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 Basic movement, posture Planning, initiation of voluntary movement Simple movement, reflex Sensory-motor integration, motor learning

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 Motor neuron pool

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

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

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