Ewald Gingl

Infrared sensitivity of thermoreceptors.

Abstract. This study compares the effects of convective and radiant heat on the discharge rates of the warm cell of a thin hair-like sensillum of the tick and of the cold cells of small peg-shaped sensilla of the locust and the cockroach. The temperature rates imposed by the convective heat contained in the air stream used for stimulation are reflected by the discharge rate of the thermoreceptors. We determined the increment in radiant heat that results in the same change in discharge rate as a given increment in temperature due to convection. The amount of infrared radiation required to produce the same effect as a 1°C change in temperature differs for the sensory cells of the tick, locust and cockroach, respectively, suggesting differences in the ability of the sensilla to take up and transfer radiant heat. The power of radiation required to modulate the discharge rates is very high and outside the biologically meaningful range in all cases. Obviously the adequate stimulus for the examined sensilla is convective heat and not radiant heat.

Problems in Hygro- and Thermoreception

Hygroreceptors have been found only in insects and a spider. They associate in antagonistic pairs of a moist and a dry cell in the same sensilla with a thermoreceptor. In insect hygro-thermoreceptive sensilla all of the three cells respond to changes in humidity and two of the three cells respond in addition to changes in temperature. Assigning the individual cells to a particular modality thus becomes a problem. Structural features of thermoreceptors have a bearing on their sensitivity. The size of the dendritic membrane area is related to the sensitivity to slowly changing and steady temperatures. It may determine the number of molecular receptors which in turn sets signal-to-noise ratio. Sensitivity to rapid changes in temperature seems to be increased by positioning the dendritic tips above the surface of the body wall. This is believed to reduce damping of heat transfer. Structural diversity among insect and spider hygro-thermoreceptive sensilla suggests different transduction mechanisms. Three models for hygroreception are discussed. In mechanical hygrometers activity is initiated by swelling or shrinking in hygroscopic sensillum structures, whereas in psychrometers the degree of cooling due to evaporation is used to measure humidity. In electrochemical hygrometers humidity affects electrolyte concentration outside the dendrites.

Olfactory receptors on the cockroach antenna signal odour ON and odour OFF by excitation

A morphologically identifiable type of olfactory sensillum on the antenna of the American cockroach contains a pair of ON and OFF cells that responds oppositely to changes in the concentration of fruit odours. The odour of lemon oil was used to study the accuracy with which these cells can discriminate between rapid step‐like, ramp‐like and oscillating changes in odour concentration. The discharge rates of both cells are not only affected by the actual concentration at particular instants in time (instantaneous concentration) but also by the rate at which concentration changes. The impulse frequency of the fruit odour ON cell is high when odour concentration is high, but higher still when odour concentration is also rising. Conversely, the impulse frequency of the fruit odour OFF cell is high when odour concentration is low and higher still when odour concentration is also falling. Thus, the effect of odour concentration on the responses of both cells is reinforced by the rate of change. Sensitivity to the rate of concentration change becomes greater when the rate is low. Because of the high sensitivity to low rates of change, these cells are optimized to detect fluctuations in fruit odour concentration. Whereas the ON cell signals the arrival and presence of fruit odour, the OFF cell detects its termination and absence. These cells provide excitatory responses for both increase and decrease in fruit odour concentration and may therefore reinforce contrast information.

Intracellular recording from a spider vibration receptor

The present study introduces a new preparation of a spider vibration receptor that allows intracellular recording of responses to natural mechanical or electrical stimulation of the associated mechanoreceptor cells. The spider vibration receptor is a lyriform slit sense organ made up of 21 cuticular slits located on the distal end of the metatarsus of each walking leg. The organ is stimulated when the tarsus receives substrate vibrations, which it transmits to the organ’s cuticular structures, reducing the displacement to about one tenth due to geometrical reasons. Current clamp recording was used to record action potentials generated by electrical or mechanical stimuli. Square pulse stimulation identified two groups of sensory cells, the first being single-spike cells which generated only one or two action potentials and the second being multi-spike cells which produced bursts of action potentials. When the more natural mechanical sinusoidal stimulation was applied, differences in adaptation rate between the two cell types remained. In agreement with prior extracellular recordings, both cell types showed a decrease in the threshold tarsus deflection with increasing stimulus frequency. Off-responses to mechanical stimuli have also been seen in the metatarsal organ for the first time.

Adaptation as a mechanism for gain control in an insect thermoreceptor.

Adaptation controls the gain of the input-function of the cockroachs cold cell during slowly oscillating changes in temperature. When the oscillation period is long, the cold cell improves its gain for the rate of temperature change at the expense of its ability to code instantaneous temperature. When the oscillation period is brief, however, the cold cell reduces this gain and improves its sensitivity for instantaneous temperature. This type of gain control has an important function. When the cockroach ventures from under cover and into moving air, the cold cell is confronted constantly with brief changes in temperature. To be of any use, a limit in the gain for the rate of change seems to be essential. Without such a limit, the cold cell will always indicate temperature change. The decrease in gain for the rate of change involves an increase in gain for instantaneous temperature. Therefore the animal receives precise information about the temperature at which the change occurs and can seek an area of different temperature. If the cockroach ventures back under cover, the rate of change will become slow. In this situation, a high gain improves the ability to signal slow temperature changes. The cockroach receives the early warning of slow fluctuations or even creeping changes in temperature. A comparison of the cold cells responses with the temperature measured inside of small, cylindrical model objects indicates that coding characteristic rather than passive thermal effects of the structures enclosing the cold cell are responsible for the observed behavior.