Biomedical subjects

S R Lockery

Publications and source records attributed to S R Lockery.

10 recordsLinked to original sources

PubMed1995

Using reflexive behaviors of the medicinal leech to study information processing.

The interneuronal network that produces local bending in the leech is distributed, in the sense that most of the interneurons involved are activated in all forms of local bending, even those in which their outputs would produce inappropriate movements. Such networks have been found to control a number of different behaviors in a variety of animals. This article reviews three issues: the physiological and modeling observations that led to the conclusion that local bending in leeches is controlled by a distributed system; what distributed processing means for this and other behaviors; and why the leech interneuronal network may have evolved to be distributed in the first place.

Animals↗

PubMed1993

A lower bound on the detectability of nonassociative learning in the local bending reflex of the medicinal leech.

Studies of neural mechanisms of learning and memory have focused on large changes at identified synapses. However, memory in distributed processing reflexes could involve widely distributed engrams characterized by small changes at every synapse in the network. To investigate this possibility, we used a neural network optimization algorithm to construct distributed engrams for nonassociative conditioning in a model of the local bending reflex of the medicinal leech (Hirudo medicinalis). The model comprised 4 sensory neurons, 10 to 40 interneurons, 8 motor neurons, and up to 480 connections. Synaptic connections in the model were first optimized to reproduce the amplitude and time course of motor neuron synaptic potentials recorded during local bending. This network, which represented the naive state before conditioning, was then reoptimized to the habituated or sensitized state. Following reoptimization, the memory for nonassociative learning was encoded by small changes dispersed across the entire network, and each change made only a small contribution to the learning. Moreover, because the changes were small, resolution of a few tenths of a millivolt, or 3-5% of an average synaptic potential, would be needed to account for half of the nonassociative learning. These results show how difficult distributed engrams can be to detect and provide a likely lower bound on the detectability of nonassociative learning in this and related networks.

Animals↗

PubMed1993

The computational leech.

The local bending reflex of the leech computes a well-defined sensorimotor input-output function in which each of several unique patterns of sensory input elicits a unique pattern of motoneuron activity. Interneurons in the reflex respond to most input patterns and contribute to most motor patterns, suggesting a distributed processing mechanism for the reflex. This suggestion is supported by models in which connection strengths are adjusted by a neural network optimization algorithm to reproduce the local bending input-output function. In addition, computational parallels between the local bending network and the perceptron, a major class of artificial neural networks, brings the functional role of local bending interneurons into question and suggests new physiological experiments.

Animals↗

PubMed1992

Distributed processing of sensory information in the leech. III. A dynamical neural network model of the local bending reflex.

The subpopulation of identified interneurons in the local bending reflex receive multiple inputs from dorsal and ventral mechanoreceptors and have outputs to dorsal and ventral motor neurons. Their connections suggest a distributed processing mechanism in which withdrawal from dorsal, ventral, or lateral stimuli is controlled by a single population of approximately 40 multifunctional interneurons, but it is unclear whether additional interneurons dedicated to particular inputs are needed to account for each kind of bend. We therefore asked whether a model could be constructed that reproduced all behaviors without dedicated interneurons. Interneurons in the model were constrained to receive both dorsal and ventral inputs. Connection strengths were adjusted by gradient descent optimization until the model reproduced the amplitude and time course of motor neuron synaptic potentials in intracellular recordings of the response to many different stimuli. After optimization, the similarity between model and identified interneurons showed that additional dedicated interneurons are not necessary to produce all forms of the behavior. Successful optimization of networks with many fewer interneurons showed that the 40-interneuron network is redundant, raising the possibility that the interneurons have additional functions. Finally, optimizing networks with additional constraints produced better matches to some of the identified interneurons and showed that local bending can be produced by two populations of interneurons: one with outputs consistent with dorsal bending, the other with ventral bending. This suggests a simple model in which two principal types of interneurons produce many different behaviors and predicts the type of interneuron that remains to be identified.

Animals↗

PubMed1992

Reconstruction of action potential development from whole-cell currents of differentiating spinal neurons.

The duration and ionic dependence of action potentials are developmentally regulated. Voltage-clamp recordings of amphibian spinal neurons have revealed alterations in five currents. To determine whether the changes in the currents are sufficient to produce the change in action potential duration and ionic dependence, we constructed a Hodgkin-Huxley model of electrical excitability of these neurons. The model shows that the equations describing the voltage-clamped currents of young and mature neurons generate action potentials appropriate in duration and ionic dependence for each developmental stage. Moreover, the observed changes in the currents are quantitatively sufficient to produce the changes in the action potential. The effect of the change in each current is detectable in the model. However, the increase in amplitude of the delayed-rectifier potassium current has the largest effect. The model further shows that changes in action potential duration could be achieved with changes in kinetics rather than amplitude of this current, or with changes in amplitudes of other currents. Thus, although increase in amplitude of the delayed rectifier plays a pivotal role in the maturation of excitability, it is not uniquely positioned to govern the action potential duration.

