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First published online February 12, 2007
Journal of Experimental Biology 210, 765-780 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02690

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Response properties of primary afferents supplying Eimer's organ

Paul D. Marasco¹ and Kenneth C. Catania^2,*

¹ Neuroscience Graduate Program, Vanderbilt University, Nashville, TN 37235, USA
² Department of Biological Sciences, Vanderbilt University, Nashville, TN 37235, USA

View larger version (48K): [in this window] [in a new window]   Fig. 1. The structure of Eimer's organ in the coast mole Scapanus orarius, viewed from the side (A) and from above (B). A central column of epithelial cells is associated with mechanoreceptive intraepidermal free nerve endings arranged in a ring (C-FNE and S-FNE). A second set of smaller nociceptive free nerve endings surrounds the central column (PER-FNE). Merkel cell–neurite complexes are arrayed at the base of the Eimer's organ (MC) and one to two lamellated corpuscles (LC) are found below the Merkel cell-neurite complexes. SC, stratum corneum. Scale bars, 20 µm. (Adapted from Marasco et al., 2006.)

View larger version (25K): [in this window] [in a new window]   Fig. 2. Single unit receptive field diagrams for three coast moles (left to right). SA, slowly adapting neurons; RA, rapidly adapting neurons; DS, directionally sensitive receptors.

View larger version (11K): [in this window] [in a new window]   Fig. 3. Peristimulus time histograms showing three receptor response profiles found during the quantitative phase of experiments in the coast mole. (A) Slowly adapting Merkel-like response with a high dynamic sensitivity and a random sustained discharge. (B) Rapidly adapting Pacinian-like (PC-like) response sensitive to changes in the dynamic phase of the indentation with no response to the static portion of the indentation. (C) Rapidly adapting response to the compressive phase of the indentation and silent through the static phase. This response is hypothesized to be from the intraepidermal free nerve endings. Imp, impulses.

View larger version (33K): [in this window] [in a new window]   Fig. 4. Peristimulus time histograms and electrophysiological traces showing responses from the star-nosed mole. (A) The receptive field (RF) for each cell is presented on a schematic of the mole's nose. (B–G) Three classes of receptor were evident, showing the following responses: (D–F) Merkel-like response; (C,G) Pacinian-like response; (B) an RA response to the onset of compression. This receptor was also directional. An arrow on the RF diagram shows the preferred direction. Imp, impulses.

View larger version (9K): [in this window] [in a new window]   Fig. 5. Plot of the frequency of maximum sensitivity for all RA cells. Two distinct populations were evident. One cluster of units was most responsive to sinusoidal compression at 250–300 Hz with displacement of 20 and 28 µm. A more diffuse cluster of units (lower right) was most responsive to sinusoidal compression ranging from 5-100 Hz at amplitudes from 100–485 µm. The upper left group was classified as Pacinian-like (PC-like) whereas the lower right group was classified as RA-unknown (RA-X). Two receptors were intermediate. Both showed peak activation at 150 Hz but one (cell 8-1) responded to a 10 µm displacement and the other (cell 14-2) responded at an amplitude of 85 µm. Inset 1 shows the response of 8-1 to static displacement in a PC-like manner. Inset 2 shows cell 14-2 was unresponsive to sinusoidal compression at 200 Hz and so was classified as RA-X.

View larger version (17K): [in this window] [in a new window]   Fig. 6. Composite drawing of the coast mole nose showing all of the RFs from the quantitative phase of the experiments. Each RF is labeled with its respective cell number and receptor sub-classification. PC-like, Pacinian-like; RA-X, rapidly adapting-unknown; RA-IC, rapidly adapting-incomplete classification.

View larger version (17K): [in this window] [in a new window]   Fig. 7. Receptor responses to indentation velocity related to the speed at which the mole touches its nose to the ground (46.2 mm s–1). (A) Lowest indentation velocity required to elicit a 1:1 for RA units (Left). A number of cells were most activated near the behaviorally relevant speed. The same graph is divided by sub-class on right. (B) Lowest velocity required to elicit any response. (Left) Most cells responded at less than 1:1 to the lower velocities. (Right) The same graph divided by sub-class. The PC-like units tended to be responsive at the lowest velocity and as such were more sensitive than the RA-X group. (C) Stimulus response relations for two cells that appeared to code indentation velocity (Left). As indentation velocity increased, the average interval frequency increased. Both cells appeared to be tuned near the behaviorally relevant velocity of 46.2 mm s–1. (Right) Responses during the ramp indentation. PC-like, Pacinian-like; RA-X, rapidly adapting-unknown; RA-IC, rapidly adapting-incomplete classification.

