What is another name for muscle spindles?

Muscle spindles are sensory receptors located within the belly of muscles that primarily detect changes in muscle length and the rate of that change. They are composed of specialized muscle fibers known as intrafusal muscle fibers. These fibers are different from the extrafusal muscle fibers, which are the muscle fibers responsible for muscle contraction.

Intrafusal muscle fibers play a crucial role in proprioception—the body’s ability to sense its position and movement in space. When a muscle is stretched, the intrafusal fibers are also stretched, prompting the muscle spindles to send signals to the central nervous system about the degree and speed of stretching. This information helps coordinate muscular actions and maintain posture and balance.

In contrast, extrafusal muscle fibers are the contractile elements of skeletal muscle that generate force during movement, while alpha motor neurons are the nerve cells that innervate these extrafusal fibers. Golgi tendon organs, on the other hand, are located at the junction between muscles and tendons and are involved in sensing changes in muscle tension, not muscle length. Therefore, the term "intrafusal muscle fibers" accurately identifies what muscle spindles are composed of and their function within the muscle system.