VNC-subdivision

Possible subdivisions in tectulum

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I tried to summarize the information of dorsal neuropils in the ventral nerve cord. Naming each subdivision may be useful to describe morphology of individual VNC neurons.

1. neck motor neuropil

The term has been used for fly descending neuron studies (Gronenberg et al., 1995; Strausfeld, 1992; Strausfeld et al., 1987; Strausfeld & Gronenberg, 1990; Flybrain neuron database), through hey do not define ‘neck motor neuropil’ explicitly. They also use the term frontal nerve motor neuron (FNM) neuropil, probabbly for the same meaning (Fig2, upper). R10B07 labels one of FNM with moderate background expression. Dendritic branches of FNM occupy almost whole dorsal part of T1, which can be used for definition (R10B07, Fig1). R37H01 labels a group of interneurons with small cell body size, connecting this are and haltere neruopil. The dendritic branches occupy the same area to FNM (Fig.3,1).

At least in nc82 staining, this area can be recognized as a portion of T1 that locate dorsally to the dorsal cervical fasciculi and fibrous area. The volume is much smaller than flight neuropil.

anterior: flanked by three tracts, ITD, MTD and MDA

posterior: flanked by flight neuropil, or just posterior edge of T2

ventral: flanked by three tracts, ITD, MTD and MDA

The area can also be defined by dendritic innervation by FNM.

2. haltere neuropil

The term is rarely used (Strausfeld, 1992; Flybrain neuron database) but some domain related to haltere might exist on the dorsal T3. Strausfeld 1992 use the term haltere neuropil as the dendritic region of haltere interneuron connecting with FNM in neck motor neuropil (Strausfeld & Seyan, 1985; Fig2, lower; Fig.3, R50H10). Also, there are some lines labeling interneuron innervating this area. Interneurons that connect with neck motor neuropil (R37H01, Fig.3,1), Interneurons that connect with flight neuropil (R20C05, Fig.4), another 2 subtypes of haltere interneurons connecting anterior or posterior part of the flight neuropil (R53A10, Fig.3,2, R91D10 [not sparse]). The area of innervation of these interneurons overlap with sensory afferent from haltere nerve (Strausfeld & Seyan, 1985).

anterior: ambiguous

ventral: ambiguous

medial: ambiguous

Nc82 signal is slightly lower than surrounding area. The boundary is ambiguous at least in nc82 signal.

3. lower area of dorsal tectulum (lower tectulum)

Some neurons specifically innervate ventral portion of dorsal tectulum, surrounded by DLV/VTV, located in T1 and T2. Flight motor neurons and interneurons in flight neuropil rarely innervate this area and neurons in this area never extend their branches into dorsal side of three dorsal tracts (ITD, MTD, MDA) (Fig.3, 3), suggesting this lower area of dorsal tectulum is distinct region from flight neuropil in terms of neuronal connectivity. Although synaptic area has been not described, the gross position is likely to be the same to the tracts of the ventral cervical fasciculi, one of the components forming dorsal tectulum (pp.374, Power 1948; also called low-stratum of dorsal tectulum, Fig.7).

Lower area of dorsal tectulum is probably overlaps with the ‘ovoid’, the region occupied by afferent projection of sense organs on the anterior wing margin (Merritt & Murphey, 1992, Fig.11; Palka et al. 1979), which correspond to aVAC in orthoptera. Also, similar pattern is observed in sensory projection from macrochaetes (Usui-Ishihara & Simpson, 2005). Other types of wing afferent might not contact with this area, because these project outside DLV, and seem not to overlap most of the interneurons shown above (R13A10).

anterior: flanked by mVAC in T1

posterior: on the posterior edge of T2, CFF?, flanked by haltere neuropil

dorsal: flanked by three tracts, ITD, MTD and MDA

lateral: ventral: tract DLV

dorsal: ambiguous, possibly ITD

4. flight neuropil

Dorsal mesothoracic neuropil from which originate motor neurons supplying direct and indirect flight muscle. The term ‘flight neuropil’ or ‘flihgt motor neuropil’ has been used for studies of descending neurons (Griss & Rowell, 1986; Strausfeld & Gronenberg, 1990; Flybrain neuron database), also called as wing motor neuropil, dorsal flight neuropil. [Strausfeld often uses the term ‘dorsal neuropils of the thoracic ganglia’, probably for the same meaning to tectulum.]

[Figure 4] R89D01 is a sparse line that labels all indirect muscle flight motor neurons (DLMs/DVMs). The innervation is limited in the flight neuropil and basically does not extend the branches toward ventral side of three dorsal tracts (ITD, MTD, MDA), suggesting these tracts can be used for the definition of flight neuropil. Steering muscle motor neuron (R19G08) and octopaminergic flight motor neurons (mesVUM, R25C01, Schlurmann & Hausen, 2003) also show similar morphological characteristics. There are several interneurons whose innervation are limited in the flight neuropil (right side).

