Nuclear bag fiber

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Nuclear bag fiber
Details
Identifiers
Latin myofibra sacculiformis
TH H3.03.00.0.00013
Anatomical terms of microanatomy

A nuclear bag fiber is one of two types of intrafusal muscle fibers in muscle spindles (the other being the nuclear chain fibers), so named because its muscle cell nuclei are congregated into a bulged section ("bag") at the middle portion of the fiber. It is innervated solely by type Ia sensory fibers (whereas nuclear chains fibers are innervated by both type Ia and type II sensory fibers) and responds to dynamic changes in muscle stretch. [1]

Contents

The diameter of nuclear bag fibers is about twice that of nuclear chain fibers. Each muscle spindle contains 1–3 nuclear bag fibers. [1]

The tautness of nuclear chain fibers is adjusted mainly by dynamic γ motor neurons to optimise the dynamic response of the muscle spindle. [2]

Types

There are two kinds of bag fibers based upon contraction speed and motor innervation:

  1. BAG2 fibers are the largest. They have no striations in middle region and swell to enclose nuclei.
  2. BAG1 fibers, smaller than BAG2.

Both bag types extend beyond the spindle capsule.

See also

References

  1. 1 2 Hall, John E.; Hall, Michael E. (2021). "Chapter 55 - Spinal Cord Motor Functions; the Cord Reflexes". Guyton and Hall Textbook of Medical Physiology (14th ed.). Philadelphia, PA: Elsevier. ISBN   978-0-323-59712-8.
  2. ↑ Hall, John E.; Hall, Michael E. (2021). "Chapter 55 - Spinal Cord Motor Functions; the Cord Reflexes". Guyton and Hall Textbook of Medical Physiology (14th ed.). Philadelphia, PA: Elsevier. ISBN   978-0-323-59712-8.


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