Ciliary Locomotion Research Articles

Coordination of Locomotion by Serotonergic Neurons in the Predatory Gastropod Pleurobranchaea californica.

Similar design characterizes neuronal networks for goal-directed motor control across the complex, segmented vertebrates, insects, and polychaete annelids with jointed appendages. Evidence is lacking for whether this design evolved independently in those lineages, evolved in parallel with segmentation and appendages, or could have been present in a soft-bodied common ancestor. We examined coordination of locomotion in an unsegmented, ciliolocomoting gastropod, the sea slug Pleurobranchaea californica, which may better resemble the urbilaterian ancestor. Previously, bilateral A-cluster neurons in cerebral ganglion lobes were found to compose a multifunctional premotor network controlling the escape swim and feeding suppression, and mediating action selection for approach or avoidance turns. Serotonergic As interneurons of this cluster were critical elements for swimming, turning, and behavioral arousal. Here, known functions were extended to show that the As2/3 cells of the As group drove crawling locomotion via descending signals to pedal ganglia effector networks for ciliolocomotion and were inhibited during fictive feeding and withdrawal. Crawling was suppressed in aversive turns, defensive withdrawal, and active feeding, but not during stimulus-approach turns or prebite proboscis extension. Ciliary beating was not inhibited during escape swimming. These results show how locomotion is adaptively coordinated in tracking, handling, and consuming resources, and in defense. Taken with previous results, they also show that the A-cluster network acts similarly to the vertebrate reticular formation with its serotonergic raphe nuclei in facilitating locomotion, postural movements, and motor arousal. Thus, the general scheme controlling locomotion and posture might well have preceded the evolution of segmented bodies and articulated appendages.SIGNIFICANCE STATEMENT Similar design in the neuronal networks for goal-directed motor control is seen across the complex, segmented vertebrates, insects, and polychaete annelids with jointed appendages. Whether that design evolved independently or in parallel with complexity in body and behavior has been unanswered. Here it is shown that a simple sea slug, with primitive ciliary locomotion and lacking segmentation and appendages, has similar modular design in network coordination as vertebrates for posture in directional turns and withdrawal, locomotion, and general arousal. This suggests that a general neuroanatomical framework for the control of locomotion and posture could have arisen early during the evolution of bilaterians.

On the Asymptotic Boundary Condition at the Free-Fluid/Porous-Medium Interface in Periciliary Layer due to the Ciliary Movement

The purpose of this study is to study the movement of the periciliary (PCL) fluid due to the ciliary locomotion. In this research, because the bundle of cilia is considered instead of a single cilium, Stokes–Brinkman equations in a macroscopic scale are employed to find the velocity of the PCL fluid. When the cilia are perpendicular to the horizontal plane, the PCL consists of only the cilia. The inclination of cilia (cilia make an angle θ θ < 90 ∘ to the horizontal plane) results in two different domains in the PCL, the regions comprising and not comprising cilia. The main objective of this study is to determine the appropriate boundary conditions of the velocity between these two regions where the PCL fluid is moved by self-propelled cilia rather than the pressure gradient. A matched asymptotic expansion method is applied to the Stokes–Brinkman equations to determine the constraints. Two boundary conditions at the interface are obtained and the velocity of the PCL fluid at the top of PCL can be used to be the boundary conditions at the bottom of the mucus layer to determine the velocity of mucus. This model can help engineers to build devices to treat patients who have problem with the respiratory system. Applications include modeling fluid flow through filters such as engine filter and rice fields.

Optimal ciliary locomotion of axisymmetric microswimmers

Many biological microswimmers locomote by periodically beating the densely packed cilia on their cell surface in a wave-like fashion. While the swimming mechanisms of ciliated microswimmers have been extensively studied both from the analytical and the numerical point of view, optimisation of the ciliary motion of microswimmers has received limited attention, especially for non-spherical shapes. In this paper, using an envelope model for the microswimmer, we numerically optimise the ciliary motion of a ciliate with an arbitrary axisymmetric shape. Forward solutions are found using a fast boundary-integral method, and the efficiency sensitivities are derived using an adjoint-based method. Our results show that a prolate microswimmer with a $2\,{:}\,1$ aspect ratio shares similar optimal ciliary motion as the spherical microswimmer, yet the swimming efficiency can increase two-fold. More interestingly, the optimal ciliary motion of a concave microswimmer can be qualitatively different from that of the spherical microswimmer, and adding a constraint to the cilia length is found to improve, on average, the efficiency for such swimmers.

