The Muscle Spindle

The driving force on lower motor neurons comes from three major sources: (1) sensory pathways from the spinal cord and brain stem that trigger reflex actions, (2) interneurons within the spinal cord that interconnect synergistic and antagonistic motor neuron pools, and (3) upper motor neurons in the motor cortex and other motor areas in the brain that provide complex motor commands. One of the major sensory inputs to the lower motor neuron is derived from specialized end organs located within the muscle itself. Two types of specialized sensory endings are present within muscle tissue, and they provide feedback control over firing rates of the alpha motor neuron. The first type, the muscle spindle, consists of a group of fine muscle fibers 4 to 10 mm long encapsulated by a fusiform or spindle-shaped connective tissue sheath (Fig. 1). The ends of the sheath are attached to adjacent muscle fibers. The central portion of the individual intrafusal muscle fiber is encircled by a specialized extrafusal muscle fiber extrafusal muscle fiber

Extrafusal Muscle Fibers

Golgi tendon organ Ib afferent

FIGURE 1 Organization of the muscle spindle and Golgi tendon organ. Muscle spindles are small, fusiform structures embedded in skeletal muscles (extrafusal muscle fibers) that monitor the length of the muscle. Spindles contain small intrafusal muscles, each with their centrally located nuclei arranged either in a cluster called a nuclear bag or in a nuclear chain. A spindle contains about four nuclear bags and two nuclear chain fibers enclosed in a capsule. (A) Two types of sensory neurons (shown in blue) innervate the spindle: (1) the annulospiral ring, or Ia afferents, sensitive to both the absolute muscle length and rate of change in length during contractions; and (2) the flower spray, or II afferents, which monitor static length of the muscle. Muscle spindles are also innervated by small myelinated neurons called gamma afferents that directly stimulate intrafusal fibers (but not extrafusal muscles) to contract and thereby regulate the tension on the annulospiral ring and flower spray afferents. (B) A second type of sensory structure, the Golgi

Golgi tendon organ Ib afferent

FIGURE 1 Organization of the muscle spindle and Golgi tendon organ. Muscle spindles are small, fusiform structures embedded in skeletal muscles (extrafusal muscle fibers) that monitor the length of the muscle. Spindles contain small intrafusal muscles, each with their centrally located nuclei arranged either in a cluster called a nuclear bag or in a nuclear chain. A spindle contains about four nuclear bags and two nuclear chain fibers enclosed in a capsule. (A) Two types of sensory neurons (shown in blue) innervate the spindle: (1) the annulospiral ring, or Ia afferents, sensitive to both the absolute muscle length and rate of change in length during contractions; and (2) the flower spray, or II afferents, which monitor static length of the muscle. Muscle spindles are also innervated by small myelinated neurons called gamma afferents that directly stimulate intrafusal fibers (but not extrafusal muscles) to contract and thereby regulate the tension on the annulospiral ring and flower spray afferents. (B) A second type of sensory structure, the Golgi mechanoreceptor nerve ending that forms a springlike configuration called the annulospiral ring. Changes in the length of fibers within the main muscle mass during contraction or relaxation passively change the tension on the intrafusal fibers through the connective tissue attachment site. This change causes compensatory stretching or relaxation of the sensory endings of the annulospiral ring. Stretching the annulospiral ring activates mechano-sensitive channels, depolarizes the sensory neuron, and increases its firing rate. The opposite effect is observed when tension on the annulospiral ring is decreased by contraction of the main muscle mass. Thus, the firing rate of the sensory nerve encodes information about the degree of muscle stretch, which is then sent back to the appropriate alpha motor neuron. In this way, the motor neuron is constantly informed about the changes in muscle length that occur in response to any previous stimulation. Because of their small size, intrafusal fibers of the muscle spindle do not make a direct contribution to the force of muscle contraction; nevertheless, their constant monitoring of the efficacy of neuronal activation of the muscle is a central component of many muscle reflexes.

Several different types of intrafusal muscle fibers can be distinguished on the basis of cellular morphology and innervation patterns. Nuclear chain intrafusal fibers have nuclei that are arranged in a chainlike configuration, whereas nuclear bag fibers have nuclei that are clumped in a baglike fashion near the center of the fiber. Both nuclear chain and nuclear bag fibers are innervated by annulo-spiral rings from large, myelinated fibers, classified as group Ia or primary afferents. These sensory neurons adapt rapidly to changes in mechanoreceptor activation and thus act as rate detectors. They signal alpha motor neurons about the rate of change in muscle length and, to a lesser degree, the absolute length resulting from contraction or relaxation.

All nuclear chain fibers and one subtype of nuclear bag fibers receive a second type of innervation, classified as group II afferents. They are medium-diameter myelinated nerves with endings in a ''flower spray'' arrangement. They tend to be tonically active and adapt slowly, continuing to fire at a characteristic frequency that encodes the degree of stretch affecting the mechanor-eceptors in the annulospiral ring. In this way, the group II afferents provide the alpha motor neuron with information about static muscle length, in contrast to the Ia fibers, which primarily provide information about the rate of change in muscle length.

tendon, is inserted between the ends of extrafusal muscle fibers and their attachments to bone. The connective tissue of the Golgi tendon organ is innervated by Ib afferents that monitor the force developed by the muscle during both contraction and passive stretch.

Group Ia fibers from the muscle spindle provide the afferent arm of an important spinal motor reflex, the stretch or myotactic reflex. Essentially, they provide the pathway by which a muscle automatically contracts immediately after it has been stretched. This reflex can be overridden by input from higher cortical centers in order to carry out purposeful movements; otherwise, it operates as a homeostatic device to resist an imposed stretch. One of its important functions is to maintain a given muscle length in order to balance the body against the pull of gravity.

The myotactic reflex pathway is monosynaptic, consisting of direct excitatory connections between Ia afferents from the muscle spindle and all the alpha motor neurons of the same or homonymous muscle. In response to Ia excitation, the motor neuron pool will fire and cause contraction of the muscle. For example, if an individual is balanced at the top of a stairway and inadvertently leans forward, the resulting stretch on the extensor muscles in the back of the leg will activate Ia fibers within the muscle and initiate a reflex contraction of the same muscle, thus returning the body to its upright position and avoiding a fall. This protective reflex relies only on the simple spinal cord stretch reflex pathway and does not require involvement of brain stem or cortical structures. In addition to its role in the stretch reflex, information from primary Ia fibers also assists higher motor centers in more complex motor computations.

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Responses

  • eulalia
    What causes the generation of action potential in the muscle spindle?
    5 months ago

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