Identification of muscle spindles in the submentalis muscle of the marine toad, Bufo marinus and its potential proprioceptive capacity in jaw-tongue coordination

Abstract

The muscle spindle has been well studied in mammals, birds, and some reptiles, but little is known about its structure and function in amphibians. Several physiological and behavioral studies in frogs and toads suggest that the submentalis muscle has an important role in jaw-tongue coordination during feeding through sensory feedback mechanisms. Evidence suggests this mechanism is generated through muscle spindles, or spindle-like structures. However, the presence of these has not been investigated. Using charged fluorescent molecules that are taken up by endocytosis at activated synapses, numerous muscle spindles were identified in the submentalis muscle of the marine toad, Bufo marinus. These were further verified with brightfield and differential interference contrast microscopy. The results provide anatomical evidence for muscle spindles in the submentalis muscle and suggest a mechanism for the submentalis muscle's role in jaw-tongue coordination in B. marinus.