Hair cell interactions in the statocyst of Hermissenda

State-dependent, visually guided behaviors in the nudibranch Berghia stephanieae

Nudibranch mollusks have structurally simple eyes whose behavioral roles have not been established. We tested the effects of visual stimuli on the behavior of the nudibranch Berghia stephanieae under different food and hunger conditions. In an arena that was half-shaded, animals spent most of their time in the dark, where they also decreased their speed and made more changes in heading. These behavioral differences between the light and dark were less evident in uniformly illuminated or darkened arenas, suggesting that they were not caused by the level of illumination. Berghia stephanieae responded to distant visual targets; animals approached a black stripe that was at least 15 deg wide on a white background. They did not approach a stripe that was lighter than the background but approached a stripe that was isoluminant with the background, suggesting the detection of spatial information. Animals traveled in convoluted paths in a featureless arena but straightened their paths when a visual target was present even if they did not approach it, suggesting that visual cues were used for navigation. Individuals were less responsive to visual stimuli when food deprived or in the presence of food odor. Thus, B. stephanieae exhibits visually guided behaviors that are influenced by odors and hunger state.

Large basolateral processes on type II hair cells are novel processing units in mammalian vestibular organs

Sensory receptors in the vestibular system (hair cells) encode head movements and drive central motor reflexes that control gaze, body movements, and body orientation. In mammals, type I and II vestibular hair cells are defined by their shape, contacts with vestibular afferent nerves, and membrane conductance. Here we describe unique morphological features of type II vestibular hair cells in mature rodents (mice and gerbils) and bats. These features are cytoplasmic processes that extend laterally from the hair cell base and project under type I hair cells. Closer analysis of adult mouse utricles demonstrated that the basolateral processes of type II hair cells vary in shape, size, and branching, with the longest processes extending three to four hair cell widths. The hair cell basolateral processes synapse upon vestibular afferent nerves and receive inputs from vestibular efferent nerves. Furthermore, some basolateral processes make physical contacts with the processes of other type II hair cells, forming some sort of network among type II hair cells. Basolateral processes are rare in perinatal mice and do not attain their mature form until 3-6 weeks of age. These observations demonstrate that basolateral processes are significant signaling regions of type II vestibular hair cells and suggest that type II hair cells may directly communicate with each other, which has not been described in vertebrates.

Functional changes in the snail statocyst system elicited by microgravity

BACKGROUND

The mollusk statocyst is a mechanosensing organ detecting the animal's orientation with respect to gravity. This system has clear similarities to its vertebrate counterparts: a weight-lending mass, an epithelial layer containing small supporting cells and the large sensory hair cells, and an output eliciting compensatory body reflexes to perturbations.

METHODOLOGY/PRINCIPAL FINDINGS

In terrestrial gastropod snail we studied the impact of 16- (Foton M-2) and 12-day (Foton M-3) exposure to microgravity in unmanned orbital missions on: (i) the whole animal behavior (Helix lucorum L.), (ii) the statoreceptor responses to tilt in an isolated neural preparation (Helix lucorum L.), and (iii) the differential expression of the Helix pedal peptide (HPep) and the tetrapeptide FMRFamide genes in neural structures (Helix aspersa L.). Experiments were performed 13-42 hours after return to Earth. Latency of body re-orientation to sudden 90° head-down pitch was significantly reduced in postflight snails indicating an enhanced negative gravitaxis response. Statoreceptor responses to tilt in postflight snails were independent of motion direction, in contrast to a directional preference observed in control animals. Positive relation between tilt velocity and firing rate was observed in both control and postflight snails, but the response magnitude was significantly larger in postflight snails indicating an enhanced sensitivity to acceleration. A significant increase in mRNA expression of the gene encoding HPep, a peptide linked to ciliary beating, in statoreceptors was observed in postflight snails; no differential expression of the gene encoding FMRFamide, a possible neurotransmission modulator, was observed.

CONCLUSIONS/SIGNIFICANCE

Upregulation of statocyst function in snails following microgravity exposure parallels that observed in vertebrates suggesting fundamental principles underlie gravi-sensing and the organism's ability to adapt to gravity changes. This simple animal model offers the possibility to describe general subcellular mechanisms of nervous system's response to conditions on Earth and in space.

Associative behavioral modification in hermissenda: cellular correlates

Three days of training consisting of trials of light paired with rotation produces a long-term modification of photopositive behavior in Hermissenda crassicornis. The behavioral modification depends on the temporal association of light and rotation. For animals that received light paired with rotation, significant increases in the spontaneous activity of type B photoreceptors were correlated with changes in photopositive behavior after training. A persistent tonic depolarization of type B photoreceptors can explain the cellular changes correlated with the long-term behavioral modification produced by the temporal association of light and rotation.

