X
|
STUDENT DIGITAL NEWSLETTER ALAGAPPA INSTITUTIONS |
|
Marshall W. Carpenter MD
Ideal speech must bracket off potentially distorting material forces and inequities heart attack 27 20 mg nifedipine otc. As a result blood pressure 4060 order nifedipine 30mg online, "public communication arteria rectalis inferior 20mg nifedipine otc, by this means at least hypertension patient education buy 30mg nifedipine otc, [became] moderated by the demands of big business and. Hegemonic domination required the bourgeois class to convince subjugated groups that they were meant to be the next moral and intellectual leaders of society by completely permeating society and the societal order, including normative values, morals, beliefs, and customs, with such messages of domination and subjugation. He referenced these post-1960s movements to show that they provided the "raw materials of the public sphere. As Habermas understands it, the analysis undertaken in the informal public sphere evidences no "attempt to link such an analysis with any remnants of a normative political theory," Habermas, supra note 102, at 421, 465, even though it may be "part of a social psychological approach to some sort of an analysis of an expressivist, somehow aesthetic, need for self-representation in public space. Finally, some theorists contend that the role of the law in society is to protect the discourse that occurs within the public sphere and to facilitate 150. As some research has shown in comparing listening to visual stimulation, "listening often imparts a sense of emotion stronger than that imparted by looking. Rothenbuhler & McCourt, supra note 39, at 367, 376 ("Television clearly eclipsed radio as the dominant broadcast medium for advertising, audiences, and investments. Hundreds of stations disaffiliated from the networks, finding their audiences and their advertising revenues in local markets. With the network abandonment, "[t]he veneer of network paternalism was stripped off. As the popular demand of such music content increased dramatically, hundreds of new recording companies developed in the late 1940s to meet such demand and to provide programming content to the growing number of independent stations (and soon-to-be disaffiliated network stations) willing to play such music. Such stations not only gave air time to Black disc jockeys, but also allowed them to air their own programming content. Following the civil unrest in Black urban America that followed Martin Luther King, Jr. See Smith, supra note 71, at 209, 211 ("The [W]hiteness of radio broadcasting grew out of unspoken, widely accepted, and long-standing conventions, but it was carefully monitored and enforced. White disc jockeys were charged with appealing to both Black and White audiences, and they for nonwhite performers, who could only find work in broadcasting by playing parts as servants or minstrels if they approximated the accents [W]hite actors, directors, and producers had popularized as `[B]lack. In 1945, famed Black poet Langston Hughes wrote of radio, [c]onsidering the seriousness of the race problem in our country. And it continues to keep alive the stereotype of the dialect-speaking amiably-moronic Negro servant as the chief representative of our racial group on the air. Savage, supra note 176, at 231, 235 (citing a letter from Hughes to historian Erik Barnouw, Mar. In the years "between 1940 and 1953 [B]lack median income rose 192 percent, and [B]lack home ownership increased by 129 percent. In most regions of the country, especially in cities, 90 percent of African Americans now owned radios. In essence, he symbolized the voice, interests, and understandings of the everyday lives and exchanges of his listening audience. There was a segment of the White listening audience, White youth, that was not offended by Black-oriented programming, but was drawn to it. As a result, eventually radio station owners hired Black personnel to serve as voice coaches for White disc jockeys who engaged in racial ventriloquy. When radio, the disc jockey, and the airing of rhythm and blues (and, subsequently, rock and roll) are viewed through the lens of theorists who adopt an understanding of participatory democracy that embraces popular cultural expression,220 they reveal much generally "about the emptiness and forced conformity of [W]hite culture. For example, as some cultural historians have pointed out, for a generation of White middle class youth (boys in particular), America at the time demanded homogeneity, obedience, and a "phony[] surface conformity that threatened to suck all the spirit and individuality"222 out of 215. Indeed, it was through the White disc jockey that the teenage audience was discovered, since what was played on radio came to be determined by what was bought in the record stores. At the time, teenage consumption of records was more voluminous than his or her adult counterpart. With a smaller targeted audience, local radio stations, through their disc jockeys, turned what was once a problem (a shrinking listening audience), into an advertising advantage. During this time, network television not only continued to perpetuate the dominantly inscribed racial stereotypes of Black Americans, but also, through its programming, replicated the phony innocence, conformity, and forced homogeneity that American youth sought to escape. Prior to the war, however, various organizations were aimed at attaining equality for Black America, and were organized around different ideologies that ranged from Black nationalist claims for a separate political and economic state to antiimperialist or anti-colonialist calls for a physical revolution. Before Elvis Presley, virtually all R&B artists that White teens