Motor Overview



Motor Overview


Lubdha M. Shah, MD



TERMINOLOGY


Abbreviations



  • Primary motor cortex (PMC)


  • Premotor cortex area (PMA)


  • Supplementary motor area (SMA)


  • Basal ganglia (BG)


IMAGING ANATOMY


Overview



  • PMC (M1, area 4) in precentral gyrus



    • Origin of majority of corticospinal tracts & corticobulbar fibers, particularly those controlling motor cranial nerves


    • Projections to thalamus and BG



      • Input from ventral lateral nucleus of thalamus, sensory cortical areas, premotor cerebral regions


      • Well-defined somatotopic organization of motor cortex


      • Movements can be generated by the lowest intensity of electrical stimulation


      • Specific movements tend to be represented rather than specific muscles


    • Parallel input from SMA, PMA, BG, cerebellum


    • Primary function in execution, as well as some planning of movement


    • Lesions produce spastic contralateral weakness, most prominent in distal extremities


  • PMA (area 6) lies anterior to M1 with many of same connections as motor cortex



    • Most output is to M1, with smaller output to brain stem and spinal cord


    • Receives input from sensory association cortex and feedback from basal ganglia via ventral anterior and ventral lateral thalamic nuclei


    • Electrical stimulation produces more complex movements and at a higher stimulus intensity than simple movements from M1


    • Primarily responsible for initiation and planning of movement



      • Generates complex motor plans in response to external cues


    • Helps to guide body movements by integrating sensory information


    • Lesions produce less severe weakness but greater spasticity than with isolated precentral gyrus lesions


  • SMA: Area in medial superior frontal gyrus (Brodmann area 6) anterior to the primary motor cortex and superior to cingulate sulcus



    • Midline defines its medial limit; its anterior boundary is defined by a line perpendicular to the rostrum of the corpus callosum


    • Divided into rostral and caudal aspects by the V line: Vertical line traversing the posterior margin of the anterior commissure



      • Rostral SMA (pre-SMA) activates during word-generation and working memory tasks


      • Caudal SMA activates during motor and sensory tasks


      • Rostral SMA is particularly active during learning of new sequential procedures


      • Caudal SMA is active during performance of sequential movements


    • SMA receives input from motor and premotor cortices and from sensory cortex


    • Projects to M1, basal ganglia, thalamus, brain stem, and contralateral SMA


    • Thought to be involved in initiation of motion, planning complex movements, coordinating movements involving both hands



      • Generates motor plans in response to internal cues, automatic motor responses


    • Lesions of this area can cause inability to initiate motions (abulia), motor apraxia, transient weakness



      • Can result in severe deficits that improve or resolve over 6 weeks (SMA syndrome)


    • During complex motor and heat sensory tasks, activation tends to occur in contralateral posterior portion of SMA


    • Word-generation and working memory tasks tend to produce activation in anterior portion of SMA, particularly on left


  • Basal ganglia



    • Overlap as well as segregation among connections of motor cortices with striatum and thalamus



      • Supports notion that neuronal information of motor cortices is funneled in control of volitional movement


    • Seem to be activated more by sequential or internally cued movement than by repetitive or externally cued movement


    • May be involved in velocity of movement



      • Basal ganglia-thalamo-motor loop plays important role in controlling rate of sequential finger movements in self-initiated movement but not in externally triggered movement


  • Thalamus

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Sep 18, 2016 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on Motor Overview

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