Chromatophore radial muscle fibers anchor in flexible squid skin

Cephalopod skin is soft, flexible, and produces rapid color changes for camouflage and signaling primarily by regulating the shapes of its numerous chromatophore organs. Each chromatophore has 10–30 radial muscle cells, termed fibers, under central nervous system control. Each fiber contains myofilaments that contract in concert to stretch the pigment‐containing cell from its punctate, spherical state to a fully expanded thin disk of color. Expansion occurs in less than one second and can result in a 14‐fold expansion in pigment cell diameter. We investigated the anchoring mechanism of radial muscle fibers that expand pigment cells in the longfin squid, Doryteuthis (Loligo) pealeii. The proximal Active Zone of a radial muscle fiber adheres to the pigment cell within an ensheathing sinus. The distal portion forms terminal arbors, thereby increasing the surface area, to adhere it to the dermal extracellular matrix (ECM). While the muscle fiber is attached to the pigment cell with haptosomes, the remainder of the fiber is adhered to the surrounding basal lamina (part of the ECM) by numerous, closely spaced, small costamere‐like projections. Branching of the radial muscle fiber termini and the costamere‐like attachments are key anatomical specializations that anchor the radial muscle fibers in the pliable skin while allowing the freedom of movement required for large changes in pigment cell diameter. We postulate that these features may be relevant for the development of soft actuation models in materials science.     

Chromatophore Organs, Reflector Cells, Iridocytes and Leucophores in Cephalopods

The chromatophore organs of Lohgo are each composed of fivetypes of cells: a central pigment cell: radially arranged, obliquelystriated muscle fibers: neuronal processes; glial cells: andan investment of sheath cells. Sheath cells are absent in Octopuschromatophore organs. The cycle of expansion and retractionof a chromatophore organ may occur within the order of a second.It is clear that the muscle fibers expand the pigment cell andspread out the pigment granules. The pigment is contained withinan unusual, filamentous, cytoplasmic compartment called thecytoelastic sacculus. This compartment has elastic properties. Reflector cells and iridocytes produce structural colors eventhough their components are colorless. Reflector cells in Octopusbear peripheral sets of leaf-like reflecting lamellae calledreflectosomes: these contain proteinaceous platelets with ahigh refractive index (1.42). In each reflectosome the reflectinglamellae are separated by gaps that are about equal to the thicknessof the lamellae, but have a lower refractive index (1.33). Reflectosomesare believed to reflect light and to function as thin-film interferencedevices. Iridocytes in squid and cuttlefish contain iridosomes that arealso composed of sets of ribbon-like platelets but these arelocated centrally within the cell body. The platelets are usuallyoriented on edge with respect to the surface of the skin. Thepossibility that dermal iridocytes may act as diffraction gratingsis discussed. Leucophores have thousands of processes that containglobules of protein with a high refractive index. These cellsscatter light of all wave lengths and appear white in whitelight.     

Electrical and mechanical responses of chromatophore muscle fibers of the squid,Loligo opalescens,to nerve stimulation and drugs

