circuit and motor control

• Overview of the motor system • Topographic relationship between spinal motor 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 pool 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 neuron (1) activation Æ contraction of same or synergistic muscle Æ{ (2) 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