Abstract
The ON and OFF olfactory receptor neurons (ORNs) on the cockroach antenna display a high sensitivity for the rate at which odorant concentration changes. That rate of change acts as a gain control signal that improves the sensitivity of both ORNs for fluctuating concentration changes. By means of extracellular recording techniques, we find in both types of ORNs an increased gain for the rate of concentration change when the duration of the oscillation period increases. During long-period oscillations with slow concentration changes, the high gain for the rate of concentration change improves the ORNs ability to detect low rates of concentration changes when the fluctuations are weak. To be useful in plume tracking, gain control must be invariant to the air flow velocity. We describe that raising the level of the flow rate has no effect on the ON-ORN responses to concentration changes down to rates of 2%/s, but exerts a slight increase on the OFF-ORN response during these extremely low rates. At 4%/s, however, the OFF-ORN response is also unaffected by the flow rate level. The asymmetry corresponds with a generally higher sensitivity of the OFF-ORN to concentration changes. Nevertheless, the gain of both ORNs for the concentration rate change is robust against the air flow velocity. This makes possible an instantaneous analysis of the rate of concentration change for both directions of change by one or the other ORN. Therefore, the ON and OFF ORNs are optimized to encode concentration increments and decrements in a turbulent odorant plume.
Highlights
The primary objective of this study was to determine what effect the rate of the air flow carrying the odorant across a cockroach’s antenna has upon the activity of olfactory receptor neurons (ORNs)
This suggests that the ORNs are optimized to detect minute changes in odorant concentration, even if they persist in one direction
Insect olfaction is usually assumed to reflect the perception of the odorant by ORNs, but it is largely dependent on the air flow across the antenna
Summary
The primary objective of this study was to determine what effect the rate of the air flow carrying the odorant across a cockroach’s antenna has upon the activity of olfactory receptor neurons (ORNs). The work leading up to this study began with the identification of pairs of ORNs in a structurally identifiable sensillum type which respond antagonistically to the same change in odorant concentration. In this way, concentration increments and decrements are encoded by excitatory signals. When odor concentration changes slowly, both ORN types improve the gain for the rate of change at the expense of the gain for the instantaneous concentration. This suggests that the ORNs are optimized to detect minute changes in odorant concentration, even if they persist in one direction
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