By D. Chenor. Georgia Southwestern State University.
The entire renal Arcuate tubule and collecting duct system consists of a single layer vein of epithelial cells surrounding fluid (urine) in the tubule or Ascending Arcuate duct lumen himcolin 30gm for sale. Cells in each segment have a characteristic his- vasa recta artery tological appearance purchase 30 gm himcolin otc. Each segment has unique transport Descending properties (discussed later). The jux- Renal From renal artery tamedullary nephrons, whose glomeruli lie in the cortex pelvis next to the medulla, comprise about one-eighth of the To renal nephron population. They differ in several ways from the vein other nephron types: they have a longer loop of Henle, FIGURE 23. A standard nomenclature for structures of the permeability and transport properties, and a different type kidney. In the outer medulla, descending and ascending vasa recta are The Kidneys Have a Rich Blood Supply grouped in vascular bundles and are in close contact with and Innervation each other. This arrangement greatly facilitates the ex- Each kidney is typically supplied by a single renal artery change of substances between blood flowing in and out of the medulla. The seg- fibers, which travel to the kidneys, mainly in thoracic mental arteries branch into interlobar arteries, which pass spinal nerves T10, T11, and T12 and lumbar spinal nerve toward the cortex between the kidney lobes (see Fig. At the junction of cortex and medulla, the interlo- renal blood vessels and a fall in renal blood flow. These, in thetic nerve fibers also innervate tubular cells and may turn, give rise to smaller cortical radial arteries, which cause an increase in Na reabsorption by a direct action on pass through the cortex toward the surface of the kidney. In addition, stimulation of sympathetic nerves Several short, wide, muscular afferent arterioles arise increases the release of renin by the kidneys. Each afferent arteriole sory) renal nerves are stimulated by mechanical stretch or gives rise to a glomerulus. The glomerular capillaries are by various chemicals in the renal parenchyma. The efferent arteriole Renal lymphatic vessels drain the kidneys, but little is then divides into a second capillary network, the per- known about their functions. Venous vessels, in general, lie parallel to the arterial ves- sels and have similar names. The Juxtaglomerular Apparatus Is the Site The blood supply to the medulla is derived from the ef- of Renin Production ferent arterioles of juxtamedullary glomeruli. These ves- sels give rise to two patterns of capillaries: peritubular Each nephron forms a loop, and the thick ascending limb capillaries, which are similar to those in the cortex, and touches the vascular pole of the glomerulus (see Fig. Tubular reabsorption ascending involves the transport of substances out of tubular urine; limb these substances are then returned to the capillary blood, Granular which surrounds the kidney tubules. Tubular secretion in- Extraglomerular mesangial cell volves the transport of substances into the tubular urine. The terms reabsorption and se- Glomerular capillary cretion indicate movement out of and into tubular urine, re- spectively. The juxtaglomerular apparatus consists of the macula the amount excreted is expressed by the following equation: densa, extraglomerular mesangial cells, and granular cells. The Juxtaglomerular Apparatus: Struc- Excreted Filtered Reabsorbed Secreted (1) ture and Function. These tests measure the rates of glomerular filtration, renal blood flow, prised of the macula densa, extraglomerular mesangial cells, and tubular reabsorption or secretion of various substances. The macula densa (dense spot) Some of these tests, such as the measurement of glomerular consists of densely crowded tubular epithelial cells on the filtration rate, are routinely used to evaluate kidney function. The extraglomerular mesangial Renal Clearance Equals Urinary Excretion Rate cells are continuous with mesangial cells of the glomerulus; Divided by Plasma Concentration they may transmit information from macula densa cells to the granular cells. The granular cells are modified vascular A useful way of looking at kidney function is to think of the smooth muscle cells with an epithelioid appearance, located kidneys as clearing substances from the blood plasma. When a substance is excreted in the urine, a certain volume These cells synthesize and release renin, a proteolytic en- of plasma is, in effect, freed (or cleared) of that substance. The renal clearance of a substance can be defined as the volume of plasma from which that substance is completely removed (cleared) per unit time. The clearance formula is: AN OVERVIEW OF KIDNEY FUNCTION V˙ Cx Ux (2) Three processes are involved in forming urine: glomerular Px filtration, tubular reabsorption, and tubular secretion (Fig. Glomerular filtration involves the ultrafiltration of substance X, U is the urine concentration of substance, P X X plasma in the glomerulus. The filtrate collects in the urinary is the plasma concentration of substance X, and V is the˙ urine flow rate. The product U Vequals the excretion˙ X rate per minute and has dimensions of amount per unit time Filtration Kidney tubule (e. The clearance of a substance can easily be determined by measuring the concentrations of a substance in urine and plasma and the urine flow rate Reabsorption Secretion Excretion (urine volume/time of collection) and substituting these values into the clearance formula. Glomerulus Peritubular capillary Inulin Clearance Equals the Glomerular Filtration Rate FIGURE 23. This highly simplified drawing shows a nephron and An important measurement in the evaluation of kidney its associated blood vessels. The clearance of iothalamate, an iodinated or- ganic compound, also provides a reliable measure of GFR. They must be infused intravenously, and because short urine collection periods are used, the bladder is usu- ally catheterized; these procedures are inconvenient. There is no such known substance, but IN = C GFR = IN creatinine comes close.
