[Factors of parallelism in the sense organs]

Factors of parallel development of the gravity receptor, traced back in phylogenesis of vertebrates and invertebrates, are presented, and associations of the receptor with the nervous system and eye are considered. The parallel development is due to the similar, hereditary fixed behaviour of animals in relation to such a definite physical factor as the gravitational field of the Earth.

[Ultrastructure of the osphradium of Viviparus sp. (Mollusca, Prosobranchia)]

By means of scanning and transmissive electron microscopy, osphradial structure has been studied in the freshwater herbivorous mollusc Viviparus sp. Ciliary supporting and microvillous cells are described; they do not form any specific for the osphradium zones on its surface. The number of the villi, belonging to one supporting cell, is within 20-40. Peripheral processes of the receptory subepithelial cells have microvilli and 1-2 kinocilia on their apical surface. Central processes of the primary sensitive receptor cells go into the neuropil of the osphradial ganglion, that is situated in the center of the osphradial torus. Among uni- and multipolar ganglionic cells typical neurosecretory cells with electron opaque granules 200 nm in diameter are revealed. Release of neurosecretion takes place in the neuropil and in the connective tissue of the osphradial ganglion. A suggestion is made on modulating role of neuropeptides in the osphradial ganglion, that enables to change sensitivity of this receptor organ depending on physiological state of the animal and on the environment.

Plasticity and proprioception in insects. II. Modes of reflex action of the locust metathoracic femoral chordotonal organ

Reflex responses of tibial motoneurones were examined during mechanical stimulation of the femoral chordotonal organ, a joint angle receptor of the locust hindleg. Step displacements of the main ligament of the organ, mimicking 10–15 degree changes in joint angle, produced different patterns of discharge in motoneurones (1) when the leg was resting against a support and (2) when the support was removed to induce active searching movements. Tibial motoneurones showed resistance reflex responses to oppose the apparent joint movement when the leg rested against a support. Resistance reflexes consisted of constant, short latency excitatory responses followed by discharges that varied in intensity (gain) and degree of tonic coupling. These variations were not due to simple summation with other inputs to motoneurones. Responses changed during periods of active searching movements. Tibial flexor motoneurones fired phasically in response to apparent joint movement in any direction. Tibial extensor motoneurones were generally inhibited by chordotonal inputs. These reflex changes are not simple reflex ‘reversals’, but represent more complex changes in reflex mode. Potential functions of each of these reflex modes and the need for plasticity in reflexes of the chordotonal organ are discussed.

Plasticity and proprioception in insects. I. Responses and cellular properties of individual receptors of the locust metathoracic femoral chordotonal organ

The metathoracic femoral chordotonal organ is a joint angle receptor of the locust hindleg. It consists of 45–55 bipolar sensory neurones located distally in the femur and mechanically coupled to the tibia. Responses of receptors of the organ were examined by extracellular and intracellular recording. The organ as a whole encodes the angle of the femorotibial joint but shows substantial hysteresis. Tonic activity is greatest at the extremes of joint position. The organ possesses no direct linkage to tibial muscle fibres and shows no response to resisted muscle contractions in most ranges of joint angle. However, responses to extensor muscle contractions are obtained when the tibia is held in full flexion due to specializations of the femoro-tibial joint. These responses could be of importance in signalling preparedness for a jump. Intracellular soma recordings of activity in individual receptors indicate that the organ contains two types of receptors: phasic units that respond to joint movement and tonic units that encode joint position and also show some response to movement. All units are directionally sensitive and respond only in limited ranges of joint angle. Some phasic units increase firing frequency with increasing rate of movement and thus encode joint velocity. Other phasic units fire only single action potentials and can encode only the occurrence and direction of joint movement. All tonic units increase activity in the extremes of joint position and show substantial hysteresis upon return to more median positions. Direct soma depolarization produces different responses in different types of units: phasic receptors show only transient discharges to current injection; tonic receptors exhibit sustained increases in activity that are followed by periods of inhibition of background firing upon cessation of current injection. Receptors of the chordotonal organ are separable into two major groups, based upon their response characteristics, soma location and dendritic orientation: a dorsal group of receptors contains tonic units that respond in ranges of joint flexion (joint angle 0–80 degrees) and phasic units that respond to flexion movements; a ventral group of sensilla contains tonic units active in ranges of joint extension (joint angle 80–170 degrees) and phasic receptors that respond to extension movements. The response properties of these receptors are discussed with reference to the potential functions of the chordotonal organ in the locust's behavioural repertoire.

