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M555 Medical Neuroscience Motor System: Descending Pathways Supraspinal Input Several Pathways (“Tracts”) Descend from the Brain descending input from cerebral cortex to brain stem and spinal cord descending input from brain stem to spinal cord neurons in several brain regions send axons to vicinity of SMNs few terminate directly on SMNs; most terminate in populations of interneurons interneurons provide input to SMNs medial and lateral vestibulospinal tracts > origins medial Vest Sp Tr lateral Vest Sp Tr > major inputs > route medial Vest Sp Tr lateral Vest Sp Tr > terminations > general roles influence activity of neck muscles, anti-gravity muscles in trunk, hips. legs help to stabilize head, maintain upright posture, postural adjustments as head and body move medial and lateral reticulospinal tracts > origins medial Ret Sp Tr lateral Ret Sp Tr > major inputs > route medial Ret Sp Tr lateral Ret Sp Tr > terminations > general role avenue for sensory input and cortical motor planning to influence motor activity involving spinal cord tectospinal tract > origin > major inputs > route > terminations > general role turning of head and eyes to novel stimuli a sensory map on upper part of superior colliculus activity in sebsory map then activates corresponding parts of underlying motor map visual world pu visual in t layer l input la a d o m lti yer mu p t u u o t r l a o t y er mo superior colliculi aqueduct tectospinal tract tectospinal tract rubrospinal tract > origin > major inputs > route > terminations > general role some uncertainty precise, well-controlled movements involving upper limb movements, esp hands appears to facilitate flexor movements more than extensor Note: all except RubroSpinal Tr influence more medial SMNs and INTNs axial muscles -neck, trunk, shoulders, hips RubroSpinal Tr (and CorticoSpinal Tr) influence more lateral SMNs and INTNs distal muscles of limbs, particularly hands Sup Coll Sup Coll Red N Red N Ret Form pons Lateral Vest N Ret Form pons Medial Vest N Medial Vest N Ret Form medulla Ret Form medulla Descending Spinal Pathways from Brain Stem Lateral Vest N A Couple of Clinical Problems Related to These Descending Tracts Decorticate Rigidity (“Decorticate Posturing”) contraction of extensor muscles in neck, trunk, legs flexion of arms at elbow level of lesion in brain stem: CorticoSpinal rigid extension (arched neck) diencephalon possible explanation for this posture damage to brain stem just rostral to superior colliculus corticospinal pathway damaged midbrain pons medulla RubroSpinal VestibuloSpinal ReticuloSpinal some flexion rigid extension (arched back) spinal cord rigid extension some descending pathways still intact and functioning rigid extension rigid extension toes point inward Decerebrate Rigidity contraction of extensor muscles in neck, trunk, legs, arms but still some flexion in hands and fingers level of lesion in brain stem: CorticoSpinal possible explanation for this posture diencephalon damage to brain stem between superior and inferior colliculus midbrain pons medulla corticospinal pathway damaged rubrospinal pathway damaged RubroSpinal VestibuloSpinal ReticuloSpinal spinal cord some descending pathways still intact and functioning rigid extension (neck arched) rigid extension rigid extension (arched back) rigid extension and pronated rigid extension rigid extension rigid extension toes point inward Additional Note a worrisome pattern: decorticate rigidity progressing to decerebrate rigidity space occuping lesion Motor System: Descending Corticospinal Pathways Descending Projections from Cerebral Cortex Corticospinal Pathways Lateral Surface of Left Cerebral Hemisphere Corticobulbar Pathways origins in several regions of cortex primary motor cortex (M1, area 4) > large contribution to CS/CB pathways > somatotopic organizatiion > stimulation - simple, isolated movements lowest threshold, shortest latency > traditional view - movement execution >cerebellum (via thalamus) is a major source of input Medial Surface of Left Cerebral Hemisphere >programs fundamental aspects of movement direction, force, change of force,velocity, joint position suplemental motor cortex (area 6) > contribution to CS/CB pathways > somatotopic organizatiion > stimulation - higher threshold, longer latency, more complex movements > active when movement is simply contemplated >cerebellum (via thalamus) is a major source of input > role: planning internally generated movement intention for movement? premotor motor cortex (area 6) > contribution to CS/CB pathways > somatotopic organizatiion > stimulation - higher threshold, longer latency, more complex movements > basal ganglia (via thalamus) is a major source of input > role: planning externally generated movement corpus callosum simplified primary motor cortex somatotopy somatopic organiztion of primary motor cortex hip leg foot arm lateral hand** face** mouth cross-section through left cerebral hemisphere from figs 18-11 and 3-28 in Nolte, 5th ed from figs 18-10 and 3-30 in Nolte, 6th ed Parietal Cx SuppMCx PrimaryMCx Prefrontal Cx trunk top PreMCx interneurons somatic motor neurons Lateral Surface of Left Cerebral Hemisphere somatosensory cortex (areas 3,1,2) > large contribution to pathways > modulation of afferent sensory input posterior parietal cortex (areas 5 and 7) > small contribution to pathways > visual guidance of movements (part nearer visual cortex) tactile guidance of movements (part nearer somatosensory cortex) Medial Surface of Left Cerebral Hemisphere corpus callosum cingulate motor area > small contribution to pathways > movements associated with an emotional context SuppMCx CingMCx PreMCx PrimaryMCx SomatSenCx interneurons interneurons SMNs sensory input frontal eye field > not usually considered part of pathways FEF superior colliculus gaze control centers in brain stem > involved in voluntary movement of eyes > unilateral lesion of frontal eye fields - oculomotor nuclei I II III IV V VI neurons and lamina of cortex > in motor cortex, pyramidal cells lamina V of cortex > largest cells = Betz cells but only a very small fraction of pathway trochlear nuclei pyramidal neuron anatomy of “projection” neurons and their “descending” axon > million axons collect in white matter below cerebral cortex > large-diameter, heavily myelinated axons of Betz cells - 2% > moderate to large diameter axons (12 – 15 microns, myelinated - 10%) > most axons - small diameter (5 microns), some myelin or no myelin (~33%) synapse on interneurons abducens nuclei in telencephalon axons leave gray matter enter corona radiata (white matter) funnel down into the internal capsule caudate genu putamen globus pallidus in midbrain in middle portion of cerebral peduncle in pons horizontal section through midbrain cerebral peduncle (crus cerebri) dispersed among pontine nuclei horizontal section through pons in medulla reassemble on face of medulla pyramids – “pyramidal tract” “pyramid” in spinal cord to LatCorSpTr influence laterally situated SMNs most axons Betz cell axons 55% to upper limbs 20% to trunk 25% to lower limbs movements in distal limbs, especially hands precise, fine movements to Ant(Ven)CorSpTr - motor decussation in lower medulla Corticospinal Tract Damage corticospinal tract alone loss of ability to make precise movements of digits more experimental primate studies than actual clinical cases corticospinal tract along with other structures strokes, tumors, traumatic brain injuries motor cortex/corticospinal tract plus other sites cerebral cortex white matter of cerebral hemisphere - cortical connections - initially, signs analogous to spinal shock , including flaccidity then, eventual onset of characteristic signs of UPPER Motor Neuron loss hypertonia hyperreflexia spastic hemiplegia, hemiparesis Babinski and Hoffmann’s signs clonus clasp-knife response corticospinal- most direct access of cerebral cortex to SMNs in spinal cord, but not the only access Cerebral Cortex corticospinal tract corticoreticular pathways Reticular Formation reticulospinal tracts interneurons Spinal Cord SMNs