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C R Fourtner

Publications and source records attributed to C R Fourtner.

18 recordsLinked to original sources
PubMed1993

Identification of a proline residue as a transduction element involved in voltage gating of gap junctions.

Gap junction channels are structurally distinct from other ion channels in that they comprise two hemichannels which interact head-to-head to form an aqueous channel between cells. Intercellular voltage differences together with increased intracellular concentrations of H+ and Ca2+ cause closure of these normally patent channels. The relative sensitivity to voltage varies with the subunit (connexin) composition of the channels. The third of four transmembrane-spanning regions (M3) in connexins has been proposed to form the channel lining, and a global 'tilting' of the hemichannel subunits has been correlated with channel closure. But specific components involved in transduction of channel gating events have not been identified in either gap junctions or other ion channel classes (however, see model in ref. 5). We have examined a strictly conserved proline centrally located in M2 of connexin proteins. Mutation of this proline (Pro 87) in connexin 26 causes a reversal in the voltage-gating response when the mutant hemichannel is paired with wild-type connexin 26 in the Xenopus oocyte system. This suggests that the unique properties associated with this residue are critical to the transduction of voltage gating in these channels.

Amino Acid Sequence↗
PubMed1990

Morphallaxis in an aquatic oligochaete, Lumbriculus variegatus: reorganization of escape reflexes in regenerating body fragments.

We describe functional and anatomical correlates of the reorganization of giant nerve fiber-mediated escape reflexes in body fragments of an aquatic oligochaete, Lumbriculus variegatus, a species that reproduces asexually by fragmentation. Since fragments from any axial position always regenerate short heads (seven or eight segments long) and much longer tail sections, segments originating from posterior fragments become transposed along the longitudinal axis and acquire, by morphallaxis, features of escape reflex organization that conform to their new anterior position. Using noninvasive electrophysiological recordings we have quantified, on a day-to-day and a segment-by-segment basis, the reorganization that occurs in sensory field arrangements of the medial (MGF) and lateral (LGF) giant nerve fibers, as well as changes in giant fiber conduction velocity and morphometry. Our results show that (1) posterior fragments, originally subserved by the LGF sensory field gradually become subserved by the MGF sensory field; (2) appropriate increases in the ratio of MGF:LGF cross-sectional area, perimeter, and conduction velocity accompany the reorganization in giant fiber sensory fields; and (3) sensory field reorganization can be repeatedly reversed by additional amputations. These results demonstrate that the functional organization of escape reflexes is highly plastic and that morphallaxis may result from the counterbalance of morphogenic influences localized within the anterior and posterior ends of regenerating body fragments.

Action Potentials↗
PubMed1988

The lateral giant fibers of the tubificid worm, Branchiura sowerbyi: structural and functional asymmetry in a paired interneuronal system.

Neuroanatomical and ultrastructural studies of the paired lateral giant nerve fibers (LGFs) in posterior segments of the tubificid worm, Branchiura sowerbyi, demonstrate that the fibers are 1) segmental in origin (two cell bodies per segment), 2) joined longitudinally and transversely to form an intersegmental syncytial network, and 3) surrounded by a myelinlike sheath. The LGFs are unique among paired giant fiber systems because of their extreme asymmetry, the diameter of the left fiber being several times greater than that of the right. Electrophysiological studies demonstrate that the small, right fiber has a high input resistance and, during mechanosensory stimulation, functions as the locus for LGF spike initiation. The larger, left fiber contributes by enhancing the speed of LGF spike conduction along the animal. One physiological benefit of this asymmetric arrangement may be optimization of escape reflex sensitivity to mechanosensory inputs.

Action Potentials↗
PubMed1988

Neural control of ventilation in the shore crab, Carcinus maenas. II. Frequency-modulating interneurons.

1. We have identified a class of nonspiking interneurons which can control the frequency of ventilation in a graded manner. These frequency modulating interneurons (FMis) also receive synaptic inputs in-phase with the ventilatory motor output providing a functional positive feedback loop in the ventilatory system. The class of FMis is composed of three morphologically and physiologically distinct interneurons, FMi1, FMi2 and FMi3. 2. Depolarization of FMi1 increases the rate of ventilation, while hyperpolarization decreases the rate (Fig. 1). This control is restricted to a single ventilatory central pattern generator (CPG), (Fig. 2), although FMi1 sends processes into the neuropils of both hemiganglionic CPGs (Fig. 3). 3. Hyperpolarization of FMi2 increases the rate of both ventilatory CPGs while depolarization of this cell slows and eventually arrests the rhythm (Figs. 5 and 6). FMi2 receives a synaptic input correlated with the motor output of each of the ventilatory CPGs (Fig. 4). During periods of reversed ventilation, this cell is abruptly hyperpolarized and continues to be driven in-phase with the ventilatory motor output (Fig. 7). 4. Hyperpolarization of FMi3 increases the rate of ventilation and depolarization decreases the rate of ventilation produced by both CPGs (Fig. 10). This control of the ventilatory rate by FMi3 is graded (Fig. 11). There is no apparent change in the membrane potential of FMi3 during reversed ventilation and it is morphologically distinct from FMi2. 5. FMi2 and FMi3 may be involved in the switch in ventilatory motor pattern from forward to reversed ventilation. Hyperpolarization of FMi2 and depolarization of FMi3 can elicit bouts of reversed ventilation from both CPGs (Fig. 13). 6. These results suggest that the FM interneurons act in parallel to control the frequency of ventilation and may act as integrating elements between spiking 'command' fibers in the circumesophageal connectives and the nonspiking interneurons of the ventilatory CPG.

