Responses to Conflicting Stimuli in a Simple Stimulus–Response Pathway

Abstract

The "local bend response" of the medicinal leech (Hirudo verbana) is a stimulus-response pathway that enables the animal to bend away from a pressure stimulus applied anywhere along its body. The neuronal circuitry that supports this behavior has been well described, and its responses to individual stimuli are understood in quantitative detail. We probed the local bend system with pairs of electrical stimuli to sensory neurons that could not logically be interpreted as a single touch to the body wall and used multiple suction electrodes to record simultaneously the responses in large numbers of motor neurons. In all cases, responses lasted much longer than the stimuli that triggered them, implying the presence of some form of positive feedback loop to sustain the response. When stimuli were delivered simultaneously, the resulting motor neuron output could be described as an evenly weighted linear combination of the responses to the constituent stimuli. However, when stimuli were delivered sequentially, the second stimulus had greater impact on the motor neuron output, implying that the positive feedback in the system is not strong enough to render it immune to further input.

Encoding of Tactile Stimuli by Mechanoreceptors and Interneurons of the Medicinal Leech

“…In line with other recent findings (Gaudry and Kristan, 2009; Palmer et al, 2014; Baljon and Wagenaar, 2015; Pirschel and Kretzberg, 2016) the results of this study indicate that the model of the local bend network needs to be revised regarding the input signals provided by mechanoreceptors and the computation performed by interneurons.…”

“…In particular, the analysis of combined encoding of multiple stimulus properties should be extended to a larger space of stimulus dimensions (e.g., velocity, shape, application angle, indentation depth, vibration, duration additionally to location and intensity). Moreover, the local bend response was reported to be modulated by feedback-loops in the network (Baljon and Wagenaar, 2015), by neuromodulators (Lockery and Kristan, 1991; Gaudry and Kristan, 2009), as well as by feeding status and environmental factors like water depth (Palmer et al, 2014). Hence, despite the low number of neurons involved in this seemingly so hard-wired network, the leech tactile system is also well suited for studies on general mechanisms underlying the flexibility of neural activity and behavior.…”

Whole-ganglion imaging of voltage in the medicinal leech using a double-sided microscope

“…Stimulating P cells causes a combination of excitation and inhibition in identified "local bend interneurons" (LBIs) 12,17 . The LBIs synapse onto several motor neurons to produce an appropriate pattern of contraction and relaxation in the local area of the body wall that depends on which location (or which P cell) was stimulated 12,17,18 .…”

VScope — Data Acquisition and Analysis for Voltage-sensitive Dye Imaging Using Multiple Cameras and Electrophysiology

“…Naturally, VScope can also be used for experiments that only involve electrophysiology (e.g., [7]), although other software (e.g., pClamp (Molecular Devices, Sunnyvale CA)) exists that is more specialized for that goal. Conversely, VScope can be used for experiments that only involve imaging.…”