Encapsulated sensory receptors in muscles and tendons provide information about the degree of tension in a muscle and its position.
The muscle spindle is the specialized stretch receptor located within the skeletal muscle
The muscle spindle is a specialized receptor unit in muscle; it consists of two types of modified muscle fibers called spindle cells and neuron terminals (Fig. 10.11). Both types of modified muscle fibers are surrounded by an internal capsule. A fluid-filled space separates the internal capsule from an outer external capsule. One type of spindle cell, the nuclear bag fiber, contains an aggregation of nuclei in an expanded midregion; the other type, called a nuclear chain fiber, has many nuclei arranged in a chain. The muscle spindle transmits information about the degree of stretching in a muscle. The sensory (afferent) nerve fibers carrying information from the muscle spindle have endings that are spirally arranged around the midregion of both types of spindle cells. In addition, spindle cells receive motor (efferent) innervation from the spinal cord and brain by 7 efferent nerve fibers, which are thought to regulate the sensitivity of the nerve nerve
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