NERVOUS SYSTEM - Axons and dendrites- Part I - Commissures

General info - Commissures in Head - Amphid Commissure- Deirid Commissure- Commissure in Tail - Back to Contents

General Description

In the worm, most of the nerve cords run as longitudinal process bundles along the main body axis. However, individual neuron processes can make transitions between these longitudinal process trajectories. These transitions which run circumferentially (dorso-ventrally) between the longitudinal nerves are called commissures . Whereas in higher animals a “commissure” normally consists of dozens or even thousands of processes, in the nematode a commissure can consist of a single process which pioneers its own route along the bodywall. The major commissures include the amphid and deirid commissures in the head, the lumbar, dorsorectal and dorsolateral commissures in the tail, and more than 40 individual commissures along the length of the body where ventral cord motor neuron processes run singly or in pairs to reach the dorsal side from the ventral nerve cord (CommFIG1a, b, c, CommFIG2 and CommFIG4). The nerve ring which comprises the largest and most complex region of neuropil in the body is essentially an enlarged commissural region encircling the pharyngeal isthmus, with some 200 processes involved, and most running a half circle around the ring (CommFIG1a). Inside the pharynx, two shorter commissures, the pharyngeal nerve ring and the terminal bulb commissure, connect pharyngeal dorsal and subventral nerve cords (See Alimentary System Part IB).
The topology of commissures within the nervous system of the body follow two types of routes around the body; medial and lateral (Durbin R.,1987; White J.G. et al., 1986; Hedgecock al., 1987; Norris C., Hall D. H. and Hedgecock E. , unpublished). The nerve ring follows a medially positioned route at the extreme basal (medial) edge of the hypodermis facing the pseudocoelom (see CommFIG1a). During development, pioneer axons for the nerve ring are postulated to extend inwardly along extensions of the hypodermis or along the inward extensions (the muscle arms) of the head muscles, which themselves seem to be organized by scaffold cells, the GLRs. These nerve processes of the nerve ring remain in direct contact with hypodermis on the outside for the most part, and inside are separated from the muscle arms by the basal laminae of the hypodermis and muscle. Other commissures following such medial routes include those from the dorsorectal ganglion to the preanal ganglion in the tail (see CommFIG11. Hall thesis, 1977; Hall and Russell, 1991) and the ventrally directed HSN processes. In these cases the axons must negotiate their way around specialized muscle elements of the rectum or vulva, and pass between narrow gaps of muscle and a thin sheet of hypodermis.

Laterally positioned commissural routes are much more common. In these cases neuron processes travel singly or in groups along a closely confined space underneath the body wall muscles, always in close apposition to the thin sheet of hypodermis that covers the muscle. Again the nerves remain separated from muscle elements by the basal laminae of these two tissues. Where the nerve processes must cross the hypodermal ridges, they tend to revert to a medial route, in some cases coming to close contact with the pseudocoelom. The right sided ventral cord motor neuron commissures reach the dorsal cord which is located on the left side of the hypodermal ridge by crossing underneath the hypodermis.

Commissures in the Head

There are four major commissures in the head; right and left amphid commissures and right and left deirid commissures (See CommFIG2 and 3).

The Amphid Commissures

The amphid commissures on both sides are located lateral to the junction of the pharyngeal isthmus and the terminal bulb and travel between the ventral body wall muscle and lateral hypodermis. These commissures are mainly made up of axon bundles of the amphid neurons as they originate from the cell somata and travel ventrally towards the nerve ring, but they also contain axons that come from the ventral cord. Axons of two such neurons, SAAV and SABV join the anterior ventral sublateral cords as they leave the amphid commissures (See CommFIG4 and 5).
The posterior section of the amphid commissure is also referred to as sublateral commissure since it is made of axons of ventral sublateral cords. Of these, anteriorly traveling PLN axons dive through the amphid commissure to join the ventral cord on their way to the nerve ring, whereas, posteriorly traveling SIBV, SMBV, SIAV, SMDV axons use amphid commissures to join the ventral sublateral cords (See CommFIG5).
It must be noted that RID axon is only in the left side amphid commissure and SABD axon is only in the right side amphid commissure.
During embryonic development amphid and sublateral commissural axons cross to the ventral ganglion between muscle basal lamina and neuron SIBD. It is suggested that SIBD at this time transiently insinuates between muscle and hypodermis, forming a bridge between the lateral and ventral ganglion and possibly provides a substrate for early sublateral and amphid commissural axons. The commissural axon growth occurs in waves such that by 430 min of embryonic development 8 amphid axons have reached the nerve ring via the amphid commissures, but the remainder of amphid axons have not yet crossed to the ventral ganglion.(Norris C. R. et al, 1997; Norris C. R. et al, 1998).
The images below show the left side amphid commissure in adult. The
arrows point to the centrifugal direction of the axons as they travel through the commissure.

