Menotaxis refers to an animal maintaining a constant angle to a stimulus. The Silkworm moth, for instance, flies at an angle perpendicular to the direction of the wind in order to pick up a scent trail.
Once the moth detects the trail, it turns upwind to find the chemical gradient of the trail. Another type of menotaxis is sun compass orientation. Many animals use the position of the sun to orient themselves. These animals can compensate for the sun moving across the sky over the course of the day. Honeybees for instance, imprint on the arc of the sun.
They can utilize polarized light, and so they can locate the position of the sun even on an overcast day. Tropotaxis refers to taking signal samples simultaneously from paired receptors. From , we can see tropotaxis literally means "turned movement. An experiment by Martin and Lindauer revealed that crossing the antennae in honeybees results in the bees traveling in the opposite direction from where the trail would lead them.
Magnetotaxis is orientation in response to magnetic cues. Wide varieties of animals use magnetic cues to navigate. The waste will be ejected from a structure called the anal pore or cytoproct. Various single-celled eukaryotes have the anal pore.
The anal pore of a paramecium is a region of the pellicle that is not covered by ridges and cilia. The thin pellicle allows vacuoles to be merged into the cell surface and emptied. As we mentioned above, the outermost layer is the soft shell of pellicle and cilia. Bound to pellicle is a narrow peripheral layer of specialized firm cytoplasm, called the ectoplasm. Below the ectoplasm lies a more fluid type of cytoplasm: the endoplasm. This region contains the majority of cell components and organelles.
In this high-resolution image of the paramecium cell, you can see two layers of cytoplasm: ectoplasm and endoplasm. Trichocysts are protective organelles embedded in the ectoplasm layer.
Compared to the rest of the cytoplasm endoplasm , ectoplasm forms a thin, dense, and clear outer layer containing trichocysts and fibrillar structures. The roots of cilia also anchor in the ectoplasm layer.
Pellicle and ectoplasm together serve as the protective skin for paramecia. Trichocyst trick-o-sists is a small spindle-like organelle situated in the ectoplasm with a minute pore opened on the pellicle surface. Trichocysts are arranged perpendicular to the ectoplasm.
Trichocysts are filled with a dense refractive fluid containing swelled substances. When the cells receive mechanical, chemical, or electric stimuli, trichocysts discharge their contents and become long, thin, stinging spikes. After they are discharged, new ones are generated from kinetosomes. The exact function of trichocysts is not quite clear, though a popular theory is that they are important for defense against predators.
Trichocysts may also help cell adhesion and support the paramecium cell body. Trichocysts are spindle-like organelles that can discharge stinging filaments as a protection against predators. Left: A TEM image showing a trichocyst embedded in the ectoplasm. When receiving outside stimuli, the core of trichocyst will swallow and push the spike out from the sheath. Image: Bannister, J. Cell Sci. Right: Highly magnified phase contrast image showing a paramecium fired its spiky trichocysts for protection.
Like a normal eukaryotic cell, enclosed inside the pellicle layer of paramecium is a jelly-like substance called cytoplasm. The cytoplasm includes the cytosol and all the organelles. The cytosol is like condensed soup inside the cell. It is a complex mixture of all kinds of substances dissolved in water. You can find small molecules like ions sodium, potassium, or calcine , amino acids, nucleotides the basic units of DNA , lipids, sugars, and large macromolecules such as proteins and RNAs.
Unlike the regular eukaryotic cells, paramecium has two nuclei , a big one and a small one. Paramecium also consists of two types of vacuoles: contractile vacuole and food vacuole , which do not exist in human cells. The most unusual characteristic of paramecia is their nuclei. They have two types of nuclei, which differ in their shape, content and function.
White and black arrowheads point symbiotic bacteria inside the cytoplasm. Photo credit: MDPI. The two types of nuclei are micronucleus and macronucleus. The micronucleus contains all of the DNA called genome that is present in the organism.
This DNA is passed from one generation to another generation during reproduction. On the other hand, the macronucleus contains a subset of DNA from the micronucleus. These DNA fragments are copied from micronucleus to macronucleus because they carry genes that are frequently needed by the paramecium cell. Genes in the macronucleus are actively transcripted to mRNA and then translated to proteins. The macronucleus is polyploid or contains multiple copies of each chromosome, sometimes up to copies.
