Review
The cutaneous sensory system
Francis McGlone a,
*, David Reilly b
a
Dept of Neurological Science, University of Liverpool, Clinical Sciences Centre for Research & Education, Lower Lane, Liverpool L9 7LJ, UK
b
Unilever Discover R&D, Colworth Science Park, UK
Contents
1. Introduction to the somatosensory system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
2. The peripheral nervous system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
2.1. Touch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
2.2. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
2.3. Pain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
2.4. Itch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
2.5. Pleasure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
2.6. Sensory transduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
3. The central projections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
3.1. Spinal cord . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
3.2. Brain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
1. Introduction to the somatosensory system
The primary sensory modality subserving the body senses is
collectively described as the somatosensory system. It comprises
all those peripheral afferent nerve fibers, and specialised receptors,
subserving proprioceptive (joint, muscle) and cutaneous sensitivity.
The former processes information about limb position and
Neuroscience and Biobehavioral Reviews 34 (2010) 148­159
A R T I C L E I N F O
Keywords:
Somatosensory
Cutaneous
C-fibres
CT-afferents
Touch
Temperature
Itch
Pain
Pleasure
A B S T R A C T
The cutaneous senses are traditionally thought to comprise four recognized submodalities that relay
tactile, thermal, painful and pruritic (itch) information to the central nervous system, but there is
growing evidence for the presence of a fifth modality that conveys positive affective (pleasant)
properties of touch. Cutaneous sensory channels can be further classified as serving predominantly
either discriminative or affective functions. The former provides information about the spatial and
temporal localisation of events on the body surface, e.g., the presence of an insect or the temperature of a
cold wind; and the latter, although widely recognised as providing the afferent neural input driving the
negative emotional experience of pain, is here posited to provide the afferent neural input driving the
positive emotional experience of affiliative touch as well. A distinction is made between the properties of
fast conducting myelinated afferents and those of slowly conducting unmyelinated afferents, with the
former subserving a sensory-discriminative role, and the latter an affective-motivational one. Here we
review the basic elements of the somatosensory system and outline evidence for the inclusion of the
`fifth' sub-modality, conveyed by low-threshold C-fiber mechanoreceptors as the counterpart of highthreshold
C-fiber nociceptors with both C-fiber systems serving opposing aspects of affective touch, yet
underpining a common mechanism for the preservation of self and species.
ß 2009 Published by Elsevier Ltd.
* Corresponding author. Tel.: +44 771 1152895.
E-mail address: francis.mcglone@liverpool.ac.uk (F. McGlone).
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muscle forces which the central nervous system uses to monitor
and control limb movements and to ensure that a planned action or
movement is executed fluently via elegant feedback and feedforward
mechanisms. This review paper will focus on sensory inputs
arising from the skin, namely cutaneous sensibility.
Sensory modalities operate within interconnecting, intermodal
and crossmodal networks, ensuring that interactions with the
environment are generally multisensory (see Calvert et al., 2004,
for review). Vision and hearing are classified as exteroceptive
senses and provide information that can be used to guide approach
or avoidance behaviours; olfaction is also able to provide such
information: think only of the smell of burning, or the aroma of
coffee. For many behaviours, a physical and/or chemical contact
sense is required in order to extract more information about
stimuli in the immediate environment, and the senses of touch and
taste provide this information. The cutaneous senses are classically
defined as including tactile, thermal, pain and itch sensing
submodalities, and there is growing evidence for an additional
cutaneous sensory channel that subserves positively affective
aspects of touch, such as those generated during grooming and
nurturing behaviours. `Touch' in this context is seen as interoceptive,
providing information about the homeostatic state of the
body, and even the sense of self (Craig, 2009). The skin is a highly
complex organ, innervated by a wide array of specialised sensory
neurones sensitive to heat, cold, pressure, irritation, itch and pain.
Touch is the first sense to develop in utero, Montagu (1978, p. 195)
reporting tactile responses to a hair stroking the cheek of a foetus
at around 8 weeks gestational age. Cutaneous sensitivity of the
embryonic body extends to the genital area by week 10, the palms
by week 11, the soles by week 12, the abdomen and buttocks by
week 17, and by week 32 every part of the body is responsive to the
gentle stroke of a single hair. This developmental hierarchy of
tactile sensitivity is reflected anatomically: the sites developing
cutaneous sensitivity first possessing the greatest number and
variety of sensory receptors in adults. Consequently, they are also
represented cortically with larger areas of primary somatosensory
cortex. In addition to demonstrating sensitivity to light touch,
prenates also respond to tissue harming stimuli. Giannakoulopoulos
et al. (1994) have reported that within 10 min of inserting a
hypodermic needle into a fetus's intrahepatic vein, for a transfusion,
there is a 590% rise in beta-endorphin and a 183% rise in
cortisol. This biochemical evidence of a physiological response to
nociception, and evidence that cutaneous C-fiber systems are
functional at a discriminative level at an early developmental
stage, raises the possibility that C-fiber systems are also functional
at an affectively positive level. The component of the cutaneous
senses that is relayed to the somatosensory cortex includes the
entire body from the neck down; sensations from the face are
relayed via cranial nerves, with both parts sharing a common
central organization. As with other sensory modalities, information
is relayed from entry level cortex to higher order neural
systems controlling perception, attention and emotion, as well as
systems that integrate this information with other sensory
modalities. This pattern of connectivity enables neural processing
systems to maximize information received from the senses about
the conditions in the external world.
2. The peripheral nervous system
The skin is the most extensive and versatile of the body's organs
and in a fully grown adult covers a surface area approaching 2 m2
.