Interstitial Annelida

Members of the following marine annelid families are found almost exclusively in the interstitial environment and are highly adapted to move between sand grains, relying mostly on ciliary locomotion: Apharyngtidae n. fam., Dinophilidae, Diurodrilidae, Nerillidae, Lobatocerebridae, Parergodrilidae, Polygordiidae, Protodrilidae, Protodriloididae, Psammodrilidae and Saccocirridae. This article provides a review of the evolution, systematics, and diversity of these families, with the exception of Parergodrilidae, which was detailed in the review of Orbiniida by Meca, Zhadan, and Struck within this Special Issue. While several of the discussed families have previously only been known by a few described species, recent surveys inclusive of molecular approaches have increased the number of species, showing that all of the aforementioned families exhibit a high degree of cryptic diversity shadowed by a limited number of recognizable morphological traits. This is a challenge for studies of the evolution, taxonomy, and diversity of interstitial families as well as for their identification and incorporation into ecological surveys. By compiling a comprehensive and updated review on these interstitial families, we hope to promote new studies on their intriguing evolutionary histories, adapted life forms and high and hidden diversity.

Комплексний вплив нітратів і біотичних чинників на активність in vitro війок фронтального миготливого епітелію зябрового апарату Sinanodonta woodiana (Mollusca, Bivalvia, Unionidae)

Effects of MPC, 2 MPC, and 3MPC water solutions of potassium nitrate and biotic factors (trematode infection and “gill pregnancy”) were studied on the duration of locomotion and beat frequency of epithelial cilia of outer demibranchs of Sinanodonta woodiаna (Lea, 1834). The species has recently invaded the system of water bodies of Ukraine. It has occupied numerous water bodies in the Kiliya distributary of Danube Delta, establishing populations of density and biomass. Statistically significant differences by the studied parameters of ciliary epithelium (duration of locomotion and beat frequency of cilia) were not observed in the control group of non-infected males and latent females of S. woodiana. In females with “gill pregnancy”, the duration of ciliary locomotion decreased by 1.5 times, and beat frequency by 1.4 times (P>99.9 %). It should be noted that the outer surface of demibranchs of female mollusks was completely covered by the filled to the brim marsupial pockets. Thus, the functional load of gills is high, leading to the development of compensatory processes in experimental animals. The processes induce increased effectiveness of protective factors that counteract them. Another biotic factor negatively affecting the experimental pond mussels was the high rates of trematode infection in the hosts’ reproductive glands which contained sporocysts and cercariae of Rhipidocotyle campanula Dujardin, 1845. Prevalence of that infection ranged from 50 to 100 %. These rates of infection were associated with sharply declining functions of cilia of the gill epithelium in studied animals. The compound pathogenic influence of parasite and “gill pregnancy” factors even more affected the functional parameters of outer gill ciliary epithelium. The MPC levels of toxicant in the water medium were accompanied with the first symptom of intoxication of the Chinese pond mussel, the stage of stimulation. At that point, the functional parameters of epithelial cilia of “pregnant” females and in moderately and weakly infected mollusks of both sexes increased in response to exposure to the toxic factor. With the growing concentrations of toxicant to 2 MPC and 3 MPC, the experimental pond mussels developed the pathological process of intoxication, which progressed from the stage of depression to sublethal and lethal stages. Accordingly, the locomotion activity of ciliary epithelium increasingly declined and, in the end, stopped. The “gill pregnancy” and high rates of trematode infection exacerbated the consequences of the process.

Axisymmetric squirmers in Stokes fluid with nonuniform viscosity

The ciliary locomotion and feeding of an axisymmetric micro-swimmer in a complex fluid whose viscosity depends on nutrient concentration are investigated numerically. The micro-swimmer is modeled as having spheroidal geometry, and ciliary beating is modeled by a slip velocity; i.e. a squirmer is adapted. Looking at the coupling between swimming and feeding of spheroidal squirmers, it is found that swimming speed and feeding are most affected by a non-uniform viscosity environment when the ratio of advection-forces to diffusion-transport, characterized by the nondimensional Pecl\'et number, is moderate ($\text{Pe}\approx 10$). These changes are correlated to significant increases in the pressure force on the surface of the squirmer. The swimming and feeding changes are found to be more significant in oblate spheroids than prolate spheroids. Most interestingly, nontrivial symmetric slip boundary conditions on the surface of the squirmer, which results in zero net motion in a constant viscosity fluid, can yield non-zero propulsion when paired with asymmetric nutrient boundary conditions. The fact that symmetric fluid boundary conditions can result in asymmetric propulsion has implications for the design of artificial micro-swimmers and other low-$\text{Re}$ squirmers in complex fluids.

