Thermosensitivity and its possible fine‐structural basis in mechanoreceptors in the beak skin of geese
Authors: Gottschaldt Kay‐Michael , Fruhstorfer Heinrich , Schmidt Wolfgang , Kräft Ingrid 
Published: January 2005

An electrophysiological study on single afferent nerve fibers in the ophthalmic nerve of the goose revealed two kinds of temperature‐sensitive mechanoreceptors in the beak, which are innervated by large‐diameter myelinated axons. Some Herbst corpuscles, which display rapidly adapting responses to mechanical stimulation, discharge tonically to cooling the receptive field. A static response maximum occurs at temperatures between 15 and 25°C. The vibration sensitivity of Herbst corpuscles decreases with temperature. Many slowly adapting Ruffini endings also respond with sustained discharges to cooling but behave like rapidly adapting mechanoreceptors on warming the receptive field. The temperature at which the maximal static response occurs in different slowly adapting units varies between 10 and 30°C.

The fine structure of Herbst corpuscles and Ruffini endings was investigated electron microscopically in order to elucidate possible morphological substrates for the specific functional properties of the two receptor types. Both Herbst corpuscles and Ruffini endings are characterized by a distinct but different combination of nervous and nonnervous (auxiliary) tissue elements and by the occurrence of special structural units at the sensory nerve endings. Such “transducer sites” presumably correspond to those areas of the receptor membrane where the mechanoelectric transduction process takes place. The most prominent constituent of a transducer site is a spurlike axon process, which in the Herbst corpuscle projects between the lamellae of the inner core, and in the Ruffini endings comes into contact with collagenous microfibrils, either directly or indirectly through the mediation of Schwann cell processes.

Two morphological variants of Ruffini endings were recognized and were found to occur at different locations in the dermis underlying the horny covering of the bill tip. One type is distinguished by the presence of a specialized terminal cell making contact with the initial branches of the arborizing receptor axon. The other type of Ruffini ending lacks the terminal cell.

The structure‐function relations in Herbst corpuscles and Ruffini endings are discussed on the basis of the electrophysiological and ultrastructural observations. It is argued that the different response characteristics of both receptor types to mechanical stimuli result from the specific kind and arrangements of the auxiliary receptor structures and from the mechanical properties of the latter. On this basis the thermosensitivity of Herbst corpuscles and Ruffini endings can be explained as a consequence of the differential effects which temperature changes have on one hand on the auxillary receptor structures and on the other hand on the mechanically elicited receptor potential. It is concluded that the temperature sensitivity of some types of mechanoreceptors may be a secondary result of that differentiation of the auxiliary receptor structures which makes the discriminative function of the different types of mechanoreceptors possible and, therefore, does not necessarily conflict with the principle of modality specificity in sensory receptors.