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week 11

by: Emma Notetaker
Emma Notetaker
GPA 3.975

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week 11
Systems Neuroscience
Laura Schrader
Class Notes
25 ?




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Popular in Nutrition and Food Sciences

This 5 page Class Notes was uploaded by Emma Notetaker on Friday April 1, 2016. The Class Notes belongs to NSCI 3320 at Tulane University taught by Laura Schrader in Spring 2016. Since its upload, it has received 12 views. For similar materials see Systems Neuroscience in Nutrition and Food Sciences at Tulane University.

Similar to NSCI 3320 at Tulane

Popular in Nutrition and Food Sciences


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Date Created: 04/01/16
Wednesday, March 30, 2016 Week 11 Motor Systems Spinal Control • hierarchical organization of neural structures involved in the control of movement level function structures high strategy association areas, basal ganglia middle tactics motor cortex, cerebellum low execution brain stem, spinal cord • lower motor neuron: from spinal cord to innervated muscles • important for muscle contraction - releases ACh onto nicotinic receptors • exits via ventral horn of spinal cord • alpha motor neurons • inputs: spinal interneurons • • help to generate central patterns • muscle spindles (sensory input) • Aa neurons get proprioceptive info from here, send to spinal cord • upper motor neurons in brain • inputs enter at dorsal root, branch repeatedly and form excitatory synapses on interneurons and alpha motor neurons in ventral horn • upper motor neuron: from brain to spinal cord • input from corticospinal tract • synapses on alpha motor neuron • muscle terminology: axial muscles: movement of trunk • • lie medially • maintenance of posture • proximal muscles: for movement of shoulder, elbow, pelvis and knee (girdle) • distal muscles: movement of hands, feet, digits • lie laterally • smooth muscle: enteric motor system • innervated by nerve fibers from autonomic nervous system • innervated by lower motor neurons • ex: digestion and blood pressure • striated: cardiac and skeletal skeletal muscle: mediates movements • • bulk of muscle mass • extensors: triceps, quads • lie laterally • flexors: biceps, hamstrings • lie laterally • motor neurons controlling flexors lie dorsal to those controlling extensors • motor neuron pool: muscle and all axons that innervate it • types of motor neurons: 1 Wednesday, March 30, 2016 • alpha: directly trigger generation of force by muscles • innervate extrafusal fibers (cause muscle contraction) gamma: innervate intrafusal fibers within muscle spindle • • intrafusal fibers contain muscle spindles • muscle spindle: stretch receptor, inside muscle fibers • spindles and associated Aa sensory neurons sense muscle length • proprioceptors Aa: largest and fastest conducting axons • contained in intrafusal fibers Aa axons contain info about muscle length • • synapse in ventral horn • myotactic reflex: • sensory neurons (Aa) synapse on alpha motor neurons: monosynaptic reflex arc • weight added to the muscle, transiently elongated (stretched) • Aa axon sends info about stretch to the lower motor neuron (depolarization of Aa) leads to discharge of action potential to alpha motor neuron • • alpha motor neuron causes muscle contraction • ex: knee-jerk reflex • tap of quadricep tendon to stretch • Aa fiber sends muscle lengthening information to dorsal horn • synapses on alpha motor neuron causes reflex contraction of quads and extends leg • • intrafusal fibers: • Aa wrap around muscle spindles, which contain intrafusal fibers • muscle spindles contain modified skeletal muscle fibers (intrafusal) • innervated by gamma motor neurons • cause contraction of intrafusal fibers to maintain firing of Aa neuron • contraction in response to shortening of extrafusal fibers (when muscle is shortened, the Aa neurons stop firing) • contraction pulls on non-contractile equatorial region and keeps Aa axons active • alpha activation decreases Aa activity, gamma activation increases Ia activity • important for maintaining sensitivity of sensory response to gauge muscle length and contraction • shortened muscle shortened intrafusal fibers • golgi tendon organs: act as strain gauge • lies in series with muscle • monitor muscle tension or the force of contraction • innervated by group Ab axons • in series with muscle (carry muscle tension info) • group Ib enters spinal cord, branch repeatedly and synapse on interneruons in ventral horn • mediates reverse myotactic reflex: • mediated by inhibitory response in alpha motor neuron • provides feedback about muscle tension • protects muscle from being overloaded • normal function: regulate muscle tension in an optimal range • reciprocal inhibition: contraction of one set of muscle is couple to relaxation of antagonistic muscle (flexors vs. extensors) • mediated by inhibitory interneuron • flexor reflex: reflex arc within spinal cord to flex limb from aversive stimulus 2 Wednesday, March 30, 2016 • sensory pain axons enter spinal cord and branch to activate interneurons at various segments excite the alpha motor neurons that control flexor muscle of affected limb —> inhibits • antagonistic muscles • crossed extensor reflex: compensates for extra load imposed by limb withdrawal on the antigravity extensor muscle of the opposite leg • important so that you maintain standing when flex one leg to withdraw from pain • excitatory input to flexors of one leg (aversive) extensors of OTHER leg excited • • provides building block for locomotion • circuits that give ride to rhythmic actions • generate central patterns • some children can come out of incomplete spinal cord injury - training • diseases: amyotrophic lateral sclerosis: Lou Gehrig’s Disease • • muscle weakness and atrophy, slow deterioration • degeneration of alpha motor