Charles R. Fourtner

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.

Locomotory activity in the extensor and flexor tibiae of the cockroach, Periplaneta americana

Abstract Electromyograms (EMGs) were recorded from the metathoracic extensor and flexor tibiae of cockroaches when the animals were: walking on a level surface, walking on a ball, or producing rhythmic leg movements while being restrained ventral surface upward. In the rapidly walking (> 2 steps/s) and restrained animals, there was reciprocity between EMGs from the extensor and flexor tibiae. In slowly walking (

Central Nervous Control of Cockroach Walking

During the past few years there has been a concerted effort to determine the neuronal circuit of and the neurophysiological basis for rhythmic leg movements in walking insects. In the cockroach there is strong evidence that rhythmic motor patterns, similar to those occurring during normal walking, spontaneously occur after complete deafferentation of the segmental ganglion. This led to the hypothesis that there are segmental rhythm generators that operate independently from the segmental sensory input. Subsequent morphological and physiological investigations of the metathoracic ganglion of the cockroach demonstrated the presence of interneurons whose membrane potentials oscillate during rhythmic leg movements. One interneuron, Interneuron I, oscillates such that its cyclic depolarizations are synchronized with the flexion phase (return stroke) of the rhythmic leg movements. When experimentally depolarized, Interneuron I drives and specifically recruits the flexion motoneurons in the same sequence as in normal walking. Interneuron I also appears to be an integral part of the neuronal circuit generating the rhythmic leg movements since short depolarizing pulses in Interneuron I can reset the cycle time of the rhythmic leg movements. An unusual phenomenon of Interneuron I and indeed of most interneurons within the metathoracic ganglion of the cockroach is that they are not excitable, that is they do not produce nerve spikes. These interneurons modulate activity in motoneurons by minute changes in membrane potentials. This is the first locomotory system in which identified nonspiking neurons have been demonstrated to play an important role in generating a specific motor pattern.

The ultrastructure of the metathoracic femoral extensors of the cockroach, Periplaneta americana

The ultrastructure of the femoral extensors of the metathoracic leg of the cockroach, Periplaneta americana was studied to determine morphological correlations with the known patterns of innervation, physiological properties and biochemical properties. Three different types of muscle fibers were described. Type 1 consisted of short sarcomeres (mean 3.7 μm), few mitochondria and sparse glycogen‐like material; Type 2, short sarcomeres (4.2 μm), numerous mitochondria, large amounts of glycogen; Type 3, long sarcomeres (7.5 μm), numerous mitochondria and large amounts of glycogen. A qualitative examination of the sarcoplasmic reticulum (SR) and transverse tubular system (TTS) revealed the density of SR and TTS to be greatest in Type 1 and least in Type 3. There were obvious correlations between the morphological features and the other known characteristics of these muscle fibers. The role of these different muscle fiber types in different locomotory behaviors was discussed. In summary, the three types of muscle fibers are used in three different behaviors: Type 1, rapid walking; Type 2, slow walking; Type 3, postural control.

Reflexes evoked by the femoral and coxal chordotonal organs in the cockroach, Periplaneta americana

Abstract 1. 1. Reflexes evoked by mechanical stimulation of isolated and identified chordotonal organs in cockroach limbs were studied. 2. 2. The femoral chordotonal organ (FCO) produced classical resistance reflexes in the flexor and extensor tibiae. The frequency response characteristics of the FCO evoked reflexes (active from 0.1 to 8 Hz) suggest a possible role in the maintenance of posture. 3. 3. The coxal chordotonal organ (CCO) produced resistance reflexes in the femoral flexors but had no effect on the femoral extensors. 4. 4. The frequency response characteristics (2.0–25 Hz) suggest that these reflexes may play a possible role in the maintenance of the rigidly fixed position of the femur during flight.

Potassium channels in pituitary cells in the teleost, Gillichthys mirabilis

Abstract We have examined whole-cell K+ currents and a Ca2+-dependent K+ channel at the single channel level in rostral pars distalis cells of Gillichthys mirabilis. Whole-cell K+ currents activated by depolarizing pulses have an inactivating component and a sustained component. The magnitude of both of these components is increased when a hyperpolarizing prepulse is delivered prior to depolarization. Both components are partially blocked by application of 5 mM TEA+. The Ca-dependent K+ channel, (K(Ca)), was sensitive to 2 mM TEA+ in outside-out patches (O/O) but not in inside-out patches (I/O). Channel open probability (P(o)) was dependent on membrane potential (Vm), with depolarization leading to an increase in P(o). Calcium on the cytoplasmic face of I/O patches increased channel P(o) in a dose-dependent manner. A portion of the single K(Ca) channels studied displayed inactivation after depolarizing pulses. These channels may be a component of the inactivating whole-cell current.

Characterization of an anion channel in pituitary cells of Gillichthys mirabilis

Abstract Patch-clamp studies on pituitary cells from Gillichthys mirabilis show the presence of anion channels with selectivity CI > F = Br = I. These channels are voltage sensitive over the physiological range of membrane potentials and have a unitary conductance of 94 ± 15 pS in symmetrical KCl. Halides other than Cl on the cytoplasmic face of the membrane cause an increase in open probability ( P (o)). DPC causes a dose dependent decrease in P (o) without affecting conductance. Sodium on the cytoplasmic face of the membrane causes a decrease in outward current.