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Brain Stem and Cortical Control of Movement

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Brainstem and Cortical Control Of Movement Richard M. Costanzo, Ph.D. OBJECTIVES After studying the material of this lecture, the student should be able to: 1. Identify fiber tracts belonging to the Pyramidal and Extrapyramidal pathways. 2. Describe the role that each of the following brainstem centers play in the control of posture and spatial orientation. a. Red nucleus b. Pontine reticular formation c. Medullary reticular formation d. Vestibular nuclei e. Superior colliculus 3. Describe the effects of a transection at different levels of the brainstem and spinal cord. 4. Explain the underlying cause of decerebrate rigidity. 5. Describe the role of the primary motor cortex (area 4) in the control of voluntary movement. 6. Describe the role of the premotor and supplementary motor area (area 6) in the control of voluntary movement I. MOTOR CENTERS AND PATHWAYS Historically, the motor system has been divided into Pyramidal and Extrapyramidal pathways. Pyramidal pathways have fibers that pass through the medullary "pyramids", and include the corticospinal and corticobulbar tracts. The corticospinal tract carries information from motor cortex directly to the spinal cord, the corticobulbar projects to centers in the brainstem. All other motor pathways are considered to be extrapyramidal and originate in centers primarily within the brainstem. These brainstem structures are responsible for the control of posture and spatial orientation. They include the red nucleus, pontine and medullary reticular formation, vestibular nuclei, and superior colliculus. Figure 1: Motor Centers and Descending Pathways (From Berne & Levy) A. Rubrospinal Tract This tract originates in the red nucleus. Fibers project to interneurons in the lateral region of the spinal cord. Stimulation of the red nucleus causes facilitation of flexors and inhibition of extensors. B. Pontine (Medial) Reticulospinal Tract This tract originates from cells in the nucleus reticularis pontis caudalis and nucleus reticularis pontis oralis located in the medial two thirds of the pons (Pontine reticular formation). Fibers project to the ventromedial spinal cord where they have a general excitatory effect on both extensor and flexor motoneurons, although maximal excitation is on the extensors. C. Medullary (Lateral) Reticulospinal Tract Cells originate in the medullary reticular formation (nucleus reticularis gigantocellularis) and terminate on spinal cord interneurons in the intermediate gray. The medullary reticulospinal tract has the opposite effect of the Pontine reticulospinal tract, in that it has a general inhibitory effect on motoneurons with a stronger inhibition on extensors. D. Lateral Vestibulospinal Tract Cells originate in the lateral vestibular nucleus (Deiters' nucleus) and project to ipsilateral motoneurons and interneurons. Stimulation of cells in Deiters' nucleus produces a powerful excitation of extensors and inhibition of flexors It plays an important role in the control of antigravity muscles and the maintenance of posture. E. Tectospinal Tract Cells of origin are in the superior colliculus. Fibers project to the cervical spinal cord where they control neck muscles involved in head movement. II. BRAINSTEM CONTROL OF POSTURE Transections at different levels of the brainstem have been used to demonstrate the importance of brainstem centers in the control of posture. Isolation of centers below the transection from central influences above, reveals the regulatory functions of the intact centers. A. Spinal Transection If the spinal cord is cut three things happen. 1. Complete loss of voluntary movements This results from the interruption of descending pathways from motor centers located in the brainstem and higher centers. Following spinal transection is a total paralysis of all muscles below the level of the lesion, a condition referred to as paraplegia. 2. Loss of conscious sensations Sensory information from the body regions below the cut (spinal dermatome regions) can not reach higher centers and those regions appear to be anesthetized. 