| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Laboratorio de Fisiología y Biología Molecular, Departamento de Fisiología y Biología Molecular y Celular, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Instituto de Fisiología, Biología Molecular y Neurociencias/Consejo Nacional de Investigaciones Científicas y Tecnológicas, Ciudad Universitaria, Pabellón II, 1428 Buenos Aires, Argentina
Submitted 10 December 2003; accepted in final form 27 January 2004
 |
ABSTRACT |
|---|
|
|
|
INTRODUCTION |
|---|
|
). This characteristic implies that all sensory modalities should possess mechanisms to regulate their own responsiveness to incoming signals, providing the animal with the ability to disregard inputs that, under specific circumstances, become irrelevant or disturbing. Different sensory systems count on a variety of peripheral and central mechanisms to filter incoming signals.
Taking as an example the visual system, one realizes that sensory signals can be filtered right at the periphery or at higher processing levels. Closing the eyelids or adjusting the iris are examples of peripheral means to regulate visual input, whereas focusing on certain objects of the scene, while ignoring others, is part of a complex processing taking place at higher levels. As to mechanosensory pathways, the majority of the studies in the past analyzed regulatory mechanisms that happen at central levels (Buschges and El Manira 1998; Rudomin 1999), while regulation of mechanosensitivity at peripheral levels has been relatively ignored (Pubol 1982). Considering the widespread exposure of the skin to the environment, it seems highly relevant to explore how the skin that holds the sensory terminals affects somatosensory sensitivity, providing a peripheral filter to external inputs.
Invertebrate organisms usually provide experimental settings where it is possible to test interactions that are of a more complex nature in higher organisms. The question of what is the role that skin surface plays in mechanoreception can be investigated directly in the leech Hirudo medicinalis. On one hand, the leech has a well-developed mechanosensory system in which one can record intracellularly the responses of well-identified sensory neurons to peripheral stimulation. There are three distinct types of mechanosensory neurons in each leech segmental ganglion: neurons sensitive to light touch (T), pressure (P), and nociceptive (N) stimuli (Nicholls and Baylor 1968). On the other hand, the skin of the leech is divided by transverse furrows into 102 annuli that can be erected through the contraction of annulus erector muscles (Mann 1962). These muscle fibers are controlled by a pair of motoneurons present in each ganglion, the annulus erector (AE) motoneurons (Stuart 1970). Thus the surface of the skin can be changed from smooth to ridged by controlling the activity of AE motoneurons. A thorough analysis of the distribution of the low-threshold T cell terminals in the leech skin showed that they are located near the skin surface, intermingled with epithelial cells (Blackshaw 1981). On the bases of the preceding information, the goal of this study was to analyze whether changes in the skin topology exert any influence on mechanosensitivity.
Despite the evidence of annulus erection in the leech, it is not yet known what induces it, and no clear function can be ascribed to such movements of the annuli. Here we show that skin surface topology and mechanosensory responses can influence each other. On one hand, activation of AE motoneurons, to levels that cause substantial annulus erection, diminished the responsiveness of T cells to mechanical stimulation. In turn, activation of T cells inhibited AE motoneuron activity; however, this mutual interaction was not symmetrical. The balance of this feedback mechanism was strongly influenced by the biophysical properties of the motoneurons. The results illustrate a potential mechanism to regulate somatosensory sensitivity at peripheral levels.
Preliminary experiments in the implementation of the localized water flow over the leech skin were carried out by C. A. Williams and E. A. Whitchurch as a student project during Neural Systems and Behavior Course (2003) at the Marine Biological Laboratory (Woods Hole, MA).
|
|
METHODS |
|---|
|
H. medicinalis, weighing 2–5 g, were obtained from a commercial supplier (Leeches USA, Westbury, NY) and maintained at 15°C in artificial pond water. The animals were not fed for at least 1 mo prior to dissection.
