Spinal mechanisms of electric organ discharge synchronization in Gymnotus carapo

Summary The duration of the electric organ discharge (EOD) in Gymnotus carapo is brief and independent of fish size. Spinal mechanisms involved in electrocyte synchronization were explored by recording spontaneous action potentials of single fibers from the electromotor bulbospinal tract (EBST). Using the field potential of the medullary electromotor nucleus (MEN) as a temporal reference we calculated the orthodromic conduction velocity (CV) of these fibers (range: 10.7–91 m/s).The CVs (in m/s) of fibers recorded at the same level of the spinal cord were significantly different in small and large fish; this difference disappeared when CV were expressed as percentage of body length/ms. Plotting these values against conduction distance (also in %) showed that low CV fibers predominate in the rostral cord while only fast fibers are found at distal levels. Moreover, antidromic stimulation of the distal cord was only effective on high CV fibers. The orthodromic CVs in the distal portion of the recorded fibers were calculated by collision experiments; no significant differences were found between proximal and distal portions.The spatial distribution of CV values within the EBST is proposed to play the main role in synchronizing the electromotoneurons' activity along the spinal cord.Abbreviations EOD electric organ discharge - EO electric organ - EBST electromotor bulbospinal tract - MEN medullary electromotor nucleus - CV conduction velocity - EMN electromotoneuron     

Electric organ activation in Gymnotus carapo: Spinal origin and peripheral mechanisms

A new technique of multiple-air-gap recording was developed to study the EO activation process in Gymnotus carapo. Using this technique, the spatiotemporal pattern of electromotive force generation was investigated in normal and spinal-lesioned animals.Our data indicate that the EOD may be considered as the result of the sequential activation of 3 defined portions of the EO: the abdominal portion (included in the rostral 25% of the fish body), the central portion (comprising the intermediate 50% of the fish body) and the tail portion (the caudal 25% of the fish body). The EOD generated at each portion is characterized by: 1) timing respect to the pacemaker nucleus discharge, 2) speed of progression within the region, 3) waveform, and 4) magnitude.Spinal sections demonstrated that EMNs serving relatively small portions of the EO are widely distributed (convergence) and that surgical exclusion of relatively small portions of the spinal cord diminishes the amplitude of the EOD along an extended portion of the EO (divergence).Abbreviations EMF electromotive force - EMN electromotor-neurons - EO electric organ - EOD electric organ discharge - PMNFP pacemaker nucleus field potential - PEN posterior electromotor nerve - PNA peripheral neural activity     

Spatial distribution of the medullary command signal within the electric organ of Gymnotus carapo

The pacemaker nucleus of Gymnotus carapo contains two types of neurons: pacemaker cells which set up the frequency of the electric organ discharge (EOD) and relay cells which convey the command signal to the spinal cord. Direct activation of a single relay cell provides enough excitation to discharge a pool of spinal electromotor neurons and electrocytes, generating a small EOD (unit EOD). Different relay cells generate unit EODs of variable size and waveform, indicating the involvement of different groups of electrocytes. A special technique of EOD recording (multiple air-gap) was combined with intracellular stimulation of relay cells to study the spatial distribution within the electric organ (EO) of the command signal arising from different relay cells. Three types of relay cells could be identified: type I commanding the rostral 10% of the EO, type II which distribute their command all along the EO and type III driving the caudal 30%. Waveform analysis of unit EODs indicates that doubly innervated electrocytes which are the most relevant for attaining the specific EOD waveform, receive a favored command from the pacemaker nucleus.Abbreviations CV conduction velocity - EMF electromotive force - EMN electromotor neuron - EO electric organ - EOD electric organ discharge - PN pacemaker nucleus - uEOD unit electric organ discharge     

Waveform generation in Rhamphichthys rostratus (L.) (Teleostei,Gymnotiformes)

