucleus fibrosus. Back pain is a pervasive problem that affects two- gen and hydrophilic proteoglycans. However, it is occasionally anterior and posterior ligaments, facet joints, vertebral the presenting symptom of systemic diseases such as can- endplates, and the peripheral layer of the anulus fibrosus cer or infection. Therefore, the disk is not usually a source with neurologic symptoms, are surgically treatable. Thus, of pain, although degeneration in the disk may lead to the major diagnostic task is to distinguish the 95% of pa- pain by stretching of ligamentous tissue, nerve compres- tients with simple back pain from the 5% with serious un- sion, or inflammation. In this article, an overview of the spectrum of degenerative disease of the spine is provided. Special emphasis is directed to the mag- Disk Degeneration netic resonance imaging (MRI) appearance of degenerative spine disorders, since MRI has become the standard of ref- With aging, the nucleus pulposus becomes dehydrated erence regarding the evaluation of patients with back pain and tears occur in the anulus fibrosus. Radial or type 3 tears are of special interest in the setting of disk Anatomical Considerations degeneration since these types of anular tears concern the entire anulus fibrosus, and they correlate with shrinkage The intervertebral disk is a complex structure consisting and disorganization of the nucleus. Hydration and an- of hyaline cartilage, fibrocartilage, and mucopolysaccha- ular integrity seem to be important for the disk to absorb ride and dense fibrous tissue, which together gives the and transmit compressive loads to the vertebral column. The layer of the hyaline As the disk ages and degenerates, it progressively loses cartilage attached to the vertebral endplate and encircled this capacity. This results in disk-space narrowing and re- by the ring apophysis is called the cartilaginous endplate. Occasionally, gas or calcifi- Within the endplate are numerous vascular channels cation develops within a degenerating disk. One of the theories of disk degeneration is that ment of disk degeneration. The signal characteristics of the degenerative changes in the vertebral endplate impair dif- disk in T2-weighted sequences reflect changes caused by fusion into and out of the disk, impeding the function of aging or degeneration. The outer ring contains the densest grades to describe the different stages of lumbar-disk de- fibrous lamellae, which display low signal intensity on generation. The grading system is based on MR signal in- T2-weighted MR images due to the absence of ground tensity, disk structure, distinction between nucleus and an- substance. The cells in the outer ring of the anulus are al- nulus, and disk height (Table 1, Fig. Unlike the outer ring of the cients for intra- and interobserver agreement were excel- anulus, the inner ring contains predominantly chondro- lent for this system; thus it is useful in daily practice. Classification of disk degeneration based on sagittal T2-weighted magnetic resonance (MR) images (according to) Grade Differentiation of nucleus pulposus Signal intensity of nucleus pulposus from anulus Disk height I Yes Homogeneously hyperintense Normal II Yes Hyperintense with horizontal dark band Normal III Blurred Slightly decreased, minor irregularities Slightly decreased IV Lost Moderately decreased, hypointense zones Moderately decreased V Lost Hypointense, with or without horizontal hyperintense band Collapsed Fig. Sagittal T2-weighted images show the different degrees of disk de- generation according to the classification system proposed by Pfirrmann et al. Several classification systems have been proposed to describe disk abnormalities. Currently, the most widely accepted terms are: normal, bulging, protrusion, extrusion and sequestration. A disk is considered normal when it does not reach beyond the border of the adjacent vertebral bodies. Bulging is defined as circumferential, symmetric disk extension around the posterior vertebral border. Protrusion is defined as focal or asymmetric extension of the disk beyond the vertebral border, with the disk origin broader than any other dimen- sion of the protrusion. Extrusion is defined as a more ex- treme extension of the disk beyond the vertebral border, Fig. T2-weighted images in with the base against the disk of origin narrower than the di- the sagittal and axial planes ameter of the extruding material and a connection between demonstrate disk extrusion at the L4/5 disk level with com- the material and the disk of origin (Fig. Sequestration is pression of the right-sided L5 defined as a free disk fragment that is distinct from the par- nerve root ent disk and has intermediate signal intensity on T1-weight- ed images but increased signal intensity on T2-weighed im- ages (Fig.