The leech nervous system is composed of 21 midbody ganglia and a head and tail brain. Each midbody ganglion innervates a body segment and contains all the corresponding sensory and motor neurons (Muller et al. 1981). The skin of each midbody segment is divided in five annuli. Two types of preparations have been used: isolated midbody ganglia and body-wall preparations, as indicated. For the first type, individual ganglia were dissected out and pinned, ventral side up, in Sylgard-coated petri dishes (Dow Corning) filled with saline solution at room temperature. The body-wall preparations consisted of a three segments section of the body wall, comprising skin and muscles, cut along its dorsal midline, of which only the central segment was left innervated by the corresponding segmental ganglion. The body wall was pinned down on the Sylgard base of a dish, and the ventral aspect of the ganglion was exposed by making a small hole in the ventral skin (Fig. 1, inset). To ease the stretching of the body wall in the chamber, this procedure was performed in a solution containing 20 mM Mg2+ and 1 mM Ca2+, which abolishes chemical transmission and minimizes muscle contractions (Baylor and Nicholls 1969). Thereafter, the solution in the chamber was exchanged with normal saline, washing out the tissue with five times the volume of the dish, to assure the effective wash out of the high Mg2+ concentration. The sheath covering the ganglion was removed in some preparations. The saline solution had the following composition (in mM): 115 NaCl, 4 KCl, 1.8 CaCl2, 1 MgCl2, 10 Tris base, and 10 glucose (pH 7.4).
|
The neuronal activity was studied by means of intracellular electrodes inserted into the somata of identified neurons. T and AE neurons were recognized by their soma location, electrophysiological properties, and function (Muller et al. 1981; Nicholls and Baylor 1968).
The microelectrodes were pulled from borosilicate capillary tubing (FHC, Brunswick, ME), filled with a 3 M potassium acetate solution, and had a resistance of 20–40 M
. The electrodes were connected to an amplifier Axoclamp 2B (Axon Instruments, Union City, CA) operating in the current-clamp configuration, and they were bridge-balanced. The recordings were digitized using a Digidata 1320 interface and acquired using Clampex protocols (pClamp 8.0.2, Axon Instruments) at sampling frequencies of 5–10 kHz.
Intracellular excitation of T cells was achieved by injecting trains of suprathreshold current pulses (2–4 nA, 5 ms, 10–15 Hz) delivered through the recording electrode using a Master 8 stimulator (A.M.P.I., Jerusalem), which was triggered by the acquisition software. Electrophysiological recordings were analyzed using Axograph 4.5 and Clampfit 8.1.0.12 [EC] (Axon Instruments).
Skin stimulation and video recording
Two types of mechanical stimulation of the skin were applied: "localized bubbling" and "localized stream." The localized bubbling method was previously described (Marín Burgin and Szczupak 2003). Briefly, air pressure pulses were delivered through a micropipette, which had a tip of around 5 µm, by a Picospritzer II pressure-pulse generator (General Valve, Fairfield, NJ) on the skin. These pressure pulses resulted in the localized bubbling of the external solution. The localized stream method consisted in directing a flow of external solution, parallel to the surface of the skin. For this purpose, we used a tube as a reservoir of saline solution and a micropipette was attached to one end, while the Picospritzer was attached to the other end. The micropipette was bent under heat so that its tip, which was around 50 µm wide, expelled the flow parallel to the surface of the skin. The flow was controlled by pressure pulses applied using the Picospritzer (6–9 psi). In preliminary experiments, the solution in the tube was colored with Fast Green, and we observed that this procedure produced a narrow stream of solution that extended for around three to five annuli, with a consequent movement of the surrounding solution, but without causing visible turbulence, bubbling, or any microelectrode movement. This kind of stimulation produced a train of T cell spikes throughout the duration of the flow that was highly repeatable on successive trials and persisted even when the external solution (in the bath and in the tube) contained 20 mM Mg2+ and 1 mM Ca2+ (n = 4), a solution that abolishes chemical synaptic transmission in leech ganglia.
In each midbody leech ganglion, right and left AE motoneurons innervate the muscle fibers of the contralateral hemisegment and sensory neurons innervate the ipsilateral body wall (Nicholls and Baylor 1968; Stuart 1970). Thus to test the interaction between T and AE cells, the experimental configuration was to record from one of the three T cells present in each hemiganglion (Fig. 1A, inset) and to excite the contralateral AE motoneuron by DC current injection through the recording electrode (bridge balanced). Annulus erection was confirmed by visual inspection of the skin through the microscope.
