ALIMENTARY SYSTEM - (Part I b) Pharyngeal Neurons

Pharyngeal Neurons - Neuron List - Back to Contents

Neurons of the pharynx

The pharyngeal nervous system is made of 20 intrinsic pharyngeal neurons which lie completely within the pharynx. 6 of these are bilaterally paired and 8 are single neurons. Most have cell bodies in one of the pharyngeal bulbs, and each extends processes anteriorly and/or posteriorly within longitudinal nerve cords. Most of these processes interact synaptically with others within a small plexus (also called the pharyngeal nerve ring) formed in the anterior bulb between the narrow region where pm4 and pm5 appose each other (AlimFIG16). The synapses made by pharyngeal neurons are of the "en passant" type similar to those seen in the somatic neurons. The pharyngeal motor neurons also form neuromuscular junctions (NMJs) onto pharyngeal muscles. There are no direct contacts between these neurons and the larger somatic nervous system except the gap junctions made between RIPL/R neurons and the pharyngeal I1's, as well as possible gap junctions between RIPL/R and the pharyngeal motor neuron, M1 ( See "Connectivity" in Albertson and Thomson, 1976, Avery L. and Thomas J. H., 1997 and the WA editors' note2 below). Ablation of RIP's results in only a minor effect on pharyngeal function such that the brief inhibition of pumping in response to light touch to body disappears (Avery L. and Thomas J. H., 1997). Though it is possible that one or more pharyngeal neurons, such as NSM's, could secrete hormonal factors into the pseudocoelom to influence the rest of the animal, pharyngeal and somatic nervous systems function independently of each other and a dissected and isolated pharynx continues its normal pumping behavior.

In fact, generation of electrical potential changes required for pharyngeal pumping is probably intrinsic to the pharyngeal muscle cells themselves since pumping behavior continues even when all the pharyngeal neurons are ablated (Avery L. and Horvitz H. R., 1989). Although pumping can still occur after complete ablation of the pharyngeal nervous system, 4 of the pharyngeal neurons, MC and M3 motor neuron pairs, are important for the regulation of the pump motion (see Feeding behavior in Part 1a).

(WA editors' note2) RIP (somatic neuron ) to M1 (pharyngeal neuron) innervation revisited: Close inspection of several animals to reconfirm the connectivity of the somatic cells, RIPL and RIPR to the pharyngeal nervous system revealed that the penetration of the RIP branches into the pharynx is somewhat variable (Hall, unpublished). RIPL enters pharynx slightly more anteriorly than RIPR-as confirmed in 3/3 animals (N2Q, N2T, and N2W from MRC archives). Gap junctions were seen to I1L in two cases, and possibly also in the third case. Additional gap junctions were seen to M1 in one case, and potential chemical synapses from M1 onto RIPL in one case. On the right side, the entry of RIPR was only found in 2/3 cases, and clearly did not happen in the third, although the I1R process migrated out very close to the basal lamina in this case. Distinctive gap junctions with I1R were seen in only one animal and possible gap junctions with M1 were seen in one case. In fact the I1 processes always made a characteristic close approach to the site where RIP entered the pharynx. Occasionally the RIP process also touched pharyngeal muscle or another cell type, but no other likely synapses were found.

Motor Neurons: Among motor neurons, M2 and M3 are paired neurons while M1, M4 and M5 are single (See single neuron images below). M2 and M3 neurons are subventrally located and each innervate one side of the subventral and dorsal muscle sectors. M3's are also suggested to have proprioceptive sensory function. M4 and M5 neurons send out two branches, each of which first innervates the subventral sectors and then turns and innervates the dorsal ones.
Interneurons: I1, I2, I3 and I6 are unbranched bipolar cells . I1 cells make electrical synapses with the somatic neurons RIPL/R and synapse onto MC's. I4 has two processes that grow anteriorly. I5 is a fairly complex cell, the processes of which make a circle within the pharyngeal nerve ring. All of these neurons except I4 have free subcuticular endings that may function as proprioceptive.
Other neurons: MC neurons are a pair of bipolar cells situated within the anterior bulb and innervate pm4 (L. Avery pers. comm.). The neurosecretory-motor neurons are a pair, NSML and NSMR, which send out branches that form varicosities and contain various diameter-sized vesicles. The processes of the NSM's run in close apposition to the pseudocoelom over most of their lengths. NSM's are serotonergic and as described above, these cells may have both neurosecretory and motor functions and may communicate the presence of food to the rest of the animal's body. Exogenous application of serotonin stimulates pumping, decreases locomotion and stimulates egg-laying which are also seen in response to the presence of bacteria in the environment. NSM's were thought to be good candidates to mediate the effects of endogenous serotonin. However, ablation of NSM's have only subtle effects on pumping suggesting they may be redundant for this function (Avery L. et al, 1993, Avery L. and Thomas J. H., 1997). The slowing of locomotion in the presence of bacteria becomes more enhanced in animals which are previously food-deprived compared to well-fed animals. This phenomenon is described as "the enhanced slowing response". When NSM's are ablated there is a small but significant decrease in this enhanced slowing of locomotion which suggests NSM's contribute to this behavior (Sawin E. R. et al, 2000). The motor-interneuron, MI, is a single unipolar cell that is situated in the dorsal side of the anterior bulb and synapses onto both muscle and other neurons.
In the anterior bulb, the processes of some of these neurons form a well-organized structure, the pharyngeal nerve ring, as they turn around and cross to the other side (See Fig25 in Albertson and Thomson, 1976, AlimFIG16). More posteriorly at the anterior portion of the terminal bulb a half ring, the terminal bulb commissure, is made by neuronal processes (AlimFIG17). For a more detailed description of each neuron, refer to individual neuron pages.
(WA editors' note3). The distinction between pharyngeal neurons as motorneurons, interneurons and other neurons is somewhat arbitrary. As noted above most of these neurons have structures that suggest mixed functions.

The processes of three of the pharyngeal neurons, M1, M2 and M3, have unusual trajectories. M1 process runs anteriorly between the muscle and the right marginal cell until it reaches the pharyngeal nerve ring (See AlimFig22 and 23 below) where it relocates to the dorsal nerve cord and continues traveling anteriorly in this location (AlimFig24-30). Unlike other neuron processes (e.g., see I4 process turn on the right side in AlimFig24), M2 and M3 processes do not make their dorsal turn within the pharyngeal nerve ring which is sandwiched between pm4 and pm5 (AlimFig24), but more anteriorly within the pm4 where they pierce through the pm4 as they travel towards the dorsal side to enter the dorsal nerve cord (AlimFig26-29). M2 neurons make synapses to pm4 along the way.

Click on the thumbnails to see enlarged TEM sections.

List of neurons of the pharynx

1.Motor neurons
2. Interneurons
2. Other neurons


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