Download Neuromuscular spindle The central nervous system continuously

Neuromuscular spindle
The central nervous system continuously monitors the position of the limbs and the
state of contraction of the various muscles. Muscles have a specialized encapsulated
sensor called the neuromuscular spindle that contains sensory and motor
components (Figure 7-16).
A neuromuscular spindle consists of 2 to 14 specialized striated muscle fibers
enclosed in a fusiformsheath or capsule of connective tissue. They are 5 to 10 mm
long and therefore much shorter than the surrounding contractile muscle fibers.
The specialized muscle fibers in the interior of the neuromuscular spindle are called
intrafusal fibers to distinguish them from the non-specialized extrafusal fibers
(Latin extra, outside; fusus, spindle), the regular skeletal muscle fibers.
There are two kinds of intrafusal fibers designated by their histological appearance:
(1) nuclear bag fiber, consisting of a central sensory (non-contractile) bag-like
region, and (2) the nuclear chain fiber, so-called because its central portion
contains a chain-like array of nuclei. The distal portion of both nuclear bag and
nuclear chain fibers consists of striated muscle with contractile properties.
The neuromuscular spindle is innervated by two types of afferent axons making
contact with the central (receptor) region of the intrafusal fibers.
Two types of anterior motor neurons of the spinal cord give rise to motor nerve fibers:
the large-diameter alpha motor neurons innervate the extrafusal fibers of muscles;
the small-diameter gamma motor neuronsinnervate the intrafusal fibers in the
spindle.
Sensory nerve endings are arranged around the central nuclear region and sense the
degree of tension of the intrafusal fibers.
The intrafusal muscle fibers of the neuromuscular spindle are in parallel with the
extrafusal muscle fibers. When the extrafusal muscle fibers contract (shorten), the
neuromuscular spindle becomes slack. If the spindle remains slack, no further
information about changes in muscle length can be transmitted to the spinal cord.
This situation is corrected by a feedback control mechanism by which the sensory
region of the spindle activates gamma motor neurons, which contract the poles of the
spindle (the contractile region). Consequently, the spindle stretches.
In addition to the neuromuscular spindle, Golgi tendon organs, located in series
with the extrafusal muscle fibers, provide information about the tension or force of
contraction of the skeletalmuscle.
The neuromuscular spindle is an example of a proprioceptor (Latin, proprius, one's
own; capio, to take), a structure that informs how the body is positioned and moves in
space.