Spinal Sensorimotor System Part II:

These are the neurons that actually drive the muscle cells. There are five classes, designated as alpha ( -MNs), static beta ( s-MNs), dynamic beta ( d-MNs), static gamma ( s-MNs), and dynamic gamma ( d-MNs). The -MNs are the main drive neurons for the extrafusal muscle fibers, and hence are known as slow (S), fast-resistant to fatigue (FR), and fast-fatiguing (FF) skeleto-motoneurons. The -MNs are actually small -MNs with very small-diameter axons. To date they have not been studied extensively, but it is believed that they share many of the same types of inputs as the -MNs. The innervation of the muscle by -MNs was illustrated in figure 6 of the “Muscles” tech brief. In recognition of their connection to both extrafusal and intrafusal fibers, they are known as skeleto-fusimotor neurons. The -MNs innervate the intrafusal spindle fibers, thereby exercising direct control on the biasing and sensitivity of the muscle afferent sensors of groups Ia and II. Their connection to muscle was also illustrated in figure 6 of “Muscles”. These motoneurons are frequently called fusimotor neurons. All motoneurons belong to what we called the motoneuron level of spinal neural network organization in Part I of this tech brief.