In other words, the function of the micronucleus is to maintain genetic stability and making sure that the desirable genes are passed to the next generation. It is also called the germline or generative nucleus. Macronucleus plays a role in non-reproductive cell functions including the expression of genes needed for the everyday function of the cell. The macronucleus is also called the vegetative nucleus.
The macronucleus acts as the random-access memory RAM which stores working data and machine codes. The computer only loads programs currently in use from hard drive to RAMs. In a paramecium cell, more active genes meaning the cell need more of these proteins encoded by these genes may have more copies in the macronucleus. By having two nuclei, if a piece of DNA is in the micronucleus but not in the macronucleus, it will be removed during the next round of cell division.
In other words, if something foreign got into the micronuclear genome, then when the next macronucleus is made, it would be removed and not included in the expressed version [transcribed] of the genome. This mechanism functions as a primitive DNA immune system; that is, surveying the genome and trying to keep out invading elements. Morphologically, macronucleus is kidney-liked or ellipsoidal in shape.
The micronucleus is found close to the macronucleus. It is a small and compact structure, spherical in shape. All paramecium species have one macronucleus. However, the number of micronuclei can vary by species. For example, P.
Vacuoles take on specific functions in a paramecium cell. Paramecium has two types of vacuoles: contractile vacuoles and food vacuoles.
One paramecium cell has two star-shaped contractile vacuoles sitting on each end of the body. They are filled with fluids and are present at fixed positions between the endoplasm and ectoplasm. Contractile vacuoles are responsible for osmoregulation , or the discharge of excess water from the cell. Beyond adaptation, there is an important literature on learning in Paramecium and other ciliates. Unfortunately, as reviewed by Applewhite , many of those studies are difficult to interpret as they lack appropriate controls or observations.
In a series of papers Gelber, , , , , a , b , Gelber showed an apparent reinforcement of behavior with a food reward see Gershman et al. A platinum wire is lowered repeatedly into a depression slide with paramecia.
If the wire is intermittently baited with bacteria, then more and more paramecia cling to the wire, even when a clean wire is finally lowered into the slide. What might be the stimulus? Gelber noted that the behavior was not observed when paramecia were tested in the dark, suggesting that perhaps paramecia, with permission developed an attraction to a reflection or shadow cast by the wire.
These observations were controversial, because it was objected that lowering the baited wire introduces bacteria in the fluid, to which paramecia are then attracted even when the wire is removed or cleaned Jensen, A plausible explanation, in line with informal observations reported in this set of studies, is that feeding reduces the activity of paramecia so that they tend to stay near the wire, and promotes thigmotaxis so that they tend to adhere more easily to the wire.
In this case, the procedure would indeed reinforce a behavior, namely the feeding behavior, but not a stimulus-specific behavior. More detailed observations seem necessary to understand the phenomenon. Another phenomenon that has attracted some attention is tube escape learning, first described by French in French, A single Paramecium is placed in a drop and a thin tube is lowered into it. The organism is drawn into the tube by capillarity. When the experiment is repeated, escape time decreases to around 15 s after a few trials.
This phenomenon has been robustly reproduced by several authors Hanzel and Rucker, ; Applewhite and Gardner, , but its basis is unclear. Applewhite and Gardner proposed that Paramecium released some substance in the tube that then influences future behavior, but this hypothesis contradicts earlier results by Hanzel and Rucker showing the same performance improvement in multiple paramecia with the same tube.
Studies of tube escape learning in Stentor , another ciliate, suggest that the phenomenon is related to gravitaxis Bennett and Francis, ; Hinkle and Wood, Performance improvement is seen only when the tube is vertical, not when it is horizontal, where escape is fast from the first trial. This suggests the following speculative explanation: in a vertical tube, paramecia are trapped near the top because of negative gravitaxis, then prolonged confinement perhaps signaled by frequent avoiding reactions inhibits the normal gravitactic behavior, so that the organism can escape to the bottom.
Finally, Hennessey et al. When a tone is played by a speaker below the slide, Paramecium shows no reaction. However, when the tone is paired with electrical stimulation triggered in the middle of the tone, Paramecium reacts to the stimulus with an avoiding reaction, then after a few trials gives an avoiding reaction at the onset of the tone, in anticipation of the electrical stimulus.
The authors demonstrate extinction reaction disappears when sound is presented alone , retention and specificity reacting specifically to a Hz tone or to a Hz tone. The physiological basis is not known. Armus and colleagues Armus et al. The bath is split into two compartments, one in the dark, the other one in light. Initially, Paramecium spends more time in the dark compartment, because of photophobia. Training consists in electrically stimulating the cell when it enters the compartment of the cathode.