To quantify annulus erection, we filmed the skin using a CCD camera Sony TR50 (Sony Corp.) mounted on a stereomicroscope Olympus SZ60 (Olympus). In body wall preparations, we focused on the profile of an annulus (Fig. 3, inset), while recording from the contralateral AE motoneuron. The AE motoneuron was stimulated to fire at different frequencies by injecting DC current pulses (20 s) of different amplitude (0–2 nA). The skin was filmed for around 10 s before and throughout the stimulus. The images were stored on magnetic tape. Two frames were digitized, one before and one corresponding to 15 s after the stimulus onset, and analyzed using a PC and Scion Image (Scion Corp., Frederick, MD). To characterize the state of the annuli, we measured their height (see Fig. 3), and calculated the ratio between the value during the stimulus over the value before the stimulus.
|
|
|
RESULTS |
|---|
|
Erection of the annuli that subdivide the skin produces a major change in the body surface topology of the leech. To analyze whether this surface change could influence mechanosensory sensitivity, we studied the responses of the low-threshold T mechanosensory cells to mechanical stimulation of the skin as the annuli were induced to erect via the excitation of the corresponding AE motoneurons (see METHODS).
Experiments were performed using body wall preparations (see METHODS) where the skin was stimulated by localized bubbling of the external solution (Marín Burgin and Szczupak 2003), and T and AE cells were recorded by means of intracellular electrodes. Stimulation of the appropriate sensory field (Lewis and Kristan 1998; Marín Burgin and Szczupak 2003; Nicholls and Baylor 1968) evoked a series of action potentials in the recorded T cell throughout the stimulus period (Fig. 1Ai). The higher-threshold mechanosensory neurons (P and N cells) do not respond to this type of stimulation (Marín Burgin and Szczupak 2003). That the T cells could be the primary sensory detectors of the applied stimuli is suggested by the fact that they respond when the whole preparation is bathed in a solution containing a high Mg2+/Ca2+ ratio, which precludes any chemical synaptic interaction (Marín Burgin and Szczupak 2003).
Excitation of the AE motoneuron by the injection of DC current caused a decrease in the T cell responses to the mechanical stimuli to a degree that depended on the AE firing frequency (Fig. 1A, ii and iii). This effect reverted as AE was returned to the initial silent condition (Fig. 1Aiv). The recordings also show that AE motoneurons received inhibitory inputs during the mechanical stimulation, and these responses will be addressed later in this manuscript. Note that the somatic recordings of AE motoneurons show very small spikes due to their passive propagation from "electrically distant" active sites. The spikes (AE traces in Fig. 1A, ii and iii) can be clearly distinguished from other subthreshold events (AE traces in Fig. 1A, i and iv) because of their stereotyped time course. These considerations also apply to subsequent figures.
Figure 1C summarizes the results of six experiments similar to that described above. The graph in Fig. 1C shows that, in all cases, T cell firing decreased as AE firing rate increased. In one case (
), the T response increased at low AE firing rates and decreased with higher AE firing frequencies. The average T cell response when AE fired at around 10 Hz (8–11 Hz) was 0.6 ± 0.2 of that measured when AE motoneuron was silent (n = 6).
In this study, we chose to stimulate the skin using the localized bubbling method rather than the more classically used touch with a stylus (Yau 1976). Our aim was to learn how the skin surface topology affected sensory responsiveness. This aim demanded the use of stimuli whose delivery did not depend on the actual contact with the skin itself. In delivering a touch stimulus with a stylus, we would have needed to consider how the shape of the tip interacted with the changing skin surface. The localized bubbling could be delivered at a constant distance from the skin, allowing the contact surface to adapt to the skin surface. However, because the observed change in T cell response could reflect a limited access of the bubbles to the ridged skin, we repeated these experiments using a different method of stimulation. This alternative method consisted of delivering a narrow steady flow of solution parallel to the surface of the skin, in a direction perpendicular to the annuli (see METHODS). The steady flow of external solution over the skin for 3 s evoked a train of action potentials in T cells (Fig. 1Bi) composed of around 50 spikes (36–95 spikes in 7 experiments). The T cells also responded to water flow over the skin surface when the preparations (n = 2) were bathed in a solution containing a high Mg2+/Ca2+ ratio (see METHODS; data not shown), suggesting that T cells are primary detectors for this type of stimulation (P and N cells did not respond). This T cell response also diminished as the corresponding AE motoneuron was induced to fire action potentials (Figs. 1B, ii and iii), and the decrease was a function of the AE firing frequency (Fig. 1D). The average T cell response when AE fired at around 10 Hz (9–12 Hz) was 0.5 ± 0.1 (n = 8) of that measured when AE was silent.
