Neuroscience: Exploring the
Brain, 3e
Chapter 13: Spinal Control of Movement
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Introduction
• Motor Programs
– Motor system: Muscles and neurons that control
muscles
– Role: Generation of coordinated movements
– Parts of motor control
• Spinal cord coordinated muscle contraction
• Brain activate motor programs in spinal cord
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The Somatic Motor System
• Types of Muscles
– Smooth: digestive
tract, arteries, related
structures
– Striated: Cardiac
(heart) and skeletal
(bulk of body muscle
mass)
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Lower Motor Neurons
• Lower motor neuron: in
ventral horn of spinal
cord
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Lower Motor Neurons
• Distribution of lower motor
neurons in the ventral
horn
– Motor neurons
controlling flexors lie
dorsal to extensors
– Motor neurons
controlling axial
muscles lie medial to
those controlling distal
muscles
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Lower Motor Neurons
• Alpha Motor Neurons
– Motor unit: Motor
neuron and all the
muscle fibers it
innervates
– Motor neuron pool: All
the motor neurons that
innervate a single
muscle
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Lower Motor Neurons
• Graded Control of Muscle Contraction by Alpha Motor Neurons
– Varying firing rate of motor neurons
– Recruit additional synergistic motor units
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Lower Motor Neurons
• Inputs to Alpha Motor Neurons
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Lower Motor Neurons
• Types of Motor Units
– Red muscle fibers: Large number of mitochondria and
enzymes, slow to contract, can sustain contraction
– White muscle fibers: Few mitochondria, anaerobic
metabolism, contract and fatigue rapidly
– Fast motor units: Rapidly fatiguing white fibers
– Slow motor units: Slowly fatiguing red fibers
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Lower Motor Neurons
• Neuromuscular Matchmaking
– Crossed Innervation Experiment: John Eccles
• Switch nerve input - switch in muscle phenotype
(physical characteristics)
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Excitation-Contraction Coupling
• Muscle Contraction
– Alpha motor neurons
release ACh
– ACh produces large
EPSP in muscle fiber
– EPSP evokes muscle
action potential
– Action potential triggers
Ca2+ release
– Fiber contracts
– Ca2+ reuptake
– Fiber relaxes
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Spinal Control of Motor Units
• Sensory feedback from muscle spindles - stretch receptor
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Spinal Control of Motor Units
• The Myotatic Reflex
– Stretch reflex: Muscle pulled tendency to pull back
– Feedback loop
– Discharge rate of sensory axons: Related to muscle
length
– Monosynaptic
– e.g., Knee-jerk reflex
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Spinal Control of Motor Units
• The Myotatic Reflex (Cont’d)
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Spinal Control of Motor Units
• Two Types of Muscle Fiber
– Extrafusal fibers:
Innervated by alpha motor
neurons
– Intrafusal fibers:
Innervated by gamma
motor neurons
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Spinal Control of Motor Units
• Gamma Loop
– Keeps spindle “on air”
– Changes set point of the myotatic feedback loop
– Additional control of alpha motor neurons and
muscle contraction
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Spinal Control of Motor Units
• Golgi Tendon Organs
– Additional proprioceptive input - acts like strain
gauge - monitors muscle tension
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Spinal Control of Motor Units
• Golgi Tendon Organs
– Spindles in parallel with fibers; Golgi tendon
organs in series with fibers
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Spinal Control of Motor Units
• Golgi Tendon Organs
– Reverse myotatic reflex function: Regulate muscle
tension within optimal range
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Spinal Control of Motor Units
• Proprioception from the joints
– Proprioceptive axons in joint tissues
– Respond to angle, direction and velocity of
movement in a joint
– Information from joint receptors: Combined with
muscle spindle, Golgi tendon organs, skin receptors
– Most receptors are rapidly adapting, bring
information about a moving joint
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Spinal Control of Motor Units
• Spinal Interneurons
– Synaptic inputs to spinal interneurons:
• Primary sensory axons
• Descending axons from brain
• Collaterals of lower motor neuron axons
• Other interneurons
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Spinal Control of Motor Units
• Inhibitory Input
– Reciprocal inhibition: Contraction of one muscle set
accompanied by relaxation of antagonist muscle
• Example: Myotatic reflex
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Spinal Control of Motor Units
• Excitatory Input
– Flexor reflex:
Complex reflex arc
used to withdraw limb
from aversive
stimulus
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Spinal Control of Motor Units
• Excitatory Input
– Crossed-extensor reflex:
Activation of extensor
muscles and inhibition
of flexors on opposite
side
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Spinal Control of Motor Units
• Generating Spinal Motor Programs for Walking
– Circuitry for walking resides in spinal cord
– Requires central pattern generators
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Spinal Control of Motor Units
• Rhythmic Activity in a Spinal Interneuron via NMDA
Receptors
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Spinal Control of Motor Units
• Possible Circuit for Rhythmic Alternating Activity
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Central Pattern Generators: Pyloric Rhythm & Endogenous
burster (Pacemaker) Neurons
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Dogfish Swimming: Reafferent modulation of CPG Rhythm.
Tail
movement
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Concluding Remarks
• Spinal control of movement
– Different levels of analysis
– Sensation and movement linked
• Direct feedback
– Intricate network of circuits
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End of Presentation
Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins
Lower Motor Neurons
• Somatic Musculature and
distribution of lower motor
neurons in spinal cord
– Axial muscles: Trunk
movement
– Proximal muscles:
Shoulder, elbow, pelvis,
knee movement
– Distal muscles: Hands,
feet, digits (fingers and
toes) movement
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Excitation-Contraction Coupling
• The Molecular Basis of Muscle Contraction
– Z lines: Division of myofibril into segments by
disks
– Sarcomere: Two Z lines and myofibril
– Thin filaments: Series of bristles
– Thick filaments: Between and among thin
filaments
– Sliding-filament model:
• Binding of Ca2+ to troponin causes myosin to
bind to action
• Myosin heads pivot, cause filaments to slide
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Excitation-Contraction Coupling
Sliding-filament Model of Muscle Contraction
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Excitation-Contraction Coupling
• Steps in Excitation-Contraction Coupling
• Ca+ binding to troponin allows myosin heads
to bind to actin. Then myosin heads pivot,
causing filaments to slide
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Spinal Control of Motor Units