By P. Kan. Millikin University. 2018.
When the force exerted by a shortening ing is physically impossible regardless of the degree of muscle continuously increases as it shortens discount toradol 10 mg with mastercard, the contrac- activation discount toradol 10mg on line. The curves labeled A and A B corre- and greater shortening with the lower weight (A) buy toradol 10 mg low cost. If weight C (to- spond to the force and shortening records during the lifting of tal weight A B C) is added to the afterload order 10mg toradol amex, the muscle those weights generic toradol 10mg overnight delivery. In each case, the adjustable platform prevents the cannot lift it, and the entire contraction remains isometric. If the muscle has not been stimulated, this resisting force is called passive The types of contraction described above provide a basis force or resting force. The isomet- The relationship between force and length is much dif- ric and isotonic mechanical behavior of muscle can be de- ferent in a stimulated muscle. The amount of active force or scribed in terms of two important relationships: active tension a muscle can produce during an isometric • The length-tension curve, treating isometric contraction contraction depends on the length at which the muscle is at different muscle lengths held. At a length roughly corresponding to the natural • The force-velocity curve, concerned with muscle per- length in the body, the resting length, the maximum force formance during isotonic contraction is produced. If the muscle is set to a shorter length and then stimulated, it produces less force. At an extremely short Isometric Contraction and the Length-Tension Curve. If the muscle is made Because it is made of contractile proteins and connective longer than its optimal length, it produces less force when tissue, an isolated muscle can resist being stretched at rest. This behavior is summarized in the length-ten- When it is very short, it is slack and will not resist passive sion curve (Fig. Normally a muscle is force produced by a series of twitches made over the range CHAPTER 9 Skeletal Muscle and Smooth Muscle 161 Passive Total force 5 4 3 2 1 0 0 7 8 9 Time Length Active 9 8 At length 8. Subtraction of the passive curve from the total curve Time yields the active force curve. These curves are further illus- Passive trated in the lower right corner of the figure. Everyday experience shows that the speed at which a mus- The total peak force from each twitch is related to each cle can shorten depends on the load that must be moved. The muscle length is changed only Simply stated, light loads are lifted faster than heavy ones. The difference between the into how the force and shortening of muscles are matched total force and the passive force is called the active force to the external tasks they perform, as well as how muscles (see inset; Fig. The active force results directly from function internally to liberate mechanical energy from their the active contraction of the muscle. The analysis is performed by arranging a The length-tension curve shows that when the muscle is muscle so that it can be presented with a series of afterloads either longer or shorter than optimal length, it produces (see Fig. Myofilament overlap is a primary factor in deter- stimulated, lighter loads are lifted quickly and heavier loads mining the active length-tension curve (see Chapter 8). If the applied load is greater than the maximal However, studies have demonstrated that at very short force capability of the muscle, known as Fmax, no shorten- lengths, the effectiveness of some steps in the excitation- ing will result and the contraction will be isometric. If no contraction coupling process is reduced—binding of cal- load is applied, the muscle will shorten at its greatest possi- cium to troponin is less and there is some loss of action po- ble speed, a velocity known as Vmax. The initial velocity—the speed with which the muscle The functional significance of the length-tension curve begins to shorten—is measured at various loads. Many skeletal locity is measured because the muscle soon begins to slow muscles are confined by their skeletal attachments to a rel- down; as it gets shorter, it moves down its length-tension atively short region of the curve that is near the optimal curve and is capable of less force and speed of shortening. In these cases, the lever action of the skeletal sys- When all the initial velocity measurements are related to tem, not the length-tension relationship, is of primary im- each corresponding afterload lifted, an inverse relationship portance in determining the maximal force the muscle can known as the force-velocity curve is obtained. Cardiac muscle, however, normally works at lengths steeper at low forces. When the measurements are made on significantly less than optimal for force production, but its a fully activated muscle, the force-velocity curve defines passive length-tension curve is shifted to shorter lengths the upper limits of the muscle’s isotonic capability. The length-tension relationship is, there- tice, a completely unloaded contraction is very difficult to fore, very important when considering the ability of cardiac arrange, but mathematical extrapolation provides an accu- muscle to adjust to changes in length (related to the volume rate Vmax value. The role of the length-tension curve in smooth a series of isotonic contractions. The initial velocity points muscle is less clearly understood because of the great di- (A–D) correspond to the contractions shown at the top. For all muscle types, however, the length-tension incomplete stimulation (e.
