Muscle spindle

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A muscle spindle is a specialized muscle structure that interacts with both sensory and motor neurons to convey proprioceptive information about the muscle to the central nervous system, and to respond to muscle stretching.

Table of contents

1 Anatomy
2 Stretch reflex
3 Development

Anatomy

Muscle spindles are found within muscles, embedded in so-called extrafusal muscle fibers. They are composed of 3-10 modified muscle fibers called intrafusal muscle fibers, of which there are two types, nuclear bag fibers and nuclear chain fibers.

The muscle spindle has both sensory and motor components. Primary and secondary sensory fibers synapse on the intrafusal fibers of the spindle, providing the sensory component of the structure. The motor component is provided by a gamma efferent motor fiber that innervates the spindle and causes a slight contraction when activated.

Stretch reflex

When a muscle is stretched, primary sensory fibers of the muscle spindle detect both the velocity and the degree of stretch, and send this information to the spinal cord. Likewise, secondary muscle fibers detect and send information about the degree of stretch (but not the velocity thereof) to the CNS. This information is transmitted across an interneuron to an alpha efferent motor fiber, which activates extrafusal fibers of the muscle to contract, thereby reducing stretch.

Development

It is also believed that muscle spindles play a critical role in sensorimotor development.