Animals↗
PubMed1988

Functional characterization of a voltage-gated anion channel from teleost fish intestinal epithelium.

An anion channel was isolated, using patch-clamp technique, from the basolateral membrane of goby intestinal epithelial cells. Single-channel conductance varied over a range from 20 to 90 pS. The channel was voltage-gated over the physiological range of cell membrane potential with depolarization increasing the proportion of time in the open state. There was no Ca2+ sensitivity. The selectivity sequence was SO4(2-) greater than Cl- greater than Mes-. The channel may function in vivo as one of several avenues of basolateral membrane Cl- exit with the voltage-gating property serving to match basolateral Cl- exit to apical entry.

Animals↗
PubMed1984

Patterned regeneration of internal femoral structures in the cockroach, Periplaneta americana L.

The morphology of the nerve and tracheal supply to the extensor tibiae muscle in normal legs was compared to that in regenerate legs. In normal femurs, the extensor nerve and trachea extend along the posterior surface of the extensor muscle. The nerve and trachea are closely associated and branch coincidently at regular intervals. In regenerate femurs, the nerve and trachea are not closely associated with each other, and both structures differ from normals in their branching patterns. The results suggest that tissue level interactions during regeneration differ from those during embryogenesis.

Animals↗
PubMed1981

Mechanical properties of a slow muscle in the cockroach.

The mechanical properties of the metacoxal muscle, 177d, in the cockroach, Periplaneta americana, was investigated. The muscle exhibited a mean resting tension of 2.6 +/- 1.3g SD. Neurally evoked tension summed with the resting tension and the relaxation phase of the evoked tension varied from less than 1 s to several minutes. This residual tension varied not only in duration but also in amplitude. Stimulation of inhibitory axons increased the rate of relaxation and thereby abolished the residual tension. However, inhibitory stimulation never reduced the resting tension. Stimulation of the main leg nerve at several times the threshold of the inhibitory axons could evoke residual tension. Recording of synaptic potentials from the two histochemically different fiber types (dorsal and ventral groups) revealed large hyperpolarizations in the ventral fibers and decreased duration and amplitude of excitatory potentials in the dorsal fibers. These results suggest that there are a variety of ways in which tension can be evoked, maintained, and controlled in these muscles.

Animals↗
PubMed1978

Specificity of afferent and efferent regeneration in the cockroach: establishment of a reflex pathway between contralaterally homologous target cells.

1. In 132 cockroaches the main leg nerve on one side (right), of the metathoracic segment was crossed to the opposite (left) side and allowed to regenerate. In 3-8 wk, 59% of the animals displayed reflex activity in the left leg (behaviorally demonstrated by leg withdrawal following tarsal stimulation). 2. EMGs from the femoral extensor revealed potentials characteristic of normal activity in the extensor, which is innervated by an identified motor neuron, Ds. 3. Intracellular recordings from processes within the right hemiganglion of the metathoracic ganglion (CNS) demonstrated 1:1 activity between a unit in the CNS recording and the EMG of the left extensor. Subsequent intracellular staining revealed that the unit was on the right side of the CNS and was identified as motor neuron Ds by the location of its soma and dendrites. This finding indicated that specific, contralateral, efferent reinnervation occurs in the cockroach. 4. In normal cockroaches a monosynaptic reflex exists between hair plate afferents and Ds. A temporal analysis (stimulus-interval histogram) indicated that the reflex is also established in the crossed-regenerated animals. These data suggested that specific contralateral afferent reinnervation also occurs in the cockroach and that the monosynaptic nature of the normal reflex was reestablished. 5. Therefore, cell-to-cell specificity in neuron-to-neuron or neuron-to-muscle interactions not only occurs in normally developing or regenerating animals but also occurs between contralaterally homologous target cells, given the proper experimental conditions. It is also suggested that this experimental procedure of redesigning pathways may be a useful tool for further studies of behavior.

Afferent Pathways↗
PubMed1977

Excitation of the common inhibitory motor neuron: a possible role in the startle reflex of the cockroach, Periplaneta americana.

