Interneurons In The Fruit Fly Visual System Work As 1400 Individual Cells

  • Neurobiologists describe a unique case of an amacrine cell called CT1 in a fruit fly. 
  • CT1 connects to approximately 1,400 regions in the fly brain and each cell area functions as a separate neuron. 
  • A computer simulation shows that CT1 reaches biophysical limits. 

Typically, a neuron can be seen as a single processing unit. It receives signals from one or multiple presynaptic cells, converts these signals, and sends one processed-signal to its postsynaptic cells.

There exist some exceptions, for instance, interneurons in the locust mesothoracic ganglion and amacrine cells in the mammalian retina represent several electrically isolated microcircuits within a single neuron.

Recently, researchers at Max Planck Society for the Advancement of Science described an extreme case of such an amacrine cell — named CT1 — in the Drosophila (a fruit fly) visual system.

Unique Visual System of Fruit Flies

The visual system of Drosophila comprises the eye with approximately 700 facets and the optic lobe. CT1 receives signals from every single cell column that connects to these facets in the brain.

Since CT1 synapses extend to two different regions of brains, they play a key role in processing dark or light edges. CT1 connects to approximately 1,400 regions in the Drosophila brain.

Reference: NCBI | DOI:10.1016/j.cub.2019.03.070 | MPI

This complex interconnection should corrupt the entire system. Every individual cell columns process light perceived by their facet. If columns’ signals were mixed, the whole image information for postsynaptic cells would be destroyed. But the loss of image information never happens, as fruit flies see very well.

In this study, neurobiologists have shown that individual contact regions of CT1 are independent functional units, electrically isolated from each other. Every single unit receives signals from its associated column and responds to the same column.

The team used computer modeling and calcium measurements to analyze highly compartmentalized retinotopic response characteristics in neighboring terminals of CT1. They found that these functional units do not interfere with one another.

CT1 from the fly brain works with its subunit | Credit: Max Planck Institute of Neurobiology

In order to be electrically isolated, the connections of cell units must be long and thin. This raises their electrical resistance. The CT1 connections have a diameter of about 100 nanometers. Moreover, these ‘cables’ form loops, making connections 10x longer than what is required to bridge the distance. According to researchers, it would be almost impossible for these connections to get much longer or thinner in the Drosophila brain.

The Mystery

Researchers are still trying to figure out why CT1 exhibits different properties compared to most other cells. Till date, only a few types of cells have been identified with such a structure. As far as CT1 is concerned amongst them, only two cells exist in the Drosophila brain (one is in the left hemisphere while the other one is in the right).

Read: AI Reveals What Neurons In the Brain’s Visual System Prefers To Look At

The exact functions of CT1 are still not well-understood. Its output signal goes to motion sensitive T5 or T4 cells, which evaluate how images in front of the eye are moving in different directions. But how these amacrine cells affect motion vision? Neurobiologists will try to answer this in their next study.

Written by
Varun Kumar

Varun Kumar is an experienced science and technology journalist interested in machines, AI, and space exploration. He received M.tech degree in computer science from Indraprastha University. To find out what his latest project is, feel free to directly email him at varunkmr09@gmail.com. 

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