Sensory hairs

Deafness KCNQ4 is expressed in vestibular system, brain, and cochlea sensory hair cells. KCNQ4 has been... 

Host marking pheromones are important in many species of parasitic hymen-optera, because they ensure that a female parasitoid focuses on non-parasitized hosts. This, in turn, ensures a more effective use of limited host resources. Marking pheromones can be internal (injected into the host at the time of oviposition) or external (applied to the host during inspection and/or ovipo-sition). The internal markers can be detected by sensory hairs on the parasitoid ovipositor [11]. The internal markers often also delay the development of the host. 

Sidi, S., Friedrich, R W. and Nicolson, T. NompC TRP channel required for vertebrate sensory hair cell mechanotrans-duction. Science 301 96-99,2003. 

Nicolson, T., Rusch, A., Friedrich, R W., Granato, M., Ruppersberg, J. P., Nusslein-Volhard, C. Genetic analysis of vertebrate sensory hair cell mechanosensation the zebra-fish circler mutants. Neuron 20 271-283,1998. 

Ramprashad, F. and K. Ronald. 1977. A surface preparation study on the effect of methylmercury on the sensory hair cell population in the cochlea of the harp seal (Pagophilus groenlandicus Erxleben, 1977). Canad. Jour. Zool. 55 223-230. 

The mechanosensory systems of fish, including the lateral line, are closely related to the mammalian hearing system [12]. Besides possessing the typical vertebrate inner ear, fish possess the lateral line organs that contain sensory hair cells. These analogies are most relevant in toxicology and dmg discovery and evaluation, as some of the pharmaceuticals already detected in aquatic ecosystems as emerging pollutants affect the auditory function in humans. 

Seiler C, Nicolson T (1999) Defective cahnodulin-dependent rapid apical endocytosis in zebrafish sensory hair cell mutants. J Neurobiol 41 424-434... 

There is no term in equation (21.3) for air flow. External air flow around organisms is sufficiently slow (subsonic) that it may usually be treated as incompressible (Vogel, 1994). This incompressibility means that the concentration of chemical stimulus molecules (n) will not be increased noticeably by the pressures that, develop adjacent to insect sensory hairs or antennae due to moving air (or moving antennae). The replacement of any captured molecules by the arrival of fresh odorant-laden air is the primary reason why air flow has such a dramatic effect on interception rate. One way of considering the influence of air flow is that at best the air flow could bring the interception rate closer to the limit predicted by equation (21.3). In order to discuss approaches more complex than that provided by equation (21.3), we have to consider the physical bases for molecular movements diffusion and convection. 

Figure 21.1 Air flow is from left. (A) Air moving relative to a solid object (such as the sensory hair shown in cross-section) is slower closer to the surface of the object and is zero at the surface (the no-slip condition). The length and orientation of each arrow represent the magnitude and direction of the air velocity at the point in space at the base of the arrow. (B) Streamlines of the moving fluid are indicated by arrows. The path of a diffusing odorant molecule is pictured crossing the streamlines.

The apparent simplicity of equations (21.5)—(21.7) can be misleading the number of exact solutions is small, and while approximate solutions fill books (e.g. Crank, 1975), their application can be problematic. Seemingly small differences in the boundary conditions will completely change the character of the solution, as will be seen below, and identification of the appropriate boundary conditions is not easy. However, in practice one can make an educated guess about the approximate boundary conditions, or can estimate the interception rate for different kinds of possible boundary conditions. Below are a number of solutions that are the most useful for the geometry most relevant to insect antennae, the cylinder. Either sensory hairs or filiform antennae can be approximated as cylindrical in shape. 

Murray (1977) uses the same formula (equation 21.18) to predict interception rates of sensory hairs on insect antennae, but provides an additional insight in its application. As long as Pe < 1 (where L is hair diameter), the odorant molecules may be assumed to strike the hair anywhere. As Pe becomes larger (Pe >1), the interception rate will differ more with downstream/upstream location on the cylinder. The same logic may be used to estimate the potential for spatially dependent interception by a filiform antenna. 

Insect antennae vary tremendously in size and shape, but two common forms are filiform (a single cylinder) and pectinate (feathery arrays of many cylinders). Most of the published morphological information on insect antennae concentrates on sensory hairs (e.g. Zacharuk, 1985 Steinbrecht, 1987, 1999 Zacharuk and Shields, 1991). Descriptions of sensory hair morphology are necessary for biomechanical analyses and interpretation, but it is equally important to have... 