After training, Paramecium spends more time than before in the cathodal half, which now only differs by its lighting. However, if stimulation is triggered in the anodal half, then after training Paramecium spends less time in that half.
Therefore, the phenomenon does not seem to be based on an association between the electrical stimulus and the light stimulus. A plausible interpretation is the following. As is known from studies of galvanotaxis Ludloff, ; Dale, , electrical stimulation makes Paramecium move toward the cathode. Stimulation in the lighted cathodal compartment then makes Paramecium spend more time in light, which results in adaptation of the photophobic behavior.
Thus, after training, Paramecium spends more time than before in the lighted compartment. In summary, although the existing literature is complex, there is clear evidence of behavioral plasticity in Paramecium. Some can be categorized as adaptation, and there is at least one documented case of learning Hennessey et al. In the absence of any stimulus, Paramecium swims in spirals. Paramecium is covered by several thousand cilia Fig. Thus, on the hidden surface further from the observer , cilia beat toward the left and rear.
This results in a forward movement with a rotation around the longitudinal axis, as in unscrewing over to the left; Fig. Spiral swimming. A , Organization of ciliary basal bodies on the oral ventral and aboral dorsal side from Iftode et al. B , Ciliary beat cycle: power stroke or effective stroke and recovery stroke Omori et al.
C , Water currents produced by cilia for different orientations of Paramecium Jennings, In the oral groove, currents are oriented toward the mouth. Cilia on parallel lines are at the same phase of the beat cycle. The curved arrow shows the direction of movement.
A possible reason is that cilia in the oral groove beat in a specific direction, toward the mouth, which counters the movement produced by the other cilia. A recent study has shown indeed that properties of oral cilia differ from other cilia Jung et al. This may explain why the trajectory describes a wide spiral, with the oral side always facing its axis Fig. Properties of spiral swimming can vary, in particular its speed and width.
Paramecium can also swim backward, with an effective stroke toward the front and slightly to the right. Thus, in backward swimming, the movement is not the symmetrical of forward swimming: the cell still rotates in the same direction. As a consequence, the swimmer stops as soon as cilia stop beating. Therefore, if cilia beating were synchronized over the entire body, then the swimmer would move forward in regular discontinuous steps. In fact, this can happen in the escape reaction: a strong heat stimulus near the posterior end induces a synchronous power stroke of the cilia as in the butterfly stroke; Hamel et al.
If on the contrary cilia beating were completely disorganized which can happen transiently in the avoiding reaction , then neighboring cilia might beat in inconsistent directions and this is not an efficient way of swimming. In fact, it has been shown that the metachronal pattern optimizes the energetic efficiency of swimming Gueron and Levit-Gurevich, ; Osterman and Vilfan, It was once postulated that ciliary coordination might be electrically controlled by the cell, but Paramecium is essentially isopotential Eckert and Naitoh, Instead, cilia coordination is mediated by hydrodynamic interactions Machemer, ; Guirao and Joanny, and mechanical coupling through the compliant body Narematsu et al.
This illustrates the concept of embodiment in motor neuroscience: part of the problem of efficient coordination is solved not by manipulating body representations, but by direct physical interaction of the body with its immediate environment Tytell et al. In the case of microorganisms such as Paramecium , the results of this physical interaction can be understood precisely, thanks to an abundant literature on the mechanics of cilia and flagella Blake and Sleigh, ; Sartori et al.
When observed in a vertical plane, trajectories are curved upward Roberts, ; Fig. The earliest explanation, the gravity-buoyancy model, postulates a mismatch between the buoyancy center and the gravity center Verworn, : this could generate a torque making the body align with gravity. Roberts Roberts, , argued that density inhomogeneities are unlikely to be sufficient to account for the observations, and instead proposed a drag-gravity model: as the posterior end is larger than the anterior end, the viscous drag differs and the posterior end falls more rapidly than the anterior end; thus, the cell turns upward.
However, Jensen and later Kuznicki observed that dead or immobilized cells fall with no preferred orientation, although this is questioned by Roberts Roberts, This would discard both passive orientation mechanisms. The propulsion-gravity model Winet and Jahn, is a more complex proposition, which links gravitaxis with ciliary beating: sedimentation introduces viscous resistance to beating that is stronger in the up phase of the helicoidal cycle than in the down phase, resulting in velocity-dependent reorientation.
