Morphology, ultrastructure and contractile properties of muscles responsible for superior tentacle movements of the snail

Bending, twitching and quivering are different types of tentacle movements observed during olfactory orientation of the snail. Three recently discovered special muscles, spanning along the length of superior tentacles from the tip to the base, seem to be responsible for the execution of these movements. In this study we have investigated the ultrastructure, contractile properties and protein composition of these muscles. Our ultrastructural studies show that smooth muscle fibers are loosely embedded in a collagen matrix and they are coupled with long sarcolemma protrusions. The muscle fibers apparently lack organized SR and transverse tubular system. Instead subsarcolemmal vesicles and mitochondria have been shown to be possible Ca2+ pools for contraction. It was shown that external Ca2+ is required for contraction elicited by high (40 mM) K+ or 10-4 M ACh. Caffeine (5 mM) induced contraction in Ca2+-free solution suggesting the presence of a substantial intracellular Ca2+ pool. High-resolution electrophoretic analysis of columellar and tentacular muscles did not reveal differences in major contractile proteins, such as actin, myosin and paramyosin. Differences were observed however in several bands representing presumably regulatory enzymes. It is concluded that, the ultrastructural, biochemical and contractile properties of the string muscles support their special physiological function.     

Secretory glands of the snail tentacle and their relation to the olfactory organ (Mollusca,Gastropoda)

Summary The epidermis of the posterior tentacles of the terrestrial snail Achatina fulica was examined by histological and histochemial methods. There are seven types of unicellular glands in the tentacle skin: three mucocytes containing either acid mucopolysaccharides or neutral mucopolysaccharides, or both; two mucocytes containing glycoproteins; a lipid gland; and a protein gland. The mucocytes are considerably more abundant along the shaft of the tentacle than at the tip, where the olfactory organ is situated. Conversely, the lipid glands and the protein glands are found almost exclusively in the olfactory organ. With minor exceptions, none of the foregoing cell types is present in the skin of the head or the foot. These observations indicate a high degree of local specificity in secretory products, consistent with a ubiquitous and generous endowment of glands in the molluscan skin. Collar cells, described by previous authors in closely related species, were not observed.     

Novel peripheral motor neurons in the posterior tentacles of the snail responsible for local tentacle movements

Three flexor muscles of the posterior tentacles of the snail Helix pomatia have recently been described. Here, we identify their local motor neurons by following the retrograde transport of neurobiotin injected into these muscles. The mostly unipolar motor neurons (15–35 µm) are confined to the tentacle digits and send motor axons to the M2 and M3 muscles. Electron microscopy revealed small dark neurons (5–7 µm diameter) and light neurons with 12–18 (T1 type) and 18–30 µm diameters (T2 type) in the digits. The diameters of the neurobiotin-labeled neurons corresponded to the T1 type light neurons. The neuronal processes of T1 type motor neurons arborize extensively in the neuropil area of the digits and receive synaptic inputs from local neuronal elements involved in peripheral olfactory information processing. These findings support the existence of a peripheral stimulus–response pathway, consisting of olfactory stimulus—local motor neuron—motor response components, to generate local lateral movements of the tentacle tip (“quiver”). In addition, physiological results showed that each flexor muscle receives distinct central motor commands via different peritentacular nerves and common central motor commands via tentacle digits, respectively. The distal axonal segments of the common pathway can receive inputs from local interneurons in the digits modulating the motor axon activity peripherally without soma excitation. These elements constitute a local microcircuit consisting of olfactory stimulus—distal segments of central motor axons—motor response components, to induce patterned contraction movements of the tentacle. The two local microcircuits described above provide a comprehensive neuroanatomical basis of tentacle movements without the involvement of the CNS.     

