Think small

Abstract

The tadpole larva of a sea squirt is only the second animal to have its entire nervous system mapped out, and the results confirm that there is still much to learn from the smallest brains.

Keywords: C. intestinalis; CNS; central nervous system; connectome; network; neuroscience; synapse.

Figure 1.. Connectome of the Ciona intestinalis tadpole larva.

(A) As adults, sea squirts like Ciona intestinalis are immobile, filter-feeding animals. The scale bar is 10 mm. (B) The tadpole-like larva of Ciona, however, can swim and has a vertebrate-like body plan with a flexible rod that runs along its back called a notochord. Ryan et al. cut a single larva into ~7,000 ultrathin slices and used these to reconstruct the larval connectome. (C) The larva also has sensory organs, including a single eyespot on the right side and an organ involved in sensing gravity (called otolith) on the left. The scale bars in B and C are 100 μm. (D) The Ciona connectome shows that the larval nervous system consists of a peripheral and a central nervous system; the latter includes a brain vesicle and a motor ganglion. The brain vesicle contains the photoreceptors for the eyespot, the coronet cells (which are thought to detect pressure), antenna cells (which are involved in sensing the position of the otolith), and relay neurons. The motor ganglion contains neurons that directly control the contractions of the muscles. The brain shows many asymmetries in its cellular anatomy and wiring. For example, there are more relay neurons on the left of the larvae, and the motorneurons in the motor ganglion (numbered 1–5) are connected differently on the left and right sides of the larvae (red arrows show asymmetric connections). Photographs in A, B and C are courtesy of Antonio Palladino.