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Lecture

U8-Sensorimotor System.docx

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Department
Psychology
Course
PSYC 271
Professor
Monica Valsangkar- Smyth
Semester
Winter

Description
U8: THE SENSORIMOTOR SYSTEM Mar/15/12 1. Principles of Sensorimotor Hierarchically Organized System function - Has a hierarchical arrangement allowing higher levels to focus on complex functions o Ex. Compared to a company – association cortex (president)  general commands  muscle + motor neurons (lower levels) - After much practice, lower levels perform well-learned tasks with little higher involvements. - This system is also characterized by functional segregation, each level of the senorimotor and company hierarchies tends to be composed of different units, each of which performs a different function. Motor Output is Guided by Sensory Input - Sensory feedback is also necessary for monitoring the body’s responses and feeding their information back into sensorimotor circuits. - The only responses that are not normally influenced by sensory feedback are ballistic movements – brief, all-or-none movements Learning changes the Nature and Locus of Sensorimotor control - When a company is starting up, decisions are made by company president after careful consideration but as the company develops, many individual actions are coordinated into sequences of prescribed procedures. - Likewise, after much practice, individual responses become organized into continuous motor programs. Posterior Parietal Association Cortex - Receives input from visual, auditory, and somatosensory systems and output goes to secondary motor cortices. - Plays a role in directing behavior by providing spatial information and in directing attention. Helps localize body and external objects in space. o Ex. Mountcastle et al: posterior parietal cortex uses tactile, visual and positional information to act as a command centre for limb, and eyes operation. o Electrical stimulation applied to inferior portions of posterior parietal cortex caused patients to perform a particular action at high levels of stimulation. - Lesions in this region produces apraxia (unable to perform movements) and contralateral neglect (inability to respond to signals on the opposite side of body) o Ex. Patients with contralateral neglect often beave as if the left side of their world doesn’t exist and they often fail to appreciate that they have a problem. o Associated with large lesions of right posterior parietal lobe o The deficit occurs for their egocentric left or the left of their own bodies. Also applies to the left side of objects. Apraxia: - Two main types of apraxia: 1. Ideomotor Apraxia - Patients can’t copy movements or make gestures (i.e. wave hello) and results from damage to left posterior parietal lobe - Can`t perform movements when requested to do so, especially when movements are out of context but they can perform the same movements under natural conditions, when they`re not thinking about doing so. 2. Constructional Apraxia - Spatial organization that is disrupted. - Patients can’t assemble puzzles, build tree houses, draw a picture or copy a series of facial movements. - Develop after injury to either the left or right posterior parietal lobe. - Both these can be thought of as resulting from disruption of parietofrontal connections that control movement. Dorsolateral Prefrontal Association Cortex - Receives projections from the posterior parietal cortex and sends projections to areas of secondary motor cortex to primary motor cortex and the frontal eye field. - Plays a role in the evaluation of external stimuli and the initiation of voluntary reactions to them - The activity of some neurons depends on the charactristics of objects; the activity of others depends on the locations of objects or the combination of both. Primary Motor Cortex - Used electrical brain stimulation techniques to map the motor Wilder Penfield homunculus in the human brain - Found that stimulation of each particular cortical site activated a particular contralateral muscle and produced a simple movement. - Found the primary motor cortex is in the precentral gyrus of the frontal lobe and is somatopically organized. - Primary motor cortex is dedicated to controlling parts of the body that are capable of intricate movements, such as the hands and mouth. - Each site in the primary cortex receives sensory feedback from receptors in the muscles and joints that the site influences. o Monkeys are an exception, with 2 different hand areas in the primary motor cortex of each hemisphere, one receives input from skin receptors instead of from muscles. o Helps with stereognosis – identifying objects by touch. - The motor homunculus has a disproportionate representation of hands and mouth o Ex. 2 areas of each primary motor cortex control the contralateral/ opposite hand - Neurons in primary motor cortex code for a preferred direction of movement, they fire most just before and during the movement and when the movement is in the preferred direction and less as the direction deviates from the preferred one. - Lesions produce contralateral astereognosia, they reduce the speed and force of contralateral movements and make it difficult to move one body part (e.g. a finger) independently of others – do not produce paralysis. Current View of Primary Motor Cortex Function - Using longer bursts of current at slightly higher intensities, this produced seemingly natural response sequences. o Ex.Stimulation at one site reliably produced a feeding response. - Sites that moved a particular body part overlapped greatly with sites that moved other body parts. - The conventional view that many primary motor cortex neurons are tuned to movement in a particular direction has also been challenged – as monkeys moved about freely the firing of many primary motor cortex neurons were most closely related to the end point of a movement, not to the direction of the movement. - If a monkey reached toward a particular location, primary motor neurons sensitive to that target location activated regardless of the direction of the movement that was needed to get to the target. Remote Control of Mind - Developed technology needed for a primate to use the activity of its Nicolelis and Chapin primary motor cortex to control the movements of a robotics arm hundreds of miles