Lateral Giant Fibers Research Articles

Investigation of the efficiency of the shape of chopped pulses using earthworm model.

In neural electrical stimulation, limiting the charge delivered during a stimulus pulse is essential to avoid nerve tissue damage and to save power. Previous experimental and modeling studies indicated that waveforms such as non-rectangular continuous pulses or rectangular chopped pulse were able to improve stimulation efficiency. The goal of this study is to evaluate if non-rectangular chopped pulses such as quarter sine and ramp are more charge efficient than rectangular chopped pulse. We performed in vivo study on 17 lumbricus terrestris and compared the charge per stimulating phase needed to activate lateral giant fibers (LGF) and medial giant fiber (MGF) using chopped non-rectangular pulses and rectangular pulse, varying stimulation duration parameters. Results indicated that non rectangular chopped pulses activated MGF and LGF with less charge than rectangular chopped pulses. For MGF (respectively LGF), the gain of charge was up to 33.9\% (resp. 17.8\%) using chopped ramp, and up to 22.8\% (resp. 18.1\%) using chopped quarter sine.

Pulsed focused ultrasound changes nerve conduction of earthworm giant axonal fibers.

This study examined the effects of pulsed focused ultrasound (FUS) in disrupting nerve conduction. FUS operating at a 210 kHz fundamental frequency was administered to the medial and lateral giant axonal nerve fibers of earthworms in a burst of pulses (1 ms tone burst duration, 20 Hz pulse repetition frequency). The magnitude and latencies of the nerve potentials induced by electrical stimulation were measured under three experimental conditions - (I) no sonication, (II) sonication at 600 mW/cm spatial-peak temporal-average intensity (Ispta), and (III) sonication at 200 mW/cm Ispta. The sonication at 600 mW/cm temporarily decreased the magnitude of the action potential peak (~16%), whereas the baseline peak level was quickly restored in postsonication sessions. Sonication administered at a lower intensity (i.e. 200 mW/cm) did not alter the peak magnitude. The sonication did not alter the nerve conduction velocity. The acoustic intensities used in the experiment did not increase the temperature of the sonicated tissue. The results indicate that axonal neurotransmission can be disrupted temporarily by the application of pulsed FUS, suggesting its potential utility in modulating the functional connectivity established by white matter tracts in the brain.

Neurotransmissional, structural, and conduction velocity changes in cerebral ganglions of Lumbricus terrestris on exposure to acrylamide.

Acrylamide (ACR), an environmental toxin though being investigated for decades, remains an enigma with respect to its mechanism/site of actions. We aim to explicate the changes in cerebral ganglions and giant fibers along with the behavior of worms on ACR intoxication (3.5-17.5mg/mL of medium/7days). Neurotransmitter analysis revealed increased levels of excitatory glutamate and inhibitory gamma amino butyrate with reduced levels of dopamine, serotonin, melatonin, and epinephrine (p < 0.001). Scanning electron microscopy showed architectural changes in cerebral ganglions at 3.5mg/mL/ACR. The learning behavior as evidenced by Pavlovian and maze tests was also altered well at 3.5mg/mL of ACR. Electrophysiological assessment showed a reduction in conduction velocity of the medial and lateral giant nerve fibers. We speculate that the observed dose/time-dependent changes in neurotransmission, neurosecretion, and conduction velocity on ACR intoxication at 17.5mg/ml, possibly, could be due to its effect on nerve fibers governing motor functions. The bioaccumulation factor in the range of 0.38-0.99mg/g of ACR causes a detrimental impact on giant fibers affecting behavior of worm. The observations made using the simple invertebrate model implicate that the cerebral ganglionic variations in the worms may be useful to appreciate the pathology of the neurological diseases which involve motor neuron dysfunction, esp where the availability of brain samples from the victims are scarce.