Action Potentials↗

PubMed1991

Two forms of sensitization of the local bending reflex of the medicinal leech.

Sensitization of the local bending reflex of the medicinal leech Hirudo medicinalis was studied in a semi-intact preparation in which behavioral and electrophysiological recordings were made simultaneously. 1. Sensitization of local bending could be produced in two ways: by repeated stimulation of the mechanoreceptor sensitive to pressure (the P cell), and by stimulation of the mechanoreceptor sensitive to noxious stimuli (the N cell). 2. Both forms of sensitization produced a central neuronal change, measured as an increase in the number of stimulus-evoked action potentials in cell 3 (an excitor of dorsal longitudinal muscles). 3. Intracellular stimulation of serotonin-containing neurons 21 and 61 mimicked the sensitizing stimuli, but stimulation of the Retzius cell, which also contains serotonin, did not. 4. Stimulation of the Leydig cell, which releases octopamine, decreased the strength of local bending.

Action Potentials↗

PubMed1990

Distributed processing of sensory information in the leech. I. Input-output relations of the local bending reflex.

Sensory processing in the local bending reflex of the leech (Hirudo medicinalis) was studied by examining the input-output relations of the reflex. Sensory cells (P cells) were stimulated singly and in pairs and the responses of the longitudinal muscle motor neurons were recorded. Each pattern of single and paired P cell stimulation produced a unique pattern of motor neuron response. In general, motor neuron response patterns were consistent with the behavioral response of the whole animal.

Afferent Pathways↗

PubMed1990

Distributed processing of sensory information in the leech. II. Identification of interneurons contributing to the local bending reflex.

Isolated midbody ganglia of the leech Hirudo medicinalis were surveyed for interneurons contributing to the dorsal component of the local bending reflex, i.e., to the excitation of dorsal excitatory motor neurons that follows stimulation of dorsal mechanoreceptors responsive to pressure (P cells). Nine types of local bending interneuron could be distinguished on physiological and morphological grounds--8 paired and 1 unpaired cell per ganglion. Synaptic latencies from sensory neurons to interneurons were consistent with a direct or possibly disynaptic pathway. Connections between interneurons appeared to be rare and hyperpolarization of individual interneurons during local bending produced small but reliable decrements in motor neuron response, suggesting that multiple parallel pathways contribute to the behavior. Paradoxically, most interneurons received substantial inputs from ventral as well as dorsal mechanoreceptors, indicating that interneurons that were distinguished by their contribution to dorsal local bending were, in fact, active in ventral and lateral bends as well. The capacity to detect a particular stimulus and produce the appropriate response cannot be localized to particular types of interneuron; rather, it appears to be a distributed property of the entire local bending network.

Animals↗

PubMed1989

Function of identified interneurons in the leech elucidated using neural networks trained by back-propagation.

Mechanical stimulation of the body surface of the leech causes a localized withdrawal from dorsal, ventral and lateral stimuli. The pathways from sensory to motor neurons in the reflex include at least one interneuron. We have identified a subset of interneurons contributing to the reflex by intracellular recording, and our analysis of interneuron input and output connections suggests a network in which most interneurons respond to more than one sensory input, most have effects on all motor neurons and in which each form of the behaviour is produced by appropriate and inappropriate effects of many interneurons. To determine whether interneurons of this type can account for the behaviour, or whether additional types are required, model networks were trained by back-propagation to reproduce the physiologically determined input-output function of the reflex. Quantitative comparisons of model and actual connection strengths show that model interneurons are similar to real ones. Consequently, the identified subset of interneurons could control local bending as part of a distributed processing network in which each form of the behaviour is produced by the appropriate and inappropriate effects of many interneurons.

Animals↗

PubMed1985

Habituation of the shortening reflex in the medicinal leech.

The shortening reflex was elicited in food-sated Hirudo medicinalis by light flashes delivered at 20-s intervals over a 40-trial session. The reflex was enhanced if animals were stored at 5-7 degrees C rather than 20 degrees C. Short-term, that is, within-sessions, habituation was readily observed. Dishabituation could be produced by a single electric shock at Trial 30. However, the shock also enhanced responding when delivered before Trial 1 (sensitization). No long-term, that is, between-sessions, habituation occurred over 6 consecutive days of testing; on the contrary, responding gradually increased.

Animals↗