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Chapter 14: Brain Control of Movement (Descending spinal pathways from…
Chapter 14: Brain Control of Movement
Motor systems are organized hierarchically
Forebrain, Brainstem, Spinal cord
Basal ganglia and cerebellum act on cerebral cortex through relay nuclei in the thalamus (VL/VA)
Final common pathway
motor neurons in spinal cord (body), brainstem cranial nerves (head)
Lower motor neuron
soma in ventral horn of spinal cord
soma in brainstem nuclei for skeletal muscles of head
innervates muscle fiber
final common pathway: lower motor neurons directly command muscle contraction
Descending spinal pathways from brain
modulate spinal reflexes or directly drive motor output
clinically referred to as "Upper Motor Neurons"
soma in cerebral cortex or brainstem
Ventromedial pathways
Control of posture, locomotion, orienting, balance
balance: vestibulospinal tract
orienting reflexes
Tectospinal: head
tectobulbar: eyes
posture and locomotion
pontine and medullary reticulospinal tract
under brainstem control
innervate axial and proximal musculature
synapse on motor neurons innervating axial muscles and extensors
Lateral pathways
Voluntary movement of distal musculature
goal directed movements
control fine movements of arms and fingers
corticospinal tract (Pyramidal tract)
limbs
Effect of corticospinal tract lesions
difficulty moving distal limbs
may recover over time, voluntary movements are also slower and less accurate
loss of ability to make independent finger movements - does not recover
rubrospinal tract (small in humans)
distal limb muscles
corticobulbar tract (bulb=brainstem)
travels with corticospinal tract and provides UMN innervation to LMNs in cranial nerve motor nuclei
face, jaw, tongue, throat
Rodman's areas 4 and 6 in frontal lobe
under direct cortical control
innervate distal musculature
synapse on motor neurons innervating distal muscles and flexors
Primary motor cortex — M1 — Area 4
Activity in M1 neurons occurs before and during a voluntary movement
activity encodes force and direction of movement
Direction vectors
higher firing rate = more force
cell fires most to leftward movement, direction vector points left
direction vector: points in the preferred direction for neuron
its length depends on firing rate over a range of directions (represents how active cell was during a particular movement)
population vector = vector sum
initiation of complex voluntary movement
Layer 5 Betz cells - origin of the corticospinal tract
Different cortexes and areas of brain
Parietal-occipital-temporal association cortex
proprioceptors - current position of body in space
analysis of sensory inputs (vision, somatosensory, auditory)
construct a representation of the world which is sent to prefrontal cortex
Prefrontal cortex
executive function - working memory, reasoning, problem solving
abstract thought
decision making
anticipating consequences of action
"Motor Planning" Area 6
Supplementary motor area, SMA
Premotor area, PMA
Motor planning
get "set"
signal to get ready to perform a specific task (neurons in PMA fire)
"go"
signal to perform a specific task (shortly after movement is initiated, PMA cell stops firing)
Loops
Basal Ganglia: and initiation of willed movements, Globus pallidus is output of basal ganglia (GP is tonically active and inhibiting the VL/VA thalamus)
excitatory connections from cortex
synapses from cortical cells excite cells in putamen
inhibitory synapses on globus pallidus neurons
VL/VA released from inhibition, excites SMA
Anatomy
Striatum: caudate/putamen
globus pallidus
sub thalamic nucleus
substantia nigra
Globus Palidus tonically inhibits VL thalamus
Anything that increases GP output leads to hypokinesis (trouble moving)
degeneration of dopaminergic cells in substantia nigra
Decreased GP output leads to hyperkinesis (lots of uncontrollable movements)
loss of tonic inhibitory output to thalamus, cell loss in subthalamus
Cerebellar Loop (error-based loop)
Axons from layer V pyramidal cells from pontine nuclei feed the cerebellum
lateral cerebellum projects back to motor cortex via a relay in Vlc
Cerebellum anatomy
learning sequencing and timing of complex movements
Projections to cerebellum
Motor cortex
intended movement
inferior cerebellar peduncle
actual movement (proprioceptors)
what was intended is compared with what actually happened
error signal is sent to cortex to correct ongoing and future movements
Parallel fibers, Purkinje cells, climbing fibers
parallel fibers connect with tens of thousands of Purkinje cells, relaying weak sensory info
Climbing fibers innervate Purkinje cell, indicating motor error and long-term weakening of co-incident parallel fiber input