Novel triplet of flexor muscles in the posterior tentacles of the snail, Helix pomatia

The anatomy of three novel flexor muscles in the posterior tentacles of Helix pomatia is described. The muscles originate from the ventral side of the sensory pad and are anchored at different sites in the base of the tentacle stem. The muscles span the tentacle and always take the length of the stem which depends on the rate of tentacle protrusion indicating that the muscles are both contractile and extremely stretchable. The three anchoring points at the base of the stem determine three space axes along which the contraction of a muscle or the synchronous contraction of the muscles can move the tentacle in space.     

Regulation of the length of the snail tentacle by the concentration-dependent contraction

The tentacle of terrestrial snail with olfactory organs on the tips display complex behavior when snail investigates the new environment. We reconstructed the trajectory of the tentacle in three dimensions from two simultaneous video recordings in freely moving snail without odor and after odor application. We found that without oder the snail displayed continuous environment scanning with elongated tentacles. Odor application elicited startle-like short-term flexions of the tentacle which were independent from odor concentration and concentration-dependent gradual tentacle contraction. Identified central motoneuron MtC3 is known to produce the most part of the central tentacle retraction to the noxious stimuli. In nose-brain preparation the MtC3 responded to odors in concentration-dependent manner similar by dynamics and duration to the concentration-dependent gradual tentacle contraction in intact snail. It suggests that the MtC3 provides the central control of the extent of the scanning area by limiting the tentacle length. The MtC3-related gradual contraction of the tentacle can be aimed to tune the olfactory behavior of the terrestrial snail to the particular odor environment.     

Excitatory actions of propofol and ketamine in the snail Lymnaea stagnalis

This study compares the actions of the intravenous anaesthetics propofol and ketamine on animal behaviour and neuronal activity in the snail Lymnaea stagnalis, particularly in relation to excitatory effects observed clinically. When injected into the whole animal, neither agent induced total anaesthesia. Rather, behavioural activity was enhanced by propofol (10(-5) M) and ketamine (10(-7) M), indicating excitatory effects. When superfused over the isolated central nervous system (CNS), differential effects were produced in two identified neurons, right pedal dorsal 1 (RPeD1) and visceral dorsal 4 (VD4). Resting membrane properties were largely unaffected. However, spike after hyperpolarisation was significantly reduced in RPeD1, but not VD4, with some evidence of increased excitability. In addition, an intrinsic bursting property (post-stimulus burst) in VD4 was altered by propofol (10(-7) M). The results suggest significant excitatory components in the actions of some intravenous anaesthetics, as well as a potential role in modifying excitation and bursting mechanisms in the CNS.     

ATP in the frog olfactory organ

The distribution of ATP in different parts of the frog olfactory organ was investigated. It is demonstrated that the highest macroerg amount is characteristic of the receptor part of olfactory mucosa. The amount of ATP decreased after the stimulation of the olfactory mucosa by odorants. The role of ATP in olfactory receptor process is discussed.     

Neural control of olfaction and tentacle movements by serotonin and dopamine in terrestrial snail

We investigated the role of serotonin (5HT) and dopamine (DA) in the regulation of olfactory system function and odor-evoked tentacle movements in the snail Helix. Preparations of the posterior tentacle (including sensory pad, tentacular ganglion and olfactory nerve) or central ganglia with attached posterior tentacles were exposed to cineole odorant and the evoked responses were affected by prior application of 5HT or DA or their precursors 5-hydroxytryptophan (5HTP) and l-DOPA, respectively. 5HT applications decreased cineole-evoked responses recorded in the olfactory nerve and hyperpolarized the identified tentacle retractor muscle motoneuron MtC3, while DA applications led to the opposite changes. 5HTP and l-DOPA modified MtC3 activity comparable to 5HT and DA action. DA was also found to decrease the amplitude of spontaneous local field potential oscillations in the procerebrum, a central olfactory structure. In vivo studies demonstrated that injection of 5HTP in freely moving snails reduced the tentacle withdrawal response to aversive ethyl acetate odorant, whereas the injection of l-DOPA increased responses to “neutral” cineole and aversive ethyl acetate odorants. Our data suggest that 5HT and DA affect the peripheral (sensory epithelium and tentacular ganglion), the central (procerebrum), and the single motor neuron (withdrawal motoneuron MtC3) level of the snail’s nervous system.     

