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After sufficient force has an isometric contraction can be studied by attaching a dis- been generated buy 180mg cardizem mastercard, the muscle will begin to shorten and lift sected muscle to an apparatus similar to that shown in Fig- the load (phase 2) cardizem 60 mg line. This arrangement provides for setting the length of because the force exerted by the muscle exactly matches the muscle and tracing a record of force versus time. In a that of the weight, and the mass of the weight does not twitch, isometric force develops relatively rapidly, and sub- vary. The dura- line representing constant force, while the muscle length tions of both contraction time and relaxation time are re- (lower tracing) is free to change. As relaxation begins lated to the rate at which calcium ions can be delivered to (phase 3), the muscle lengthens at constant force because it and removed from the region of the crossbridges, the actual is still supporting the load; this phase of relaxation is iso- sites of force development. During an isometric contrac- tonic, and the muscle is reextended by the weight. When tion, no actual physical work is done on the external envi- the muscle has been extended sufficiently to return to its ronment because no movement takes place while the force original length, conditions again become isometric (phase is developed. The muscle, however, still consumes energy 4), and the remaining force in the muscle declines as it to fuel the processes that generate and maintain force. In almost all situations encountered in daily life, isotonic contraction is preceded Isotonic Contraction. When conditions are arranged so by isometric force development; such contractions are the muscle can shorten and exert a constant force while do- called mixed contractions (isometric-isotonic-isometric). In the simplest conditions, this constant force is traction varies, depending on the afterload. At low after- CHAPTER 9 Skeletal Muscle and Smooth Muscle 159 Isometric twitch Rise of isometric Force force transducer 3 Isometric relaxation Muscle 2 1 4 1 Force is constant during isotonic phases 0 Stimulator Stimulus Isotonic Isotonic shortening relaxation 5 2 3 6 Length is constant during isometric 7 phases Weight 8 Contraction Relaxation 0. The pen at during the final relaxation, conditions are again isometric because the lower end of the muscle marks its length, and the weight at- the muscle no longer lifts the weight. The dotted lines in the force tached to the muscle provides the afterload, while the platform and length traces show the isometric twitch that would have re- beneath the weight prevents the muscle from being overstretched sulted if the force had been too large (greater than 3 units) for the at rest.

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Note that the superior view of the cerebellum (A) correlates closely The Brain: Gross Views discount cardizem 60 mg without a prescription, Vasculature cheap cardizem 60 mg line, and MRI 33 A B II,III V II,III IV I V Midbrain (Mid) Primary fissure (PriFis) PriFis Basilar pons (Bpon) VI Mid VII VII Fourth Bpon ventricle (ForVen) ForVen Medulla Med VIII (Med) VIII X X IX IX Posterolateral fissure (PostLatFis) II,III IV V C PriFis Mid VI Bpon VII ForVen Med X IX VIII 2-32 A median sagittal view of the cerebellum (A) showing its re- Lobules I-V are the vermis parts of the anterior lobe; lobules VI-IX lationships to the midbrain, pons, and medulla. This view of the cere- are the vermis parts of the posterior lobe; and lobule X (the nodulus) bellum also illustrates the two main fissures and the vermis portions of is the vermis part of the flocculonodular lobe. Designation of these lobules follows the method devel- larities between the gross specimen (A) and a median sagittal view of oped by Larsell. Peduncles Middle cerebellar Superior cerebellar Inferior colliculus Trochlear nerve Flocculus Crus cerebri Trigeminal nerve: Sensory root Motor root Basilar pons 2-33 Lateral and slightly rostral view of the cerebellum and brain- relative positions of, and distinction between, motor and sensory roots stem