Signal Detection

Detection of stimuli by mechanoreceptors is a three-step process: coupling, transduction, and encoding (French, 1988). Coupling refers to the deformation of the sensory neuron's dendritic membrane caused by movement of the hair in its socket, the inward movement of the cuticular dome in campaniform sensilla, or the stretching of the chordo-tonal sensillum. Coupling in Type II proprioceptors (stretch receptors and peripheral nerve nets) presumably also results in distortion of the neuronal membrane, though these systems are much less studied. Transduction is the generation, followed by its flow through the dendritic membrane, of the receptor (generator) current. It results from the stretching of the membrane and the opening of transduction (stretch-activated) channels contained therein. Studies with Drosophila mutants have elegantly demonstrated that the channels include specific proteins that serve as "gates" (Walker et al., 2000). When the gates are opened as a result of membrane distortion, K+ions rush in from the extra-dendritic space, creating the receptor current. Encoding, the final step, is the transfer of information from the sensillum to the central nervous system. In common with typical neurons, this is seen as a train of action potentials induced by the receptor current.

Hairs differ in their sensitivity; long, delicate hairs respond to the slightest force, even air-pressure changes, whereas shorter, thicker spines (sometimes called sensilla chaetica) require considerable force for stimulation. Associated with this varied sensitivity are differences in the electrophysiology of the hairs. Delicate hairs typically adapt quickly, that is, rapidly lose their sensitivity to a continuously applied stimulus. More strongly built hairs, however, adapt only very slowly. Most hairs respond to a stimulus only while moving and 379

are said to be "velocity-sensitive" and the response is "phasic." Such hairs are found on + + 1 ■ , TT-, -A A 1 + SENSORY SYSTEMS

structures that "explore" the environment. The remainder responds continuously to a static deformation ("pressure-sensitive" forms with a "tonic" response). These are found usually as hair plates, at joints or on genitalia, which gather information on position with respect to gravity or posture. In these situations they are serving as proprioceptors.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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