Excitatory local circuits and their implications for olfactory processing in the fly antennal lobe

Conflicting views exist of how circuits of the antennal lobe, the insect equivalent of the olfactory bulb, translate input from olfactory receptor neurons (ORNs) into projection-neuron (PN) output. Synaptic connections between ORNs and PNs are one-to-one, yet PNs are more broadly tuned to odors than ORNs. The basis for this difference in receptive range remains unknown. Analyzing a Drosophila mutant lacking ORN input to one glomerulus, we show that some of the apparent complexity in the antennal lobe's output arises from lateral, interglomerular excitation of PNs. We describe a previously unidentified population of cholinergic local neurons (LNs) with multiglomerular processes. These excitatory LNs respond broadly to odors but exhibit little glomerular specificity in their synaptic output, suggesting that PNs are driven by a combination of glomerulus-specific ORN afferents and diffuse LN excitation. Lateral excitation may boost PN signals and enhance their transmission to third-order neurons in a mechanism akin to stochastic resonance.     

Excitatory interactions between olfactory processing channels in the Drosophila antennal lobe

Each odorant receptor gene defines a unique type of olfactory receptor neuron (ORN) and a corresponding type of second-order neuron. Because each odor can activate multiple ORN types, information must ultimately be integrated across these processing channels to form a unified percept. Here, we show that, in Drosophila, integration begins at the level of second-order projection neurons (PNs). We genetically silence all the ORNs that normally express a particular odorant receptor and find that PNs postsynaptic to the silent glomerulus receive substantial lateral excitatory input from other glomeruli. Genetically confining odor-evoked ORN input to just one glomerulus reveals that most PNs postsynaptic to other glomeruli receive indirect excitatory input from the single ORN type that is active. Lateral connections between identified glomeruli vary in strength, and this pattern of connections is stereotyped across flies. Thus, a dense network of lateral connections distributes odor-evoked excitation between channels in the first brain region of the olfactory processing stream.     

Signal transduction mechanism responsible for changes in axoplasmic transport caused by neurotransmitters

Transduction mechanism for modulation of axoplasmic transport by neurotransmitters was studied using cultured mouse superior cervical ganglion cells. The transported particles were analyzed with a computer-assisted video-enhanced differential interference contrast microscope system. Acetylcholine depressed and adrenaline increased axoplasmic transport. GTP-binding proteins linked with both receptors activate or inactivate adenylyl cyclase, thereby altering the intracellular concentration of cyclic AMP. The cyclic AMP activates protein kinase A, which phosphorylates certain enzymes and the enzymes in turn phosphorylate motor proteins. An inhibitor of protein kinase A, KT5720, decreases the number of the transported particles. In a stable state the cyclic AMP level stays at a normal level. Treatment with neurotransmitters causes a change in this level, which changes the activity of protein kinase A and thus decreases or enhances the phosphorylation of motor proteins. These changes are involved in the modulation of axoplasmic transport. In honor of Dr. Sidney Ochs.     

Twitching and quivering of the tentacles during snail olfactory orientation

In Helix aspersa the posterior tentacles house a sensitive olfactory organ. We studied two types of tentacular movements, twitch and quiver. A twitch is a brief retraction (mean duration, 4.1 s); a quiver is a rapid lateral movement (350 ms) unaccompanied by retraction. We videotaped the tentacles while snails explored an open field. When an attractive odor source, linalool, was present at one side of the arena, the snails consistently moved towards it. By contrast, if only the carrier substance was present the snails moved in random directions. Twitching was 50 times more frequent during linalool trials than during control trials, while quivering was 1.4 times more frequent. Twitching increased steadily and dramatically as snails approached the linalool source and, in the temporal dimension, the maximum rate of twitching occurred when the snails arrived at the odor source. Quivers occurred at a fairly constant rate. Twitching is interpreted as a mechanism to remove odor molecules trapped in the liquid covering of the olfactory epithelium, thus resulting in better temporal resolution for olfactory perception. Quivering may be a mechanism to increase access of odor molecules to receptors by decreasing the boundary layer at the surface of the tentacle. Accepted: 24 May 1997     