Taken together these results show that the low-threshold mechanosensory neurons could detect local bubbling and steady flow over the surface of the skin, and these responses were inhibited by the activation of the motoneuron whose firing raised the skin into ridges.
The effect of AE activity on T responsiveness could be due to phenomena taking place at central or peripheral sites. To test whether AE firing affected the excitability of T cells at central levels, we performed experiments in isolated ganglia (see METHODS). T cells were stimulated through the recording electrode by ramp current pulses with a peak amplitude of 2 nA, while AE was set to fire at different rates (Fig. 2A, i and ii). The ramp current step depolarized the T cell and caused it to fire as threshold potential was reached, generating a series of spikes throughout the rest of the electrical stimulus. The threshold potential at which T cells fired the first spike and the total number of spikes fired throughout the ramp were not affected by AE spike frequency (Fig. 2B). This result indicates that AE activity did not affect T cell excitability centrally, and therefore, the effect of AE spiking on T cell response to mechanical stimulation of the skin (Fig. 1) was the consequence of a phenomenon that took place at the periphery. Our hypothesis is that this phenomenon was the annulus erection caused by AE firing.
|
Although it has been clearly established that excitation of AE motoneurons is responsible for the contraction of muscles that result in annulus erection (Stuart 1970), the relationship between AE activity and annulus erection has not been described in detail. Because our interpretation of the results presented in Fig. 1 is that AE firing affected mechanosensitivity through the consequent change in skin topology, it is important to determine the relationship between AE firing frequency and annulus erection. To achieve this goal we measured annulus erection by performing video recordings of the skin as AE motoneurons were shifted to different firing frequencies (see METHODS).
Figure 3A shows representative photos focusing on an annulus in two states: before the current pulse injection (Fig. 3Ai) and when the AE motoneuron was firing at a mean frequency of 7 Hz (Fig. 3Aii). In this study, we have not analyzed the dynamics of the annulus erection throughout the pulse injection, but a preliminary inspection showed that, for each electrical stimulus level, the annulus erection reached a plateau at around 3 s. Figure 3Bi describes the change in annulus height as a function of AE firing frequency, measured 15 s after the onset of the current pulse to capture a steady annulus state. The figure shows the results from different preparations, revealing that annulus erection was evoked after reaching a minimal firing frequency of around 5 Hz and increased as AE firing rate increased, up to certain maximal value. The maximal annulus height varied from preparation to preparation and ranged from 1.5 to 3 times the resting height. Figure 3Bii shows the same data as in Fig. 3Bi, but the height was made relative to the maximal value obtained in each case. This graph shows that the relationship between the AE motoneuron firing frequency and annulus erection could be represented as a sigmoid: for annulus height to show a measurable increase, AE had to fire at a minimal frequency of 5 Hz, and it increased steeply in the range between 5 and 15 Hz, reaching a plateau value beyond which further increase in AE firing rate caused no further measurable increase. The average frequency value at the inflection point of the sigmoid fitting (see METHODS) of all the curves was around 10 Hz (9.7 ± 0.7 Hz).
Note that the same range of AE spike frequency that cause annulus erection (Fig. 3Bii) diminishes the mechanosensory sensitivity of T cells (Fig. 1, C and D).
