The structure of tonic flexor motoneurons in crayfish abdominal ganglia

Abstract

1. The tonic flexor motoneurons were filled with cobalt dye via the cut ends of their axons. All six physiologically defined cells were identified anatomically (Figs. 2–4). 2. The cell somata are widely scattered in the ventral rind of the ganglia; three cells have ipsilateral and three cells have contralateral somata in reference to their axons; cells with contralateral somata tend to be more rostral in the ganglion (Figs. 2, 11). 3. All cells have bilateral dendritic domains (Figs. 5, 6, 8, 10). Each soma is connected to a long, thin neurite which travels dorsally and enlarges into a thick process (neuropilar segment) that crosses the midline in the posterior outer commissure (Figs. 8, 9) except for fl, which crosses anteriorly (Fig. 10). Many branches emerge from the neuropilar segment; the proximal portions of these branches and the neuropilar segment contribute to the coarse dorsal neuropile. Distal branches are found in the fine ventral neuropile (Figs. 9, 10). No recurrent collaterals were observed. 4. Most of the neuropilar segments converge into a narrow arc that sweeps across the dorsal neuropile (Figs. 8, 9). The close correspondence of the processes, especially of contralateral homologues (Figs. 10, 12) provides an anatomical basis for functional interactions among the tonic flexors. The main masses of finer processes that run along the longitudinal axis are located laterally. 5. Bilateral, serial and interanimal homologies of soma position all showed similar degrees of variation. Positions were relatively constant but might vary by up to 100 μ relative to external landmarks (Fig. 11). 6. Dendritic geometries were sufficiently similar to permit unambiguous identification of homologues, but variability in the number and shape of branches is common.