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.

Pavlovian conditioning in Hermissenda: a circuit analysis

An understanding of associative learning requires (1) an adequate description of the experimental conditions under which learning is produced, (2) a knowledge of what is learned or the determination of the content of learning, and (3) an explanation of how learning generates changes in behavior (Rescorla, 1980). These basic issues are being addressed at both the behavioral and cellular/molecular levels by the analysis of associative learning in animals with relatively uncomplex nervous systems. Use of Pavlovian conditioning of invertebrates as a model for associative learning has led to the identification of cellular and synaptic mechanisms underlying the formation of basic associations. However, an understanding of the associative processes that form the basis for Pavlovian conditioning requires an explanation not only of the mechanisms of temporal contiguity or predictability between the conditioned stimulus (CS) and the unconditioned stimulus (US), but also of how changes produced in the nervous system by conditioning are expressed in behavior. Studies with invertebrates have provided the opportunity to examine how associative learning is expressed in the neural circuitry that supports the generation of learned behavior.

Potassium currents in isolated statocyst neurons and RPeD1 in the pond snail, Lymnaea stagnalis

To begin to determine the underlying neural mechanisms of memory formation, we studied two different cell types that play important roles in different forms of associative learning in Lymnaea. Statocyst neurons (hair cells) mediate classical conditioning, whereas RPeD1 is a site of memory formation induced by operant conditioning of aerial respiration. Because potassium (K(+)) channels play a critical role in neuronal excitability, we initiated studies on these channels in the aforementioned neurons. Three distinct K(+) currents are expressed in the soma of both the hair cells and RPeD1. In hair cells and RPeD1, there is a fast activating and rapidly inactivating 4-aminopyridine (4-AP)-sensitive A current (I(A)), a tetraethyl ammonium (TEA)-sensitive delayed rectifying current, which exhibits slow inactivation kinetics (I(KV)), and a TEA- and 4-AP-insensitive Ca(2+)-dependent current (I(Ca-K)). In hair cells, the activation voltage of I(A); its half-maximal steady-state activation voltage and its half-maximal steady-state inactivation were at more depolarized levels than in RPeD1. The time constant of recovery from I(A) inactivation was slightly faster in hair cells. I(A) in hair cells is also smaller in amplitude than in RPeD1 and is activated at more depolarized potentials. In like manner, I(KV) is smaller in hair cells and is activated at more depolarized potentials than in RPeD1.

Pavlovian conditioning of Hermissenda: current cellular, molecular, and circuit perspectives

The less-complex central nervous system of many invertebrates make them attractive for not only the molecular analysis of the associative learning and memory, but also in determining how neural circuits are modified by learning to generate changes in behavior. The nudibranch mollusk Hermissenda crassicornis is a preparation that has contributed to an understanding of cellular and molecular mechanisms of Pavlovian conditioning. Identified neurons in the conditioned stimulus (CS) pathway have been studied in detail using biophysical, biochemical, and molecular techniques. These studies have resulted in the identification and characterization of specific membrane conductances contributing to enhanced excitability and synaptic facilitation in the CS pathway of conditioned animals. Second-messenger systems activated by the CS and US have been examined, and proteins that are regulated by one-trial and multi-trial Pavlovian conditioning have been identified in the CS pathway. The recent progress that has been made in the identification of the neural circuitry supporting the unconditioned response (UR) and conditioned response (CR) now provides for the opportunity to understand how Pavlovian conditioning is expressed in behavior.

Statocyst Hair Cell Activation of Identified Interneurons and Foot Contraction Motor Neurons in Hermissenda

