Introduction
Physical body
Interior structure
Larval Brain
The central nervous system (CNS) of the Drosophila larva develops during embryogenesis. It is therefore fully functional when the larva hatches. The larval CNS originates from an invariant set of neuronal stem cells, called neuroblasts, which delaminate during early embryonic stages from the neuroectoderm and subsequently divide to first produce the neurons of the larval nervous system (primary neurons), followed by adult specific secondary neurons which are born during larval stages. The CNS is divided into an anterior region, often referred to as the brain, and a posterior ventral nerve chord (VNC). For Drosophila larvae, CNS = Brain + VNC.
Developmentally the brain is derived from two ganglia, each consisting of three neuromeres: the supraesophageal and subesophageal ganglion (SOG). The supraesophageal ganglion consists of the protocerebrum, deuterocerebrum, and tritocerebrum; the SOG consists of the mandibular, maxillary, and labial neuromeres. The VNC develops from the ventral neurectoderm, the SOG from the neuroectoderm of gnathal segments, and the supraesophageal ganglion from the procephalic neuroectoderm.
An alternative and more accessible subdivision relies on neuro- anatomical markers. Here, the CNS can be subdivided into two compartments: a peripheral cortex formed primarily by cell bodies and a synapse dense central neuropile, where axonal and dendritic domains overlap.
Olfactory system
Visual system
The visual system of the larva is comparably simple. The larval eye consists of 12 photoreceptor neurons (PRs), which are subdivided into two types: four PRs express the blue-sensitive rhodopsin5 (rh5), while the remaining eight express the green-sensitive rhodopsin6 (rh6). Larval PRs project their axons to the larval optic neuropile(LON): a small, distinct neuropile compartment located medio-laterally to the central brain. In contrast with the LAL comparably little is known about the downstream circuit of the LON. The best characterized target neurons are the main pacemakers of the clock circuit, termed lateral neurons (LNs) marked by the expression of the neuropeptide pigment-dispersing factor (Pdf). The larval brain comprises four Pdf-expressing LNs per brain hemisphere. Clock neurons have been show to modulate the light-response of larvae in a circadian fashion. The fi fth Pdf-negative LN is also part of the clock circuit and extends its dendritic arbors into the LON. The LON is further innervated by two serotonergic neurons. These serotonergic neurons are important for general responsiveness to light. How these neurons are connected to the larval PRs remains elusive. Likewise, their function is not yet known. Lastly, a set of three neurons known as optic lobe pioneers (OLPs) has been shown to project into the LON. The function of the OLPs is also unknown.
Neural circuits
Open questions
- Why synapses form the dense central neuropile? Would it be more efficient to transmit the information flow in the neural circuit?