The Control of Action
Movement control is exerted at separate and distinct level (components of the motor system)
Muscles & motoneurons
MN cell body located along spinal chord and in brain stem
axons to on muscles, innervates a number of muscles fibres
MN + muscle fibres = motor unit
MNs activated by
sensory afferent neurons
interneurons
descending tracts from forebrain and brain stem
Alpha motor neurons
originate in spinal chord, exit via ventral root, terminate in muscle
action potential --> acetylcholine
Spinal Chord
MN's innervate skeletal muscle
SC is first/final point for sensorimotor integration
Interneurons
Termination of descending pathways
Sensorimotor integration in the spinal chord
stretch reflex
E.g. tendon tap
autogenic
monosynaptic
muscle spindle
Ia different
synapse alpha MN
Acetylcholine release
muscle contracts
Apraxia
loss of ability to execute or carry out learned purposeful movements
common after damage to L hemisphere
type depends on exact region- common in frontal and parietal cortices
higher order motor deficit
classification subject to ongoing debate, arts of body, type of movement etc.
Supplementary motor complex
planning
sequencing
SMC projects to ipsilateral and contralateral motor cortex, and contralateral SMC
Monkey SMA & SEF neurons active before movement
neurons fire before specific sequence
Cerebellum
inputs from cortex, brain stem , and spinal chord
outputs to spinal chord, motor cortex, oculomotor nuclei
integration
Comparator
well learning automatic movements
Basal Ganglia
receives input from cortex
sends output to cortex
no direct sensory inputs
no direct output to spinal chord
critical for movement control (imitation, selection, inhibition)
Primary motor cortex
conscious voluntary movement
early stages of learning
Distal- contralateral
Proximal- bilateral
brain stem
Indirect pathways via brain stem