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Motor Systems Functional Hierarchy of Motor Paths Parallel Organization

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Plan • Components of the motor systems – Focus on spinal control of limbs and trunk Introduction to the Motor Systems – Same principles apply to to head control via brain stem • Basic principles of movement control – What is helpful for understanding basic motor system organization John H. Martin, Ph.D. • Motor programs for voluntary movement Center for Neurobiology and Behavior • Descending motor pathways Cortical motor areas Functional Hierarchy of Motor Paths Motor Basal ganglia Systems Descending Cerebellum Motor execution: force & cortical motor Descending direction paths brain stem paths Spinal cord: Intermediate zone Ventral horn PNS Fig. 33-12 Muscle Motor Cortical Parallel Organization areas Association & limbic cortex Internal capsule Basal ganglia from Cerebellum Basal ganglia from Cerebellum 1 Hierarchical & Parallel Organization of the motor systems Organization of Movements • Top down organization of the motor pathways-- • Hierarchical: 3 major types opposite that of sensory paths –Reflexes • Subcortical motor centers--cerebellum & basal ganglia--access cortical motor areas via the – Postural adjustments thalamus (not just sensory) – Voluntary movements • Organization of multiple subcortical and cortical • …from simple to complex motor circuits-reminiscent of parallel sensory pathways • Diverse & adaptive – Purposeful Organization of Movements Reflexes • Hierarchical: 3 major types – Reflexes Spinal cord circuits – Postural adjustments Spinal & Brain stem • Stimulus-evoked involuntary muscle – Voluntary movements Spinal cord, Brain contraction stem, & cortex • Monosynaptic (+) reflex Postural adjustments & voluntary movements – Knee-jerk depend more on cerebellar and basal ganglia –Jaw-jerk function than reflexes • Simple neural representation (circuit) Dual purpose: 1) upcoming lectures; 2) context for motor pathways Knee Jerk Reflexes From muscle stretch receptors • Stimulus-evoked involuntary motor muscle contraction • Monosynaptic (+) reflex – Knee-jerk –Jaw-jerk Ventral horn • Disynaptic reflex (+) – withdrawal to muscle 2 Greater control: Why Disynaptic? from periphery • Greater control (neural gate) from higher – Very simple context centers • More complex response to muscle Response blocked by inhibition Motor I/O Postural adjustments • Context important Knee-jerk – Can reorganize depending on context • Feedback control-reactive Automatic – Error correction postural – Response lags stimulus; sometimes too late; adjustments sometimes vicious circle • Balance • Feed-forward control-predictive • Limb support – Response anticipates stimulus ¾Flexible than – More timely, but depends on practice reflexes (greater #; each w/control) • Depends on cerebellum, brain stem ¾Constrained pathways & spinal cord than voluntary • More complex neural representation The goal of voluntary movements is Voluntary movements represented… somewhere • Organized around purposeful acts • Motor equivalence • Flexible input-output relationships – Individual motor actions share important – Limitless characteristics even when performed in different – Price to pay: whole brain ways • Recruits all motor systems components & • Abstract representation; effector independent much of the association cortex – Hand writing – Soccer • Goal representation Discuss: • ??Association & Premotor cortex • Goal representation • Motor programs 3 Kinematic & Dynamic Programs Voluntary movements are in Reaching organized by motor programs • Translate goal into action • Reach to target--(Sensation to Action) – Formation of a movement representation, or – Visual cortex-->Association cortex-->Premotor-->1° motor motor program • Distinct kinematic and dynamic programs •??Premotor cortex --> Primary motor cortex – Reach up • Program • Against gravity – To produce the desired goal, which muscles should • More force to achieve goal contract and when – Reach down • 2 Key movement characteristics that are • Gravity assists programmed • Less force to achieve goal – Flexible control – Spatial (hand path; joint angles) Kinematic program –Force Dynamic program Cortical motor areas Summary Motor • Motor behavior hierarchy Pathways 1° motor cortex – Reflexes Premotor cortex – Postural adjustments – Voluntary movements Red nucleus • Internal/neural representations Reticular formation Vestibular nuclei – Reflexes simple; invariant Descending – Postural adjustments Superior colliculus cortical motor – Voluntary movements complex; flexible Descending paths • Voluntary movements brain stem – Goal representation paths Spinal cord: – Kinematic and dynamic programs – No wonder why voluntary movement recruit entire Intermediate zone motor system Ventral horn PNS Fig. 33-12 Muscle Origins of motor paths Premotor Premotor areas areas Indirect Direct Motor Pathways Hierarchy Motor Pathways Hierarchy 4 Premotor Premotor areas areas Motor Pathways Hierarchy Motor Pathways Hierarchy Premotor areas Motor pathways organized around the motor nuclei Motor Pathways Hierarchy Spinal Motor Columns Lateral pathways: limb control From brain Segmental interneuron Medial pathways: trunk control NTA Fig. 10-2 Motor neuron Short Ventral Horn Long Organization: Proximal - distal Propriospinal-- rule Motor columns Intersegmental-- PNS Fig. 33-13 (motor neurons) neurons 5 Brain Stem Motor Paths Brain Stem Pathways Medial Lateral • Lateral – Rubrospinal tract: distal limb control; crude (red nucleus) Tectum Red nucleus •Medial Reticular – Tectospinal tract: eye-head coordination formation (superior colliculus) Vestibular Tectospinal – Reticulospinal tract: automatic postural nuclei tract adjustments and movements (hip; shoulder) Reticulospinal (reticular formation) Vestibulospinal tracts Rubrospinal tract – Vestibulospinal tract: balance (axial muscles); tracts automatic postural adjustments (vestibular nuclei) PNS Fig. 33-14 Bilateral Contralateral Cortical Medial Lateral Motor Cortical motor paths Paths • Lateral corticospinal tract – Limb control mostly Vestibular & • Ventral corticospinal tract Reticular Red nucleus nuclei – Proximal muscle control; mostly upper body Medial Rubro- brain stem spinal • For cranial muscle control: paths tract Pyramidal X Corticobulbar tract Ventral Lateral Cortico- – with medial and lateral components corticospinal PNS Fig. 33-15 tract spinal Bilateral tract Contralateral Why bother study the motor Origins of cortical motor paths pathways? • Anatomical substrates: How it works • Multiple parallel paths & diversity of spinal connections • Primary motor cortex – Damage to 1° motor cortex and pre-motor cortex • Premotor cortex projections recover some lost functions – Damage to cortex and brain stem paths recover • Supplementary motor area (SMA) some lost functions • Cingulate motor area (CMA) – With spinal cord injury. loss of monosynaptic connections and alternate paths via segmental and intersegmental interneurons can recover some lost functions 6.
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