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CHAPTER_ 14.docx

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Wilfrid Laurier University
Connell Mc Cluskey

INTRODUCTION The central motor system is arranged as a hierarchy of control levels, with the forebrain at the top and the spinal cord at the bottom (see Table 14.1).  The highest level concerned with strategy – the goal of the movement and the movement strategy that best achieves that goal  The middle level is concerned with tactics – the sequences of muscle contractions, arranged in space and time, required to smoothly and accurately achieve the strategic goal  The lowest level is concerned with execution – activation of the motor neuron and interneuron pools that generate the goal-directed movement and make any necessary adjustments of posture The proper functioning of each level of the motor control hierarchy relies so heavily on sensory information that the motor system of the brain might be considered a sensorimotor system.  At the highest level, sensory information generates a mental image of the body and its relationship to the environment  At the middle level, tactical decisions are based on the memory of sensory information from past movements  At the lowest level, sensory feedback is used to maintain posture, muscle length, and tension before and after each voluntary movement. DESCENDING SPINAL TRACTS How does the brain communicate with the motor neurons of the spinal cord? Axons from the brain descend through the spinal cord along two major groups of pathways (see Fig. 14.2):  One is the lateral column of the spinal cord o The lateral pathways are involved in voluntary movement of the distal musculature and are under direct cortical control  The other is the ventromedial column o The ventromedial pathways are involved in the control of posture and locomotion and are under brain stem control The Lateral Pathways The most important component of this pathway is the corticospinal tract (see Fig. 14.3a). Originating in the neocortex, it is the longest and one of the CNS tracts.  Two-thirds of the axons in the tract originate in areas 4 and 6 and is called the motor cortex.  The remaining axons derive from the somatosensory areas of the parietal lobe to regulate the flow of somatosensory information to the brain. A much smaller component of the lateral pathways is the rubrospinal tract, which orginates in the red nucleus of the midbrain (see Fig. 14.3b). A major source of input to the red nucleus is the region of the front cortex that contributes to the corticospinal tract.  It contributes to motor control in many mammalian species o In humans it appears to be reduced, most of its functions subsumed by the corticospinal tract. The Effects of Lateral Pathway Lesions Experimental lesions in both corticospinal and rubrospinal tracts in monkeys rendered them unable to make fractionated movements of arms and hands (i.e. could not move their shoulders, elbows, wrists, and fingers independently).  Voluntary movements were also slower and less accurate Lesions in the corticospinal tracts alone caused a movement deficit as severe as that observed after lesions in the lateral columns.  However, many functions gradually reappeared  The only permanent deficit was some weakness of the distal flexors and an inability to move the fingers independently  Lesion in the rubrospinal tract completely reversed this recovery o This suggests that the corticorubrospinal pathway was able to partially compensate for the loss of the corticospinal tract input The Ventromedial Pathways It contains four descending tracts that orginate in the brain stem and terminate among the spinal interneurons controlling proximal and axial muscles:  Vestibulospinal tract  Tectospinal tract  Pontine reticulospinal tract  Medullary reticulospinal tract The ventromedial pathways use sensory information about balance, body position, and the visual environment to reflexively maintain balance and body posture The Vestibulospinal Tracts The vestibulospinal and tectospinal tracts function to keep the head balanced on the shoulders as the body moves and to turn the head in response to new sensory stimuli. The vestibulospinal tracts originate in the vestibular nuclei of the medulla, which relay sensory information from the vestibular labyrinth (see Fig. 14.4a).  