Rhamphichthys rostratus (L.) emits brief pulses (2 ms) repeated very regularly at 50 Hz. The electric organ shows a heterogeneous distribution of the electrocyte tubes and the occurrence of three electrocyte types (caudally innervated, rostrally innervated and marginallycaudally innervated). In the sub-opercular region the electric organ consists of a pair of tubes containing only caudally innervated electrocytes. At the abdominal region the EO consists of three pairs of tubes. Each pair contains one of the described electrocyte types. The number of electrocyte tubes increases toward the tail to reach nine or ten pairs in the most caudal segments. In the intermediate region most tubes contain doubly innervated electrocytes except the ventral pair that contains caudally innervated electrocytes. The caudal 25% contains exclusively caudally innervated electrocytes. The electric organ discharge consists of five wave components (V1 to V5). Electrophysiological data are consistent with the hypothesis that V1 results from the activity of the rostral faces of rostrally innervated electrocytes. V2 results from the activities of rostral faces of marginally-caudally innervated electrocytes while V3 results from the activities of caudal faces of most electrocytes. Curarization experiments demonstrated that V4 and V5 result from action potential invasion and are not directly elicited by neural activity.Abbreviations AEN1 anterior electromotor nerve 1 - AEN2 anterior electromotor nerve 2 - BMB boraxic methylene blue - CIE caudally innervated electrocytes - EMF electromotive force - EO electric organ - EOD electric organ discharge - I current amplitude - MCIE marginally-caudally innervated electrocytes - MT medial tubes - PEN posterior electromotor nerve - R _n internal impedance - RIE rostrally innervated electrocytes - Rl load resistor - SAT short abdominal tubes - V voltage amplitude     

Mauthner cell-initiated abrupt increase of the electric organ discharge in the weakly electric fishGymnotus carapo

Stimulation of the spinal cord of the electric fish Gymnotus carapo, evoked an abrupt increase in the discharge rate of the electric organ. At the maximum of this response, the rate increased an average of 26 ± 11.8%. The duration of the response was 4.9 ± 2.12 s; its latency was 10.4 ± 1.1 ms. Activation of the Mauthner axon played a decisive role in this phenomenon as indicated by the following: (1) recordings from the axon cap of the Mauthner cell demonstrated that the response was evoked if the Mauthner axon was antidromically activated and (2) a response that was similar to that produced by spinal cord stimulation, was elicited by intracellular stimulation of either Mauthner cell. Stimulation of the eighth nerve could also increase the discharge rate of the electric organ. The effect was greater if a Mauthner cell action potential was elicited. The findings described in the present report, indicate the existence of a functional connection between the Mauthner cell and the electromotor system in Gymnotus carapo. This connection may function to enhance the electrolocative sampling of the environment during Mauthner-cell mediated behaviors. This is a novel function for the Mauthner cell.Abbreviations EHP extrinsic hyperpolarizing potential - EOD electric organ discharge - M-AIR Mauthner initiated abrupt increase in rate - M-cell Mauthner cell - M-axon Mauthner axon - PM pacemaker nucleus - PM-cell pacemaker cell - PPn prepacemaker nucleus - SPPn sublemniscal prepacemaker nucleus     

Waveform generation of the electric organ discharge inGymnotus carapo

Summary The electric organ (EO) ofGymnotus carapo was studied using different neurohistological techniques including conventional electron microscopy. The electric tissue extends along the fish body from the pectoral girdle to the tip of the tail, constituting a single, undivided organ. However, taking into account the number, arrangement, and innervation of the electrocytes, it is possible to divide the EO into three different portions. The more rostral portion is included within the ventral wall of the abdominal cavity. It consists of singly and doubly innervated electrocytes arranged in two rows at each side of the midline. Innervation of this zone is supplied by the first 5–7 segmental nerves and by the anterior electromotor nerves. Segmental nerves terminate on the rostral faces of doubly innervated electrocytes; axons stemming from the anterior electromotor nerves end on the caudal faces of both doubly and singly innervated electrocytes. There is an intermediate body-tail region in which the electrocytes are arranged in four dorsoventral tubes (tubes 1 to 4) on each side of the midline. In this zone, doubly innervated electrocytes (confined within tube 1) coexist together with singly innervated ones, receiving nerve terminals on their caudal faces (tubes 2, 3, and 4). The innervation characteristics appear modified at more distal portions of the tail where the doubly innervated electrocytes of tube 1 are replaced by singly innervated units. The most distal portion of the EO (approximately its terminal 30%) consists of numerous, homogeneously innervated electrocytes with nerve endings distributed exclusively on their caudal faces. Nerve supply to the intermediate and distal regions derives from the posterior electromotor nerves (PENs) which appear as well-defined anatomical entities beyond the level of metamere XXVII. At the bodytail and more distal regions the innervation pattern of the EO is particularly complex. Thin nerve trunks arise from the PENs and project ventrally toward the electrocyte tubes. Before reaching the electric tissue the electromotor axons branch frequently. Our anatomical studies indicate that the EO is heterogeneous, a feature consistent with most recent electrophysiological and biophysical experiments.Abbreviations AEN anterior electromotor nerve - EMN electromotoneurons - EO electric organ - EOD electric organ discharge - LLN lateral line nerve - PEN posterior electromotor nerve     

