The second part of this tech brief deals with modeling the intrafusal muscle fibers and the sensory neurons. We begin with the mechanical model of the intrafusal muscle fibers. From the “Muscles” tech brief we recall that intrafusal fibers are too weak to play any mechanical role in the force exerted by a muscle. Instead their role has been described as that of a “sophisticated strain gauge.” Figure 1, taken from Matthews, is an illustration of a muscle spindle. A muscle spindle contains two major components: the bag and/or chain fibers, which are innervated by axons from gamma motoneurons and contain contractile elements; and the receptor tissues in which are intertwined nerve ends from the primary (group Ia) or secondary (group II) sensory neurons. Spindle fibers are generally a few mm in length, with large ones sometimes exceeding 10 mm, and they are attached to in parallel to the extrafusal fibers. They are thus subject to the same stretch (or contraction) as the whole muscle. The portion of the spindle containing the receptor makes up about one to two tenths of the total resting length of the spindle.