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B and C, Reflex control of skeletal muscletension in response to active contraction varies depending on state of activation of γ-motor neurons (andits effects on muscle spindles) and degree of stretch of Golgi tendon organs.■■the spinal cord, with excitatory synapses on motorneurons innervating axial muscles involved in postureand support of the body against gravity.Medullary reticular nuclei, through projections descending in the medullary reticulospinal tract of the lateralcolumn of the spinal cord, inhibit motor neurons thatinnervate axial muscles.Vestibular nuclei on the floor of the fourth ventriclehave projections that descend by way of the anteriorcolumns of the spinal cord in the lateral and medialvestibulospinal tracts.
These nuclei receive input from■■the vestibular apparatus and have an important role incontrolling muscles to maintain equilibrium, working inconjunction with the pontine reticular nuclei to regulateaxial muscles.The superior colliculus of the mesencephalon receivesinput from visual nuclei; through the tectospinal tract,it is involved in control of head, neck, and eyemuscles.The red nucleus of the mesencephalon receives inputfrom the motor cortex. It sends projections in the rubrospinal tract through the lateral columns of the spinalThe Somatic Motor SystemFrom extensor spindlereceptor (Ia, II fibers)81From extensor tendonorgan (Ib fibers)Axosomatic oraxodendriticinhibitory synapseInhibitory synapseExcitatory synapseExcitatory synapseTo extensorsTo extensorsTo flexorsTo flexorsA.
Stretch reflexB. Tendon organ reflexNociceptive fibersIpsilateral flexionContralateral extensionInhibitory synapseExcitatory synapseExcitatory synapseInhibitory synapseTo extensorsTo flexorsTo extensorsC. Flexor withdrawal reflexTo flexorsFigure 6.4 Spinal Reflex Pathways for Stretch, Tendon Organ, and Flexor WithdrawalReflexes Reflexes elicited by stretch of muscle spindle receptor (A) or Golgi tendon organs (B) associatedwith an extensor muscle are illustrated.
Opposite effects on flexors and extensors occur for reflexes initiatedby stretch of a flexor muscle or tendon organ. In the flexor withdrawal reflex, nociceptive stimuli produceipsilateral flexion and contralateral extension through the pathways illustrated (C).cord and is involved in voluntary control of largemuscles, particularly in the limbs. It can be consideredin some ways as an alternative pathway to the corticospinal tract for controlling voluntary muscle movement.This is ordinarily a relatively minor pathway in humans,compared with other mammals, but may graduallyassume an expanded role when the corticospinal tract isinjured.CerebellumThe cerebellum (“little brain” in Latin) has an importantaccessory role to the motor cortex, in coordination and controlof posture, balance and movement, and in planning and initiation of motion.
These effects are accomplished through theactivity of its three lobes (Fig. 6.6):■The archicerebellum (also known as the vestibulocerebellum) is involved in the regulation of posture andbalance, and in control of eye and head movement. Itreceives afferent signals from the vestibular apparatusand sends efferent signals through the relevant descending efferent pathways (Fig.
6.7).■■The paleocerebellum (also called the spinocerebellum)has an important role in the regulation of proximal limbmovement. Afferent sensory signals regarding the position and movement of limbs are utilized to “fine-tune”limb motion through the relevant descending pathways(see Fig. 6.7).The neocerebellum (also known as the pontocerebellum) has a coordinating role in the regulation of distallimb movement, based on input from the cerebral cortex(via the pontine nuclei). It aids in the planning andinitiation of motor activity through its efferent fibers(see Fig. 6.7).Alternatively, the lobes of the cerebellum can be conceptualized as anterior, middle, and flocculonodular lobes (seeFig.
6.6).The cerebellum’s role in muscle control involves coordination and fine-tuning of motion. Deficits in cerebellar function are expressed as deficiency in fine motor activity,coordination, and equilibrium.HipKneeMotorcortexAnkleInternalcapsuleElbowWrisFitngersThe Nervous System and MuscleTrunkShoulder82bumTh eckNBrowEyelidNaresLipsTongueLarynxToesLateral aspect of cerebralcortex to showtopographic projection ofmotor centers onprecentral gyrus.MidbrainBasispedunculiPonsMotor systemFibers originate in motor cortex anddescend via posterior limb of internalcapsule to basis pedunculi of midbrain.Longitudinal bundles branch uponentering basis pontis and rejoin to enterpyramids of medulla.BasispontisAt lower medulla, bulk of fibers crossmedian plane to form lateral corticospinaltract; some fibers continue downward inipsilateral lateral corticospinal tract; othersdescending ipsilateral anteriorcorticospinal tract.MedullaPyramidsSynapse occurs at spinal level: Lateralcorticospinal fibers synapse on ipsilateralanterior horn cells; anterior corticospinalfibers synapse on contralateral anteriorhorn cells.MedullaDecussationof pyramidsAbove midthoraciclevelMotorendplateSpinalcordBelow midthoraciclevelAnterior corticospinal tractLateral corticospinal tractMotorendplateFigure 6.5 Corticospinal Tract The corticospinal tract, also called the pyramidal tract, is the majordescending pathway in voluntary motor activity and is particularly important in fine motor activity.