Effect of mechanosensory neurons on the AE motoneuron
Throughout the preceding experiments, we analyzed the effects that AE activity produced on T mechanosensory neurons, ignoring the fact that the sensory stimulus evoked inhibitory responses in the motoneuron (see Fig. 1, A, ii and iii, and B, ii and iii). Figure 4A presents an additional example obtained in a body wall preparation showing that a stimulus that excited the corresponding T cell evoked an inhibitory response in the AE motoneuron. In all these examples, and in the rest of the experimental observations that constitute this study, it was possible to observe that the synaptic AE hyperpolarizations were markedly reduced when the motoneuron was depolarized and fired at higher rates (Fig. 4A, i and ii). As the motoneuron was depolarized, it was possible to observe a decrease in the duration and in the maximal amplitude of the hyperpolarizing response. However, because increasing AE activity diminished T cell responses (Fig. 1), the observed decline in AE response could plainly reflect a decline in synaptic input from the sensory neurons.
|
). Therefore it was reasonable to assume that the inhibitory responses produced by light mechanical stimuli observed in Figs. 1A and 4A were due to a synaptic interaction between T and AE motoneurons at the ganglion level. As expected, stimulation of T cells evoked inhibitory responses in AE motoneurons, and these responses also declined as the AE membrane potential was depolarized, increasing their firing frequency (Fig. 4B, i and ii). These results indicate that activation of T cells evoked inhibitory responses in AE motoneurons, but this inhibitory synaptic input was restricted by the depolarization of the motoneuron. For a more detailed study, we analyzed the AE motoneuron responses in an extended range of membrane potentials. Figure 5A exhibits a set of traces showing the responses of an AE motoneuron to spike trains elicited in T mechanosensory neurons as the AE motoneuron was set at a wide range of membrane potentials. These recordings illustrate that AE motoneurons exhibited a hyperpolarizing response that reached a maximum value at an AE membrane potential of around -30 mV and decreased when the membrane potential was shifted both to more positive and to more negative values. Figure 5B summarizes the results obtained for a series of experiments like the ones described in Fig. 5A. The graph shows that the responses increased in the voltage range between -80 and -30 mV, reaching a maximal value at around -30 mV, and decreasing on further depolarization. The reversal potential of the hyperpolarization was around -60 mV.
|
|
; Wessel et al. 1999). However, in terms of the hyperpolarizing synaptic response, more significant is the modulation of input resistance on depolarization of AE motoneuron. As AE was depolarized, one would have expected to see an increase in the amplitude of a synaptic response that had a reversal potential at around -60 mV. A possible explanation for the observed decrease in input resistance is the impact of voltage-activated conductances active during spiking activity. Figure 6C demonstrates the correlation between AE firing frequency and membrane potential, showing that the firing increased steadily in the range between -50 and -10 mV, in step with the steady decrease in input resistance observed in this voltage range. |
|
DISCUSSION |
|---|
|
It is generally accepted that leeches possess two separate types of receptors to relay mechanical signals from their environment: the tactile endings of T, P, and N cells in the skin (Blackshaw 1981; Blackshaw et al. 1982; Yau 1976) and the ciliated sensory cells in the sensilla (Derosa and Friesen 1981; Friesen 1981; McVean et al. 1990; Phillips and McVean 1982). This division implied that activation of the first type required the actual indentation of the skin due to contact with an external solid object, while the second type was activated by water movement over the skin surface.
In a previous study, we showed that T cells are activated by the light touch produced by air bubbling onto the skin (Marín Burgin and Szczupak 2003), and in this study, we show that these low-threshold mechanosensory neurons were also excited by flow of water parallel to the surface of the skin, as first demonstrated by Nicholls and Baylor (1968). It was suggested that water motion evoked T cell activity through excitatory synaptic input from sensillar water movement receptors (SMRs), rather than by direct stimulation of its terminals (Friesen 1981). However, that the responses of T cells to the mechanical stimuli applied throughout this study were due to direct activation of its sensory terminals is suggested by two main facts: 1) almost all the spikes in our T cell recordings arose directly from the baseline, resembling the spikes produced during direct activation of mechanosensory terminals by touching the skin with a stylus, while the T cell spikes evoked during SMR activation are preceded by small depolarizing prepotentials (Friesen 1981); 2) the responses of T cells were also observed in a solution that impairs chemical synaptic transmission in the leech. Definite determination of whether T cells act as primary receptors, or are excited by sensillar receptors (e.g., through electrical junctions), remains to be tested. However, this study clearly indicates that the low-threshold tactile receptor cells were sensitive to movements of water surrounding the leech, indicating that these mechanoreceptors can be used by the animal to sense its own movement or the movement of other nearby objects. T cells did not appear to be active during swimming (Yu et al. 1999), but it is possible that these sensory neurons become phasically active at the initial stages of movement, and adaptation takes place as movement proceeds. According to this interpretation, the T mechanosensory neurons could serve, together with the sensillar movement receptors, as an early source of information on the leech surroundings.
