Plan Components of the motor systems Focus on spinal control of - PDF document

Plan • Components of the motor systems – Focus on spinal control of limbs and trunk – Same principles apply to to head control Introduction to the Motor Systems via brain stem • Basic principles of movement control – What is helpful for understanding basic motor system organization • Motor programs for voluntary movement John H. Martin, Ph.D. Center for Neurobiology and Behavior • Descending motor pathways Cortical motor areas Functional Hierarchy of Motor Paths Motor Basal ganglia Systems Cerebellum Descending 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 Basal ganglia Cerebellum from Cerebellum 1

Hierarchical & Parallel Organization Organization of Movements of the motor systems • Top down organization of the motor pathways-- • Hierarchical: 3 major types opposite that of sensory paths – Reflexes • Subcortical motor centers--cerebellum & basal – Postural adjustments ganglia--access cortical motor areas via the thalamus (not just sensory) – Voluntary movements • Organization of multiple subcortical and cortical • …from simple to complex motor circuits-reminiscent of parallel sensory • Diverse & adaptive pathways – Purposeful Organization of Movements Reflexes • Hierarchical: 3 major types – Reflexes Spinal cord circuits • Stimulus-evoked involuntary muscle – Postural adjustments Spinal & Brain stem contraction – Voluntary movements Spinal cord, Brain 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 • Disynaptic reflex (+) horn – withdrawal to muscle 2

Greater control: Why Disynaptic? from periphery from • Greater control (neural gate) higher – Very simple context centers • More complex response Response blocked to muscle 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 • Feed-forward control-predictive • Balance • Limb support – Response anticipates stimulus � Flexible than – More timely, but depends on practice reflexes (greater #; each w/control) • Depends on cerebellum, brain stem � Constrained 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 ways – Price to pay: whole brain • Abstract representation; effector independent • Recruits all motor systems components & 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 • Reach to target--(Sensation to Action) • Translate goal into action – Visual cortex-->Association cortex-->Premotor-->1° motor – Formation of a movement representation , or motor program • Distinct kinematic and dynamic programs • ?? Premotor cortex --> Primary motor cortex – Reach up • Program • Against gravity • More force to achieve goal – To produce the desired goal, which muscles should – Reach down contract and when • Gravity assists • 2 Key movement characteristics that are • Less force to achieve goal programmed – Flexible control – Spatial (hand path; joint angles) Kinematic program – Force Dynamic program Cortical motor areas Summary Motor 1° motor cortex • Motor behavior hierarchy Pathways Premotor cortex – Reflexes – Postural adjustments Red nucleus – Voluntary movements Reticular formation • Internal/neural representations Vestibular nuclei – Reflexes simple; invariant Descending Superior colliculus – Postural adjustments cortical motor Descending – Voluntary movements complex; flexible paths brain stem • Voluntary movements paths – Goal representation Spinal cord: – Kinematic and dynamic programs Intermediate zone – No wonder why voluntary movement recruit entire 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 Ventral Horn Short Organization: Long 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 • Medial Red nucleus Reticular – Tectospinal tract: eye-head coordination formation (superior colliculus) Vestibular Tectospinal – Reticulospinal tract: automatic postural nuclei tract adjustments and movements (hip; shoulder) (reticular formation) Reticulospinal Rubrospinal tract – Vestibulospinal tract: balance (axial muscles); Vestibulospinal tracts automatic postural adjustments tracts (vestibular nuclei) Bilateral Contralateral PNS Fig. 33-14 Cortical Medial Lateral Motor Cortical motor paths Paths • Lateral corticospinal tract – Limb control mostly Vestibular & • Ventral corticospinal tract Reticular Red nucleus – Proximal muscle control; mostly upper nuclei body Rubro- Medial spinal • For cranial muscle control: brain stem tract paths Corticobulbar tract Pyramidal X Lateral Ventral – with medial and lateral components Cortico- corticospinal PNS Fig. 33-15 spinal tract tract Bilateral Contralateral Why bother study the motor pathways? Origins of cortical motor paths • 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