Glutamic acid can be used in the amination of other -keto acids to form the corresponding Retinol/RBP amino acids discount v-gel 30gm with mastercard worldwide herbals. It can also be converted to glutamine by cou- complex Chylomicron remnant pling with ammonia purchase v-gel 30 gm line herbals on demand coupon, a reaction catalyzed by glutamine containing retinyl ester synthetase. After urea, glutamine is the second most im- Chylo- portant metabolite of ammonia in the liver. It plays an im- micron Lipoprotein lipase portant role in the storage and transport of ammonia in the blood. Through the action of various transaminases, gluta- The metabolism of vitamin A (retinol) by mine can be used to aminate various keto acids to their cor- FIGURE 28. It also acts as an important oxida- tive substrate, and in the small intestine it is the primary substrate for providing energy. Retinol (an alcohol) is transported in chylomicrons mainly as an ester of long-chain fatty acids (see Chapter 27). When chylomicrons enter the circula- THE LIVER AS A STORAGE ORGAN tion, the triglyceride is rapidly acted on by lipoprotein li- pase; the triglyceride content of the particles is signifi- Another important role of the liver is the storage and me- cantly reduced, while the retinyl ester content remains tabolism of fat-soluble vitamins and iron. Receptors in the liver mediate the rapid uptake ble vitamins, particularly vitamin B12, are also stored in the of chylomicron remnants, which are degraded, and the liver. These stored vitamins are released into the circulation retinyl ester is stored. When the vitamin A level in blood falls, the liver mobi- lizes the vitamin A store by hydrolyzing the retinyl ester The Liver Has a Central Role in (see Fig. The retinol formed is bound with retinol- Regulating Coagulation binding protein (RBP), which is synthesized by the liver before it is secreted into the blood. The amount of RBP se- Liver cells are important both in the production and the creted into the blood is dependent on vitamin A status. Most of the known clot- tamin A deficiency significantly inhibits the release of RBP, ting factors and inhibitors are secreted by hepatocytes, whereas vitamin A loading stimulates its release. In addition, several coagulation Hypervitaminosis A develops when massive quantities and anticoagulation proteins require a vitamin K–depend- of vitamin A are consumed. Since liver is the storage organ ent modification following synthesis, specifically factors II, for vitamin A, hepatotoxicity is often associated with hy- VII, IX, and X and proteins C and S, to make them effective. The continued ingestion of excessive The monocyte-macrophage system of the liver, pre- amounts of vitamin A eventually leads to portal hyperten- dominantly Kupffer cells, is an important system for clear- sion and cirrhosis. Distur- Vitamin D is thought to be stored mainly in skeletal bances in liver perfusion and function result in the muscle and adipose tissue. However, the liver is responsible ineffective clearance of activated coagulation proteins, so for the initial activation of vitamin D by converting vitamin patients with advanced liver failure may be predisposed to D to 25-hydroxy vitamin D , and it synthesizes vitamin D- 3 3 developing disseminated intravascular coagulation. Vitamin K is a fat-soluble vitamin important in the he- patic synthesis of prothrombin. Prothrombin is synthesized Fat-Soluble Vitamins Are Stored in the Liver as a precursor that is converted to the mature prothrombin, Vitamin A comprises a family of compounds related to a reaction that requires the presence of vitamin K retinol. Vitamin K deficiency, therefore, leads to im- tenance of epithelia, and reproduction. The dietary vitamin K requirement is extremely small and is adequately supplied by the average North American diet. Bacteria in the GI tract also provide vitamin genase releases iron from the heme, which then enters the K. This appears to be an important source of vitamin K be- free iron pool and is stored as ferritin or released into the cause prolonged administration of wide-spectrum antibi- bloodstream (bound to apotransferrin). It is un- vitamin K absorption is dependent on normal fat absorp- clear whether the iron from the hemosiderin granules is ex- tion, any prolonged malabsorption of lipid can result in its changeable with the free iron pool. The vitamin K store in the liver is relatively lim- It was long believed that Kupffer cells were the only ited, and therefore, hypoprothrombinemia can develop cells involved in iron storage, but recent studies suggest within a few weeks. Vitamin K deficiency is not uncommon that hepatocytes are the major sites of long-term iron stor- in the Western world. Transferrin binds to receptors on the surface of hepa- K usually provides a cure. The apotransferrin The Liver Is Important in the Storage (not containing iron) is recycled back to the plasma, and and Homeostasis of Iron the released iron enters a labile iron pool. The iron from transferrin is probably the major source of iron for the he- The liver is the major site for the synthesis of several pro- patocytes, but they also derive iron from haptoglobin-he- teins involved in iron transport and metabolism. When hemo- tein transferrin plays a critical role in the transport and globin is released inside the hepatocytes, it is degraded in homeostasis of iron in the blood.