The physiological characteristics of T cells resemble those of rapidly adapting mechanosensory neurons of the vertebrates (Johnson 2001) in several aspects: 1) their endings lie just beneath the epidermis (Blackshaw 1981); 2) they are sensitive to minute skin deformations, as evidenced by their sensitivity to bubbling or to water flow over the surface of the skin; 3) they show a fast adaptation to steady touch (Carlton and McVean 1995); and 4) they are more sensitive to dynamic skin deformation (Carlton and McVean 1995), and as such, they provide a neural image of motion signals.
Annulus erection regulated T cell sensitivity to peripheral stimulation
Previously no clear function has been assigned to annulus erection in the leech. The observation that annulus erection occurred during the contraction phase of crawling led to the speculation that annulus erection could be used to give the animal better traction against the substrate (Eisenhart et al. 2000). On the other hand, flattening the annuli is a prerequisite for swimming, probably because a smooth skin surface offers hydrodynamic advantages (Friesen 1981). In this study, we have investigated whether erection of the annuli could influence the sensory responses to mechanical stimuli applied onto the skin. The results from this study show that the responsiveness of the low-threshold mechanosensory T cells was sharply decreased by activation of the motoneurons that regulate raising the skin ridges in the leech, in a way that correlated with the motoneuron firing frequency. Thus the data suggest that regulation of skin ridging could modulate the sensitivity of the animal to external stimuli. The interpretation that the effect of AE activity on T cell response was due to the resulting annulus erection was supported by two additional observations: 1) AE motoneuron activation had no effect on T cell central excitability (Fig. 2) while strongly affecting responsiveness to peripheral mechanostimuli (Fig. 1); and 2) the AE firing frequency that produced effective annulus erection was similar to that which affected T cell responses. However, our results do not rule out the possibility that AE motoneurons affect T cells in the periphery by chemical means, i.e., through the release of the neurotransmitter or a substance co-released with it (Gascoigne and McVean 1991). In an investigation on the fine structure of the T cell terminals, Blackshaw (1981) reported that branches of the T cell axon run together with about four other axons until they leave this group of axons and enter the intercellular spaces between the epithelial cells. These other axons have not been identified yet, but if they are branches of the AE motoneurons, they would be in a position to exert a direct effect on the electrophysiological properties of the T cell terminals by chemical release.
According to the "mechanical interpretation", T cells would decrease their responsiveness as the skin becomes ridged due to changes in the viscoelastic properties of the skin (i.e., compressibility, creep etc.) that, in turn, could alter the effectiveness of mechanical stimuli impinging on the skin (Pubol 1982). These results suggest that skin topology can exert a strong influence on mechanosensory sensitivity extending what has been usually considered a central phenomenon through presynaptic inhibition of sensory neurons (Clarac et al. 2000; Rudomin 1999) to a peripheral scenario. The type of peripheral modulation proposed in our interpretation would be analogous to the one exerted by gamma efferents on the sensory afferents in the muscle-spindle stretch reflex (Appelberg et al. 1983) in that gamma motoneurons exert peripheral changes that modify the response of sensory neurons to muscle length.
An equivalent modulation in the visual system is the one exerted by the eyelids (or in a more subtle way by the iris) that causes visual suppression by limiting the input of external stimuli on the primary sensory neurons (Volkmann et al. 1980). Given the similarities between leech T sensory neurons and rapidly adapting mechanosensory neurons of vertebrates, the present results on the effect of skin topology on mechanosensitivity could be of relevance to the vertebrate somatosensory system.