Pheromone receptors in Bombyx mori and Antheraea pernyi. II. Morphometric analysis

Sensilla trichodea of the silk moths, Antheraea pernyi and Bombyx mori, were reconstructed from serial sections after freeze substitution. The volume and surface area of the different sensillar cells were calculated from the area and circumference of consecutive section profiles. A. pernyi and B. mori differ largely in the size of the sensory hair and the larger outer dendritic segments as well as in the volume of the receptor lymph within the hair, while there are only small differences regarding inner dendritic segments, receptor-cell somata, trichogen and tormogen cells and the volume of the receptor lymph below the hair base. In each sensillum the two (or three) receptor-cell somata, dendrites, and initial axonal segments differ significantly in volume and surface. The apical cell membranes of the trichogen and tormogen cells, which border the receptor-lymph cavity and which are the presumed site of electrogenic cation pumps, are deeply invaginated and enlarged by microlamellae and microvilli, so that their area is twice that of the remaining basolateral cell membrane. In contrast to mechanoreceptors, the trichogen cell is the largest auxiliary cell and has the largest apical membrane surface. The morphometric data are discussed with regard to recent electrophysiological observations.

Pheromone receptors in Bombyx mori and Antheraea pernyi. I. Reconstruction of the cellular organization of the sensilla trichodea

The cellular organization of freeze-substituted antennal sensilla trichodea, which contain the sex pheromone receptors, was studied in male silkmoths of two species (Bombyx mori, Bombycidae; Antheraea pernyi, Saturniidae). The cellular architecture of these sensilla is complex, but very similar in both species. A three-dimensional reconstruction of a sensillum trichodeum of B. mori is presented. Two receptor cells (in A. pernyi 1-3) and three auxiliary cells are present. Of the latter, only the thecogen cell forms a true sheath around the receptor cells. A unique thecogen-receptor cell junction extends over the entire area of contact. Septate junctions occur between all sensillar cells apically, and in the region of the axonal origin basally. Gap junctions are also found between all cells except the receptor cells. The trichogen and tormogen cells show many structural indications of secretory activity and are thought to secrete the receptor lymph. Their apical membrane bordering the receptor-lymph space is enlarged by microvilli and microlamellae, but only those of the trichogen cell show regularly arranged membrane particles (portasomes), indicating secretory specialization among the auxiliary cells. Epidermal cells are found as slender pillars between sensilla, but extend apically along the non-sensillar cuticle and basally along the basal lamina.

Development of the lateral line system in Xenopus laevis. I. Normal development and cell movement in the supraorbital system

During development of Xenopus laevis, the supraorbital lateral line system (i.e. the parietal and supraorbital lines of organs and the anterior auditory group of organs) is all derived from a single primordium located in the ear region of the epidermis. The primordium elongates first by active movement along the dorsal margin of the eye. Individual primary organs are then formed by progressive fragmentation of the streak-like primordium. After fragmentation, passive displacement of the organs due to skin growth seems to play the main role in altering the arrangement of the line system. Transplantation experiments confirmed that non-placodal epidermal cells are not incorporated into the developing system. The active elongation of the primordium is due to cell multiplication, and not due to cell rearrangement or change in cell shape or size. Cell multiplication is not confined to a growth zone, but dividing cells are randomly distributed throughout the primordium. All cells of a primordium have to change position during its elongation.

Development of the lateral line system in Xenopus laevis. II. Cell multiplication and organ formation in the supraorbital system

Cell multiplication was studied during development of the supraorbital lateral line system in Xenopus laevis. The increase in cell number is biphasic. The first phase extends from the beginning of primordial elongation to the end of primary organ formation. Cell number increases linearly during this interval. Throughout this phase, a constant number of cells is in S phase of the cell cycle at a given time, despite a more than 10-fold increase in total cell number. After their formation, the number of the primary organs remains essentially constant. The individual primary organs are not clones of cells. Different organs grow at different rates, and become more and more heterogeneous in size. The second phase which is correlated with accessory organ formation is characterized by an elevated growth rate. This phase was not studied in detail. If developing larvae are starved, growth is normal up to completion of the first growth phase but is arrested at this point. The frequency distribution of the sizes of such growth-arrested organs approximates a binominal distribution. From its characteristics, a detailed model of cell proliferation and organ formation can be deduced: cell multiplication occurs through asymmetrically dividing stem cells, which become allocated to the forming organs at random and go through a fixed number of cell divisions.

