Muscle spindles are sensory nerves found in the muscles of the body. They consist of specialized muscle fibers that lie parallel to the extrafusal, or regular, muscle tissue. Muscle spindles are also known as intrafusal muscle fibers to distinguish them from regular muscle tissue. Muscle spindles communicate with both the surrounding muscle tissue and the brain to carry out multiple functions.
Proprioception
Muscle spindles play a critical role in proprioception, or the awareness of where your body is in space, according to the University of Washington. The muscle spindles work with other small organs in the muscles, like the Golgi tendon organ, to provide information to the brain on the muscle's state of contraction, length, and joint angle. The brain uses this information to determine where the body is in relationship to itself.
Stretch Protection
The muscle spindle also helps protect the muscle from being stretched too far or too quickly. When the muscle is stretched, the muscle spindle senses the stretching. If the stretch happens too quickly or is too intense, the muscle spindle signals the muscles to contract to prevent further stretching. According to the University of Washington, this signal is known as the myotatic reflex.
Static and Dynamic Signaling
The International Fitness Association (IFA) notes that muscle spindles use both static and dynamic signaling to coordinate and control muscle contraction. A dynamic signal comes from the spindle's nuclear bag fibers and only lasts for a moment. Its purpose is to send a quick signal to the muscles to contract. A static signal comes from the spindle's nuclear chain fibers. These fibers signal a stretch is occurring when the muscle stretches gradually. The signals allow the brain to maintain the body's balance by engaging other parts of the body to respond to the stretching muscle.
