Labyrinthine Organs Research Articles

Morphology and vascular anatomy of the accessory respiratory organs of the air-breathing climbing perch, Anabas testudineus (Bloch).

The vascular organization and endothelial cell specialization of the air-breathing organs of Anabas testudineus were examined by light and scanning electron microscopy of fixed tissue and vascular corrosion replicas. The vessels supplying blood to the lining of paired suprabranchial chambers and the plicated labyrinthine organs within the chambers are tripartite, having a median artery and paired, lateral veins. Hundreds of respiratory islets, the functional units of gas exchange, cover the surfaces of both the chamber and labyrinthine organ. A median islet artery supplies the central aspect of each islet and gives rise to numerous short arterioles from which the transverse channels are formed. Transverse channels are parallel capillary-sized vessels that extend in two rows away from the medial arterioles and drain laterally into one of two lateral islet veins. Basally situated single rows of endothelial cells lining the transverse channels form thick, evaginated, tongue-like cytoplasmic processes that project freely into the lumen from the tissue side of the channel. Other thin, septate, cytoplasmic extensions of the same cells form valve-like septa that extend across the channel. Both the septa and tongue-like processes appear to direct the red blood cells to the epithelial side of the channel and thus decrease the diffusion distance between the air and red cell. A large sinusoidal space lies under the transverse channels and may support the channels and even elevate them during increased oxygen demand. The epithelium covering the transverse channels is smooth, which enhances air convection and minimizes unstirred layer effects. The epithelium between the channels contains microvilli that may serve to trap bacteria or particulates and to humidify the air chambers.

Scanning electron microscopy of the respiratory organs of juvenile and adult climbing perch,Anabas testudineus

The gills and air-breathing organs of fingerlings (1–2.5 g) and adult (30 g)Anabas testudineus have been studied by scanning electron microscopy. The labyrinthine organs bear only one or two leaf-like plates in fingerlings of 1–2.5 g body weight. In very small finglerlings (1 g) no proper development of respiratory islets could be seen on the labyrinthine plates. There is evidence of the suprabranchial chambers being used for aquatic respiration in juvenile stages. In adults the inhalent and exhalent apertures become well defined with shutters developed on the first pair of gills, when they become obligatory air-breathers.

Functional organization of the peripheral efferent vestibular system in the frog

The functional organization of the efferent vestibular system (EVS) was studied in the isolated frog labyrinth. To ascertain whether, besides the efferent branching fibres that innervate several end-organs, the EVS is also endowed with efferent non-branching axons which might control a given population of sensory units in each end-organ, the 8th nerve and one of its branchlets were electrically stimulated while recordings of the spontaneous activity arising from the different sensors were made by impaling single afferent axons in all the 8th nerve branchlets. The results demonstrated that the vast majority of the sensory units whose activity was modified by stimulating the whole 8th nerve was also affected by stimulating an 8th nerve branchlet. These findings therefore rule out the possibility that the EVS is endowed with projective fibres and strengthen the view that the EVS is a highly divergent system with collaterals arising from single parent axons that innervate several end-organs. These experiments have also shown that the percentage of sensory units which are actually controlled by the EVS varies amongst the different labyrinthine organs. It is maximal in the sacculus (ca. 90%), somewhat lower in canal organs (ca. 80%) and the utriculus (ca. 70%) and considerably lower in the lagena (ca. 50%). This EVS arrangement therefore might allow information arising from some organs to be modified more extensively than that from others.

Specificity of the K-channels in labyrinthine organs: effects of Rb+ and Cs+ on the conversion process in frog semicircular canals

The specificity of the K-channels which are present in the ciliated membrane of ampullar receptors has been investigated by replacing K+ in the fluids bathing the canal with Rb+ and Cs+. Results show that, unlike Cs+, Rb+ is able to substitute K+ in maintaining the receptor function. These findings favour the hypothesis that the transduction channels which allow the receptor current to flow across sensory cell bodies are specific K-channels. The effects of Rb+ and Cs+ on primary sensory neuron endings were also studied.

