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Com- pared with fast-twitch fibers purchase sildalis 120 mg mastercard effexor xr impotence, their glycogen content is low cheap 120mg sildalis erectile dysfunction female doctor. The slow-twitch fibers develop force slowly but maintain contractions longer than fast-twitch muscle. The fast-twitch fibers, or type II, can be subdivided as type IIa or IIb. Type IIb fibers (also called fast-glycolytic) have few mitochondria and low levels of myo- globin (hence, they appear white). They are rich in glycogen and use glycogenoly- sis and glycolysis as their primary energy source. These muscles are prone to fatigue, because a continued reliance on glycolysis to produce ATP leads to an increase in lactic acid levels, resulting in a drop in the intracellular pH. As the pH drops, the ability of the muscle to produce ATP also diminishes. However, fast- twitch muscle can develop greater forces than slow-twitch muscle, such that con- tractions occur more rapidly. Type IIa fibers (also called fast-oxidative glycolytic) have properties of both type I and IIb fibers and thus display functional character- istics of both fiber types. The properties of types I, IIa, and IIb fibers are summa- rized in Table 47. Muscles are a mixture of the different fiber types, but depending on the function a muscle could have a preponderance of one fiber type over another. Type I fibers are found in postural muscles such as the psoas in the back musculature or the soleus in the leg. The percentage of type I to type II will vary with the muscle. Type II fibers are more prevalent in the large muscles of the limbs that are responsible for sudden, powerful movements. Extraoc- ular muscles would also have more of these fibers than type I. Smooth Muscle Cells Smooth muscle cells are found in the digestive system, blood vessels, bladder, air- ways, and uterus. The cells have a spindle shape with a central nucleus (see Fig. The designation of smooth refers to the fact that these cells, which contain a single nucleus, display no striations under the microscope. The contraction of smooth muscle is controlled involuntarily (the cells contract and relax without any conscious attempt to have them do so; examples of smooth muscle activity include moving food Table 47. Properties of Muscle Fiber Types Type I Fibers Type II Fibers Type IIa Type IIb • Slow-twitch (slow speed • Intermediate-twitch (fast • Fast-twitch (fast speed of of contraction) speed of contraction) contraction) • Slow-oxidative (low glyco- • Fast-oxidative glycolytic • Fast-glycolytic (high glyco- gen content) fibers (intermediate gen content) glycogen levels) • High myoglobin content • Intermediate fiber • Low myoglobin content (appear red) diameter (appear white) • Small fiber diameter • High myoglobin content • Low mitochondrial (appear red) content • Increased concentration • Increased oxidative • Limited aerobic of capillaries surrounding capacity on training metabolism muscle (greater oxygen • Intermediate resistance to • Large fiber diameter delivery) fatigue • High capacity for aerobic • More sensitive to fatigue metabolism as compared with other fiber types • High resistance to fatigue • Least efficient use of energy, primarily glycolytic • Used for prolonged, • Used for sprinting and aerobic exercise resistance tasks CHAPTER 47 / METABOLISM OF MUSCLE AT REST AND DURING EXERCISE 865 along the digestive tract, altering the diameter of the blood vessels, and expelling urine A reduced flow of oxygen-rich from the bladder). In contrast to skeletal muscle, these cells have the ability to main- blood to the heart muscle may lead tain tension for extended periods, and do so efficiently, with a low use of energy. The amount of ATP that can be gen- erated by glycolysis alone is not sufficient to C. Cardiac Muscle Cells meet the energy requirements of the con- The cardiac cells are similar to skeletal muscle in that they are striated (contain fibers), tracting heart. The multicellular contacts allow the cells to act as a common unit and to con- tract and relax synchronously. Cardiac muscle cells are designed for endurance and consistency. They depend on aerobic metabolism for their energy needs because they contain many mitochondria and very little glycogen. These cells thus generate only a small amount of their energy from glycolysis using glucose derived from glycogen. NEURONAL SIGNALS TO MUSCLE For an extensive review of how muscle contracts or a detailed view of the signaling to allow muscle contraction, consult a medical physiology book. The ryanodine receptors are cal- The nerve–muscle cell junction is called the neuromuscular junction (Fig. This binding stim- coplasmic reticulum of muscle cells. One ulates the opening of sodium channels on the sarcolemma. The massive influx of type of receptor can be activated by a depo- larization signal (depolarization-induced cal- sodium ions results in the generation of an action potential in the sarcolemma at the cium release).

