Somatic Motor Function and Motor Neurons

outer (nonvisceral) regions of the body
somatic motor portion of the peripheral nervous system
responsible for carrying neural messages from the spinal cord to skeletal muscle fibers.
neural messages
signals for muscular contraction to occur
motor neuron
somatic neuron that innervates skeletal muscle fibers. alpha motor neuron
motor unit
each motor neuron and all the muscle fibers that it innervates
innervation ratio
the number of muscle fibers innervated by a single motor neuron. low for muscle groups that require fine motor control (23/1), high for large muscles not involved in fine motor control (1000/1)
motor unit recruitment
progressive activation of more and more motor neurons to increase the force of muscle contraction
size principle
orderly and sequential recruitment or larger motor units. first motor units to fire are small. when more force is required, there is a progressive increase in the recruitment of more large motor neurons to increase muscle force production
type s (slow)
small motor neurons innervate the slow and high oxidative muscle fibers. fibers are called type 1 muscle fibers
type FR (fast, fatigue resistant)
larger motor neurons innervate the intermediate muscle fibers called type 2a fibers
type FF (fast, fatigable)
largest motor neurons and innervate the fast muscle fibers called type 2x fibers