Summary The muscle fibers of brown and red chromatophores in the skin of the squid, Loligo opalescens, respond to motor nerve stimulation with non-propagating excitatory postsynaptic potentials (e.p.s.p.'s) of fluctuating amplitude. Depending on the strength of stimulation several size classes of e.p.s.p.'s are found, indicating polyneuronal innervation. Facilitation and summation are minimal even though the reversal potential of the e.p.s.p.'s is close to zero.Acetylcholine (ACh) and 5-hydroxytryptamine (5-HT) have no effect on membrane characteristics of the muscle fiber, but ACh greatly augments the spontaneous quantal release of transmitter [increase in the frequency of miniature postsynaptic potentials (m.p.s.p.'s)] and thereby causes tonic contraction (miniature tetanus). 5-HT reduces the frequency of miniature potentials and abolishes tonic contraction. Inhibition of cholinesterase by eserine does not affect the amplitude or time course of e.p.s.p.'s and of m.p.s.p.'s. High concentrations of cholinergic blocking agents (atropine, banthine) reduce the postsynaptic effects of nerve stimulation in some cases. The natural transmitter substance of the motoneurones may not be ACh. The action of 5-HT appears to be intracellular.Neighboring muscle fibers are electrically coupled through low resistance pathways. These are most likely provided by the close junctions that form part of the myo-muscular junctions. The specific membrane resistance of the regular muscle fiber membrane was found to range from 1,056 to 1,320 Ohm×cm², that of the close junctions ranges from 12.8 to 22.6 Ohm×cm². The area occupied by close junctions is small, however, and only 10% of the current injected into one cell passes into the next. Some of the e.p.s.p.'s observed in a given muscle fiber most likely represent the electrotonic spread of the e.p.s.p.'s of the neighbor fibers. Of the six classes of e.p.s.p.'s observed in some muscle fibers, only two may originate in these fibers themselves.Chromatophores in aged preparations often exhibit pulsations. These are caused by spike potentials arising within muscle fibers whose membranes have become electrically excitable. Each spike is preceded by a generator depolarization. The electrical coupling of neighboring muscle cells permits conduction of the spike potentials throughout the set of muscle fibers of a pulsating chromatophore. Altered conditions within such preparations also lead to tonic contractions and contractures that are not necessarily accompanied by electrical activity. Several arguments are presented in support of the hypothesis that the tonic condition of nerve terminals (characterized by enhanced spontaneous transmitter release) and of muscle fibers (characterized by inability to relax) is due to an abnormal condition of intracellular calcium (lack of Ca-binding by sarcoplasmic reticulum or other storage sites).No evidence could be found for an inhibitory innervation of the chromatophore muscles. The nerve-induced relaxation of tonically contracted muscle fibers is caused by the action of motoneurones.Preliminary experiments on muscle fibers of the anterior byssus retractor muscle of Mytilus support the hypothesis that the tonic behavior (catch) of other molluscan muscles is due to mechanisms similar to those found in the chromatophore muscles.This investigation was supported by Public Health Service Grant No. NB 04145 from the National Institute of Neurological Diseases and Blindness. We are grateful to the director of the Friday Harbor Laboratories, Prof. R. L. Fernald for providing space and facilities for this investigation.Supported by a Training Grant GM 1194 from the National Institute of General Medical Sciences.     

Increased effectiveness of a motorneuron after partial denervation of its target muscle in the cricket Teleogryllus oceanicus

Fibers of the metathoracic extensor tibia muscle of the cricket Teleogryllus oceanicus are innervated by a slow excitatory axon (slow fibers), a fast excitatory axon (fast fibers), or by both slow and fast axons (dual fibers). Sectioning metathoracic nerve 5 removes the fast axon input to the muscle but not that of the slow axon. Following such partial denervation, the mechanical responses initiated by the slow axon increase progressively for at least 30 days; twitch tensions reach 5–10 times those of control muscles and tetanic tensions 10–30 times control values. After sectioning nerve 5, resting membrane potentials decrease in those fibers which originally received fast axon input and the input resistance of all fiber types increases, including that of slow fibers which are not innervated through nerve 5. Excitatory junctional potentials (EJPs) initiated by the slow axon become larger following partial denervation, accounting in part for the larger contraction amplitudes. The increased input resistance is adequate to account for the larger EJPs in slow fibers but not for the proportionally greater increase in EJP amplitude in fibers which were formerly dually innervated. The change in EJP amplitude is abrupt in slow fibers and gradual in formerly dual fibers.     

Motor innervation of the pharynx levator muscle of the snail, Helix pomatia: physiological and histological properties

(1) Motor innervation of the pharynx levator muscle of Helix pomatia was investigated with intracellular recording and axonal iontophoresis of cobalt chloride. (2) Muscle fibers respond to direct electrical stimulation of the muscle with active graded responses or non-overshooting spike potentials. (3) Each fiber is innervated via the external and internal lip nerves by several (mostly 3) excitatory nerve fibers each. Two types of EPSPs can be distinguished according to amplitude, duration, and facilitation. (4) Axonal CoCl2-staining via an external lip nerve branch revealed many nerve fibers entering the muscle and branching there into a rich network of blebbed fibers of various diameters.     