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Mechanisms That Affect Glycogen and Triacylglyc- high K m erol Synthesis in Liver ADP After a meal purchase sildalis 120mg amex erectile dysfunction when pills don work, the liver synthesizes glycogen and triacylglycerol sildalis 120 mg fast delivery erectile dysfunction psychological treatment. The level of glyco- Glucose-6-P gen stored in the liver can increase from approximately 80 g after an overnight fast to a limit of approximately 200–300 g. Although the liver synthesizes triacylglycerol, it does not store this fuel but rather packages it in very-low-density lipoprotein Fructose-6-P (VLDL) and secretes it into the blood. The fatty acids of the VLDL triacylglycerols phosphofructokinase-1 secreted from the liver are stored as adipose triacylglycerols. Adipose tissue has an AMP , F-2,6-BP ,+ + almost infinite capacity to store fat, limited mainly by the ability of the heart to pump ATP , Citrate– – blood through the capillaries of the expanding adipose mass. Although we store fat throughout our bodies, it tends to accumulate in places where it does not interfere too Fructose-1, 6-P much with our mobility: in the abdomen, hips, thighs, and buttocks. Both the synthesis of liver glycogen and the conversion by the liver of dietary glucose to triacylglycerol (lipogenesis) are regulated by mechanisms involving key enzymes in these pathways. GLUCOKINASE pyruvate (cAMP-dependent) kinase – Alanine After a meal, glucose can be converted to glycogen or to triacylglycerol in the liver. Because of the enzyme’s low affinity for glucose, this enzyme is most active in the fed state, when Fig. Regulation of glucokinase, PFK-1, the concentration of glucose is particularly high because the hepatic portal vein and pyruvate kinase in the liver. Synthesis of synthase– P glucokinase is also induced by insulin (which is elevated after a meal) and (inactive) repressed by glucagon (which is elevated during fasting). In keeping with the protein ADP protein liver’s function in maintaining blood glucose levels, this system is set up such that kinase A phosphatase the liver can only metabolize glucose when sugar levels are high, and not when + Glucagon + Insulin ATP sugar levels are low. GLYCOGEN SYNTHASE (active) In the conversion of glucose 6-phosphate to glycogen, the key regulatory enzyme is Glycogen UDP–Glucose glycogen synthase. This enzyme is activated by the dephosphorylation that occurs when insulin is elevated and glucagon is decreased (Fig. PHOSPHOFRUCTOKINASE-1 AND PYRUVATE KINASE For lipogenesis, glucose 6-phosphate is converted through glycolysis to pyruvate. Glucose–6–P Key enzymes that regulate this pathway in the liver are phosphofructokinase-1 (PFK-1) and pyruvate kinase. PFK-1 is allosterically activated in the fed state by Glucose fructose 2,6-bisphosphate and adenosine monophosphate (AMP) (see Fig. Phosphofructokinase-2, the enzyme that produces the activator fructose 2,6- This enzyme is phosphorylated by a series of bisphosphate, is dephosphorylated and active after a meal (see Chapter 22). Pyru- kinases, which are initiated by the cAMP- vate kinase is also activated by dephosphorylation, which is stimulated by the dependent protein kinase, under fasting condi- increase of the insulin/glucagon ratio in the fed state (see Fig. It is dephosphorylated and active after a meal, and glycogen is stored. Circled P phosphate; a circled sign activated by; a 4. PYRUVATE DEHYDROGENASE AND PYRUVATE CARBOXYLASE circled sign inhibited by. The conversion of pyruvate to fatty acids requires a source of acetyl CoA in the cytosol. Pyruvate can only be converted to acetyl CoA in mitochondria, so it enters mitochondria and forms acetyl CoA through the pyruvate dehydrogenase (PDH) reaction. This enzyme is dephosphorylated and most active when its supply of sub- P strates and adenosine diphosphate (ADP) is high, its products are used, and insulin pyruvate is present (Fig. The enzyme that catalyzes this reac- inactive tion, pyruvate carboxylase, is activated by acetyl CoA. Because acetyl CoA cannot directly cross the mitochondrial membrane to form fatty acids in the cytosol, it con- ADP denses with oxaloacetate, producing citrate. The citrate that is not required for tri- carboxylic acid (TCA) cycle activity crosses the membrane and enters the cytosol. ADP – kinase phosphatase + Ca2+ As fatty acids are produced under conditions of high energy, the high NADH/NAD+ ratio in the mitochondria inhibits isocitrate dehydrogenase, which leads to citrate accumulation within the mitochondrial matrix. As the citrate accu- ATP P mulates, it is transported out into the cytosol to donate carbons for fatty acid i synthesis.

