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 4.