Apart from its role as a sensory organ the skin contains in excess of
2 million sweat glands and 5 million hairs, that may be either fine
vellous types covering all surfaces apart from the soles of the feet
and the palms of the hands (glabrous skin). Skin consists of an
outer stratified squamous epithelium of ectodermal origin ­ the
epidermis ­ and an inner, thicker, supporting layer of connective
tissue of mesodermal origin--the dermis. The thickness of this
densely innervated layer varies from 0.5 mm over the eyelid to
>5.0 mm in glabrous skin. Afferent nerve impulses are conveyed
by fibers of primary sensory neurons located in trigeminal and
dorsal root ganglia, which are comprised of a heterogeneous
population comprising of cell bodies of all the peripheral afferents
innervating the skin. Efferent axons of dorsal root ganglia neurons
terminate in the skin where they innervate a variety of cutaneous
structures such sweat glands, hair follicles, Merkel cells, Meissner's
corpuscles and blood vessels. The nerve bundles course through
the dermis vertically, forming a horizontal sub-epidermal neural
plexuses before losing their Schwann cell covering at the dermoepidermal
junction and penetrating the epidermal basement
membrane, ascending between the keratinocytes and terminating
as free nerve endings. Cutaneous innervation consists mainly of
unmyelinated fibers, accounting for around 90% of all dermal nerve
fibers (Ebenezer et al., 2007).
2.1. Touch
Most primate research into skin sensory processing has focused
on the glabrous surface of the hand, in particular the digits, and a
description of this somatic site will provide for a general
understanding of somatosensation (Johansson, 1976; Vallbo
et al., 1979; Darian-Smith, 1984a,b; Willis and Coggeshall, 1991;
Gescheider et al., 1992; Greenspan and Lamotte, 1993). Of the four
`classical' submodalities of the somatosensory system the tactile
one subserves the perception of pressure, vibration, and texture,
and relies upon four different receptors in the digit skin: (1)
Pacinian corpuscles, (2) Meissner's corpuscles, (3) Merkel's disks,
and (4) Ruffini endings, collectively known as low-threshold
mechanoreceptors (LTMs), a class of cutaneous receptors that are
specialised to transduce mechanical forces impinging the skin into
nerve impulses (Fig. 1). The first two are classified as fast adapting
(FA) as they respond to the initial and final contact of a mechanical
stimulus on the skin, and the second two are classified as slowly
adapting (SA), continuing to fire during a constant mechanical
stimulus. A further classification relates to the LTM's receptive field
(RF), i.e., the surface area of skin to which they are sensitive which
is determined by the LTM's anatomical location within the skin.
Those near the surface, at the dermal/epidermal boundary
(Meissner's corpuscles and Merkel's disks) possessing small RFs,
and those lying deeper within the dermis, (Pacinian corpuscles and
Ruffini endings), having large RFs.
Psychophysical procedures have traditionally been used to
study the sense of touch and, as in hearing research where the
sensory receptor is another type of specialised mechanoreceptor,
differing frequencies of vibration are used to quantify the response
properties of this sensory system. George von Bekesy (1939) was
the first to use vibratory stimuli as an extension of his research
interests in audition. In a typical experiment participants would be
asked to respond with a simple button-press when they were just
able to detect the presence of a vibration presented to a digit
within one of two time periods. This two alternative forced choice
paradigm (2-AFC) generates a threshold-tuning curve, the slopes of
which provide information about a particular class of LTM's
response properties. As can be seen from Fig. 2, a `U'-shaped
function is generated, with detection thresholds increasing in
sensitivity as vibrotactile frequency increases to a `peak' at around
300 Hz, at which point the curve begins to increase again as
sensitivity decreases (Table 1).
By carefully controlling the stimulus parameters of the
vibrating probe (spatial configuration, frequency, amplitude,
duration and skin surface temperature) as well as the use of
various masking techniques, Bolanowski et al. (1988) proposed
F. McGlone, D. Reilly / Neuroscience and Biobehavioral Reviews 34 (2010) 148­159 149
that there are four distinct psychophysical channels mediating
tactile perception in the glabrous skin of the hand. This model
proposes that each psychophysically determined channel is
represented by one of the four anatomical end organs and nerve
fiber subtypes. Frequencies in the 40­500 Hz range provide a sense
of `vibration', transmitted by Pacinian corpuscles (PC channel or
FAII), Meissner corpuscles transmit a sense of `flutter' in the 2­
40 Hz range (NPI channel or FAI), while `pressure' is mediated by
Merkel's disks in the 0.4­2.0 Hz range (NPIII or SAI) and Ruffini end
organs produce a `buzzing' sensation in the 100­500 Hz range
(NPII or SAII). Neurophysiological studies have by and large
supported this model. See Table 2 for a summary of the properties
of these LTMs.
There have been relatively few studies of tactile sensitivity on
the hairy skin, the cat being the animal of choice for most of these
studies. Mechanoreceptive afferents (Ab fibers) have been
Fig. 1. A cross-sectional perspective of glabrous (A) and hairy (B) skin (with permission of the artist R.T. Verrillo).
F. McGlone, D. Reilly / Neuroscience and Biobehavioral Reviews 34 (2010) 148­159150
described that are analogous to those found in human glabrous
skin (FAI, FAII, SAI, SAII). Essick and Edin (1995) have described
sensory fibers with these properties in human facial skin, however,
the relationship between these sensory fibers and tactile perception
is still uncertain, and this is exemplified by the response
properties of SAI afferents. Harrington and Merzenich (1970)
reported that these afferents are responsive to levels of stimulation
that are below perceptual thresholds. Meanwhile, Jarvilehto et al.
(1976) describe high levels of activity in human hairy skin SAIs that
are not perceivable, in contrast to the responses of this class of
afferent in glabrous skin where SAI nerve activity is directly
correlated with a sense of pressure.
Sensory axons are classified according to their degree of
myelination, the fatty sheath that surrounds the nerve fiber. The
degree of myelination determines the speed with which the axon
can conduct nerve impulses, and hence the nerve's conduction
velocity. The largest and fastest axons are called Aa, and include
some of the proprioceptive neurons, such as the muscle stretch
receptors. The second largest group, called Ab, includes all of the
discriminative touch receptors being described here. Pain and
temperature include the third and fourth groups, A-d and C-fibers,
and will be discussed in Section 2.2 (see Table 2).
