Radial Muscle Fibres Research Articles

Isolated Giant Smooth Muscle Fibres in Beroe Ovata: Ionic Dependence of Action Potentials Reveals two Distinct Types of Fibre

ABSTRACT The ionic dependence of action potentials evoked in giant smooth muscle fibres isolated by enzymatic digestion from the body wall of the marine invertebrate Beroe ovata (Ctenophora) has been investigated using conventional electrophysiological techniques. Differences were observed in the two fibre types studied. The resting membrane potential was –60 ± 1·35 mV (N = 25) in longitudinal muscle fibres and –66 ± 1·37 mV (N =32) in radial fibres. Action potentials had a short plateau in longitudinal fibres but not in radial fibres. The action potential overshoot of both fibre types was decreased in Ca2+-free artificial sea water (ASW). In Na+-deficient ASW, action potentials could not be generated in radial fibres and showed a reduced overshoot in longitudinal fibres. Tetrodotoxin (10−smoll−1) added to ASW or Ca2+-free ASW did not affect the action potentials of either type of fibre. Action potentials of both fibres were partially blocked by Co2+ (20-50 mmol 1−1) or Cd2+ (1-2 mmol 1−1). Action potentials of longitudinal fibres in Na+-deficient ASW were abolished by Co2+ (20 mmol 1−1). In Ca2+-free ASW, the action potential overshoots of both sets of fibres were restored following the addition of Sr2+ or Ba2+. In longitudinal fibres, Sr2+ increased the duration of the action potential plateau. In both longitudinal and radial muscle fibres, Ba2+ prolonged the action potential. In longitudinal fibres exposed to tetraethylammonium chloride (TEACI) or 4-aminopyridine (4AP), the action potential was slightly prolonged. In these fibres, TEA+ or 4AP added to Ca2+-free ASW induced only a long-lasting depolarizing plateau. In radial fibres, the action potential duration was slightly increased in the presence of TEA+; it was unaffected by 4AP. In Ca2+-free ASW, TEA+ and 4AP induced an oscillating membrane response which appeared to be dependent on the intensity of the injected current pulse. It is concluded that (a) there are significant differences between the action potentials of longitudinal and radial muscle fibres but that both are dependent on Na+ and Ca2+, (b) in longitudinal fibres, a Ca2+-activated K+ conductance and a TEA+-sensitive voltage-activated K+ conductance contribute to the repolarizing phase of the action potential, the former being predominant, (c) in radial fibres, the repolarizing phase of action potentials probably involves different membrane K+ conductances among which is a TEA+-sensitive K+ conductance.

Cholinergic Innervation of Muscle Fibres in Squid

INTRODUCTIONThe small squidAlloteuthis, and the largerLoligohave elongate mantles chiefly made up of two distinct types of circular muscle fibres, partitioned by thin sheets of radial muscle fibres (Bone, Pulsford & Chubb, 1981).Nerve terminals on both types of circular fibre and upon radial fibres contain 50 nm electron-lucent vesicles; preliminary pharmacological investigations (Bone & Howarth, 1980) indicate that L-glutamate is the excitatory transmitter at the terminals upon the circular fibres. In other cephalopods, such asSepia, and the squidsLepidoteuthisandTaningia(Clarke & Maul, 1962; Clarke, 1967) there are in addition to the circular and radial fibres, layers of longitudinal muscle fibres in the mantle. Such longitudinal fibre layers are lacking over the greater part of the mantle inAlloteuthisandLoligo.In this note, we show that they are present only around the anterior margin of the mantle in these two squid, and that both these anterior longitudinal fibres and the radial fibres of the mantle are insensitive to L-glutamate, but contract in response to acetylcholine, as do the retractor muscles of the head and arms, and the siphon muscles.

THE FEEDING BIOLOGY OFPODARKE PUGETTENSIS(POLYCHAETA: HESIONIDAE)

1. Podarke pugettensis is carnivorous, ingesting a wide variety of small benthic invertebrates, including members of at least 11 polychaete families. Harpacticoid copepods constituted the bulk of the diet in the field population examined. Food items are limited to those that are small relative to the predator and can fit through its pharynx. Plant material and sediment are not ingested purposely, but may enter the gut by being sucked in with the prey, or by being within the gut of the prey.2. Ingested animals appear to be thoroughly digested by the time they reach the hindgut, while plant material appears to be undigested. The gut passage time is generally 12 to 24 hr, indicating a slow movement and thorough digestion of animal material. The long gut passage time may be necessary because Podarke is limited in the food it consumes by relative prey availability, and so must digest food as thoroughly as possible.3. The unarmed pharynx acts as a double-action pump, with two thick-walled muscular regions and two thinner-walled valves. The massive walls of the muscular regions consist mostly of radial muscle fibers which can rapidly open the pharynx, sucking in prey.4. Podarke pugettensis appears to utilize vibration sensing, direct touching, and chemoreception to detect and evaluate potential prey. Size of the prey appears to be the most critical factor influencing prey selection.5. Calculations of feeding rate of Podarke indicate that this predator could consume a significant portion of the production of co-occuring benthic copepod populations.