heard on the radio were Black artists. Here, Habermas is partially correct in asserting that informal publics can be influential to form more organized movements that overtly challenge the ruling authority or state apparatus. He underestimates, however, the power of these informal publics to challenge the ruling ideologies in and of themselves. Black disc jockeys were much more explicit about the racial issues of the day that affected the Black community. This audience found pleasure in the visibility and attention given to Black musical and cultural expression (even if coded and subverted) since, for so long, they had been completely ignored and objectified on radio, and were continuing to be ignored on television. With rhythm-and-blues-infused rock and roll music played on the air symbolizing an "imagined" racial interaction on air, and with the literal and spontaneous everyday interactions on the dance floor between Black and White youth, mainstream racial segregationist norms "were starting to buckle, and a huge new generation of young people was beginning to flex its demographic muscle. Alan Freed was known to kiss Black female performers, share the stage with and embrace Black male performers, and "was even seen sharing a cigarette or a drink with these performers after the show. He also helped to make radio a center of business in the entertainment industry, at least as it related to rock and roll-the music genre that most influenced popular culture at the time and exemplified the intergenerational and interracial battle over identity and identity formation. Rock-and-roll disc jockeys were targeted as the culprits for instigating and fueling the desires for such transformative cultural understandings, which, within a decade, advanced to a demand for change of [B]lack rock and pop stars disrupted the old patterns of segregated shows, and this was especially revolutionary in the South, where segregated facilities were commonplace. Now [B]lacks and [W]hites would enter the same building to hear the same R&B group they had heard on the radio, but they were separated from each other by ropes or other dividers. Once everyone started dancing, however, these barricades often fell, and there they would be, dancing together. Popular movies such as Hairspray, the Frankie Avalon Story, Ray, and Cadillac Records touch on this American cultural phenomenon that was fueled by the radio airplay of such music. Top 40, at the time, was not as scientific as it has come to be in terms of being based on national surveys and market research. The number forty originally represented "the approximate number of songs a deejay could play in a three-hour shift. It reflected the music tastes and preferences of the local listening audiences as determined by the disc jockey, who surveyed what music and records were being bought in the local community record store, which, during this time, were primarily rhythm and blues and rock and roll records purchased by teenagers. In fact, payola dated back to the 1930s in one form or another to when songwriters offered band leaders certain incentives to play one of their songs in a set or performance on radio. As disc jockeys gained in popularity and control over the content that was aired on the radio, they began to receive paid incentives from an endorsing record company to play a particular record. The stage was set for a national inquiry determined to bring Top 40 radio back within the control of corporate leaders. Kielbowicz & Lawson, supra note 187, at 352 ("Station managers reined in deejays by imposing more centralized control over programming, which led, according to some observers, to the rise of formula play lists such as Top 40 formats. Now it was music directors and station managers, rather than deejays, who made deals with record companies and their distributors. Moreover, smaller independent record labels would be hurt considerably with fewer opportunities for their songs to get airplay due to the subsequent development of Top 40 music play lists- which were based on national surveys, including music listings in Billboard magazine-and to a reduction in the number of songs played on the radio airwaves, an increase in advertising jingles, and rapid-fire disc jockey talk. This exodus played out repeatedly on broadcast radio as different subversive voices on radio found their way onto the airwaves only to be eventually commercialized or co-opted-a situation not too different from the current status of radio. These other factors are no longer present in the deregulated and ownership-consolidated radio (and music) industry in which radio now exists. The government needs to reinvigorate a localism policy that ensures that radio, in particular, given its unique qualities, is more representative and inclusive of contesting voices, especially those of the underserved. Continued adherence to the predominant market-based analysis of the public interest obligations imposed on broadcasters, where buying power of a particular demographic is the dispositive force, will not lead to such inclusion, as evidenced by the current state of radio. Deregulation and Its Effect on Music Content on Radio the deregulatory efforts that began in the 1980s, and were cemented with the passage of the Telecommunications Act of 1996, have virtually eliminated many of the factors that were once present and relevant to the rise of rock and roll on radio. In addition, the increased competition among these independent radio stations for content and a listening audience gave rise to a number of smaller, independent record labels that provided such content and to the rise of the local disc jockey, who was intimately connected with his listening audience.
