Problems in Hygro- and Thermoreception
273
the base of the sensillum (Fig. 1A,C). With this electrode placement biphasic action potentials are recorded from the three sensory cells contained within the sensillum (Fig. IB). The sensillum is a small cuticular peg which is enclosed by a cuticular wall (Fig. 1 D). The dendrites of two sensory cells extend into the lumen of
the sensillar peg. The difference in their diameters is small, as are the amplitudes
of the recorded action potentials of two cells (Fig. 1 B). Two of the three cells are
bimodal, responding to changes in both temperature and vapor pressure. Stimulation is provided by three air streams emerging at 2.5 ms· 1 from jets 7 mm in diameter (Fig. 1 E). Each air stream can be directed separately onto the preparation.
Two of the air streams (A,B) are at the same temperature (T) but at different partial pressure of water vapor (Pw), another pair (A, C) is at the same Pw but at different T. A change from stream A to C produces a change in T at constant Pw, a
change from A to B produces a change in Pw at constant T.
Simultaneous recordings from the three receptor cells in the hygrothermoreceptive sensillum revealed that each cell responds to changes in Pw and
two of them to changes in T. Nevertheless the differences in the responses, especially in the direction of change in impulse frequency (F), provide criteria for their
individual identification. In Fig. IF I, when T rapidly decreases at constant Pw,
two cells respond with an increase in F. These two cells could be regarded as cold
cells. However, they could equally well be considered receptors for relative humidity (Hr). Because Hr increases when T decreases and Pw remains constant.
Rapid warming causes both cells to interrupt their discharge (Fig. 1 F 2). These responses likewise are consistent with both interpretations because the increase in T
implies a decrease in Hr. If the increase in F of these two cells is indeed a response
to an increase in Hr, one would expect that F increases when Hr increases as a result of an increase in Pw with T held constant. However, when Pw increases at
constant T, only one of the two cells responds as expected; impulse frequency of
the other cell decreases with this increase in Hr (Fig. IF 3).
These opposite responses reveal fundamental differences between the two cells.
If it should tum out that the responses to the two modalities are governed by a single parameter in each case, they could be regarded as unimodal. In fact, only one
of the two cells meets the criterion for a unimodal hygroreceptor. Unlike the first
cell, the second cell responds with an increase in impulse frequency to an increase
in Hr regardless of whether the increase in Hr is brought about by a decrease in T
at constant Pw or an increase in Pw at constant T. This cell is designated moist
cell.
Its function as a hygroreceptor, however, is conceivable only if it is capable of
taking up water from the air or giving it off to the air. The sensillum surface must
be hygroscopic. A hygroscopic surface of the sensillum is important for the interpretation of the responses of the first cell as well. This cell behaves like a cold cell
in that impulse frequency is increased upon cooling the sensillum and decreased
by warming it.
273
the base of the sensillum (Fig. 1A,C). With this electrode placement biphasic action potentials are recorded from the three sensory cells contained within the sensillum (Fig. IB). The sensillum is a small cuticular peg which is enclosed by a cuticular wall (Fig. 1 D). The dendrites of two sensory cells extend into the lumen of
the sensillar peg. The difference in their diameters is small, as are the amplitudes
of the recorded action potentials of two cells (Fig. 1 B). Two of the three cells are
bimodal, responding to changes in both temperature and vapor pressure. Stimulation is provided by three air streams emerging at 2.5 ms· 1 from jets 7 mm in diameter (Fig. 1 E). Each air stream can be directed separately onto the preparation.
Two of the air streams (A,B) are at the same temperature (T) but at different partial pressure of water vapor (Pw), another pair (A, C) is at the same Pw but at different T. A change from stream A to C produces a change in T at constant Pw, a
change from A to B produces a change in Pw at constant T.
Simultaneous recordings from the three receptor cells in the hygrothermoreceptive sensillum revealed that each cell responds to changes in Pw and
two of them to changes in T. Nevertheless the differences in the responses, especially in the direction of change in impulse frequency (F), provide criteria for their
individual identification. In Fig. IF I, when T rapidly decreases at constant Pw,
two cells respond with an increase in F. These two cells could be regarded as cold
cells. However, they could equally well be considered receptors for relative humidity (Hr). Because Hr increases when T decreases and Pw remains constant.
Rapid warming causes both cells to interrupt their discharge (Fig. 1 F 2). These responses likewise are consistent with both interpretations because the increase in T
implies a decrease in Hr. If the increase in F of these two cells is indeed a response
to an increase in Hr, one would expect that F increases when Hr increases as a result of an increase in Pw with T held constant. However, when Pw increases at
constant T, only one of the two cells responds as expected; impulse frequency of
the other cell decreases with this increase in Hr (Fig. IF 3).
These opposite responses reveal fundamental differences between the two cells.
If it should tum out that the responses to the two modalities are governed by a single parameter in each case, they could be regarded as unimodal. In fact, only one
of the two cells meets the criterion for a unimodal hygroreceptor. Unlike the first
cell, the second cell responds with an increase in impulse frequency to an increase
in Hr regardless of whether the increase in Hr is brought about by a decrease in T
at constant Pw or an increase in Pw at constant T. This cell is designated moist
cell.
Its function as a hygroreceptor, however, is conceivable only if it is capable of
taking up water from the air or giving it off to the air. The sensillum surface must
be hygroscopic. A hygroscopic surface of the sensillum is important for the interpretation of the responses of the first cell as well. This cell behaves like a cold cell
in that impulse frequency is increased upon cooling the sensillum and decreased
by warming it.