Gravitactic behavior of Paramecium. A , Upwardly curved trajectories of Paramecium in a vertical chamber from Roberts, , with permission. B , Velocity change corrected for sedimentation as a function of cell orientation from Nagel and Machemer, , with permission , open circles correspond to a morphologic mutant. C , Avoiding reaction frequency as a function of acceleration in a centrifuge microscope, after 4 h of equilibration from Nagel and Machemer, , with permission.
Triangles indicate cell direction. In addition to these hydrodynamic mechanisms, physiological mechanisms have been postulated.
It has been observed that Paramecium swims slightly faster upwards than downwards, once sedimentation has been subtracted Machemer et al. Although spurious correlations should be ruled out e. As there is a spatial gradient of mechanosensitivity between the front and rear, the transduced current would be hyperpolarizing when the anterior end is upward increased pressure on the rear end and depolarizing when the anterior end is downward.
In support of this hypothesis, a cell vertically immobilized between two horizontal electrodes can spontaneously turn upward or downward, and small membrane potential changes with the expected sign are observed, although with long latency on the order of 20 s; Gebauer et al.
These physiologically induced changes in mean velocity and avoiding reaction rate likely represent a small contribution to gravitaxis, compared with the reorientation of the cell Roberts, , but it is conceivable that reorientation itself occurs by physiological modulation of velocity within the helicoidal cycle Mogami and Baba, In the avoiding reaction, Paramecium swims backward if the reaction is strong then turns before it swims forward again.
Backward swimming occurs because cilia reorient, with the power stroke oriented toward the anterior end instead of the posterior end, but how can Paramecium turn? Turning requires some inhomogeneity in the ciliary beating pattern. First, anterior and posterior cilia do not revert synchronously during the avoiding reaction Fig. When the avoiding reaction is initiated, all cilia simultaneously strike forward, which moves the cell backward 2.
The beating pattern then progressively reorganizes into the metachronal pattern as the cell swims backward 3—5. Reorientation of the cell starts when the anterior end reverts to the forward metachronal pattern 6—8. Thus, anterior and posterior ends show different metachronal patterns, respectively, of forward and backward swimming. Details of the avoiding reaction. A , Reorganization of the ciliary beating pattern during the avoiding reaction after Machemer, B , Cross-section of Paramecium seen from the anterior end, during forward swimming a , corresponding to step 1 and during reorientation b , corresponding to step 6 , according to Jennings The arrows correspond to the induced movement of the body opposite to the beating direction.
It is not obvious, however, how this asynchronous pattern would make the cell turn. If the beating pattern were axisymmetric, then the net force produced by either group of cilia anterior or posterior should be directed along the main axis.
Jennings claims that cilia in the oral groove may also reverse, i. This could make Paramecium turn toward its aboral dorsal side, as observed, but Jennings and Jamieson observed that when Paramecium was cut in two pieces below the oral groove, both pieces could turn in a similar way Jennings and Jamieson, Jennings also mentions that cilia of the anterior end do not all strike to the right: instead, they strike toward the oral groove Jennings, ; Fig.
As a result, the cell turns toward the aboral side. This is supported by more recent observations in a flattened ciliary sheet from Paramecium Noguchi et al. Thus, turning likely results from inhomogeneity in the response of different groups of cilia, but details are still lacking.
When Paramecium touches an obstacle, mechanosensitive channels open, depolarize the membrane and trigger a calcium-based action potential Eckert, The entry of calcium then triggers the reorientation of cilia, so that Paramecium swims backwards. Then calcium is buffered or pumped out Plattner et al. Historically, Paramecium electrophysiology has been studied by placing the cell in a tiny droplet, letting the fluid evaporate until the cell is captured by surface tension, then inserting sharp microelectrodes and covering with extracellular medium Naitoh and Eckert, A recent method immobilizes the cell by suction against a filter Kulkarni et al.
Paramecium is an isopotential cell, as demonstrated with two-electrode measurements Eckert and Naitoh, ; Dunlap, ; Satow and Kung, , which is a particularly favorable situation for electrophysiological modeling.