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The cause may be infection concentration of blood calcium order sildalis 120mg on-line erectile dysfunction self injection, and the condition is or autoimmunity buy 120mg sildalis otc erectile dysfunction at the age of 25, that is, abnormal production of anti- called tetany (TET-ah-ne). This low calcium tetany bodies to the thyroid gland. Hashimoto disease is an au- should not be confused with the infection called tetanus toimmune thyroiditis that may be hereditary and may (lockjaw). The disease results in enlargement of the thyroid (goiter) and hypothyroidism. It is treated with thyroid hormone replacement and, in some cases, surgery. Checkpoint 12-6 What is the effect of thyroid hormones on Thyroid cells? These very sensitive tests are used to detect abnormal thy- roid function and to monitor response to drug therapy. A Esophagus test for the level of thyroid-stimulating hormone (from the pituitary) is frequently done at the same time. Further Trachea testing involves giving a person oral radioactive iodine Figure 12-6 Parathyroid glands (posterior view). The four and measuring the amount and distribution of radiation small parathyroid glands are embedded in the posterior surface that accumulates in the thyroid gland. THE ENDOCRINE SYSTEM: GLANDS AND HORMONES 253 In contrast, if there is excess production of PTH, as ◗ Increase in the heart rate may happen in tumors of the parathyroid glands, calcium ◗ Increase in the metabolic rate of body cells is removed from its normal storage place in the bones and ◗ Dilation of the bronchioles, through relaxation of the released into the bloodstream. The loss of calcium from smooth muscle of their walls the bones leads to fragile bones that fracture easily. Be- cause the kidneys ultimately excrete the calcium, the for- Checkpoint 12-8 The main hormone from the adrenal medulla also functions as a neurotransmitter in the sympathetic nervous mation of kidney stones is common in such cases. Checkpoint 12-7 What mineral is regulated by calcitonin and parathyroid hormone (PTH)? Hormones From the Adrenal Cortex There are three main groups of hormones secreted by the adrenal The Adrenal Glands cortex: The adrenals are two small glands located atop the kid- ◗ Glucocorticoids (glu-ko-KOR-tih-koyds) maintain the neys. Each adrenal gland has two parts that act as sepa- carbohydrate reserve of the body by stimulating the rate glands. The inner area is called the medulla, and the liver to convert amino acids into glucose (sugar) in- outer portion is called the cortex (Fig. The production of these hormones in- creases in times of stress to aid the body in responding Hormones From the Adrenal Medulla The hor- to unfavorable conditions. They raise the level of nutri- mones of the adrenal medulla are released in response to ents in the blood, not only glucose, but also amino stimulation by the sympathetic nervous system. The prin- acids from tissue proteins and fatty acids from fats 12 cipal hormone produced by the medulla is epinephrine, stored in adipose tissue. Glucocorticoids also have the also called adrenaline. Another hormone released from ability to suppress the inflammatory response and are the adrenal medulla, norepinephrine (noradrenalin), is often administered as medication for this purpose. The closely related chemically and is similar in its actions to major hormone of this group is cortisol, which is also epinephrine. These two hormones are referred to as the called hydrocortisone. We have already learned about important in the regulation of electrolyte balance. They these hormones in studying the autonomic nervous sys- control sodium reabsorption and potassium secretion tem. When released from nerve endings instead of being by the kidney tubules. The major hormone of this released directly into the bloodstream, they function as group is aldosterone (al-DOS-ter-one). Some of their effects are as follows: ◗ Sex hormones are secreted in small amounts, having ◗ Stimulation of the involuntary muscle in the walls of little effect on the body. Hypersecretion of cortisol results in a condition known as Cushing syndrome, the symptoms of which in- Cortex clude obesity with a round (“moon”) face, thin skin that Medulla bruises easily, muscle weakness, bone loss, and elevated blood sugar. Use of steroid drugs also may produce these symptoms. If aldosterone is secreted in excess, as a result of hyperfunction of the adrenal cortex, the condition is termed aldosteronism.

