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Even when at rest the fibers of a muscle are
at least partially contracted, possessing a small degree of tension which is
termed muscle tone or tonus. Muscle tone is controlled by neuronal impulses and
is influenced by receptors found in the muscle and tendons.
This influence leads to the generation of
reflexes in the spinal cord, such as the immediately obvious knee jerk but also
including key functions such as the maintenance of posture or the proper function
of the digestive system.
The main regulator of muscle tone is the
muscle spindle, a small sensory unit that is closely associated with, and lies parallel
to a muscle. Connecting to the endomysium of a muscle fiber, muscle spindles are composed of
nuclear bag fibers and nuclear chain fibers. Both are similar to muscle fibers
in that they contain actin and myosin myofilaments which allows them to stretch
with the muscle. However, unlike skeletal muscle fibers where the nuclei are spread
out and located at the periphery of the cell, in nuclear bag and nuclear chain
fibers the nuclei are located in a central region which is enlarged in nuclear
Both cells of the muscle spindle contain
sensory neurons and when stretched muscle spindles become activated triggering impulses
to the spinal cord which can generate an immediate reflex. Spindles can also
trigger impulses to the cerebral cortex providing information about the degree
of stretch within the muscle.
To maintain tone spindles also operate a
feedback loop through by directly triggering motor neurons linked to their
associated muscle. If tone decreases and the muscle stretches the spindles
trigger an impulse resulting in the contraction of the muscle. With this contraction
the spindle is no longer stretched and so stops triggering further contraction.
A similar system is found in the tendons
attaching muscle to bone where distinct stretch receptors termed golgi tendon
organs assess the level of stretch within the tendon. The sensitivity of the
golgi tendon organ is significantly less than that of the spindle and so it is
thought they exist to prevent damage rather than control muscle tone.
Smooth and Cardiac Muscles
Smooth and cardiac muscles do not have specialized
muscle spindles, rather tone is maintained through autonomous feedback from the
muscle fibers, neurons and associated tissues.