Chapter 14 nroc64.docx

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22 Apr 2012
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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
- 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
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.
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
The Ventromedial Pathways
- Four descending tracts that originate in the brain stem and terminate among the spinal
interneurons controlling proximal and axial muscles
- The Vestibulospinal Tracts
Functions to keep head balanced on shoulders as the body moves, and to turns head in
response to new sensory stimuli (true for tectospinal tract as well)
Originate in the vestibular nuclei of the medulla, which relay sensory information from
the vestibular labyrinth in the inner ear
One component of the vestibulospinal tracts projects bilaterally down the spinal cord and
activates the cervical spinal circuits that control neck and back muscles guide head
The other component projects ipsilaterally as far down as the lumbar spinal cord
maintains upright and balanced posture by facilitating extensor motor neurons of the legs
- The Tectospinal Tract
Originates in the superior colliculus of midbrain, which receives input from the retina
Optic tectum = superior colliculus
Superior colliculus receives input from retina, visual cortex, and afferent axons carrying
somatosensory and auditory information
Helps orient towards new stimulus
- The Pontine and Medullary Reticulospinal Tracts
Reticulospinal tracts arise mainly from reticular formation of brain stem
Reticular formation receives information from many sources
Reticular formation divided into two parts that give rise to two different descending
a) Pontine (medial) Reticulospinal Tract: enhances antigravity reflexes of the spinal
cord, helps maintain a standing posture by resisting the effects of gravity
b) Medullary (lateral) Reticulospinal Tract: liberates the antigravity muscles from
reflex control
Motor Cortex
- Region of the frontal lobe
- Area 4 lies anterior to central sulcus on the precentral gyrus
- Area 6 lies anterior to area 4
- Area 4 = primary motor cortex = M1
- Area 6: specialized for skilled voluntary movement, comprised of the premotor area (PMA) in a
lateral region and the supplementary motor area (SMA) in a medial region both perform
similar functions but on different groups of muscles
- SMA sends axons that innervate distal motor units directly
- PMA connects primarily with reticulospinal neurons that innervate proximal motor units
Contributions of Posterior Parietal and Prefrontal Cortex
- Mental image of body in space in relation to surrounding objects is generated by somatosensory,
proprioceptive and visual inputs to the posterior parietal cortex
- Two areas of interest in the posterior parietal cortex: area 5, receives inputs from primary
somatosensory cortical areas 3, 1, 2; and area 7, receives input from higher-order visual cortical
areas such as MT
- Lesions in these areas of parietal lobes in patients may results in simply neglecting the side of the
body opposite the parietal lesion
- Prefrontal cortex important for abstract thought, decision making, and anticipating the
consequences of action
- Prefrontal and parietal cortex represent highest levels motor control hierarchy; both send axons
that converge on cortical area 6
- When asked to perform finger movements from memory, somatosensory and posterior parietal
areas, parts of prefrontal cortex (area 8), area 6, and area 4 showed increased blood flow;
however, when asked to mentally rehearse the movements, area 6 was active but area 4 was not
Neuronal Correlates of Motor Planning
- Area 6 (SMA and PMA) plays an important role in eh planning of movement, particularly
complex movement sequences of the distal musculature
- A selective inability to perform complex (but not simple) motor acts is called apraxia
- “Ready” depends on activity in the parietal and frontal lobes
- “Set” may reside in the SPA and PMA, where movement strategies are devised and held until
- “Go” command implemented with the participation of major subcortical input to area 6; origin of
GO command, the primary motor cortex
- Major subcortical input to area 6 arises in a nucleus of the dorsal thalamus, called the ventral
lateral (VL) nucleus
- Input this part of VL, VLo, arises from the basal ganglia, buried deep within the telencephalon
- Basal ganglia is a target of the cerebral cortex, particularly the frontal, prefrontal and parietal
- LOOP: cortex basal ganglia thalamus back to cortex (SMA)
- Function of loop: selection and initiation of willed movements
Anatomy of the Basal Ganglia