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Chapter 8 - PSYC2410 Fall - Choleris

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PSYC 2410
Elena Choleris

PSYC2410 – Chapter 8 The Sensorimotor System 8.1 Three Principles of Sensorimotor Function  The sensorimotor system is hierarchically organized o Main advantage of hierarchical organization is that higher levels of the hierarchy are left free to perform more complex functions. o It has parallel hierarchy which enables the association cortex to exert control over the lower levels of the hierarchy in more than one way o Has functional segregation: each level of sensorimotor hierarchies tend to be composed of different units which perform different functions o In sensorimotor system, information mainly flows down  Motor Output is guided by Sensory input o Sensory feedback plays an important role in directing the continuation of the responses that produced it. Ballistic movements (brief, all or none high-speed movements) are not controlled by sensory input o Many adjustments in motor output in response to sensory feedback are controlled unconsciously by the lower levels of the sensorimotor hierarchy without the involvement of the higher levels  Learning Changes the nature and locus of Sensorimotor Control o In initial stages of motor learning, each individual response is performed under conscious control, then after practice individual responses become organized into continuous integrated sequences of action that flow smoothly and are adjusted by sensory feedback without conscious regulation.  A general model of Sensorimotor system function o Starts at association cortex, motor signals descend to the skeletal muscles that perform the movements 8.2 Sensorimotor Association Cortex Two major areas of sensorimotor association cortex:  Posterior Parietal Association Cortex o Plays a key role in integrating original positions of the parts of the body that are to be moved and the position of external objects, in directing behaviour by providing spatial information and in directing attention. o It receives info from the 3 sensory systems: visual, auditory and somatosensory. o Output of the posterior parietal cortex goes to areas of motor cortex: dorsolateral prefrontal association cortex, secondary motor cortex and frontal eye field (small area of prefrontal cortex that controls eye movements) o Studies suggest that posterior parietal cortex comprises a mosaic of small areas specialized for guiding particular movements of eyes, head, arms or hands o Damage to posterior parietal cortex produces deficits in perception and memory of spatial relationships, in accurate reaching and grasping, in the control of eye movement and in attention. o Apraxia: disorder of voluntary movement not attributable to simple motor deficit or to any deficit in comprehension or motivation. Have trouble performing when requested to, but can do it naturally. Symptoms are bilateral, but often caused by unilateral damage to the left posterior parietal lobe or its connections. o Contralateral neglect: a disturbance of the ability to respond to stimuli on the side of th body opposite to the side of a brain lesions, in the absence of simple sensory or motor deficits. Associated with large lesions of the right posterior parietal lobe. Fail to respond to objects on their egocentric left (left of their own body) as well as the left side of objects regardless of where they are in their visual field  Dorsolateral prefrontal Association Cortex o Receives projections from the posterior parietal cortex and sends projections to areas of secondary motor cortex, primary motor cortex and frontal eye field. o Plays a role in evaluation of external stimuli and initiation of voluntary reactions to them o Response properties of dorsolateral prefrontal neurons suggest that decisions to initiate voluntary movements may be made in this area of cortex, but depend on critical interactions with posterior parietal cortex PSYC2410 – Chapter 8 8.3 Secondary Motor Cortex Secondary motor cortex: areas that receive input from association cortex (posterior parietal cortex and dorsolateral prefrontal cortex) and send output to primary motor cortex. Areas of secondary motor cortex: supplementary motor area wraps over the top of the frontal lobe and extends down its medial surface into the longitudinal fissure and premotor cortex runs in a strip from the supplementary motor area to the lateral fissure.  Identifying the Areas of Secondary Motor Cortex o 3 supplementary motor areas: SMA, preSMA and supplementary eye field o 2 premotor areas: dorsal and ventral o 3 small areas: cingulater motor areas in the cortex of the cingulate gyrus o To qualify as secondary motor cortex, an area must be appropriately connected with association and secondary motor areas. o Electrical stimulation of secondary motor cortex elicits complex movements involving both sides of the body o Neurons in secondary motor cortex become more active just prior to initiation of and during voluntary movements  Mirror Neurons o Neurons that fire when an individual performs a particular goal-directed hand movement or when she or he observes the same goal-directed movement performed by another. o Provide a possible mechanism for social cognition (knowledge of the perceptions, ideas, and intentions of others). o These neurons respond to the understanding of an action, not so some superficial aspect of it, they responded fully to the particular action regardless of how it was detected. o Many ventral premotor mirror neurons fire when a monkey doesn’t see the full action, but has enough clues to create a mental representation of it o Have been found in the inferior portion of the posterior parietal lobe: respond to the purpose of an action rather than the action itself. o Existence of mirror neurons in humans not yet confirmed, but indirect evidence suggests they do  fMRI studies have found parts of human motor cortex that are active when a person performs, watches, or imagines a particular action 8.4 Primary Motor Cortex Primary motor cortex: located in the precentral gyrus of the frontal lobe. It is the major point of convergence of cortical sensoriomotor signals and is the major point of departure of sensorimotor signals from the cerebral cortex.  Conventional View of Primary Motor Cortex Function o Primary motor cortex is organized somatotopically according to a map of the body. The somatotopic layout of the human primary motor cortex is commonly referred to as the motor homunculus. o Most of it is dedicated to controlling parts of the body that are capable of intricate movements such as the hands and mouth o Each site in the primary motor cortex receives sensory feedback from receptors in the muscles and joints that the site influences.  