“a motor neuropil, contains branches from motor neurons that drive wing elevators and depressors but also receives projections from flight proprioceptors­­‑wing hinge receptors and wing sensory hairs ” (Leise,1991)

“Some neuropils, however, like the dorsal flight neuropils of insect thoracic ganglia, have no visible histological boundaries and can only be recognized as discrete neuropils from the overlapping arborizations of functionally related neurons” (Leise,1991)

anterior: flanked by neck motor neuropil

posterior: CFF, flanked by haltere neuropil, or just posterior edge of T2

ventral: flanked by three tracts, ITD, MTD and MDA

5. medial ventral association center

Medial ventral association center (mVAC) has been identified as terminal region of chorodotonal organs or auditory afferent and known as ‘auditory neuropil’ in other insects, which is also referred to as the “anterior intermediate sensory neuropil” (Römer et al., 1988). R11G06 labels a single population of sensory neurons to mVAC. Type B femoral campaniform sensilla project to posterior to the mVAC decaussation, Type C & D project dorsal to mVAC (Merritt & Murphey, 1992). I have not yet found any paper for sensory neuron directly into mVAC in Drosophila. Sensory projection from auditory organ in parasitoid fly possibly project to mVAC (Oshinsky & Hoy, 2002)?

There are several neurons that have specific innervation to this area and not to dorsal tectulum and leg neuropil: interneurons connecting mVAC at 6 different locations (R11A07, Fig5), ascending neuron to the brain (R10C12), suggesting this area is distinct region in terms of neuronal connectivity.

“a region of fine neuropil crossing the midline of each thoracic neuromere and fanningout posteriorly.” (Merritt & Murphey, 1992)

“Auditory and vibrational receptors of locusts and bushcrickets project primarily to a medial

region of the ganglion traditionally called the anterior ring tract (aRT; Tyrer and Gregory,

1982). Pflüger et al. (1988) proposed that the area be renamed the medial ventral association

centre (mVAC) to conform with well established terminology, and to indicate that the area is a

neuropil in the true sense and not a tract. Almost simultaneously, Römer et al. (1988) proposed

that this same area be termed the anterior intermediate sensory neuropil (aISN). For consistency

with the bulk of other literature describing the anatomy of the CNS, the term mVAC should be

used.” (Field & Matheson, 1998)

anterior: T1: ambiguous

T2: flanked by ventral accessory commissure of mesothoracic neuromere

T3: basically on the anterior edge, flanked by unknown small structure neuropil (~10µm)

posterior: T1: gap (weak nc82 signal) with MsLN

T2: gap with MsLN

T3: gap with MTD

dorsal: T1: flanked by DLV

T2: ambiguous, adjacent to lower area of dorsal tectulum (fibrous sturcture)

T3: partially flanked by DLV

Blagburn JM (2008) Engrailed expression in subsets of adult Drosophila sensory neurons: an enhancer-trap study. Inv Neurosci 8:133-146.

Field LH, Matheson T (1998) Chordotonal organs of insects. Adv Insect Physiol 27:1-228.

FLYBRAIN Neuron Database http://flybrain-ndb.iam.u-tokyo.ac.jp/brainregion-list.html

Griss C, Rowell CH (1986) Three descending interneurons reporting deviation from course in the locust. I. Anatomy. J Comp Physiol A 158:765-774.

Gronenberg W, Milde JJ, Strausfeld NJ (1995) Oculomotor control in calliphorid flies: Organization of descending neurons to neck motor neurons responding to visual stimuli. J Comp Neurol 361:267-284.

Leise EM (1991) Evolutionary trends in invertebrate ganglionic structure. Seminars in Neuroscience 3:369-377.

Merritt DJ, Murphey RK (1992) Projections of leg proprioceptors within the CNS of the fly Phormia in relation to the generalized insect ganglion. J Comp Neurol 322:16-34.

Milde JJ, Strausfeld NJ (1990) Cluster organization and response characteristics of the giant fiber pathway of the blowfly Calliphora erythrocephala. J Comp Neurol 294:59-75.

Palka J, Lawrence PA, Hart HS (1979) Neural projection patterns from homeotic tissue of Drosophila studied in bithorax mutants and mosaics. Dev Biol 69:549-575.

Pfluger HJ, Braunig P, Hustert R (1988) The organization of mechanosensory neuropiles in locust thoracic ganglia. Philos Trans Roy Soc Lond B 321:1-26.

Römer H, Marquart V, Hardt M (1988) Organization of a sensory neuropile in the auditory pathway of two groups of Orthoptera. J Comp Neurol 275:201-215.

Schlurmann M, Hausen K (2003) Mesothoracic ventral unpaired median (mesVUM) neurons in the blowfly Calliphora erythrocephala. J Comp Neurol 467:435-453.

Smith SA, Shepherd D (1996) Central afferent projections of proprioceptive sensory neuron in Drosophila revealed with the enhancer-trap technique. J Comp Neurol 364:311-323.

Strausfeld NJ (1992) The head-neck system of the blowfly Calliphora: 1 Anatomic organization of neck muscles, motor neurons, and multimodal and visual inputs. In P.P. Vidal (eds): The head-neck sensory motor system. New York: Oxford University Press, pp.56-63.

Strausfeld NJ, Gronenberg W (1990) Descending neurons supplying the neck and flight motor of Diptera: organization and neuroanatomical relationships with visual pathways. J Comp Neurol 302:954-972.

Strausfeld NJ, Seyan HS (1985) Convergence of visual, haltere, and prosternal inputs at neck motor neurons of Calliphora erythrocephala. Cell Tissue Res 240:601-615.

Strausfeld NJ, Seyan HS, Milde JJ (1987) The neck motor system of the fly Calliphora erythrocephala. I. Mudcles and motor neurons. 160:205-224.

Usui-Ishihara A, Simpson P (2005) Differences in sensory projections between macro- and microchaetes in Dsophila flies. Dev Biol 277:170-183.