View larger version (11K): [in this window] [in a new window]   Fig. 8. Graphs of receptor response to multiple static displacement amplitudes for 13 RA receptors divided by subclass. The Pacinian-like (PC-like) group tended to be more sensitive to smaller displacements at absolute levels whereas the rapidly adapting-unknown (RA-X) group was not responsive to displacements smaller than 125 µm.

View larger version (15K): [in this window] [in a new window]   Fig. 9. Receptive fields of directional receptors isolated during the qualitative phase. The arrows indicate the preferred direction. Two cells appeared to be directional but were lost before a specific direction of highest activity could be established.

View larger version (44K): [in this window] [in a new window]   Fig. 10. Representative normalized and rotated circular histograms of directional activity for all three sub-classes of receptor (A–C). The receptive field (RF) and stimulus directions, including the direction of maximum activity (MAX) are depicted to the right (a1–d1). (A) Receptor 17-1 was representative of the Pacinian-like (PC-like) population. This receptor was moderately directionally tuned by the Rayleigh statistic but showed low scores for the tuning ratio (TR) and tuning index (TI) (see Table 1). (B) Receptor 7-16 was the only slowly adapting response and was strongly directionally tuned. (C) Receptor 13-1 was representative of the rapidly adapting-unknown (RA-X) population. This receptor was strongly directional and responses were inverted when the stimulator was rotated 180° (c2). (D) The most highly directionally tuned receptor found in the study. This receptor scored high on all measures of directionality and responses were inverted when the stimulator was rotated 180° (d2). Scale bars, 1 mm.

View larger version (10K): [in this window] [in a new window]   Fig. 11. A graphical comparison of directional tuning between the normalized and rotated average response magnitudes for the Pacinian-like (PC-like) and rapidly adapting-unknown (RA-X) populations. The RA-X population was strongly directionally tuned. The PC-like population was more broadly tuned than the RA-X population.

View larger version (22K): [in this window] [in a new window]   Fig. 12. Cycle histograms relating the precision of temporal patterning to vibrotactile stimuli at peak frequency of activation. (A) Representative cycle histograms from each of the three receptor classes (N=100). (B) Three units with the highest degree of phase locking (N=100). (C) There was adequate data present to calculate cycle histograms across each stimulus frequency for unit number 14-4. As frequency increased the receptor became less phase locked, however, all impulses fell within a 1 ms time frame. V, variance; R, length of mean vector.

View larger version (20K): [in this window] [in a new window]   Fig. 13. Receptive field (RF) diagrams for units that were visually attributed to Eimer's organs. (A) RF for an afferent covering a single Eimer's organ in the coast mole. (B) RF for an afferent covering four separate Eimer's organs in the coast mole (1–4). (C) RF for an afferent covering a single Eimer's organ in a star-nosed mole. In A and B the right panels show responses to stimulation of the RF with and insect pin attached to the Chubbuck stimulator. In C responses are shown to a hand-held probe.

View larger version (106K): [in this window] [in a new window]   Fig. 14. The hypothetical function of Eimer's organ. Each organ is depicted schematically as a stack of epithelial cells with the free nerve endings running up each side. This structural configuration may allow for a tactile `snapshot' each time the nose touches a surface. Maximally active free nerve endings are indicated with an arrow. (A) The Eimer's organs with no tactile stimulus. (B) Hypothetical response to small, spherical surface features. (C) Hypothetical response to larger contours. (D) The free nerve terminals at the surface of an Eimer's organ in the star-nosed mole revealed with DiI. Each satellite terminal is given a number to represent position. (E) Schematic representation of the numbered terminals color-coded and arranged in a hexagon. (F) A scanning electron micrograph of the surface of the nose of the star-nosed mole showing Eimer's organs in a hexagonal array. (G) Schematized Eimer's organs with color-coded nerve terminals in a hexagonal array. (H) Eimer's organs compressed by a cylinder and a sphere. The color of the deflected hexagon reflects the direction of maximal displacement. (I) The two objects have been made translucent. The deflected Eimer's organs generate a stereotypic output signaling the shape and contours of the applied stimulus.