The Neurophysiology of Touch149EpidermisDermisSubcutisRuffiniPacinianMerkelMeissnerFigure 6.1Vertical section of the glabrous skin of the human hand,
schematically demonstrating the locations of the four types of mechanore-
ceptors; the major layers of human skin are also shown.Source:After
Johansson & Vallbo (1983; Figure 4). Reprinted with permission.and adaptation responses to sustained and transient stimula-
tion (see Table 6.1).
Thetwofast-adaptingpopulations(FAunits)showrapid
responsestotheonset,andsometimestheoffset,ofskinde-
formation.Inaddition,FAI(fastadaptingtypeI)unitshave
verysmall,well-definedreceptivefields,whereasFAII(fast
adaptingtypeII)unitshavelargereceptivefieldswith
poorlydefinedboundaries.FAIunitsrespondparticularly
welltorateofskindeformation,andtheyarepresumedto
endinMeisner’scorpuscles.FAIIunitsrespondreliablyto
boththeonsetandoffsetofskindeformation,particularly
accelerationandhigher-derivativecomponents,andhave
beenshowntoterminateinPaciniancorpuscles.Thetwo
slow-adaptingpopulations(SAunits)showacontinuousre-
sponsetosustainedskindeformation.SAI(slowadapting
typeI)unitsdemonstrateastrongdynamicsensitivity,as
wellasasomewhatirregularresponsetosustainedstimula-
tion.TheyarepresumedtoendinMerkelcellneuritecom-
plexes(seeFigure6.1).SAII(slowadaptingtypeII)units
showlessdynamicsensitivitybutamoreregularsustained
discharge,aswellasspontaneousdischargesometimesin
theabsenceofskindeformation;theyarepresumedtoend
inRuffiniendings.Bolanowski,Gescheider,Verrillo,and
Checkosky(1988)havedevelopedafour-channelmodelof
mechanoreception,whichassociatespsychophysicalfunc-
tionswiththetuningcurvesofmechanoreceptorpopula-
tions.Eachofthefourmechanoreceptorsispresumedto
producedifferentpsychophysicalresponses,constitutinga
sensorychannel,sotospeak.
Response to thermal stimulation is mediated by several
peripheral cutaneous receptor populations that lie near the
body surface. Researchers have documented the existence of
separate“warm” and “cold” thermoreceptor populations in
the skin; such receptors are thought to be primarily responsi-
ble for thermal sensations. Nociceptor units respond only to
extremes (noxious) in temperature (or sometimes mechani-
cal) stimulation, but these are believed to be involved in pain
rather than temperature sensation.
Response to noxious stimulation has received an enor-
mous amount of attention. Here, we simply note that two
populations of peripheral afferent fibers (high-threshold noci-
ceptors) in the skin have been shown to contribute to pain
transmission: the larger, myelinated A-delta fibers and the
narrow, unmyelinated C fibers.
Mechanoreceptors in the muscles, tendons, and joints (and
in the case of the hand, in skin as well) contribute to the
kinesthetic sense of position and movement of the limbs.
With respect to muscle, the muscle spindles contain two
types of sensory endings: Large-diameter primary endings
code for rate of change in the length of the muscle fibers, dy-
namic stretch, and vibration; smaller-diameter secondary
endings are primarily sensitive to the static phase of muscle
activity. It is now known that joint angle is coded primarily
by muscle length. Golgi tendon organs are spindle-shaped
receptors that lie in series with skeletal muscle fibers. These
receptors code muscle tension. Finally, afferent units of the
joints are now known to code primarily for extreme, but not
intermediate, joint positions. As they do not code for inter-
mediate joint positions, it has been suggested that they serve
mainly a protective function—detecting noxious stimulation.
The way in which the kinesthetic mechanoreceptor units me-
diate perceptual outcomes is not well understood, especiallyTABLE6.1FourMechanoreceptorPopulations in the Glabrous Skin
of the Human Hand, with TheirDefining Characteristics
Adaptation Response
Fast; No response toSlow; Responds to
Receptive Fieldsustained stimulationsustained stimulation
Small, well definedFAISAI
Large, diffuseFAIISAII
Note:FA=fast adapting; SA=slow adapting; and I and II index types
within each classification.