However blood pressure ranges nhs nifedipine 30 mg on-line, this type of reaction characteristically produces a great quantity of heat that is released into the surrounding environment blood pressure medication on steroids nifedipine 20mg sale. To prevent this phenomena from becoming problematic arteria hyaloidea persistens buy discount nifedipine 20 mg, an oxygen analyzer is placed within the circle to measure the percentage of oxygen within the breathable air blood pressure 50 over 30 30mg nifedipine free shipping. When the percent oxygen within the circle system decreases below a preset level, the oxygen analyzer, which is electronically coupled to a tank of high-pressure 100% oxygen (not air) within the unit, releases a measured quantity of oxygen into the circle system through the second stage (low-pressure) reducing valve. The first stage high-pressure reducing regulator is directly attached to the outlet valve of the oxygen tank. Advantages of the closed-circuit (circle) system include a longer period of usage without refilling the oxygen tank, usually in the neighborhood of 4 hour, as opposed to a maximum usage of 1 hour with an open system employing compressed air. Disadvantages would include the heavy weight of the system, although the open circuit is also heavy, and the exothermic (heat producing) reaction that takes place between carbon dioxide and soda lime or other carbon dioxide-removing substances. Oxygen readily supports combustion which is an Personal Protective Equipment: Practical and Theoretical Considerations 555 obvious disadvantage, as well as the necessity of having to periodically change the carbon dioxidefiltering material. If changing the carbon dioxide-filtering material is ignored, carbon dioxide levels within the circle system will continue to increase. As levels of carbon dioxide increase initially a cerebral vasodilatory effect (dilatation of blood vessels within the brain, which increases pressure on the brain) will occur, the extent of which will be directly proportional to the level of carbon dioxide. This will be expressed clinically with the development of an increasingly severe headache. A chemical reaction occurs within the filtering material when it becomes saturated with carbon dioxide and needs to be changed. The color of this material will undergo a color change, usually from white to blue. This water would then have to be drained from the system through a one-way valve mechanism. The exact composition of an air-purifying filter will be discussed later in this chapter. What is important is that the filter employed be able to remove the contaminant and in the concentrations present. Air is usually introduced into it by two one-way valves (one on each side) located inside the nosecup. Some respirators, especially those of British origin, lack the one-way valves, but strategically place the interconnection between inside and outside of the nosecup in a location that will not interfere with proper functioning of the nosecup. If this latter were to occur the temperature and humidity content of the air on the inside of the eye lens would be significantly greater than that on the outside of the eye lens, with resulting condensation of water vapor to liquid water on the inside of the eye lens, with the resulting obscuring of vision. It totally covers the head, has an opening for the facepiece of the respirator that is surrounded by an elasticized band to tightly secure the hood to the respirator facepiece, and additionally covers the shoulders and the upper chest (both front and rear aspects), and is secured by two straps that extend from the outside of the bottom of the rear aspect of the hood, with one strap extending under each axilla (armpit), and subsequently both straps attaching to the outside of the lower margin of the front aspect of the garment. If the task is performed in a sunny area, then radiant energy from the sun in the form of infrared rays provides additional heat accumulation within the ensemble. The following medications decrease blood flow to the skin: diuretics by producing fluid loss and dehydration with accompanying constriction of blood vessels, beta blockers by decreasing cardiac output, and alpha agonists by constricting blood vessels. Weight-loss medications have multiple effects, including increasing the rate of metabolism and hence heat production; and anticholinergic drugs decrease or prevent sweating thus decreasing or eliminating evaporative cooling. Consequences of heat stress would include reduced psychological functions including reduction in speed of thought, alertness, and reaction time. Resting body metabolism will produce heat energy equivalent to that produced by a 100 W light bulb, moderate work increases this to about 250 watts, whereas extreme work, such as running, increases this to approximately 1000 watts. The body attempts to dissipate heat production by increasing blood flow to the skin and increasing sweating. Cardiac output initially increases, but will fall as circulating volume decreases by sweating, and syncopal (fainting) episodes may result. When heat production exceeds heat dissipation, body temperature will begin and continue to rise. This method of cooling relies on the ability of the surrounding atmosphere to accept water vapor. Hence, the lower the humidity of the surrounding air, the more humidity it will accept, and the greater the degree of evaporative cooling that will occur. Air that is 100% saturated with water vapor will no longer accept additional moisture, and hence evaporative cooling comes to a halt. The latter normally constitutes the principal method by which body-generated heat is lost to the environment. If patients experience heat exhaustion they are still capable of sweating and controlling thermoregulation (temperature control of body). This less-severe diagnosis of heat exhaustion is a diagnosis of exclusion, whose symptoms include exhaustion, weakness, headache, fatigue, light-headedness, and dizziness. Signs of heat exhaustion may also include lethargic, depression, irritability, and confusion, elevated pulse rate, and orthostatic hypotension. Heat stroke is a true medical emergency, and serious medical consequences, including death, may result if external cooling of the body is not immediately instituted. Signs of heat stroke include delirium, disorientation, combativeness, seizures, collapse (fainting), low blood pressure, and a compensating weak, rapid pulse. Anatomical dead space (the volume of air in the respiratory tree that must be moved before fresh air can be admitted to the alveoli) is usually 1 cc per pound of body weight. Tidal volume and hence minute ventilation increase significantly to compensate for this added dead space. However, the increase in tidal volume and minute ventilation is usually not as great as the increase in dead space; consequently alveolar ventilation decreases slightly. Usually respiratory rate decreases slightly with initial respirator usage, although this will increase as the level of exercise increases, and the corresponding need for oxygen increases. Air-purifying respirators increase expiratory resistance far less than atmosphere-supplied pressure-demand respirators. The latter maintains a significant positive pressure throughout the entire expiratory cycle. This in turn keeps more alveoli ventilated (opened) during expiration, which increases the oxygenation of the blood. This lowered cardiac output at worst could result in syncope (fainting), and at best a decreased endurance at increased levels of exercise. Negative-pressure air-purifying respirators (gas masks) increase inspiratory resistance to a far greater extent than expiratory resistance. With negative-pressure air-purifying respirators, as minute ventilation increases, inspiratory and expiratory resistances increase geometrically thus individuals may decrease their peak inspiratory and expiratory flow rates to decrease the resistance to air flow. Moreover, respirator users must increase their respiratory rate (Hodous, 1989) to meet the demands of increased minute ventilation. Personal Protective Equipment: Practical and Theoretical Considerations 559 airflow. Bronchospasm (narrowing of the bronchi) may result in hypoxia (decreased oxygen) and hypercarbia (increased carbon dioxide) in the blood. An increase in respiratory work, due to increased expiratory resistance as described above, will also increase cardiac work by a ripple down effect especially as work loads increase. In summary, the increased tidal volume, increased inspiratory and expiratory airway resistances (the magnitude of which vary depending on the type of respirator employed), increased anatomical dead space, as well as the initial decrease in respiratory rate, result in decreased alveolar ventilation, increased work of breathing, and decreased endurance at maximum work loads. Generally negative-pressure air-purifying respirators tend primarily to increase respiratory work that may then later have a ripple down effect to increase cardiac work in individuals with metabolic compromise. Atmosphere-supplied pressure-demand respirators generally increase cardiac work significantly due to the increased weight of the respiratory equipment. Additionally, increased respiratory work is generated by the increased muscular effort needed to continue breathing against the positive pressure created by the respirator during the entire expiratory cycle. The maximum peak positive oral pressure generated during very heavy exercise with a respirator in the positive pressure mode. This process produces further dehydration while requiring additional energy utilization. Some respirators come equipped with a voicemitter to overcome this difficultly, and although this is an improvement, much is left to be desired. Heightened emotions in the form of anxiety, as well as a feeling of claustrophobia and isolation frequently occur. In addition to a decrease in visual fields, the sensation of touch is decreased, and smell and taste are also compromised.
20mg nifedipine overnight delivery. How Do I Calibrate My Manual Blood Pressure Cuff - Zeroing the Gauge.
