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How You Do What You Do 1. 2. 3. The sensorimotor system is hierachically organized Motor output is guided by sensory input Learning can change the nature and locus of sensorimotor control 1. Hierarchical organization ◦ Association cortex at the highest level, muscles at the lowest i.e from general goals (cortical level) to specific details of action (lower levels). ◦ Parallel structure – signals flow between levels over multiple paths ◦ Information flow is down, while in the Sensory system informtion flows through the hierarchy. 2. Motor output guided by sensory input ◦ Sensory feedback plays an important role in the control of movement (exception: ballistic movements). 3. Learning (experience) changes the nature and locus of sensorimotor control. ◦ E.g. from Conscious behavior to automatic. From conscious control (cortical level) to "Automatic Pilot" (lower levels). Sensory information is integrated in Association cortex. Two Major areas of Sensorimotor Association Cortex are:◦ Posterior parietal association cortex ◦ Dorsolateral prefrontal association cortex Each composed of several different areas with different functions This cortex receives input from ◦ ◦ ◦ The somatosensory system, The visual system and The auditory system. This information specifies the initial conditions for the programming of action: The original position of the body parts to be moved. The position of external objects Damage to the posterior parietal cortex causes Apraxia and Contralateral neglect. Apraxia: difficulty in executing a movement when ordered to do so, but able to do it when not thinking about it. Lesion is often on the left side. Contralateral Neglect: Patient does not respond to sensory stimulation from the side opposite to the lesion of the parietal cortex (usually on the right side). The output of the Posterior Parietal Association Cortex goes to the Dorsolateral Prefrontal Association Cortex, and to the Frontal Eye Field (Fig. 8.2). This cortex receives input from The somatosensory system, The visual system and The auditory system. This information specifies the initial conditions for the programming of action: The original position of the body parts to be moved. The position of external objects Damage to the posterior parietal cortex causes Apraxia and Contralateral neglect. Apraxia: difficulty in executing a movement when ordered to do so, but able to do it when not thinking about it. Lesion is often on the left side. Contralateral Neglect: Patient does not respond to sensory stimulation from the side opposite to the lesion of the parietal cortex (usually on the right side). The output of the Posterior Parietal Association Cortex goes to the Dorsolateral Prefrontal Association Cortex, and to the Frontal Eye Field (Fig. 8.2). Integrates information about ◦ Body part location ◦ External objects Receives visual, auditory, and somatosensory information Outputs to motor cortex Apraxia – disorder of voluntary movement – problem only evident when instructed to perform an action – usually a consequence of damage to the area on the left Contralateral neglect – unable to respond to stimuli contralateral to the side of the lesion - usually seen with large lesions on the right Input from posterior parietal cortex Output to secondary motor cortex, primary motor cortex, and frontal eye field Evaluates external stimuli and initiates voluntary reactions – supported by neuronal responses Input mainly from association cortex Output mainly to primary motor cortex At least 7 different areas ◦ 2 supplementary motor areas SMA and preSMA ◦ 2 premotor areas dorsal and ventral ◦ 3 cingulate motor areas Subject of ongoing research May be involved in programming movements in response to input from dorsolateral prefrontal cortex Many premotor neurons are bimodal – responding to 2 different types of stimuli Precentral gyrus of the frontal lobe Major point of convergence of cortical sensorimotor signals Major point of departure of signals from cortex Somatotopic – more cortex devoted to body parts which make many movements Motor homunculus Control of hands involves a network of widely distributed neurons Stereognosis – recognizing by touch – requires interplay of sensory and motor systems Some neurons are direction specific – firing maximally when movement is made in one direction Interact with different levels of the sensorimotor hierarchy Coordinate and modulate May permit maintenance of visually guided responses despite cortical damage 10% of brain mass, > 50% of its neurons Input from 1° and 2° motor cortex Input from brain stem motor nuclei Feedback from motor responses Involved in fine-tuning and motor learning May also do the same for cognitive responses A collection of nuclei Part of neural loops that receive cortical input and send output back via the thalamus Modulate motor output and cognitive functions 2 dorsolateral ◦ Corticospinal ◦ Corticorubrospinal 2 ventromedial ◦ Corticospinal ◦ Cortico-brainstem-spinal tract Both corticospinal tracts are direct Most synapse on interneurons of spinal gray matter Corticospinal - descend through the medullary pyramids, then cross ◦ Betz cells – synapse on motor neurons projecting to leg muscles ◦ Wrist, hands, fingers, toes Corticorubrospinal – synapse at red nucleus and cross before the medulla ◦ Some control muscles of the face ◦ Distal muscles of arms and legs Corticospinal ◦ Descends ipsilaterally ◦ Axons branch and innervate interneuron circuits bilaterally in multiple spinal segments Cortico-brainstem-spinal ◦ Interacts with various brain stem structures and descends bilaterally carrying information from both hemispheres ◦ Synapse on interneurons of multiple spinal segments controlling proximal trunk and limb muscles Dorsolateral one direct tract, one that synapses in the brain stem Terminate in one contralateral spinal segment Distal muscles Limb movements Ventromedial one direct tract, one that synapses in the brain stem More diffuse Bilateral innervation Proximal muscles Posture and whole body movement Motor units – a motor neuron + muscle fibers, all fibers contract when motor neuron fires Number of fibers per unit varies – fine control, fewer fibers/neuron Muscle – muscle fibers bound together by a tendon Acetylcholine released by motor neurons at the neuromuscular junction causes contraction Motor pool – all motor neurons innervating the fibers of a single muscle Fast muscle fibers – fatigue quickly Slow muscle fibers – capable of sustained contraction due to vascularization Muscles are a mix of slow and fast Flexors – bend or flex a joint Extensors – straighten or extend Synergistic muscles – any 2 muscles whose contraction produces the same movement Antagonistic muscles – any 2 muscles that act in opposition Golgi tendon organs ◦ Embedded in tendons ◦ Tendons connect muscle to bone ◦ Detect muscle tension Muscle spindles ◦ Embedded in muscle tissue ◦ Detect changes in muscle length Knee-jerk reflex Stretch reflex – monosynaptic, serves to maintain limb stability Withdrawal reflex – multisynaptic Reciprocal innervation – antagonistic muscles interact so that movements are smooth – flexors are excited while extensors are inhibited, etc. Perhaps all but the highest levels of the sensorimotor system have patterns of activity programmed into them and complex movements are produced by activating these programs Cerebellum and basal ganglia then serve to coordinate the various programs A given movement can be accomplished various ways, using different muscles Central sensorimotor programs must be stored at a level higher than the muscle (as different muscles can do the same task) Sensorimotor programs may be stored in 2° motor cortex Programs for many species-specific behaviors established without practice Fentress (1973) – mice without forelimbs still make coordinated grooming motions Practice can also generate and modify programs ◦ Response chunking ◦ Shifting control to lower levels Response chunking ◦ Practice combines the central programs controlling individual response Shifting control to lower levels ◦ Frees up higher levels to do more complex tasks ◦ Permits greater speed