Neuronal coordination of motile cilia in locomotion and feeding.

Efficient ciliary locomotion and transport require the coordination of motile cilia. Short-range coordination of ciliary beats can occur by biophysical mechanisms. Long-range coordination across large or disjointed ciliated fields often requires nervous system control and innervation of ciliated cells by ciliomotor neurons. The neuronal control of cilia is best understood in invertebrate ciliated microswimmers, but similar mechanisms may operate in the vertebrate body. Here, we review how the study of aquatic invertebrates contributed to our understanding of the neuronal control of cilia. We summarize the anatomy of ciliomotor systems and the physiological mechanisms that can alter ciliary activity. We also discuss the most well-characterized ciliomotor system, that of the larval annelid Platynereis. Here, pacemaker neurons drive the rhythmic activation of cholinergic and serotonergic ciliomotor neurons to induce ciliary arrests and beating. The Platynereis ciliomotor neurons form a distinct part of the larval nervous system. Similar ciliomotor systems likely operate in other ciliated larvae, such as mollusc veligers. We discuss the possible ancestry and conservation of ciliomotor circuits and highlight how comparative experimental approaches could contribute to a better understanding of the evolution and function of ciliary systems.This article is part of the Theo Murphy meeting issue ‘Unity and diversity of cilia in locomotion and transport’.

Effects of nonuniform viscosity on ciliary locomotion

The swimming efficiency of an idealized microswimmer is theoretically examined where the pointwise viscosity depends weakly on the surrounding nutrient. Results show that optimal swimming efficiency is achieved differently in the constant- and variable-viscosity cases.

Ciliomotor circuitry underlying whole-body coordination of ciliary activity in the Platynereis larva.

Ciliated surfaces harbouring synchronously beating cilia can generate fluid flow or drive locomotion. In ciliary swimmers, ciliary beating, arrests, and changes in beat frequency are often coordinated across extended or discontinuous surfaces. To understand how such coordination is achieved, we studied the ciliated larvae of Platynereis dumerilii, a marine annelid. Platynereis larvae have segmental multiciliated cells that regularly display spontaneous coordinated ciliary arrests. We used whole-body connectomics, activity imaging, transgenesis, and neuron ablation to characterize the ciliomotor circuitry. We identified cholinergic, serotonergic, and catecholaminergic ciliomotor neurons. The synchronous rhythmic activation of cholinergic cells drives the coordinated arrests of all cilia. The serotonergic cells are active when cilia are beating. Serotonin inhibits the cholinergic rhythm, and increases ciliary beat frequency. Based on their connectivity and alternating activity, the catecholaminergic cells may generate the rhythm. The ciliomotor circuitry thus constitutes a stop-and-go pacemaker system for the whole-body coordination of ciliary locomotion.

小惑星探査ロボットのための繊毛推進機構の繊毛の傾きと長さを考慮した運動解析

小惑星探査ロボットのための繊毛推進機構の繊毛の傾きと長さを考慮した運動解析

An optimal control approach to ciliary locomotion

We consider a class of low Reynolds number swimmers, of prolate spheroidal shape, which can be seen as simplified models of ciliated microorganisms. Within this model, the form of the swimmer does not change, the propelling mechanism consisting in tangential displacements of the material points of swimmer's boundary. Using explicit formulas for the solution of the Stokes equations at the exterior of a translating prolate spheroid the governing equations reduce to a system of ODE's with the control acting in some of its coefficients (bilinear control system). The main theoretical result asserts the exact controllability of the prolate spheroidal swimmer. In the same geometrical situation, we consider the optimal control problem of maximizing the efficiency during a stroke and we prove the existence of a maximum. We also provide a method to compute an approximation of the efficiency by using explicit formulas for the Stokes system at the exterior of a prolate spheroid, with some particular tangential velocities at the fluid-solid interface. We analyze the sensitivity of this efficiency with respect to the eccentricity of the considered spheroid and show that for small positive eccentricity, the efficiency of a prolate spheroid is better than the efficiency of a sphere. Finally, we use numerical optimization tools to investigate the dependence of the efficiency on the number of inputs and on the eccentricity of the spheroid. The ``best'' numerical result obtained yields an efficiency of $30.66\%$ with $13$ scalar inputs. In the limiting case of a sphere our best numerically obtained efficiency is of $30.4\%$, whereas the best computed efficiency previously reported in the literature is of $22\%$.