For P. At rest, the membrane is permeable to many cations Naitoh and Eckert, a. One reason might be that the extracellular medium fresh water typically has very low ionic content, so that the cytosolic ions exert a large osmotic pressure on the membrane. In Paramecium and other protozoa, this osmotic imbalance is regulated by specialized organelles, the contractile vacuoles, which expel water that invades the cell by osmosis Allen and Naitoh, When Paramecium is mechanically stimulated on the front, or a current is injected, the membrane is depolarized Fig.
If the stimulus is strong enough, this depolarization triggers a graded action potential, with a stimulus-dependent amplitude all-or-none spikes can occur if extracellular calcium is partially replaced by barium; Naitoh and Eckert, b. This action potential is due to calcium voltage-gated channels distributed over the cilia and delayed rectifier potassium channels located in the somatic membrane; this can be demonstrated by removing the cilia with ethanol and shaking Machemer and Ogura, Membrane potential responses to mechanical stimulation with a glass stylus on the front A and on the rear B ; from Naitoh and Eckert, , with permission; top traces: voltage command to the piezoelectric actuator.
Action potential currents in P. A , Current recorded in voltage-clamp with different depolarization steps above resting potential. The first and last peaks are capacitive transients. The early negative transient is mediated by calcium; the late positive current is mediated by potassium.
B , Early and late currents versus membrane potential relative to rest. Recovery from inactivation takes a few tens to a hundred of milliseconds Naitoh et al. This is a common form of inactivation of calcium channels in neurons, which has been discovered first in Paramecium Brehm and Eckert, a.
It involves calmodulin, a highly conserved calcium sensor that is found across all species Ben-Johny and Yue, There is also a calcium-activated current, which develops more slowly Satow and Kung, It is involved in repolarization after sustained stimulation Saimi et al. Genetic analysis has identified in particular SK channels located in the cilia Valentine et al.
All these channels have homologs in mammalian neurons. Cilia are highly conserved structures. Motile cilia are found not only in swimming microorganisms but also in multicellular organisms including humans, where they are involved in moving fluids, for example the cerebrospinal fluid Faubel et al. The cilium contains a cytoskeleton called the axoneme, composed of nine microtubule doublets arranged in a ring around a central pair of microtubules Porter and Sale, Dynein motors make microtubule doublets slide on each other, which bends the cilium Walczak and Nelson, The activity of these motors is regulated by second messengers, in particular calcium and cyclic nucleotides cAMP and cGMP.
Thus, stronger current pulses trigger larger and faster spikes, resulting in larger calcium increase and therefore longer reversed beating Machemer and Eckert, Electromotor coupling. A , Calcium uncaging in cilia circle triggers local ciliary reversal from Iwadate, , with permission. B , Beating frequency filled: positive; open: negative as a function of membrane potential in voltage clamp from Machemer, , with permission.
Reversal is indicated by dots. Squares and circles are two different permeabilized models, circles being more physiological. Cilia reverse at the minimum beating frequency.
D , Cell length versus pCa in a permeabilized cell from Nakaoka et al. Beating frequency also changes with voltage Machemer and Eckert, ; Fig. In particular, cilia beat faster when the command voltage is increased above resting potential. Early work in permeabilized cells indicated that calcium controls ciliary reorientation but not beating frequency Naitoh and Kaneko, , but this was later argued to be because of unphysiological aspects of the permeable models Nakaoka et al.
In more physiological permeabilized cells, an increase in ciliary calcium concentration above the resting level triggers ciliary reorientation and an increase in beating frequency, matching the effect of depolarization Fig. Note that swimming velocity does not exactly follow this frequency increase, because it also depends on the coordination of cilia, which is disrupted when cilia reorient.
For small depolarizations, not all cilia reorient Machemer and Eckert, , which may explain how the organism turns. The cell also contracts when calcium concentration increases Fig. Mechanoreception in Paramecium and other ciliates has been the object of several reviews Naitoh, ; Machemer, ; Machemer and Deitmer, ; Deitmer, Touching the anterior part of Paramecium results in membrane depolarization, while touching the posterior part results in membrane hyperpolarization Naitoh and Eckert, Six genes of the Piezo family Coste et al.
Ionic channels mediating mechanosensitivity are located on the basal membrane; a deciliated cell is still mechanosensitive Ogura and Machemer, Cilia are not directly involved in transduction, but they are involved in the mechanical transfer and filtering of stimuli.
In the middle region, a mixed current can be observed, with an outward then inward component, indicative of a superposition of two ionic channel responses. There are also graded changes in mechanosensitivity along the oral-aboral dorsoventral axis.