Localization of neurons innervating the columellar muscles of the snail

The topographic arrangement of large and small neurons participating in the mechanism of the defensive reflex was studied in the circumesophageal nerve ring ofHelix pomatia by a modified retrograde cobalt ion transport method. Comparison of the results with those of previous electrophysiological investigations of the mechanism of the defensive relfex leads to the conclusion that this reflex is effected by a system of neurons consisting of nine large and 60–80 small nerve cells.Research Institute of Neurocybernetics, State University, Rostov-on-Don. Translated from Neirofiziologiya, Vol. 12, No. 6, pp. 637–641, November–December, 1980.     

Neuronal background of positioning of the posterior tentacles in the snail Helix pomatia

The location of cerebral neurons innervating the three recently described flexor muscles involved in the orientation of the posterior tentacles was investigated by applying parallel retrograde Co- and Ni-lysine tracing via the olfactory and the peritentacular nerves. Their innervation patterns in the flexor muscles were studied by applying anterograde neurobiotin tracings via these nerves. The labeled neurons are clustered in eight groups in the cerebral ganglion. They send both common and distinct innervation pathways to the flexor and the tegumental muscles and to the tentacular retractor muscle. The common pathway reaches the muscles via the olfactory nerve, whereas the distinct pathways innervate via the internal and external peritentacular nerves. The three anchoring points of the three flexor muscles at the base of the tentacle outline the directions of three force vectors generated by the contraction of the muscles and enable the protracted tentacle to bend around a basal pivot. In the light of earlier physiological and the present anatomical findings, we suggest that the common innervation pathway to the muscles is required for tentacle withdrawal and the retractor mechanism, whereas the distinct pathways primarily serve the bending of the protracted posterior tentacles during foraging.     

Epimuscular myofascial force transmission occurs in the rat between the deep flexor muscles and their antagonistic muscles

The goal of the present study was to test the hypothesis that epimuscular myofascial force transmission occurs between deep flexor muscles of the rat and their antagonists: previously unstudied mechanical effects of length changes of deep flexors on the anterior crural muscles (i.e., extensor digitorum longus (EDL), as well as tibialis anterior and extensor hallucis longus muscle complex (TA + EHL) and peroneal (PER) muscles were assessed experimentally. These muscles or muscle groups were kept at constant length, whereas, distal length changes were imposed on deep flexor (DF) muscles before performing isometric contractions. Distal forces of all muscle-tendon complexes were measured simultaneously, in addition to EDL proximal force. Distal lengthening of DF caused substantial significant effects on its antagonistic muscles: (1) increase in proximal EDL total force (maximally 19.2%), (2) decrease in distal EDL total (maximally 8.4%) and passive (maximally 49%) forces, (3) variable proximo-distal total force differences indicating net proximally directed epimuscular myofascial loads acting on EDL at lower DF lengths and net distally directed loads at higher DF lengths, (4) decrease in TA + EHL total (maximally 50%) and passive (maximally 66.5%) forces and (5) decrease in PER total force (maximally 51.3%). It is concluded that substantial inter-antagonistic epimuscular myofascial force transmission occurs between deep flexor, anterior crural and peroneal muscles.In the light of our present results and recently reported evidence on inter-antagonistic interaction between anterior crural, peroneal and triceps surae muscles, we concluded that epimuscular myofascial force transmission is capable of causing major effects within the entire lower leg of the rat. Implications of such large scale myofascial force transmission are discussed and expected to be crucial to muscle function in healthy, as well as pathological conditions.