Morphology and physiology of motor neurons innervating labral muscles of locusta migratoria

Major animal taxa, known as phyla, each represents a basic bauplan. Among the diversity of phyla, arthropods are the most dominant on the earth. The conspicuous feature of the arthropod bauplan is the grouping of body segments into specialized units by the phenomenon of tagmosis which in insect has given rise to three body regions; head, thorax and abdomen. But the number and the nature of segments involved in composition of the insect head is still a matter of dispute among today’s evolutionary biologists. From the beginning of the dispute, there was general agreement that three mouthparts segments (mandible, maxillae, and labium) take part in the head composition, and their ganglia fuse to form the suboesophageal ganglion of the insect nervous system. However, the nature of the segments anterior to the mandible (labral/ocular, antennal, and intercalary segments) remained controversial. Modern molecular, palaeontological, and neuroanatomical studies showed that the insect head is composed of six segments; occular, antennal, intercalary, and three gnathal segments. But conflicting views are still present regarding the nature and segmental affiliation of the insect labrum. Several such modern studies imply that the labrum is a highly modified appendage of the intercalary segment, and sensory innervation of the embryonic and the adult labrum shares many similarities with that of head and thoracic appendages. However, defining criterion of an appendage implies that every appendage must be innervated by its own segmental ganglion, and the innervation pattern must include both sensory and motor elements. The aim of thesis is to investigate the motor innervation pattern of muscles of the labrum in the adult locust to reveal the evolutionary changes underlying neuronal circuitry, which might help to decide whether the labrum represents a structure that has developed during the evolution by fusion of paired appendages associated with the intercalary segment. Using Neurobiotin as a retrograde neuronal tracer, specific motor nerves of individual labral muscles were stained. Results show that all labral muscles receive innervation from both the tritocerebral lobes of the brain and the suboesophageal ganglion, except for M1, the labral compressor muscle. M1 is innervated by two motor neurons. Both tritocerebral lobes contain one soma. The axons of both neurons branch in the periphery to innervate ipsi- and contralateral muscles. The labral anterior retractor muscles, M2, are innervated by 6 tritocerebral motor neurons, with 3 somata located in each tritocerebral lobe. Their axons cross the midline in a distinct commissure between the two muscles. The labral posterior retractor muscles, M3 and anterior dilator muscles of foregut, M38, are innervated from the ipsilateral tritocerebrum only. Both muscles together receive 8 motor axons and there are some common motor neurons that innervate both muscles. In addition, M2, M3 and M38 also receive common innervation from neurons located in the suboesophageal ganglion (SOG). In relation to the muscle, their somata are located on the ipsi- and the contralateral sides of the SOG. Their axons ascend through the circumoesophageal connectives and proceed into the frontal connectives without forming ramifications in the tritocerebral lobes. These axons also cross the midline in the periphery to reach muscles on both sides of the head. They either cross within the commissure between muscles M2, or through the frontal ganglion. Further more, electrophysiology of labral muscles in isolated insect heads was investigated by intracellular recordings from the fibers of individual muscles. The recordings showed that all postsynaptic potentials were excitatory and no inhibitory synaptic inputs to labral muscles were found during the present study. Similarly simultaneous intracellular recording from fibers of labral muscles and suboesophageal ganglion motor neurons also revealed no inhibitory synaptic inputs to labral muscles, and showed that suboesophageal neurons are common excitatory motor neurons. The innervation pattern was compared with that of the other body appendages of locust, and it was concluded that labral muscles are innervated from both sides of the CNS and their innervation pattern is bilaterally symmetric as that of other appendages. The participation of both tritocerebral and suboesophageal ganglion motor neurons supports the notion that the labrum is innervated by the so-called intercalary segment. The innervation pattern also supports the idea that the labrum derives from fused appendages: There are two symmetrical sets of motor neurons. Some of them cross the midline in the periphery. This indicates that during the evolution of the labrum its motor innervation pattern was reconfigured in the periphery, while the original bilateral sets of central neurons remained unchanged.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2011

Author dc:creator

• Alvi, Abid Mahmood

Contributors dc:contributor

• Bräunig, Peter-Michael

• info:eu-repo/classification/ddc/570
• Zoologie
• Neuroanatomie
• Morphologie
• Gehirn
• Elektrophysiologie
• Biowissenschaften, Biologie
• konfokale Mikroskopie
• labrum
• labral muscles
• Locusta
• motor neurons

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-121658%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:59917