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CHAPTER 14. BRAIN CONTROL OF MOVEMENT
the motor control hierarchy has three levels
→ the actions of a baseball pitcher preparing to pitch to a batter.
→ strategies
vision, audition, somatic sensation, and proprioception.
how to move the body to the state which a pitch is delivered and the
desired outcome is attained.
decision is made, based on experience.
→ tactics
make the tactical decision (to throw a curve ball).
issue instructions to the brain stem and spinal cord.
→ execution
Descending spinal tracts
How does the brain communicate with the motor neurons of the spinal cord?
the lateral pathway → involved in voluntary movement of the distal muscles
and are under direct cortical control.
the ventromedial pathway → involved in the control of posture and locomotion
and are under brain stem control.
The lateral pathways
(1) cortiocospinal tract
→ originate in the neocortex.
→ the longest and one of the largest CNS (106 axons).
→ 2/3 of the axons in the tract originate in areas 4 and 6 of the frontal lobe.
areas 4 and 6 of the frontal lobe = motor cortex
→ others derive from the somatosensory areas of the parietal lobe.
regulate the flow of somatosensory information to the brain.
→ axons from the cortex pass through the internal capsule.
→ course through the base of the cerebral peduncle in the midbrain.
→ then pass through the pons and collect to form a tract
at the base of medulla.
pyramidal tract → the tract’s cross section is roughly triangular.
→ decussation at the junction of the medulla and spinal cord.
→ as the axons cross, they collect in the lateral column of the spinal cord.
the lateral spinal corticospinal tract (CST).
→ CST axons terminate in the dorsolateral region of the ventral horns and
intermediate gray matter → control the distal muscles (the flexor).
(2) rubrospinal tract
→ originate in the red nucleus of the midbrain. “rubro” → red.
a major source of input to the red nucleus → the motor cortex
→ immediately decussate in the pons.
→ join axons in the corticospinal tract in the lateral column of the spinal cord.
→ its contributions to motor control is reduced in human.
the effects of lateral pathway lesions
→ experimental lesions in both CST and RST in monkeys.
can not move their shoulders, elbows, wrists, fingers independently.
voluntary movements are also slower and less accurate.
could sit upright and stand with normal posture.
→ experimental lesions in CST.
a movement deficit → severe.
many functions gradually reappear over the months.
permanent deficit → weakness of the distal flexors and an inability to move
the fingers independently.
lesion in RST reverse this recovery.
the corticorubrospinal pathway compensate for the loss of the CST input.
stalk
these components of the lateral pathway control fine movements
of the arms and fingers
The ventromedial pathways
→ contain four descending tracts originating in the brain stem.
→ terminate among the spinal interneurons controlling proximal and
axial muscles.
→ use sensory information about balance, body position and the vision to
maintain balance and body posture.
(1) The vestibulospinal tracts
→ originate in the vestibular nuclei of the medulla.
→ relay sensory information from the VL in the inner ear.
→ bilateral projection to the cervical spinal cord.
control neck and back muscles and thus guide head movement.
→ ipsilateral projection to the lumbar spinal cord.
maintain an upright and balanced posture by facilitating extensor motor
neurons of the legs.
(2) The tectospinal tract
→ originate in the superior colliculus of the midbrain
→ direct input from the retina, visual cortex, and others.
→ directs the head and eyes to move so that the appropriate point of space
is imaged on the fovea.
the reticulospinal tracts
→ reticulospinal tracts arise from the reticular formation of the brain stem.
reticular formation
a complex meshwork of neurons and fibers
runs the length of the brain stem
receives input from many sources and participates in many functions.
(3) the pontine (medial) reticulospinal tract
→ enhances the antigravity reflexes of the spinal cord.
→ facilitate the extensors of the lower limbs.
→ helps maintain a standing posture
(4) the medullary (lateral) reticulospinal tract
→ liberates the antigravity muscles from reflex control (opposite effect).
→ activity in both reticulospinal tracts is controlled by descending signals from
the cortex.
The planning of movement by the cerebral cortex
→ the control of voluntary movement engages almost all of the neocortex.
Motor Cortex
area 4 → lies just anterior to the central sulcus on the precentral gyrus.
area 6 → lies just anterior to area 4.
W. Penfield → weak electrical stimulation of area 4 in the precentral gyrus would
elicit a twitch of the muscles in a particular region of the body on the
contralateral side.
there is a somatotopic organization in this region.
area 4 → the primary motor cortex (M1)
area 6 → “higher motor area” in humans.
electrical stimulation evoke complex movements of either side of the body.
premotor area (PMA)
→ motor maps in a lateral region.
→ reticulospinal neurons that innervate proximal motor units.
supplementary motor area (SMA)
→ motor maps in a medial region
→ directly innervate distal motor units.
The contributions of posterior parietal and prefrontal cortex
two areas in the posterior cortex
→ body image and the perception of spatial relations.
area 5
a target of inputs from S1 areas 3, 1, and 2.
area 7
a target of higher-order visual cortical areas such as MT.
prefrontal area
→ abstract thought, decision making, and anticipating the consequences
of action
extensively connected with the parietal lobes.