Ultrastructure and function of cephalopod chromatophores.

synopsis. Each chromatophore organ consists of a pigment cell and of several radial muscle fibers that represent separate cells. The pigment granules are contained within an elastic sacculus within the pigment cell. The sacculus is attached around the equator of the chromatophore to the cell membrane by zonal haptosomes. In turn, the cell membrane is attached to the radial muscle fibers by a dense basal lamina. The cell membrane of the retracted chromatophore is highly folded. Contraction of the radial muscle fibers is initiated by (a) excitatory junction potentials, (b) miniature potentials, or (c) spike potentials. The latter arise spontaneously in the muscle fibers when these have undergone some internal (metabolic?) change. The contraction of the muscle fibers causes expansion of the pigment-containing sacculus. Relaxation of the muscle fibers permits the sacculus to assume its original lenticular or near-spherical shape; the energy for this is stored within the expanded elastic components of the sacculus. In normal skin the chromatophore organs are entirely under the control of the central nervous system, the muscle fibers being activated only by local, excitatory postsynaptic potentials initiated by motor nerve impulses. That postsynaptic potentials are non-propagating insures that individual motor fibers can be activated individually, thus permitting a delicate control of skin color by recruitment as well as by frequency. Tonic contractions and pulsations, involving spontaneous release of transmitter from nerve terminals and spike generation within the muscle fibers, respectively, are the result of altered, abnormal conditions within the skin.

Ultrastructure of cephalopod chromatophore organs

The chromatophore organs of Loligo opalescens are composed of five different types of cells: the chromatophore proper; radial muscle fibers; neuronal processes (axons); glial cells; and chromatophoral sheath cells. The surface of a retracted chromatophore is extensively folded, but upon contraction of the radial muscle fibers it becomes flattened and the folds of the surface disappear. The cell membrane cannot be responsible for the elasticity of the chromatophore as claimed by earlier investigators. The pigment granules are confined within a filamentous compartment (cytoelastic sacculus) throughout the cycle of expansion and retraction. The sacculus decreases in thickness during expansion and increases in thickness during retraction and does not become folded. The elastic properties of the chromatophore are attributed to the cytoelastic sacculus. Primary infoldings of the chromatophore surface are anchored to the sacculus at points called focal haptosomes. The periphery of the sacculus attaches to the plasmalemma of the equatorial part of the chromatophore, opposite the area of attachment of the radial muscle fibers (myochromatophoral junction) by a zonular haptosome. The regular, obliquely striated muscle fibers that expand the chromatophore are associated with axons and glial cell processes. Adjacent muscle fibers may be electrically coupled through close junctions. The entire chromatophore and the muscle fibers are surrounded by sheath cells.

Cytochemical and Electron Microscope Studies on the Pharynx and Intestinal Epithelium of Nippostrongylus brasiliensis

Pharynx and intestinal epithelium were studied with various cytochemical methods and the electron microscope. The pharynx possesses the characteristic triradiate lumen and is lined internally with cuticle. Its wall is muscular and myofilaments of two types, about 7 m,a and 15 my/ in diameter, can be seen in cross section. Besides the myofilaments, large branching mitochondria are present with many cristae. A pharyngeal gland is also present appearing in cross section as a single dense triangular cell, along the ventral margin. Cells of the intestinal epithelium are so large that a typical cross section of the intestine passes through only two or three cells. These show some of the properties of intestinal mucosa, such as a microvillar border, terminal web, and terminal bars at the cell margin. The nucleus is usually situated toward the base of the cell where most of the mitochondria also occur. The mitochondria contain cristae with characteristic triangular or forked apices. The cell also contains endoplasmic reticulum, Golgi apparatus, secretory granules, free ribosomal particles, and glycogen. The presence of a large quantity of ribosomes and also secretory granules in the cytoplasm suggests that the cell is involved in the synthesis and secretion of digestive enzymes. Several additional kinds of inclusion bodies are also present, suggestive of intracellular bacteria and lysosome-like bodies. The pharynx or esophagus of nematodes is a cylindrical tube with an internal cuticular lining (Hyman, 1951) and has a lumen which is characteristically triradiate. The pharyngeal wall contains radial muscle fibers and the pharyngeal gland as well as other cell components. Immediately posterior to the muscular pharynx is the relatively simple, tubular, thinwalled intestine. Studies on the intestinal epithelium of nematodes have shown it to be composed of a single layer of columnar epithelial cells whose most striking feature is the presence of a bacillary layer (brush border) on the internal surface. This bacillary layer has been regarded as immobilized (Hyman, 1951). In other electron microscopic studies were also described in the bacillary layer of Ancylostoma duodenale (by Browne and Chowdhury, 1959) and in Trichinella spiralis (by Beckett and Received for publication 25 May 1966. * These investigations were supported in part by Grant AI-00884 of the U. S. Public Health Service, NIAID, and in part by Grant HD-1242 of the National Institute of Child Health and Human Development, and U. S. Public Health Service General Research Support Grant FR-5367. The author was supported by the Mr. and Mrs. Frank G. Logan Fellowship Fund of the University of Chicago. t Present address: Division of Microbiology, Medical Research Center, Brookhaven National Laboratory, Upton, Long Island, New York 11973. Boothroyd, 1960). It is doubtful, however, that these projections should be considered as cilia in the usual sense. Several investigators have demonstrated the presence of microvilli in the intestinal epithelium of Ascaris (Bretschneider, 1954; Joyon and Collin, 1962; Kessel et al., 1961; Sheffield, 1964; Tokin, 1959). Microvilli have also been reported in other nematodes such as Capillaria hepatica (by Wright, 1963) and Rhabditis strongyloides (by Peebles, 1957). The intestinal epithelium of the nematode presumably has a dual role acting in secretion as well as food absorption. In keeping with its secretory role, several zymogen-like granules are usually present (Chitwood and Chitwood, 1950). The intestinal epithelium also acts as a storage place for glycogen, and is thus involved in the carbohydrate metabolism of the organism (von Brand, 1960). MATERIALS AND METHODS Nippostrongylus brasiliensis was maintained in laboratory rats. Adult worms were obtained from the intestine of the rats after 10 days of infection. For light microscopy the worms were fixed in hot formalin-acetic-alcohol and Zenker's fluid, subsequently dehydrated and embedded in paraffin. The sections were stained with hematoxylin-eosin and Mallory's triple stain (Humason, 1962) for general histology and in Prussian blue (Lillie, 1954) to detect the presence of iron. A slide of opossum prostate tissue known to contain iron was stained as control. One-,A sections of Epon-embedded material were stained in azure B to study the presence