Morphological basis for tonotopy in the anuran amphibian papilla

The amphibian papilla of the more-recently derived frogs and toads is similar to the mammalian cochlea in at least three ways: 1. Its sensory surface is a slender, curved structure. 2. It exhibits tonotopy, with the highest-frequency sensitivity located at the end apparently closest to the source of acoustical excitation. 3. Its single-axon tuning curves exhibit extremely steep high-frequency slopes and gentle low-frequency slopes, consistent with selection by a distributed-parameter, low-pass filter with cutoff frequency that decreases as one moves farther from the source of acoustical excitation. The filter operation in the cochlea is centered around the basilar membrane, a structure whose profound taper is largely responsible for the decreasing cutoff frequency. Although the amphibian papilla lacks a basilar membrane, it does possess a conspicuously tapered tectorial membrane, which might serve a similar function. In this paper, after reviewing briefly an old and simple model of the cochlear filter and the morphology of the amphibian papilla and its tectorial membrane, we are unable to reconcile the structure of the latter with the topological requirements for realization of a filter analogous to the former. In fact, we are unable to deduce the principles of frequency selectivity in the frog auditory endorgan.

Cupular secretion by Xenopus laevis line organs: autoradiographic evidence for incorporation of 3H-glucose and 35S-sulfate

Autoradiographic evidence for incorporation of 3H-glucose and 35S-sulfate into the cupulae of Xenopus laevis (African clawed toad) lateral line organs was obtained after injection into the dorsal lymph sacs of adult animals. Time intervals of 15 minutes to 4 hours after administration of these labeled metabolic precursors were used to examine the time course of the apparent mechanism of growth of the cupulae. Our results suggest that the two layers of accessory cells (the sustentacular cells and inner layer of mantle cells), concentrically arranged around the organ's central sensory (hair) cells, elaborate distinct cupular components. Sustentacular cells, immediately adjacent to the sensory cells, appear to produce and extrude at their exposed apices a cupular "core" substance labeled by 3H-glucose, but not by 35S-sulfate. The layer of inner mantle cells, external to the sustentacular cells, was labeled by both precursors and is spatially situated to secrete a cupular sheath enclosing the cupular core. Ultrastructural differences between the secretory products within the two cell types were marked. Electron microscopic autoradiography of toads killed 4 hours after 3H-glucose injection showed that silver grains were associated with accumulations of the respective secretory products in sustentacular and inner mantle cells, and label was found over the cupular trough area, where the bases of the cupulae are attached. These results suggest that the cupular core and sheath may both contain mucopolysaccharide, and the sheath, a sulfated mucopolysaccharide.

Sensory neurones recognise defined pathways in Drosophila central nervous system

An analysis of the central projection of various sensory neurones in the homoeotic mutant bithorax postbithorax, and in flies where an adult nerve had been experimentally misrouted, reveals that neurones are able to develop a normal projection even if they enter the central nervous system at an unusual place.

Neuronal and synaptic organization in the gravity receptor system of the statocyst of Octopus vulgaris

The neuronal and synaptic organization of the sensory epithelium (macula) of the gravity receptor system of Octopus vulgaris was investigated by serial electron microscopic reconstruction. Three different types of afferent neurons, unipolar, bipolar, and multipolar, are described. Afferent synapses exist between the secondary sensory cells (hair cells) and the afferent neurons. Consequently, the neurons are first-order neurons. Two morphologically distinct types of afferent synapses could be identified: the most common type, present on every hair cell, has a finger-like postsynaptic process; the second type, which does not occur on every hair cell, has a flat or somewhat curved postsynaptic process. As a rule, the hair cells each form synapses with more than one afferent neuron. The neurons, in turn, form synapses with more than one hair cell. A complicated arrangement of efferent synapses was found at the level of both the hair cells and the neurons. The results are discussed with reference to their physiological consequences.

The connections of the sensory organ of Bellonci with the brain in Isopoda (crustacea)

The peduncle linking the organ of Bellonci with the brain was examined in Sphaeroma serratum and Anilocra frontalis. This peduncle, in its extension to the brain, becomes a nerve-like tract with bundles of pedicles originating from the sensory cell bodies located in the organ of Bellonci. It ends at the level of the medulla interna in an alveolar region resulting from the swelling of the sensory pedicle terminations. At this level three types of connections have been observed. The first is characterized by afferent synapses to the brain with, in the sensory pedicle endings, structures similar to the presynaptic ribbons noted by some authors in photoreceptors of arthropods. The two other types include nerve fibres originating from the brain, one with small electron lucent vesicles, a second displaying larger vesicles with a core of medium density. These fibres form efferent synapses to the organ of Bellonci. The sensory differentiation of the organ of Bellonci in Isopoda is confirmed but its true role is not specified.