Movement, characteristic of all animals, is the end product of skeletal muscle contraction.

It is orches­trated by the central nervous system (CNS) through its control of the motor unit (see Chapter 6), which has been referred to as movement’s “final common path." To control body movement appropriately, the CNS must (I) assess the effect of gravity on the many muscles of the body, (2) deter­mine the initial position of the body parts to be moved, and (3) detect any discrepancy between the intended movement and the movement that actually occurs.

Two important receptor systems have evolved in the skeletal muscles of mammals to provide the CNS with the afore­mentioned information: the muscle spindle and the Golgi tendon organ (Figure 8-1). The muscle spindles, arranged in parallel to the contracting skeletal muscle fibers, provide information about muscle length. The Go⅛i tendon organ, arranged in series with the contracting skeletal muscle fibers, detects muscle tension. This chapter discusses the anatomy and physiology of these two receptor organs; Chapter 10 discusses how the CNS uses the information gathered from these organs to coordinate posture and locomotion. Some of this informa­tion is used in reflex arcs of the type described in Chapter 7.