Stretch Receptors

Stretch receptors are proprioceptors that send information to the spinal cord and brain concerning the lengths and tensions of muscles. The two main kinds of stretch receptors are muscle spindles and Golgi ten don organs; however, no sensation results when they are stimulated.

Muscle spindles are located in skeletal muscles near their junctions with tendons. Each spindle consists of one or more small, modified skeletal muscle fibers (intrafusal fibers) enclosed in a connective tissue sheath. Near its center, each intrafusal fiber has a specialized nonstriated region with the end of a sensory nerve fiber wrapped around it (fig. 12.5a).

The striated portions of the intrafusal fiber contract to keep the spindle taut at different muscle lengths. Thus, if the whole muscle is stretched, the muscle spindle is also stretched, triggering sensory nerve impulses on its nerve fiber. Such sensory impulses travel into the spinal cord and onto motor fibers leading back to the same muscle, contracting it. This action, called a stretch reflex, opposes the lengthening of the muscle and helps

Blood Vessel Receptors

Figure

(a) Increased muscle length stimulates muscle spindles, which stimulate muscle contraction. (b) Golgi tendon organs occupy tendons, where they inhibit muscle contraction.

Figure

(a) Increased muscle length stimulates muscle spindles, which stimulate muscle contraction. (b) Golgi tendon organs occupy tendons, where they inhibit muscle contraction.

maintain the desired position of a limb in spite of gravitational or other forces tending to move it (see chapter 11, p. 403).

Golgi tendon organs are found in tendons close to their attachments to muscles. Each is connected to a set of skeletal muscle fibers and is innervated by a sensory neuron (fig. 12.5fo). These receptors have high thresholds and are stimulated by increased tension. Sensory impulses from them produce a reflex that inhibits contraction of the muscle whose tendon they occupy. Thus, the Golgi tendon organs stimulate a reflex with an effect opposite that of a stretch reflex. The Golgi tendon reflex also helps maintain posture, and it protects muscle attachments from being pulled away from their insertions by excessive tension. Table 12.2 summarizes the somatic receptors and their functions.

H Describe a muscle spindle.

Explain how muscle spindles help maintain posture.

Where are Golgi tendon organs located?

What is the function of Golgi tendon organs?

tmyW/l Somatic

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