W. O. Friesen

Modulation of swimming behavior in the medicinal leech

The effects of serotonin on the electrical properties of swim-gating neurons (cell 204) were examined in leech (Hirudo medicinalis) nerve cords. Exposure to serotonin decreased the threshold current required to elicit swim episodes by prolonged depolarization of an individual cell 204 in isolated nerve cords. This effect was correlated with a more rapid depolarization and an increased impulse frequency of cell 204 in the first second of stimulation. In normal leech saline, brief depolarizing current pulses (1 s) injected into cell 204 failed to elicit swim episodes. Following exposure to serotonin, however, identical pulses consistently evoked swim episodes. Thus, serotonin appears to transform cell 204 from a gating to a trigger cell.Serotonin had little effect on the steady-state currentvoltage relation of cell 204. However, serotonin altered the membrane potential trajectories in response to injected current pulses and increased the amplitude of rebound responses occurring at the offset of current pulses. These changes suggest that serotonin modulates one or more voltage dependent conductances in cell 204, resulting in a more rapid depolarization and greater firing rate in response to injected currents. Thus, modulation of intrinsic ionic conductances in cell 204 may account in part for the increased probability of swimming behavior induced by serotonin in intact leeches.

Cellular analysis of theBulla ocular circadian pacemaker system

SummaryThe isolatedBulla eye expresses a circadian rhythm in optic nerve impulse frequency. In an effort to learn more about the organization of theBulla retina and, specifically, about the organization of retinal elements involved in the circadian pacemaker system, we have recorded both intracellularly and extracellularly from retinal cells, as well as examined thick sections and scanning electron micrographs of the eye. We report that:1.TheBulla retina contains approximately 1000 large photoreceptors with distinct villousbearing distal segments which form a layer around a solid lens. There is also a population of approximately 100 neurons which surround a neuropil at the base of the retina.2.Electrical activity in the optic nerve consists of large compound action potentials and lower amplitude activity. Compound action potentials occur spontaneously in darkness and both types of optic nerve activities can be induced by light pulses.3.Intracellular recording from the photoreceptor layer reveals four types of responses: (a) cells which depolarize in response to a light pulse and then transiently hyperpolarize before returning to resting levels, (b) cells which depolarize and then return to resting levels without a hyperpolarization, (c) spontaneously active cells which transiently hyperpolarize and then depolarize during a light pulse and (d) cells which depolarize upon illumination with the production of action potentials.4.Intracellular recording from cells at the base of the retina reveals neurons which are spontaneously active and fire action potentials in exact synchrony with compound impulses in the optic nerve. These basal retinal neurons are electrically coupled to one another and are responsible for the compound optic nerve impulse.5.We find that the most common type of photoreceptors (R-type) are electrically coupled to one another but we find no evidence that these photoreceptors make contact with basal retinal neurons.6.Localized illumination of retinal layers with miniature light guides reveals that the photoreceptor layer is responsible for light-induced low amplitude optic nerve impulses. In constrast, the light-induced compound action potential response is generated by light sensitive neurons at the retinal base.7.The photoreceptor layer exerts an inhibitory effect on basal retinal neurons. Illumination of the photoreceptor layer leads to a hyperpolarization in basal retinal neuron membrane potential. We think it is likely that this inhibition is mediated by a particular class of retinal cells, similar to H-type cells in theAplysia retina.8.An explicit model for the organization of theBulla retina is proposed.

Responses of the medicinal leech to water waves

Summary1.Sensitivity of leeches,Hirudo medicinalis, to low-amplitude water movements has been previously suspected on the basis of casual observations and qualitative experiments. We report here the results of quantitative experiments designed to test the capability of medicinal leechesHirudo medicinalis to detect and respond to the stimulation provided by water waves.2.Selected unfed leeches responded to low-amplitude surface waves (about 1 mm high) by initiating swimming activity in 86% of all tests in one set of experiments and in 95% of all tests in another. Swimming responses occurred less frequently in fed leeches (60% of all tests).3.The orientation of locomotory responses (either crawling or swimming) to wave stimulation was non-random. In most instances the evoked locomotory movements were directed into the waves, towards the stimulus source.4.We conclude that surface water waves are an adequate stimulus to direct leech movements towards their animal prey.

Systems-level modeling of neuronal circuits for leech swimming.

This paper describes a mathematical model of the neuronal central pattern generator (CPG) that controls the rhythmic body motion of the swimming leech. The systems approach is employed to capture the neuronal dynamics essential for generating coordinated oscillations of cell membrane potentials by a simple CPG architecture with a minimal number of parameters. Based on input/output data from physiological experiments, dynamical components (neurons and synaptic interactions) are first modeled individually and then integrated into a chain of nonlinear oscillators to form a CPG. We show through numerical simulations that the values of a few parameters can be estimated within physiologically reasonable ranges to achieve good fit of the data with respect to the phase, amplitude, and period. This parameter estimation leads to predictions regarding the synaptic coupling strength and intrinsic period gradient along the nerve cord, the latter of which agrees qualitatively with experimental observations.

