Extended Data Fig. 6: Details of the threshold mapping of mechanosensitive afferents.
From: Emergence of a brainstem somatosensory tonotopic map for substrate vibration
a-c, Mechanosensitive afferents were stimulated by applying a series of sinusoidal mechanical stimuli. These were presented at the receptive field center of individual units,at linearly increasing amplitude and with increasing frequencies (0.3–1600 Hz). Examples of responses evoked by full range of the stimulation of mechanosensitive afferents potentially innervating Merkel cells (a) and Meissner (b) and Pacinian corpuscles (c), respectively. The threshold was determined by the first spike. d, Frequency dependent threshold curves (mechanical activation threshold versus stimulation frequency) of the three mechanosensitive afferents above. Data represent the mean ± S.E.M. Trials were repeated for 5 times. The grey lines show the mean threshold curve of other nerve fibres. e, The threshold curves of all different groups of nerve fibres with a logarithmic scale. We found that mechanical thresholds were highly consistent within the groups, regardless of the receptive field of the afferent within the hindlimb. Plus signs (+) indicate the stimulation parameter specific to preferentially activate only one defined group of mechanosensitive end-organs: Merkel cell-like (MERK), 1 Hz, 300 μm; Meissner-like (MEIS), 30 Hz, 100 μm; Pacinian-like (PACI), 600 Hz, 3μm. Note that they were below the threshold of all the mechanosensitive afferents which were identified as hair follicles-like (HAIR) or proprioceptors (PROP), most likely muscle spindles. Hair follicle-like afferents can be identified by air puff and hair deflection, while proprioceptive afferents can be identified by moving the joints without touching the skin. Though our physiological recordings suggested our stimulation is not ideal to activate hair follicle afferents, we cannot fully rule out the possibility that hair follicle afferents were activated. For proprioceptive afferents with spontaneous firing rates, threshold was determined by the moment that the firing rate within 0.5 s bins exceeded 2 standard deviations above the spontaneous firing rate. Note that the frequency used for threshold mapping was sometimes different even within the same types of afferents, but the results were consistent. f, Functional clusters shown in a t-SNE space. g, Adaptation characteristics were defined by the rate of action potentials in response to a static indentation. The number of the mechanosensitive afferents or neurons is indicated on the bars. Middle groups of bars show the DCN neurons responding to only one of the stimuli defined in e.