Smoke from diesel fuel is generated by injecting the fuel into the exhaust manifold of a vehicle that results in vaporization of the fuel which subsequently condenses in the atmosphere to produce particles blood pressure 150100 generic nifedipine 20 mg on-line, 0 hypertension treatment jnc 7 purchase nifedipine 30 mg otc. The material deposits and accumulates in the lung blood pressure numbers mean discount nifedipine 30 mg visa, being retained long enough to induce an inflammatory response heart attack like symptoms cheap 30 mg nifedipine overnight delivery. Dodecachlorodiphenyl has been used as a dosimetric tracer for aerosols of diesel fuel (Dalbey and Lock, 1982; Dalbey et al. The 1 h exposure caused lacrimation, increased oral and nasal secretions, nasal hemorrhages, and tremors in guinea pigs; rats demonstrated cyanosis, and mice had piloerection. Histopathological lesions found in the respiratory tract were congestion in nasal turbinates, bronchitis, peribronchiolitis, peribronchiolar lymphocyte infiltration, pulmonary histiocytosis, bronchopneumonia, and pulmonary congestion, edema, and hemorrhage. Mild-to-moderate pulmonary congestion was seen compared with unexposed controls, with minimal inflammatory changes in the nasal turbinates and trachea. Body weight decreased until the fourth exposure week, after which there were body weight gains. Functional residual capacity, vital capacity, and peak flow rate were not affected. These figures were divided by an uncertainty factor of 10 for nonpermanent health impairment, and a further uncertainty factor of 10 for interspecies variations. Fog-oil smoke is generated by injecting mineral oil into a heated manifold, and the vapor so produced condenses in the atmosphere into a cloud of suspended obscuring particles. Mineral oils produce slight-to-moderate irritation by standard rabbit skin irritation tests (Beck et al. As with diesel-fuel smokes, a proportion of generated particulates are respirable, and the most sensitive toxic end point following acute or repeated exposures is respiratory tract toxicity. For a repeated exposure study, male Sprague-Dawley rats were exposed to concentrations of 0. Lung lavage had an increased number of polymorphonuclear leukocytes, alveolar macrophages, total cells, and increase in lavage fluid proteins. Hematology showed a dose-related decrease in mean corpuscular volume and mean corpuscular hemoglobin that correlated with increased erythrocyte count. Behavioral studies, clinical chemistry, and immune function studies showed no significant effects. The subchronic (13 week) exposure study showed a decrease in body weight with increased lung weights at 0. Colored smokes are used for identification purposes (friendly units or targets), signaling, coordination purposes during operations, and simulation of explosions. Such smokes are generated by volatilizing and subsequently condensing a pyrotechnic mixture containing an organic dye. A variety of dyes, singly or in combination, are used or have been suggested for use in screening smokes (Table 18. Some examples of these are reviewed below with respect to the major factors relevant to use in screening smokes. Known commonly as Disperse Red 9, this dye comprises up to 40% of red smoke grenades. It is also used in a violet smoke grenade when mixed with 1,4-diamino-2,3dihydroanthraquinone and as a screening smoke mixed with Solvent Yellow 33 and Solvent Green 3. The acute inhalation toxicity of red smoke generated pyrotechnically from the following composition was studied: Disperse Red 9, 40%; sodium bicarbonate, 25%; potassium chlorate, 26%; sulfur, 9% (Owens and Ward, 1974). After exposure animals demonstrated signs of nasal irritation, salivation, gagging, and respiratory difficulty. An in vivo screen for carcinogenicity, based on the sensitivity of the mouse mammary gland to certain carcinogens, was conducted by Griswold et al. A repeated exposure inhalation study to pyrotechnically generated smoke from a dye mixture of Disperse Red 9, Solvent Yellow 33, and Solvent Green 3 was conducted by Marrs et al. Excess mortality was seen with highconcentration exposure in mice and guinea pigs. Rats had marked collections of macrophages, and this species also showed adenocarcinomas of the breast and biliary hyperplasia. Mice showed increased alveolar macrophages and fatty change was noted in the livers. This is a yellow dye (Vat Yellow 4), often combined with 7-H-benz[de]anthracene-7-one for a yellow smoke, or with benzanthrone and Solvent Green 3 to give a green smoke. No tumors were reported in several studies in which dibenzo[b,def]chrysene7,14-dione was given by subcutaneous injection and by cutaneous dosing to mice (Hartwell, 1957). This dye, also known as benzanthrone, is often combined with dibenzo[b,def]chrysene-7,14-dione to produce a yellow dye smoke, or with Solvent Green 3 and Vat Yellow 4 for a green screening smoke. It is not irritating to the skin of guinea pigs (Owens and Ward, 1974), but may cause irritant dermatitis and skin pigmentation in humans (Uebelin