Figure 123

Convergence of axons onto a local interneuron. %

muscles, and (3) the tendons, joints, and skin surrounding the muscles.

These receptors monitor the length and tension of the muscles, movement of the joints, and the effect of movements on the overlying skin. In other words, the movements themselves give rise to afferent input that, in turn, influences the movements via negative feedback. As we shall see next, their input not only provides negative-feedback control over the muscles but contributes to the conscious awareness of limb and body position as well.

Length-Monitoring Systems Absolute muscle length and changes in muscle length are monitored by stretch receptors embedded within the muscle. These receptors consist of peripheral endings of afferent nerve fibers that are wrapped around modified muscle fibers, several of which are enclosed in a connective-tissue capsule. The entire structure is called a muscle spindle (Figure 12-4). The modified muscle fibers within the spindle are known as intrafusal fibers, whereas the skeletal-muscle fibers that form the bulk of the muscle and generate its force and movement are the extrafusal fibers.

Within a given spindle, there are two kinds of stretch receptors: One responds best to how much the muscle has been stretched, the other to both the

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Golgi Tendon Organs

Capsule

Intrafusal muscle fibers

Stretch receptor

Extrafusal muscle fiber

Golgi tendon organ

Tendon

FIGURE 12-4

A muscle spindle and Golgi tendon organ. Note that the muscle spindle is parallel to the extrafusal muscle fibers. The Golgi tendon organ will be discussed later in the chapter.

Adapted from Elias, Pauly, and Burns.

(a) Muscle stretch

Capsule n

Intrafusal muscle fibers

Stretch receptor

Extrafusal muscle fiber

Golgi Tendon Organ

Golgi tendon organ

Tendon

FIGURE 12-4

A muscle spindle and Golgi tendon organ. Note that the muscle spindle is parallel to the extrafusal muscle fibers. The Golgi tendon organ will be discussed later in the chapter.

Adapted from Elias, Pauly, and Burns.

Time

Time

(b) Muscle contraction

Muscle spindle Stretch receptor

Afferent nerve fiber

Extrafusal muscle fiber

Intrafusal muscle fiber

Action potential response

Contraction

Time

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