away. - Other work involves developing a brain machine interface (BMI) for seizure control that would function somewhat like a heart pacemaker. It would monitor the brain’s electrical activity for patterns that indicate an imminent attack. - It would then deliver an electrical stimulus to quench the storm or release antiepileptic medication. Cerebellum and Basal Ganglia - Doesn’t participate directly in transmission of signals to spinal cord. They integrate and coordinate the activity of structures at various levels of the sensorimotor system - The interconnections between sensory and motor areas via the cerebellum and basal ganglia are the reason why damage to cortical connections between visual cortex and frontal motor areas doesn’t abolish visually guided responses. Cerebellum - Constitutes only 10% of brain mass, it contains more than half of the brain’s neurons - Receives information from primary and secondary motor cortex, information about descending motor signals from brain stem motor nuclei and feedback from motor responses via the somatosensory and vestibular systems. - Cerebellum compares these 3 sources of input and correct ongoing movements deviating from intended course. - Cerebellum functions in fine-tuning and learning of cognitive Aside: Witch trials in Salem had women responses as well as motor responses. who had spastic movements were witches. Basal Ganglia - Basal ganglia receives information from various parts of the cortex and transmits it back to motor cortices via thalamus. - Perform a modulatory function, contributing few fibres to descending motor pathways, instead they are part of neural loops that receive cortical input from various cortical areas and transmit it back to the cortex via the thalamus. - Many of these loops carry signals to and from the motor areas of the cortex. - In addition to their role in modulating motor output, they also have cognitive functions - Have been shown to participate in learning to respond correctly in order to obtain reward and avoid punishment. Parkinson’s Disease - Attacks 0.5% of the population and develops in people in their 50s and 60s. - Characterized by involuntary movements, tremor at rest, muscular rigidity, disorders of posture, slowness of movement, disturbances in speech and a masklike face but no intellectual deterioration. - Unknown cause, but associated with the degeneration of dopamine in cells in some of the nuclei in basal ganglia as well as neurons in the substantia nigra. - Treatment involves using L-Dopa, the precursor to dopamine to be taken up by dopaminergic neurons and is converted to dopamine Huntington’s Chorea - Characterized by ceaseless, involuntary, jerky movements and progressive dementia caused by a loss of cells in the basal ganglia - Unlike Parkinsons, it is associated with dementia. - Disease is caused by a single gene so 50% of all offspring of a Huntington’s disease is caused by a single dominant gene so 50% of all offspring of a Huntington’s parent will get it. - Symptoms don’t appear until after reproduction, so this disease is passed on. 2. Secondary Motor Cortex - Areas of the secondary motor cortex are those that receive much of their input from the association cortex and send much of their output to primary motor cortex - Only 2 areas of secondary motor cortex were known: the supplementary motor area and the premotor cortex 1. Supplementary motor area: - Wraps over the top of the frontal lobe and extends down its medial surface into the longitudinal fissure 2. Premotor cortex - Runs in a strip from the supplementary motor area to the lateral fissure - There are at least 8 areas of secondary motor cortex in each hemisphere, each with its own subdivisions. o Ex. Three different supplementary motor areas, 2 premotor areas (dorsal and ventral), 3 small areas (cingulate motor areas) - Areas of secondary motor cortex are involved in the programming of specific patterns of movements after taking general instructions from dorsolateral prefrontal cortex. o Ex. Brain imaging studies in which the patterns of activity in the brain have been measured while the subject is either imagining his or her own performance of a series of movements. Mirror Neurons - Mirror neurons are neurons that fire when an individual performs a particular goal-directed hand movement or when they observe the same goal-directed hand movement performed/copied by another person - This discovery is significant because they provide a mechanism for social cognition – knowledge of the perceptions, ideas, and intentions of others - Mapping the actions of others onto one’s own action repertoire would facilitate social understanding, cooperation and imitation. - Mirror neurons reacted to the sight of an action that made a sound were found to respond just as robustly to the sound alone (e.g. cracking a peanut) - Mirror neurons have also been found in the inferior portion of the posterior parietal lobe, which respond to the purpose of an action rather than to the action itself. - Although not confirmed in humans, fMRI studies have found areas of human motor cortex that are active when a person performs, watches or imagines a particular action. 3. Descending Motor Pathways - Neural signals conduct from primary motor cortex to the motor neurons of the spinal cord over 4 different pathways: 1. 2 pathways descend in the dorsolateral region of spinal cord 2. 2 descend in the ventromedial region of the spinal cord Dorsolateral Corticospinal Tract and Dorsolateral corticospinal tract Dorsolateral Corticorubrospinal Tract - Group of axons descending from primary motor cortex through the medullary pyramids (2 bulges on the ventral surface of the medulla)  decussates  descend in the contralateral dorsolateral spinal white matter. - Betz cells are extremely large pyramidal neurons of the primary motor cortex are most notable neurons of these axons. - Axons synapse on small interneurons of spinal grey matter  synapse onto motor neurons of distal muscles of wrist, hands, fingers and toes. - Second group of axons that descends from primary motor cortex synapses in the red nucleus of the midbrain  decussate  descend through the medulla where some of them terminate in the nuclei of the cranial nerves to control muscles of the face. Dorsolateral Corticorubrospinal Tract - The rest of the axons continue to d
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