Vallbo and Johansson (1978) developed an electrophysiological
technique called microneurography to study the function of single
peripheral nerve fibers innervating the human hand, which has
provided a generally accepted model of touch that relates the four
anatomically defined types of cutaneous or subcutaneous sense
organs to their neural response patterns. Microneurography
involves inserting a fine tungsten microelectrode, tip diameter
<5 mm, through the skin of the wrist and into the underlying
median nerve, which innervates the thumb and first two digits. A
sensitive biological amplifier records and amplifies the spike
discharges conveyed by the axons and feeds these to a loudspeaker
to enable the experimenter to hear the spike activity and `hone-in'
on a single unit. Skilled manual micromanipulation of the
electrode, coupled with stroking across the hand to stimulate
LTMs, results first in a population response being recorded, i.e.,
neural activity in a nerve fascicle containing hundreds of
peripheral axons until finally, sometimes after many hours, a
single axon is isolated. At this stage the threshold force for
activation and receptive field (RF) of the single unit are mapped
with thin nylon filaments (Von-Frey hairs') and the unit subtype
(i.e. FA or SA) is identified. Once this stage is completed, a small
pulsed current of a few microamps (typically <7 mA) is delivered
to the nerve to provides additional perceptual confirmation of the
unit subtype. If, for example, a FA unit has been isolated,
microstimulation is perceived as a `flutter' or `vibration', depending
on the frequency of the electrical pulses, and is perceptually
localised to the previously mapped RF. Fig. 3 depicts the
relationships between RF, adaptation rate and unit type from
studies carried out on the human hand (Westling, 1986).
2.2. Temperature
The cutaneous somatosensory system detects changes in
ambient temperature over an impressively wide range, initiated
when thermal stimuli that differ from a homeostatic set-point
excite temperature specific sensory nerves in the skin (see
Ringkamp, in this issue). Within the innocuous thermal sensing
range there are two populations of thermosensory fibers, one
responding to warmth and the other to cold, and include fibers
from the Ad and C range. Specific cutaneous cold and warm
receptors have been defined as slowly conducting units that
exhibit a steady-state discharge at constant skin temperature and a
dynamic response to temperature changes (Hensel and Boman,
1960; Hensel, 1973). Cold-specific and warm-specific receptors
can be distinguished from nociceptors that respond to noxious low
and high temperatures (<20 8C and >45 8C) (Torebjo¨rk and Hallin,
1976; Campero et al., 1996), and also from thermo-sensitive
mechanoreceptors (Hensel and Boman, 1960; Konietzny, 1984).
Konietzny recorded from 13 cold-specific units in humans using
microneurography, and measured conduction velocities (CVs) in
the C-fiber range (0.43­2.04 m s1
). Serra et al. (1999) uncovered a
number of spontaneously active fibers with microneurography,
Fig. 2. Absolute detection thresholds for sinusoidal stimuli (from Bolanowski et al.,
1988) where it can be seen that as vibration frequency increases detection
thresholds decrease (note­log axis).
Table 1
Summarizes the major findings in Bolanowski et al. (1988) and previous work done by these researchers at the Institute for Sensory Research, Syracuse University (Verrillo,
1963; Gescheider et al., 1982, 1983, 1985).
Channel Pacinian NPI NPII NPIII
Frequency response 40­80 Hz 3­100 Hz 15­400 Hz <0.3­>100 Hz
Threshold (re 1 mm) <20 dB @ 300 Hz 28 dB @ 3 Hz 10 dB @ 300 Hz 28 dB @ 3 Hz
Sensation Vibration Flutter Not known Pressure
Temporal summation Yes No Yes No
Spatial summation Yes No Not known No
Receptor type FAI Pacinian corpuscle FAII Meissner's corpuscle SAII Ruffini end organ SAI Merkel's disk
Table 2
Summarizes the main characteristics of primary sensory afferents innervating
human skin.
Sensory afferent nerves
Class Modality Axonal
diameter
(um)
Conduction
velocity
(m/s)
Myelinated
Aa Proprioceptors from muscles
and tendons
20 120
Ab Low-threshold mechanoreceptors 10 80
Ad Cold, noxious, thermal 2.5 12
Unmyelinated
C-pain Noxious, heat, thermal 1 <1
C-tactile Light stroking, gentle touch 1 <1
C-autonomic Autonomic, sweat glands,
vasculature
1 <1
F. McGlone, D. Reilly / Neuroscience and Biobehavioral Reviews 34 (2010) 148­159 151
which were sensitive to small temperature changes and that were
described as cold-specific units, but all had CVs in the C-fiber range
(0.43­1.27 m s1
). Textbooks describe the cutaneous cold sense in
man as being mediated by myelinated A-fibers with CVs in the
range 12­30 m s1
(Darian-Smith, 1984a,b). However, Campero
et al. (2001) have found that either human cold-specific afferent
fibers are incompletely myelinated `BC' fibers, or are C as well as Acold
fibers, with the C-fiber group contributing little to sensation.
Duclaux et al. (1976) described `BC' fibers as having electrophysiological
and morphological properties of C-fibers in their
distal part of the axon process, and B fibers at their proximal end.
An example of a feature of these units can be seen in Fig. 4 where
the resting activity at room temperature (21 8C), which is
characterized by a low frequency discharge ($1 Hz), is suppressed
by the sudden warming of the RF and is increased by cooling.
Free nerve endings for cold-sensitive or warm-sensitive nerve
fibers are located just beneath the skin surface, and the terminals
of an individual temperature-sensitive fiber do not branch
profusely or widely. Rather, the endings of each fiber form a
small, discretely sensitive point, separated from the sensitive
points of neighboring fibers. The total area of skin occupied by the
receptor endings of a single temperature-sensitive nerve fiber is
relatively small ($1 mm in diameter) with the density of these
thermo-sensitive points varying in different body regions. For
example, there are approximately 20 cold points per square
centimeter in the lips, 4 in the finger, and less than 1 cold point per
square centimeter in trunk areas. There is also a differential
innervation of cold and warm neurons with at least 5 times as
many cold-sensitive points as warm-sensitive points. It is well
established from physiological and psychophysical testing that
warm- and cold-sensitive nerve fibers differ in both structure and
function.