Both the prefrontal and the parietal cortex send axons that converge on
cortical area 6 and this plays an important role in higher-order motor planning
the discharge of a cell in the
premotor area before a movement
→ instruction stimulus
→ “ Get set, monkey”
→ trigger stimulus
→ “ Go, monkey”
→ performance.
The Basal Ganglia (기저핵)
→ buried deep within the telencephalon.
→ one of the functions is to be the selection and initiation of willed movements.
→ consists of the four structures
caudate nucleus, putamen, globus pallidus, subthalamic nucleus
substantia nigra (a midbrain structure) is reciprocally connected with
basal ganglia.
→ caudate nucleus + putamen = striatum
the target of the cortical input to the basal ganglia.
→ the globus pallidus is the source of the output to the ventral lateral nucleus of
(VL) the thalamus.
→ It appears that the basal ganglia participate in many circuits, including
motor, memory and cognitive function.
“GO, Monkey”
the motor loop; cortex → striatum → globus pallidus → VLo → Cortex (SMA)
from cortex to cells in the putamen → excitatory synapses
from putamen to cells in globus pallidus → inhibitory synapses
from globus pallidus to cells in VLo → inhibitory synapses
from VLo to SMA in area 6 → excitatory synapses
→ the cortical activation of the putamen is excitation of SMA by VLo.
Basal Ganglia disorders
Hypokinesia (a paucity of movement)
→ increased inhibition of the thalamus by the basal ganglia.
(ex) Parkinson’s disease
→ degeneration of substantia nigra inputs to the striatum.
→ the depletion of dopamine closes the funnel that feeds activity to
SMA via the basal ganglia and VLo.
Hyperkinesia (an excess of movement)
→ decreased basal ganglia output
(ex) Huntington’s disease
→ profound loss of neurons in the caudate nucleus, putamen, and globus
pallidus and other brain regions.
→ loss of its inhibitory output to the thalamus.
→ hyperkinesia or dyskinesia.
→ “chorea” → uncontrollable and purposeless movement with rapid
irregular motion of various parts of the body.
(ex) ballism → damage to the subthalamic nucleus.
the subthalamic nucleus excites neurons in the globus pallidus that
project to VLo.
The initiation of movement by primary motor cortex
area 4 has strong synaptic connections with the motor neurons.
M1 → the motor strip; the ribbon of cortex stretches the full length of the
precentral gyrus.
The Input-output Organization of M1
→ the pathway from motor cortex to lower motor neurons
originate in cortical layer V.
→ a population of pyramidal neurons in layer V.
Betz cells → the largest cells
→ the layer V pyramidal cells in M1 receive input from two sources.
other adjacent cortical areas
areas 6, 3, 1, and 2
thalamic input
a part of the ventral lateral nucleus called
VLc, which relays information from the
cerebellum.
The coding of movement in M1
→ recording from M1 neurons in behaving animals.
a burst of activity occurs immediately before and during a voluntary movement.
encode two aspects of the movement → force and direction.
→ M1 cells fire most vigorously during movement in one direction but also
discharge during movements that varied +/- 45 degrees.
→ movement direction is encoded by the collective activity of a population of
neurons.
the activity of each cell → direction vector
a population vector
→ the vectors representing each cell’s activity are averaged.
The malleable motor map
plasticity of the adult motor cortex
(ex) the regions of cortex that used to evoke movement of the vibrissae cause
muscle movement in the forelimb or around the eyes after the motor
nerve supplying the vibrissae was cut.
The Cerebellum
→ cerebellar lesions cause “ataxia”
movements become uncoordinated and inaccurate.
dyssynergia → decomposition of synergistic multijoint movement.
dysmetric
(ex) ethanol intoxication → depression of cerebeller circuits.
Anatomy of Cerebellum
→ sits on the peduncles that rise from the pons.
→ a thin sheet of cortex that is repeatedly folded.
folia → a series of shallow ridges and run transversely (from side to side)
lobules → deeper transverse fissures (sagittal slice), 10 lobules.
→ contains more than 50% of the total number of neurons in the CNS.
→ deep cerebeller nuclei
relay most of the cerebeller cortical output to various brain stem structures.
→ vermis
the midline region.
separates the two lateral cerebellar hemispheres from each other.
The motor loop through the lateral cerebellum
→ axons from layer V pyramidal cells in the S1, M1, areas 4, 6, and posterior
parietal area form a massive projection to clusters of cells in the pons,
the pontine nuclei.
→ the pontine nuclei, in turn, feed the cerebellum.
→ corticopontocerebeller projection contains about 20 million axons.
→ the lateral cerebellum projects back to the motor cortex via a relay in the VLc
of the thalamus.
→ appears to instruct the primary motor cortex with respect to movement
direction, timing, and force.
Programming the Cerebellum
→
→
→
→
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think about the process of learning a new skill.
practice makes it perfect !
you master the skill.
the creation of a new motor program.
you perform them almost unconsciously.