THE RELATION BETWEEN EXCITATION-CONTRACTION COUPLING AND FINE-STRUCTURE OF A MOLLUSCAN MUSCLE, THE RADULAR RETRACTOR OF THE WHELK, BUCCINUM-UNDATUM

The Buccinum radula is of the rachiglossate type with two outer rows o f fierce hook-like attack teeth and a medial row of straight sharp-poi nted shredding teeth. Individual cells of the radular retractor muscle are 10-12 mu m in diameter and separated at the closest by gaps of on ly 40 nm, providing areas of potential electrical contact. The cell me mbranes are heavily invested with long finger-like invaginations, asso ciated with sarcoplasmic reticular cisternae, and surface caveolae; th e latter are associated with the numerous dense body membrane attachme nt plaques found in this muscle. The radular retractor muscle possesse s a significant sarcoplasmic reticulum of peripheral cisternae and dee per vesicles associated with mitochondria. The surface caveolae may re sult from myofilament force exerted via attachment plaques at the cell membrane, while deeper invaginations may constitute a rudimentary tra nsverse tubular system to relay surface depolarization to associated s arcoplasmic reticular cisternae inducing calcium release to effect exc itation-contraction coupling. The radular retractor muscle possesses t he usual thick paramyosin and thin actin myofilaments, the latter asso ciated with dense bodies and attachment plaques presumably to transduc e force to the cell membrane. The mitochondria are unusually large and packed into dense central clusters surrounded by large deposits of gl ycogen granules. The nerve endings on the radular retractor muscle fib res show four different types of transmitter vesicle, presumably relat ed to the four kinds of agonist action in this muscle, cholinergic, se rotonergic, peptidergic and purinergic. All nerve endings have mixed v esicle populations, clear evidence of co-transmission. In this muscle we see a modification of usual smooth muscle structure to effect fast sustained contractions, an ultrastructural configuration functionally designed for the muscle's central role in the feeding cycle.