Numbers, distribution, and types of neurons in the pedal disk of Hydra based on a serial reconstruction from transmission electron micrographs

The numbers, distribution, and types of neurons in a pedal disk of Hydra littoralis were determined from electron micrographs of 567 serial sections approximately 0.12 micron thick. Of 248 neurons counted, we found 234 ganglion cells in the epidermis and 14 in the gastrodermis. No sensory cells with surface projecting cilia were observed in either epithelial layer of the foot region. We found ciliary structures in 196 (84%) of the epidermal neurons: 55 had a well defined cilium-stereociliary complex, 30 had a cilium lacking stereocilia, and 111 could not be classified. In contrast, 38 epidermal neurons lacked evidence of ciliary structures; 10 of the 14 gastrodermal neurons had one or more centrioles, some with an elaborate pericentriolar rootlet system, but no cilium or stereocilia. Neuronal perikarya could be classified into those with dense heterochromatic nuclei and those with light granular nuclei; often these two nuclear variations were observed in paired or triad arrangements of epidermal neurons. In addition, 68 (29%) of the epidermal neurons were characterized by the presence of small dense granules (115-178 nm in diameter) in the cytoplasm around the periciliary space. Although 32 pairs and 5 triads of contiguous neuronal perikarya were present in the epidermis, only two paired neuronal perikarya were present in the gastrodermis. The major concentration of neurons was approximately midway between the basal surface and the region of transition of epitheliomuscular cells into glandulomuscular cells. There was no evidence of large neuronal aggregations suggestive of ganglia in the pedal disk.     

Factors involved in the generation of tension during contraction to high potassium in the rat vas deferens

A large number of studies indicate that K⁺-induced contractions of smooth muscle depend on extracellular calcium. If these contractions depend exclusively on extracellular calcium then contractile responses to 140 mM K⁺, which are larger than the response to 35 mM K⁺, should be associated with a larger influx of ⁴⁵Ca. This is not the case in the vas deferens from reserpine pretreated rats. During a 2 min interval, ⁴⁵Ca influx induced by 140 mMK⁺ was identical to that produced by 35 mM K⁺. This suggests that a second mechanism may be involved in responses to high K⁺. Indeed, 140 mM K⁺ caused an approximately 300% increase above control in the formation of inositol trisphosphate (IP₃) in tissues prelabelled with ³H-myoionositol whereas 35 mM K⁺ did not increase IP₃. IP₃ is thought to cause the release of calcium from internal stores which is consistent with our finding of an increase in ⁴⁵Ca efflux into calcium-free medium from tissues prelabelled with ⁴⁵Ca and stimulated with 140 mM K⁺. Stimulation with 35 mM K⁺ did not influence ⁴⁵Ca efflux. We conclude that in the rat vas deferens high K⁺ promotes tension development by smooth muscle by a dual mechanism: influx of extracellular calcium and release of calcium from internal stores via a IP₃ mechanism.     

Ultrastructure of the tentacle nerve plexus and putative neural pathways in sea anemones

Abstract. Neurons of sea anemone tentacles receive stimuli via sensory cells and process and transmit information via a plexus of nerve fibers. The nerve plexus is best revealed by scanning electron microscopy of epidermal peels of the tentacles. The nerve plexus lies above the epidermal muscular layer where it appears as numerous parallel longitudinal and short interconnected nerve fibers in Calliactis parasitica . Bipolar and multipolar neurons are present and neurites form interneuronal and neuromuscular synaptic contacts. Transmission electron microscopy of cross sections of tentacles of small animals, both C. parasitica and Aiptasia pallida , reveals bundles of 50–100 nerve fibers lying above groups of longitudinal muscle fibers separated by intrusions of mesoglea. Smaller groups of 10–50 slender nerve fibers are oriented at right angles to the circular muscle formed by the bases of the digestive cells. The unmyelinated nerve fibers lack any glial wrapping, although some bundles of epidermal fibers are partially enveloped by cytoplasmic extensions of the muscle cells; small gastrodermal nerve bundles lie between digestive epithelial cells above their basal myonemes. A hypothetical model for sensory input and motor output in the epidermal and gastrodermal nerve plexuses of sea anemones is proposed.