Nitric Oxide Mediates the Glutamate-dependent Pathway for Neurotransmission in Sepia officinalis Chromatophore Organs

Chromatophore organs are complex and unique structures responsible for the variety of body coloration patterns used by cephalopods to communicate and camouflage. They are formed by a pigment-containing cytoelastic sacculus, surrounded by muscle fibers directly innervated from the brain. Muscle contraction and relaxation are responsible for expansion and retraction of the pigment-containing cell. Their functioning depends on glutamate and Phe-Met-Arg-Phe-NH₂-related peptides, which induce fast and slow cell expansion, respectively, and 5-hydroxytryptamine, which induces retraction. Apart from these three substances and acetylcholine, which acts presynaptically, no other neuroactive compounds have so far been found to be involved in the neuroregulation of chromatophore physiology, and the detailed signaling mechanisms are still little understood. Herein, we disclose the role of nitric oxide (NO) as mediator in one of the signaling pathways by which glutamate activates body patterning. NO and nitric-oxide synthase have been detected in pigment and muscle fibers of embryo, juvenile, and adult chromatophore organs from Sepia officinalis. NO-mediated Sepia chromatophore expansion operates at slower rate than glutamate and involves cGMP, cyclic ADP-ribose, and ryanodine receptor activation. These results demonstrate for the first time that NO is an important messenger in the long term maintenance of the body coloration patterns in Sepia.     

Evidence for L-glutamate as a transmitter substance of motoneurons innervating squid chromatophore muscles

Motor nerve branches were stimulated in the dermis layer prepared from isolated pieces of dorsal mantle skin of the squid Lolliguncula brevis and the contractions of chromatophore muscle fibers were recorded with the aid of a photo-electric transducer. L-Glutamate (L-Glu), kainate and quisqualate caused a contracture and often repetitive twitch-like contractions. These effects were readily reversible. In the case of L-Glu application, twitches induced by single stimuli applied to motor nerves were enhanced and prolonged. The glutamate antagonists glutamic acid gamma-methyl ester, glutamic acid diethyl ester, D,L-2-amino-4-phosphonobutyrate and gamma-D-glutamylglycine prevented both nerve induced and L-Glu induced contractions. The NMDA-receptor agonists N-methyl-D-aspartate, L-aspartate and D-glutamate, and their antagonists alpha-aminoadipate and D,L-2-amino-5-phosphonovalerate were found ineffective. With the aid of saline media of different Ca and Mg content, it was possible to selectively eliminate one or all components of the effect of L-Glu. Tetrodotoxin abolished nerve induced contractile responses but did not interfere with the contracture caused by L-Glu. Intracellular electrical recording indicated that nerve stimulation causes EPSPs which do not give rise to spike discharges. The results are compatible with the hypothesis that L-Glu is a transmitter substance of the motoneurons that innervate chromatophore muscle fibers.     

Cephalopod chromatophores: neurobiology and natural history

The chromatophores of cephalopods differ fundamentally from those of other animals: they are neuromuscular organs rather than cells and are not controlled hormonally. They constitute a unique motor system that operates upon the environment without applying any force to it. Each chromatophore organ comprises an elastic sacculus containing pigment, to which is attached a set of obliquely striated radial muscles, each with its nerves and glia. When excited the muscles contract, expanding the chromatophore; when they relax, energy stored in the elastic sacculus retracts it. The physiology and pharmacology of the chromatophore nerves and muscles of loliginid squids are discussed in detail. Attention is drawn to the multiple innervation of dorsal mantle chromatophores, of crucial importance in pattern generation. The size and density of the chromatophores varies according to habit and lifestyle. Differently coloured chromatophores are distributed precisely with respect to each other, and to reflecting structures beneath them. Some of the rules for establishing this exact arrangement have been elucidated by ontogenetic studies. The chromatophores are not innervated uniformly: specific nerve fibres innervate groups of chromatophores within the fixed, morphological array, producing 'physiological units' expressed as visible 'chromatomotor fields'. The chromatophores are controlled by a set of lobes in the brain organized hierarchically. At the highest level, the optic lobes, acting largely on visual information, select specific motor programmes (i.e. body patterns); at the lowest level, motoneurons in the chromatophore lobes execute the programmes, their activity or inactivity producing the patterning seen in the skin. In Octopus vulgaris there are over half a million neurons in the chromatophore lobes, and receptors for all the classical neurotransmitters are present, different transmitters being used to activate (or inhibit) the different colour classes of chromatophore motoneurons. A detailed understanding of the way in which the brain controls body patterning still eludes us: the entire system apparently operates without feedback, visual or proprioceptive. The gross appearance of a cephalopod is termed its body pattern. This comprises a number of components, made up of several units, which in turn contains many elements: the chromatophores themselves and also reflecting cells and skin muscles. Neural control of the chromatophores enables a cephalopod to change its appearance almost instantaneously, a key feature in some escape behaviours and during agonistic signalling. Equally important, it also enables them to generate the discrete patterns so essential for camouflage or for signalling. The primary function of the chromatophores is camouflage. They are used to match the brightness of the background and to produce components that help the animal achieve general resemblance to the substrate or break up the body's outline. Because the chromatophores are neurally controlled an individual can, at any moment, select and exhibit one particular body pattern out of many. Such rapid neural polymorphism ('polyphenism') may hinder search-image formation by predators. Another function of the chromatophores is communication. Intraspecific signalling is well documented in several inshore species, and interspecific signalling, using ancient, highly conserved patterns, is also widespread. Neurally controlled chromatophores lend themselves supremely well to communication, allowing rapid, finely graded and bilateral signalling.     

