Properties of dorsal root unmedullated fibers on the two sides of the ganglion

As an aid in the interpretation of the physiological properties of unmedullated nerve fibers, particularly those having their cells of origin in the dorsal root ganglia, more precise information about their morphology has been acquired through employment of the electron microscope. The appearance of the fibers in the skin nerves is described, with special reference to the structure of their sheaths; and a notation is made about the bearing of the axon-sheath relationship on the biophysical mechanism of conduction (p. 714). There is no basic difference between the sheath systems of the d.r.C and the s.C fibers. Attention is called to a point of similarity between the sheaths of unmyelinated and myelinated axons (p. 715). An assessment was made of the likelihood of interaction between the fibers. In action potentials showing temporal dispersion at several distances, the elevations appeared in their calculated positions. A model of a group of Schwann sheaths was constructed from successive electron microscope sections, showing that the lengths of the sheath branches are short in comparison with the wave lengths of the action potentials. Supported by these and other considerations, the argument is strongly in favor of the conclusion that among d.r.C fibers, as in other fibers, there is no cross-excitation between the axons. A new analysis of the size distribution of the fibers in a sural nerve was made from electron microscope pictures; and from the measurements the action potential was constructed. The result confirmed the view, previously expressed, that the velocities of conduction in the fibers can be precisely accounted for by multiplying the diameters by a constant. In the dorsal roots, the striking change that takes place in the appearance of the fibers and their disposition in the Schwann sheaths can be seen in Fig. 11. The axons partake of the special properties of the peripheral branches, which necessitated the creation of the subdivision of d.r.C fibers. But, their diameters are much smaller. At a set of reduced conduction velocities the configuration of the compound action potential in the nerves is repeated in the roots, with the root velocities still conforming to the size-velocity rule derived from nerve axons.     

Modification by temperature of conduction and ganglionic transmission in the gastropod nervous system

The pedal ganglia of the terrestrial gastropod Ariolimax contain junctions between nerve fibers which are shown to be preferential points of fatigue and which exhibit facilitation (summation) of preganglionic impulses to produce a postganglionic spike. These characteristics in conjunction with others previously reported (reversible susceptibility to nicotine, convergence of preganglionic impulses, and inhibition of transmission through setting up a refractory state in the postganglionic fiber) are considered sufficient to indicate synaptic transmission in the pedal ganglia. The mean conduction velocity of the fastest fibers in the pedal nerves is 0.52 meter per second for preganglionic and 0.50 meter per second for postganglionic fibers at 7.56°C. The conduction rates at 21.76°C. are respectively 0.80 meter per second and 0.83 meter per second. The mean ganglionic delay is 0.033 second at 7.56°C. and 0.019 second at 21.76°C. The mean Q₁₀'s for conduction velocity are thus 1.37 for preganglionic and 1.42 for postganglionic fibers. The mean Q₁₀ for ganglionic delay is 1.49. If the assumption is made that the Q₁₀ for ganglionic delay is that of a limiting reaction, this figure then represents a value below which the Q₁₀ for synaptic delay is statistically improbable.     

Evidence for GABAergic fibers entering the superior cervical ganglion of rat from the preganglionic nerve trunk

Summary The origin of gamma-aminobutyric acid immunoreactive (GABA-IR) nerve fibers present in the superior cervical ganglion (SCG) of rat was investigated. With immunocytochemical techniques many nerve fibers showed GABA-like positivity in the cervical sympathetic trunk, whereas similar staining could not be revealed in the internal carotid nerve or in the external carotid nerve. Ligation of the cervical sympathetic trunk for 24 h resulted a dramatic reduction in the staining density in the ganglion and in the cervical sympathetic trunk distal to the ligature. After transection of the preganglionic nerve fibers for eleven days or more, very few fibers staining for GABA were seen in the ganglion. The immunohistochemical results suggest that a major source of GABA within the SCG is a population of GABAergic axons entering from the preganglionic trunk.     

The localization of the β-subtype of protein kinase C (PKC-β) in rat sympathetic neurons

Summary The localization of PKC- was studied in rat sympathetic neurons using a polyclonal antibody specific for the ₁- and ₂-subspecies. The tissues studied included the superior cervical (SCG) and hypogastric (HGG) ganglia and the target tissues of the SCG and HGG neurons: the submandibular gland, iris, prostate and vas deferens. PKC--LI was found in nerve fibers in both ganglia. A proportion of the fibers in the SCG disappeared after decentralization, suggesting that the fibers were of both pre- and postganglionic origin. The somata of the HGG and SCG neurons expressed varying amounts of PKC--LI, the majority of SCG neurons being labelled only after colchicine treatment. In all target tissues there were PKC--immunoreactive nerve fibers in bundles, but the most peripheral branches of the fibers were negatively labelled. The results show that PKC--LI is widely present in sympathetic postganglionic neurons with mainly quantitative differences. The lack of PKC- in the most peripheral branches of nerve fibers might be a general feature of sympathetic postganglionic neurons, suggesting that the participation of PKC- in neurotransmitter release and in other functions in nerve terminals in sympathetic adrenergic neurons is unlikely.     