Vigor- growth-promoting actions on progenitor cells or stem cells generic himcolin 30 gm without a prescription, ous exercise also stimulates GH secretion generic himcolin 30 gm overnight delivery. Obesity results such as prechondrocytes in the growth plates of bone and in reduced GH secretion. GH stimulates such pro- Changes in the circulating levels of metabolites also af- genitor cells to differentiate into cells with the capacity to fect GH secretion. An important action of GH on the tion stimulates GH secretion, whereas hyperglycemia in- differentiation of progenitor cells is stimulation of the ex- hibits it. Growth hormone secretion is also stimulated by an pression of the IGF-I gene; IGF-I is produced and released by these cells. IGF-I exerts an autocrine mitogenic action on the cells that produced it or a paracrine action on neighbor- ing cells. In response to IGF-I, these cells undergo division, 20 causing the tissue to grow mainly through cell replication. As mentioned earlier, GH deficiency in childhood causes 14-year-old boy a decrease in the rate of body growth. Individuals with this condition may be deficient in GH only, or they may have multiple anterior pituitary hormone deficiencies. GH defi- 10 ciency can be caused by a defect in the mechanisms that con- trol GH secretion or the production of GH by soma- totrophs. In some individuals, the target cells for GH fail to 5 respond normally to the hormone because of several differ- ent mutations in the GH receptor. When excessive GH secretion occurs in an adult, further linear growth does not occur because the growth plates of the long 25-year-old man bones have calcified. Instead, it causes the bones of the face, 15 hands, and feet to become thicker and certain organs, such as the liver, to undergo hypertrophy. This condition, known as acromegaly, can also be caused by the chronic administration 10 of excessive amounts of GH to adults. Although the main physiological action of GH is on body growth, it also has important effects on certain as- 5 pects of fat and carbohydrate metabolism. Its main action on fat metabolism is to stimulate the mobilization of triglycerides from the fat depots of the body. This process, known as lipolysis, involves the hydrolysis of triglycerides 8 AM Noon 4 PM 8 AM Mid- 4 AM 8 AM to fatty acids and glycerol by the enzyme hormone-sensi- night tive lipase. The fatty acids and glycerol are released from Pulsatile GH secretion in an adolescent boy adipocytes and enter the bloodstream. In the adult, GH levels are re- lipolysis is not understood, but most evidence suggests that duced as a result of smaller pulse width and amplitude rather than it causes adipocytes to be more responsive to other lipoly- a decrease in the number of pulses. CHAPTER 32 The Hypothalamus and the Pituitary Gland 593 CLINICAL FOCUS BOX 32. How- Growth hormone (GH) is species-specific, and humans do ever, a random blood sample may be useful to detect GH not respond to GH derived from animals. In the past, the resistance, a syndrome in which the patient exhibits symp- only human GH available for treating children who were toms of GH deficiency but presents with high GH levels in GH-deficient was a very limited amount made from human the blood. This problem was solved when measure the levels of IGF-I, IGF-II, and the IGF-binding pro- the gene for human GH was cloned in 1979 and then ex- tein 3 (IGFBP3) in the blood. The production of large amounts of re- mitogenic effects of GH on tissues in the body. IGF-I and combinant human GH, with all the activities of the natural IGF-II bind to IGFBP3 in the blood. During the 1980s, careful life of the IGFs, transports them to target cells, and facili- clinical trials established that recombinant human GH was tates their interaction with IGF receptors. In children with GH deficiency, the con- Despite the availability of recombinant GH, the diagno- centration of IGFs and IGFBP3 are low. GH is combinant GH will increase IGF-I, IGF-II, and IGFBP3 in the released in periodic bursts, the greatest of which occur in blood, which will result in increased long bone growth. Between pulses of secretion, the The epiphyseal growth plate in the bone becomes less re- blood concentration of GH is nearly undetectable by most sponsive to GH and IGF-I several years after puberty, and techniques. For these reasons, a random measure of GH in long bone growth stops in adulthood (see Chapter 36). GH is also thought to function as one of the counter- Gonadotropins Regulate Reproduction regulatory hormones that limit the actions of insulin on The testes and ovaries have two essential functions