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Lbm 2 Modul Tht Chusna

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Excitatory-Inhibitory Antagonism BetweenPontine and Medullary Reticular NucleiThe reticular nuclei are divided into two major groups:(1) pontine reticular nuclei, located slightly posteriorlyand laterally in the pons and extending into the mesencephalon,and (2) medullary reticular nuclei, whichextend through the entire medulla, lying ventrally andmedially near the midline. These two sets of nucleifunction mainly antagonistically to each other, withthe pontine exciting the antigravity muscles and themedullary relaxing these same muscles.Pontine Reticular System. The pontine reticular nucleitransmit excitatory signals downward into the cordthrough the pontine reticulospinal tract in the anteriorcolumn of the cord, as shown in Figure 55–8.The fibersof this pathway terminate on the medial anteriormotor neurons that excite the axial muscles of thebody, which support the body against gravity—that is,the muscles of the vertebral column and the extensormuscles of the limbs.The pontine reticular nuclei have a high degree ofnatural excitability. In addition, they receive strongexcitatory signals from the vestibular nuclei, as well asfrom deep nuclei of the cerebellum. Therefore, whenthe pontine reticular excitatory system is unopposedby the medullary reticular system, it causes powerfulexcitation of antigravity muscles throughout the body,so much so that four-legged animals can be placed ina standing position, supporting the body againstgravity without any signals from higher levels of thebrain.Medullary Reticular System. The medullary reticularnuclei transmit inhibitory signals to the same antigravityanterior motor neurons by way of a different

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tract, the medullary reticulospinal tract, located in thelateral column of the cord, as also shown in Figure55–8. The medullary reticular nuclei receive stronginput collaterals from (1) the corticospinal tract, (2)the rubrospinal tract, and (3) other motor pathways.These normally activate the medullary reticularinhibitory system to counterbalance the excitatorysignals from the pontine reticular system, so that undernormal conditions, the body muscles are not abnormallytense.Yet some signals from higher areas of the brain can“disinhibit” the medullary system when the brainwishes to excite the pontine system to cause standing.At other times, excitation of the medullary reticularsystem can inhibit antigravity muscles in certain portionsof the body to allow those portions to performspecial motor activities. The excitatory and inhibitoryreticular nuclei constitute a controllable system that ismanipulated by motor signals from the cerebral cortexand elsewhere to provide necessary backgroundmuscle contractions for standing against gravity and toinhibit appropriate groups of muscles as needed sothat other functions can be performed.Role of the Vestibular Nuclei to Excite theAntigravity MusclesAll the vestibular nuclei, shown in Figure 55–7, functionin association with the pontine reticular nuclei tocontrol the antigravity muscles. The vestibular nucleitransmit strong excitatory signals to the antigravitymuscles by way of the lateral and medial vestibulospinaltracts in the anterior columns of the spinalcord, as shown in Figure 55–8.Without this support ofthe vestibular nuclei, the pontine reticular systemwould lose much of its excitation of the axial antigravitymuscles.The specific role of the vestibular nuclei, however,is to selectively control the excitatory signals to the differentantigravity muscles to maintain equilibrium in

response to signals from the vestibular apparatus

Vestibulocerebellum—Its Function inAssociation with the Brain Stem andSpinal Cord to Control Equilibrium andPostural MovementsThe vestibulocerebellum originated phylogeneticallyat about the same time that the vestibular apparatusin the inner ear developed. Furthermore, as discussedin Chapter 55, loss of the flocculonodular lobes andadjacent portions of the vermis of the cerebellum,which constitute the vestibulocerebellum, causesextreme disturbance of equilibrium and postural

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movements.We still must ask the question, what role does thevestibulocerebellum play in equilibrium that cannotbe provided by other neuronal machinery of the brainstem? A clue is the fact that in people with vestibulocerebellardysfunction, equilibrium is far more disturbedduring performance of rapid motions thanduring stasis, especially so when these movementsinvolve changes in direction of movement and stimulatethe semicircular ducts. This suggests that thevestibulocerebellum is especially important in controllingbalance between agonist and antagonistmuscle contractions of the spine, hips, and shouldersduring rapid changes in body positions as required bythe vestibular apparatus.One of the major problems in controlling balance isthe amount of time required to transmit positionsignals and velocity of movement signals from the differentparts of the body to the brain. Even when themost rapidly conducting sensory pathways are used, upto 120 m/sec in the spinocerebellar afferent tracts, thedelay for transmission from the feet to the brain is still15 to 20 milliseconds. The feet of a person runningrapidly can move as much as 10 inches during thattime. Therefore, it is never possible for return signalsfrom the peripheral parts of the body to reach thebrain at the same time that the movements actuallyoccur. How, then, is it possible for the brain to knowwhen to stop a movement and to perform the nextsequential act, especially when the movements areperformed rapidly? The answer is that the signals fromthe periphery tell the brain how rapidly and in whichdirections the body parts are moving. It is then thefunction of the vestibulocerebellum to calculate inadvance from these rates and directions where thedifferent parts will be during the next few milliseconds.The results of these calculations are the key to thebrain’s progression to the next sequential movement.Thus, during control of equilibrium, it is presumedthat information from both the body periphery and thevestibular apparatus is used in a typical feedbackcontrol circuit to provide anticipatory correction ofpostural motor signals necessary for maintaining equilibriumeven during extremely rapid motion, including

rapidly changing directions of motion.

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