Evidence for GABAergic fibers entering the superior cervical ganglion of rat from the preganglionic nerve trunk

The origin of gamma-aminobutyric acid immunoreactive (GABA-IR) nerve fibers present in the superior cervical ganglion (SCG) of rat was investigated. With immunocytochemical techniques many nerve fibers showed GABA-like positivity in the cervical sympathetic trunk, whereas similar staining could not be revealed in the internal carotid nerve or in the external carotid nerve. Ligation of the cervical sympathetic trunk for 24 h resulted a dramatic reduction in the staining density in the ganglion and in the cervical sympathetic trunk distal to the ligature. After transection of the preganglionic nerve fibers for eleven days or more, very few fibers staining for GABA were seen in the ganglion. The immunohistochemical results suggest that a major source of GABA within the SCG is a population of GABAergic axons entering from the preganglionic trunk.     

Temperature Effect on Proximal to Distal Gradient of Quantal Release of Acetylcholine at Frog Endplate

The conduction velocity of the nerve terminal, mean quantal content, and release latencies of uniquantal endplate currents (EPCs) were recorded in proximal, central, and distal parts of the terminal by extracellular pipettes located 5, 50, and 100 mm from the end of myelinated nerve trunk. The spike conduction velocity, minimal latency, modal value of the latency histograms, and time interval during which 90% of EPCs released (P₉₀) at distal, central, and proximal part of the frog nerve terminal have different temperature dependency between 10° and 28°C. As shown by the size and time-course of reconstructed multiquantal EPCs, the secretion synchronization, which is greatest in distal parts, compensates at least partly for the progressive slowing of spike conduction velocity in the proximodistal direction, in particular at lower temperatures.     

Olfactory nerve fibers

Cross sections of olfactory nerves present a unique appearance. They indicate the presence of large numbers of very small nerve fibers, with a modal diameter of about 0.2 µ and a narrow range for their size variation. From one side of the nasal septum of a pig the yield of fibers was estimated at 6,000,000; the number arising from the turbinates would be considerably larger. The fibers are attached to the membranes of the Schwann sheaths in large bundles through mesaxons longer and more branched than those that have been seen in other nerves. Continuity of the axons between the nerves and the bipolar cells was traced in an examination of the olfactory mucous membrane; and the indication of a one-to-one relationship between cells and axons was reinforced by a comparative count. After the axons leave the bipolar cells they become incased in the central projections of the sustentacular cells. Where the latter come into contact with the basal cells the axons emerge to push back the plasma membranes of the basal cells in the first step in acquiring their nerve sheaths. Later steps are described. When the axons are delivered by the basal cells to the collecting Schwann tubes, they are already aggregated into small bundles with sheaths fundamentally the same as those they will possess until they are delivered to the glia in the olfactory bulb. Some of the aspects of the cytology of the bipolar cells and adjoining sustentacular cells are described. A survey of the physiological properties of olfactory nerve fibers was made in some experiments on the olfactory nerve of the pike. Almost all of the action potential is encompassed within a single elevation, manifesting at its front a conduction velocity of 0.2 m./sec. For a comparison, the last elevation in the C action potential in the sciatic nerve of the frog is cited as an example of conduction at the same velocity. Though expressed through long time constants, the properties of the pike olfactory fibers conform to the generalized schema for properties of vertebrate nerve fibers. This conformity signalizes that they differ from the exceptional properties of the unmedullated fibers of dorsal root origin. An afferent function for unmedullated nerve fibers does not imply that the fibers concerned are alike in their physiological properties.     

