55-101 Lecture Notes - Spinal Cord, Phantom Limb, Somatotopic Arrangement

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Outline of Lecture 39 (03-06 B; Shadmehr)
Motor System IV – Motor Cortex
I. Functions and properties of premotor cortex, motor cortex, and supplementary motor area
- The premotor cortex codes object position with respect to arm, independent of fixation point and
hand configuration
- Primary motor cortex is sensitive to forces needed to perform a reaching movement
- Like the somatosensory cortex, the motor cortex has a somatotopic map
- The motor map can change after amputation, a result of loss of strength of inhibitory
interneurons connecting two areas in the motor cortex
- Following amputation, the size of the motor region bordering amputated region enlarges;
phantom limb pain may be a result of size imbalance between the two hemispheres
- Following a lesion of a motor region, forced use of damaged region (constrained motion
rehabilitation) helps restore some motor function
- Some primary motor cortex neurons respond to direction of muscle forces, thus primary motor
cortex is responsible for determining required muscle activity to generate desired movement
- Supplementary motor area (SMA) is responsible for memory-guided movements whereas
premotor cortex is responsible for visually-guided movements
Summary of major points
- Basic summary of lectures 38 and 39 (motor system III and IV):
1) PPC: compute hand and target position in eye centered coordinates
2) Premotor cortex: determine hand displacement trajectory
3) Primary motor cortex: transform desired movement to muscle activity pattern
4) Spinal cord: relay signals to muscles
5) Muscles: do the movement
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