When a muscle moves, it is important for the brain to keep track of that movement. It is also important to track the state of a muscle at any time to determine the position and location of the joint it operates. Muscle spindles provide this information to the central nervous system.
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Form and Function
Muscle spindles are specialized sensory organs. Each muscle spindle is a feedback mechanism that detects muscle length and any changes to muscle length by increasing the number of electrical signals it generates as its sensory nerve endings are stimulated. Each spindle consists of a tiny collagen capsule, tapered at both ends. It contains many specialized muscle fibers called intrafusal fibers. They are parallel with the main muscle fibers within the skeletal muscle tissue that actually provide movement to the muscle. These are extrafusal fibers and are stimulated to move by alpha motor neurons.
At either end of each of the fibers are contractile proteins that are controlled by nerve endings called gamma motor neurons. These neurons are responsible for maintaining the sensitivity and tension of the muscle spindle, regardless of its length. When extrafusal muscle fibers receive an impulse to move from the central nervous system, the same impulse is sent to the muscle spindle through the gamma motor neurons. This impulse causes the contractile proteins within the muscle spindle to readjust the length of the intrafusal fibers. This then tightens or loosens the tension on the center of each fiber.
Annulospiral Nerve Endings
The central area of each intrafusal fiber has no contractile proteins. Instead, there are sensory nerve endings called annulospiral endings. They are named this because each nerve ending spirals around the center of the intrafusal fiber. As the tension on each intrafusal fiber increases or decreases, the nerve endings detect and transmit these changes to the central nervous system.
Primary Nerve Ending
The primary annulospiral nerve ending is the largest and the fastest firing in the body. This nerve ending detects movement. When the muscle stretches, the nerve ending fires in rapid succession. When the muscle stops moving, the nerve ending readjusts to the change and stops firing.
Secondary Nerve Ending
The secondary annulospiral nerve ending is attached to the intrafusal fiber and detects position. This nerve ending also fires when the muscle moves. However, when the muscle stops moving, the nerve continues to fire at a specific rate providing constant feedback.
Communication to Central Nervous System
All the information from the muscle spindle is transmitted to the central nervous system to help determine the angle and position of a joint, the velocity of movement and the amount of tension on a muscle to provide a view constantly of the relative position of a body part at any time.