2.3. Pain
Here we consider a system of peripheral sensory nerves that
innervate all cutaneous structures and whose sole purpose is to
protect the skin against potential or actual damage. These primary
afferents comprise Ad and C-fibers that respond selectively and
linearly to levels of thermal, mechanical and chemical intensity/
strength that are tissue threatening or damaging. This encoding
mechanism is termed nociception and describes the sensory
process detecting any overt, or impending, tissue damage (see
Auvray et al., in this issue). Pain is described in terms of an
`experience' rather than just a simple sensation. Within the
nociceptive system there are submodalities which are evident at
the peripheral anatomical level are evident with respect to the
degree of nerve fiber myelination (see Table 1). Ad fibers are thin
(1­5 um), myelinated axons of mechanical and thermal nociceptors,
with average CVs of 12 m/s. C-fibers are thin (<1 um),
unmyelinated, slowly conducting axons (<1 m/s). Mechanical
Fig. 3. The four types of low-threshold mechanoreceptors in human glabrous skin are depicted. The four panels in the centre show the nerve firing responses to a ramp and
hold indentation and in % the frequency of occurrence and putative morphological correlate. The black dots in the left panel show the RFs of Type I (top) and Type II (bottom)
afferents. The right panel shows the average density of Type I (top) and Type II (bottom) afferents with darker areas depicting higher densities (after Westling, 1986).
Fig. 4. Resting discharge of a C cold fiber at room temperature. (A) The resting
discharge is suppressed by warming of the receptive field (RF) from 31 8C to 35 8C.
(B) From a holding temperature of 35 8C, at which the unit is silent, activity is
initiated by cooling the RF to 31 8C. (Time bar: 5 s) (see Campero et al., 2001 for
single unit responses to a range of temperatures).
F. McGlone, D. Reilly / Neuroscience and Biobehavioral Reviews 34 (2010) 148­159152
nociceptors in the Ad range and possess RFs distributed as 5­20
small sensitive spots over an area approximately 2­3 mm in
diameter. In many cases activation of these spots depends upon
stimuli intense enough to produce tissue damage, such as a pinprick.
Ad units with a short latency response to intense thermal
stimulation in the range 40­50 8C have been described as well as
other units excited by heat after a long latency--usually with
thresholds in excess of 50 8C.
Over 50% of the unmyelinated axons of a peripheral nerve
respond not only to intense mechanical stimulation, but also to
heat and noxious chemicals, and are therefore classified as
polymodal nociceptors (Bessou and Perl, 1969) or C-mechanoheat
(CMH) nociceptors (Campbell et al., 1989). A subgroup of
polymodal nociceptors have been reported to respond to extreme
cold, however, many of these units develop an excitatory response
to cooling after prior exposure to noxious heat. A small number of
C-fibers have mechanical thresholds in the nociceptor range with
no response to heat while others have been found that respond
preferentially to noxious heating. RFs of these C-fiber units consist
of single zones with distinct borders and in this respect they differ
from Ad nociceptors that have multipoint fields. Innervation
densities are high and responses have been reported to a number of
irritant chemicals such as dilute acids, histamine, bradykinin and
capsaicin. Schmidt et al. (1995) described not only CMH responsive
units, but a novel class of C-fiber nociceptors responding only to
mechanical stimuli (CM), units responding only to heating (CH),
and units that were insensitive to mechanical and heating stimuli
and also to sympathetic provocation tests (CMiCHi). Some CM, CH,
and CMiCHi units can be sensitised to thermal and/or mechanical
stimuli after topical application of skin irritants such as mustard oil
or capsaicin--these units then acquire responsiveness to stimuli to
which they were previously unresponsive. Recruitment of these
`silent' nociceptors implies spatial summation to the nociceptive
afferent barrage at central levels, and may therefore contribute to
primary hyperalgesia after chemical irritation and to secondary
hyperalgesia as a consequence of central sensitisation (see below).
The axon terminals of nociceptive axons do not possess
specialised end organ structures and for that reason are referred
to as free nerve endings. This absence of any encapsulation renders
them sensitive to chemical agents, both intrinsic and extrinsic.
Inflammatory mediators released at a site of injury can initiate or
modulate activity in surrounding nociceptors over an area of
several millimetres leading to two types of hyperalgesia
responses--the phenomenon of increased sensitivity of damaged
areas to painful stimuli. Primary hyperalgesia occurs within the
damaged area while secondary hyperalgesia occurs in undamaged
tissues surrounding this area.
2.4. Itch
The sensation of itch has, in the past, been thought to be
generated by the weak activation of pain nerves, but with the
recent finding of primary afferent neurons in humans (Schmelz
et al., 1997; Schmelz, in this issue), and spinal projection neurons
in cats (Andrew and Craig, 2001), that have response properties
matching those subjectively experienced after histamine application
to the skin, it is now recognised that separate sets of neurons
mediate itch and pain, and that the afferent neurons responsible
for histamine-induced itch in humans are unmyelinated C-fibers.
Until relatively recently is was though that histamine was the final
common mediator of itch, but clinical observations in which itch
can be induced mechanically or is observed without an accompanying
flare reaction, cannot be explained as being mediated by
histamine sensitive pruriceptors. These observations support the
existence of histamine-independent types of itch nerves (Ikoma
et al., 2005) in which itch is generated, without a flare reaction, by
cowhage spicules. As with the existence of multiple types of pain
afferents, different classes of itch nerves are also likely to account
for the various experiences of itch reported by patients (Yosipovitch
et al., 2002).
2.5. Pleasure
It is generally accepted that human tactile sensibility is solely
mediated by LTMs with fast conducting large myelinated afferents
(as described above). However, in recent years a growing body of
evidence has been accumulating, from anatomical, psychophysical,
electrophysiological and neuroimaging studies, for the presence of
a population of C-fibers, found only hairy skin, that are neither
nociceptive nor pruritic, but that respond preferentially to low
force, slowly moving mechanical stimuli traversing their RFs.
These nerve fibers have been classified as C-tactile afferents (CTafferents),
and were first described by Johansson et al. (1988) using
microneurography. Evidence of a more general distribution of CTafferents
has subsequently been found in the arm and the leg, but
they have never been found in glabrous skin sites (Vallbo et al.,
1979). It is well-known that mechanoreceptive innervation of the
skin of many mammals is subserved by A-fiber and C-fiber
afferents (Zottermann, 1939; Bessou and Perl, 1969; Iggo and
Korhuber, 1977), but until the observations made by Nordin
(1990), C-fiber mechanoreceptive afferents in human skin
appeared to be lacking entirely.
