Secondary type II sensory fibers (medium diameter) end adjacent to the central regions of the static bag and chain fibres.

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

Efferent nerve fibers of gamma motor neurons 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.

Likewise, secondary type II sensory fibers respond to muscle length changes (but with a smaller velocity-sensitive component) and transmit this signal to the spinal cord.

The Ia afferent signals are transmitted monosynaptically to many alpha motor neurons of the receptor-bearing muscle.

The reflexly evoked activity in the alpha motor neurons is then transmitted via their efferent axons to the extrafusal fibers of the muscle, which generate force and thereby resist the stretch.

Hypertonia may be the result of over-sensitivity of alpha motor neurons and interneurons to the Ia and II afferent signals.