|
|
|
Properties of Touch Receptors in the Skin
The physics behind the sense of touch is poorly understood, and very little non-invasive research into the stimulation and distribution of touch receptors has been performed. The aim of this investigation is to image the Pacinian corpuscle touch receptors in human fingertips using Magnetic Resonance Imaging (MRI) techniques, in order to map their spatial distribution. Further experiments will include perceptual investigations involving stimulation of the skin by applying low-level vibrations from tactile arrays to the fingertip. At a later stage, quasi-static loads will be applied to the fingertip and the results imaged. A greater understanding of the physical processes behind signal generation in Pacinian corpuscles, together with more detailed information about tissue deformation during stimulation will be achieved as a result of these experiments. This will lead to a better appreciation of the sense of touch as a whole, with potential applications in the development of more sensitive tactile arrays, potentially leading to the creation of three-dimensional virtual environments.
|
|
|
fig. 2 Cross-section of a Pacinian corpuscle
(Adapted from an image from website www.scf-online.com)
Introduction
For centuries it has been known that there are specialised nerves and nerve endings in the skin, which are responsible for our perception of the sense of touch. However, very little is known about the physics of these nerve endings, or ‘touch receptors’.
There are four types of touch receptor in hairless skin: Merkel receptors, which are pressure and low-frequency vibration receptors, sensitive in the range 0.3–3.0 Hz; Ruffini corpuscles, which are thermoreceptors, sensitive to frequencies in the range 15-400 Hz; Meissner corpuscles, which are tactile and touch receptors for the frequency range 3-40 Hz; and Pacinian corpuscles which can detect pressure, acceleration and frequency over the range 20-1000 Hz although they are most sensitive to frequencies in the range 250-500 Hz (Lindblom, 1965; Talbot et al., 1968; Johansson et al.,1982; Vallbo and Johansson, 1984; Bolanowski et al., 1988).
Each receptor is sensitive to vibrations in a different range of frequencies. The receptors closest to the skin’s surface are sensitive to low frequency stimuli, whilst those located deeper in the skin are sensitive to higher frequency stimuli. When vibrations of sufficient amplitude and frequency reach the touch receptors, it triggers a sequence of events within their internal membranes that culminates in the generation of an electrical action potential. This action potential propagates along the nerve attached to the receptor and sent to the brain.
|
fig. 3 Distribution of Pacnian corpuscles in the left hand of a 76-year-old female specimen, B. Stark et al., 1998
Pacinian corpuscles have a central, unmyelinated neurite surrounded by 10-60 layers of lamellae (composed of flattened neurolemmocyte cells) that are separated by interstitial fluid and collagen fibres. Pressures applied to the skin’s surface are transmitted through the skin, reach the Pacinian corpuscles and deform the layers of lamellae and fluid. The resulting increase in pressure upon the neurite’s membrane alters the configuration of pressure-sensitive sodium channels located therein. This causes the channels to open, and positive sodium ions flow across them and into the neurite.
The influx of positive ions causes the transmembrane potential to become more positive. This increase in potential is known as a generator potential. Typical resting potentials (i.e. the normal potential measured across the membrane, maintained by the cell when no stimulus is present) in human nerves are ~-80mV. The greater the deformation of the corpuscle (i.e. amplitude of vibration), the greater the generator potential that is produced in the neurite’s membrane. Once the generator potential reaches a threshold level, a series of action potentials are generated at the node of Ranvier (fig. 2). Larger or more rapid stimuli cause action potentials to be generated at higher frequencies.
Of the four types of touch receptor in hairless skin, Pacinian corpuscles are the largest, ranging from 3-4mm in length in adult humans (Cauna and Mannan, 1958). They have an onion-like structure, and can be found deep in the subcutaneous fat (fig.2), surrounding nerve endings in clusters resembling bunches of grapes. The corpuscles tend to be aligned around a common axis within each cluster (Spencer and Schaumburg, 1973). Their presence has been noted in many areas of the human body; genitals (Shehata,1970), tendons (Rauber, 1865), near peripheral nerves (Shanthaveerappa and Bourne, 1963), near knee joints (Skoglund, 1956; Zimny and Wink, 1991), in the hands and feet etc. (J. Bell et al. 1993).
In human hands, the clusters of Pacinian corpuscles tend to be grouped most densely around the metacarophalangeal joints of the fingers (B. Stark et al. 1998). However, the Pacinian corpuscles are closest to the surface of the skin in the fingertips (fig.3).
|
|
|
|
fig. 4 T2-weighted images of phantom 1, (iii) photograph of phantom 1 blahbbbThe red circles indicate possible sites of Pacinian corpuscles
|
T2-weighted images of phantom 2
|
|
|
|
|
I got a first for my project, and my results were displayed in the form of a poster at the Enactive conference this year at Montpelier. My poster was selected to be in the best six!
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