neurons • loss of superoxide disputes: can’t break down free radicals • excitoxicity: glutamate overstimulation • treatment targets release of glutamate muscular dystrophy: progressive weakness and muscle deterioration • • genetic loss of dystrophin • myasthenia gravis: autoimmune disease • nAChRs Central control of movement • when pitcher is throwing, controlled by • lateral pathways: initiation of movement, cause muscle contractions (through alpha motor neurons) corticospinal tract: command for skilled movements, correction of motor patterns from • spinal cord • rubrospinal tract: command for skilled movements, correction of motor patterns from spinal cord • ventromedial pathways: maintain posture, control balance • reticulospinal: activation of spinal programs for stepping and stereotypic movements vestibulospinal: utonic sctivity in antigravity muscles • • descending motor tracts in spinal cord: (look at diagram slide 4) • lateral voluntary movement control of distal musculature (primarily flexors) • corticopinal: aka pyramidal tract • controls distal muscles • lateral spinal cord particularly controls extensors • • lesions cause inability to move limbs separately • slow and inaccurate voluntary movement • sometimes rubrospinal flexor control may compensate for this • starts in primary motor cortex and goes through internal capsule • goes through base of midbrain and crosses at medullary pyramids (decussation) 3 Wednesday, March 30, 2016 • synapses on alpha motor neurons of contralateral spinal cord • 10% of fibers remain ipsilateral rubrospnial: • • primarily flexors of upper extremities • central input from cortex • starts in red nucleus of midbrain • crosses in pons, travels contralaterally • next to corticospinal tract (lateral spinal cord) ventromedial: originiate in brainstem • • integrate sensory information about balance and position to maintain balance and posture • tectospinal: head movement • starts in superior colliculus (tectum) • head and neck posture and movement vestibulospinal: where things are in space • • originates in vestibular nuclei, sends bilateral outputs through spinal cord • remains ipsilateral • vestibular labyrinth cranial nerve VIII • projects to lumbar areas to facilitate extensors of legs (hamstrings) • control of neck and back muscles cerebellar inputs (mainly inhibitory) • • ALMOST NO CORTICAL INPUT • medial longitudinal fasciculus: projections from vestibular nuclei to cranial nerves • reticulospinal tracts: primarily innervate gamma motor neurons • influenced by corticoreticular and ascending spinoreticular tract • sensory info from cortex and spinal cord • pontine: starts in pons, travels down spinal cord • enhances antigravity reflexes of spinal cord • facilitates extensors • maintains upright position, movements and tone • medullary: starts in medulla, travels down spinal cord • opposite effects of pontine • relieves antigravity muscles from reflex control • inhibits extensors • function of ventromedial pathways in absence of cortical inputs • lower motor neuron lesions: weakness and muscle atrophy • upper motor neuron lesions (from cortex or brainstem): still have innervation from alpha and gamma motor neurons • no muscle atrophy • mainly causes extensor rigidity • lesion that extends into brainstem • decerebrate rigidity: • essentially inhibited everything in cerebral area • rigidity in all extensors (whole body tenses) • reticular spinal tract still innervating gamma motor neurons, which still cause muscle contraction • transection of dorsal root can relieve some extensor hypertonus - blocks input to alpha motor neuron so that it stops sending signals (relaxes rigidity) • decorticate rigidity: corticospinal tract NOT intact, but rubrospinal tract is intact 4 Wednesday, March 30, 2016 • abnormal flexion of upper arms (due to issues with corticospinal) • rigidity and extension of lower limbs descending motor information: • • input originates in motor cortex • area 4: primary motor cortex (M1) • Wilder Penfield: experimented with stimulation of the cortex which led to movement or twitching in arm or limb • actual movement of muscles area 6: premotor area (PMA) • • skilled voluntary movement • planning of movement • integrates sensory input • sensorimotor associations • projections to reticulospinal to innervate proximal muscles also sends info to primary motor cortex • • activity encodes intention and planning • supplementary motor area (SMA): • sequences and bimanual coordination • ex: buttoning shirt • innervation to distal muscles lesions: apraxia (inability to perform skilled movements) • • experiment with monkey: receives stimulus and has to push a button • ready: parietal and frontal lobes (attention) • set: SMA and PMA (PMA active during mental rehearsal/planning) • go: M1 • precentral gyrus organization: homunculus (similar to somatosensory cortex) • face and hands: lateral • trunk and lower limbs: medial • areas requiring higher motor control take up more space on the cortex • activity of M1 neurons • area 4: lower threshold for elicitation of movement by electrical stimulation • entire cortex may be active for specific movement • direction and force preference • pathways starts in layer V (Betz cells) • gets inputs from other areas and thalamus • sends info down to somatosensory areas 3, 1, 2 • cortical areas outside M1 use wide variety of sensory cues to select and guide • monkey trained to move stimulus to the right • firing of a large part of the motor cortex - major choice of those cells causes movement • encoding direction in motor cortex: • most of cortex active for every movement (not just the areas specific for certain body parts) • activity of each cell codes a choice for particular movement (force and direction preference) • direction of each movement determined by averaging choices of many cells (“votes”) 5


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