Network interneurons underlying ciliary locomotion in Hermissenda.

In the nudibranch mollusk Hermissenda, ciliary locomotion contributes to the generation of two tactic behaviors. Light elicits a positive phototaxis, and graviceptive stimulation evokes a negative gravitaxis. Two classes of light-responsive premotor interneurons in the network contributing to ciliary locomotion have been recently identified in the cerebropleural ganglia. Aggregates of type I interneurons receive monosynaptic excitatory (I(e)) or inhibitory (I(i)) input from identified photoreceptors. Type II interneurons receive polysynaptic excitatory (II(e)) or inhibitory (II(i)) input from photoreceptors. The ciliary network also includes type III inhibitory (III(i)) interneurons, which form monosynaptic inhibitory connections with ciliary efferent neurons (CENs). Illumination of the eyes evokes a complex inhibitory postsynaptic potential, a decrease of I(i) spike activity, a complex excitatory postsynaptic potential, and an increase of I(e) spike activity. Here, we characterized the contribution of identified I, II, and III(i) interneurons to the neural network supporting visually guided locomotion. In dark-adapted preparations, light elicited an increase in the tonic spike activity of II(e) interneurons and a decrease in the tonic spike activity of II(i) interneurons. Fluorescent dye-labeled type II interneurons exhibited diverse projections within the circumesophageal nervous system. However, a subclass of type II interneurons, II(e(cp)) and II(i(cp)) interneurons, were shown to terminate within the ipsilateral cerebropleural ganglia and indirectly modulate the activity of CENs. Type II interneurons form monosynaptic or polysynaptic connections with previously identified components of the ciliary network. The identification of a monosynaptic connection between I(e) and III(i) interneurons shown here suggest that they provide a major role in the light-dependent modulation of CEN spike activity underlying ciliary locomotion.

Paramecium swimming in capillary tube

Swimming organisms in their natural habitat need to navigate through a wide range of geometries and chemical environments. Interaction with boundaries in such situations is ubiquitous and can significantly modify the swimming characteristics of the organism when compared to ideal laboratory conditions. We study the different patterns of ciliary locomotion in glass capillaries of varying diameter and characterize the effect of the solid boundaries on the velocities of the organism. Experimental observations show that Paramecium executes helical trajectories that slowly transition to straight lines as the diameter of the capillary tubes decreases. We predict the swimming velocity in capillaries by modeling the system as a confined cylinder propagating longitudinal metachronal waves that create a finite pressure gradient. Comparing with experiments, we find that such pressure gradient considerations are necessary for modeling finite sized ciliary organisms in restrictive geometries.

Origin and early evolution of neural circuits for the control of ciliary locomotion

Behaviour evolved before nervous systems. Various single-celled eukaryotes (protists) and the ciliated larvae of sponges devoid of neurons can display sophisticated behaviours, including phototaxis, gravitaxis or chemotaxis. In single-celled eukaryotes, sensory inputs directly influence the motor behaviour of the cell. In swimming sponge larvae, sensory cells influence the activity of cilia on the same cell, thereby steering the multicellular larva. In these organisms, the efficiency of sensory-to-motor transformation (defined as the ratio of sensory cells to total cell number) is low. With the advent of neurons, signal amplification and fast, long-range communication between sensory and motor cells became possible. This may have first occurred in a ciliated swimming stage of the first eumetazoans. The first axons may have had en passant synaptic contacts to several ciliated cells to improve the efficiency of sensory-to-motor transformation, thereby allowing a reduction in the number of sensory cells tuned for the same input. This could have allowed the diversification of sensory modalities and of the behavioural repertoire. I propose that the first nervous systems consisted of combined sensory-motor neurons, directly translating sensory input into motor output on locomotor ciliated cells and steering muscle cells. Neuronal circuitry with low levels of integration has been retained in cnidarians and in the ciliated larvae of some marine invertebrates. This parallel processing stage could have been the starting point for the evolution of more integrated circuits performing the first complex computations such as persistence or coincidence detection. The sensory-motor nervous systems of cnidarians and ciliated larvae of diverse phyla show that brains, like all biological structures, are not irreducibly complex.

