The abdominal motor system of the crayfish, Cherax destructor. II. Morphology and physiology of the deep extensor motor neurons

Two opposing muscle systems underlie abdominal contractions during escape swimming in crayfish. In this study we used extracellular and intracellular stimulation, recording and dye-filling to systematically identify each of the five deep extensor excitors and single inhibitor of the crayfish, Cherax destructor. Functional associations of each neuron were characterised by recording its responses to sensory and abdominal cord inputs, its extensor muscle innervation pattern, and its relationships with other neurons. Each excitor receives excitatory input from the tonic abdominal stretch receptors and the largest neuron also receives input from the phasic stretch receptor. The two largest excitors innervate the muscle bundle containing the fastest fibres and may be electronically coupled. The smaller neurons may also be electronically coupled and innervate the remaining deep extensor fibres which display dynamic characteristics from fast to medium-fast. The inhibitor does not receive input from the stretch receptors, but is strongly excited by tactile afferents. The implications of these findings for the current models of the control of abdominal tailflips and swimming are discussed. Accepted: 21 June 1998     

The abdominal motor system of the crayfish, Cherax destructor. I. Morphology and physiology of the superficial extensor motor neurons

Using extracellular and intracellular stimulation, recording and dye-filling, we identified and studied the superficial extensor motor neurons of the crayfish, Cherax destructor. Functional associations of each neuron were characterised by recording its responses to sensory and abdominal cord inputs, its extensor muscle innervation pattern and its relationships with other neurons. Two clear associations were found among the six neurons of each segment. A medium-sized excitor (no. 3), that innervates a substantial percentage of extensor muscle fibres, and the largest excitor (no. 6), recruited during peak, excitation, were inhibited by input from unknown interneurons that excited the common inhibitor (no. 5). Likewise, these excitors received excitatory input when the inhibitor was silent. Another medium-sized neuron (no. 4) that innervates many muscle fibres was co-active with one of the small excitors (no. 2). The two medium-sized neurons were never active at the same time, and these two groupings may be determined by pre-motor interneurons. The implications of these findings for our understanding of motor control in this system are discussed. Accepted: 21 June 1998     

The structure and function of serially homologous leg motor neurons in the locust. II. Physiology

Simultaneous intracellular recordings were made from pairs of motor neurons in the pro- or mesothoracic ganglion of the locust. Though central connections were sought between pairs of motor neurons, none were found. This is in sharp contrast to the findings that flexor and extensor tibiae neurons in the metathoracic ganglion make certain connections between themselves (Hoyle and Burrows, 1973; Heitler and Burrows, 1977a). As the previously mentioned authors believed that the metathoracic flexor-extensor connections were used as part of the motor program for jumping and kicking, the present results strongly support their hypothesis. Common PSPs have been found in a variety of pairs of motor neurons. Of note are common PSPs of the same sign to antagonists. Different innervation patterns have been found for the flexor and extensor muscles. It is proposed that serially homologous motor neurons serving similar functions are, to a first approximation, similar in the locust. Serially homologous motor neurons serving different functions will, in most cases, have altered structures and/or functions.     

The structure and function of serially homologous leg motor neurons in the locust. I. Anatomy

Twenty-one prothoracic and 17 mesothoracic motor neurons innervating leg muscles have been identified physiologically and subsequently injected with dye from a microelectrode. A tract containing the primary neurites of motor neurons innervating the retractor unquis, levator and depressor tarsus, flexor tibiae, and reductor femora is described. All motor neurons studied have regions in which their dendritic branches overlap with those of other leg motor neurons. Identified, serially homologous motor neurons in the three thoracic ganglia were found to have: (1) cell bodies at similar locations and morphologically similar primary neurites (e.g., flexor tibiae motor neurons), (2) cell bodies at different locations in each ganglion and morphologically different primary neurites in each ganglion (e.g., fast retractor unguis motor neurons), or (3) cell bodies at similar locations and morphologically similar primary neurites but with a functional switch in one ganglion relative to the function of the neurons in the other two ganglia. As an example of the latter, the morphology of the metathoracic slow extensor tibiae (SETi) motor neurons was similar to that of pro- and mesothoracic fast extensor tibiae (FETi) motor neurons. Similarly the metathoracic FETi bears a striking resemblance to the pro- and the mesothoracic SETi. It is proposed that in the metathoracic ganglion the two extensor tibiae motor neurons have switched functions while retaining similar morphologies relative to the structure and function of their pro- and mesothoracic serial homologues.     