Rotenone causing dysfunctional mitochondria and lysosomes in cerebral ganglions of Lumbricus terrestris degenerate giant fibers and neuromuscular junctions

Rotenone is well–documented to cause neurodegenerative condition such as Parkinson's, in the exposed systems. However, its detrimental effect on particular sites of neuronal pathway is still under investigation. We aimed at elucidating the impact of rotenone on cerebral ganglions (CG) of Lumbricus terrestris which control movement and behaviour of the worms. Worms were exposed to 0–0.4 ppm/mL of rotenone. Mitochondrial and lysosomal integrities were found to be affected beyond 0.2 ppm/mL of rotenone. Activities of cholinergic enzymes and the expression of tyrosine hydroxylase showed an impaired neuronal transmission in CGs at the dose of 0.2 ppm/mL of rotenone. Histopathological and immunoflourescent analyses showed neuronal apoptosis, reduced nucleic acid content and inhibited of neurosecretion at 0.3 ppm/mL. Electron microscopy showed that the neurons and neuromuscular junctions were affected at 0.2 ppm/mL. Dose-dependent changes were also observed in the motor function such as burrowing behaviours and locomotion. Conduction velocity (CV) and locomotion assessment showed that the CV of lateral giant fiber (LGF) was reduced while that of MGF remains unaffected at 0.2 ppm, the dose at which the burrowing behaviour was also not affected. LGF, cholinergic enzymes and tyrosine hydroxylase are primarily targeted by rotenone affecting locomotion at 0.2 ppm/mL while MGF, neuropile and the burrowing behaviour were affected at 0.3 ppm/mL. We demonstrate, in addition to dose-dependent effects, that the bioaccumulation factors range 0.28–0.32 ppm/μg of rotenone cause degenerative impact on giant fibers affecting neuronal behaviors/locomotion of worms. We also propose worms for studying mechanisms of neuronal pathology caused by chemicals prevailing in earth's atmosphere.

Rapid neural circuit switching mediated by synaptic plasticity during neural morphallactic regeneration

The aquatic oligochaete, Lumbriculus variegatus (Lumbriculidae), undergoes a rapid regenerative transformation of its neural circuits following body fragmentation. This type of nervous system plasticity, called neural morphallaxis, involves the remodeling of the giant fiber pathways that mediate rapid head and tail withdrawal behaviors. Extra- and intracellular electrophysiological recordings demonstrated that changes in cellular properties and synaptic connections underlie neurobehavioral plasticity during morphallaxis. Sensory-to-giant interneuron connections, undetectable prior to body injury, emerged within hours of segment amputation. The appearance of functional synaptic transmission was followed by interneuron activation, coupling of giant fiber spiking to motor outputs and overt segmental shortening. The onset of morphallactic plasticity varied along the body axis and emerged more rapidly in segments closer to regions of sensory field overlap between the two giant fiber pathways. The medial and lateral giant fibers were simultaneously activated during a transient phase of network remodeling. Thus, synaptic plasticity at sensory-to-giant interneuron connections mediates escape circuit morphallaxis in this regenerating annelid worm.

Electron tomographic analysis of gap junctions in lateral giant fibers of crayfish

Innexin-gap junctions in crayfish lateral giant fibers (LGFs) have an important role in escape behavior as a key component of rapid signal transduction. Knowledge of the structure and function of characteristic vesicles on the both sides of the gap junction, however, is limited. We used electron tomography to analyze the three-dimensional structure of crayfish gap junctions and gap junctional vesicles (GJVs). Tomographic analyses showed that some vesicles were anchored to innexons and almost all vesicles were connected by thin filaments. High densities inside the GJVs and projecting densities on the GJV membranes were observed in fixed and stained samples. Because the densities inside synaptic vesicles were dependent on the fixative conditions, different fixative conditions were used to elucidate the molecules included in the GJVs. The projecting densities on the GJVs were studied by immunoelectron microscopy with anti-vesicular monoamine transporter (anti-VMAT) and anti-vesicular nucleotide transporter (anti-VNUT) antibodies. Some of the projecting densities were labeled by anti-VNUT, but not anti-VMAT. Three-dimensional analyses of GJVs and excitatory chemical synaptic vesicles (CSVs) revealed clear differences in their sizes and central densities. Furthermore, the imaging data obtained under different fixative conditions and the immunolabeling results, in which GJVs were positively labeled for anti-VNUT but excitatory CSVs were not, support our model that GJVs contain nucleotides and excitatory CSVs do not. We propose a model in which characteristic GJVs containing nucleotides play an important role in the signal processing in gap junctions of crayfish LGFs.

