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Chapter 14

Chapter 14- Brain Control.doc

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It useful to think of this motor control hierarchy as having three levels: • The highest level, represented by the association areas of neocortex and basal ganglia of the forebrain is concerned with strategy: o The goal of the movement and the movement strategy that best achieves the goal. • The Middle level, represented by the motor cortex and cerebellum is concerned with tactics: o The sequences of muscle contractions, arranged in space and time, required to smoothly and accurately achieve the strategic goal. • The lowest level, represented by the brain stem and spinal cord is concerned with execution: o 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 properly be considered a sensorimotor system o At the highest level, sensory information generates a mental image of the body and its relationship to the environment. o At the middle level, tactical decisions are based on the memory of sensory information from past movements o At the lowest level, sensory feedback is used to maintain posture, muscle length and tension before and after each voluntary movement. Descending Spinal Tracts: Axons from the brain descend through the spinal cord along two major groups: • Lateral column of the spinal cord o Remember: lateral pathways are involved in voluntary movement of the distal musculature and are under direct cortical control • 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 the lateral pathways is the corticospinal tract • Originating in the neocortex, it is the longest and one of the largest CNS tracts. • 2/3 of the axons in the tract originate in areas 4 and 6 of the frontal lobe, connectively called motor cortex. • Most of the remaining axons in the corticospinal tract derive from the somatosensory areas of the parietal lobe and serve to regulate the flow of somatosensory information to the brain. Axons from the cortex pass through the internal capsule bridging the telencephalon and thalamus, course through the base of the cerebral peduncle, a large collection of axons in the midbrain, then pass through the pons and collect to form a tract at the base of the medulla. • The tract forms a bulge called the medullary pyramid, running down the ventral surface of the medulla. At the junction of the medulla and spinal cord, the pyramidal tract crosses or decussates at the pyramidal decussation. • This means that the right motor cortex directly commands the movement of the left side of the body and the left motor cortex controls the muscles on the right side • As the axons cross they collect in the lateral column of the spinal cord and form the lateral corticospinal tract o The corticospinal tract axons terminate in the dorsolateral region of the ventral horns and intermediate gray matter, the location of the motor neurons and interneurons that control the distal muscles particularly the flexors. • A much smaller component of the lateral pathways is the rubrospinal tract which originates in the red nucleus of the midbrain. o Axons from the red nucleus decussate in the pons, and join those in the corticospinal tract in the lateral column of the spinal cord. o A major source of input to the red nucleus is the very region of frontal cortex that contributes to the corticospinal tract. The Ventromedial Pathways: • Contains four descending tracts that originate in the brain stem and terminate among the spinal interneurons controlling proximal and axial muscles o These tracts are the vestibulospinal tract, the tectospinal tract, the pontine reticulospinal tract and the medullary reticulospinal tract. These 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 through space and to turn the head in response to new sensory stimuli. o The vestibulospinal tracts originate in the vestibular nuclei of the medulla, which relay sensory information from the vestibular labyrinth in the inner ear.  The vestibular labyrinth consists of fluid-filled canals and cavities in the temporal bones that are closely associated with the cochlea. • One component of the vestibulospinal tracts projects bilaterally down the spinal cord and activates the cervical spinal circuits that control neck and back muscles and thus guide head movement. o Stability of the head is important because the head contains our eyes, and keeping the eyes stable, even as our body moves, ensures that our image of the world remains stable. • Another component of the vestibulospinal tracts projects ipsilaterally as far down as the lumbar spinal cord. o It helps us maintain an upright and balance posture by facilitating extensor motor neurons in the legs. Box 14.1: Paresis, Paralysis, Spasticity and Babinski: The neural components of the motor system extend from the highest reaches of the cerebral cortex to the farthest terminals of the motor axons in muscles. • The site of motor system damage has a big effect on the types of deficits a patient experiences. o Damage to the lower parts of the motor system – alpha motor neurons or their motor axons- leads to easily predicted consequences.  Partial damage may cause paresis ( weakness)  Complete severing of a motor nerve l
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