Nuclear chain fiber
Encyclopedia
A nuclear chain fiber is a specialized sensory
organ
contained within a muscle
. Nuclear chain fibers are intrafusal fibers which, along with nuclear bag fiber
s, make up the muscle spindle
responsible for the detection of changes in muscle length.
There are 3–9 nuclear chain fibers per muscle spindle that are half the size of the nuclear bag fiber
s. Their nuclei are aligned in a chain and they excite the secondary nerve. They are static while the nuclear bag fibers are dynamic in comparison. The name "nuclear chain" refers to the structure of the central region of the fiber, where the sensory axons wrap around the intrafusal fibers.
The secondary nerve association involves an efferent and afferent pathway that measure the stress and strain placed on the muscle (usually the extrafusal fibers connected from the muscle portion to a bone). The afferent pathway resembles a spring wrapping around the nuclear chain fiber and connecting to one of its ends away from the bone. Again, depending on the stress and strain the muscles sustains, this afferent and efferent coordination will measure the "stretch of the spring" and communicate the results to the central nervous system
.
Also note that a similar structure attaching one end to muscle and the other end to a tendon is known as a Golgi tendon organ. However, Golgi tendon organs differ from nuclear chain and nuclear bag fibers in that they are considered in series rather than in parallel to the muscle fibers.
innvervation and receive efferent
innervation. The afferent innervation is via Group II and Ia neurons. These project to the nucleus proprius
in the dorsal horn of the spinal cord
. Efferent innervation is via static γ neurons
. Stimulation of γ neurons causes the nuclear chain to shorten along with the extrafusal muscle fibers. This shortening allows the nuclear chain fiber to be sensitive to changes in length while its corresponding muscle is flexed.
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
contained within a muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
. Nuclear chain fibers are intrafusal fibers which, along with nuclear bag fiber
Nuclear bag fiber
A nuclear bag fiber is a type of intrafusal muscle fiber that lies in the center of a muscle spindle. Each has a large number of nuclei concentrated in bags and they cause excitation of both the primary and secondary nerve fibers....
s, make up the muscle spindle
Muscle spindle
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...
responsible for the detection of changes in muscle length.
There are 3–9 nuclear chain fibers per muscle spindle that are half the size of the nuclear bag fiber
Nuclear bag fiber
A nuclear bag fiber is a type of intrafusal muscle fiber that lies in the center of a muscle spindle. Each has a large number of nuclei concentrated in bags and they cause excitation of both the primary and secondary nerve fibers....
s. Their nuclei are aligned in a chain and they excite the secondary nerve. They are static while the nuclear bag fibers are dynamic in comparison. The name "nuclear chain" refers to the structure of the central region of the fiber, where the sensory axons wrap around the intrafusal fibers.
The secondary nerve association involves an efferent and afferent pathway that measure the stress and strain placed on the muscle (usually the extrafusal fibers connected from the muscle portion to a bone). The afferent pathway resembles a spring wrapping around the nuclear chain fiber and connecting to one of its ends away from the bone. Again, depending on the stress and strain the muscles sustains, this afferent and efferent coordination will measure the "stretch of the spring" and communicate the results to the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
.
Also note that a similar structure attaching one end to muscle and the other end to a tendon is known as a Golgi tendon organ. However, Golgi tendon organs differ from nuclear chain and nuclear bag fibers in that they are considered in series rather than in parallel to the muscle fibers.
Innervation
As intrafusal muscle fibers, nuclear chain fibers both send afferentAfferent
Afferent is an anatomical term with the following meanings:*Conveying towards a center, for example the afferent arterioles conveying blood towards the Bowman's capsule in the Kidney. Opposite to Efferent.*Something that so conducts, see Afferent nerve fiber...
innvervation and receive efferent
Efferent
Efferent is an anatomical term with the following meanings:*Conveying away from a center, for example the efferent arterioles conveying blood away from the Bowman's capsule in the kidney. Opposite to afferent....
innervation. The afferent innervation is via Group II and Ia neurons. These project to the nucleus proprius
Nucleus proprius
The Nucleus proprius is a layer of the spinal cord adjacent to the substantia gelatinosa. Nucleus proprius constitutes the bulk of the dorsal horn and receives inputs from the dorsal root ganglions that carry sensory information, such as light touch, as well as pain and temperature information...
in the dorsal horn of the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
. Efferent innervation is via static γ neurons
Gamma motoneurons
Gamma motoneurons , also called gamma motor neurons, are the efferent component of the fusimotor system, the system by which the central nervous system controls and modifies muscle spindle sensitivity. The fusimotor system refers to the combination of muscle spindles and γ-motoneurons...
. Stimulation of γ neurons causes the nuclear chain to shorten along with the extrafusal muscle fibers. This shortening allows the nuclear chain fiber to be sensitive to changes in length while its corresponding muscle is flexed.