Androgen binding in the brain and electric organ of a mormyrid fish

Summary The mormyrid fish of Africa produce a weak electric pulse called an Electric Organ Discharge (EOD) that functions in electrical guidance and communication. TheEOD waveform describes the appearance of a single pulse which is produced by the electric organ's excitable cells, the electrocytes. For some species, there is a sex difference in the appearance and duration of the EOD waveform, which is under the control of gonadal steroid hormones. We now show, using biochemical techniques, that the steroid-sensitivity of the myogenic electric organ correlates with the presence of comparatively high levels of androgen-binding activity in the cytosol of electrocytes.TheEOD rhythm describes the rate at which the electric organ fires and is under the control of a central electromotor pathway. Sex differences have also been described for the EOD rhythm. Using steroid autoradiographic techniques, we found uptake of tritium-labelled dihydrotestosterone (³H-DHT) by cells within the reticular formation that lie adjacent to the medullary relay nucleus which innervates the spinal electromotoneurons that excite the electric organ. However, no DHT-binding was observed in the relay or electromotor nuclei.Steroid-concentrating cells were also found in several other brainstem regions, the hypothalamus, and the thalamus. In particular, a group of DHT-concentrating, motoneuron-like cells were observed in the caudal medulla and were identified as aswimbladder orsonic motor nucleus.The biochemical data suggest that the electric organ has evolved a sensitivity to gonadal steroid hormones that may underlie the development of known sex differences in the EOD waveform. The autoradiographic results suggest that if steroids do affect the development of sex differences in the EOD rhythm, it is at some level removed from known spinal and medullary electromotor nuclei.Abbreviations ac anterior commissure - AD area dorsalis telencephali - AV area ventralis telencephali - CBL cerebellum - DT dorsal thalamus - E electromotoneuron - En entopeduncular nucleus,ef lateral line efferent nucleus - EG eminentia granularis - ELLL electroreceptive lateral line lobe,EO electric organ,FV folded part of valvula of cerebellum - H hypothalamus - M mesencephalon - MO medulla oblongota - OB olfactory bulb - OT optic tectum - PO preoptic area - R medullary relay nucleus - rf reticular formation - SC spinal cord - SMN sonic motor nucleus - T telencephalon - TP posterior tuber of diencephalon - TS torus semicircularis - UV unfolded part of valvula of cerebellum,v ventricle - VT ventral thalamus     

Cytoarchitectural organization of the electromotor system in the electric catfish (Malapterurus electricus)

Summary The cytoarchitectural organization of the electromotor system of the electric catfish (Malapterurus electricus) was investigated in order to obtain insight into the neuronal reorganization accompanying the functional transition of a presumptive previous motor system to an electromotor system eliciting electric organ discharge. The electric catfish possesses two giant electromotoneurons situated within the rostral spinal cord. Intracellular dye injections have revealed the enormous extension of the dendritic tree of electromotoneurons. About 50 primary dendrites span the entire lateral funicle and intermediate grey matter, and reveal an extensive contralateral projection. The giant dendritic tree (1.2 mm in rostrocaudal direction) presumably receives inputs from all ascending and descending pathways of the spinal cord. Electromotoneurons and motoneurons receive the same type of fibre inputs, and electromotoneurons and interneurons are connected through common presynaptic elements. The innervation pattern of the electromotoneurons and spinal motoneurons is similar. Synaptic terminals with round synaptic vesicles often reveal chemical contacts and gap junctions. Furthermore, dendrites of the two electromotoneurons form juxtapositions (ephapses) with each other and also with spinal interneurons. Our results suggest that the two electromotoneurons are homologous to median (primary) spinal motoneurons and are the central structures of the electromotor system within the central nervous system of the electric catfish. A high capability of information processing can be attributed to the giant dendritic trees from functional considerations. This presumably enables the electromotoneurons to elicit an electric organ discharge in different behavioural contexts with a minimum of functional reorganization.     