with the middle and superior cerebellar peduncles exposed. See page 40, Figure 2-41D for an MRI show- the relationship of the trochlear nerve to the inferior colliculus and the ing the trochlear nerve. Figure 3-10 on page 61 also dashed line on the left represents the position of the sulcus limitans and shows a comparable view of the brainstem and the posterior portions the area of the inferior cerebellar peduncle is shown on the right. The Brain: Gross Views, Vasculature, and MRI 35 Vessels Structures Choroid plexus, third ventricle Pineal Habenula Medial thalamus Brachium of superior colliculus Thalamogeniculate arteries Superior Lateral thalamus colliculus Pulvinar nucleus Internal capsule Choroid plexus, lateral ventricle Medial and lateral Lateral geniculate body posterior choroidal arteries Medial geniculate body Quadrigeminal artery Brachium of inferior colliculus Superior cerebellar artery: Crus cerebri Medial branch Trochlear nerve (IV) Lateral branch Inferior colliculus Superior cerebellar peduncle Anterior medullary velum Facial colliculus Vestibular area Inferior cerebellar peduncle Middle cerebellar peduncle Choroid plexus, fourth ventricle Hypoglossal trigone Anterior inferior cerebellar artery Glossopharyngeal nerve (IX) Vagal nerve (X) Posterior inferior Accessory nerve (XI) cerebellar artery Restiform body Vagal trigone Trigeminal tubercle (tuberculum cinereum) Cuneate tubercle Posterior spinal artery Gracile tubercle Gracile fasciculus Cuneate fasciculus 2-35 Dorsal view of the brainstem and caudal diencephalon show- tion to serving the medulla, branches of the posterior inferior cerebel- ing the relationship of structures and some of the cranial nerves to ar- lar artery also supply the choroid plexus of the fourth ventricle. The vessels shown in this view have originated ventrally and tuberculum cinereum is also called the trigeminal tubercle. Medial eminence Superior cerebellar peduncle of fourth ventricle Facial colliculus Middle cerebellar peduncle Superior fovea Vestibular area Striae medullares Lateral recess Foramen of Luschka Hypoglossal trigone Sulcus limitans Vagal trigone Restiform body Cuneate tubercle Inferior fovea Gracile tubercle Tela choroidea (cut edge) 2-37 The floor of the fourth ventricle (rhomboid fossa) and imme- diately adjacent structures. The Brain: Gross Views, Vasculature, and MRI 37 Fornix Choroid plexus, third ventricle Optic tract Posterior choroidal arteries Thalamogeniculate artery Lateral geniculate body Dorsal thalamus Posterior cerebral artery Mammillary body Medial geniculate body Quadrigeminal artery Superior colliculus Posterior communicating artery Crus cerebri Internal carotid artery Brachium of inferior colliculus Inferior colliculus Oculomotor nerve Superior cerebellar artery Trochlear nerve Trigeminal nerve Motor root Sensory root Superior cerebellar peduncle Anterior medullary velum Basilar artery Middle cerebellar peduncle Anterior inferior cerebellar artery Vestibulocochlear nerve Facial nerve Labyrinthine artery Abducens nerve Posterior inferior cerebellar artery Glossopharyngeal nerve Choroid plexus, Vagus nerve fourth ventricle Hypoglossal nerve Restiform body Accessory nerve Cuneate tubercle Gracile tubercle Posterior inferior cerebellar artery Posterior spinal artery Anterior spinal artery Vertebral artery 2-38 Lateral view of the brainstem and thalamus, which shows the shown as dashed lines. Arteries that distribute to dorsal structures orig- relationship of structures and cranial nerves to arteries. Compare with Figure 2-36 on the fac- mate positions of the labyrinthine and posterior spinal arteries, when ing page. Rupture of aneurysms at this location is one of the are shown in axial (B, T1-weighted; D, T2-weighted) and in oblique more common causes of spontaneous subarachnoid hemorrhage. Note the similarity between the ax- proximity of these vessels to optic structures and the hypothalamus (D) ial planes, especially (B), and the gross anatomical specimen. In addi- explain the variety of visual and hypothalamic disorders experienced by tion, note the relationship between the anterior cerebellar artery, an- these patients.