A three-dimensional flexible microprobe array for neural recording assembled through electrostatic actuation

We designed, fabricated and tested a novel three-dimensional flexible microprobe to record neural signals of a lateral giant nerve fiber of the escape circuit of an American crayfish. An electrostatic actuation folded planar probes into three-dimensional neural probes with arbitrary orientations for neuroscientific applications. A batch assembly based on electrostatic forces simplified the fabrication and was non-toxic. A novel fabrication for these three-dimensional flexible probes used SU-8 and Parylene technology. The mechanical strength of the neural probe was great enough to penetrate into a bio-gel. A flexible probe both decreased the micromotion and alleviated tissue encapsulation of the implant caused by chronic inflammation of tissue when an animal breathes or moves. The cortex consisted of six horizontal layers, and the neurons of the cortex were arranged in vertical structures; the three-dimensional microelectrode arrays were suitable to investigate the cooperative activity for neurons in horizontal separate layers and in vertical cortical columns. With this flexible probe we recorded neural signals of a lateral giant cell from an American crayfish. The response amplitude of action potentials was about 343 µV during 1 ms period; the average recorded data had a ratio of signal to noise as great as 30.22 ± 3.58 dB. The improved performance of this electrode made feasible the separation of neural signals according to their distinct shapes. The cytotoxicity indicated a satisfactory biocompatibility and non-toxicity of the flexible device fabricated in this work.

Experimental validation of the nerve conduction velocity selective recording technique using a multi-contact cuff electrode

The earthworm ( Lumbricus terrestris) is presented as an in vitro model of a peripheral nerve containing only two fibers each with distinctly different conduction velocities, the median and lateral giant fibers (MGF and LGF). The worm model is used with a multi-contact cuff electrode to validate the spatial–temporal filtering effect of different electrode contact configurations and the effect of applying a delay adder and matched filter tuned to either the MGF or LGF action potential (AP) to extract conduction direction and velocity from the recording. The results confirmed the known effect of inter-electrode spacing and bipolar and tripolar recording configuration on the AP amplitude. It also demonstrates a crossover point where the amplitude of the tripolar recording is larger than the monopolar recording, an effect of the slower action potential conduction velocities in the worm. The delay adder was found to be an effective velocity sensitive filter, able to discriminate units based on conduction velocity. The matched filter to be an effective means to eliminate artifact and increase signal to noise ratios, however was not found to improve selectivity.

Neurochemical and electrophysiological diagnosis of reversible neurotoxicity in earthworms exposed to sublethal concentrations of CL-20.

Background, aim, and scopeHexanitrohexaazaisowurtzitane (CL-20) is a relatively new energetic compound sharing some degree of structural similarity with hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), a known neurotoxic compound. Previously, we demonstrated using a noninvasive electrophysiological technique that CL-20 was a more potent neurotoxicant than RDX to the earthworm Eisenia fetida. In the present study, we investigated the effect of CL-20 exposure and subsequent recovery on muscarinic acetylcholine receptors (mAChRs) to further define the mechanism of reversible neurotoxicity of CL-20 in E. fetida.Materials and methodsWe used a noninvasive electrophysiological technique to evaluate neurotoxicity in CL-20-treated worms, and then measured how such exposures altered levels of whole-body mAChR in the same animals.Results and discussionA good correlation exists between these two types of endpoints. Effect on mAChR levels was most prominent at day 6 of exposure. After 7 days of recovery, both conduction velocity and mAChR were significantly restored. Our results show that sublethal concentrations of CL-20 significantly reduced mAChR levels in a concentration- and duration-dependent manner, which was accompanied with significant decreases in the conduction velocity of the medial and lateral giant nerve fibers. After 7-day post exposure recovery, worms restored both neurochemical (mAChR) and neurophysiological (conduction velocity) endpoints that were reduced during 6-day exposures to CL-20 concentrations from 0.02 to 0.22 µg/cm2.Conclusions and perspectivesOur findings support the idea that CL-20 induced neurotoxic effects are reversible, and suggest that CL-20 neurotoxicity may be mediated through the cholinergic system. Future studies will investigate other neurotransmission systems such as GABA, glutamate, and monoamine. Ion channels in the nerve membrane should be examined to further define the precise mechanisms underlying CL-20 neurotoxicity.

Electrical Synapses: Rectification Demystified

Some electrical synapses rectify - they pass current preferentially in one direction. A new study argues that rectifying junctions result when the two sides of the junction contribute hemichannels with different properties to the gap junction.