Efficiency optimization and symmetry-breaking in a model of ciliary locomotion

A variety of swimming microorganisms, called ciliates, exploit the bending of a large number of small and densely packed organelles, termed cilia, in order to propel themselves in a viscous fluid. We consider a spherical envelope model for such ciliary locomotion where the dynamics of the individual cilia are replaced by that of a continuous overlaying surface allowed to deform tangentially to itself. Employing a variational approach, we determine numerically the time-periodic deformation of such surface which leads to low-Reynolds locomotion with minimum rate of energy dissipation (maximum efficiency). Employing both Lagrangian and Eulerian points of views, we show that in the optimal swimming stroke, individual cilia display weak asymmetric beating, but that a significant symmetry-breaking occurs at the organism level, with the whole surface deforming in a wavelike fashion reminiscent of metachronal waves of biological cilia. This wave motion is analyzed using a formal modal decomposition, is found to occur in the same direction as the swimming direction, and is interpreted as due to a spatial distribution of phase differences in the kinematics of individual cilia. Using additional constrained optimizations, as well as a constructed analytical ansatz, we derive a complete optimization diagram where all swimming efficiencies, swimming speeds, and amplitudes of surface deformation can be reached, with the mathematically optimal swimmer, of efficiency one-half, being a singular limit. Biologically, our work suggests therefore that metachronal waves may allow cilia to propel cells forward while reducing the energy dissipated in the surrounding fluid.

Network model for visually mediated ciliary locomotion in Hermissenda

The overall goal of this study is to investigate the ways in which learning modifies behavior. A combination of computational and empirical studies is being used to address this issue. Empirical studies investigate learning from a cellular and synaptic perspective in the relatively simple nervous system of the nudibranch mollusk Hermissenda [1-3]. Pavlovian conditioning produces light-elicited inhibition of normal positive phototaxis in Hermissenda. Learning changes both cellular excitability and synaptic strength in the neural circuit that supports phototaxis. In the present study, a model of the circuit that supports visually mediated locomotion (Fig. ​(Fig.1A)1A) was developed. Consistent with empirical observations, simulated responses to light increased the level of VP1 spike activity (Fig. 1B1), which is equivalent to positive phototaxis. Simulations indicated that phototaxis resulted from disinhibition of VP1. Light increased activity in Ie and decreased activity in Ii (Fig. 1B2). The net result was less activity in IIIi and disinhibition of VP1 (Fig. 1B2). Simulations also indicated that disinhibition produced phototaxis only if VP1 had a high level of tonic firing. The model is being refined and expanded, and will be used to investigate the generation of other behaviors (e.g., foot contraction), the responses to other sensory inputs (e.g., gravity), and the influence of learning-induced plasticity (e.g., increased Ie excitability and decreased VP1 tonic firing). Simulations also will help identify features of the model that warrant further empirical investigation. Figure 1 Network model for visually mediated ciliary locomotion. A: The model had six cells: LB, lateral B-type photoreceptor; Ib, Ie, Ii and IIIi, interneurons; VP1, ciliary motor neuron. The cells were Hodgkin-Huxley-like neurons. Membrane conductances also ...

Muscular waves contribute to gliding rate in the freshwater gastropod Lymnaea stagnalis

This study revises the mechanisms of ciliary locomotion and demonstrates muscular contribution to locomotion rate in Lymnaea stagnalis. L. stagnalis sticks to the substratum by the foot sole and moves smoothly with no visible contractions of the foot. A ciliated epithelium covering the sole is underlain by smooth muscle cells containing giant mitochondria. It is shown here that slow (basal) locomotor activity (measured as the flow rate of physiological saline over isolated sole) appears spontaneously or is induced by 10(-8)-10(-7) M 5-HT. 5-HT (10(-7)-10(-4) M) facilitates locomotor activity dose-dependently, and KCN (an inhibitor of mitochondrial respiration) decreases these effects to the basal level. 5-HT and KCN have no effect on the frequency of ciliary beat (stroboscopic measurements), and blockers of anaerobic glycolysis inhibit ciliary motility. Under anaerobic conditions locomotion of a snail is slow and insensitive to 5-HT in contrast to that in aerobic environments. It is concluded that glycolysis supplies energy to ciliated cells and respiration to sole muscle cells; 5-HT stimulates ciliary beating in an all-or-none fashion and muscular waves in a dose-dependent manner; cilia provide slow (basal) gliding, and locomotory rate up to 80% above the basal level is determined by muscular waves.