Feedback interactions between the sensory neuron and the motoneuron
While AE motoneuron activity could cause a decrease in sensory response to light mechanosensory signals, these inputs could counteract this effect by inhibiting the motoneuron. It is worth noting that activation of sensillar movement receptors also cause inhibitory responses in AE neurons (Friesen 1981). The results show that AE motoneurons received inhibitory signals from skin stimuli and from direct activation of T cells. However, this negative feedback loop was limited by the biophysical properties of the motoneuron.
The input resistance of AE motoneurons displayed an inverted V-shape relationship with the membrane potential. The inverse correlation between input resistance and firing frequency of AE motoneurons, at the -60 to -10 mV range (Fig. 6), suggests that the spiking activity was responsible for input resistance modulation. AE spikes are initiated within the ganglionic arborizations of the neuron (Gu et al. 1991), and therefore the voltage-dependent conductances underlying action potentials could influence the conductance of the central arborizations of the motoneuron, where the synaptic inputs take place. However, the drop in resistance could be also caused by additional voltage-dependent conductances, as the persistent sodium current described in leech neurons (Angstadt 1999; Opdyke and Calabrese 1994). As a consequence, the inhibitory responses caused by mechanosensory stimuli diminished as the membrane potential of the cell was shifted to positive values, despite the fact that this shift entailed an increase in driving force of the synaptic conductance. In addition, the drop in resistance produced a decrease in the time constant of the motoneuron, curtailing the duration of the inhibitory response. These observations imply that the more active the motoneuron, the less sensitive it became to inhibitory signals.
Taken together, the data suggest that the mechanosensory-induced inhibition would be relevant when the motoneuron is firing at a low rate, devoid of a strong excitatory drive (whose origin has still to be determined). Conversely, when brought to fire, AE motoneurons would stay "committed" to a high activity level, because at this state they became relatively insensitive to synaptic inputs. In turn, this would have a strong impact on sensory responsiveness to incoming mechanical signals. The diagram in Fig. 7 summarizes the outlines of our interpretation.
|
|
|
ACKNOWLEDGMENTS |
|---|
|
GRANTS
This work was supported by grants from Fundación Antorchas (Argentina) and Agencia de Promoción Científica y Tecnológica (Argentina).
|
|
FOOTNOTES |
|---|
Address for reprint requests and other correspondence: L. Szczupak, Ciudad Universitaria. Pabellón II, piso 2, 1428 Buenos Aires, Argentina (E-mail: szczupak{at}mail.retina.ar).
|
|
REFERENCES |
|---|
|
Angstadt JD and Friesen WO. Modulation of swimming behavior in the medicinal leech II. Ionic conductances underlying serotonergic modulation of swim-gating cell 204. J Comp Physiol A 172: 235-248, 1993.[Medline]
Appelberg B, Hullinger M, Johansson H, and Sojka P. Actions of 
-motoneurones elicited by electrical stimulation of group III muscle efferent fibres in the hind limb of the cat. J Physiol 335: 275-292, 1983.
Baylor DA and Nicholls JG. Chemical and electrical synaptic connexions between cutaneous mechanoreceptor neurones in the central nervous system of the leech. J Physiol 203: 591-609, 1969.
Blackshaw SE. Morphology and distribution of touch cell terminals in the skin of the leech. J Physiol 320: 219-228, 1981.
Blackshaw SE, Nicholls JG, and Parnas I. Physiological responses, receptive fields and terminal arborizations of nociceptive cells in the leech. J Physiol 326: 251-260, 1982.
Buschges A and El Manira A. Sensory pathways and their modulation in the control of locomotion. Curr Opin Neurobiol 8: 733-739, 1998.[CrossRef][Web of Science][Medline]
Carlton T and McVean A. The role of touch, pressure and nociceptive mechanoreceptors of the leech in unrestrained behavior. J Comp Physiol A 177: 781-791, 1995.
Clarac F, Cattaert D, and Le Ray D. Central control components of a 'simple' stretch reflex. Trends Neurosci 23: 199-208, 2000.[CrossRef][Web of Science][Medline]
Derosa YS and Friesen WO. Morphology of leech sensilla: observations with the scanning electron microscope. Biol Bull 160: 383-393, 1981.
