The muscles, joints, and ligaments are innervated with sensory receptors that inform the central nervous system about body position and muscle activity. Skeletal muscles contain muscle spindles, Golgi tendon organs, free nerve endings, and some Pacinian corpuscles. Joints contain Ruffini endings and Pacinian corpuscles,- joint capsules contain nerve endings,- ligaments contain Golgi tendon-like organs. Together, these are the proprioceptors, providing sensation from the deep somatic structures. These sensations, which may not reach a conscious level, include the position of the limbs and the force and speed of muscle contraction. They provide the feedback that is necessary for the control of movements.
Muscle spindles provide information about the muscle length and the velocity at which the muscle is being stretched. Golgi tendon organs provide information about the force being generated. Spindles are located in the mass of the muscle, in parallel with the extrafusal muscle fibers. Golgi tendon organs are located at the junction of the muscle and its tendons, in series with the muscle fibers (Fig. 5.4).
Muscle Spindles. Muscle spindles are sensory organs found in almost all of the skeletal muscles. They occur in
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- Nuclear bag fiber
Muscle spindle and Golgi tendon organ structure. A, Muscle spindles are located parallel to extrafusal muscle fibers,- Golgi tendon organs are in series. B, This enlarged spindle shows nuclear bag and nuclear chain types of intrafusal fibers, afferent innervation by Ia axons, which provide primary endings to both types of fibers, type II axons, which have secondary endings mainly on chain fibers, and motor innervation by the two types of gamma motor axons, static and dynamic. C, An enlarged Golgi tendon organ. The sensory receptor endings interdigitate with the collagen fibers of the tendon. The axon is type Ib.
greatest density in small muscles serving fine movements, such as those of the hand, and in the deep muscles of the neck. The muscle spindle, named for its long fusiform shape, is attached at both ends to extrafusal muscle fibers. Within the spindle's expanded middle portion is a fluid-filled capsule containing 2 to 12 specialized striated muscle fibers entwined by sensory nerve terminals. These intra-fusal muscle fibers, about 300 ^m long, have contractile filaments at both ends. The noncontractile midportion contains the cell nuclei (Fig. 5.4B). Gamma motor neurons innervate the contractile elements. There are two types of intrafusal fibers: nuclear bag fibers, named for the large number of nuclei packed into the midportion, and nuclear chain fibers, in which the nuclei are arranged in a longitudinal row. There are about twice as many nuclear chain fibers as nuclear bag fibers per spindle. The nuclear bag type fibers are further classified as bag1 and bag2, based on whether they respond best in the dynamic or static phase of muscle stretch, respectively.
Sensory axons surround both the noncontractile mid-portion and paracentral region of the contractile ends of the intrafusal fiber. The sensory axons are categorized as primary (type Ia) and secondary (type II). The axons of both types are myelinated. Type Ia axons are larger in diameter (12 to 20 |xm) than type II axons (6 to 12 |xm) and have faster conduction velocities. Type Ia axons have spiral shaped endings that wrap around the middle of the intra-fusal muscle fiber (see Fig. 5.4B). Both nuclear bag and nuclear chain fibers are innervated by type Ia axons. Type II axons innervate mainly nuclear chain fibers and have nerve endings that are located along the contractile components on either side of the type Ia spiral ending. The nerve endings of both primary and secondary sensory axons of the muscle spindles respond to stretch by generating action potentials that convey information to the central nervous system about changes in muscle length and the velocity of length change (Fig 5.5). The primary endings temporarily cease generating action potentials during the release of a muscle stretch (Fig. 5.6).
Golgi Tendon Organs. Golgi tendon organs (GTOs) are 1 mm long, slender receptors encapsulated within the tendons of the skeletal muscles (see Fig. 5.4A and C). The distal pole of a GTO is anchored in collagen fibers of the tendon. The proximal pole is attached to the ends of the extrafusal muscle fibers. This arrangement places the GTO in series with the extrafusal muscle fibers such that contractions of the muscle stretch the GTO.
A large-diameter, myelinated type Ib afferent axon arises from each GTO. These axons are slightly smaller in diameter than the type Ia variety, which innervate the muscle spindle. Muscle contraction stretches the GTO and generates action potentials in type Ib axons. The GTO output provides information to the central nervous system about the force of the muscle contraction.
Information entering the spinal cord via type Ia and Ib axons is directed to many targets, including the spinal in-terneurons that give rise to the spinocerebellar tracts. These tracts convey information to the cerebellum about the status of muscle length and tension.
Gamma Motor Neurons. Alpha motor neurons innervate the extrafusal muscle fibers, and gamma motor neurons innervate the intrafusal fibers. Cells bodies of both alpha and gamma motor neurons reside in the ventral horns of the spinal cord and in nuclei of the cranial motor nerves. Nearly one third of all motor nerve axons are destined for intrafusal muscle fibers. This high number reflects the complex role of the spindles in motor system control. Intrafusal muscle fibers likewise constitute a significant portion of the total number of muscle cells, yet they contribute little or nothing to the total force generated when the muscle con-
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