Intrafusal muscle fiber

Intrafusal muscle fibers are not to be confused with extrafusal muscle fibers, which contract, generating skeletal movement and are innervated by alpha motor neurons.

Structure

Types

There are two types of intrafusal muscle fibers: nuclear bag fibers and nuclear chain fibers. They bear two types of sensory ending, known as annulospiral and flower-spray endings. Both ends of these fibers contract, but the central region only stretches and does not contract.

Intrafusal muscle fibers are walled off from the rest of the muscle by an outer connective tissue sheath consisting of flattened fibroblasts and collagen. This sheath has a spindle or "fusiform" shape, hence the name "intrafusal".

Innervation

They are innervated by gamma motor neurons and beta motor neurons. Gamma efferents from small multipolar neurons from anterior gray column innervate it. These form a part of neuromuscular spindles.