Chapter 43 Group II Spindle Afferent Fibers in Humans: their Possible Role in the Reflex Control of Stance

Abstract

Publisher Summary Since the first description by Ruffini of primary and secondary spindle endings, it has been tacitly assumed that they perform different reflex functions. While the role played in the monosynaptic response by group Ia afferents originating from the primary endings is generally agreed upon, some textbooks attribute to the small diameter myelinated group II afferent fibers, originating from the secondary endings, a role in producing flexor reflex effects. Accordingly, these fibers are classified as flexor reflex afferents together with high-threshold skin and joint afferents. Back in his preface to the 1972 Monograph of the Physiological Society, however, Matthews presented the notion that spindle group II fibers from secondary endings played a part in reflexly producing the hypertonia of the decerebrate preparation instead of merely contributing to a generalized flexor reflex. Group II afferent fibers that innervate muscle spindles discharge in response to muscle stretch and Ia fibers and are subjected to similar gamma and descending effects. Group II fibers can make both monosynaptic and oligosynaptic connections with homonymous and synergist hind-limb alpha motoneurons. A large number of spindle group II afferent fibers exist in the cat, and these fibers are roughly the same in number as the Ia fibers in all muscles in which they have been counted. It is highly possible that the same is true in humans, judging from the distribution of fibers of different diameter in the dorsal roots of the human lumbar spinal cord.