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Even when at rest, muscle fibers 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
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 reaction but also
including key functions such as the posture maintenance and proper digestive system function..
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 that allow 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. When stretched, muscle spindles become activated, triggering impulses
to the spinal cord that 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 by directly triggering motor neurons linked to their
associated muscles. If tone decreases and the muscle stretches the spindle, an impulse results in a muscle contraction. With this contraction, the spindle is no longer stretched.
A similar system is found in the tendons
attaching muscle to bone. Distinct stretch receptors called 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, 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. Tone is maintained through autonomous feedback from the
muscle fibers, neurons, and associated tissues.