The complex synaptic pathways onto a looming‐detector neuron revealed using serial block‐face scanning electron microscopy

Abstract

The ability of locusts to detect looming stimuli and avoid collisions or predators depends on a neuronal circuit in the locust's optic lobe. Although comprehensively studied for over three decades, there are still major questions about the computational steps of this circuit. We used fourth instar larvae of Locusta migratoria to describe the connection between the lobula giant movement detector 1 (LGMD1) neuron in the lobula complex and the upstream neuropil, the medulla. Serial block‐face scanning electron microscopy (SBEM) was used to characterize the morphology of the connecting neurons termed trans‐medullary afferent (TmA) neurons and their synaptic connectivity. This enabled us to trace neurons over several hundred micrometers between the medulla and the lobula complex while identifying their synapses. We traced two different TmA neurons, each from a different individual, from their synapses with the LGMD in the lobula complex up into the medulla and describe their synaptic relationships. There is not a simple downstream transmission of the signal from a lamina neuron onto these TmA neurons; there is also a feedback loop in place with TmA neurons making outputs as well as receiving inputs. More than one type of neuron shapes the signal of the TmA neurons in the medulla. We found both columnar and trans‐columnar neurons connected with the traced TmA neurons in the medulla. These findings indicate that there are computational steps in the medulla that have not been included in models of the neuronal pathway for looming detection.

Three-dimensional SEM, TEM, and STEM for analysis of large-scale biological systems

“… 2014 ), allowing the examination of the neuronal connectivity in blocks of considerable lengths (e.g., ~ 400 µm) (Wernitznig et al. 2022 ) and volumes of up to 500,000 µm 3 (Motta et al. 2019 ).…”

“…Because each section is discarded, this allows for a lower section thickness than in array tomography (e.g., 20-30 nm Helmstaedter 2013; Briggman et al 2011). This route is faster than focused ion beam SEM (FIB-SEM) (described below), and covers larger volumes at the same time (Zankel et al 2014), allowing the examination of the neuronal connectivity in blocks of considerable lengths (e.g., ~ 400 µm) (Wernitznig et al 2022) and volumes of up to 500,000 µm 3 (Motta et al 2019).…”