Eisenhart FJ, Cacciatore TW, and Kristan WB. A central pattern generator underlies crawling in the medicinal leech. J Comp Physiol A 186: 631-643, 2000.[CrossRef][Medline]
Engel AK, Fries P, and Singer W. Dynamic predictions: oscillations and synchorny in top-down processing. Nat Rev Neurosci 2: 704-716, 2001.[CrossRef][Web of Science][Medline]
Friesen WO. Physiology of water motion detection in the medicinal leech. J Exp Biol 92: 255-275, 1981.
Gascoigne L and McVean A. Neuromodulatory effects of acetylcholine and serotonin on the sensitivity of leech mechanoreceptors. Comp Biochem Physiol 99: 369-374, 1991.[CrossRef]
Gu X, Muller KJ, and Young SR. Synaptic integration at a sensory-motor reflex in the leech. J Physiol 441: 733-754, 1991.
Iscla IR, Arini PD, and Szczupak L. Differential channeling of sensory stimuli onto a motor neuron in the leech. J Comp Physiol 184: 233-241, 1999.[CrossRef]
Johnson KO. The roles and functions of cutaneous mechanoreceptors. Curr Opin Neurobiol 11: 455-461, 2001.[CrossRef][Web of Science][Medline]
Lewis JE and Kristan WB. A neuronal network for computing population vectors in the leech. Nature 391: 76-79, 1998.[CrossRef][Medline]
Mangan PS, Curran GA, Hurney CA, and Friesen WO. Modulation of swimming behavior in the medicinal leech. III. Control of cellular properties in motor neurons by serotonin. J Comp Physiol A 175: 709-722, 1994.[Medline]
Mann KH. Leeches (Hirudinae). Their Structure, Physiology, Ecology and Embryology. New York: Pergamon Press, 1962.
Marín Burgin A and Szczupak L. Network interactions among sensory neurons. J Comp Physiol A 189: 59-67, 2003.
McVean A, Gascoigne L, and Page A. The external structure and distribution of sensilla in the medicinal leech. Acta Zoologica 71: 161-167, 1990.
Muller KJ, Nicholls JG, and Stent GS. Neurobiology of the Leech. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory, 1981.
Nicholls JG and Baylor DA. Specific modalities and receptive fields of sensory neurons in CNS of the leech. J Neurophysiol 31: 740-756, 1968.
Opdyke CA and Calabrese RL. A persistent sodium current contributes to oscillatory activity in heart interneurons of the medicinal leech. J Comp Physiol A 175: 781-789, 1994.[Medline]
Phillips CE and McVean A. Ultrastructure of the water-movement-sensitive sensilla in the medicinal leech. J Neurobiol 13: 473-486, 1982.[CrossRef][Web of Science][Medline]
Pubol BH. Factors affecting cutaneous mechanoreceptor response: I. Constant-force versus constant-displacement stimulation. J Neurophysiol 47: 515-529, 1982.
Rudomin P. Presynaptic inhibition in the vertebrate spinal cord revisited. Exp Brain Res 129: 1-37, 1999.[CrossRef][Web of Science][Medline]
Stuart AE. Physiological and morphological properties of motoneurones in the central nervous system of the leech. J Physiol 209: 627-646, 1970.
Volkmann FC, Riggs LA, and Moore RK. Eyeblink and visual suppression. Science 207: 900-902, 1980.
Wessel R, Kristan WB, and Kleinfeld D. Supralinear summation of synaptic inputs by an invertebrate neuron: dendritic gain is mediated by an "inward rectifier" K+ current. J Neurosci 19: 5875-5888, 1999.
Yau K.-W. Receptive fields, geometry and conduction block of sensory neurones in the central neurvous system of the leech. J Physiol 263: 513-538, 1976.
Yu X, Nguyen B, and Friesen WO. Sensory feedback can coordinate the swimming activity of the leech. J Neurosci 19: 4634-4643, 1999.
This article has been cited by other articles:
|
|
 |
|
  M. A. Calvino, I. R. Iscla, and L. Szczupak Selective Serotonin Reuptake Inhibitors Induce Spontaneous Interneuronal Activity in the Leech Nervous System J Neurophysiol, May 1, 2005; 93(5): 2644 - 2655. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
TOP
ABSTRACT
INTRODUCTION
10 Hz (Aii and Bii), 
78% of the observed variance.