Polysensory interneuronal projections to foot contractile pedal neurons in Hermissenda.

A Pavlovian-conditioning procedure may produce modifications in multiple behavioral responses. As an example, conditioning may result in the elicitation of a specific somatomotor conditioned response (CR) and, in addition, other motor and visceral CRs. In the mollusk Hermissenda conditioning produces two conditioned responses: foot-shortening and decreased locomotion. The neural circuitry supporting ciliary locomotion is well characterized, although the neural circuit underlying foot-shortening is poorly understood. Here we describe efferent neurons in the pedal ganglion that produce contraction or extension of specific regions of the foot in semi-intact preparations. Synaptic connections between polysensory type Ib and type Is interneurons and identified foot contractile efferent neurons were examined. Type Ib and type Is interneurons receive synaptic input from the visual, graviceptive, and somatosensory systems. Depolarization of type Ib interneurons evoked spikes in identified tail and lateral foot contractile efferent neurons. Mechanical displacement of the statocyst evoked complex excitatory postsynaptic potentials (EPSPs) and spikes recorded from type Ib and type Is interneurons and complex EPSPs and spikes in identified foot contractile efferent neurons. Depolarization of type Ib interneurons in semi-intact preparations produced contraction and shortening along the rostrocaudal axis of the foot. Depolarization of Is interneurons in semi-intact preparations produced contraction of the anterior region of the foot. Taken collectively, the results suggest that type Ib and type Is polysensory interneurons may contribute to the neural circuit underlying the foot-shortening CR in Hermissenda.

Development of the 5-HT-like Immunoreactive Pedal Plexus in the Pond SnailLymnaea stagnalis appressa

In many gastropods, a serotonin-like immunoreactive axon plexus lies over ciliary cells on the pedal sole. The origin and function of axons in this plexus is uncertain. By using serotonin antibodies in the direct-developing embryo of the pond snail, the axons that initially form this plexus were traced from seven large neurons in each pedal ganglion. Soon after metamorphosis begins, the first immunoreactive pedal ganglion neuron sends multiple branched neurites to lie directly over pedal sole ciliary cells. By 70% of the 11 days required for hatching, axons from the seven neuron pairs form a plexus over ciliary cells in the whole sole. The axon from each of the seven neurons is guided to a specific area of the pedal plexus where ciliary cells are developing. Axons from two pairs of these neurons, which form the pedal plexus in the posterior part of the foot, are in the unpaired nerve that comes from the pedal ganglia ventral commissure. It is likely that these two developing neuron pairs are homologs of the two neuron pairs in Lymnaea stagnalis that have axons in this ventral commissure nerve. Identification of these neurons and the other five neuron pairs with axons in the pedal plexus will provide a basis for future studies of the relation between the plexus and pedal ciliary locomotion.

Sensory Regulation of Network Components Underlying Ciliary Locomotion inHermissenda

Ciliary locomotion in the nudibranch mollusk Hermissenda is modulated by the visual and graviceptive systems. Components of the neural network mediating ciliary locomotion have been identified including aggregates of polysensory interneurons that receive monosynaptic input from identified photoreceptors and efferent neurons that activate cilia. Illumination produces an inhibition of type I(i) (off-cell) spike activity, excitation of type I(e) (on-cell) spike activity, decreased spike activity in type III(i) inhibitory interneurons, and increased spike activity of ciliary efferent neurons. Here we show that pairs of type I(i) interneurons and pairs of type I(e) interneurons are electrically coupled. Neither electrical coupling or synaptic connections were observed between I(e) and I(i) interneurons. Coupling is effective in synchronizing dark-adapted spontaneous firing between pairs of I(e) and pairs of I(i) interneurons. Out-of-phase burst activity, occasionally observed in dark-adapted and light-adapted pairs of I(e) and I(i) interneurons, suggests that they receive synaptic input from a common presynaptic source or sources. Rhythmic activity is typically not a characteristic of dark-adapted, light-adapted, or light-evoked firing of type I interneurons. However, burst activity in I(e) and I(i) interneurons may be elicited by electrical stimulation of pedal nerves or generated at the offset of light. Our results indicate that type I interneurons can support the generation of both rhythmic activity and changes in tonic firing depending on sensory input. This suggests that the neural network supporting ciliary locomotion may be multifunctional. However, consistent with the nonmuscular and nonrhythmic characteristics of visually modulated ciliary locomotion, type I interneurons exhibit changes in tonic activity evoked by illumination.