Download Motor Systems II Loops and Tracts

Motor Systems II
Loops and Tracts
Reading:
BCP Chapter 14
VA
Motor System
Intent: posterior parietal
association cortex
Plan of action: dorsolateral
prefrontal association cortex
Specific set of instructions:
secondary motor cortex
Execution: primary motor cortex
Cortical Loops and Descending Tracts
Two major cortical loops:
• one through the basal ganglia
and secondary motor cortex that
selects and initiates action;
• one through the cerebellum and
primary motor cortex that
modulates and sequences
muscle contractions while a
movement is in progress.
Four major descending pathways
(mainly from primary motor cortex):
• two in the dorsolateral region of
the spinal cord; and
• two in the ventromedial region.
VA
The Basal Ganglia
A collection of inter-connected,
midline, nuclei located lateral to
the thalamus
Includes the striatum (caudate
and putamen), globus pallidus
(external and internal),
subthalamic nucleus and
substantia nigra
External, GPe
Internal, GPi
Basal Ganglia Pathways
Two major functional pathways
through basal ganglia, direct and
indirect/hyper-direct, with
opposite net effects on thalamic
targets. Proper function: balance
Direct: excitation
Facilitates motor (or cognitive)
programs in the cerebral cortex
that are adaptive for the
present task
Indirect/hyper-direct: inhibition
Inhibits the execution of
competing motor programs.
VA
Basal Ganglia: Parkinson’s Disease
Parkinson’s disease is characterized
by slowness or absence of movement
(bradykinesia or akinesia), rigidity,
and a resting tremor (hands, fingers)
Cause: the loss of the dopaminergic
neurons in the substantia nigra
Direct pathway striatal neurons have
D1 dopamine receptors, which cause
depolarization, whereas indirect
pathway striatal neurons have D2
dopamine receptors, which cause
hyperpolarization. The nigrostriatal
pathway thus produces net excitation
of cortex in two ways. In Parkinson’s
disease, balance is tipped in favor of
the indirect inhibitory pathway.
frontiersin.org
Basal Ganglia: Huntington’s Disease
The symptoms of Huntington’s disease
are in many respects the opposite of the
symptoms of Parkinson’s disease.
Huntington’s disease is characterized by
choreiform movements: involuntary,
jerky movement of the body, especially of
the extremities and face.
Huntington’s disease results from the
selective loss of striatal neurons in the
indirect pathway. Thus, the balance
between the direct and indirect pathways
becomes tipped in favor of the direct
pathway. Without their normal inhibitory
inputs, thalamic neurons can fire
randomly and inappropriately, causing the
motor cortex to execute motor programs
without proper control.
Cerebellum
Gross Anatomy: The visible part of the
cerebellum is actually a single thin sheet
of folded cortex. It is characterized by a
series of shallow ridges call folia.
Altogether, the cerebellum constitutes
about 10% of brain mass, but more than
50% of its neurons
•
Cerebellar Cortex: Subdivided into
lobes, lobules, vermis (midline region)
and lateral hemispheres.
•
Cerebellar nuclei: Embedded deep in
the white matter of the cerebellum;
communicate cerebellar cortical output
to other brain centers including motor
cortex, descending motor pathways,
and vestibular nuclei (balance)
Anterior
lobe
Cerebellar
cortex
Posterior
lobe
Motor Loop through Cerebellum
Layer V sensorimotor cortical cells
(secondary and primary motor cortex,
somatosensory cortex, and posterior
parietal cortex) project to the pontine
nuclei (in the pons).
Pontine nuclei send massive input to
cerebellar cortex.
Lateral cerebellum projects to cortex via
lateral nucleus of thalamus (VLc).
Function:
• To modulate and sequence muscle
contractions for voluntary movements.
• To evaluate disparities between
intention and action.
• To correct output of cortical and
subcortical motor systems while
movement is in progress.
VA
Sensorimotor Adaptation
The cerebellum is important in
reprogramming movements to
compensate for sensory
disturbances.
– Example: Adaptation of
motor output to altered
visuospatial experience
(e.g., ocular prisms).
The cerebellum is also important
for learning new motor skills; that
is, practice improves efficiency,
speed, and precision of motor
performance.
Cerebellar Disorders
Ataxia:
intention tremor
disturbances in rate and regularity of
alternating movements.
Asynergia (dysynergia):
prolonged reaction times
“decomposition” of multi-joint
movements.
Dysmetria:
Undershoot or overshoot of
movements toward objects.
Descending Motor Pathways
Four major descending
pathways: two in the dorsolateral
region of the spinal cord; two in
the ventromedial region. In each
region, one is direct and one is
indirect.
Motor
homunculus
Upper motor
neuron
Lower motor
neuron
Dorsolateral tracts
• corticospinal (direct)
• corticorubrospinal
Motor
nuclei
Medulla
Ventromedial
• corticospinal (direct)
• cortico-brainstem-spinal
Dorsolateral
corticospinal
Ventromedial
corticospinal
Dorsolateral Tracts
Corticospinal tract:
descends from primary motor cortex
(e.g., Betz cells) through the
medullary pyramids, then crosses
the midline (decussates) and
synapses directly in the cord. Wrist,
hands, fingers, toes
Corticorubrospinal tract:
synapses at red nucleus, and
crosses before medulla. Face, arms
and legs.
Dorsolateral tracts tend to synapse
on small interneurons in the cord
that, in turn, synapse on motor
neurons of distal targets
Ventromedial Tracts
Corticospinal tract:
descends ipsilaterally and directly
to the spinal cord, then branches
and innervates interneuron circuits
bilaterally in multiple spinal
segments
Cortico-brainstem-spinal tract:
synapses on various brainstem
structures and descends bilaterally,
carrying information from both
hemispheres
Ventromedial tracts synapse on
small interneurons in the cord that,
in turn, synapse on motor neurons
of the trunk and proximal limb
muscles
Division of Labor
Dorsolateral tracts
• one direct tract, one that
synapses in the brain stem
• terminate in one contralateral
spinal cord segment
• distal muscles
• limb movements
Ventromedial tracts
• one direct tract, one that
synapses in the brain stem
• terminate in multiple, bilateral
spinal cord segments
• proximal muscles
• posture and whole body
movements
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