Spinal cord circuit and motor control Overview of

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, motor learning Simple movement, reflex Basic movement, posture

Dorsal horn Ventral horn

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

Muscle force • Firing rate of motor neurons • Progressive recruitment of motor units • S FR FF

Stretch reflex Muscle stretch activation of dorsal root ganglion neuron { (1) Alpha motor neuron activation contraction of same or synergistic muscle (2) Interneuron activation inhibition of another motor neuron reduced contraction of antagonist muscle

The dorsal root ganglion neuron (proprioceptor) is activated when intrafusal muscle fiber (spindle) detects changes in muscle length

Another example of stretch reflex: Knee jerk