Spinal Control of Movement

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Spinal Control of Movement

• Lecture 21

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Spinal Motor Circuits

• Local control of stereotyped movement
• reflexes rhythmic motor patterns
• based on spinal interneurons a motor neurons
• Same circuits used for voluntary movements
• controlled by descending fibers
• upper motor neurons

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Spinal Motor Circuits

• Sensory feedback
• somatosensory receptors
• cutaneous receptors
• Golgi tendon organs
• muscle spindles
• intrafusal fibers
• g motor neurons

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Spinal Cord

• Muscles their neurons not evenly distributed
• C3 - T1 Arms
• cervical brachial plexuses
• L1 - S3 Legs
• lumbar sacral plexuses
• Spinal cord enlarged
• more neurons in ventral horns

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a Motor Neurons

• Cell body in ventral horn
• Emerge from ventral root
• Innervate extrafusal fibers
• Uninterrupted to muscle fibers
• final common pathway
• Only excitatory input to muscles
• Inhibition at spinal cord
• Cholinergic

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ACh
a motor neuron
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Input to Alpha Motor Neurons

• 3 sources only
• 1. DRG neurons
• sensory neurons (proprioception)
• feedback from muscle spindles
• 2. Upper motor neurons
• primarily from M1
• 3. Spinal interneurons
• most input (excitatory inhibitory)
• generation of motor programs

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Inputs to Alpha Motor Neurons
Sensory neurons
Dorsal
DRG
Spinal interneurons
Upper motor neurons - M1
Ventral
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Movement of Limbs

• Flexors and extensors are ANTAGONISTIC
• muscles and are reciprocally innervated
• Limb flexion
• flexors excited extensors inhibited
• Limb extension
• extensors excited flexors inhibited
• Disynaptic inhibition

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Upper Motor Neurons

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Monosynaptic Stretch Reflex

• Postural adjustments
• Muscle tone
• knee-jerk reflex
• Muscle spindles
• length detectors
• Low threshold
• Sensory neuron ---gt alpha motor neurons
• monosynaptic excitation
• disynaptic inhibition

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Intrafusal Muscle Fibers

• Control tension on muscle spindles
• Not force for movement
• Innervated by g motor neurons
• active concurrently with a motor neurons
• Load the Spindles

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M S




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APs
tension
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Muscle Stretched
APs
tension
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Muscle Contracts
APs
contraction
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Muscle Contracts
APs
contraction
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Muscle Contraction

• alpha ----gt extrafusal
• gamma ----gt intrafusal
• alpha gamma fire together

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M S

gamma

alpha


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Polysynaptic reflexes
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Myotactic Reflex

• Golgi Tendon Organ
• Gauges muscle tension
• controlled contractions
• safety mechanism
• Inhibits a motor neurons
• disynaptic inhibition

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Dorsal
Inhibits a motor neuron
-

Ventral
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Withdrawal Reflex

• Flexion
• remove limb from noxious stimulus
• Polysynaptic reflex
• sensory neuron
• interneurons
• motor neuron
• 2 or more synapses
• slower than monosynaptic

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Polysynaptic withdrawal reflex
R
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Dorsal




Ventral
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Withdrawal Reflex

• also influences other muscles
• e.g. Step on tack
• flexion of ipsilateral limb
• simultaneous extension of contralateral leg
• excite extensor
• inhibit flexor

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Generation of Rhythmic Motor Patterns
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Central Pattern Generators

• Half-center Model
• alternating activity in flexor extensor
• Step-cycle has 2 phases
• swing phase
• foot off ground flexing forward
• stance phase
• foot planted leg extending
• Each limb has own pattern generator

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Half-center Model
Flexor
a
Tonic input
a
Extensor
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Rhythmic Patterns Sensory Feedback

• Not necessary for locomotion
• but slower, less coordinated
• Stumble correction reaction
• during swing phase
• tactile stimulus on dorsal foot ---gt flexion
• Reflex reversal
• override during extension
• flexion would cause collapse

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Goal-directed Locomotion

• Requires intact supraspinal systems
• Coordination of antigravity muscles
• Brainstem motor nuclei
• modulation of contraction strength
• Reticulo-, rubro-, corticospinal
• Balance
• Vestibulospinal tract (Ventromedial)
• Also cortical subcortical areas

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Movement DisordersSpinal Cord
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Spinal Cord Pathology

• Poliomyelitis
• virus attacks motor neurons
• wasting of muscles, paralysis
• may have virus but not polio
• still contagious
• vaccine, but still cases
• concerns of developing in tropics

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Spinal Cord Pathology

• Trauma
• bruised or severed
• injury worsened by
• inflamation
• endorphins
• excitoxicity
• Treatment - regeneration
• implant peripheral nerve
• neural tissue transplants