Nervous control and spontaneous activity of the chromatophores of a cephalopod, Loligo opalescens

1. 1. The radial muscle fibers of the chromatophores receive only a motor innervation. In those of ventral skin, one motor fiber innervates the muscles of several chromatophores. In the dorsal skin, each motor fiber innervates only part of the muscle complement of a number of chromatophores; a bundle of motor axons innervate the same skin territory, thus permitting stepwise expansion of chromatophores through tetanic contractions of increasing numbers of their muscle fibers. 2. 2. Single nerve impulses cause twitch-like contractions. Above 2/sec summation of individual contractions occurs with transition to a smooth tetanus at frequencies above 12/sec. 3. 3. ACh causes tonic contractions and “pulsations”. 5-Hydroxytryptamine antagonizes spontaneous and ACh-induced tonic contractions. It does not interfere with nerve-induced contractions but augments the rate of shortening and of relaxation. Tetrodotoxin does not interfere with spontaneous tonic contractions but abolishes the effects of nerve stimulation. Banthine blocks the action of motor fibers and of ACh. 4. 4. Spontaneous and ACh-induced tonic contractions are accompanied by local potentials resembling miniature potentials. Spontaneous contractions are preceded by spikes which arise from generator depolarization. Nerve stimulation results only in local junction potentials. This explains why single muscle fibers can be independently activated by motoneurons even though spontaneous contractions may occur nearly simultaneously in many or all muscle fibers of a chromatophore.

The rôle of the body fluid in the movement of soft-bodied invertebrates - II. The extenstion of the siphons of Mya arenaria L. and Scrobicularia plana (da Costa)

It is widely held that molluscan organs are protruded and extended by a forceful influx of blood, but observations and experiments on Mya and Scrobicularia show that this is certainly not true for the movements of the siphons of these two lamellibranchs. The siphons of Mya are extended by water being forced into them from the mantle cavity by the action of the adductor muscles of the shell. Changes in length and shape of the siphons of Scrobicularia are effected solely by an intrinsic mechanism consisting of longitudinal and radial muscle fibres together with beds of collagen. The muscles are antagonized through the blood contained within the siphonal walls which at all times retain a constant volume.

CHROMATOPHORES

Summary.Chromatophores, the parts concerned with the colour changes in animals, are best developed in the cephalopods, the crustaceans, and the three lower classes of vertebrates, the fishes, amphibians, and reptiles. In the cephalopods chromato‐ phores are really diminutive organs in that each one consists of a central coloured cell which is expanded against its own elasticity by radial muscle fibres. In the crustaceans a cellular complex often with several colours makes up each chromato‐ phore. This expands and contracts slowly and thus changes the tint of the animal. In the vertebrates a coloured background in the skin is exposed to view or covered up by motile black pigment cells whose melanin granules are made to migrate within the containing cell. Most animals show relatively uniform changes of colour, but in a few, as, for instance, flat‐fishes, a pattern may be imitated showing that the central control of the chromatophore system must be somewhat differentiated in the direction in which the musculature is.