Neuromuscular Systems in Molluscs

AMER. ZOOL., 13:247-270 (1973). Neuromuscular Systems in Molluscs CAROLYN B. HEYER AND STANLEY B. KATER Department of Zoology, University of Iowa, Iowa City, lorua 52240 AND ULF L. KARLSSON SYNOPSIS. Molluscs have become increasingly popular in the study of central neural mechanisms. More recently, there have been attempts to relate activity in central neurons with behavior in animals of this phylum. The latter studies necessitate an understanding of the effectors of such behaviors. This requires not only information about the neuromuscular junction, but also an awareness of the capabilities of the muscles themselves. Therefore, we have discussed some structural and related functional characteristics of molluscan muscle. We suggest that invertebrate mucles might be compared on three scales: the amount of myofilament organization, the amount of vesicular specialization and organization, and the amount of paramyosin. We have considered some characteristics of (he widely-studied sustained contraction, known as "catch." Finally, we have discussed the neuromuscular junction—the types of junctions, the multiplicity of innervation, and some aspects of pharmacology. The results of such a study indicated many areas in which further research is essential before we can understand behavior in terms of activity in the central nervous system. vation: looking at neuromuscular junctions alone often gives an incomplete and even There are several reasons for attacking perhaps a misleading picture of the meproblems of neuromuscular control in mol- chanical condition of the muscle. Thereluscs. Characterization of neuromuscular fore, we will begin this discussion with a junctions is important for the information brief characterization of molluscan musin itself and for comparative studies. cle, including both structural and related There is, however, another source of inter- functional aspects. A comparison of the est in the subject. Molluscs have been used structure of muscles in molluscs with that extensively for the study of central neural of muscles in animals of other phyla may mechanisms. Recently, there has been in- provide valuable insights into the funccreasing interest in aspects of behavior me- tional significance of structural variations, diated by those central neurons. In order and also some indication of the mechanito understand behavior in terms of neural cal capabilities of the muscles. Then we interactions, we must first look at the pe- will consider in more detail the most wideriphery and deal with the effectors. Fur- ly studied functional characteristic of molther, we must look at the capabilities of luscan muscle—catch. Finally, we will look muscles themselves, as well as their inner- at the nervous control of molluscan muscle. This includes characterization of the We thank Dr. L. Van Orden for preparing neuromuscular junction in terms of its muscles for the fluorescence histochemistry and morphology as well as its effects on muscle Jack Rued for invaluable assistance during all aspects of preparation of this manuscript. This refibers. In this context the number of neusearch was supported by an NSF predoctoral fel- rons innervating each muscle fiber, the lowship to C. B. H., PHS giants 1 R01 NS09696functional types of neuromuscular rela01 and XSO-3354 of XINDS, and, in part, by tions, and the pharmacology of neuromusUniversity Development Grant GU 2591 from NSF. INTRODUCTION 247 Department of Anatomy (Medicine) and Division of Ultras true ture (Dentishy), University of Iowa, Iowa City, Iowa 52240 HEYER, KATER, AND KARLSSON 248 NEUROMUSCULAR SYSTEMS IN MOLLUSCS 249 Further, thin filaments isolated from the several molluscan muscles studied do not contain such regulatory molecules as tropoSTRUCTURE nin and tropomyosin, indicating that calcium dependence in vivo is not controlled General by thin filaments (Kendrick-Jones et al., Although the structure of molluscan 1970). At this time it appears that mollusmuscle seems to vary widely, there are a can thin filaments contain only actin, alnumber of components which are common though there is one suggestion that actin not only to molluscs, but also to a wide may be complexed with /}-actinin (Suzuki variety of other organisms. The diameter et al., 1971). of molluscan fibers is small, frequently less The diameter of molluscan thick myothan 10 /x. Organelles, such as the nucleus, filaments varies from 100 A in the squid Golgi apparatus, and mitochondria, may chromatophore muscle (Cloney and Florey, be organized as a central core, at the pe- 1968) to 1500 A in tonic muscles such as riphery, or a combination of the two the ABRM (Hanson and Lowy, 1960), and places. In many molluscan muscles, how- the tonic adductors of several other pelecyever, the contractile apparatus nearly fills pods (Kalamkarova and Kryukova, 1966; the whole fiber (see Fig. 1). Kryukova, 1968b). The thick myofilaments are frequently quite long in the less wellorganized m,uscles (e.g., up to 30 p in the Myofilaments ABRM; Twarog, 1967c), but undoubtedly Unlike vertebrate smooth muscle and are not continuous throughout the length like vertebrate striated muscle, two types of muscle fibers (Hanson and Lowy, 1959). of myofilaments are easily identified in The myofilaments appear to contain myomolluscan muscle. Thin filaments vary sin and tropomyosin A (or paramyosin; from 40 to 100 A in diameter, depending Elliott et al., 1957). The results of extracon the muscle studied and the method of tion procedures indicate that paramyosin fixation. X-ray diffraction data indicate probably forms the core of the myofilathat the actin repeat of the anterior byssus ment, with myosin on the surface (Kahn retractor muscle (ABRM) of the pelecypod and Johnson, 1960; Hanson and Lowy, Mytilns edulis and frog sartorius muscle 1964; Szent-Gyorgyi et al., 1971). Although are the same, suggesting structural simi- the extracted myosin has many properties larity (Lowy and Vibert, 1967). Actin from similar to myosin extracted from vertebrate molluscan muscle can combine in vitro striated muscle, molluscan myosin is with the relaxing proteins of rabbit stri- unique in its ability to bind calcium and ated muscle and display the properties of thereby regulate calcium-dependent actothin filaments isolated from vertebrate myosin interactions (Kendrick-Jones et al., striated muscle (Kendrick-Tones et al., 1970). Initially, major longitudinal peri1970). However, actin isolated from clam odicities of 720 A or one of three submulmuscle differs from that isolated from rab- tiples, 320, 240, or 145 A (Bear and Selby, bit striated muscle in a number of bio- 1956), were used to identify paramyosinchemical measures (Suzuki et al., 1971). containing muscles. More recently, a wide cular junctions need to be considered. ate buffer, dehydrated in a graded series of acetone and embedded in Vcstopal W. 500-1000 A sections were obtained with an LKB Ultratomc III, stained with solutions of ura (...truncated)