Mechanisms of direct and neural excitability in electroplaques of electric eel

1. Current flow outward through the caudal, reactive membrane of the cell causes direct stimulation of the electroplaque. The electrical response in denervated as well as in normal preparations recorded with internal microelectrodes is first local and graded with the intensity of the stimulus. When membrane depolarization reaches about 40 mv. a propagated, all-or-nothing spike develops. 2. Measured with internal microelectrodes the resting potential is 73 mv. and the spike 126 mv. The latter lasts about 2 msec. and is propagated at approximately 1 M.P.S. 3. The latency of the response decreases nearly to zero with strong direct stimulation and the entire cell may be activated nearly synchronously. 4. Current flow inward through the caudal membrane of the cell does not excite the latter directly, but activation of the innervated cell takes place through stimulation of the nerve terminals. This causes a response which has a latency of not less than 1.0 msec. and up to 2.4 msec. 5. The activity evoked by indirect stimulation or by a neural volley includes a prefatory potential which has properties different from the local response. This is a postsynaptic potential since it also develops in the excitable membrane which produces the local response and spike. 6. On stimulation of a nerve trunk the postsynaptic potential is produced everywhere in the caudal membrane, but is largest at the outer (skin) end of the cell. The spike is initiated in this region and is propagated at a slightly higher rate than is the directly elicited response. Strong neural stimulation can excite the entire cell to simultaneous discharge. 7. The postsynaptic potential caused by neural or indirect stimulation may be elicited while the cell is absolutely refractory to direct excitation. 8. The postsynaptic potential is not depressed by anodal, or enhanced by cathodal polarization. 9. It is therefore concluded that the postsynaptic potential represents a membrane response which is not electrically excitable. Neural activation of this therefore probably involves a chemical transmitter. 10. The nature of the transmitter is discussed and it is concluded that this is not closely related to acetylcholine. 11. Paired homosynaptic excitation discloses facilitation which is not present when the conditioning stimulus is direct or through a different nerve trunk. These results may be interpreted in the light of the existence of a neurally caused chemical transmitter or alternatively as due to presynaptic potentiation. 12. The electrically excitable system of the electroplaque has two components. In the normal cell a graded reaction of the membrane develops with increasing strength of stimulation until a critical level of depolarization, which is about 40 mv. 13. At this stage a regenerative explosive reaction of the membrane takes place which produces the all-or-nothing spike and propagation. 14. During early relative refractoriness or after poisoning with some drugs (eserine, etc.) the regenerative process is lost. The membrane response then may continue as a graded process, increasing proportionally to the stimulus strength. Although this pathway is capable of producing the full membrane potential the response is not propagated. 15. Propagation returns when the cell recovers its regenerative reaction and the all-or-nothing response is elicited. 16. Excitable tissues may be classified into three categories. The axon is everywhere electrically excitable. The skeletal muscle fiber is electrically excitable everywhere except at a restricted region (the end plate) which is only neurally or chemically excitable. The electroplaque of the eel, and probably also cells of the nervous system have neurally and electrically excitable membrane components intermingled. The electroplaques of Raia and probably also of Torpedo as well as frog muscle fibers of the "slow" system have membranes which are primarily neurally and chemically excitable. Existence of a category of invertebrate muscle fibers with graded electrical excitability is also considered. 17. In the eel electroplaque and also probably in the cells of neurons, tests of the mode of neural activation carried out by direct or antidromic stimulation cannot reveal the neurally and chemically activated component. The data of such tests though they appear to prove electrical transmission are therefore inadequate for the detection and study of the chemically initiated process.     

