Muscle Spindle Apparatus

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In order for the nervous system to control skeletal movements properly, it must receive continuous sensory feedback information concerning the effects of its actions. This sensory information includes (1) the tension that the muscle exerts on its tendons, provided by the Golgi tendon organs, and (2) muscle length, provided by the muscle spindle apparatus. The spindle apparatus, so called because it is wider in the center and tapers toward the ends, functions as a length detector. Muscles that require the finest degree of control, such as the muscles of the hand, have the highest density of spindles.

Each spindle apparatus contains several thin muscle cells, called intrafusal fibers (fusus = spindle), packaged within a connective tissue sheath. Like the stronger and more numerous "ordinary" muscle fibers outside the spindles—the extrafusal fibers—the spindles insert into tendons on each end of the muscle. Spindles are therefore said to be in parallel with the extrafusal fibers.

Unlike the extrafusal fibers, which contain myofibrils along their entire length, the contractile apparatus is absent from the central regions of the intrafusal fibers. The central, noncontracting part of an intrafusal fiber contains nuclei. There are two types of intra-fusal fibers. One type, the nuclear bag fibers, have their nuclei arranged in a loose aggregate in the central regions of the fibers. The other type of intrafusal fibers, called nuclear chain fibers, have their nuclei arranged in rows. Two types of sensory neurons serve these intrafusal fibers. Primary, or annulospiral, sensory endings wrap around the central regions of the nuclear bag and chain fibers (fig. 12.26), and secondary, or flower-spray, endings are located over the contracting poles of the nuclear chain fibers.

Since the spindles are arranged in parallel with the extrafusal muscle fibers, stretching a muscle causes its spindles to stretch. This stimulates both the primary and secondary sensory endings. The spindle apparatus thus serves as a length detector because the frequency of impulses produced in the primary and secondary endings is proportional to the length of the muscle. The primary endings, however, are most stimulated at the onset of stretch, whereas the secondary endings respond in a more tonic (sustained) fashion as stretch is maintained. Sudden, rapid stretching of a muscle activates both types of sensory endings, and is thus a more powerful

Muscle 349

Skeletal muscle

Peripheral nerve (motor and sensory nerve fibers)

Skeletal muscle

Peripheral nerve (motor and sensory nerve fibers)

Flower Spray Endings

Extrafusal fibers

Intrafusal fibers:

Nuclear chain fibers

Secondary fiber

Flower-spray endings

Efferent nerve fibers (motor):

Gamma fiber

Waldrop

Extrafusal fibers

Intrafusal fibers:

Nuclear chain fibers

Nuclear bag fiber

Connective tissue sheath

Afferent nerve fibers (sensory):

Primary fiber

Annulospiral endings

Secondary fiber

Flower-spray endings

Efferent nerve fibers (motor):

Gamma fiber

Alpha fiber

Motor end plates

Waldrop

Nuclear bag fiber

Connective tissue sheath

Afferent nerve fibers (sensory):

Primary fiber

Annulospiral endings

Alpha fiber

Motor end plates

Muscle Spindle Apparatus

■ Figure 12.26 The location and structure of a muscle spindle. (a) A muscle spindle within a skeletal muscle. (b) The structure and innervation of a muscle spindle.

stimulus for the muscle spindles than a slower, more gradual stretching that has less of an effect on the primary sensory endings. Since the activation of the sensory endings in muscle spindles produces a reflex contraction, the force of this reflex contraction is greater in response to rapid stretch than to gradual stretch.

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Responses

  • Fiorella
    What is a muscle spindle apparatus?
    8 years ago
  • BO
    What is a sensory and motor nerve fibres endings?
    8 years ago
  • eric
    What type of sensory neuron is associated with a muscle spindle apparatus?
    8 years ago
  • heribald
    Which tissue types are components of the spindle apparatus?
    8 years ago
  • izaak
    What is the function of the muscle spindle apparatus?
    8 years ago
  • tewelde
    Is the muscle spindle arranged parallel to the extra fusal fibers?
    8 years ago
  • Markus
    What are primary and secondary sensory endings?
    7 years ago
  • James
    Which muscles contain spindle shaped cells?
    7 years ago
  • elsa
    What is the muscle spindle apparatus associatedwith?
    6 years ago
  • Charles
    WHAT IS THE ROLE OF SPINDLE APPARATUS IN MUSCLE FUNCTION?
    5 years ago

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