Nuclear bag fiber

Nuclear Bag Fiber

File:Nuclear bag fiber diagram.jpg

Diagram of a nuclear bag fiber

The Nuclear Bag Fiber is a type of muscle fiber found in intrafusal muscle fibers, which are part of the muscle spindle that is involved in the sensation of muscle stretch. It is named for its distinctive appearance, with nuclei clustered in a central "bag".

Structure

Nuclear bag fibers are larger than nuclear chain fibers, another type of intrafusal muscle fiber. They are characterized by a central region, or "bag", where the nuclei of the fiber are clustered. This region is surrounded by a capsule. The bag is flanked by non-contractile regions that contain sensory nerve endings.

Function

Nuclear bag fibers are sensitive to both the rate and degree of muscle stretch. They are innervated by both Type Ia sensory fibers and Type II sensory fibers. Type Ia fibers wrap around the central region of the nuclear bag fiber and are particularly sensitive to the rate of muscle stretch. Type II fibers innervate the ends of the fiber and are sensitive to the degree of stretch.

Clinical Significance

Abnormalities in nuclear bag fibers can contribute to muscle weakness and muscle wasting, as well as disorders of muscle tone such as hypotonia and hypertonia.

See Also

• Muscle spindle
• Intrafusal muscle fiber
• Nuclear chain fiber