Distribution and behaviour of glabrous cutaneous receptors in the human foot sole

Abstract

To document the activity of cutaneous mechanoreceptors in the glabrous skin of the foot sole, tungsten microelectrodes were inserted through the popliteal fossa and into the tibial nerve of thirteen healthy human subjects. A total of 104 cutaneous mechanoreceptors were identified in the glabrous skin of the foot. This sample consisted of 15 slow adapting type I (14 %), 16 slow adapting type II (15 %), 59 fast adapting type I (57 %), and 14 fast adapting type II units (14 %). The location of the receptors and the outline of the receptive fields were determined by using nylon monofilaments perpendicularly applied against the surface of the skin. This revealed that the receptors were widely distributed without an accumulation of receptors in the toes. There were also larger receptive fields predominantly isolated on the plantar surface of the metatarsal-tarsal region of the foot sole. Furthermore, with the foot in an unloaded position, there was no background discharge activity in any of the cutaneous receptors in the absence of intentionally applied stimulation. These findings suggest that skin receptors in the foot sole behave differently from those receptors found on the glabrous skin of the hand. This may reflect the role of foot sole skin receptors in standing balance and movement control.

Figure 1. Multi-unit innervation territories in the foot sole

Fascicular receptive fields in the medial and lateral plantar nerves were mapped during multi-unit stimulation prior to single-unit recordings in all subjects. In the plantar surface of the foot, nine multi-unit regions were observed.

Figure 2. Types of SA responses in the foot sole

In response to a maintained force application, two slow adapting responses were observed in the foot sole. An example of the discharge activity and the location of the receptive fields for a SAI receptor (A) and a SAII receptor (B), as well as their adaptation properties are presented. Ten consecutive waveforms are aligned and overlapped to demonstrate that these were indeed single-unit recordings. It is important to note that there was no spontaneous activity in the absence of any intentionally applied stimulation in any of the cutaneous receptors in the foot sole.

Figure 3. Distribution of cutaneous mechanoreceptors in the foot sole

A, the receptive field for each receptor type in the foot sole is illustrated. The receptive field was outlined with a monofilament 4–5 times greater than the initial threshold value. B, the approximate position of the afferent unit in the foot sole for all receptor types is depicted. C, distribution of the total number of documented receptors and the accompanying threshold levels per unit in the foot sole (n = 104).

Figure 4. Measuring directional sensitivity of the SAII receptor in the heel

An example of directional sensitivity for one of the SAII receptors located in the heel is presented. A hand-held force transducer stretched the plantar skin of the foot in an attempt to ensure reproducible amounts of stretch. This figure shows the level of stretch applied to the skin and the corresponding discharge activity in response to anterior stretch (A), medial stretch (B), posterior stretch (C) and lateral skin stretch (D).