Computation predicts rapidly adapting mechanotransduction currents cannot account for tactile encoding in Merkel cell-neurite complexes
Fig 3
An ultra-slowly inactivating (USI) current is essential to drive the slow adaptation in firing in the sustained response.
In contrast, neither the modification of the SI and RI currents of generator function, nor the skin’s viscoelastic relaxation, adequately account for the slow adaptation in firing in the sustained response. Instantaneous firing frequency (IFF) plots over time are generated (panel B) similar to those observed in electrophysiological recordings (panel A). The data for wildtype animals were originally reported in Maksimovic, et. al. 2014 [3]. The two traces per plot represent two magnitudes of ramp and hold stimulation. The need for the USI component is shown in panel C. Without the USI component, the output IFF reaches a plateau and does not adapt as is typically observed for SAI afferents. Another potential way to achieve adaptation is to increase the time constant on the SI component of the model from 200 to 570 ms. However, the duration of this time constant is well outside observed bounds. For the case of the low magnitude stimulus, in panels D—F, increasing generator function parameters τRI, τSI, and KSI_Peak can increase receptor current in ramp-up, early hold, and late hold phases, respectively, as well as their corresponding IFFs (S2 Fig). They do not however impact the plateau in the sustained hold. In particular, current traces with different (D) τRI values show the impact upon the peak current produced, (E) τSI values show the impact during the early hold phase, and (F) KSI_Peak values show the impact of modulating the steady state magnitude relative to the peak. Note that in panels D—F, the USI component is included. In panel G, currents are shown that correspond to IFFs in panel C. Data similar to panel C, but for the case of high magnitude stimulation, are given in S3 Fig. The tau values are in units of ms.