in hu- muscle, adipose tissue, and the liver. The first is to produce sperm cells and hibits glucose use by muscle and adipose tissue and in- ova (egg cells), respectively. These effects are array of steroid and peptide hormones, which influence vir- opposite those of insulin. Also, GH makes muscle and fat tually every aspect of the reproductive process. Thus, GH nor- nadotropic hormones FSH and LH regulate both of these mally has a tonic inhibitory effect on the actions of insulin, functions. The production and secretion of the go- much like the glucocorticoid hormones (see Chapter 34). The regulation of human repro- large amounts of GH for an extended time. They may de- duction by this hypothalamic-pituitary-gonad axis is dis- velop insulin resistance and an elevated insulin level in the blood. They may also have hyperglycemia caused by the underutilization and overproduction of glucose.
These results have been borne out by preliminary clinical trials of buspirone discount 30gm himcolin mastercard, used in combination with neuroleptics discount 30 gm himcolin free shipping, and several novel 5-HT1A agonists (e. BSF 190555) are currently under development for this clinical application (Meltzer 1999). However, there seem to be regional differences in the extent to which this population of receptors is tonically activated by extracellular 5-HT and so the literature describing the effects of 5-HT1B antagonists on 5-HT release is somewhat confusing. Because of the dearth of 5-HT1B selective ligands, the gross physiological effects that result from activation of these receptors is largely uncharacterised. Activation of these receptors could also contribute to the anti- migraine effects of sumatriptan, a non-selective 5-HT1B/1D agonist (but see below). An interesting development in this area is the possibility that there could be an endogenous ligand, for these receptors: 5-HT-moduline. This is a tetrapeptide that is released from neurons and is claimed to be the first allosteric modulator of a G protein- coupled receptor to be identified so far. Functionally, 5-HT-moduline behaves like a 5-HT1B antagonist and so increases terminal release of 5-HT (Massot et al. Soon after characterisation of the 5-HT1D receptor, which was found in certain species (e. These receptors were therefore regarded as species variants and came to be described as the 5-HT1B/1D subtype. Since then, another 5-HT1 receptor subtype has been identified and current nomenclature dictates that this is the (new) 5-HT1D receptor. So far, little is known about this novel 5-HT1D receptor but, in the rat and human, its mRNA is found, albeit in low concentrations, in the basal ganglia, nucleus accumbens, hippocampus, frontal cortex and Raphe nuclei. It is negatively coupled to adenylyl cyclase and is possibly located presynaptically, on both the 5-HT neuronal cell body and terminals, but this has yet to be confirmed. Interest in this receptor has been generated by the possibility that its activation accounts for the anti-migraine effects of the non-selective 5-HT1B/1D agonist, sumatriptan. The exact process(es) that account for this action are unresolved but 5-HYDROXYTRYPTAMINE 201 favoured possibilities include vasoconstriction of cerebral arteries and/or blockade of neurogenic pain and inflammation generated by vascular afferents within the trigeminal nucleus. Because mRNA for the 5-HT1B receptor is not found in this nucleus in humans, activation of the 5-HT1D receptor is thought to be responsible for these effects. Radioligand binding and the distribution of mRNA for the 5-ht1E receptor suggest that it is found mainly in the striatum but it also seems to be present in the amygdala, frontal cortex and globus pallidus, to some extent. However, the lack of specific ligands has prevented their pharmacological characterisation. Recently, the anti-migraine drug, sumatriptan, has been found to bind to 5-ht1F receptors with an affinity similar to that for 5-HT1B/1D receptors and so they might have a role in migraine. They are coupled to phospholipase A and have an excitatory effect on the host cell as a result of the ensuing reduction in K conductance. The many well-known agonist ligands for this receptor include DOI (1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride), DOM (2,5-dimethoxy-4-methylamphetamine) and LSD. Preclinical studies reinforce the view that 5-HT2A (and possibly 5-HT2C) receptor activation underlies the hallucinogenic effects of these compounds (Krebs- Thomson, Paulus and Geyer 1998). This is consistent with recent evidence that all the atypical neuroleptics, such as clozapine, risperidone