Preparation of affinity-purified,biotinylated tetanus toxin,and characterization and localization of cell surface binding sites on nerve growth factor-treated PC12 cells

Biotinylated derivatives of tetanus toxin were prepared and isolated by chromatofocusing and ganglioside-affinity chromatography. Biotinylation was monitored by the appearance of a 210,00 dalton complex upon SDS-polyacrylamide gel electrophoresis in the presence of avidin, and by selective binding to an avidin-Sepharose gel. At molar biotin:toxin ratios from 11 to 201 only biotinylated derivatives with low toxicity were obtained; these derivatives, however, retained 60–80% of their specific binding affinity for brain synaptosomes. A biotinylated tetanus toxin derivative purified by ganglioside-affinity chromatography was used to identify and localize tetanus toxin binding sites on PC12 cells. Electron microscopic analysis with streptavidin-gold revealed very low levels of tetanus toxin binding sites on the surface of untreated cells, and the appearance of such binding sites during the second week of nerve growth factor-induced differentiation. Examination of micrographs of the differentiated cells indicated that the tetanus toxin binding sites sites are concentrated on the neurites, with relatively few appearing on the cell bodies. Cognate studies using¹²⁵I-labeled, affinity-purified tetanus toxin revealed an increase in PC12 binding capacity from about 0.07 nmol/mg protein in untreated cells to 0.8 nmoles/mg protein in cells treated for 14 days with nerve growth factor. Cells treated in suspension for 2–3 weeks with nerve growth factor do not express tetanus toxin binding sites; upon plating, these cells required one week for the appearance of binding sites, although neurites grew much more rapidly from these primed cells. The high binding capacity of these tetanus toxin sites, as well as their sensitivity to neuraminidase, is indicative of a polysialoganglioside structure. The advantages of biotinylated tetanus toxin derivatives are discussed and the significance of nerve growth factor-differentiated PC12 cells grown as monolayers as a model for the study of the development, localization, and function of neuraminidase-sensitive tetanus toxin binding sites is presented.Abbreviations PBS phosphate-buffered saline - STS sucrose-Tris-serum solution - NGF nerve growth factor - C collagen - PL polylysine - BBG bovine brain ganglioside mixture - GM₁ gafactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosyl ceramide - GD_1a [N-acetylneuraminyl]-galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosyl ceramide - GT_1a [N-aceylneuraminyl]-galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosyl ceramide - GD_1b galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl-N-acetylneuraminyl]-galactosylglucosyl ceramide - GT_1b [N-acetylneuraminyl]-galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl-N-acetylneuraminyl] galactosylglucosyl ceramide - NANA N-acetylneuraminic acid     

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)     

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.     

Water transport in invertebrate peripheral nerve fibers

Osmotic and diffusion permeabilities (P_f and P_d) of invertebrate nerve fibers to tritiated water were measured to determine what water flux studies could reveal about "the nerve membrane" and to directly test the possibility of active transport of water into or out of invertebrate nerve fibers. P_f/P_d ratios for lobster walking leg nerve fibers were found to be about 20 ± 7 at 14°C. P_d measurements were made for squid giant axons at 25°C. and found to yield a value of 4 x 10^–4 cm.^–1 sec.^–1. When combined with the data of D. K. Hill for P_f, a P_f/P_d ratio of 21 ± 5 is obtained. These P_f/P_d ratios correspond to "effective pore radii" of about 16 ± 4 angstrom units, according to theories developed by Koefoed-Johnsen and Ussing and independently by Pappenheimer and his colleagues. Variations of water flux ratios with temperatures were studied and apparent activation energies calculated for both diffusion experiments and osmotic filtration experiments using the Arrhenius equation, and found to be close to 3 to 5 cal. per mole of water transferred. Cyanide (5 x 10^–3 molar) and iodoacetate (1 x 10^–3 molar) poisoned lobster leg nerve fibers showed no appreciable change in diffusion or osmotic filtration water effluxes. Caution in interpreting these proposed channels as simple pores was emphasized, but the possibility that such channels exist and are related to ionic flow is not incompatible with electrophysiological data.     

Immunohistochemical localization of tryptophan hydroxylase and serotonin transporter in the carotid body of the rat

It has been proposed that serotonin (5-HT) facilitates the chemosensory activity of the carotid body (CB). In the present study, we investigated mRNA expression and immunohistochemical localization of the 5-HT synthetic enzyme isoforms, tryptophan hydroxylase 1 (TPH1) and TPH2, and the 5-HT plasma membrane transport protein, 5-HT transporter (SERT), in the CB of the rat. RT-PCR analysis detected the expression of mRNA for TPH1 and SERT in extracts of the CB. Using immunohistochemistry, 5-HT immunoreactivity was observed in a few glomus cells. TPH1 and SERT immunoreactivities were observed in almost all glomus cells. SERT immunoreactivity was seen on nerve fibers with TPH1 immunoreactivity. SERT immunoreactivity was also observed in varicose nerve fibers immunoreactive for dopamine beta-hydroxylase, but not in nerve fibers immunoreactive for vesicular acetylcholine transporters or nerve terminals immunoreactive for P2X₃ purinoreceptors. These results suggest that 5-HT is synthesized and released from glomus cells and sympathetic nerve fibers in the CB of the rat, and that the chemosensory activity of the CB is regulated by 5-HT from glomus cells and sympathetic nerve fibers.     