Monkey have two different hand areas in the PMC of each hemisphere, one receives input from receptors in the skin, and one from muscles and joints.  This facilitates stereognosis: process of identifying objects by touch  Current View of Primary Motor Cortex Function o Used longer bursts of current which produced complex natural-looking response sequences. o Instead of individual muscle contraction, have complex species-typical movements and sites that moved a body part overlapped greatly with sites for other parts o Also challenged that PMC neurons were tuned to movement in a particular direction: firing of many PMC neurons was most closely related to the end point of a movement, not to the direction of the movement. In monkeys reaching toward a particular location, neurons sensitive to that target location tended to become active regardless of the direction of the movement that was needed to get to the target. PSYC2410 – Chapter 8 o The signals from every site in the PMC diverge greatly, so each particular site has the ability to get a body part to a target location regardless of the starting position o The sensorimotor system is inherently plastic. Each location in the PMC can produce innumerable patterns of muscle contraction required to get a body part from any starting point to a target location. The route that neural signals follow from a given area of PMC is extremely plastic and is presumably determined at any point in time by somatosensory feedback.  Effects of Primary Motor Cortex Lesions o Extensive damage to PMC has less effect than one would expect. Large lesions may disrupt a patient’s ability to move one body part independently of others, may produce astereognosia and may reduce the speed, accuracy and force of a patient’s movements but they don’t eliminate voluntary movement because of parallel pathways that descend directly from secondary motor areas to subcortical motor circuits without passing through PMC. 8.5 Cerebellum and Basal Ganglia Cerebellum and basal ganglia interact with different levels of the sensorimotor hierarchy and coordinate and modulate its activities. Interconnections between sensory and motor areas via the cerebellum and basal ganglia are thought to be one reason why damage to cortical connections between visual cortex and frontal motor areas does not abolish visually guided responses.  Cerebellum o Although it constitutes only 10% of the mass of the brain, it contains more than half of the brain’s neurons o 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. o It is thought to compare these 3 sources of input and correct ongoing movements that deviate from their intended course and is thus believed to play a key role in motor learning: learning of sequences of movements in which timing is a critical factor. o Consequences of diffuse cerebellar damage for motor function are devastating: loss of ability to control precisely the direction, force, velocity and amplitude of movements and the ability to adapt patterns of motor output to changing conditions; difficult to maintain steady posture, leads to tremor; disturbances of balance, gait, speech and control of eye movement; difficulty learning new motor sequences. o Cerebellum functions in the fine-tuning and learning of cognitive responses in the same way that it functions in the fine-tuning and learning of motor responses o Organized systematically in lobes, columns and layers  Basal Ganglia o Complex heterogeneous collection of interconnected nuclei o Performs modulatory functions o Contributes few fibers to descending motor pathways; instead, 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 the loops carry info to and from the motor areas of the cortex. o Thought to be involved in cognitive functions in addition to the modulation of motor output because basal ganglia areas are known to project to cortical areas with cognitive functions (prefrontal lobes). Response learning, reward 8.6 Descending Motor Pathways Neural signals are conducted from the PMC to the motor neurons of the spinal cord over 4 different pathways. 2 descend in the dorsolateral region of the spinal cord, 2 descend in the ventromedial region of the spinal cord.  Dorsolateral Corticospinal Tract (direct) o Dorsolateral Corticospinal tract neurons descend from PMC through the medullary pyramids (two bulges on the ventral surface of the medulla) then decussate and continue to descend in the contralateral dorsolateral spinal white matter. PSYC2410 – Chapter 8 o Beltz cells: extremely large pyramidal neurons of the PMC o Most axons synapse on small interneurons of the spinal gray matter, which synapse on the motor neurons of distal muscles of the wrist, hands, fingers and toes.  Animals that are capable of moving their digits independently have dorsolateral Corticospinal tract neurons that synapse directly on digit motor neurons  Dorsolateral Corticorubrospinal Tract (indirect) o Second group of axons that descends from the PMC synapse in the red nucleus of the midbrain, then axons of neurons in the red nucleus decussate and descend through the medulla where some terminate in the nuclei of the crainial nerves that control muscles of the face, the rest continue to descend in the dorsolateral portion of the spinal cord. o Axons synapse on interneurons that synapse on motor neurons that project o the distal muscles of the arms & legs  Ventromedial Corticospinal Tract (direct) o Long axons that descend ipsilaterally from the PMC directly into the ventromedial areas of the spinal white matter. o As each axon descends, it branches diffusely and innervates the interneuron circuits in several different spinal segments on both sides of the spinal gray matter. o  Ventromedial Cortico-brainstem-spinal Tract (indirect) o Comprises motor cortex axons that feed into a complex network of brain stem structures. o The axons of some of the neurons in this complex brain stem motor network then descend bilaterally in the ventromedial portion o the spinal cord. o Each side carries signals from both hemispheres and each neuron synapses on the interneurons of several different spinal cord segments that control the proximal muscles of the trunk and limbs. o Four brain stem structures interact with the ventromedial cortico-brainstem-spinal tract:  Tectum: receives auditory and visual information about spatial location  Vestibular nucleus: receives information about balance from receptors in the semicircular canals of the inner ear  Reticular formation: among other things, it contains motor programs that regulate complex species- typical movements such s walking, swimming, jumping  Motor nuclei of the cranial nerves: control muscles of the face  Comparison of the Two Dorsolateral Motor Pathways and the Two Ventromedial Motor Pathways o Similar in that each contains two major tracts, one whose axo
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