2023 65 B3

Towards Characterizing the Neural Circuitry Mediating the Defensive Strike Response in Manduca sexta

By: Christian Valtierra

Department: Biology

Faculty Advisor: Dr. Megumi Fuse

Nociceptive sensitization is an evolutionarily ancient adaptive mechanism that is characterized by a heightened state of arousal and a decrease in the sensory threshold required to engage in defensive behaviors following exposure to an intensely noxious or harmful stimulus. Along these lines, the tobacco hornworm, Manduca sexta, displays a robust defensive strike behavior that becomes sensitized following injury. Previous electrophysiological studies suggest that the state of sensitization in M. sexta is encoded at the level of the central nervous system rather than by changes in peripheral nervous system as evidenced by paired extracellular recordings of the dorsal nerve (sensory input) and ascending ventral nerve chord (upstream of the ganglion receiving sensory input) during mechanical stimulation. Although the CNS alterations are evident, the current model may be overly simplistic as it is based in the comparison of overall nerve activity rather than of single cell activity of known populations of sensory and/or higher order neurons. Therefore, it is worth re-examining the possibility of presynaptic plasticity. I propose leveraging techniques from digital signal processing, unsupervised machine learning and the analysis of time series ("spike sorting") on the extracellular recordings, to extract the spiking activity of individual neurons in the peripheral and central nervous system during stimulation, in injured and non-injured preparations. Because sensory information travels along labeled lines, it is hypothesized that (1) only sorted units corresponding to the pressure sensitive β-MD neuron and the γ-MD nociceptive neuron(s) will (1) yield significant peaks in their respective sensory unit-VNC cross-correlation histograms, indicating functional coupling, and (2) systematically vary their firing during mechanical stimulation as a function of stimulus intensity. Moreover, if peripheral sensitization occurs in M. sexta, then only the unit corresponding to the nociceptor will show a shift in its stimulus evoked activity relative to the nociceptors of non-injured controls.