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PS263 - Ch 8 Textbook.docx

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Todd Ferretti

PS263 – Chapter 8  The Sensorimotor System Three Principles of Sensorimotor Function 1. Hierarchical Organization: Sensorimotor is directed by commands that cascade down through the levels of a hierarchy – from the association (highest) to the muscles (lowest) Information flows down a. Advantage: Higher levels of the hierarchy are left free to perform more complex functions (parallel – signals flow between & over multiple paths) b. Functional Segregation: Each level of the system & company hierarchies tends to be composed of different units, each of which performs a different function. 2. Motor Output guided by Sensory Input: Eyes, organs of balance, and receptors in skin monitor body’s responses & feed their information back into the sensorimotor circuit. a. Sensory Feedback plays an important role in directing the continuation of the responses that produced it. b. Ballistic Responses: not normally influenced by sensory feedback – brief, all-or-none, high-speed movements (I.e., swatting a fly) 3. Learning Changes the Nature & Locus: During the initial stages of motor learning, each response is performed under conscious control – after much practice the indicidual responses become organized into continuous integrated sequences of action that flow smoothly & are adjusted by sensory feedback without conscious regulation. Sensorimotor Association Cortex  At the top of the sensorimotor hierarchy, divided into two major areas: 1. Posterior Parietal Association Cortex: Posterior to the primary somatosensory cortex, important role in integrating: directing behaviour by providing spatial information and in directing attention; receives substantial information, input from more than one system but from 3 systems (visual, audio & somatosensory) a. Output goes to areas of the motor cortex in the frontal cortex (dorsolateral prefrontal association cortex to secondary motor cortex and frontal eye field: small area of prefrontal, controls eye movement) b. Damage  deficits in perception & memory of special relationships, accurate reaching and grasping, controlling eye movement & attention c. Apraxia: Disorder of voluntary movement that is not attributable to a simple motor deficit or to any deficit in comprehension or motivation. d. Contralateral Neglect: Disturbance of a patients ability to response to stimuli on the side of the body opposite to the brain lesion in the absence of simple sensory or motor deficits – left side of world gone: i. Egocentric Left: when patients tilt their heads, their field of neglect is not normally tilted with it - Lesions on the right posterior parietal lobe 2. Dorsolateral Prefrontal Association Cortex: Receives projections from the posterior parietal cortex and sends them to areas of the secondary & primary motor cortex as well as the frontal eye field. Plays a role in the evaluation of external stimuli & initiation of voluntary reactions to them. PS263 – Chapter 8  The Sensorimotor System a. Activity of some neurons depends on the characteristics of objects – activity depends on the locations of objects & combo of both (response rather than the object) Neurons fire until complete. Secondary Motor Cortex  Areas that receive much of their input from the association cortex & send their output to the primary motor cortex -- two areas visible on frontal lobe: 1. Supplementary Motor Area: Wraps over the top of the frontal love & extends down its medial surface into the longitudinal fissure (SMA, preSMA, eye field) 2. Premotor Cortex: Runs in a strip from the supplementary motor area to the lateral fissure (dorsal & ventral) 3. Cingulate Motor Area: In the cortex of the cingulate gyrus  These areas are thought to be involved in the programming of specific patterns of movement after taking general instruction from dorso-prefrontal-cortex.  Mirror Neurons: Neurons that fire when an individual performs a particular goal-directed hang movement of when she or he observes the same goal- directed movement performed by another (early 1990s by Rizzolati) Primary Motor Cortex  Located in the precentral gyrus of the frontal lobe – major point of convergence of cortical sensorimotor signals and the major point of departure of sensorimotor signals from cerebral cortex.  Somatotopic layout of the human primary motor cortex is referred to as the motor homunculus (little man)  Penfield, 1937  Stereognosis: process of identifying objects by touch (close eyes & touching) Cerebellum & Basal Ganglia  Interact with different levels of the hierarchy and coordinate & modulate its activities – the interconnections are thought to be the reason why damage to cortical connections between visual & frontal motor does not abolish visually guided responses.  Cerebellum: Receives information from primary & secondary cortex, information about descending motor signals from brain stem motor nuclei and feedback from motor responses via the somatosensory & vestibular. Major role in motor learning – sequences of movements. o Damage  Patient loses the ability to control direction, force, velocity and amplitude of movements & the ability to adapt patterns of motor output to changing conditions.  Basal Gangli
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