Pavlovian conditioning of Hermissenda produces both light-elicited inhibition of normal positive phototactic behavior and conditioned stimulus (CS)-elicited foot-shortening. Rotation, the unconditioned stimulus (US) elicits foot-shortening and reduced forward ciliary locomotion. The neural circuit supporting ciliary locomotion and its modulation by light is known in some detail. However, the neural circuits responsible for rotation-elicited foot-shortening and reduced forward ciliary locomotion are not known. Here we describe components of the neural circuit in Hermissenda that produce anterior foot contraction and ciliary activation mediated by statocyst hair cells. We have characterized in semi-intact preparations newly identified pedal ventral contraction motor neurons (VCMNs) and interneurons (Ib). Type Ib interneurons receive polysynaptic input from statocyst hair cells and project directly to VCMNs and cilia-activating motor neurons. Depolarization of VCMNs with extrinsic current in normal artificial seawater (ASW) and high-divalent cation ASW, and under conditions where central synaptic transmission was suppressed with 5 mM Ni2+ ASW, elicited a contraction of the ipsilateral anterior foot measured from videotape recordings. Mechanical displacement of the statocyst or depolarization of identified statocyst hair cells with extrinsic current elicited spikes and complex excitatory postsynaptic potentials (EPSPs) in type Ib interneurons and complex EPSPs and spikes recorded in VCMNs. Type Ib interneurons are electrically coupled and project to VCMNs and VP1 cilia-activating motor neurons located in the contralateral pedal ganglia. The results indicate that statocyst hair-cell-mediated anterior foot contraction and graviceptive ciliary locomotion involve different interneuronal circuit components from the circuit previously identified as supporting light modulated ciliary locomotion.

Dual sensory-motor function for a molluskan statocyst network

In mollusks, statocyst receptor cells (SRCs) interact with each other forming a neural network; their activity is determined by both the animal's orientation in the gravitational field and multimodal inputs. These two facts suggest that the function of the statocysts is not limited to sensing the animal's orientation. We studied the role of the statocysts in the organization of search motion during hunting behavior in the marine mollusk, Clione limacina. When hunting, Clione swims along a complex trajectory including numerous twists and turns confined within a definite space. Search-like behavior could be evoked pharmacologically by physostigmine; application of physostigmine to the isolated CNS produced "fictive search behavior" monitored by recordings from wing and tail nerves. Both in behavioral and in vitro experiments, we found that the statocysts are necessary for search behavior. The motor program typical of searching could not be produced after removing the statocysts. Simultaneous recordings from single SRCs and motor nerves showed that there was a correlation between the SRCs activity and search episodes. This correlation occurred even though the preparation was fixed and, therefore the sensory stimulus was constant. The excitation of individual SRCs could in some cases precede the beginning of search episodes. A biologically based model showed that, theoretically, the hunting search motor program could be generated by the statocyst receptor network due to its intrinsic dynamics. The results presented support for the idea that the statocysts are actively involved in the production of the motor program underlying search movements during hunting behavior.

Gap junctional communication in the vibration-sensitive response of sea anemones

Although gap junctions occur in auditory and vestibular systems, their function is unclear. Here we present evidence for gap junctional communication in transmitting mechanosensory signals in a sea anemone model system. Hair bundles on anemone tentacles are vibration-sensitive mechanoreceptors that regulate discharge of nematocyst from effector cells. We find that vibration-dependent nematocyst discharge is selectively and reversibly blocked by the gap junction uncouplers, heptanol and arachidonic acid. Epidermal cells within excised tentacles exhibit a low level of dye coupling which is significantly enhanced upon deflection of overlying hair bundles. Dye coupling is inhibited both by gap junction uncouplers and by agents that interfere with mechanotransduction, including streptomycin and elastase. Electrophysiological data suggest gap junctional communication between cells giving rise to different hair bundles. When hair bundles are stimulated with a sweep of vibrations, individual cells show responses to five to eight frequencies. The number of responsive frequencies is reduced to one or two by heptanol and essentially abolished with streptomycin treatment. Immunoreactivity to the gap junction protein, connexin 43, is abundant in the tentacle epidermis and localized to membranes at junctions between several cell types. Small areas of close membrane apposition are observed between these cell types with intermembrane clefts of 4-7 nm. Of the several membrane proteins isolated from tentacles, immunoreactivity to connexin 43 is observed in a single band with an apparent molecular weight of approximately 46 kDa.

Synaptic interactions between crista hair cells in the statocyst of the squid Alloteuthis subulata

Intracellular injections of the fluorescent dye Lucifer yellow into the various cell types within the anterior transverse crista segment of the statocyst of squid revealed that the primary sensory hair cells and both large and small first-order afferent neurons have relatively simple morphologies, each cell having a single, unbranched axon that passes directly into the small crista nerve that innervates the anterior transverse crista. However, the small first-order neurons have short dendritic processes occurring in the region of the sensory hair cells. The secondary sensory hair cells have no centripetal axons, but some have long processes extending from their bases along the segment. Simultaneous intracellular recordings from pairs of the different cell types in the anterior transverse crista segment demonstrated that electrical coupling is widespread; secondary sensory hair cells are coupled electrically along a hair cell row, as are groups of primary sensory hair cells. Secondary sensory hair cell also are coupled to neighboring small first-order afferent neurons. However, this coupling is rectifying in that it only occurs from secondary sensory hair cells to first-order afferent neurons. Direct electrical stimulation of the small crista nerve to excite the efferent axons revealed efferent connections to both the primary sensory hair cells and the small first-order afferent neurons. These efferent responses were of three types: excitatory or inhibitory postsynaptic potentials and excitatory postsynaptic potentials followed by inhibitory postsynaptic potentials. The functional significance of the cell interactions within the crista epithelium of the statocyst of squid is discussed and comparisons drawn with the balance organs of other animals.

