Chapter 14
- Central Motor System has 3 levels:
a) Association areas of neocortex and basal ganglia of forebrain: strategy – goal of the
movement and movemtn strategy that best achieves the goal
b) Motor cortex and cerebellum: tactics – the sequences of muscle contractions, arranged in
space and time to achieve the goal
c) Brain stem and spinal cord: execution – activation of motor neuron and interneuron pools that
generate goal-directed movement and make necessary adjustments to posture
DESCENDING SPINAL TRACTS
- Axons from brain descend through spinal cord along two major groups of pathways
- Lateral Pathways: involved in voluntary movement of distal musculature and are under direct
cortical control
- Ventromedial Pathways: involved in control of posture and locomotion and are under brain
stem control
The Lateral Pathways
- Corticospinal Tract
Originates in the neocortex
Two thirds of the axons in the tract originate in areas 4 and 6 of frontal lobe, aka motor
cortex
Most of the remaining axons in the corticospinal tract derive from somatosensory areas of
the parietal lobe and serve to regulate the flow of somatosensory information to brain
Axons from Cortex pass through internal capsule bridging the telencephalon and thalamus base
of the cerebral peduncle (collection of axons in the midbrain) through the pons collect to form a
tract at base of medulla (forming a bulge – medullary pyramid at ventral surface of medulla) aka
pyramidal tract at junction of medulla and spinal cord, pyramidal tract crosses at pyramidal
decussation axons collect in the lateral column of the spinal cord and form the lateral corticospinal
tract corticospinal tract axons terminate in the dorsolateral region of the ventral horns and
intermediate gray matter (location of motor neurons and interneurons that control the distal muscles i.e.
flexors)
The right motor cortex directly commands the movement on the left side of the body and
vice versa
- Rubrospinal Tract
Originates in the red nucleus of the midbrain
Axons from red nucleus decussate in the pons lateral column of the spinal cord
Major source of input to the red nucleus is the same region that also contributes to the
corticospinal tract
- Effects of Lateral Pathway Lesions
Lesions in both corticospinal and rubrospinal tracts in monkeys rendered them unable to
make fractionated movements of the arms and hands: could not move shoulders, elbows,
wrists, and fingers independently
slower voluntary movements
however, could sit upright and stand with normal posture