Consider sensory hairs 2 pm in diameter that are 20 pm apart. Inserting these values into equation (21.22) and assuming that D is 2.5 x 10 6 m2/s (the diffusion coefficient for bombykol, the main component of the commercial silkmoth sex pheromone Adam and Delbriick, 1968) results in the prediction that these hairs are likely to interfere with each other s odorant interception when the air speed between the hairs is below 0.0125 m/s. This is not a discontinuous function - the sensory hairs will interfere with each other more at slower speeds and less at faster speeds. Another way of appreciating what this means quantitatively is to recognize that the root mean square displacement of a molecule (considering movement in one dimension) is... 

A change in interception rate with air flow has been identified as a characteristic of flux detectors such as insect sensory hairs (Kaissling, 1998). However, in some instances (low Pe), an increase in flow will have only a negligible impact on the flux rate, even for a flux detector. This was shown by Berg and Purcell (1977) in the context of cells swimming through a liquid environment. That is, the flow rate doesn t matter if the molecules can walk themselves around just as quickly. For single cylindrical sensors (isolated hairs or filiform antennae), the slower the flow (the lower the Re and Pe), the less the interception rate is expected to increase with an increase in air speed (equations 21.18 and 21.20). 

Unfortunately, the air flow in the vicinity of these microscopic sensory hairs is extremely difficult to calculate. Cheer and Koehl (1987b) provide a solution for the flow field in the vicinity of two parallel and infinitely long cylinders. Even for this simple geometry, the solution (expressed as a stream function) has enough terms that it takes up most of a printed journal page, and the reader must differentiate the provided stream function with respect to the spatial variables in order to solve for the velocities at different points in space. Finite hairs usually experience less flow between them than predicted assuming infinite length because fluid can go around the tips as well as the sides of an array (Koehl, 2001). 

Antennal morphologies that slow the air flow in the vicinity of the sensory hairs, such as pectinate antennae that typically pass only about 10 percent of the approaching air, will distort the flow in a particular way - they will cause spreading (divergence) of the air stream as it approaches an antenna. This distortion of the air means that a small patch of odorant molecules will strike a much larger number of sensory hairs than would be predicted on the basis of the undistorted patch size (Loudon and Davis, unpublished). 

If a chemical stimulus consists of multiple chemical components which differ in the magnitude of their diffusion coefficients, it is of interest to consider whether or not these components will be intercepted by the sensory hairs at the ratios in which they are available in the air. This question may be addressed using the equations supplied above. 

Example calculation for diffusion of multiple chemicals Consider a mix of chemicals that differ greatly in molecular mass (and hence in their diffusion coefficients), such as a 3 1 ratio of ethanolihexadecanol in the air surrounding a sensory hair or filiform antenna. The 16-carbon alcohol will be approximately eight times as massive as the 2-carbon alcohol. The diffusion coefficients (D) are 1.32 x 10 5 m2/s for ethanol (Welty el al., 1984) and 2.5 x 10 6 m2/s for hexadecanol (using the value for bombykol), both in air at 298 K. What will the rate of interception be at the level of a sensory hair for these two chemicals The answer (and choice of equation) depends on the boundary conditions. 

If the sensory hair is suddenly immersed in a homogeneous air sample that contains the two chemicals, the ratio of the rates at which the hair takes up the two compounds will be directly proportional to the product of their molecular concentrations and the square root of the ratio of the diffusion coefficients (approximating using the first term in equation 21.16). That is, if ethanol and hexadecanol had similar diffusion coefficients, the 3 1 ratio in their molecular concentration would be reflected in an expected 3 1 ratio in interception by the hair. The diffusion coefficients actually differ by a factor of 5.3, and therefore the odorant with the smaller diffusion coefficient (ethanol, in this case), will be taken up at a rate of approximately 2.3 times what would be expected on the basis of their molecular concentrations. Thus, the 3 1 ethanol hexadecanol ratio would be expected to result in an interception ratio of 6.9 1. This boundary condition corresponds approximately to the case of a filiform antenna suddenly immersed in a cloud of odorant in still air. 

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