and Buess, 1951; Trivedi and Niyogi, 1968). Benzanthrone did not show dominant-lethal activity in a mouse test (Epstein et al. Benzanthrone is not carcinogenic by skin painting or subcutaneous injection (Hartwell, 1957; Owens and Ward, 1974). Also known as Solvent Green 3, it is used in screening smokes with Solvent Yellow 33 and with benzanthrone=Vat Yellow 4. Studies have indicated that inhaled Solvent Green 3 has a long residence time in the lung. Known also as Solvent Yellow 33, this dye, sometimes in combination with Solvent Green 3, is used in colored screening smokes. It has been shown to produce allergic contact dermatitis in both laboratory animals and humans (Larsen, 1974; Calnan, 1976; Noster and Hausen, 1978). Monitors for adverse effects included body weights, respiratory function, biochemical estimation of lung injury, and histology (Sun et al. Little dye was retained in the lungs, 490 Chemical Warfare Agents: Chemistry, Pharmacology, Toxicology, and Therapeutics but that present was related to the exposure concentration. After exposure lung compliance, functional residual capacity, and forced vital capacity were significantly increased for the high exposure concentration group, indicating loss of elastic recoil without airflow obstruction. There were no biologically significant changes in serum biochemistry and no abnormal histology. At the end of the 13 week exposure period, the high-concentration group had body weights 5% lower than unexposed controls. Little dye was recovered from the lungs, but that detected did not increase linearly with increasing aerosol concentration, probably due to lower deposition efficiency for the larger sized particles. The macrophages accumulated in alveoli adjacent to terminal bronchioles and alveolar ducts, and the pigment was present as irregular masses ranging from <1 to >15 mm diameter. At this concentration, there were effects on body weights (reduction) and accumulation of vacuolated alveolar macrophages. Compared to unexposed controls, body weight was 7% lower for 4 weeks and 9% less after 13 weeks.
It also keeps the body in a stable position through postural and righting reflexes blood pressure in spanish generic nifedipine 30 mg amex. Fine motor control thus depends on the continuous interaction of multiple centers responsible for the planning (efferent copy) and execution of movement blood pressure medication xanax purchase 20mg nifedipine with visa. The primary motor area (area 4) regulates the force of muscle contraction and the goaloriented direction of movement; it mainly controls distal muscle groups hypertension natural remedies cheap nifedipine 30 mg without prescription. The supplementary motor area (medial area 6) plays an important role in complex motor planning blood pressure chart uk pdf purchase 30mg nifedipine visa. The premotor area (lateral area 6) receives nerve impulses from the posterior parietal cortex and is concerned with the visual and somatosensory control of movement; it mainly controls trunk and proximal limb movement. The cerebellum coordinates limb and eye movements and plays an important role in the maintenance of balance and the regulation of muscle tone. The basal ganglia have a close anatomic and functional connection to the motor cortex and participate in the coordination of limb and eye movement. Reflex Movements Withdrawing a foot from a noxious stimulus or spreading the arms when falling are examples of reflex movements. Intrinsic muscle reflexes regulate muscle tone and elasticity and are important for postural control and coordination of muscle groups. Specific functions such as joint stabilization or adjustment of contraction strength are achieved with the aid of inhibitory spinal interneurons. Extrinsic reflexes include protective reflexes (flexor response to noxious stimulus, corneal reflex) and postural reflexes (extensor reflex, neck reflex). Motor Function 42 Rhythmic Movements Walking, breathing, and riding a bicycle are rhythmic movements. They are subserved both by spinal reflex arcs and by supraspinal influence from the brain stem, cerebellum, basal ganglia, and motor cortex. Voluntary Movements Voluntary movements depend on a sequence of contractions of numerous different muscles that is planned to achieve a desired result (motor program). Hence different parts of the body are able to carry out similar movements (motor equivalence) more or less skillfully. Voluntary movements incorporate elements of the basic reflex and rhythmic movement patterns; their smooth execution depends on afferent feedback from the visual, vestibular, and proprioceptive systems to motor centers in the spinal cord, brain stem, and cerebral cortex. Motor Function 43 Centromedian nucleus Motor Execution muscles of the trunk and proximal portions of the limbs that maintain the erect body posture. Because of the bilateral innervation, paresis due to interruption of these pathways recovers more readily than distal paresis due to a pyramidal lesion. Lesions of the pyramidal tract usually involve the adjacent