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Lecture

CHAPTER 14

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Department
Physiology
Course
PSL440Y1
Professor
A
Semester
Winter

Description
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 fr
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