The functional role of CT-afferents is not fully understood
(Mackenzie et al., 1975), but their neurophysiological response
properties, fiber class, and slow conduction velocities preclude
their role in any form of rapid mechanical discriminative or
cognitive tasks, and point to a more limbic function, particularly
the emotional aspects of tactile perception (Vallbo et al., 1993;
Essick et al., 1999). However, the classification of a population of
afferent low-threshold C-fiber mechanoreceptors responding
preferentially to low velocity and low force mechanical stimulation,
and the assignment of a functional role in human skin, has
only recently been achieved as described by Olausson et al., in this
special issue, and in Loken et al. (2009).
The recognition that cutaneous sensitivity can serve both
discriminative and affective functions is best exemplified in the
case of pain, in which two independent systems of cutaneous
nerves serve two very different qualitative perceptual and
emotional states, known as 1st and 2nd pain (Cross, 1994). The
former is experienced as sharp or pricking sensation and is
conveyed to the central nervous system by fast conducting
myelinated Ad afferents. 1st pain is responsible for controlling
withdrawal reflexes such as when a potentially tissue threatening
stimulus contacts the body surface. The sensations evoked by
transitory stimulation are experienced immediately and are
qualitatively devoid of any lasting emotional distress, serving a
primary discriminative function--something is damaging the skin.
The latter, 2nd pain, is conveyed to the central nervous system by
C-fibers and generates far more qualitatively complex and,
importantly, temporally delayed sensations and emotions, comprising
qualities such as dull, throbbing, radiating and burning
(Melzack, 1975). Stimulation of fast conducting A-fibers provides
information for discriminative purposes, whereas stimulation of
the C-fibers evokes emotional responses. Pain is defined as `an
unpleasant sensory and emotional experience associated with
actual or potential tissue damage, or described in terms of such
damage'. Use of the terms `sensory' and `emotional' refers to the
dual nature of pain, with descriptions like ``throbbing, prickly, hot,
dull'' referring to the sensory component of pain, and descriptions
like ``torturing, annoying, frightful, sickening'' referring to the
emotional qualities of pain. Pain is always a subjective psychological
state ­ see Auvray et al., this issue ­ and it is an accepted fact
F. McGlone, D. Reilly / Neuroscience and Biobehavioral Reviews 34 (2010) 148­159 153
that cutaneous `pain' has both sensory/discriminative and
affective/motivational properties ­ qualities that are critically
dependent upon two classes of peripheral afferents ­ myelinated
A-fibers and unmyelinated C-fibers. We propose that `touch' is also
characterised by sensory/discriminative and affective/motivational
components, and that there are two touch systems parallel
to the two pain systems. 1st touch is subserved by fast conducting
Ab afferents responsible for rapid identification of the physical
properties of a tactile stimulus. The sensory information is
primarily discriminative and non-emotional, conveying qualitative
states such as ``wet, hard, rough, etc.'' and is essentially `immediate'
in terms of conscious awareness. By contrast, 2nd touch, mediated
by slowly conducting mechanosensitive C-fibers (CTs), conveys
information related to tactile inputs associated with affiliative and
affective touch, such as those gentle and slow stroking touches
experienced during grooming or nurturing behaviours. Importantly,
as these inputs are conveyed via C-fibers, they do not reach
immediate conscious awareness, generating the temporally
delayed positive emotional attributes of touch in a similar manner
to the delayed onset and qualitative properties of 2nd pain.
Work is in progress to identify this class of C-fibers histologically,
with a study using the pan-neuronal marker PGP9.5 and
confocal laser microscopy to identify a population of free nerve
endings located solely within the epidermis that may represent the
putative anatomical substrate for this sub-modality (Reilly et al.,
1997). Recent evidence from Anderson's group has shown, in a
mouse model, a molecular genetic visualization of a rare subset of
unmyelinated sensory neurons that they suggested may detect
gentle touch (Liu et al., 2007). Using a genetically encoded tracer,
they found that Mas-related G protein-coupled receptor B4
(MrgprB4) marks a subpopulation of unmyelinated, non-peptidergic
sensory fibers that exclusively, and importantly in terms of
the human microneurographic data, only innervate hairy skin.
These fibers terminate in large arborisations similar in size and
distribution to human C-fiber tactile afferent's RFs, suggesting that
MrgprB4 may provide genetic access to these elusive neurons in
mice and enable the elucidation of their receptor molecular
neurobiology.
2.6. Sensory transduction
Signalling of stimuli such as touch, temperature, pain and
pleasure requires molecular recognition of stimulus and mobilization
of a response in the form of an electrical signal. However, the
relationship between stimulus and transduction pathway is
anything but simple, and it is clear that perception of a single
stimulus often requires several transduction mechanisms. Conversely,
a given protein can contribute to several senses, e.g., heat
and touch (Lumpkin and Caterina, 2007). The sensitivity of sensory
circuits is further influenced and tonally regulated by extrinsic (e.g.
UV radiation) and intrinsic (e.g. nerve growth factor) mediators.
Touch and tactile sensitivity require rapid and direct signalling
that is provided by ion channels via interaction with both
intracellular cytoskeletal and extracellular matrix proteins (Gillespie
and Walker, 2001). The key mammalian ion channel
candidates studied to date are the epithelial sodium channels
and the acid-sensing ion channels (ASICs), both of which belong to
the Degenerin/epithelial amiloride sensitive Na+ channel (DEG/
ENaC) superfamily of ion channels, reviewed recently by Bonsch
and Lewin (2006).
Although the fundamental role of ion channels as the molecular
basis of mechanotransduction was first established in the
nematode worm Caenorhabditis Elegans, and the fruitfly Drosophila
melanogaster, a related vertebrate channel from mammalian tissue
was later identified by Canessa et al. (1993). The first neural
channel identified was the brain sodium channel (BNC1, also
known as ASIC2), and it is one of several mammalian DEG/ENaC
channels known to form homo- and hetero-multimers and is the
basis of voltage-insensitive sodium channels which are expressed
in both small and large dorsal root ganglia sensory neurons (Price
et al., 2000). These amiloride sensitive sodium channels also
respond to protons, although when expressed in low-threshold
mechanoreceptors, the ASIC2 channels are not gated by low pH,
possibly due to the requirement for activation of intact cytoskeletal
support structures to allow gating. Likewise, in ASIC2 knockout
mice, phenotypic testing shows that only LTMs are affected (Price
et al., 2000; Driscoll and Tavernarakis, 2000; McIlwrath et al.,
2005).