Primary afferent fibers conduct impulses in both directions under physiological stimulus conditions

Summary In electric fish of the family Mormyridae some primary afferent fibers conduct impulses not only from electroreceptors to the brain but also from the brain to the receptors. The efferent impulses may be elicited by electrical stimulation which is within the physiological range, i.e., by stimulation which is similar in amplitude and duration to the stimulation that is caused by the fish's own electric organ discharge. Afferent and efferent impulses in the same afferent fiber were identified by: simultaneously recording from a fiber at two different points, at the receptor and at the nerve trunk (Figs. 2C-H; 3B-D); by cutting the afferent fiber between the brain and the recording site as well as between the recording site and the periphery; and by intra-axonal recording from the afferent fiber near its entry into the brain (Fig. 4). The efferent impulses result from the central integration of a corollary discharge of the electric organ motor command with excitatory and inhibitory input from several different receptors near the one from which afferent impulses originate (Fig. 4). The centrally originating impulse may be capable of modifying the effect of signals originating in the periphery.Abbreviations ELLL electrosensory lateral line lobe - EOCD electric organ corollary discharge - EOD electric organ discharge - epsp excitatory postsynaptic potential - NPLL posterior lateral line nerve     

Retrograde labeling of regenerated electromotor neurons with HRP in a teleost fish,Sternarchus albifrons: Relation to cell death

Summary Back-labeling of regenerated electromotor neurons in the teleost Sternarchus albifrons was performed to test the hypothesis that, in regenerated spinal cord, incorrectly located electromotor neurons are eliminated because their axons do not reach the correct target area (electric organ). In each cross section examined, all of the regenerated electromotor neurons ipsilateral to the implantation site were labeled with horseradish peroxidase, including those ectopic cells located at the edge of the cord, which are later eliminated by selective cell death. Retrograde labeling of these ectopic neurons demonstrates that their axons do extend into the correct target area (the regenerated electric organ). Thus total misdirection of the axons cannot be the cause of their subsequent cell death. We conclude that selective neuronal death in this system does not reflect the absence of axonal projection to the correct target area.A preliminary report on this work has been presented in Soc. Neurosci. Abstracts 10:48 (1984)     

Crustacean cardioactive peptide in the nervous system of the locust,Locusta migratoria: an immunocytochemical study on the ventral nerve cord and peripheral innervation

Summary Crustacean cardioactive peptide-immunoreactive neurons occur in the entire central nervous system of Locusta migratoria. The present paper focuses on mapping studies in the ventral nerve cord and on peripheral projection sites. Two types of contralaterally projecting neurons occur in all neuromers from the subesophageal to the seventh abdominal ganglia. One type forms terminals at the surface of the thoracic nerves 6 and 1, the distal perisympathetic organs, the lateral heart nerves, and on ventral and dorsal diaphragm muscles. Two large neurons in the anterior part and several neurons of a different type in the posterior part of the terminal ganglion project into the last tergal nerves. In the abdominal neuromers 1–7, two types of ipsilaterally projecting neurons occur, one of which gives rise to neurosecretory terminals in the distal perisympathetic organs, in peripheral areas of the transverse, stigmata and lateral heart nerves. Four subesophageal neurons have putative terminals in the neurilemma of the nervus corporis allati II, and in the corpora allata and cardiaca. In addition, several immunoreactive putative interneurons and other neurons were mapped in the ventral nerve cord. A new in situ whole-mount technique was essential for elucidation of the peripheral pathways and targets of the identified neurons, which suggest a role of the peptide in the control of heartbeat, abdominal ventilatory and visceral muscle activity.Abbreviations AG abdominal ganglia - AM alary muscle - AMN alary muscle nerve - CA corpus allatum - CC corpus cardiacum - dPSO distal perisympathetic organ - LHN lateral heart nerve - LT CCAP-immunoreactive lateral tract - NCA nervus corporis allati - NCC nervus corporis cardiaci - NM neuromer - PMN paramedian nerve - PSO perisympathetic organ - SOG subesophageal ganglion - VDM ventral diaphragm muscles - VNC ventral nerve cord     

Steroid influences upon the electrosensory system of weakly electric fish: direct effects upon discharge frequencies with indirect effects upon electroreceptor tuning