Extensor motor neurons of the crayfish abdomen

Summary The somata of five deep extensor motoneurons of the third abdominal ganglion of the crayfish(Procambarus clarkii) were located and identified. The positions of these somata within the ganglion and their distal distribution to muscles have been mapped and were constant. The soma of the extensor inhibitor was noted to touch the soma of the flexor inhibitor. Three of the excitatory neurons were clustered near their exit route.Sensory and cord routes of activation of the extensor motoneurons were also found and were constant from preparation to preparation. Sub-threshold recording showed that these motoneurons exhibited radically different types of post-synaptic response to stimuli at different sites in the nervous system. No interaction between extensor motoneurons or between the extensor and flexor motoneurons was observed.     

A novel nuclear structure containing the survival of motor neurons protein.

Spinal muscular atrophy (SMA) is a common, often fatal, autosomal recessive disease leading to progressive muscle wasting and paralysis as a result of degeneration of anterior horn cells of the spinal cord. A gene termed survival of motor neurons (SMN), at 5q13, has been identified as the determining gene of SMA (Lefebvre et al., 1995). The SMN gene is deleted in > 98% of SMA patients, but the function of the SMN protein is unknown. In searching for hnRNP-interacting proteins we found that SMN interacts with the RGG box region of hnRNP U, with itself, with fibrillarin and with several novel proteins. We have produced monoclonal antibodies to the SMN protein, and we report here on its striking cellular localization pattern. Immunolocalization studies using SMN monoclonal antibodies show several intense dots in HeLa cell nuclei. These structures are similar in number (2-6) and size (0.1-1.0 micron) to coiled bodies, and frequently are found near or associated with coiled bodies. We term these prominent nuclear structures gems, for Gemini of coiled bodies.     

The functional role of octopaminergic neurons in insect motor behavior

The role of efferent, octopaminergic dorsal unpaired median (DUM) neurons in insects is examined by recording from them during motor behaviour. This population of neuromodulatory neurons is divided into sub-populations which are specifically activated or inhibited during ongoing motor behavior. These neurons are always activated in parallel to the respective motor circuits, and in addition to their modulatory effects on synaptic transmission may also cause metabolic changes in their target tissues.     

Agrin-like molecules in motor neurons

According to the agrin hypothesis molecules that mediate the nerve-induced aggregation of acetylcholine receptors and acetylcholinesterase on developing and regenerating skeletal muscle fibers are similar or identical to agrin, a protein extracted from the electric organ of marine rays. Here we present evidence that agrin is highly concentrated in the cell bodies of motor neurons and is transported to axon terminals which is consistent with the agrin hypothesis.     

Experience-dependent development of spinal motor neurons

Locomotor activity in many species undergoes pronounced alterations in early postnatal life, and environmental cues may be responsible for modifying this process. To determine how these events are reflected in the nervous system, we studied rats reared under two different conditions-the presence or absence of gravity-in which the performance of motor operations differed. We found a significant effect of rearing environment on the size and complexity of dendritic architecture of spinal motor neurons, particularly those that are likely to participate in postural control. These results provide evidence that neurons subserving motor function undergo activity-dependent maturation in early postnatal life in a manner analogous to sensory systems.     

A gene expression fingerprint of C. elegans embryonic motor neurons

Background

ADP-ribosylation is an enzyme-catalyzed posttranslational protein modification in which mono(ADP-ribosyl)transferases (mARTs) and poly(ADP-ribosyl)transferases (pARTs) transfer the ADP-ribose moiety from NAD onto specific amino acid side chains and/or ADP-ribose units on target proteins.