A model for intersegmental coordination in the leech nerve cord

The neuronal circuits that generate swimming movements in the leech were simulated by a chain of coupled harmonic oscillators. Our model incorporates a gradient of rostrocaudally decreasing cycle periods along the oscillator chain, a finite conduction delay for coupling signals, and multiple coupling channels connecting each pair of oscillators. The interactions mediated by these channels are characterized by sinusoidal phase response curves. Investigations of this model were carried out with the aid of a digital computer and the results of a variety of manipulations were compared with data from analogous physiological experiments. The simulations reproduced many aspects of intersegmental coordination in the leech, including the findings that: 1) phase lags between adjacent ganglia are larger near the caudal than the rostral end of the leech nerve cord; 2) intersegmental phase lags increase as the number of ganglia in nervecord preparations is reduced; 3) severing one of the paired lateral connective nerves can reverse the phase lag across the lesion and 4) blocking synaptic transmission in midganglia of the ventral nerve cord reduces phase lags across the block.

Termination of leech swimming activity by a previously identified swim trigger neuron

Cell Tr2 is a neuron in the subesophageal ganglion of the leech that can trigger swim episodes. In this report, we describe the ability of Tr2 to terminate ongoing swim episodes as well as to trigger swimming. Stimulation of Tr2 terminated ongoing swim episodes in nearly every preparation tested, while Tr2 stimulation triggered swim episodes in only a minority of the preparations. We suggest that the primary role of Tr2 is in the termination rather than the initiation of swimming activity.The swim trigger neuron Tr3 and a swim-gating neuron, cell 21, hyperpolarized during Tr2-induced swim termination. Another swim-gating neuron, cell 204 was sometimes slightly excited, but more often, hyperpolarized during Tr2-induced swim termination. In contrast to these cells, Tr2 stimulation excited another swim-gating neuron, cell 61. The responses of the swimgating cells were variable in amplitude and sometimes not evident during Tr2-induced swim termination. Hence, the effects of Tr2 stimulation on swim-gating neurons seem unlikely to be the direct cause of swim termination.Oscillator cells examined during Tr2-induced swim termination include: 27, 28, 33, 60, 115, and 208. The largest effect seen in an oscillator neuron was in cell 208, which was repolarized by up to 10 mV during Tr2 stimulation. Tr2 stimulation did not produce any obvious synaptic effects in motor neurons DI-1, VI-1, and DE-3. Our findings indicate that other, yet undiscovered, connections are likely to be important in Tr2-induced swim termination. Therefore, we propose that cell Tr2 is probably a member of a distributed neural network involved in swim termination.

Positive feedback loops sustain repeating bursts in neuronal circuits

Voluntary movements in animals are often episodic, with abrupt onset and termination. Elevated neuronal excitation is required to drive the neuronal circuits underlying such movements; however, the mechanisms that sustain this increased excitation are largely unknown. In the medicinal leech, an identified cascade of excitation has been traced from mechanosensory neurons to the swim oscillator circuit. Although this cascade explains the initiation of excitatory drive (and hence swim initiation), it cannot account for the prolonged excitation (10–100xa0s) that underlies swim episodes. We present results of physiological and theoretical investigations into the mechanisms that maintain swimming activity in the leech. Although intrasegmental mechanisms can prolong stimulus-evoked excitation for more than one second, maintained excitation and sustained swimming activity requires chains of several ganglia. Experimental and modeling studies suggest that mutually excitatory intersegmental interactions can drive bouts of swimming activity in leeches. Our model neuronal circuits, which incorporated mutually excitatory neurons whose activity was limited by impulse adaptation, also replicated the following major experimental findings: (1) swimming can be initiated and terminated by a single neuron, (2) swim duration decreases with experimental reduction in nerve cord length, and (3) swim duration decreases as the interval between swim episodes is reduced.

Analysis of impulse adaptation in motoneurons.

Animal locomotion results from muscle contraction and relaxation cycles that are generated within the central nervous system and then are relayed to the periphery by motoneurons. Thus, motoneuron function is an essential element for understanding control of animal locomotion. This paper presents motoneuron input–output relationships, including impulse adaptation, in the medicinal leech. We found that although frequency-current graphs generated by passing 1-s current pulses in neuron somata were non-linear, peak and steady-state graphs of frequency against membrane potential were linear, with slopes of 5.2 and 2.9xa0Hz/mV, respectively. Systems analysis of impulse frequency adaptation revealed a static threshold nonlinearity at −43xa0mV (impulse threshold) and a single time constant (τxa0=xa088xa0ms). This simple model accurately predicts motoneuron impulse frequency when tested by intracellular injection of sinusoidal current. We investigated electrical coupling within motoneurons by modeling these as three-compartment structures. This model, combined with the membrane potential–impulse frequency relationship, accurately predicted motoneuron impulse frequency from intracellular records of soma potentials obtained during fictive swimming. A corollary result was that the product of soma-to-neurite and neurite-to-soma coupling coefficients in leech motoneurons is large, 0.85, implying that the soma and neurite are electrically compact.

Systems approach to modeling the neuronal CPG for leech swimming

This paper proposes a mathematical model of the neuronal central pattern generator (CPG) for leech swimming. The model is developed through the systems approach where dynamical components and their connections are first identified through input/output data from physiological experiments and then integrated into a chain of nonlinear oscillators. Our approach leads to a model of moderate complexity when compared with existing models developed through biophysical principles. We show through numerical simulations that our model can successfully reproduce the phase coordination observed in the isolated nerve cord of the leech CPG. As a byproduct, a prediction is obtained for the intrinsic period gradient along the nerve cord.