Clinical complications of body fluid and elec- adaptations within exercising muscle that would increase trolyte balance buy generic toradol 10mg online. Body Fluid Bal- muscle oxidative capacity and reduce lactic acid production purchase toradol 10mg fast delivery. Boca Raton 10mg toradol fast delivery, FL: CRC Press order toradol 10 mg, By reducing the ventilatory demands of exercise buy 10mg toradol overnight delivery, these 1996;297–317. PART IX Endocrine Physiology CHAPTER Endocrine Control Mechanisms Daniel E. Hormones are chemical substances, involved in cell-to-cell transported in the bloodstream bound to carrier proteins, communication, that promote the maintenance of home- whereas most peptide and protein hormones are soluble in ostasis. Steroid hormones and thyroid hormones are generally docrine abnormalities. The word “hormone” is derived gan systems by a group of specialized chemical substances from the Greek hormaein, which means to “excite” or to “stir called hormones. Certain endocrine disease nate a variety of different physiological processes. The states, such as diabetes mellitus, thyroid disorders, and re- processes that hormones regulate fall into four areas: (1) the productive disorders, are fairly common in the general pop- digestion, utilization, and storage of nutrients; (2) growth ulation; therefore, it is likely that they will be encountered and development; (3) ion and water balance; and (4) repro- repeatedly in the practice of medicine. In addition, because hormones either directly or indi- rectly affect virtually every cell or tissue in the body, a num- ber of other prominent diseases not primarily classified as Hormones Regulate and endocrine diseases may have an important endocrine com- Coordinate Many Functions ponent. Atherosclerosis, certain forms of cancer, and even certain psychiatric disorders are examples of conditions in It is difficult to describe hormones in absolute terms. As a which an endocrine disturbance may contribute to the pro- working definition, however, it can be said that hormones gression or severity of disease. They Feedback Regulation Is an Important are highly potent, specialized, organic molecules produced Part of Endocrine Function by endocrine cells in response to specific stimuli and exert The endocrine system, like many other physiological sys- their actions on specific target cells. The mecha- equipped with receptors that bind hormones with high nism is usually negative feedback, although a few positive affinity and specificity; when bound, they initiate charac- feedback mechanisms are known. Both types of feedback teristic biological responses by the target cells. Although many hormones travel by just its rate of hormone secretion to produce the desired this mechanism, we now realize that there are many hor- level of effect, ensuring the maintenance of homeostasis. Since negative feedback is their target cells by diffusion through the interstitial fluid. First-order feedback regulation Hormone Receptors Determine Whether a is the simplest type and forms the basis for more complex Cell Will Respond to a Hormone modes of regulation. In this example, an endocrine cell se- In the endocrine system, a hormone molecule secreted cretes a hormone that produces a specific biological effect into the blood is free to circulate and contact almost any in its target tissue. As the biological response in- that possess specific receptors for the hormone, will re- creases, the amount of hormone secreted by the endocrine spond to that hormone. More commonly, feedback biological effects characteristic of that hormone are initi- regulation in the endocrine system is complex, involving ated. Therefore, in the endocrine system, the basis for second- or third-order feedback loops. For example, multi- specificity in cell-to-cell communication rests at the level ple levels of feedback regulation may be involved in regu- of the receptor. Similar concepts apply to autocrine and lating hormone production by various endocrine glands un- paracrine mechanisms of communication. The A certain degree of specificity is ensured by the re- regulation of target gland hormone secretion, such as adre- stricted distribution of some hormones. For example, sev- nal steroids or thyroid hormones, begins with production eral hormones produced by the hypothalamus regulate of a releasing hormone by the hypothalamus. These hor- hormone stimulates production of a trophic hormone by mones are carried via small blood vessels directly from the the anterior pituitary, which, in turn, stimulates the pro- hypothalamus to the anterior pituitary, prior to entering duction of the target gland hormone by the target gland. In addition, the trophic hormone may inhibit the distribution of active hormone is the local transforma- releasing hormone secretion from the hypothalamus, and tion of a hormone within its target tissue from a less active in some cases, the releasing hormone may inhibit its own to a more active form. Dihydrotestos- to provide certain advantages compared with the simpler sys- terone is a much more potent androgen than testosterone. Theoretically, it permits a greater degree of fine-tuning Because the enzyme that catalyzes this conversion is found of hormone secretion, and the multiplicity of regulatory steps only in certain locations, its cell or tissue distribution minimizes changes in hormone secretion in the event that partly localizes the actions of the androgens to these sites. Therefore, while receptor distribution is the primary fac- It is important to bear in mind the normal feedback rela- tor in determining the target tissues for a specific hor- tionships that control the secretion of each individual hor- mone, other factors may also focus the actions of a hor- mone are discussed in the chapters that follow.