Results

Using a combination of database search tools we identified the genes encoding recognizable pART domains in the public genome databases. In humans, the pART family encompasses 17 members. For 16 of these genes, an orthologue exists also in the mouse, rat, and pufferfish. Based on the degree of amino acid sequence similarity in the catalytic domain, conserved intron positions, and fused protein domains, pARTs can be divided into five major subgroups. All six members of groups 1 and 2 contain the H-Y-E trias of amino acid residues found also in the active sites of Diphtheria toxin and Pseudomonas exotoxin A, while the eleven members of groups 3 - 5 carry variations of this motif. The pART catalytic domain is found associated in Lego-like fashion with a variety of domains, including nucleic acid-binding, protein-protein interaction, and ubiquitylation domains. Some of these domain associations appear to be very ancient since they are observed also in insects, fungi, amoebae, and plants. The recently completed genome of the pufferfish T. nigroviridis contains recognizable orthologues for all pARTs except for pART7. The nearly completed albeit still fragmentary chicken genome contains recognizable orthologues for twelve pARTs. Simpler eucaryotes generally contain fewer pARTs: two in the fly D. melanogaster, three each in the mosquito A. gambiae, the nematode C. elegans, and the ascomycete microfungus G. zeae, six in the amoeba E. histolytica, nine in the slime mold D. discoideum, and ten in the cress plant A. thaliana. GenBank contains two pART homologues from the large double stranded DNA viruses Chilo iridescent virus and Bacteriophage Aeh1 and only a single entry (from V. cholerae) showing recognizable homology to the pART-like catalytic domains of Diphtheria toxin and Pseudomonas exotoxin A.

Conclusion

The pART family, which encompasses 17 members in the human and 16 members in the mouse, can be divided into five subgroups on the basis of sequence similarity, phylogeny, conserved intron positions, and patterns of genetically fused protein domains.     

Soldier-specific modification of the mandibular motor neurons in termites

Social insects exhibit a variety of caste-specific behavioral tendencies that constitute the basis of division of labor within the colony. In termites, the soldier caste display distinctive defense behaviors, such as aggressively attacking enemies with well-developed mandibles, while the other castes retreat into the colony without exhibiting any aggressive response. It is thus likely that some form of soldier-specific neuronal modification exists in termites. In this study, the authors compared the brain (cerebral ganglion) and the suboesophageal ganglion (SOG) of soldiers and pseudergates (workers) in the damp-wood termite, Hodotermopsis sjostedti. The size of the SOG was significantly larger in soldiers than in pseudergates, but no difference in brain size was apparent between castes. Furthermore, mandibular nerves were thicker in soldiers than in pseudergates. Retrograde staining revealed that the somata sizes of the mandibular motor neurons (MdMNs) in soldiers were more than twice as large as those of pseudergates. The enlargement of MdMNs was also observed in individuals treated with a juvenile hormone analogue (JHA), indicating that MdMNs become enlarged in response to juvenile hormone (JH) action during soldier differentiation. This enlargement is likely to have two functions: a behavioral function in which soldier termites will be able to defend more effectively through relatively faster and stronger mandibular movements, and a developmental function that associates with the development of soldier-specific mandibular muscle morphogenesis in termite head. The soldier-specific enlargement of mandibular motor neurons was observed in all examined species in five termite families that have different mechanisms of defense, suggesting that such neuronal modification was already present in the common ancestor of termites and is significant for soldier function.     

Electrical characteristics of neurons in the cat motor cortex

Electrical characteristics of motor cortical neurons were studied in acute experiments on immobilized cats. Values of the input resistances varied from units to tens of megohms (mean 11.11±3.93 MΩ). The threshold current is a hyperbolic function of input resistance of the corresponding neurons and negative correlation was found between the axonal conduction velocity and input resistance. The time constant (τ₀) of the membrane was 7.1±3.46 msec. A time constant τ₁, of 1.65±0.36 msec, could also be distinguished in some neurons. Electrotonic lengths of dendrites of the cortical neurons were calculated by the use of Rall's model: mean 3.66±0.94 (in units of length constant).