Spinal cord circuit and motor control
• Overview of the motor system • Topographic relationship between spinal motor neurons and muscles • Motor unit and muscle force • Reflex circuit
Spinal cord circuit and motor control Planning, initiation of voluntary movement
Sensory-motor integration, Basic movement, posture motor learning
Simple movement, reflex
Dorsal horn
Ventral horn
1 Motor neuron pool Motor neuron pool • In a mature animal, each muscle fiber receives input from a single motor neuron • Each motor neuron innervates multiple muscle fibers • All motor neurons innervating a muscle is called a motor neuron pool • All motor neurons in a motor neuron pool are clustered together in spinal cord muscle
Topographical relationship between motor neurons and muscles
• Motor neuron pools are organized within the ventral horn with those innervating distal muscle groups located laterally and those innervating the proximal muscles located medially
• Different segments of the spinal cord innervate muscles in different parts of the body (see diagram on the right)
Motor Unit •A motor unit consists of a motor neuron and all the muscle fibers it innervates
• Three types: • Fast fatigable (FF) • Powerful, but fatigue with repetitive stimulation • muscle fiber thick, large number • Motor neuron large, fast conduction, but higher firing threshold • Slow (S) • (opposite to FF) • Fast, fatigue-resistant (FR) • Intermediate between FF and S
2 Muscle force
• Firing rate of motor neurons
• Progressive recruitment of motor units •S Æ FR Æ FF
Stretch reflex Muscle stretch Æ activation of dorsal root ganglion neuron (1) Alpha motor neuron activation Æ contraction of same or synergistic muscle Æ{ (2) Interneuron activation Æ inhibition of another motor neuron Æ reduced contraction of antagonist muscle
The dorsal root ganglion neuron (proprioceptor) is activated when intrafusal muscle fiber (spindle) detects changes in muscle length
3 Another example of stretch reflex: Knee jer
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