E toradol 10mg line, R cheap toradol 10 mg online, C buy toradol 10 mg without prescription, K effective toradol 10 mg, and W are positive if they represent energy Factors other than body size that affect metabolism at losses from the body and negative if they represent energy rest include age and sex (Fig 10mg toradol. The ratio of metabolic rate to surface area is high- temperature neither rises nor falls. When the body is not est in infancy and declines with age, most rapidly in child- in heat balance, its mean tissue temperature increases if S hood and adolescence and more slowly thereafter. This situation have high metabolic rates in relation to surface area because commonly lasts only until the body’s responses to the tem- of the energy used to synthesize the fats, proteins, and other perature changes are sufficient to restore balance. Similarly, a ever, if the thermal stress is too great for the thermoregu- woman’s metabolic rate increases during pregnancy to sup- latory system to restore balance, the body will continue to ply the energy needed for the growth of the fetus. However, gain or lose heat until either the stress diminishes suffi- a nonpregnant woman’s metabolic rate is 5 to 10% lower ciently or the animal dies. Because so many fac- Relative Masses and Metabolic Heat tors affect metabolism at rest, metabolic rate is often meas- TABLE 29. Metabolic rate measured under these % of conditions is called basal metabolic rate (BMR). The com- Heat Production monly accepted conditions for measuring BMR are that the % of person must have fasted for 12 hours; the measurement Body Mass Rest Exercise must be made in the morning after a good night’s sleep, be- Brain 2 16 1 ginning after the person has rested quietly for at least 30 Trunk viscera 34 56 8 minutes; and the air temperature must be comfortable, Muscle and skin 56 18 90 about 25 C (77 F). Basal metabolic rate is “basal” only dur- Other 8 10 1 ing wakefulness, since metabolic rate during sleep is some- what less than BMR. Heat exchange with the environment can be measured directly by using a human calorimeter. In this insulated bly because a higher proportion of the female body is com- chamber, heat can exit only in the air ventilating the cham- posed of fat, a tissue with low metabolism. By measuring the flow of air and water and their that have the greatest effect on metabolic rate. Cate- temperatures as they enter and leave the chamber, one can cholamines cause glycogen to break down into glucose determine the subject’s heat loss by conduction, convec- and stimulate many enzyme systems, increasing cellular tion, and radiation. Hypermetabolism is a clinical feature of of air entering and leaving the chamber, one can determine some cases of pheochromocytoma, a catecholamine-se- heat loss by evaporation. Thyroxine magni- calorimetry, and though conceptually simple, it is cumber- fies the metabolic response to catecholamines, increases some and costly. The metabolic rate is typically 45% above nor- try, which is based on measuring a person’s rate of O2 con- mal in hyperthyroidism (but up to 100% above normal in sumption, since virtually all energy available to the body severe cases) and 25% below normal in hypothyroidism depends ultimately on reactions that consume O2. Con- (but 45% below normal with complete lack of thyroid suming 1 L of O2 is associated with releasing 21. Other hormones have relatively minor effects kcal) if the fuel is carbohydrate, 19. An A resting person’s metabolic rate increases 10 to 20% af- average value often used for the metabolism of a mixed ter a meal. The ratio of CO2 food (formerly known as specific dynamic action), lasts produced to O2 consumed in the tissues is called the res- several hours. In a steady state where CO2 is ex- haled from the lungs at the same rate it is produced in the tissues, RQ is equal to the respiratory exchange ratio, R (see Chapter 19). One can improve the accuracy of indi- 54 rect calorimetry by also determining R and either estimat- 62 52 ing the amount of protein oxidized—which usually is 60 58 50 small compared to fat and carbohydrate—or calculating it 56 48 from urinary nitrogen excretion. Even 44 50 48 42 during mild exercise, the muscles are the principal source of 46 Males 40 metabolic heat, and during intense exercise, they may ac- 44 38 count for up to 90%. Moderately intense exercise by a 42 36 healthy, but sedentary, young man may require a metabolic 40 34 rate of 600 W (in contrast to about 80 W at rest), and in- 38 Females 36 32 tense activity by a trained athlete, 1,400 W or more. Be- 34 30 cause of their high metabolic rate, exercising muscles may 28 be almost 1 C warmer than the core. Blood perfusing these 0 5 1015202530354045 50 55 60 65 70 75 muscles is warmed and, in turn, warms the rest of the body, Age (yr) raising the core temperature. Effects of age and sex on the basal meta- Muscles convert most of the energy in the fuels they FIGURE 29. During here is expressed as the ratio of energy consumption to body sur- phosphorylation of ADP to form ATP, 58% of the energy face area.
