The angular acceleration receptor system of the statocyst of Octopus vulgaris : morphometry, ultrastructure, and neuronal and synaptic organization

The angular acceleration receptor system (crista/cupula system) of the statocyst of Octopus vulgaris has been thoroughly reinvestigated, and detailed information is presented regarding its morphometry, ultrastructure, and neuronal and synaptic organization. In each of the nine crista sections, some receptor hair cells are primary sensory cells with an axon extending from their base. Also, there are large and small secondary sensory hair cells without axons, which make afferent synapses with large and small first-order afferent neurons. The afferent synapses are of two morphologically distinct types, having either a finger-like or a flat postsynaptic process; both can be seen in the same hair cell. In addition to the afferents, there is a dense plexus of efferent fibres in each crista section, and efferent synapses can be seen at the level of the hair cells and of the neurons. The morphometric analysis of the nine crista sections shows obvious differences between the odd-numbered (C1, C3, C5, C7, C9) and the even-numbered (C2, C4, C6, C8) crista sections: they differ in length, in the number of the small primary sensory cells and in the number of the small first-order afferent neurons. Centrifugal cobalt filling of the three crista nerves revealed a disproportionate innervation of the nine crista sections: the anterior crista nerve innervates section C1 and the first half of section C2, the medial crista nerve innervates the second half of section C2, sections C3, C4, C5, and the first half of section C6, and the posterior crista nerve innervates the second half of section C6, and sections C7, C8 and C9. In each of the three crista nerves, only 25% of the total number of axons are afferent fibres, the remaining 75 % are efferent. To each of the nine crista sections a cupula is attached. In the form and size of the cupulae there is again a conspicuous difference between the odd and the even crista sections: a small widebased cupula is attached to each of the odd crista sections, whereas the even crista sections each have a large narrow-based cupula with a small area of attachment. The results are discussed with reference to their functional consequences.

Epithelial sensory structures of trochids

The ciliated epithelial sensory cells of the tentacle papillae and of the concave epipodial sense organ (e.s.o.) surface of the trochids Margaretes helicinus, Gibbula cineraria, G. umbilicalis, Monodonta lineata, Diloma nigerrima and D. zealandica are primary sensory cells with intra-epithelial nuclei. The 5–7 cells of a single papilla bear many cilia. Each e.s.o. cell has one cilium surrounded by 8–9 specialized microvilli. Both kinds of sensory cell show structural similarities with known mechanoreceptors.

Elektronenmikroskopische Untersuchungen am Geruchsorgan von Ophicephalus ( Channa) obscurus GUENTHER, 1861 (Teleostei, Ophicephalidae) / Electron Microscopical Studies of Olfactory Organ of Ophicephalus (Channa) obscurus GUENTHER, 1861 (Teleostei, Ophicephalidae)

The regio olfactoria of Ophicephalus was studied by transmission electron microscopy. Two types of receptor cells were found: ciliary receptor cells and microvillous receptor cells. Both of these types represent primary sensory cells. So one can regard the light microscopical findings of Kapoor and Ojha as disproved.

The Initiation and Conduction of Action Potentials in the Optic Nerve of Tritonia

1. The optic nerve of Tritonia contains axons of the five primary sensory cells. It joins a cerebral nerve about 2.0 mm from the eye and then travels another 2.5 mm to the central ganglia. 2. Large DC responses of positive polarity were recorded with suction electrodes in the presence of light. These graded responses are generator potentials passively conducted from a site of origin in or near the receptor somata. DC responses to light were not recorded at points central to the junction of the optic nerve with the cerebral nerve. 3. The shape of extracellular spike waveforms and the temporal relationship between soma and nerve spikes support the conclusion that action potentials are initiated in the optic nerve. In the,dark, spikes originate in portions of the nerve distant from the eye. When the eye is illuminated, the trigger zone shifts about 700 µm more proximal to the eye. 4. The shift in the spike trigger zone during illumination probably reflects an habitual accommodation of proximal portions of the nerve under the conditions of these experiments, and the prevalence of partially or completely silent optic nerves is probably due to more severe consequences of sustained depolarization. The sensitivity of the receptors, in combination with the passive properties of the nerve, makes the nerve susceptible to debilitating effects of maintained illumination. 5. The excitability of optic nerve fibres is extremely low. Absolute refractory periods are 25 msec, and relative refractory periods are as long as several hundred msec. When stimulated with just-suprathreshold voltages the nerve cannot support action potentials at frequencies greater than 1 Hz. 6. The Tritonia optic nerve appears to be transitional between transmission by graded responses and transmission by action potentials.