Comparative neurotoxicity of two energetic compounds, hexanitrohexaazaisowurtzitane and hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine, in the earthworm Eisenia fetida

Hexanitrohexaazaisowurtzitane (CL-20) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), both energetic compounds, share some degree of structural similarity. A noninvasive electrophysiological technique was employed to assess the impacts of acute sublethal exposures on impulse conduction in medial (MGF) and lateral (LGF) giant nerve fiber pathways of the earthworm Eisenia fetida and to evaluate the reversibility of neurotoxic effects. Earthworms were exposed to either 0.02 to 2.15 microg/cm2 of CL-20 or 0.04 to 5.35 microg/cm2 of RDX, for 1 to 14 d, on moistened filter paper. Conduction velocities of MGF and LGF were recorded on a digital oscilloscope before and after exposure. Results indicate that at exposure levels as low as 0.02 microg/cm2 of CL-20 or 0.21 microg/cm2 of RDX, worms exhibited physiological impacts such as retardation, stiffness, and body shrink. Both MGF and LGF conduction velocities were negatively correlated with increasing doses of CL-20 or RDX. However, such neurotoxic effects were alleviated or even eliminated within a few days after exposed worms were transferred to an uncontaminated environment, indicating that the neurotoxicity is reversible even after 6-d exposure. The CL-20 is more potent than RDX, which is consistent with previous studies on lethality, growth, and reproduction endpoints in soil oligochaetes.

The Vitamin E analog Trolox reduces copper toxicity in the annelid Lumbriculus variegatus but is also toxic on its own

The ability of the water-soluble Vitamin E analog, Trolox, to prevent the toxic effects of copper exposure on the behavior and neuronal physiology of the freshwater oligochaete Lumbriculus variegatus was examined. Trolox produced a concentration-dependent increase in the 24 h LC 50 for copper exposure, with 100 μM Trolox elevating the LC 50 by almost seven-fold (from 0.36 to 2.43 μM). Copper exposure (0.2 μM) for 24 h produced a reduction in the conduction velocity of the medial and lateral giant nerve fibers, which was prevented by 100 μM Trolox. Copper exposure (0.2 μM) for 24 h also reduced the effectiveness of substrate vibration in eliciting giant nerve fiber spikes. Trolox prevented this reduction in sensory responsiveness. Trolox (100 μM) partially reversed the copper-induced (0.4 μM) decrease in touch-evoked helical swimming behavior, but had no effect on the copper-induced decrement in touch-evoked body reversal. Copper exposure (0.2 μM) for 24 h reduced the amount of spontaneous locomotion (crawling); however, Trolox did not reverse this effect. However, Trolox exposure alone produced a decrease in the distance crawled that was similar in magnitude to copper exposure. In normal worms, rapid spiking activity of the medial giant nerve fiber produces facilitation in the amplitude of the resulting muscle potentials produced by the longitudinal body wall muscles. Copper exposure had no effect on the amount of muscle potential facilitation, but Trolox exposure (100 μM) produced a significant decrease in facilitation. The results of this study indicate that many of the toxic effects of copper exposure on Lumbriculus are prevented or reduced by the antioxidant Trolox. However, the results of this study also indicate that Trolox has toxic effects on behavior and neuronal physiology. The results presented here document one of the few published reports of the detrimental effects of Vitamin E or its analogs on nervous system function or behavior.

Long-lasting potentiation of excitatory synaptic signaling to the crayfish lateral giant neuron

The neural circuit that underlies the lateral giant fiber (LG)-mediated reflex escape in crayfish has provided findings relating synaptic change to nonassociative learning such as sensitization and habituation. The LGs receive sensory inputs from the primary sensory afferents and a group of mechanosensory interneurons (MSIs). An increase of excitability by suprathreshold repetitive excitation of this circuit, which is similar to Hebbian long-term potentiation (LTP), has been reported. This potentiation was previously thought to result from the enhancement of transmission at cholinergic synapses between primary afferents and MSIs but not the electrical synapses onto LG. In this study, we found that potentiation of synaptic signaling at the electrical synapse onto LG can also be induced when the synapse was activated with subthreshold repetitive pulses or with a few strong suprathreshold shocks. LG LTP was induced in the preparation which had received pulses at limited frequency range. Although whether this LTP is involved in the learning process of escape behavior in crayfish is not clear, the intensity and amount of sensory stimulation used here mimicked those that could easily be produced by a predator trying to catch a crayfish and could be of adaptive significance in life.