In many species discount 30 gm v-gel otc herbals on wholesale, a sharp decline in the circulating levels of progesterone and The duration of pregnancy in women averages 270 14 a concomitant rise in estrogen precede birth discount 30gm v-gel visa herbs during pregnancy. Parturition or the onset progesterone does not fall significantly before delivery. Uncoordinated uterine contractions start about rise in placental progesterone-binding protein or by a de- 1 month before the end of gestation. The termination of cline in the number of myometrial progesterone receptors. They increase intracellular calcium ine muscle is regulated by hormones and by mechanical concentrations of myometrial cells and activate the actin- factors. The hormones include progesterone, estrogen, myosin contractile apparatus. The mechanical fac- parturition, the concentration of prostaglandins in amni- tors include distension of the uterine muscle and stretching otic fluid rises abruptly. Aspirin and in- Progesterone hyperpolarizes myometrial cells, lowers domethacin, inhibitors of prostaglandin synthesis, delay or their excitability, and suppresses uterine contractions. Estrogen, in gen- tions, and its release from both maternal and fetal pitu- eral, has the opposite effects. Oxytocin is used clinically to CHAPTER 39 Fertilization, Pregnancy, and Fetal Development 697 Indifferent drogen precursors. Injections of ACTH and cortisol in late stages pregnancy do not induce labor. Interestingly, the adminis- tration of estrogens to the cervix causes ripening, probably by increasing the secretion of prostaglandins. Genital fold Genital swelling POSTPARTUM AND PREPUBERTAL PERIODS Genital tubercle Lactation is controlled by pituitary and ovarian hormones, requires suckling for continued milk production, and is the major source of nutrition for the newborn. As the child grows, puberty will occur around age 10 to 11 because the hypothalamus activates secretion of pituitary hormones Male Female that cause secretion of estrogens and androgens from the Glans gonads and adrenals during that time. Alterations in hor- Fused Glans mone secretion lead to abnormal onset of puberty and go- urogenital Urethral groove nadal development. Several hormones participate in Prepuce mammogenesis, the differentiation and growth of the mam- Body of mary glands, and in the production and delivery of milk. Galac- Urethral orifice Scrotal topoiesis, the maintenance of lactation, is regulated by PRL. The mammary glands begin to differentiate in the pectoral region as an ectodermal thickening on the epidermal ridge during weeks 7 to 8 of induce labor (see Clinical Focus Box 39. The prospective mammary glands lie along bilat- significance of oxytocin is that it helps expel the fetus from eral mammary ridges or milk lines extending from axilla to the uterus, and by contracting uterine muscles, it reduces groin on the ventral side of the fetus. Most of the ridge dis- uterine bleeding when bleeding may be significant after de- integrates except in the axillary region. Interestingly, oxytocin levels do not rise at the time mals with serially repeated nipples, a distinct milk line with of parturition. Mam- corpus luteum and the decidua, assists parturition by soft- mary buds are derived from surface epithelium, which in- ening the cervix, permitting the eventual passage of the vades the underlying mesenchyme. However, the the buds elongate, branch, and sprout, eventually forming relative role of relaxin in parturition in humans is unclear, the lactiferous ducts, the primary milk ducts. The ducts unite, laxin reaches its peak during the first trimester, declines to grow, and extend to the site of the future nipple. The pri- about half, and remains unchanged throughout the re- mary buds give rise to secondary buds, which are separated mainder of pregnancy. In sheep, the rounded by myoepithelial cells derived from epithelial pro- concentration of ACTH and cortisol in the fetal plasma rise genitors. In response to oxytocin, myoepithelial cells will during the last 2 to 3 days of gestation. The nipple and are- lamb pituitary or removal of the adrenals prolongs gestation, ola, which are first recognized as circular areas, are formed while administration of ACTH or cortisol leads to premature during the eighth month of gestation. Cortisol enhances the conversion of progesterone the mammary glands in utero appears to be independent of to estradiol, changing the progesterone-to-estrogen ratio, hormones but is influenced by paracrine interactions be- and increases the production of prostaglandins. Although underdeveloped, they have the capacity lack a pituitary and have atrophied adrenal glands, have an to respond to hormones, revealed by the secretion of small unpredictable length of gestation.
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