Using advanced technologies, a team of scientists  have revealed the nervous system connectivity of ctenophores, one of the most ancient animal lineages. Reconstructing neurons from the nerve net through 3D electron microscopy, they discovered an extraordinary architecture: a continuous neural network. These findings challenge our understanding of nervous systems and their evolution.

Ever since the work of scientists Santiago Ramón y Cajal and Fridtjof Nansen in the 19^th century, neurobiology research is interpreted through the lens of the neuron doctrine. This theory states that nervous systems are composed of discrete individual cells.
  
Camillo Golgi challenged this theory by putting forward the idea that neurons within a nervous system are connected as a continuous network. Cajal and Golgi shared the Nobel Prize in 1906 for their extraordinary findings, though they were fierce competitors throughout their career.

Cajal’s theory was finally proven correct by identifying neuronal junctions, so called synapses, through the invention of electron microscope in the 1950’s, thereby disproving Golgi’s theory. These new findings now prove that Golgi was also right.

Ctenophores, also known as comb jellies, are fascinating organisms that have been living in the world’s oceans for approximately 600 million years. When the first animals evolved, ctenophores were one of the first animal l­ineages on the planet. Within early evolution of neurons and nervous systems, multiple ways to make a nervous system were possibly established.

Previous attempts to describe ctenophore nervous systems connectivity had proven difficult because the organisms are delicate and very fragile, and investigating their anatomy was very challenging.

The researchers made an important observation that one single neuron in the ctenophore nerve net had made a small network by fusing its neuronal processes, also known as neurites, to each other.

Curious to explore this irregularity, the authors collected a much larger 3D dataset.

“We found fundamental differences between the nerve net of ctenophores and that of cnidarians and other animals”, says the author.

“This is extremely exciting. One could argue: is it even a nervous system?” Despite its unique architecture, the ctenophore nerve net displays key characteristics found in nervous systems such as neuropeptides and ion channels generating membrane potentials.

The characterization of the ctenophore nerve net has the potential to provide key information on the evolutionary origin of nervous systems. Through revealing the unique and unusual operating principles of ctenophore neurons, the teams offer a novel way of thinking about nervous system architectures, thereby paving the way for a new period of comparative neuroscience research.

https://www.science.org/doi/10.1126/science.ade5645

http://sciencemission.com/site/index.php?page=news&type=view&id=publications%2Fsyncytial-nerve-net-in&filter=22