The reversal potential is the membrane potential records drawn to illustrate isolated portions of the response; in an at which net current flow is zero (i toradol 10mg. As a brane and causes voltage-gated sodium channels to open toradol 10 mg mastercard, result of the altered permeabilities buy toradol 10 mg visa, a net inward current cheap 10 mg toradol, bringing the membrane to threshold purchase 10 mg toradol. This leads to an ac- known as the endplate current, depolarizes the postsynap- tion potential in the muscle membrane. This voltage change is called the endplate potential is propagated along the muscle cell membrane by potential. The voltage at which the net membrane current regenerative local currents similar to those in a nonmyeli- would become zero is called the reversal potential of the nated nerve fiber. If some circumstance causes reduced ACh release, the To complete the circuit, the current flowing inward at amount of depolarization at the endplate could be corre- the postsynaptic membrane must be matched by a return spondingly reduced. This current flows through the local muscle cyto- ever, the endplate potential is much more than sufficient to plasm (myoplasm), out across the adjacent muscle mem- produce a muscle action potential; this reserve, referred to brane and back through the extracellular fluid (Fig. As as a safety factor, can help preserve function under abnor- CHAPTER 9 Skeletal Muscle and Smooth Muscle 155 A receptors. This binding does not result in opening of the Motor axon action potential C ion channels, however, and the endplate potential is re- Muscle action duced in proportion to the number of receptors occupied potential by curare. Although the muscle can be directly stimulated electrically, nerve stimulation is ineffective. External muscular junction in a slightly different way; this molecule transmitter release return current binds to the receptors and causes the channels to open. Be- cause it is hydrolyzed very slowly by AChE, its action is long lasting and the channels remain open. This prevents resetting of the inactivation gates of muscle membrane sodium channels near the endplate region and blocks sub- 4. Drugs that produce extremely current long-lasting endplate potentials are referred to as depolar- 2. In carefully controlled doses, they can temporarily alleviate symptoms of myasthenia gravis, an autoimmune condition that results in a loss of postsynaptic ACh receptors. Longitudinal principal symptom is muscular weakness caused by end- B myoplasmic current plate potentials of insufficient amplitude. Partial inhibition Endplate potential of the enzymatic degradation of ACh allows ACh to remain effective longer and, thus, to compensate for the loss of re- ceptor molecules. A, The inward membrane current is car- to the endplate region of a muscle. If accidental denerva- ried by sodium ions through the channels associated with ACh tion occurs (e. The other currents are nonspecific and are carried by tire muscle becomes sensitive to direct application of ACh appropriately charged ions in the myoplasm and extracellular fluid. B, The endplate potential is localized to the endplate re- within several weeks. C, The muscle action potential is propagated along the sur- to the synthesis of new ACh receptors, a process normally face of the muscle. Arti- ficial electrical stimulation has been shown experimentally to prevent the synthesis of new receptors, by regulating mal conditions. The rate of rise of the endplate potential is transcription of the genes involved. If reinnervation occurs, determined largely by the rate at which ACh binds to the the extrasynaptic receptors gradually disappear. Muscle at- receptors, and indirect clinical measurements of the size rophy also occurs in the absence of functional innervation, and rise time of the endplate potential are of considerable which also can be at least partially reversed with artificial diagnostic importance. OF SKELETAL MUSCLE The variety of controlled muscular movements that humans Neuromuscular Transmission Can Be can make is remarkable, ranging from the powerful con- Altered by Toxins, Drugs, and Trauma tractions of a weightlifter’s biceps to the delicate move- ments of the muscles that position our eyes as we follow a The complex series of events making up neuromuscular moving object. In spite of this diversity, the fundamental transmission is subject to interference at several steps. The drug hemicholinium interferes with choline up- take by the presynaptic terminal and, thus, results in the de- The Timing of Muscle Stimulation Is a pletion of ACh. Botulinum toxin interferes with ACh Critical Determinant of Contractile Function release. This bacterial toxin is used to treat focal dystonias (see Clinical Focus Box 9. A skeletal muscle must be activated by the nervous system Postsynaptic blockade can result from a variety of cir- before it can begin contracting.
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