The motorhomunculus (top center panel) illustrates the location and approximate relative size of areas in the motorcortex controlling muscle activity in various regions of the body.The Somatic Motor System83Superior cerebellar peduncleMiddle cerebellar peduncleTo contralateral cerebellar cortexCortical inputLegNucleusreticularistegmentipontisArmFacPrimary fissureePontine nuclei(contralateral)Spinal inputInferior oliveUpper partof medullaoblongataSpinal inputTo nodule and flocculusVestibularnucleiInferior cerebellar peduncleFunctional Subdivisions of CerebellumHemisphere VermisReticulocerebellartractVestibular nerveand ganglionCuneocerebellartractLower partof medullaoblongataGracile nucleusCortical inputLateral reticularnucleusSpinal inputMain cuneatenucleus (relayfor cutaneousinformation)External cuneate nucleus(relay for proprioceptiveinformation)Cervical partof spinal cordFrom skin (touchand pressure)Motor interneuronFrom muscle (spindlesand Golgi tendon organs)RostralspinocerebellartractSpinal border cellsMotor interneuronLumbar partof spinal cordClarke’s columnVentral spinocerebellar tractFrom skin anddeep tissues(pain and Golgitendon organs)InterLateral mediatepartpartLeg zoneArm zoneFace zoneAnterior lobePrimaryfissureMiddle(posterior)lobe2nd spinalprojectionarea (gracilelobule)Archicerebellum(vestibulocerebellum)PosterolateralfissureFlocculonodular lobeLingulaFlocculusNodulePaleocerebellum(spinocerebellum)UvulaPyramidVermisNeocerebellum(pontocerebellum)Middle vermisHemisphereFrom skin (touchand pressure)and from muscle(spindles andGolgi tendonorgans)Dorsal spinocerebellar tractFigure 6.6 Cerebellar Afferent Pathways and Functional Subdivisions of Cerebellum Thecerebellum acts as an accessory to the motor cortex in regulation of posture, balance, movement, andplanning and initiation of motion, based on afferent sensory signals and cortical input.
Specific functionsascribed to subdivisions of the cerebellum are illustrated in the right panel or discussed in the text.Schema oftheoretical“unfolding”of cerebellarsurface inderivation ofabove diagram84The Nervous System and MuscleExcitatory endingsMotor and premotor cerebral cortexInhibitory endings of Purkinje cellsInternal capsuleVentral anterior and ventrallateral nuclei of thalamusCerebral peduncleMesencephalic reticular formationDecussation of superior cerebellar pedunclesRed nucleusDescending fibers from superior cerebellar pedunclesFastigial nucleusGlobose nucleiHook bundle of RussellEmboliform nucleusDentate nucleusSection A–B viewed from belowCerebellar cortexSection B–C viewed from aboveVestibular nucleiInferior cerebellar peduncleInferior oliveLateral reticular nucleusMedulla oblongataPontomedullary reticular formationAPlanes of section:B red arrows indicatedirection of viewCFigure 6.7 Cerebellar Efferent Pathways The cerebellar efferent pathways originate from its deepnuclei, which receive inhibitory input from Purkinje cells of the cerebellar cortex.
Axons project from thedeep fastigial, globose, emboliform, and dentate nuclei to various nuclei in the brainstem, midbrain, andthalamus, and modulate activity of descending motor pathways, resulting in coordination and fine-tuning ofmotion.Cerebellar CortexAll efferent signals of the cerebellar cortex are inhibitory andemanate from Purkinje cells residing in the middle of threelayers of the cortex.
Signals from Purkinje cells are conductedto deep cerebellar nuclei (see Fig. 6.7). The three layers of thecortex contain various cell types that, through complex interactions between each other and with fibers projecting fromother sites, regulate the inhibitory efferent output of Purkinjecells (Fig. 6.8):■The inner granular layer, containing granule cells,Golgi cells, and glomeruli. Granule cells within thislayer are the most numerous type of neuron in theCNS.
The glomeruli are the sites of synapses betweenaxons of mossy fibers from the spinocerebellar, vestibulocerebellar, and pontocerebellar tracts with the dendrites of the granule and Golgi cells. Granule cells,which receive excitatory input from the mossy fibers,send axons through the Purkinje cell layer to the outermolecular layer (discussed later). They provide excitatory signals to Purkinje cell dendrites and dendrites ofcells of the molecular layer.
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