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Transcript
Control of Movement
Patterns of Connections Made by Local Circuit Neurons in
the Intermediate Zone of the Spinal Cord Gray Matter
Long distance interneurons project bilaterally,
innervate multiple segments.
Used for posture control.
Short distance interneurons project over a few
segments, remain ipsilateral.
Used for distal muscle groups.
Descending Projections from the Brainstem to the Spinal Cord
Feed forward processing
• Able to predict changes in posture, and generate an appropriate
stabilizing response.
• Some muscles fire in anticipation of a need for postural adjustment.
– For example, the gastrocnemius muscle needs to adjust for the
anticipation of contracting the biceps that would naturally pull the
body forward – stabilizing response
• Reticular formation and Reticulospinal tract important for this.
– Lesion or pharmacologically block it in a cat and compensatory
muscle changes do not occur.
• Stimulate motor cortex in the right place can induce paw lifting, also
induces other limbs’ muscles to fire. Inhibit reticulospinal tract, paw
still moves but other legs do not.
Anticipatory Maintenance of Body Posture
EMG= elecromyography. Measure muscle APs
Gastrocnemius stimulation happens before biceps
cut reticulospinal tract Biceps will fire but not Gastrocnemius
Feedforward and Feedback
Mechanisms of Postural Control
• PN17050.JPG
Direct and indirect projections
form motor cortex
Motor cortex makes two types of projections.
• A direct pathway to the ventral lateral spinal cord
• An indirect pathway to the reticular formation (which
subsequently goes to medial spinal cord).
• For example a direct pathway will move the hand and the
indirect pathway will posture the body.
• Cutting the direct pathway, leaving the indirect pathways
intact will not affect ability to walk, run, etc, but will
prevent the distal parts of limbs from being used.
Pathways from the Motor Cortex to the Spinal Cord
Direct pathway:
lateral corticospinal and
rubrospinal tracts, distal
fine motor movements.
“Pyramidal System”
Pathways from the Motor Cortex to the Spinal Cord
Indirect pathway:
postural adjustments.
Ventral corticospinal tract,
cortico-reticulospinal tract.
“Extrapyramidal System”
Motor cortex
•
•
•
•
•
•
in the frontal lobe
several interconnected areas
Primary motor cortex in the precentral gyrus.
Gets input from basal ganglia, cerebellum and other cortical areas.
Has 6 layers, layer V is the output layer (pyramidal cells or Betz cells).
Primary pathway- the pyramidal system.
Architecture of
motor cortex is
different than the
sensory cortex.
Betz cells
The corticospinal tract.
Motor fields
• Stimulation of a neuron in primary motor
cortex activates several muscles, and
inhibits other muscles.
• Multiple neurons can activate the same
muscle.
• The “motor field” of a cortical motor
neuron has to do with organized movements
rather than specific muscle groups.
Directional Tuning of an Upper Motor
Neuron in the Primary Motor Cortex
Monkey trained to move joystick in response to light
Directional Tuning of an Upper Motor
Neuron in the Primary Motor Cortex
movement starts
at 0.
Yellow – activity
of neuron is
increased.
Purple – activity
is decreased.
Neurons have
“preferred
direction”
Activity of a single neuron in motor cortex
anticipates future movement.
Directional Tuning of an Upper Motor Neuron
in the Primary Motor Cortex
Individual neurons tuned too broadly
to accurately predict movement
direction.
Populations of neurons can calculate a
direction. Summed population responses
give the actual direction of movement.
Premotor cortex
• Rostral to primary motor cortex
• Extensive reciprocal connections with primary
motor cortex
• Also projects directly to spinal cord (30% of axons
in the pyramidal tract).
“Supplementary” premotor cortex
• mediates selection of movements
• specified by internal rather than external cues.
• Cells fire when just thinking about an event.
Lateral Premotor Cortex• Neurons fire earlier than primary motor cortex.
Important in conditional motor tasks that pair a
movement with a visual cue.
• Neurons fire before initiation of the task. Used for
intentions.
• Lesions in monkey prevent vision-conditioned
tasks, although vision is ok and the task could be
performed in other ways.
Motor cortex areas
The Primary Motor Cortex and Premotor Area in the
Human Cerebral Cortex
Photograph of sagittal section through brain stem and cerebellum
Cerebellar cortex
Cerebellar circuits