and olanzepine, act as antagonists at this receptor, an action that could well contribute to their therapeutic effects in schizophrenia. In addition to their psychotropic effects, activation of 5-HT2A receptors induces hyperthermia, which could explain this dangerous action of MDMA. Finally, an unusual feature of these receptors is that they are downregulated by prolonged exposure to antagonists, as well as agonists. The reason for this is uncertain but it could suggest that drugs which hitherto have been regarded as antagonists are, in fact, inverse agonists. However, little is known about these receptors, mainly because of the shortage, until recently, of selective ligands, their low density and the limited distribution of their mRNA in the brain. However, as information accrued from cloning studies, pharmacological characterisation, and discovery of its second messenger system, it became evident that this receptor shared the characteristics of the 5-HT2 receptor family, rather than those of a 5-HT1 receptor. The switch in classification from a 5-HT1C to a 5-HT2C receptor explains the gap in the 5-HT1 receptor family. The 5-HT2C receptor was first found in the choroid plexus, where it is thought to regulate the formation of CSF, but it has since been found in cortical and limbic areas as well as the basal ganglia. In the choroid plexus, at least, its actions seem to be mediated by activation of phospholipase C with a resulting depolarisation of the host cell. Like the 5-HT2A subtype, 5-HT2C receptors are downregulated by prolonged exposure to antagonists (inverse agonists? The discovery that 5-HT2C receptor mRNA is subject to posttranslational changes suggests that there could be several different isoforms of this receptor and it cannot be assumed that they are functionally the same. As far as can be certain, given the lack of selective ligands, their activation elsewhere in the brain is thought to culminate in reduced locomotor activity and hyperthermia. However, interest in these receptors as possible therapeutic targets is fostered by evidence that their agonists, such as mCPP, appear to be profoundly anxiogenic (see Chapter 19) and reduce food intake (see below). Instead, they comprise a pentameric complex of subunits that incorporates an ion channel. This is selective for the cations Na and K which, when opened, leads to depolarisation of the host cell. However, they are also found elsewhere in lower concentrations, notably in the cortex, amygdala and hippocampus, where they are thought to be associated mainly with GABAergic neurons. Interestingly, their function is modified by many agents that allosterically modify GABAA receptor function (e.
The superior cerebellar peduncle (brachium (A15) himcolin 30gm generic, and wing of the central lobule (A16) generic himcolin 30 gm overnight delivery. It receives fibers from the cortex of the vermis, the vestibular nuclei, and the olive. It sends fibers to the vestibular nuclei and other nuclei of the medulla oblongata. The globose nucleus (B19), too, is thought to receive fibers from the cortex of the vermis and to send fibers to the nuclei of the medulla oblongata. Fibers of the cerebellar cortex from the region between vermis and hemisphere (intermediate part) are thought to terminate at the hilum of the dentate nu- cleus in the emboliform nucleus (B20). The fibers of the latter nucleus run through the Kahle, Color Atlas of Human Anatomy, Vol. Cerebellar Peduncles and Nuclei 155 4 15 2 16 5 14 1 6 7 24 23 3 22 8 12 11 10 9 13 A Anterior view 18 19 21 20 25 B Cerebellar nuclei 26 27 33 17 28 24 22 23 29 30 31 32 C Cerebellar peduncles (according to Büttner) Kahle, Color Atlas of Human Anatomy, Vol. Projection of the convoluted relief of (primary and secondary dendrites) have a the human cerebellum onto a plane results smooth surface (C7) and are covered with in an expanse of 1m in length in the oro- synapses. The fine terminal branches are caudal dimension (from the lingula to the dotted with short spines (C8). The cortex is regularly structured cell carries approximately 60000 spinous throughout all regions of the cerebellum. Different fiber systems terminate consists of three layers: at the smooth and spiny sections of the cell:! The molecular layer (A1) lies beneath the The axon (B9) departs from the base of the surface; it contains few cells and consists Purkinje cell and extends through the mainly of unmyelinated fibers. The neurons we can distinguish the outer stel- axons of Purkinje cells terminate at neurons late cells (lying close to the surface) and the of the cerebellar nuclei (p. Purkinje cells use