Protons can activate members of the ASIC family, generating a
perception of sting and pain. However, other candidates have also
been identified in recent years. Stimuli such as temperature, pain
or chemical challenges acting on nociceptors are controlled
peripherally via a complex regulation of activity in another series
of ion channels, the thermoTRPs (Transient Receptor Potentials).
One of the earliest proteins associated with heat pain was the
vanilloid receptor subtype-1 (VR1, also referred to as TRPV1),
identified as the molecular target for the pungent irritant,
capsaicin (Caterina et al., 1997). TRPV1 is a classical cation channel
and is expressed in cutaneous sensory nerve fibers, mast cells and
epithelial cells of appendage structures (Stander et al., 2004).
Interestingly, activity for temperature (hot and cold), pain and
chemesthetic activity can all be explained in terms of the plasticity
of a family of thermoTRP cation channels (Montell et al., 2002)
which consist of 6-transmembrane polypeptide units that
assemble as tetramers to form cation-permeable pores (Clapham,
2003). The presence of multiple TRP channels, with distinct
localisation on sub-sets of C- and Ad-sensory neurons allows for a
wide spectrum of physiological activities to be regulated by these
channels and accounts, at least in part, for the complexity of these
transducer systems (Minke and Cooke, 2002). The gating
mechanism for stimuli such as radiant heat remains to be
elucidated, but again cytoskeletal components are believed to be
crucial for activation of these cation channels.
Development of transgenic mouse models lacking expression of
the VR1 gene shows that phenotypic characteristics in VR1 null (/
) mice support a functional role for VR1 in sensory transduction
of nociceptive stimuli, although it was apparent that other
receptors could partially compensate for the loss of VR1 function
(Caterina et al., 2000; Davis et al., 2000). As an understanding of the
process involved in sensing temperature and chemical stimulation
of nociceptors has evolved, it has become apparent that there are
additional non-TRP proteins and receptors which also play a role in
nociception, e.g., ASICs and the P2-X3 ATP receptor (Askwith et al.,
2001; Souslova et al., 2000).
A key question in recent years has been whether the sensory
neurons are the primary transduction element, or whether nonneuronal
cells can act as the key signalling pathway, with
subsequent activation of adjacent nerve terminals or neuronal
structures resulting in a perception of touch, temperature, pain, or
pleasure. Specialised epithelia structures such as hair cells, Merkel
cells, and receptors on taste buds are known to play a role in
sensory transduction, but recent evidence suggests that other
candidates such as keratinocytes may also be primary transducers
of mechanical stimuli (Lumpkin and Caterina, 2007). This
emerging hypothesis stems from the observation, typically by
immunohistochemical visualization, of mechano-, thermo- and
chemo-sensitive receptors such as TRPV1 on epidermal keratinocytes
(Inoue et al., 2002; Chung et al., 2004) and other nonneuronal
cell types. The presence of sensory receptors on
epidermal keratinocytes suggested a functional role in terms of
permeability barrier homeostasis and it has been shown that
TRPV1 agonists delay barrier recovery, whereas TRPV4 accelerates
F. McGlone, D. Reilly / Neuroscience and Biobehavioral Reviews 34 (2010) 148­159154
barrier recovery (Denda et al., 2007a). However, Denda et al.
(2007b) further suggested that keratinocytes could be the primary
transduction pathway, using signal transduction cascade mechanisms
such as intracellular Ca2+
fluxes to evoke a response in
adjacent C-fibers. Putative keratinocyte­neuron interactions,
intermediate molecules and 2nd messenger cascades have been
proposed and await validation (Lumpkin and Caterina, 2007).
A tonal balance in terms of mechanotransduction is achieved
via several interconnected mechanisms, e.g., modulation of growth
factors and receptors; 2nd messenger signalling pathways;
interaction with cytoskeletal elements; alteration of nerve firing
thresholds following presentation of the stimulus and consequent
perceptual processing (e.g. the increase in touch sensitivity and
hyperalgesia following inflammation reactions such as sunburn).
Without this intricate level of control the sensory system would be
swamped with redundant signals, or worse, would fail to recognise
noxious and threatening stimuli and would thus fail to act to
remove, neutralise or repair the threat. This ensures that at all
times an appropriate response is mounted by the organism,
whether it be in response to touch, temperature, pain or pleasure.
3. The central projections
The submodalties of skin sensory receptors and nerves that
convey information to the brain about mechanical, thermal, and
painful/pruritic stimulation of the skin are grouped into three
different pathways in the spinal cord and project to different target
areas in the brain. They differ in their receptors, pathways, and
targets, and also in the level of decussation (crossing over) within
the CNS. Most sensory systems en route to the cerebral cortex
decussate at some point, as projections are mapped contralaterally,
e.g., the discriminative touch system crosses in the medulla, where
the spinal cord joins the brain, while the affective pain system
crosses at the point of entry into the spinal cord.
3.1. Spinal cord
All the primary sensory neurons described above have their cell
bodies situated outside the spinal cord in the dorsal root ganglion,
with there being one ganglion for every spinal nerve. Unlike most
neurons the nerve signal does not pass through the cell body of a
sensory neuron: with the cell body sitting off to one side the signal
passes directly from the distal axon process to the proximal
process, which enters the dorsal half of the spinal cord.
Tactile primary afferents, or first order neurons, immediately
turn up the spinal cord towards the brain, ascending in the dorsal
white matter and forming the dorsal columns. In a cross-section of
the spinal cord, at cervical levels, two separate tracts can be seen:
the midline tracts comprise the gracile fasciculus conveying
information from the lower half of the body (legs and trunk), and
the outer tracts comprise the cuneate fasciculus conveying
information from the upper half of the body (arms and trunk).
Primary tactile afferents make their first synapse with second
order neurons at the medulla where fibers from each tract synapse
in a nucleus of the same name: the gracile fasciculus axons synapse
in the gracile nucleus, and the cuneate axons synapse in the
cuneate nucleus. The neurons receiving the synapse provide the
secondary afferents and cross the midline immediately to form a
tract on the contralateral side of the brainstem ­ the medial
lemniscus ­ which ascends through the brainstem to the next relay
station in the midbrain, specifically, in the thalamus.