Potassium currents in presynaptic hair cells of Hermissenda

Apart from their primary function as balance sensors, Hermissenda hair cells are presynaptic neurons involved in the Ca(2+)-dependent neuronal plasticity in postsynaptic B photoreceptors that accompanies classical conditioning. With a view to beginning to understand presynaptic mechanisms of plasticity in the vestibulo-visual system, a locus for conditioning-induced neuronal plasticity, outward currents that may govern the excitability of hair cells were recorded by means of a whole-cell patch-clamp technique. Three K+ currents were characterized: a 4-aminopyridine-sensitive transient outward K+ current (IA), a tetraethyl ammonium-sensitive delayed rectifier K+ current (IK,V), and a Ca(2+)-activated K+ current (IK,Ca). IA activates and decays rapidly; the steady-state activation and inactivation curves of the current reveal a window current close to the apparent resting voltage of the hair cells, suggesting that the current is partially activated at rest. By modulating firing frequency and perhaps damping membrane oscillations, IA may regulate synaptic release at baseline. In contrast, IK,V and IK,Ca have slow onset and exhibit little or no inactivation. These two K+ currents may determine the duration of the repolarization phase of hair-cell action potentials and hence synaptic release via Ca2+ influx through voltage-gated Ca2+ channels. In addition, IK,Ca may be responsible for the afterhyperpolarization of hair cell membrane voltage following prolonged stimulation.

GABA-induced synaptic facilitation at type B to A photoreceptor connections in Hermissenda

Gamma-aminobutyric acid (GABA) is a prevalent neurotransmitter in both vertebrate and invertebrate systems. Here we report that, in addition to its usual inhibitory actions, GABA induced synaptic facilitation at type B to A photoreceptor connections of the marine mollusk Hermissenda when applied transiently to the isolated nervous system. Synaptic facilitation also occurred in response to mechanical stimulation of the GABAergic hair cells, which are normally activated by rotational unconditioned stimuli during behavioral training of the intact animal. This synaptic facilitation represents a novel form of GABA-induced neuromodulation which may contribute to learning-dependent suppression of phototaxis in Hermissenda.

Electrical coupling between primary hair cells in the statocyst of the squid, Alloteuthis subulata

Intracellular recordings were made from primary sensory hair cells located on the dorsal side of the anterior crista segment of the squid statocyst. These hair cells were electrophysiologically identified by the occurrence of an antidromic action potential after electrical stimulation of the crista nerve. Two types of subthreshold, depolarising potentials were observed in the primary sensory hair cells. Firstly, those due to efferent inputs onto the primary hair cells and secondly those correlated one-to-one with action potentials in neighbouring primary hair cells. The former depolarising potentials could be blocked by bath applied cobalt, indicating chemical transmission, while the latter could not. Injection of a depolarising or hyperpolarising current into a primary hair cell depolarised or hyperpolarised, respectively, a neighbouring primary hair cell implying that the hair cells are electrically coupled with an electrical coupling coefficient of up to 0.4.

Electrical coupling between secondary hair cells in the statocyst of the squid Alloteuthis subulata

The cephalopod angular acceleration receptor system has sensory response characteristics similar to those of the vertebrate semicircular canal system and, unusual for an invertebrate, contains secondary receptor hair cells. The experiments reported use intracellular recordings from pairs of hair cells to show that at least one subset of the hair cells is electrically coupled along the entire length of the crista section. The coupling can be reduced by application of heptanol or octanol. Intracellular injection of H+ ions into a hair cell reduces the coupling of cells on the opposite site of the injected hair cell but does not abolish it completely. It is proposed that the coupling is likely to result in an improvement in the signal-to-noise ratio of the receptor system, a reduction in overall frequency response, but an increase in the low frequency sensitivity.