layer (ganglionic layer) (A2) is formed by GABA as neurotransmitter. It is very rich in cells, consisting of densely packed, small neurons, the granule cells. Purkinje Cells (B–D) The Purkinje cell represents the largest and most characteristic cell of the cerebellum. The Nissl stain shows the pear-shaped cell body (B4) filled with coarse Nissl bodies. Also visible are the basal portions of two or three dendrites (B5) at the upper pole of the cell. However, the cell’s entire expanse with all its processes can only be visualized by Golgi impregnation or intracellular staining. The primary stems of the dendrites ramify into further branches, and these again into finearborizationsthatformthedendritictree (B6). The dendrites spread in a two-dimen- sional plane like the branches of an espalier tree. The Purkinje cells are arranged in a strictly geometric fashion; spaced at rela- tively regular intervals, they form a row be- tween granular and molecular layers and sendtheirdendritictreesintothemolecular layer toward the surface of the folium. Cerebellar Cortex 157 1 2 3 A Cerebellar folium, Nissl stain 5 6 4 B Purkinje cell, silver im- pregnation (according to Fox) and Nissl stain 9 7 8 D Arrangement of the Purkinje cells C Detail of B within a folium Kahle, Color Atlas of Human Anatomy, Vol. Their dendritic trees, neurons run in all directions and reach ap- which ramify predominantly in the molecu- proximately 12 Purkinje dendritic trees. The cells have short axons, which In the lower third of the molecular layer lie either terminate in a glomerulus or ramify the slightly larger basket cells (A1). The Golgi long axons run horizontally above the cells belong to the inhibitory interneurons. Purkinje cell bodies and give off collaterals, the terminal branches of which form net- Glia (D) works (baskets) around the Purkinje cell Apart from the regular glial cell types, such bodies. The electron-microscopic image as the oligodendrocytes (D5) and proto- shows that the basket cell fibers form plasmic astrocytes (D6) commonly found in numerous synaptic contacts (B2) with the the granular layer, there are also glial cells Purkinje cell, namely, at the base of the cell that are characteristic for the cerebellum: body (axon hillock) and at the initial seg- Bergmann’s glia and the penniform glia of ment of the axon up to where the myelin Fañanás. The rest of the Purkinje cell body is enveloped by Bergmann’s glial cells ThecellbodiesoftheBergmann’scells(D7)lie (B3). The positioning of the synapses at the between the Purkinje cells and send long axon hillock indicates the inhibitory supporting fibers vertically toward the sur- character of the basket cells. Granule Cells (C) The supporting fibers carry leaflike processes and form a dense scaffold. Berg- These small, densely packed neurons form mann’s glia begins to proliferate where the granular layer. The Fañanás cells (D8) the Golgi impregnation shows three to five have several short processes with a charac- short dendrites which carry clawlike thick- teristic penniform structure. The thin axon (C4) of the granule cell ascends verti- cally through the Purkinje cell layer into the molecular layer, where it bifurcates at right angles into two parallel fibers (p. The granular layer contains small, cell-free islets (glomeruli), in which the clawlike dendritic endings of the granule cells form synaptic contacts with the axon terminals of afferent nerve fibers (mossy fibers, p. The electron-microscopic image shows complex, large synapses (glomeru- lus-like synaptic complexes), which are en- veloped by glial processes. Cerebellar Cortex 159 11 33 A Basket cell (according to Jacob) 44 22 C Granule cell 22 B Purkinje cell with basket cell synapses, 88 electron-microscopic diagram (according to Hámori Szentágothai) 77 66 55 D Glial cells of the cerebellum E Golgi cell (according to Jacob) Kahle, Color Atlas of Human Anatomy, Vol. There are two excitation of one row of Purkinje cells inhib- different types of terminals: climbing fibers its all neighboring Purkinje cells; this sharp- and mossy fibers. The climbing fibers (AC1) terminate at the Purkinje cells by splitting up and attaching Functional Principle of the Cerebellum (D) like tendrils to the ramifications of the den- dritic tree. Each climbing fiber terminates at The axons of the Purkinje cells (D4) termi- a single Purkinje cell and via axon collaterals nate at the neurons of the subcortical nuclei also at some stellate and basket cells. The (D10) (cerebellar nuclei and vestibular nu- climbing fibers originate from neurons of clei).
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