Physiological properties of the penis retractor muscle of Aplysia

The properties of the penis retractor muscle of Aplysia have been studied using intracellular, sucrose gap and tension recording. The fibers are of the invertebrate smooth muscle type and exhibit slow contractions which occur spontaneously or in response to stretch in isolated preparations. Individual muscle fibers are innervated by excitatory and inhibitory axons. A variety of sizes of excitatory and inhibitory junctional potentials can be recorded from them. The innervation is probably diffuse and functionally polyneuronal. The fibers are electrically coupled, permeable to potassium and chloride at rest, and exhibit no overshooting active responses. The muscle shows graded responses of depolarization and contraction proportional to strength of nerve stimulation. Facilitation and depression of junctional potentials are seen with various frequencies of nerve stimulation. Post-tetanic potentiation occurs with nerve stimulation at frequencies from 2 to 50 Hz and is suppressed in the presence of increased extracellular calcium concentrations.     

Crayfish motor nerve terminal's response to serotonin examined by intracellular microelectrode

Measurements of resting potential and action potential in presynaptic branches of the excitatory motor axon to the crayfish opener muscle were made with intracellular microelectrodes during application of serotonin (10(-9)-10(-3) M). A 5-min exposure to 10(-6) M serotonin produced enhancement of excitatory junction potentials (EJPs) lasting about 1 h. The membrane potential of the presynaptic terminal was depolarized by about 5 mV; the depolarization subsided within 1/2 h. Concomitant reduction in amplitude of the presynaptic action potential, not accompanied by spike broadening, was observed. The presynaptic depolarization, and the enhancement of EJPs, were dependent on the presence of extracellular sodium but not extracellular calcium. A possible mechanism for serotonin's effect involves initial entry of sodium into the nerve terminal, with consequent increased availability of intracellular calcium. The subsequent long-lasting phase of EJP enhancement may result from an additional effect on the metabolism of the nerve terminal.     

Lobster muscle and synapse respond to cortisol, a vertebrate hormone

Cortisol (0.28 mumol X L-1) applied to lobster (Homarus americanus) neuromuscular preparations produces a hyperpolarization in muscle fibers and an increase in amplitude of excitatory postsynaptic potentials. The effect appears to be surface-mediated, because of its rapid onset (within seconds). It is also Na+-K+ ATPase dependent, because ouabain blocks the effects. The effects are relatively short-lasting, and gradually subside within 15 min. The increase in excitatory postsynaptic potentials is attributed in part to increased quantal output of transmitter, and not to changes in muscle fiber membrane resistance. The effects of cortisol on neuromuscular transmission and membrane potential indicate that cortisol may have a physiological role in crustaceans.     

Penis-retractor muscle of Aplysia: excitatory motor neurons

Cobalt axonal iontophoresis and intracellular recordings were used to identify a cluster of several motor neurons innervating the penis-retractor muscle of Aplysia. Intracellularly recorded motor neuron action potentials elicited direct, one-for-one, constant latency excitatory junctional potentials (ejps) in individual muscle fibers. The axons of motor neurons could be recorded extracellularly in the penis-retractor nerve and stimulation of the nerve backfired the motor neurons. Perfusion of the ganglion, the muscle, or both with solutions of either increased Mg++/decreased Ca++ or increased Ca++ sea water indicated that the presumed motor neuron impaled was not a sensory cell and that interneurons were not intercalated in the pathway. Innervation of muscle fibers was found to be functionally polyneuronal and diffuse. The ejps were found to undergo marked facilitation with repetitive motor-neuron stimulation. The motor neurons were isolated in a distinct cluster in the right pedal ganglion. Their electrical activity was characterized by spontaneous irregular action potentials and a moderate input of postsynaptic potentials.     