An analysis of the interactions between sympathetic nerve fibers and smooth muscle cells in tissue culture

The interactions between sympathetic nerve fibers and smooth muscle cells and fibroblasts from the newborn guinea pig vas deferens were studied in tissue culture with phase contrast microscopy, time-lapse microcinematography, catecholamine fluorescence histochemistry and scanning and transmission electron microscopy. The amount of sympathetic nerve fiber growth, its catecholamine fluorescence reaction and the size of the nerve cell bodies and their nuclei all increased in the presence of vas deferens tissue. Specific growth of nerve fibers to large clumps of vas deferens tissue was seen from distances of up to 2 mm. In contrast, no specific growth from a distance occurred to single cells or small groups of cells. However, random contact with a muscle cell often led to close, extensive, and long-lasting associations. Contact with fibroblasts was always transitory.The rate of sympathetic nerve fiber growth over individual muscle cells was faster than over fibroblasts, which, in turn, was faster than over the collagen-coated surface of the coverslip. Palpation of a muscle cell by a nerve fiber growth cone increased the rate of spontaneous contraction of the muscle cell, the extent of the increase being dependent on the number of nerve fibers involved. Multiple innervation of a smooth muscle cell occurred if nerve fibers reached the cell at about the same time, but not if there was a close association already established. These results are discussed in relation to possible interactions of sympathetic nerve fibers with smooth muscle cells in vivo.     

Spike train structure following application of nociceptive potassium chloride solutions to the cat saphenous nerve

The structure of the spike train coding the nociceptive signal was investigated by a cross-correlation method. The action of nociceptive solutions of potassium chloride (125–1000 mM) on the saphenous nerve caused excitation of both myelinated and nonmyelinated fibers. The density of the spike train increased during the action of the stimulus up to a threshold sufficient for the appearance of nociceptive reflexes. The maximum of these reflexes coincided with the appearance of synchronous pulsations of discharges in the group of unmyelinated fibers. The nociceptive signal evoked by direct action of highly concentrated potassium chloride solutions on the nerve is thus coded by a high density of the spike train generated by the nerve fibers. Synchronous pulsations may be present in the spike train under these circumstances.Research Institute of Applied Mathematics and Cybernetics, N. I. Lobachevski Gor'kii State University. Translated from Neirofiziologiya, Vol. 9, No. 6, pp. 598–605, November–December, 1977.     

Changes in the duration of the electric response of single nerve fibers following repetitive stimulation

Single nerve fibers were isolated from the nerve innervating the sartorius or semitendinosus muscle of the toad (Bufo marinus). Single nerve fiber responses were recorded with three arrangements of the "bridge insulator" method. During stimulation at 50 to 150 pulses per second for 20 to 140 minutes the spike duration was progressively increased. After tetanization the spike duration usually continued to increase at a more rapid rate. Within 5 to 60 minutes further prolongation stopped and within 1 to 10 hours the spike duration was normal. The duration of the response of tetanized fibers was from 2.5 to more than 10 times the spike duration of untetanized fibers. Prolongation was observed in nerve fibers isolated from nerves tetanized in vivo.     

Conduction Properties Distinguish Unmyelinated Sympathetic Efferent Fibers and Unmyelinated Primary Afferent Fibers in the Monkey

Background

Different classes of unmyelinated nerve fibers appear to exhibit distinct conductive properties. We sought a criterion based on conduction properties for distinguishing sympathetic efferents and unmyelinated, primary afferents in peripheral nerves.

Methodology/Principal Findings

In anesthetized monkey, centrifugal or centripetal recordings were made from single unmyelinated nerve fibers in the peroneal or sural nerve, and electrical stimuli were applied to either the sciatic nerve or the cutaneous nerve endings, respectively. In centrifugal recordings, electrical stimulation at the sympathetic chain and dorsal root was used to determine the fiber''s origin. In centrifugal recordings, sympathetic fibers exhibited absolute speeding of conduction to a single pair of electrical stimuli separated by 50 ms; the second action potential was conducted faster (0.61 0.16%) than the first unconditioned action potential. This was never observed in primary afferents. Following 2 Hz stimulation (3 min), activity-dependent slowing of conduction in the sympathetics (8.6 0.5%) was greater than in one afferent group (6.7 0.5%) but substantially less than in a second afferent group (29.4 1.9%). In centripetal recordings, most mechanically-insensitive fibers also exhibited absolute speeding to twin pulse stimulation. The subset that did not show this absolute speeding was responsive to chemical stimuli (histamine, capsaicin) and likely consists of mechanically-insensitive afferents. During repetitive twin pulse stimulation, mechanosensitive afferents developed speeding, and speeding in sympathetic fibers increased.