As with the tactile system, pain and thermal primary afferents
synapse ipsilaterally and the secondary afferents cross, but the
crossings occur at different levels. Pain and temperature afferents
enter the dorsal horn of the spine and synapse within one or two
segments, forming Lissauer's tract. The dorsal horn is a radially
laminar structure, the thin outermost layer is the posterior
marginalis layer, the second layer the substantia gelatinosa, and
the layer medial to that, the nucleus proprius. The two types of pain
fibers, C and Ad, enter different layers of the dorsal horn. Ad fibers
enter the posterior marginalis and the nucleus proprius, and
synapse on a second set of neurons which are the secondary
afferents which relay the signal to the thalamus. The secondary
afferents from both layers cross to the opposite side of the spinal
cord and ascend in the spinothalamic tract. C-fibers enter the
substantia gelatinosa and synapse on interneurons--neurons
which do not project out of the immediate area, but relay to
secondary afferents in either the posterior marginalis, or the
nucleus proprius. The spinothalamic tract ascends the entire
length of the spinal cord and the entire brainstem, and on reaching
the midbrain is continuous with the medial lemniscus. These tracts
enter the thalamus together.
It is important to note that although the bulk of afferent input
adheres to the plan outlined above a degree of mixing occurs
between the tracts, for example, with some light touch information
traveling in the spinothalamic tract, with the result that damage to
the dorsal columns does not completely remove touch and
pressure sensation. Some proprioceptive information also travels
in the dorsal columns, and follows the medial lemniscus to the
cortex providing conscious awareness of body position and
movement. The pain and temperature system also has multiple
targets in the brainstem and other areas.
Having now covered the basic anatomy of the part of the
somatosensory system that serves the trunk and limbs, the
peripheral and central anatomy/neurophysiology of facial skin will
be briefly summarised here, as there are gross similarities in its
innervation. The trigeminal (Vth) nerve innervates all facial skin
structures (including the oral mucosa) and, as with the spinal
afferents, these neurones have their cell bodies outside of the CNS
in the trigeminal ganglion, with their proximal processes entering
the brainstem. As in the spinal cord, the four modalities of touch,
temperature, pain and itch have different receptors in the facial
skin, travel along different tracts, and have different targets in the
brainstem--the trigeminal nuclei extending from the midbrain to
the medulla. The large diameter (Ab) fibers enter directly into the
main sensory nucleus of the trigeminal nuclei and as with the
somatosensory neurons of the body, synapse and then decussate,
with the secondary afferents joining the medial lemniscus as it
projects to the thalamus. The small diameter fibers conveying pain
and temperature enter the midbrain with the main Vth cranial
nerve, but then descend through the brainstem to the caudal
medulla where they synapse and cross the midline. These
descending axons form a tract, the spinal tract of V, and synapse
in the spinal nucleus of V, so-called because it reaches as far down
as the upper cervical spinal cord, comprising three regions along its
length: the subnucleus oralis, the subnucleus interpolaris, and the
subnucleus caudalis. The secondary afferents from the subnucleus
caudalis cross to the opposite side and join the spinothalamic tract
where somatosensory information from the face joins that from
the body, entering the thalamus in a separate nucleus, the
ventroposterior medial nucleus (VPM).
In summary of this section, somatosensory paths are located in
the dorsal columns and spinothalamic tracts, with axons in the
former transmitting tactile, pressure, vibration and proprioception
impulses, and in the latter pain and temperature. Which pathway
CT-afferents travel in is not yet known, but low-threshold tactile
inputs to spinothalamic projection cells have been documented
(Applebaum et al., 1975), lending credence to reports of subtle,
contralateral deficits of touch detection in human patients following
destruction of these pathways after chordotomy procedures (White
and Sweet, 1969). As will be seen in the next section, we have better
knowledge of the cortical projections of CT-afferents.
F. McGlone, D. Reilly / Neuroscience and Biobehavioral Reviews 34 (2010) 148­159 155
3.2. Brain
Although we are only considering the peripheral to central
pathways of the somatosensory system, the thalamus, often called
the `gateway' to the cerebral cortex, also acts as a relay structure for
all other senses--even those from the archaic sense of smell pass
through this structure (Herrrero et al., 2002). It should be noted,
however, that the thalamus is not simply a `relay' structure: it plays
a major integrative role prior to projecting to the overlying primary
sensory cortices. Its sensory inputs are both discriminative and
affective, and, the thalamus is critical in adjusting affective scale, as
is evidenced by lesions of this structure causing chronic pain for
example (Jones, 2002).
The third order thalamocortical afferents (from thalamus to
cortex) travel up through the internal capsule to reach the primary
somatosensory cortex, located in the post-central gyrus, a fold of
cortex just posterior to the central sulcus (Fig. 5A).
The thalamocortical afferents convey all of the signals, whether
from VPL or VPM, to primary somatosensory cortex where sensory
information from all contralateral body surfaces is mapped in a
somatotopic (body-mapped) manner (Penfield and Rasmussen,
1952; Maldjian et al., 1999), with the legs represented medially, at
the top of the head, and the face represented laterally (Fig. 5B).
Within the cortex there are up to eight separate areas primarily
subserving somatosensation. Primary somatosensory cortex. SI,
comprising four sub-regions (2, 1, 3a and 3b), secondary
somatosensory cortex, SII, located along the superior bank of the
lateral sulcus (Woolsey, 1946; Maeda et al., 1999; Coghill et al.,
1994; Francis et al., 2000; McGlone et al., 2002), the insular cortex
(Schneider et al., 1993), and the posterior parietal cortex, areas 5
and 7b (Fig. 6). The secondary somatosensory cortex receives input
primarily from SI and in turn projects to the somatic sensory fields
in the insular region. Olausson et al. (2002) have also shown that
CT-afferents project to the dorsal posterior part of the insular,
presumably bypassing SI, since patients with no Ab afferents
demonstrate a lack of activation to brush stroking of hairy skin in
this region (see Olausson et al., in this special issue). In addition to
the primary and secondary somatosensory cortices, the posterior
parietal lobe also receives somatic inputs. This region is a higher
order sensory cortex, similar in function to an association cortex, it
relates sensory and motor processing and is concerned with
integrating the different somatic sensory modalities necessary for
perception.
