Innervation and membrane specialization at neuromuscular junctions in submaxillaris muscle of the frog

Ultrastructure of the larval tentacle and its skeletal muscle in Xenopus laevis

During premetamorphic development, tadpoles of Xenopus laevis possess a transitory pair of long, slender, mobile tentacles situated at the corners of the mouth. Microscopic examination of the larval tentacle typically reveals three distinct compartments: a central core of cartilage, a laterally situated skeletal muscle, and a nerve supply medially. Along the length of each tentacle, the epidermis is supplied by many unmyelinated nerve fibers, presumably sensory in nature, which terminate as naked axons in close association with the epidermal cells. The striated tentacular muscle, in the proximal region of the lateral compartment, consists of extrafusal muscle fibers of varying size which range in number from 36 to 48 per tentacle (n = 10). Using morphometric criteria, we have classified the skeletal muscle fibers of the larval tentacular muscle into three types: large (30-50 microns), intermediate (20-30 microns), and small (10-20 microns). By electron microscopy, each type displays characteristic sarcomeric banding patterns, sarcotubular and mitochondrial disposition, and motor endplate ultrastructure. Our morphological observations indicate that the tentacles of the Xenopus tadpole are complex mobile facial extensions which may play roles in mechanoreception and/or chemoreception during the waterborne stages of development. Because of its transitory nature, the Xenopus tentacle may be a useful experimental model in future studies of neuromuscular development and subsequent regression in a relatively short period of time.

Acetylcholinesterase in the alimentary canal of fish: light and electron microscopic detection, quantitative distribution in different segments of the gut

The acetylcholinesterase activity was measured and histochemically localized in the alimentary tract of 2 fish species, carp (Cyprinus carpio) and tench (Tinca tinca). A comparison was made of the activities in the different gut segments. Light and electron microscopic histochemistry revealed acetylcholinesterase-positive cell bodies along the entire length of the alimentary canal in both species, between the muscular layers. Acetylcholinesterase-positive, cholinergic motor endplates were frequent in the esophagus of both carp and tench, and they were also present in the striated muscular layers of the tench stomach and midgut. The enzyme activity detected by the method of Ellman et al. (1961) was highest (16.3 U/mg protein) in the tench foregut and midgut, while it was at the same lower level (9.5 U/mg protein) in each segment of the carp gut and in the tench hindgut. The morphological findings and the higher acetylcholinesterase activity in the tench foregut and midgut suggest that the enteric striated musculature is endowed with denser cholinergic innervation than the enteric smooth musculature.

Enteric neuromuscular junctions: comparison of ultrastructural features in different phylogenetic groups

The enteric neuromuscular junctions of snail (Helix pomatia), locust (Locusta migratoria migratorioides), cockroach (Periplaneta americana), carp (Cyprinus carpio) and tench (Tinca tinca) were studied by means of different light and electron microscopic methods. The nitroblue tetrazolium staining revealed that the myenteric plexuses of the above species are composed of nerve cells, a network of varicose nerves and nerve bundles. Instead of highly organized ganglia, single neurons or small groups of 2-4 cells are characteristic of the invertebrates and fish studied. Catecholaminergic fluorescence induced by glyoxylic acid was detected in the muscular layer of the entire alimentary tract in snail and the hindgut of tench. Fluorescent nerves and perikarya were frequent in the snail gut, while only nerves and no perikarya were found in tench. A close contact between enteric muscles and nerves is the most common form of enteric neuromuscular junction in both the smooth (i.e. the molluscan and fish gut) and the striated (i.e. the insect gut) musculature. The striated musculature (i.e. the insect gut, the oesophagus of carp, and the oesophagus, stomach and the midgut of tench) also receives a synaptic input. Cytochemical evidence is provided of the cholinergic character of fish motor endplates. The ultrastructural appearance and vesicle population of certain nerve terminals suggest a universal role of aminergic and peptidergic control in gut motility.

Characteristics of synaptic currents in frog muscle fibers of different types

A study was made of synaptic currents in voltage-clamped muscle fibers of the frog. Fast, submaxillaris, and slow muscle fibers are innervated by nerve fibers of different conduction velocities. To avoid spatial complications, transmitter release by nerve impulses was restricted to the site of recording and reduced to single quanta (unitary endplate currents: uepc). Following nerve stimulation, the time course of transmitter release was longer and more variable in slow and submaxillaris muscle fibers than in the fast fibers. The time constant of decay of uepc in submaxillaris and slow fibers was, respectively, about 1.8 and 2.9 times slower than the decay of uepc in fast fibers. This is due mainly to differences in the lifetime of the channels opened by acetylcholine. The neuromuscular junctions in submaxillaris muscle fibers are bouton-like or longer branched contacts; and the unitary currents in the bouton junctions have a slower time course. It is concluded that the synaptic membrane in the different types of muscle fibers has synaptic acetylcholine-operated channels that have different kinetic properties, and that these properties are determined by the type of axon that innervates the muscle fiber.