Muscle Spindle - Anatomy

Anatomy

Muscle spindles are found within the belly of muscles, embedded in extrafusal muscle fibers. Note that "fusus" is the Latin word for spindle. Muscle spindles are composed of 3-12 intrafusal muscle fibers, of which there are three types:

• dynamic nuclear bag fibers (bag₁ fibers)
• static nuclear bag fibers (bag₂ fibers)
• nuclear chain fibers and the axons of sensory neurons.

Axons of gamma motoneurons also terminate in muscle spindles; they make synapses at either or both of the ends of the intrafusal muscle fibers and regulate the sensitivity of the sensory afferents, which are located in the non-contractile central (equatorial) region.

Muscle spindles are encapsulated by connective tissue, and are aligned parallel to extrafusal muscle fibers, unlike Golgi tendon organs, which are oriented in series.

The muscle spindle has both sensory and motor components.

• Primary and secondary sensory nerve fibers spiral around and terminate on the central portions of the intrafusal muscle fibers, providing the sensory component of the structure via stretch-sensitive ion-channels of the axons.
• In mammals including humans, the motor component is provided by up to a dozen gamma motoneurons and to a lesser extent by one or two beta motoneurons. Gamma and beta motoneurons are called fusimotor neurons, because they activate the intrafusal muscle fibers. Gamma motoneurons only innervate intrafusal muscle fibers, whereas beta motoneurons innervate both extrafusal and intrafusal muscle fibers and so are referred to as skeletofusimotor neurons.
• Fusimotor drive causes a contraction and stiffening of the end portions of the intrafusal muscle fibers.

Fusimotor neurons are classified as static or dynamic according to the type of intrafusal muscle fibers they innervate and their physiological effects on the responses of the Ia and II sensory neurons innervating the central, non-contractile part of the muscle spindle.

• The static axons innervate the chain or bag₂ fibers. They increase the firing rate of Ia and II afferents at a given muscle length (see schematic of fusimotor action below).
• The dynamic axons innervate the bag₁ intrafusal muscle fibers. They increase the stretch-sensitivity of the Ia afferents by stiffening the bag₁ intrafusal fibers.