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When the skin over the lateral plantar vertigo and lend support to a tentative diagnosis of area of the right foot is stroked discount cardizem 180mg amex, the first toe extends in- Ménière’s disease buy cardizem 60mg without a prescription, as would the presence of tinnitus and voluntarily. The purpose of the application of water is to provide a ther- slowness of movement initiation, increased muscle resist- mal stimulus that will heat or cool the endolymph in the ance to passive movement, and overactive myotatic re- semicircular canals and cause convection currents that flexes on the right side one month after stroke onset. Explain why the toes extend on the right side and flex on ature would not produce this effect, and no symptoms the left in response to plantar stimulation. Warmer or cooler water would each pro- Answers to Case Study Questions for Chapter 5 duce symptoms of vertigo. This observation tends to rule out cerebral ischemia as a re- higher CNS levels initiate voluntary muscle action and sult of circulatory (vascular) or heart problems, factors that also regulate the sensitivity of the muscle stretch (my- would also be more likely in an older patient. The symptoms produced by the rotation are severe because the alpha motor neuron pools results in weakness and of the simultaneous involvement of both sets of vestibular slowness of initiation of voluntary movement. The corti- apparatus and the resulting heavy neural input, which is cospinal tract deficit also produces an increased sensitiv- likely to be greater than that produced by his underlying ity of the spinal reflex pathways, resulting in overly vigor- condition. The use of salt restriction and diuretics would reduce the slight resistance to passive movement that is detectable in overall hydration state of his body and tend to reduce ab- a relaxed muscle, becomes greatly increased and demon- normal pressure within the labyrinthine system. Spastic tone is antimotion sickness drugs would interfere with the natural most evident in the flexor muscles of the arm and the ex- neural compensation that would, it is hoped, reduce the tensor muscles of the leg. The extensor movement of the first toe in response to Reference stroking the plantar aspect of the foot, termed Babinski Drachman DA. JAMA sign, is thought to occur because of modification of flexor 1998;280:2111–2118. The Cushing response (described by famous neurosurgeon flex when the plantar surface is stimulated. Harvey Cushing) consists of the development of hyperten- The neurophysiological details of how the deficit in sion, bradycardia, and apnea in patients with increased in- corticospinal input actually produces these commonly tracranial pressure most often a result of tumors or other le- encountered abnormalities in muscle tone and reflex sions, such as hemorrhage, that compress the brain. A current theory is pressure is transmitted downward to the brainstem and dis- that the disturbance of central control reduces the torts the medulla, where the centers for blood pressure, threshold of the stretch reflex but does not alter its gain. Correct interpre- References tation of these abnormalities in vital signs permits begin- Lance JW. The control of muscle tone, reflexes, and move- ning treatments that reduce intracranial pressure.

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Between these extreme limits effective cardizem 180 mg, varia- tant in smooth muscle buy generic cardizem 60mg on-line, is of much greater consequence in tions in the internal calcium concentration can cause corre- cardiac muscle; see Chapter 10. Such modula- tion of smooth muscle contraction is essential for its regulatory functions, especially in the vascular system. The contractile proteins of smooth muscle, like those of Smooth Muscle Relaxation. The biochemical processes skeletal and cardiac muscle, are controlled by changes in controlling relaxation in smooth muscle also differ from the intracellular concentration of calcium ions. Likewise, those in skeletal and cardiac muscle, in which a state of in- the general features of the actin-myosin contraction system hibition returns as calcium ions are withdrawn from being are similar in all muscle types. In smooth muscle, the phosphorylation tractile proteins themselves that important differences ex- of myosin is reversed by the enzyme myosin light-chain ist. Because the control of contraction in skeletal and car- phosphatase (MLCP). The activity of this phosphatase ap- diac muscle is associated with thin filament proteins, it is pears to be only partially regulated; that is, there is always called actin-linked regulation. The thin filaments of some enzymatic activity, even while the muscle is contract- smooth muscle lack troponin; control of smooth muscle ing. During contraction, however, MLCK-catalyzed phos- contraction relies instead on the thick filaments and is, phorylation proceeds at a significantly higher rate, and therefore, called myosin-linked regulation. When the cytoplas- regulation, the contractile system is in a constant state of mic calcium concentration falls, MLCK activity is reduced inhibited readiness and calcium ions remove the inhibi- because the calcium dissociates from the calmodulin, and tion. In the myosin-linked regulation of smooth muscle, the myosin dephosphorylation (catalyzed by the phosphatase) role of calcium is to cause activation of a resting state of the predominates. The general outlines of this process are affinity for actin, the reactions of the crossbridge cycle can well understood and appear to apply to all types of smooth no longer take place.