Peripheral organization of the vestibular efferent system in the frog: an electrophysiological study

The distribution and the properties of efferent fibers in vestibular nerve were studied in the isolated frog labyrinth. Electrical stimulation of the central stump of any vestibular nerve branchlet elicited compound action potentials in all the other eighth nerve branchlets, indicating the existence of neural links between the various vestibular organs. The same experimental paradigm, when repeated in frogs with chronic section of the eighth nerve roots, demonstrated that these pathways are efferent collaterals extending to all vestibular organs. There are more collaterals linking the 3 semicircular canals than the otolith organs and the otoliths with the canal organs. Efferent connections in the eighth nerve were preserved in full after ablation of the ipsilateral hemi-cerebellum, suggesting that the efferent pathways probably originate in the brainstem. Intracellular recordings from single afferent fibers of both canal and otolith organs revealed that efferent fiber activation could elicit either inhibition or facilitation of the receptor discharge. It was concluded that the frog efferent vestibular system is endowed with non-selective control channels which allow single neurons to influence the receptor activity of different labyrinthine organs.

Ionic Composition of the Endolymph and Sensory Transduction in Labyrinthine Organs

The role of various ions in stimuli conversion has been investigated in isolated frog semicircular canals, by replacing the endolymph with solutions deprived in turn of Na+, K+, Cl- and Ca++. Mechanical and electrical stimuli were employed and slow ampullar and nerve potentials were recorded, together with the afferent discharge of impulses in the eighth nerve. The results support evidence that K+ is the carrier of the receptor current, as postulated in Davis' hypothesis, while Ca++ is essential in the processes which allow the current to be modulated during cupula deflections and therefore endon hair cells with mechanical sensitivity.

Surface area of the respiratory organs of the Climbing perch, Anabas testudineus (Pisces: Anabantidae)

Measurements have been made of the surface area of the gills and accessory respiratory organs of Anabas in the weight range 1–120 g, and the data analysed with respect to body weight using logarithmic transformations. The slope of the regression line for total gill area (0–615) is less than that found in most fish, the number of secondary lamellae/mm decreased more rapidly with body weight than for most water‐breathing species (h = ‐0.152). The gill area of Anabas is relatively small but when the area of the accessory organs is added, the total respiratory area is of the same order as inactive water‐breathing fish. The regression coefficient for combined areas of labyrinthine organs and lining of the suprabranchial chambers (0.713) exceeds that for the gills and together with other evidence (including estimates of diffusing capacity from morphological measurements), indicates an increasing importance of air‐breathing of larger specimens. The average surface area of the accessory organs available for 1 ml of air within the suprabranchial chambers was found to be 2226 mm2.

Fine structure of the respiratory surfaces of an air-breathing fish, the climbing perch Anabas testudineus (Bloch).

THREE principal surfaces are used for respiratory gas exchange in the anabantoid fish: (1) secondary lamellae of the gills; (2) plates of the labyrinthine organs lying dorsal to the gills; and (3) the respiratory membrane lining the suprabranchial cavity which contains the labyrinthine organs. Secondary gill lamellae are the sites of exchange between the blood and water, but the labyrinthine plates and respiratory membrane function during exchange between blood and the air contained within the suprabranchial chamber.

The accessory respiratory organs of Anabas testudineus (Bloch) (Anabantidae, Pisces)

SUMMARY 1The accessory respiratory organs of Anabas testudineus consists of the following structures: (i) the supra-branchial chamber, (ii) the labyrinthine organ, and (iii) the respiratory membrane. 2The incomplete division of the supra-branchial chamber into outer and inner recesses, the structure and action of the shutter, the manner of communication between the two recesses and communication with the pharynx and opercular chamber by means of inhalent and exhalent apertures are described. 3The labyrinthine organ, made up of several saucer-shaped plates, is composed of and derived from modified, abbreviated and fused primary and secondary gill lamellae. The surface of the labyrinthine organ is divided into respiratory ‘islets’, and injected preparations have disclosed the presence of capillaries resembling those of a typical gill lamella. 4The respiratory membrane lining the supra-branchial chamber and the labyrinthine organ consists of vascular and non-vascular areas. The vascular part of the membrane comprises small and large ‘islets’ distributed over the surface. The presence of pilaster cells and the specialized character of the blood capillaries strongly suggests that the ‘islets’ are derived from the histological elements of the gill lamellae. The islets make intriguing configurations on account of the fusion of neighbouring primary gill lamellae and the flattening of the secondary lamellae. Associated with each islet are two efferent and one median afferent vessel, and several transverse channels interconnecting them. The non-vascular part of the membrane is represented by the irregular ‘lanes’ present between the islets. This represents the interlamellar areas between contiguous gill lamellae. 5The branchial, hyoid and mandibular muscles have become modified in connexion with the change-over from aquatic to aerial respiration. The adductor arcus palatini, a muscle of the hyoid segment, is modified to form the contractile floor of the anterior part of the respiratory air chamber. The branchial muscles associated with the first four gill arches and the respiratory sac have been studied and a list of them is given.