Bimodal sensilla

Fig. 6.1 Artist s drawing of a crayfish showing some locations for important chemosensory sensilla. Above the crayfish the antennule is where the unimodal (olfactory) chemosensory aesthetascs are situated. To the left of the crayfish on the tips of the walking legs, bimodal chemo- and mechanosensory sensilla may be present e.g., as hedgehog hairs (above left) and smooth, squamose setae (below left). Bimodal sensilla show a considerable structural variety and can also be found e.g., on the mouthparts of the animal (not shown). Drawing by artist Jorge A. Varela Ramos...

Fig. 6.2 Bimodal sensilla. (a) Hedgehog sensilla from the walking leg of Homarus gammarus. Scale bar 100 gm. (b) Antennules of Pacifastacus leniusculus with both unimodal chemosensory aesthetascs (arrows) and slender bimodal chemo- and mechanosensory sensilla (arrowheads ). Scale bar 50 pm. (c) Transverse section through the dendritic outer segments (dos) of a bimodal sensillum from the antennule of Lophogaster typicus. There are three gustatory cells (gu) and one mechanosensory cell (me). The latter has a dense array of microtubules. Scale bar 500 nm...

The bimodal sensilla are mainly involved in close-range location and evaluation of food, but may also be active when contact pheromones are used in social contexts (Bauer, Chap. 14). Bimodal sensilla are also found in insects, with a morphology slightly diverging from crustaceans (Zacharuk 1985). 

The matching dichotomy of sensilla construction and neuroanatomical organization of sensory neuropils suggests that in crustaceans chemical information is received and processed in two fundamentally different modes. One mode is Olfaction which we define as chemoreception mediated by the aesthetasc - OL pathway the second mode is Distributed Chemoreception, which we define as chemoreception mediated by bimodal sensilla on all appendages and the associated striated neuropils that serve as local motor centers. Distributed chemoreception not only comprises taste, which we define as contact chemoreception in the context of... 

The sensitivity of ORNs has not been analyzed systematically. One study found that the threshold concentration of some amino acid-sensitive ORNs ranged from 10 6 to 10 4 M with a working range of 1-3 orders of magnitude (Michel et al. 1993) suggesting that ORNs may be less sensitive than CRNs of neighboring bimodal sensilla. 

The aesthetascs are olfactory sensilla and usually all chemoreception mediated by the lateral antennular flagella (or the entire antennules) was attributed to them and hence considered as olfaction. This interpretation is flawed because other chemosensory sensilla are associated with the aesthetascs and likely the activity of dCRNs in these bimodal sensilla was analyzed in most axon recordings from the lateral flagella. 

The available data sets about the physiological properties of dCRNs and ORNs and about the functional organization of chemosensory pathways in the CNS are incongruent. While much more is known about the physiology of dCRNs of bimodal sensilla than of aesthetasc ORNs, the olfactory pathway in the brain is far better analyzed than any pathway receiving chemosensory input from bimodal sensilla. 

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