Light paired with serotonin mimics the effect of conditioning on phototactic behavior of Hermissenda

A conditioning procedure consisting of pairing-specific stimulation of the eyes and gravity-detecting statocysts in Hermissenda results in a long-term modification of normal positive phototactic behavior. The learning is expressed by a significant suppression of the initiation of locomotion in the presence of light. We now report that an analogue of the classical conditioning procedure, consisting of light paired with serotonin (5-HT) applied directly to the exposed circumesophageal nervous system of otherwise intact animals, mimics the effect of conditioning on long-term changes in phototactic behavior. The effect of the conditioning analogue on behavior shows some specificity with 5-HT since light paired with dopamine or octopamine does not significantly affect phototactic behavior. The conditioning analogue exhibits pairing specificity since unpaired light and 5-HT and 5-HT applied in the dark do not produce behavioral suppression. Animals that initially received unpaired light and 5-HT do show behavioral suppression after receiving paired light and 5-HT. These results indicate that light (the conditioned stimulus) paired with the putative transmitter of the unconditioned stimulus pathway (5-HT) is sufficient to produce long-term phototactic suppression.

Modification of the initiation of locomotion in Hermissenda: behavioral analysis

We have examined the effect of a conditioning procedure on various components of visually guided locomotion in Hermissenda. Temporally specific stimulation of the visual system and gravity detecting system (statocysts) with light and rotation produced long-term changes in locomotor behavior. We found that the latency to initiate locomotion in the presence of light was significantly increased for the conditioned group as compared to baseline pre-test latencies and groups that received random presentations of the conditioning stimuli. The variability in the time taken by animals to enter a central illuminated area as reported in earlier studies can be accounted for by the increase in the latency to initiate locomotion. The modifications of visually influenced locomotion exhibits stimulus (CS) specificity since locomotor behavior is not changed following conditioning in the absence of light. In addition, conditioned animals remained in the brightest part of the light gradient significantly less than pre-test measurements. Since this response (initiation of locomotion) can be studied in a semi-intact preparation, it should be possible to investigate how this example of learning generates changes in behavior.

A common origin of voltage noise and generator potentials in statocyst hair cells

Voltage noise, generator potentials, and hair movements in the Hermissenda statocyst were analyzed. Motile hairs on the cyst's luminal surface moved as rods through +/- 10 degrees Hz when free and at 7 Hz when loaded with the weight of the statoconia (at 120 degrees C). For hair cells oriented opposite to a centrifugal force vector, rotation caused depolarization and increase of voltage noise variance. The depolarizing generator potential and the increase in voltage noise variance were similarly reduced by perfusion with zero external sodium or chloral hydrate. Cooling, perfusion with zero external sodium or chloral hydrate reduced the movement frequencies of the hairs but increased their range of motion. The same treatments reduced voltage noise variance and increased input resistance of the hair cell membrane. The results indicate that voltage noise and hair cell generator potential have a common origin: exertion of force on statocyst hairs by the weight of statoconia. The collision of statoconia with the motile hairs, not the hairs' bending, produces most of the voltage noise.

Retention of an associative behavioral change in Hermissenda

The nudibranch mollusk Hermissenda crassicornis is normally attracted to a test light. Three days of training consisting of 50 trials per day of light paired with a rotational stimulus led to a significant increase, lasting for days, in the animal's response latency to enter a test light. The group that received light associated with rotation was significantly different from groups subjected to nonassociative control procedures. Modifications of well-known sensory networks may be related to a behavioral change that shares several operational features with associative learning.

Interaction of chemosensory, visual, and statocyst pathways in Hermissenda crassicornis

Neurons in the cerebropleural ganglia (CPG), photoreceptors in the eye, optic ganglion cells, and statocyst hair cells of the nudibranch mollusk Hermissenda crassicornis responded in specific ways, as recorded intracellularly, to stimulation of the chemosensory pathway originating at the tentacular chemoreceptors as well as to stimulation of the visual pathway originating at the photoreceptors. Synaptic inhibition of photoreceptors occurs via the chemosensory pathway. The possible significance of such intersensory interaction is discussed with reference to preliminary investigation of the animal's gustatory behavior and possible neural mechanisms of behavioral choice.