nonpyramidal tracts as well and cause spastic paralysis; the rare isolated pyramidal lesions cause flaccid paralysis (p. Corticopontine fibers originate in the frontal, temporal, parietal, and occipital cortex and descend in the internal capsule near the pyramidal tract. The rubrospinal tract originates in the red nucleus, decussates immediately, forms synapses with interneurons in the brain stem, and descends in the spinal cord to terminate in the anterior horn. Rubrospinal impulses activate flexors and inhibit extensors, as do impulses conducted in the medullary portion of the reticulospinal tract. On the other hand, impulses conducted in the pontine portion of the reticulospinal tract and in the vestibulospinal tract activate extensors and inhibit flexors. The fibers descend through the posterior portion of the internal capsule through the cerebral peduncle, pons, and medulla, forming a small bulge (pyramid) on the anterior surface of the medulla. Most of the fibers cross the midline in the decussation of the pyramids and then descend through the spinal cord in the lateral corticospinal tract. Among the minority of fibers that do not cross in the pyramidal decussation, most continue in the ipsilateral anterior corticospinal tract, crossing the midline in the anterior spinal commissure only once they reach the level of their target motor neurons. In the brain stem, the pyramidal tract gives off fibers to the motor nuclei of the cranial nerves (corticopontine and corticobulbar tracts). Motor Function Motor Unit A motor unit is the functional unit consisting of a motor neuron and the muscle fibers innervated by it. The motor neurons are located in the brain stem (motor nuclei of cranial nerves) and spinal cord (anterior horn). The innervation ratio is the mean number of muscle fibers innervated by a single motor neuron. The action potentials arising from the cell body of a motor neuron are relayed along its axon to the neuromuscular synapses (motor end plates) of the muscle fibers. The force of muscle contraction depends on the number of motor units activated and on the frequency of action potentials. Innervation ratios vary from 3 for the extraocular muscles and 100 for the small muscles of the hand to 2000 for the gastrocnemius. Nonpyramidal Motor Tracts Other motor tracts lead from the cerebral cortex via the pons to the cerebellum, and from the cerebral cortex to the striatum (caudate nucleus and putamen), thalamus, substantia nigra, red nucleus, and brain stem reticular formation. Motor Function 45 Central Paralysis or areas deep to the cortex, cause spasticity and possibly an associated sensory deficit. It may be difficult to determine by examination alone whether monoparesis is of upper or lower motor neuron type (p. Involvement of corticopontine fibers causes (central) facial paresis, and impairment of corticobulbar fibers causes dysphonia and dysphagia. Unilateral lesions in the rostral brain stem cause contralateral spastic hemiparesis and ipsilateral nuclear oculomotor nerve palsy (crossed paralysis). Involvement of the pons or medulla causes an initial quadriplegia; in the later course of illness, spinal automatisms may be seen in response to noxious stimuli. Voluntary movement of paretic limbs requires greater effort than normal and causes greater muscular fatigue. Moreover, rapid alternating movements are slowed by hypertonia in the opposing agonist and antagonist muscles of paretic limbs. There may be synkinesia (involuntary movement of paretic limbs associated with other movements. Paralysis that is initially total usually improves with time, but recovery may be accompanied by other motor disturbances such as tremor, hemiataxia, hemichorea, and hemiballism. The defining feature of spasticity is a velocity-dependent increase of muscle tone in response to passive stretch. The "clasp-knife phenomenon" (sudden slackening of muscle tone on rapid passive extension) is rare. Spasticity mainly affects the antigravity muscles (arm flexors and leg extensors). The intrinsic muscle reflexes are enhanced (enlargement of reflex zones, clonus) and the extrinsic reflexes are diminished or absent. Isolated lesions of the primary motor cortex (area 4) cause flaccid weakness of the contralateral face, hand, or leg. In the last-named syndrome, hemisection of the spinal cord causes ipsilateral spastic paresis, vasomotor paresis, anhidrosis, and loss of position and vibration sense and somatosensory two-point discrimination, associated with contralateral loss of pain and temperature sensations (the so-called dissociated sensory deficit). Transverse cord lesions at T1 can produce Horner syndrome and atrophy of the intrinsic muscles of the hand. Radicular lesions produce segmental pain radiating in a band from back to front on one or both sides. Localized back pain due to spinal cord lesions is often incorrectly attributed to spinal degenerative disease until weakness and bladder dysfunction appear. Lesions at L1 to L3 cause flaccid paraplegia and bladder dysfunction (automatic bladder, p.
References