The most important aspect by far of the use of anterior supports is that the mechanical function of these Figure 6 order 120mg sildalis fast delivery erectile dysfunction liver cirrhosis. Many children need some chest systems is to pull the shoulders superiorly and posteriorly discount sildalis 120mg erectile dysfunction venous leak, which means the restraint at some time to assist with upright superior straps must be fixed above and behind the shoulders when the sitting. There are a variety of different de- children are sitting upright. Many vendors, therapists, and parents see these signs, although almost all fix over the shoul- straps as suspenders holding the wheelchair up against a child’s bottom. It is very im- portant that the chair back be kept high su- der and encourages the children’s spines to roll into kyphosis, exactly the op- periorly so the shoulder straps do not depress posite of the desired goal. Also, when children are growing fast, they should the shoulders, because the goal is to pull the have the shoulder harness attachment adjusted every 6 to 9 months to main- shoulders posteriorly, not for the straps to tain a proper fit. The inferior attachment of the anterior shoulder support act as suspenders to hold the wheelchair onto needs to be fixed posteriorly to assist in creating a posterior vector at the the child (B). Durable Medical Equipment 221 shoulder; however, this attachment point is not as crucial to good function as the proximal attachment. Another option that can occasionally be used in adolescence is a strap attached to the chest lateral crossing in front of the chest wall. These anterior chest straps only work if the force or tendency to fall into kyphosis is not very strong. These straps are especially useful in fe- male adolescents with large breasts for whom the harness type of restraint is hard to use. The strap is placed immediately inferior to the breasts. Another very important aspect of controlling the anterior fall of the trunk is to have a lap tray and armrest placed in an elevated position. By using the upper extremity on the arm rest or lap tray, children are encouraged to sit upright. In some children, the tray may be placed as high as the nipple line, which will greatly encourage sitting upright. Headrests Headrests provide two functions, first, to provide support for individuals with poor head control, and second, as a safety feature when riding in a ve- hicle. For children who have good head control but sit in a wheelchair while riding in a van or school bus, the headrest may be needed only during vehi- cle transportation. This headrest can be a simple flat extension of the back that can flip down or be removed easily when not needed. For individuals who need head control, a more elaborate system may be needed. If the only head control needed is to prevent hyperextension, a simple flat or mildly contoured headrest only may be required. If a lateral support is needed, a lateral extension, usually coming inferiorly and anteriorly, is preferred. These anterior extensions should be inferior far enough to avoid causing irritation to the ears (Figure 6. Proper anterior trunk control is important for the best function of these head restraints. To restrain the severe anterior drop of the head, a mobile forehead strap may be used. This system only works if the forehead has a shape with some ledge or protrusion, which will allows the strap to stay in place. A forehead shape with a posterior slope does not allow Figure 6. Another approach to preventing anterior drop of the tems available, and often trial and error is re- head is to use cervical collars that place the support under the mandible. Many Some of these are attached to the chair posteriorly and some are free float- headrest systems have modular posterior ing on the children.