Membrane electrical properties of motoneurons innervating the masseter muscles in rats

Membrane potentials and action potentials evoked by antidromic and direct stimulation were investigated in motoneurons of the trigeminal nucleus in rats innervating the masseter muscle. This motor nucleus was shown to contain cell populations with high and low membrane potentials. The responses of cells of the first group had shorter latent periods of their antidromic action potentials, a longer spike duration, and a lower amplitude and shorter duration of after-hyperpolarization than responses of cells of the second group, and the input resistance of their membrane also is lower. The bimodal character of distribution of electrophysiological parameters of motoneurons in the trigeminal nucleus indicates that "fast" and "slow" fibers of the masseter muscles may be innervated by different types of nerve cells.N. A. Semashko Moscow Medical Stomatological Institute. Translated from Neirofiziologiya, Vol. 13, No. 3, pp. 270–274, May–June, 1981.     

Changes in the electrical activity of the rabbit proximal colon in vivo by stimulation of the vagus and splanchnic nerves]

1. Using extracellular electrodes placed on the serosa, we recorded the modifications of the electrical activity of the colonic muslce fibers caused by the stimulation of vagal and splanchnic nerve fibers. 2. Vagal stimulation produces two types of junction potentials: excitatory junction potentials (EJPs) and inhibitory junction potentials (IJPs). The IJPs are elicited by stimulation of vagal fibers which innervate intramural non-adrenergic inhibitory neurons. 3. The conduction velocity of the nerve impulse along the vagal pre-ganglionic fibers is 1.01 m/sec for excitatory fibers and 0.5. m/sec for inhibitory fibers. 4. Splanchnic fiber stimulation causes EJP disappearance, blocking transmission between preganglionic fibers and intramural excitatory neurons, and a decrease in IJP amplitude that most likely indicates a previous hyperpolarization of the smooth muscle. 5. IJP persistence during splanchnic stimulation proves that sympathetic inhibition does not modify the transmission of the vagal influx onto the non-adrenergic inhibitory neurons of the intramural plexuses. 6. Through a comparative study of proximal and distal colonic innervation, we are able to show that there is a similar organization of both regions, that is a double inhibitory innervation: an adrenergic one of a sympathetic origin, and a non adrenergic one of a parasympathetic origin.     

Desensitization of Gamma Aminobutyric Acid (GABA) Receptors in Muscle Fibers of the Crab Cancer borealis

Carcinus muscle fibers respond to γ-aminobutyric acid (GABA) with a conductance increase that subsides rather rapidly. In the larger fibers which have low input resistance the decrease may disappear within 2 min. The inhibition of the excitatory postsynaptic potentials (EPSP's) by GABA nevertheless persists as long as the drug is applied. The subsidence of the increased conductance indicates that the membrane of the inhibitory synapses has become desensitized to GABA. The persistence of inhibition of the EPSP's appears to be due to an action of the drug on the presynaptic terminals of the excitatory axons which reduces or blocks the secretory activity that releases the excitatory transmitter.     

Excitation of cat spinal cord dorsal column fibers during stimulation of different fiber groups of cutaneous and somatic nerves

The effects of electrically stimulating different groups of nerve fibers supplying the skin and muscle on evoked potentials in cat spinal cord dorsal columns were studied. Significant differences in the configuration of dorsal column potentials recorded in response to stimulation of these nerves were found. It was shown that cutaneous nerve unmyelinated fibres were connected to unmyelinated dorsal column fibers. In addition, excitation of cutaneous C-fibers lead to activation of dorsal column fibers with the maximum conduction velocity. The somatic nerve was only connected to myelinated dorsal column fibers, and excitation of its non-myelinated fibers did not cause other types of dorsal column fibers to be activated. It is suggested that the acceleration of cutaneous signal transmission in the dorsal column system may be brought about by the necessity for rapid warning of potentially harmful stimuli.Medical Institute, Russian Federation Ministry of Public Health, Nizhny Novgorod. Translated from Neirofiziologiya, Vol. 24, No. 5, pp. 625–635, September–October, 1992.     