3. Initial loss of reflexes Immediately following transection the sudden loss of tonic background facilitation provided by descending pathways results in a loss of muscle tone and the limbs become flaccid. If the spinal transection is high (above C3) respiratory muscles will be disconnected from control centers in the brainstem and breathing will stop. This is a rather serious condition and, without a respirator, death due to anoxia will occur. If the cut were to occur around the level of C7, sympathetic tone to the heart would decrease and bradycardia and hypotension would develop. The loss of reflexes and flaccid limbs following spinal cord lesion is called spinal shock. It is the direct result of removal of strong background facilitation provided by higher centers, primarily on alpha and gamma motoneurons. Partial recovery from spinal shock may occur after a few weeks with the return of some of the basic reflexes (knee jerk, Babinski's reflex). B. Decerebrate Rigidity (Mid-Collicular Transection) Two brainstem centers that are very important to the maintenance of muscle tone in antigravity muscles (primarily extensors) are the pontine reticular formation (medial reticulospinal tract), and Deiter's nucleus (lateral vestibulospinal tract). Both centers have an excitatory influence on extensors. Stimulation of cells in the pontine reticular formation has a very powerful excitatory effect on extensors, but its activity is normally modulated (inhibited) by central (cortical) projections. If the spinal cord is cut above the level of the pontine reticular formation (mid collicular), the inhibitory influence is removed and there is an exaggerated activation of muscle tone in extensors (antigravity muscles). This produces a rigid posture which is referred to as decerebrate rigidity. In humans arms and legs are extended, back is arched, head dorsiflexed, and feet ventroflexed (curling of toes lifts against gravity). This stiff posture does not permit joints to bend and the body is capable of standing upright. This is very different from spinal transection, where extensor muscle tone is abolished and the body becomes limp. 1. Gamma rigidity Cutting the dorsal roots abolishes decerebrate rigidity. Cutting the Ia spindle afferents interrupts the myotatic stretch reflex. This demonstrates that the decerebrate rigidity was primarily due to the hypersensitivity of muscle spindles resulting from descending excitation of gamma motoneurons. Removal of the gamma contribution abolishes the rigidity. Therefore, decerebrate rigidity is considered primarily a gamma rigidity. 2. Alpha rigidity A selective increase in alpha motoneuron activity can produce what is referred to as alpha rigidity. This can be demonstrated after reversing decerebrate rigidity caused by gamma excitability (cutting the dorsal roots) and increasing the excitation of alpha motoneurons. Since cells in the lateral vestibular nucleus (Deiters' nucleus) are normally inhibited by projections from the cerebellum, removal of cerebellar projections increases the activity of these cells. The result is an increase in descending excitation of extensors and rigidity is restored by alpha motoneurons (gammas may fire too, but they are ineffective since the dorsal roots have been cut). 3. Positional (postural) reflexes Changing the position of the head can alter the distribution of muscle tone throughout the body. There are two reflexes that do this. The are antagonistic to one another. a. Tonic neck reflexes - align body with head b. Tonic labyrinth reflexes - restore head to a normal (vertical) position C. Midbrain Animal (Transection above midbrain) Transection of the brainstem above the level of the red nucleus does not result in decerebrate rigidity. This is the direct result of the inhibitory influences of the red nucleus (via the rubrospinal tract) on extensor activity. This inhibitory influence on extensors offsets the excitatory influence of lower brainstem centers. In addition, the rubrospinal tract has a facilatory effect on flexors muscles. A significant feature of the midbrain animal is its ability to reflexively right itself from any abnormal position. This is referred to as the righting reflex. This reflex occurs in stages. First the input from the vestibular organs allow the head to orient into a normal vertical position. Next distortion from the neck muscles provide information to allow the trunk to come into alignment with the head. Thus the midbrain animal is capable of maintaining normal body posture and balance reflexively (without voluntary control from higher centers). Figure 2: Diagram illustrating major brain stem centers controlling posture. III. CONTROL OF VOLUNTARY MOVEMENT Three major brain centers work together to control voluntary movement. They are: 1. the motor regions of the frontal lobe (premotor cortex, supplementary motor area, and primary motor cortex) 2. the basal ganglia, and 3. the cerebellum. These centers have direct (pyramidal system) or indirect (extrapyramidal system) projections to the "lower motor neurons". The lower motor neurons in turn make direct connections with muscle fibers (via neuromuscular junctions) and provide the "final common pathway" for all motor movement. IV. CORTICAL MOTOR CENTERS Cortical centers that control voluntary movement are located in the frontal lobe (anterior to the central sulcus). They include the primary motor cortex (area 4), premotor cortex and supplementary motor area (area 6). Although the prefrontal
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