As with studies of the peripheral nervous system, outlined
above, the technique of microneurography has again been used in
somatosensory research, in this case to study the relationship
between skin sensory nerves and their central projections, as
evidenced by the use of concurrent functional magnetic resonance
imaging (fMRI). Microstimulation of individual LTM afferents,
projecting to RFs on the digit, produces robust, focal and orderly
(somatotopic) haemodynamic (BOLD) responses in both primary
and secondary somatosensory cortices (Trulsson et al., 2000)--in
accordance with the findings of Penfield and Boldrey (1937). It is
expected that this technique will permit the study of many
different topics in somatosensory neurophysiology, such as
sampling from FA and SA mechanoreceptors and C-fibers with
neighboring or overlapping RFs on the skin, and quantifying their
spatial and temporal profiles in response to electrical chemical
and/or mechanical stimulation of the skin areas they innervate, as
well as perceptual responses to microstimulation.
Finally, the forward projections from these primary somatosensory
areas to limbic and prefrontal structures have been studied
with fMRI and PET in order to understand the affective
representations of skin stimulation for both pain and pleasure.
Evidence for the representation of pleasant touch in the brain has
been provided by Francis et al. (1999). They showed that the
Fig. 5. (A) Outline of the somatosensory pathways from the digit tip to primary
somatosensory cortex, via the dorsal column nuclei and the thalamus. (B) Penfield's
(Jasper and Penfield, 1954) somatosensory homunculus. Note the relative
overrepresentation of the hands and lips, and the relative under-representation
of the trunk and arms.
Fig. 6. Cortical areas subserving somatosensation. Primary somatosensory cortex
(SI) is located in the posterior bank of the central sulcus and the posterior gyrus and
comprises Brodmann Areas 2, 1, 3a and 3b. Secondary somatosensory cortex (SII) is
located in the upper bank of the lateral sulcus and comprises Brodmann Areas 43
with two further somatosensory regions in the posterior parietal cortex, Brodmann
Areas 5 and 7b.
F. McGlone, D. Reilly / Neuroscience and Biobehavioral Reviews 34 (2010) 148­159156
discriminative and affective aspects of touch are processed in
different brain areas, by stroking the body with either a wooden
dowel or a piece of velvet. Activation of primary somatosensory
cortex was found to be greater to the wood stimulus, whereas the
orbitofrontal cortex (an area of the frontal lobes involved in
emotion) was activated more by the velvet stimulus. This area has
also been shown to represent painful as well as pleasant touch,
demonstrating the relevance of this brain region for representing
the emotional dimensions of skin sensitivity--the positive and the
negative (Rolls et al., 2003).
4. Conclusion
This overview of the cutaneous senses provides a landscape
view of the system's structure and function, with the following
review papers in this Special Issue highlighting specific aspects and
properties of the skin senses and their roles in sensation, affect and
cognition. Cutaneous sensitivity is central to human functional,
emotional and social life, as is evidenced by it being the most
developed sensory modality at birth, contributing to brain and
cognitive development throughout infancy and childhood (Stack,
2001; Hertenstein, 2002), and continuing to play a vital role into
old age. That the skin senses can serve both a discriminative as well
as affective role is well known from our understanding of pain. The
different conduction speeds with which tissue-damaging cutaneous
sensations are conveyed to the CNS, by myelinated (Ad) and
unmyleninated (C) fibers, leads to the distinction of 1st and 2nd
pain, with the former having a discriminative quality and the latter
an emotional one. Here we have suggested a similar dualism for
touch with discriminative touch being conveyed by myelinated Ab
afferents, and emotional touch by CT-afferents leading to the
description of 1st and 2nd touch.
The results of anatomical, psychophysical, behavioural, neurophysiological
and neuroimaging studies have shown that separate
information processing channels, each with its own neurobiological
mechanism exist for the perception of tactile, thermal,
pruritic and painful stimuli. Evidence is also presented here (and
elsewhere in this Special Issue) for a specific `fifth' channel, coding
for the perception of the rewarding aspects of touch. However,
fundamental questions remain concerning the nature of how these
channels, with their individual properties, operate together in the
perception of the various stimuli naturally encountered by the
skin. Co-activation of channels is the norm: mechanical stimuli
also activate thermal channels, scratching reduces itch while
rubbing reduces pain, and with all forms of affective and affiliative
touch there is co-activation of mechanosensitive A-fibers as well
as, in hairy skin, mechanosensitive CT fibers. An adequate test of
the hypothesis that the perception of any complex cutaneous
stimulus involves the interaction of individual channels requires
that we fully understand the characteristics of each channel, and
determine the mechanisms by which they interact. Evidence for
such interactions driving the perception of complex skin sensations
comes from the early work of Bentley (1900), who showed
that the ``touch blend'' of pressure and coldness leads to an
emergent perceptual experience of wetness. The more recent
discovery of linear summation of perceived magnitudes from
different mechanosenstive channels provides further partial
confirmation of the hypothesis (Gescheider et al., 2003), but there
are many other possible ways in which these and the other
channels outlined in this paper could interact that must be
investigated before the hypothesis becomes a general principle of
cutaneous sensory information processing.
The musical analogy of a piano keyboard can best describe the
distinctions between activation of single channels, and coactivation
of a number of channels. Stimulation of a single SAI
channel (note on the keyboard) for example, such as occurs with
intra-neural microstimulation, leads to a distinct sensation of
pressure emanating from the RF of the unit. We know from the
work of Bentley (1900) that if a cold sensing unit were also able to
be co-activated (playing a second note on the keyboard), then a
`chord' is struck, perceptually, that relates to neither of the specific
channel's coding properties (pressure and temperature), but
generates the percept of wetness. The richness of perceived bodily
sensations (far more than a channel specific view would serve) is
dependent upon the diversity of the many channels of cutaneous
sensory input to the CNS, as well as to the integrative properties of
the various stages at which these inputs are processed, from the
dorsal horn to the sensory awareness stages in SI/SII, to the
affective representation in insula and orbitofrontal cortices. It has
been recognised for some time that the mind can affect the skin
(O'Donovan, 1927; Stokes and Beerman, 1940; Arck et al., 2006),
we are now recognising that the skin can affect the mind . . ..
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