Gaps in spindle physiology: Why the tandem spindle?

Most reviews of spindle structure and function have focussed on the operation of a “model” muscle spindle, which exists as a single encapsulated receptor with a single sensory zone and a predictable pattern of nerve supply. The description accurately depicts the majority of muscle spindles in cat limb muscles where most studies of muscle spindles have been carried out. It also provides a model which fits many of our developing concepts about the role of the muscle spindle in motor control. Why, then, are there muscle spindles which do not exist as single encapsulations but as groups of receptors in different types of assemblages? One such muscle spindle, the tandem muscle spindle, has been recently examined in detailed anatomical studies. The results suggest that the tandem spindle may be a specialized proprioceptive organ, but neither the physiology nor the central projections of this receptor are yet understood.