Conclusions/Significance

The presence of absolute speeding provides a criterion by which sympathetic efferents can be differentiated from primary afferents. The differences in conduction properties between sympathetics and afferents likely reflect differential expression of voltage-sensitive ion channels.     

Differentiation of Nerve Terminals in the Crayfish Opener Muscle and Its Functional Significance

Junctional potentials (jp's) recorded from superficial distal fibers of the crayfish opener muscle are up to 50 times larger than jp' in superficial central fibers when the single motor axon that innervates the muscle is stimulated at a frequency of 1/sec or less. At 80/sec, in contrast, central jp's are up to four times larger than those observed in distal fibers. The tension produced by single muscle fibers of either type is directly proportional to the integral of the time-voltage curve minus an excitation-contraction coupling threshold of 3 mv. Distal fibers therefore produce almost all the total muscle tension at low frequencies of stimulation and central fibers add an increasingly greater contribution as their nerve endings begin to facilitate in response to increased rate of motor discharge. Differentiation of muscle membrane characteristics (input resistance, space constant, time constant) cannot account for these differences in facilitation ratios. The mechanism of neuronal differentiation is not based upon the size or effectiveness of transmitter quanta, since equal sized jp's have equal variances;: mjp sizes and variances are also equal. No differences were found between fiber types in rates of transmitter mobilization, density of innervation, or the relationship between transmitter release and terminal depolarization. Single terminals on distal fibers were found to release transmitter with a greater probability than central terminals. More effective invasion of distal terminals by the nerve impulse at low frequencies can account for the difference.     

Electrophysiological investigation of pathways in the middle cervical sympathetic ganglion of the turtle

Single unit responses in the middle cervical sympathetic ganglion ofEmys orbicularis to stimulation of other nerves and changes in these responses during the action of sympathetic blocking agents on the ganglion were investigated. The results showed that some fibers of the cervical sympathetic trunk of the turtle are interrupted in this ganglion. Postganglionic fibers pass out of the ganglion and enter the lateral branch and the sympathetic trunk. Other fibers pass through the ganglion without interruption and, together with postganglionic fibers, leave the ganglion in the cervical sympathetic trunk in a cranial direction. The velocity of conduction of excitation along the preganglionic fibers is between 4–3 and 2–1.5 m/sec and along the postganglionic fibers between 4–2.6 and 0.7–0.5 m/sec (fibers of types B₂ and C). Synaptic delay in the fast-conducting fibers averages 6.6 msec. Preganglionic fast-conducting fibers form synaptic contacts on neurons with type B₂ axons, while preganglionic slow-conducting fibers form contacts on neurons with type C axons. Terminals of two preganglionic fibers differing very slightly in their threshold of excitability, and probably constituting the same group, converge on some neurons.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukranian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 1, pp. 83–89, January–February, 1972.     

Electroreceptor mechanisms of the ampullae of lorenzini in skates

Responses of the receptor epithelium of single electrically isolated ampullae of Lorenzini and spike responses of nerve fibers connected to them to electrical stimulation under voltage clamping conditions were studied in experiments on the Black Sea skateRaja clavata. The preparations had an input resistance of 200–800 KΩ, a transepithelial resting potential of between 0 and ?2 mV, and the usual spontaneous spike activity in their afferent fibers. Thresholds of the electroreceptors were 2–10 µV (current about 10^?11 A). Within the working range of the electroreceptors (up to ±500 µV, current up to 1 nA) the current-voltage characteristic curve of the epithelium was linear without any evidence of spike or wave activity of the receptor cells. With negative currents of over 1–10 nA a regenerative spike appeared on the epithelium and was accompanied by an uncharacteristic pattern of spike discharge of the nerve fiber. It is concluded that, contrary to Bennett's hypothesis, spike or oscillatory activity bears no relationship to normal working of electroreceptors. It is postulated that "secondarily sensitive" receptor cells share a common functional organization, which is based on a chemical synapse with high electrical sensitivity.