Mechanosensitive responses measured as a function of stimulation position A: anterior; P: posterior in a P. Mechanical responses have been studied mainly by deflecting a thin glass stylus onto the membrane with a piezo-electric actuator.
In another ciliate, Stylonichia , the transduced current increases linearly with the deflection amplitude of the probe; the resulting potential may saturate for strong stimuli, near the reversal potential. Faster deflections reduce response latency without changing the amplitude.
When a mutant with defective ciliary calcium channels is mechanically stimulated, ciliary reversal is observed only at the site of stimulation on the anterior membrane Takahashi and Naitoh, : this indicates that mechanical stimulation only recruits local mechanoreceptors these can trigger ciliary reversal because the transduced current is carried by calcium.
Stimulations integrate both spatially and temporally, with no sign of refractoriness. Finally, the duration of the deflection has no effect on the response. Thus, the integration of mechanical stimuli is analog to synaptic integration in a neuron: stimulation at a site produces a transient current through ionic channels, transduced currents are integrated both spatially and temporally, and the resulting potential response may trigger an action potential if it is large enough.
When Paramecium is mechanically stimulated on the rear, the membrane is hyperpolarized Fig. The electrophysiological response is shaped by several hyperpolarization-activated channels.
If that concentration is very low, a regenerative hyperpolarization can be obtained Satow and Kung, This current is similar to inward rectifiers found in other species Doupnik et al.
A calcium current activates with hyperpolarization, and the entry of calcium then mediates an increase in beating frequency Nakaoka and Iwatsuki, ; Preston et al.
This current actually activates within a few tens of ms, and decays more slowly through calcium-dependent inactivation Preston et al. It actually consists of two pharmacologically distinct currents located in the somatic membrane, one of which is sustained Nakaoka and Iwatsuki, The magnitude of the hyperpolarization-activated calcium current is directly related to the increase in beating frequency, and blocking this current also blocks the frequency increase Nakaoka and Iwatsuki, Thus, it appears that beating frequency is controlled by calcium concentration in the somatic membrane, presumably at the base of cilia, in line with studies in other ciliary systems Tamm, This contradicts several earlier hypotheses: that beating frequency increases with a hyperpolarization-induced decrease in ciliary calcium concentration Machemer, , by a iontophoretic mechanism in the cilia Brehm and Eckert, b , or by regulation by cyclic nucleotides Satir et al.
The latter hypothesis did receive some support Bonini et al. Gomez-Marin and Ghazanfar described three fundamental biological principles of behavior that highlight the need for integrated approaches in neuroscience: materiality, agency and historicity Gomez-Marin and Ghazanfar, Materiality refers to the role of body and environment in behavior.
That is, the relation between neural activity and behavior is not just a case of correspondence the coding view; Brette, , but also of physical causality: spikes cause particular physiological effects, the results of which are determined by the structure of the body and the environment it interacts with Tytell et al.
For example, in Paramecium , cilia are under electrical control but efficient motor coordination is partly achieved by hydrodynamic interactions between cilia. Agency refers to the fact that action and perception form a closed loop in the service of goals, rather than a linear stimulus-reaction chain. For example, when Paramecium meets an obstacle, the mechanosensory signal is determined not just by the object but also by the motor response that the signal causes, in a closed loop.
Historicity refers to the fact that organisms are individuals: variability is best understood not as a noisy deviation around a norm but as a functional result of their history. In Paramecium , this is evident for example in long-term adaptation to new environments, but also in some exploratory behaviors such as tube escape. Addressing these three principles requires studying an entire organism in an environment, rather than isolated subsystems. Computational neuroethology is a subfield of computational neuroscience focusing on the modeling of autonomous behavior Beer, , which has been investigated in particular artificial organisms Beer and Gallagher, and robots Webb, More recently, integrated models of C.
Those model organisms have certain obvious advantages over Paramecium , namely the fact that they have a nervous system, with interacting neurons. But Paramecium has great assets for integrative modeling of a whole organism, relating physiology and behavior. First, there is an extensive literature on Paramecium , covering detailed aspects of behavior, genetics, electrophysiology, cell and molecular biology.
This literature has highlighted similarities with metazoans, in particular nervous systems, not only functionally but also at genetic and molecular levels Connolly and Kerkut, ; Hinrichsen and Schultz, ; Beisson et al. Second, it benefits from various tools, for example genetic tools such as RNA interference Galvani and Sperling, , proteomics Yano et al. Finally, it is easy to culture Beisson et al.