The responses of the widespread common inhibitory motor neuron (CI) to tactile stimulation of the cercus and the abdomen and electrical stimulation of the cercal nerve and the abdominal connectives are investigated. Tactile stimulation produces high frequency (greater than 500 impulses/s) spike discharge in CI with the onset of CI activity preceding the discharge of the excitatory motor neurons. Electrical stimulation of the connectives demonstrates a monosynaptic connection between at least one intermediate sized fiber (conduction velocity =3.7 m/s) in the abdominal connective and the ipsilateral CIs in the meso-and metathoracic ganglia. Electrical stimulation of the cercal nerve suggests a disynaptic path from cercal nerve to CI. Arguments are presented for a cercal afferent-to-CI reflex and the possible functional role of early excitation of CI is discussed.

Animals↗
PubMed1976

Connexions between hair-plate afferents and motoneurones in the cockroach leg.

1. The trochanteral hair-plate afferents in the metathoracic leg of the cockroach, Periplaneta americana, were stimulated electrically and at the same time intracellular recordings were made from either motoneurones, interneurones or afferent terminals within the methathoracic ganglion. 2. Activity in the hair-plate afferents evoked short latency excitatory postsynaptic potentials (EPSPs) in femur flexor motoneurones. The latency of the IPSPs was on average 1-8 ms longer than the latency ofthe EPSPs. 3. Intracellular recordings from terminal branches of the hair-plate afferents showed that the delay between the peak of the afferent terminal spike and the beginning of the EPSPs is about 0.4 ms. This finding, together with the observations that the amplitude of the EPSPs is increased by the passage of hyperpolarizing current and decreased following high-frequency stimulation, indicates that the EPpSPs are evoked via-monosynaptic chemical synaptic junctions. 4. The observations of the long latency of the IPSPs, the need for a number of afferents to be simultaneously acive for them to be evoked and the occasional variability in latency, all indicate that the IPSPs are evoked via a disynaptic pathway...

Action Potentials↗
PubMed1976

Stretch-sensitive neural units in the body wall of the earthworm, Lumbricus terrestris L.

1. Sensory neural units responding to sinusoidal stretching of the body wall were studied in the earthworm, Lumbricus terrestris L. 2. A phasic stretch-sensitive unit found in segmental nerve I responded optimally to stretching at frequencies of 4-6/min. 3. The number of spikes per stretch and the spike frequency in the unit were directly related to the amplitude of the applied stretch within a range of 0-2-0-7 mm stretch/segment. 4. The ranges of amplitude and frequency sensitivity for the unit in isolated preparations corresponded closely to stretch parameters seen during peristaltic locomotion in intact animals. 5. Stretch-sensitive responses in segmental nerve II-III were more variable; some units responded to longitudinal stretch while others responded to relaxation.

Action Potentials↗
PubMed1975

Nonspiking interneurons in walking system of the cockroach.

Intracellular recordings were made from the neurites of interneurons and motoneurons in the metathoracic ganglion of the cockroach, Periplaneta americana. Many neurons were penetrated which failed to produce action potentials on the application of large depolarizing currents. Nevertheless, some of them strongly excited and/or inhibited slow motoneurons innervating leg musculature, even with weak depolariziing musculature, even with weak depolarizing currents. Cobalt-sulfide-straining of these nonspiking neurons showed them to be interneurons with their neurites contained entirely within the metathoracic ganglion. Two further characteristics of these interneurons were rapid spontaneous fluctuations in membrane potential and a low resting membrane potential. One nonspiking neuron, interneuron I, when depolarized caused a strong excitation of the set of slow levator motoneurons which discharge in bursts during stepping movements of the metathoracic leg. During rhythmic leg movements the membrane potential of interneuron I oscillated with the depolarizing phases occurring at the same time as bursts of activity in the levator motorneurons. No spiking or any other nonspiking neuron was penetrated which could excite these levator motoneurons. From all these observations we conclude that oscillations in the membrane potential of interneuron I are entirely responsible for producing the levator bursts, and thus for producing stepping movements in a walking animal. During rhythmic leg movements, bursts of activity in levator and depressor motoneurons are initiated by slow graded depolarizations. The similarity of the synaptic activity in these two types of motoneurons suggests that burst activity in the depressor motoneurons is also produced by rhythmic activity in nonspiking interneurons. The fact that no spiking neuron was found to excite the depressor motoneurons supports this conclusion. Interneuron I is also an element of the rhythm-generating system, since short depolarizing pulses applied to it during rhythmic activity could reset the thythm. Long-duration current pulses applied to interneuron I in a quiescent animal did not produce rhythmic activity. This observation, together with the finding that during rhythmic activity the slow depolarizations in interneuron I are usually terminated by IPSPs, suggests that interneuron I alone does not generate the rhythm. No spiking interneurons have yet been enccountered which influence the activity in levator motoneurons. Thus, we conclude that the rhythm is generated in a network of nonspiking interneurons. The cellular mechanisms for generating the oscillations in this network are unknown. Continued.

Action Potentials↗