Copper-induced changes in locomotor behaviors and neuronal physiology of the freshwater oligochaete, Lumbriculus variegatus

The behavioral and neurotoxic effects of copper exposure were examined in the freshwater oligochaete, Lumbriculus variegatus. The 24 h LC 50 for worms exposed to copper sulfate in an artificial pond water was 0.45 μM. Almost all animals that died due to copper exposure died during the first day of exposure. Immersion in water containing 0.2 or 0.4 μM copper produced time- and concentration-dependent reductions in the ability of tactile stimulation to evoke two stereotyped locomotory behaviors, body reversal and helical swimming. Helical swimming was more severely affected by copper exposure than was body reversal behavior. Upon return to clean water, both behaviors returned to normal levels within 1–2 days. Noninvasive electrophysiological testing indicated that copper exposure produced time- and concentration-dependent reductions in the conduction velocities of the medial and lateral giant nerve fibers. An 8 h exposure to 0.2 μM copper produced significant reductions in giant fiber conduction velocities that returned to normal levels within 3 days of return to clean water. It is likely that copper exposure can significantly degrade the ability of aquatic oligochaetes to avoid predators.

Behavioral plasticity and central regeneration of locomotor reflexes in the freshwater oligochaete, Lumbriculus variegatus. I: Transection studies

Abstract. Access to the ventral nerve cord in living specimens of Lumbriculus variegatus, an aquatic oligochaete, is normally impossible because surgical invasion induces segmental autotomy (self‐fragmentation). We show here that nicotine is a powerful paralytic agent that reversibly immobilizes worms, blocks segmental autotomy, and allows experimental access to the nerve cord. Using nicotine‐treated worms, we transected the ventral nerve cord and used non‐invasive electrophysiological recordings and behavioral analyses to characterize the functional recovery of giant nerve fibers and other reflex pathways. Initially, after transection, medial giant fiber (MGF) and lateral giant fiber (LGF) spikes conducted up to, but not across, the transection site. Reestablishment of MGF and LGF through‐conduction across the transection site occurred as early as 10 h (usually by 20 h) after transection. Analyses of non‐giant‐mediated behavioral responses (i.e., helical swimming and body reversal) were also made following nerve cord transection. Immediately after transection, functional reorganization of touch‐evoked locomotor reflexes occurred, so that the two portions of the worm anterior and posterior to the transection site were independently capable of helical swimming and body reversal responses. Similar reorganization of responses occurred in amputated body fragments. Reversion back to the original whole‐body pattern of swimming and reversal occurred as early as 8 h after transection. Thus, functional restoration of the non‐giant central pathways appeared slightly faster than giant fiber pathways. The results demonstrate the remarkable plasticity of locomotor reflex behaviors immediately after nerve cord transection or segment amputation. They also demonstrate the exceptional speed and specificity of regeneration of the central pathways that mediate locomotor reflexes.

VENTRAL NERVE CORD TRANSECTION IN CRAYFISH: A STUDY OF FUNCTIONAL ANATOMY

ABSTRACT In crayfish, neural degeneration and regeneration in the ventral nerve cord occur in one of two ways, depending on the injured fiber. Most fibers degenerate in 1 or 2 weeks, while giant fibers degenerate slowly. Although degenerative changes are similar in both cases, they do not seem to correlate with motor behavioral alterations. The aim of this work was to characterize the time course of behavioral and anatomical changes following ventral nerve cord (VNC) transection in crayfish. The behavioral analysis was focused on the righting reflex whose changes were correlated with morphological studies performed on longitudinal sections and analyzed with transmission (TEM) and scanning electron microscope (SEM). Latency for the righting reflex increased after VNC transection and then slowly decreased toward control values. Anatomically, degenerative changes began to appear 10 days after VNC transection. Disruption in membrane arrangement, subcellular organelles, and a strong increase in glia appeared in small fibers. To a lesser degree, similar changes could be detected in medial and lateral giant fibers. Glial growth reconnected the transected VNC where regeneration signs were detected in small fibers. Both stumps were reconnected at least by glial tissue 90 days after transection, while giant axons were still degenerating; at this time, the righting reflex returned to control values.