Summary Previous studies indicated that gonadal steroids can induce changes in both motor and sensory aspects of the electrosensory system of weakly electric fish: androgens decrease the electric organ discharge frequencies and electroreceptor best frequencies of the South American gymnotoidSternopygus. The relationship between these two effects, however, was not known. In the present study, electric organ discharges (EODs) ofSternopygus dariensis were eliminated by means of spinal cord transections. This was done in order to allow an independent assessment of the influences of gonadal steroids upon electroreceptor tuning and those structures in the CNS responsible for establishing the discharge frequency. Transection alone affected neither the rhythmic discharges of the pacemaker nucleus that normally controls the discharge frequency, nor the best frequencies of electroreceptors. Similarly, administration of the androgen 5-dihydrotestosterone (DHT) to transected animals also had no significant effect upon electroreceptor tuning. DHT did, however, cause significant decreases in the discharge rates of the pacemaker nucleus. Thus, the effects of gonadal steroids upon discharge frequencies in intact animals are a direct consequence of CNS influences, while effects upon electroreceptor tuning likely arise as a secondary consequence of the changed discharges of hormone-treated animals.Abbreviations ALLN anterior lateral line nerve - BF best frequency - DHT 5-dihydrotesterone - EOD electric organ discharge     

Species-specific differences in sensorimotor adaptation are correlated with differences in social structure

Here, we report a species difference in the strength and duration of long-term sensorimotor adaptation in the electromotor output of weakly electric fish. The adaptation is produced by changes in intrinsic excitability in the electromotor pacemaker nucleus; this change is a form of memory that correlates with social structure. A weakly electric fish may be jammed by a similar electric organ discharge (EOD) frequency of another fish and prevents jamming by transiently raising its own emission frequency, a behavior called the jamming avoidance response (JAR). The JAR requires activation of NMDA receptors, and prolonged JAR performance results in long-term frequency elevation (LTFE) of a fish’s EOD frequency for many hours after the jamming stimulus. We find that LTFE is stronger in a shoaling species (Eigenmannia virescens) with a higher probability of encountering jamming conspecifics, when compared to a solitary species (Apteronotus leptorhynchus). Additionally, LTFE persists in Eigenmannia, whereas, it decays over 5–9 h in Apteronotus.     

Immunohistochemical demonstration of calbindin-D 28K (CABP28K) in the spinal cord motoneurons of teleost fish

Summary The distribution and localization of the calciumbinding protein, calbindin-D 28K (CaBP28K), in the spinal cord motoneurons of larvae of the teleost fish, Apteronotus leptorhynchus (Gymnotidae) and Pollimyrus isidori (Mormyridae), and in the adult goldfish, Carassius auratus (Cyprinidae), were determined by means of immunohistochemistry. Sections of whole larvae and goldfish spinal cord were reacted with a polyclonal antibody to rat renal CaBP28K. CaBP28K was located by the PAP technique (Sternberger). It was found in the soma, dendrites, axons and axon terminals of spinal motoneurons but not in those of electromotoneurons of Apteronotus leptorhynchus, whereas it occurred in both motoneurons and electromotoneurons of the larval electric organ of Pollimyrus isidori. In these species CaBP28K was also present in the electromotoneuron axon terminals that make synaptic contacts with the pedicles of the electrocytes. In adult Carassius auratus, CaBP28K was found in the soma, dendrites and axons of certain spinal motoneurons. The results indicate that, in teleosts, the motoneurons containing CaBP28K may represent a well-defined population within the spinal cord; the role of this protein in these cells remains to be determined.     

Changes in nuclear DNA and RNA during epidermal keratinization

Summary The morphology of the oval nucleus of neonatal Torpedo marmorata is described at the light and electron microscopic level of examination. The nucleus is unique relative to other central electromotor centers of electric fish so far described being bilaterally symmetrical, composed of two nerve cell types, and possessing no gap junctions between neurons and their processes. This particular structural plan presents difficulties in accounting for presumed synchronous discharge since it has been strongly argued that electrotonic coupling by means of gap junctions is the primary process by which synchronization is accomplished. Close membrane apposition and dendritic bundling, common features within the nucleus, are discussed as possible alternative structural correlates.     

Vitamin B12 status does not influence central motor conduction time in asymptomatic elderly people: A transcranial magnetic stimulation study

Introduction: Vitamin B12 deficiency causes neurologic and psychiatric disease, especially in older adults. Subacute combined degeneration is characterized by damage to the posterior and lateral spinal cord affecting the corticospinal tract.