The trajectory of one of the axons that make up the left amphid commissure, the interneuron RIBL, is shown below (CommFIG6). After it originates from RIBL cell body, the axon first travels ventrally and slightly posteriorly within the commissure until it reaches the junction of hypodermis and ventral body wall muscle. From here it makes a medial turn and travels on the ventral side of the axon bundle anteriorly and medially until it meets the ventral cord. Once it reaches its position in the ventral cord, the axon then turns anteriorly and travels towards the nerve ring. In the picture below, the amphid commissure bundle is marked by yellow arrows or a dashed yellow line.

The Deirid Commissures

The deirid commissures are lateral to the posterior part of the terminal bulb of pharynx. After originating from their neuronal cell somata, the axons within the deirid commissures travel first postero-ventrally and then medially among the cells of the retrovesicular ganglia until they join the ventral cord (CommFIG7). They turn anteriorly in the ventral cord and travel to the nerve ring.
AQR is present only in the right side deirid commissure

Commissures in the Tail

There are three pairs of commissures in the tail; right and left lumbar commissures, right and left dorsorectal commissures and right and left dorsolateral commissures (CommFIG8 and 9 ). The dorsolateral commissures are formed by processes of PDA (right side comm.), DA9 (right side comm. See CommFIG10) and DA8 (left side comm.) motor neurons, which travel posterodorsally to the lumbar ganglia (LG) along the lumbar commissures and then continue their dorsal trajectory to dorsal cord along the dorsolateral commissures. The lumbar commissures are made of processes of DA8 (left side), PQR (left side), DA9 (right side), PDB (right side), PDA (right side), PVR (right side), PHAL/R, PHBL/R, PHCL/R, PVQL/R, LUAL/R, PVCL/R, PVWL/R, PVNL/R. The dorsorectal commissures contain processes from DVA (right side comm.), DVB (left side comm. See CommFIG11) and DVC (left side comm.) neurons which are located in the dorsorectal ganglion (DRG). In some animals the right side dorsorectal commissure contains an extra axon, possibly from AVFR neuron (Hall D. H. and Russell R. L, 1991).

Commissures in the Body

Of the 46 ventral cord motor neurons, 44 (7DA, 7DB, 6DD, 13VD and 11AS) send their processes to dorsal cord via body commissures (CommFIG12), whereas DA8 and DA9 send processes to dorsal cord via tail commissures and not through body commissures (see below). The commissural processes in the body are located outside of the muscle basal lamina and sandwiched between muscle and hypodermis as they travel along the lateral body wall (CommFIG1a, b, c). Most of these processes travel on the right side of the animal, however, 11 of them (DA1, DA3-7, DB2, DB4, DB5, DD1, VD2) make left sided commissures. They may travel alone or at times two of them may join together to grow as a single commissure (CommFIG13a, b Note the left fascicle of VNC is not seen in these images, since the neurons that make up (L) VNC do not express the GFP marker of this strain which is GABAergic neuron-specific). The anteriormost right (made by VD1 and SABD processes) and left side (made by DD1, VD2, DB2, DA1) commissures are located at the posterior of the terminal bulb of pharynx. The posteriormost body commissure which is made by AS11 neuron process is close to the preanal ganglion in the tail and runs on the right side. Along the body there are other neuron processes that travel dorsally or ventrally to reach longitudinal process tracts and make shorter commissures. These include;
On the right side:
SDQR dorsal process to dorsal sublateral tract,
AVM ventral process to VNC,
HSNR ventral process to VNC,
PLMR ventral process to VNC,
PDER ventral process to VNC,
PVDR ventral process to VNC,
On the left side:
SDQL dorsal process to dorsal sublateral tract,
HSNL ventral process to VNC,
PLML ventral process to VNC,
PDEL ventral process to VNC,
PVDLventral process to VNC,
PVM ventral process to VNC

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