Neuromuscular transmission of pectoral fin muscles of the goldfish Carassius auratus

The electrical properties and neuromuscular transmission of white and red fibers of pectoral fin muscles of the goldfish Carassius auratus were studied using an intracellular recording technique. The pectoral fin muscles consist mainly of white and red fibers. Almost all of white fibers elicited action potentials with overshoot by direct stimulation, but graded responses appeared in the red fibers. However, overshooting action potentials were often recorded from the red fibers in saline containing 20 microM tetraethylammonium (TEA) chloride. In response to single nerve stimulations, excitatory (EJPs) and inhibitory junction potentials (IJPs) were obtained from both white and red fibers in common. Both EJPs and IJPs were blocked completely or partially by d-tubocurarine, a nicotinic acetylcholine (ACh) receptor antagonist. Nicotine, a nicotinic ACh receptor agonist, and oxotremorine, a muscarinic ACh receptor agonist, depolarized both fiber types. The results suggest that white and red fibers receive double innervation from excitatory and inhibitory nerves, and have nicotinic and muscarinic ACh receptors. In the resting muscle, miniature excitatory junction potentials were generated spontaneously in both white and red fibers. Occasionally, miniature inhibitory junction potentials were recorded from the red fibers. The results indicate that the release of both excitatory and inhibitory transmitters is quantal in nature.     

Skin color in the squids Loligo pealii and Loligo opalescens

Summary Rapid, physiological color changes seen in the skin of cephalopods are due to a unique anatomical system composed of chromatophore organs and iridophores. The morphology and ultrastructure of the chromatophores was studied in the squids Loligo pealii Lesueur and Loligo opalescens Berry. A three-dimensional model of a brown chromatophore was reconstructed from serial sections for the electron microscope.The chromatophore organ is composed of a central nucleated pigment cell, 10–30 obliquely striated muscle cells (radially arranged on the equator of the pigment cell), axons, Schwann cells, and sheath cells. The pigment cell consists of a central aggregation of pigment granules and surrounding peripheral cytoplasmic compartments. These regions are incompletely separated by an electron-dense, sac-like structure, the pigment container. Proximal portions of a muscle cell contact the pigment cell in regions called myo-chromatophore junctions. Neuromuscular and myo-muscular junctions are also present.The results presented are discussed in terms of previous morphological and physiological studies of chromatophores.Part of a study submitted in partial fulfillment of the requirement for the degree of Ph. D. (Anatomy), the Graduate School of Basic Medical Sciences, New York Medical College, New York, N.Y. 10029.The research reported here was in part supported by grants from the Health Research Council of the City of New York (U-1008) and United States Public Health Service, General Research Grant No. FR-05398.Report on some of this material was given at the Annual Meeting of the American Association of Anatomists, Philadelphia, Pennsylvania, April 19–22, 1971.     

Encoding of phase spectra by the peripheral auditory system of the bullfrog

In this study we have examined the sensitivity of auditory nerve fibers in the bullfrog (Rana catesbeiana) to changes in the phase spectrum of an equal-amplitude multi-harmonic stimulus which spanned the bullfrog's range of hearing. To assess peripheral auditory phase sensitivity, changes in the response properties of VIIIth nerve fibers were measured when the relative phase angle of a single harmonic component nearest a unit's best excitatory frequency was systematically varied. The results revealed that shifts in the phase spectrum are encoded in at least J different ways by the peripheral auditory system of the bullfrog: 1) by changes in the degree of spike synchronization of fibers from both inner ear organs (the amphibian papilla and the basilar papilla) to the fundamental waveform period; 2) by changes in the shapes of period histograms of fibers from both organs; and 3) by changes in the spike rates of amphibian papilla fibers. The presence of phase sensitivity in the peripheral auditory system of the bullfrog indicates that information regarding the fine-temporal waveshape and the underlying phase spectrum of an acoustic signal is contained within the spike trains of VIIIth nerve fibers. Similar sensitivities to changes in the phase spectra and temporal waveshapes of acoustic signals may also be present in the peripheral auditory system of other vertebrates. Such studies could provide valuable insight into the role that phase spectra and temporal waveshape may play in bioacoustic communication.Abbreviations BEF best excitatory frequency - BEC best excitatory component - CS_{f 1} synchronization to the fundamental period Portions of this study have been summarized in abstract form (Bodnar and Capranica 1991)