The motion of fluid in the labyrinth (accompanying head movement) activates hair cells that signal the vestibular nuclei via cranial nerve VIII. One component of the vestibulospinal tracts projects bilaterally down the spinal cord and activates cervical spinal circuits that control neck and back muscles (guide head movement). Another component projects ipsilaterally down to the lumbar spinal cord. It helps us maintain an upright and balanced posture by facilitating extensor motor neurons of the legs. The Tectospinal Tract The tectospinal tract originates in the superior colliculus of the midbrain, which receives direct input from the retina (see Fig. 14.4b). It also receives projections from visual cortex and afferent axons with somatosensory and auditory information. The superior colliculus uses this information to construct a map of the world around us  Stimulation at one site of the map will orient a response that directs the head and eyes to move to the appropriate point of space The Pontine and Medullary Reticulospinal Tracts The reticulospinal tracts arise mainly from the reticular formation of the brain stem which may be divided into two parts giving rise to two different descending tracts (see Fig. 14.5):  Pontine (medial) reticulospinal tract  Medullary (lateral) reticulopsinal tract The pontine reticulospinal tract enhances the antigravity reflexes of the spinal cord by facilitating the extensors of the lower limbs, which helps maintain a standing posture by resisting the effects of gravity. The activity of ventral horn neurons maintains muscle length and tension. The medullary reticulospinal tract has the opposite effect. It liberates the antigravity muscles from reflex control. Activity in both tracts is controlled by descending signals from the cortex (see Fig. 14.6). THE PLANNING OF MOVEMENT BY THE CEREBRAL CORTEX Motor Cortex It is a region of the frontal lobe consisting of Area 4 (often referred to as primary motor cortex or M1) and Area 6 (see Fig. 14.7). There is speculation that area 6 might be specialized for skilled voluntary movement. It was later shown that electrical stimulation of area 6 could evoke complex movements of either side of the body.  Wilder Penfield found two somatotopically organized motor maps in area 6: o One in a lateral region called the premotor area (PMA)  innervate proximal motor units o One in a medial region called the supplementary motor area (SMA)  Innervate distal motor units directly The Contributions of Posterior Parietal and Prefrontal Cortex A mental body image seems to be generated by somatosensory, proprioceptive, and visual inputs to the posterior parietal cortex. Two areas of interest in the posterior parietal cortex are:  Area 5 – target of inputs from the primary somatosensory cortical areas 3, 1, and 2  Area 7 – target of higher-order visual cortical areas such as MT Patients with lesions in these areas show bizarre abnormalities of body image and the perception of spatial relations The parietal lobes are extensively interconnected with regions in the anterior frontal lobe that in humans are thought to be important for abstract though, decision making, and anticipating the consequences of action.  These „prefrontal‟ areas and the posterior parietal cortex send axons that converge on area 6 and represent the highest levels of the motor control hierarchy  Area 6 lies at the junction where singles encoding what actions are desired are converted into signals that specify how the action will be carried out Neuronal Correlates of Motor Planning Cells in the SMA typically increase their discharge rates about a second before the execution of a hand or wrist movement. An important feature of this activity is that it occurs in advance of the movements of either hand.  This means that supplementary areas of the two hemispheres are closely linked via the corpus callosum  Therefore, movement deficits following an SMA lesion on side is particularly pronounced for tasks required the coordinated actions of two hands  In humans, a selective inability to perform complex (but not simple) motor acts is called apraxia Consider the expression “Ready, set, go.”  The readiness depends on activity in the parietal and frontal lobes along with contributions from brain centers that control levels of attention and alertness  “Set” may reside in the supplementary and premotor areas, where movement strategies are devised and held until they are executed  “Go” appears to be implemented with participation of a major subcortical input to area 6. See Figure 14.9 Weinrich and Wise monitored the discharge of a neuron in the PMA as a monkey performed a task requiring a specific arm movement to a target  The neuron in the PMA began firing if the instruction was to move the arm left and it continued to discharge until the trigger stimulus came on and the movement was initiated  If the instruction was to move to the right, this neuron did not fire (presumably another population of PMA cells became active under this condition) The activity of this PMA neuron reported the direction of the upcoming movement and continued to do so until the movement was made THE BASAL GANGLIA The major subcortical input to area 6 arises in a nucleus called the ventral lateral (VL) nucleus located in the dorsal thalamus. The input to this location, called VLo, arises from the basal ganglia which in turn are targets of the cerebral cortex (i.e. frontal, prefrontal and parietal).  There is a loop where information cycles from the cortex through the basal ganglia and thalamus and then back to the cortex (see Fig. 14.10).  This loop functions to select and initiate willed movements Anatomy of the Basal Ganglia It consists of the caudate nucleus, the putamen, t
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