Response of Aplysia statocyst receptor cells to physiologic stimulation

1. The electrical responses of Aplysia statocyst receptor cells were investigated using intracellular micro-electrodes. These ciliated mechanoreceptor cells were stimulated by downward tilting about a horizontal axis. 2. Tilting so that the receptor cell was excited produced a depolarizing receptor potential which, if large enough, could generate action potentials. 3. Large fluctuations in membrane potential were evident during depolarizing receptor potentials and were reduced or sometimes absent when a cell was tilted upward. Power-density spectra of the noise voltage revealed that most of the energy added by downward tilt is contained in frequency components below 3 Hz. 4. Removing synaptic input to the receptor cells by cutting the statocyst nerve or adding excess Mg2+ to the bath did not abolish the increase in fluctuations caused by downward, excitatory tilts. 5. The depolarizing receptor potential was often associated with a decrease in membrane resistance as measured with constant current pulses using a bridge circuit. 6. Replacing most of the Na+ in the bath with either Tris or Mg2+ abolished both potential and resistance changes caused by downward tilt. These results indicate that an increased permeability to Na+ underlies the receptor potential.

Rectification in Aplysia statocyst receptor cells

1. Membrane slope resistance of Aplysia statocyst receptor cells was measured by passing constant current pulses, using a bridge circuit. In response to downward tilt all cells which responded exhibited depolarization but this could be accompanied by either decrease, increase or no measurable change in slope resistance, depending on resting membrane potential. 2. By altering membrane potential with d.c. and measuring slope resistance with constant current pulses, these cells are shown to exhibit both anomalous and delayed rectification. Either hyperpolarization or depolarization from one potential can cause the slope resistance to decrease by as much as a factor of 5. 3. The response to standard tilt can be changed from an increase in slope resistance to a decrease, or vice versa, by altering membrane potential. 4. When membrane potential was held constant during downward tilt, the slope resistance always decreased. 5. Slope resistance, the voltage response to standard tilts and the amplitude of membrane potential fluctuations all vary with average membrane potential in a similar manner. 6. These findings are incorporated into a circuit model in which anomalous and delayed rectification are represented by voltage-controlled elements. the response to tilt is always modelled as introducing a parallel conductance pathway with a large positive reversal potential. 7. The model demonstrates that slope resistance can be increased by adding a parallel shunt pathway if the latter brings the membrane out of the anomalous rectification region. 8. The model also demonstrates how delayed rectification can greatly alter the reversal potential inferred from measurements at potentials below actual reversal.

Signal transformation with pairing of sensory stimuli

Rotation of the isolated nervous system of Hermissenda in a caudal orientation causes a synaptic hyperpolarization accompanied by elimination of impulse activity during the steady-state phase of type A but not type B photoreceptors' responses to light. Rotation of the isolated nervous system in a cephalic orientation causes a synaptic depolarization with increase of impulse activity during the steady-state phase of both type A and type B photoreceptors' responses to light. These effects of rotation on photorecptors are explained by known synaptic interactions. Sufficient redundancy is found to be provided by the neural organization of the visual system and its interaction with the statocyst to preserve much of the visual information in spite of signal transformation in specific photorecptors resulting from pairing of rotation with light.

Responses of hair cells in the statocyst of Hermissenda

Responses to mechanical stimulation were recorded from hair cells in the statocyst of Hermissenda crassicornis. The response to a brief stimulus is a depolarizing wave which reaches peak in about 25 msec and decays slowly. 2. Hyperpolarization by extrinsic currents increases the amplitude of the response; depolarization decreases it and eventually reverses its polarity. It is inferred from these results that the primary outcome of the transduction process is an increase of membrane conductance and that the voltage change (generator potential) follows as a secondary event. 3. The features of the conductance change were reconstructed from the time course of the generator potential and the passive properties of the membrane. It was found that the increase of membrane conductance develops slowly and is roughly proportional to the energy delivered by the stimulus. 4. The time course of the conductance change required to reproduce the generator potential is similar to the output of a model involving a sequence of transformations. 5. The generator potential is sensitive to temperature, becoming faster as temperature is raised. This effect is reproduced by the model if the transition rates are assumed to be temperature-dependent, with a Q10 of about 2. 6. It is concluded that a chain of temperature-sensitive processes is interposed between the stimulus and the increase of membrane conductance.

Neural correlates of associative training in Hermissenda

Hair cells in Hermissenda respond to illumination of the ipsilateral and contralateral eyes. These responses are modified by associative training of the animal. The observed electrophysiological changes appear to result from changes in the photoreceptors' synaptic input to the hair cells.

Associative training of Hermissenda

Reflex behavior of Hermissenda in response to visual and rotational stimuli is described. It is shown that repeated association of light with rotation modifies the subsequent responses of the animals to light. This modification does not occur after the same period of light or rotation alone. The effect of the associative training is strongly dependent on the amount of daily light with which the animals are maintained.