Monosynaptic excitation of lateral giant fibres by proprioceptive afferents in the crayfish

Giant interneurones mediate a characteristic `tail flip' escape response of the crayfish, Procambarus clarkii, which move it rapidly away from the source of stimulation. We have analysed the synaptic connections of proprioceptive sensory neurones with one type of giant interneurone, the lateral giant. Spikes in sensory neurones innervating an exopodite-endopodite chordotonal organ in the tailfan, which monitors the position and movements of the exopodite, are followed at a short and constant latency by excitatory postsynaptic potentials in a lateral giant interneurone (LG) recorded in the terminal abdominal ganglion. These potentials are unaffected by manipulation of the membrane potential of LG, by bath application of saline with a low calcium concentration, or by one containing the nicotinic antagonist, curare. The potentials evoked in LG by chordotonal organ stimulation are thus thought to be monosynaptic and electrically mediated. This is the first demonstration that LG receives input from sensory receptors other than exteroceptors in the terminal abdominal ganglion.

Touch sensitivity in oligochaete giant fibers is transiently enhanced by a single spike

Weak tactile stimulation of posterior segments in the freshwater oligochaete Lumbriculus variegatus evokes a single lateral giant fiber (LGF) spike but no overt escape shortening. After initiation of a single spike, giant-fiber excitability is increased, as reflected by a period of enhanced conduction velocity for a second LGF spike that follows 5–50 ms after the first. Using non-invasive recordings from intact worms and a biofeedback arrangement for stimulus delivery, it was shown that the period of enhanced velocity is associated with a marked increase in sensitivity to a second touch stimulus. Enhanced touch sensitivity is distributed within the LGF sensory field to loci remote from the original site of stimulation, leading to an increased likelihood that a second, weak stimulus will elicit rapid escape withdrawal.

Structural and functional post-embryonic development of a non-rectifying electrical synapse in the crayfish

The post-embryonic development of the non-rectifying septate synapse between homologous lateral giant (LG) fibre segments has been investigated using electron microscopy and electrophysiology. In adults, the LG-LG synapse is characterized by closely apposed membranes (approximately 4 nm separation) traversed by regularly spaced particles, and large (60-80 nm) spherical vesicles on both sides of the junction. In newly hatched crayfish the junction between lateral giant fibre segments comprises regions of close membrane apposition as seen in the adult along with non-specialized areas of wide (10-15 nm) membrane separation. Vesicles associated with these junctions are small (25-40 nm) and pleomorphic. The number of vesicles is low by comparison with adult junctions; in most sections of hatchling junctions there are normally fewer than five vesicles, although as many as 30 have occasionally been seen. During development the non-specialized areas of wide membrane separation become rare and the vesicle population changes to a mixture of small pleomorphic forms and larger (60-80 nm) spherical ones. However even at two months the number of large spherical vesicles is markedly less than that at the adult synapse, while small pleomorphic vesicles are still abundant. Despite the difference between the adult and hatchling vesicle populations, intracellular recordings have shown that the synapse is fully functional as a non-rectifying electrical junction on hatching and that the intracellular marker Lucifer Yellow can pass between adjacent lateral giant fibre neurons.

Reorganization of escape reflexes during asexual fission in an aquatic oligochaete, Dero digitata

AbstractAsexual reproduction in the aquatic oligochaete Dero digitata involves 1) formation of a mid‐body fission zone and transverse fission plane separating the worm into anterior and posterior zooids; 2) development of four new head segments in the posterior zooid, just behind the fission plane; and 3) development of new tail segments in the anterior zooid, just anterior to the fission plane. Electrophysiological recordings from normal (i.e., non‐fissioning) and prefission worms indicate similar proportioning of giant nerve fiber sensory fields; the medial (MGF) and lateral (LGF) giant fiber fields encompass the anterior‐most 30% and posterior‐most 70%, respectively, of the worm's length. Throughout prefission stages both MGF and LGF spikes were reliably through‐conducted, though at reduced velocity, across the fission zone. This agrees with anatomical results showing continuity and reduced diameter of MGF and LGF in the fission zone. Within seconds after fissioning, sensory fields in the rostral segments of posterior zooids (= 18% of zooid length) switch from LGF to MGF sensitivity. Further changes in giant fiber sensory fields and conduction velocities in the posterior zooid, during the next 7 days, lead to a functional organization of escape reflex pathways comparable to that in normal worms. Our results suggest that an escape reflex switching mechanism, along with strong modulatory influences, ensures appropriate and adaptively significant escape reflex functioning in posterior zooids both before and after fissioning.