As outlined in this review, a number of neuroscientific themes can be addressed and revisited in Paramecium. One such theme is the physiological basis of behavior and the relation between perception and action. Another key issue is that framing neural activity as responses to stimuli denies any autonomy to the organism.
By its relative simplicity, Paramecium offers the possibility to study the physiological basis of autonomous behavior outside the frame of the classical sandwich, because it seems feasible to develop closed-loop dynamical systems models of the organism behaving autonomously in an environment, where spikes are not symbols but actions Brette, Motor control is a related theme where Paramecium may provide some insights.
Embodiment is the idea that the body can contribute to motor control, beyond the mere execution of central commands. In Paramecium , cilia beat in a coordinated fashion in the absence of central command, by hydrodynamic and mechanical interactions, yielding efficient swimming.
More generally, the mechanical properties of its body contribute to its navigation abilities, as when navigating in confined spaces, and more generally when interacting with surfaces.
As it turns out, Paramecium appears to use neither of the two mainstream concepts in motor control, planning or feedforward control; Wolpert and Ghahramani, and feedback control Powers, Instead, it uses another way to produce goal-directed behavior, based on the Darwinian insight that random exploration and elimination of unsuccessful attempts can produce adapted behavior.
While the physiological basis of learning is classically framed in terms of stimulus association, Paramecium may offer the possibility to address it in a more ecological context, that is, autonomous learning of a task. Tube escape might be such a task; however, the learning capabilities of Paramecium are still somewhat unclear.
As Paramecium is both an organism and a cell, it also offers the opportunity to investigate the relation between cellular plasticity and behavioral plasticity. Intrinsic plasticity is well documented in neurons Daoudal and Debanne, , but it remains very challenging to understand its functional implications for the organism.
Thus, it is classically interpreted in terms of homeostasis of cellular properties e. In Paramecium , since the relation between cellular physiology and behavior is more direct than in brains, it becomes possible to relate intrinsic plasticity with behavioral plasticity.
For example, ionic channel properties adapt to changes in temperature in such a way as to preserve normal motor behavior Nakaoka et al. Similarly, developmental plasticity can be addressed by investigating the physiological and behavioral changes after fission Iftode et al. Indeed, as ionic channels are spatially organized for example depolarizing mechanoreceptors at the front , this organization is disrupted by fission and must be somehow restored.
Decisions are customarily a result of the Reviewing Editor and the peer reviewers coming together and discussing their recommendations until a consensus is reached. When revisions are invited, a fact-based synthesis statement explaining their decision and outlining what is needed to prepare a revision will be listed below. The following reviewer s agreed to reveal their identity: Kirsty Wan. This manuscript provides a detailed review of what can be considered an extreme version of a reductionist neuronal system - a single-cell organism.
When searching for simpler systems to study physiology and behaivour, none can be simpler than the paramecium. And yet both the behaviour and physiology of this single-celled organism are also very rich. This work describes in detail the many behaviours of mainly two paramecia P. This extensive review is well written, thought provoking and should of interest to a wide scientific community.
Below are some specific comments on the manuscript. Neuroscience is often, almost by definition, predicated upon knowledge and insights gained from various animal models with well-studied nervous systems. The simplest organisms are often overlooked, yet are capable of surprisingly complex behaviours.
The ethology of unicells had been the subject of intense study several decades ago, but is now experiencing something of a resurgence in interest. This comprehensive review summarises and synthesises much of the data available in the literature about the bioelectrical basis of behaviour in the model ciliate Paramecium. The work will be of interest to a broad audience, particularly biophysicists working on cell motility, while serving to re acquaint neuroscientists with this unique model system.
Some mechanisms are vague or only hypothetical at this stage, but this is understandable given lack of data. The work is very much a synthesis of old results - all figures are taken directly from the old literature.
There is some mention of this in the last section, but this can be expanded, especially for the intended neuroscience readership. The entire paper focuses on Paramecium, while this may be justified, it would be useful to highlight any conserved features or comparisons with other systems in the discussions but also throughout - this may include other ciliated organisms, or other model organisms typically featured in neuroscience.
For instance, details of the capacitance of the cell are given 3. Which receptors are shared by paramecium and metazoan neuron-types? Again, many descriptions originate from stylised accounts by Jennings, it would be good to check if these observations indeed hold true, e.
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