Objective: To test corticospinal tract projections using motor evoked potentials (MEPs) by transcranial magnetic stimulation (TMS) in asymptomatic older adults with low vitamin B12 (B12) levels.

Methods: Cross-sectional study of 53 healthy older adults (>70 years). MEPs were recorded in the abductor pollicis brevis and tibialis anterior muscles, at rest and during slight tonic contraction. Central motor conduction time (CMCT) was derived from the latency of MEPs and peripheral motor conduction time (PMCT). Neurophysiological variables were analyzed statistically according to B12 status.

Results: Median age was 74.3?±?3.6 years (58.5% women). Twenty-six out of the 53 subjects had low vitamin B12 levels (B12?p?=?0.014).

Conclusions: No subclinical abnormality of the corticospinal tract is detected in asymptomatic B12-deficient older adults. The peripheral nervous system appears to be more vulnerable to damage attributable to this vitamin deficit. The neurophysiological evaluation of asymptomatic older adults with lower B12 levels should be focused mainly in peripheral nervous system evaluation.     

On the electromotor neurons of both electric organs of Pollimyrus isidori (Mormyridae, Teleostei)

The electromotor neurons (EMNs) of the adult electric organ of Pollimyrus isidori and the "giant neurons" (GNs) found in the more rostral part of the spinal cord were investigated both with light and electron microscopical methods. The two kinds of neuron are more or less similar in size, about 30 microns, and are found in the more dorsal part of the spinal cord. The EMNs show somatosomatic gap junctions, as do the GNs. In addition, chemical synapses could be found between nerve fibres and both types of neurons. Histochemical investigations show that the EMNs and the GNs are both AChE positive. On the basis of the arguments presented in the discussion, we believe that the "giant neurons" are the electromotor neurons of the larval electric organ of Pollimyrus isidori.     

Immunohistochemical localization of a synaptic-vesicle antigen in a cholinergic neuron under conditions of stimulation and rest

Summary An antiserum against a specific component (a glycosamino glycan) of the cholinergic synaptic-vesicle of Torpedo marmorata has been used to investigate the localization of the component in the cell body, its movement within the electromotor axon and its fate within the nerve terminal upon electrical stimulation. After immunofluorescent staining, spots are observed throughout the cytoplasm of the lobe perikarya, although they are concentrated in the region of the axon hillock. Ligation of the electromotor nerves leading from the lobe to electric organ produces a proximal build-up of material which stains readily with the antivesicle antiserum, indicating that the vesicle antigen is transported from the cell body to the nerve terminal. A marked increase in indirect immunofluorescent staining of the electric organ is observed in the nerve ending upon electrical stimulation. We interpret this result as fusion of the vesicles with the presynaptic plasma membrane and exteriorization of the vesicle antigen to the extracellular space, thereby facilitating its staining. After recovery of the system the fluorescence declines, a result that is consistent with the reinternalization of the vesicle antigen into the core of reformed vesicles. The results support a mechanism whereby vesicles recycle within the nerve terminal and transmitter is released by exocytosis.     

Membrane proteins of synaptic vesicles and cytoskeletal specializations at the node of Ranvier in electric ray and rat

Summary Binding sites for antibodies against membrane proteins of synaptic vesicles have been shown to be enhanced at nodes of Ranvier in electromotor axons of the electric ray Torpedo marmorata and sciatic nerve axons of the rat, using indirect immunofluorescence and monoclonal antibodies against the synaptic vesicle transmembrane proteins SV2 and synaptophysin (rat) or SV2 (Torpedo). In the electric lobe of Torpedo, vesicle-membrane constituents occurred at higher density in the proximal axon segments covered by oligodendroglia cells than in the distal axon segments where myelin is formed by Schwann cells. Antibody binding sites were enhanced at nodes forming the borderline of the central and peripheral nervous systems. Filamentous actin was present in the Schwann-cell processes covering both the nodal and the paranodal axon segments as suggested by the pattern of phalloidin labelling. Furthermore, in rat sciatic nerve, Schmidt-Lanterman incisures were intensely labelled by phalloidin. A similar nodal distribution was found for binding sites of antibodies against actin and myosin. Binding of antibodies to tubulin was enhanced at nodes in Torpedo electromotor axons. The apparent nodal accumulation of constituents of synaptic vesicle membranes and the presence of filamentous actin and of myosin are discussed in relation to the substantial constriction of the axoplasm at nodes of Ranvier.