Chapter 13: Touch
Touch: The skin-based receptor system. The entire surface of the body on which there
is living tissue (skin) is a potential receptive surface for the touch system. However, the
most active and sensitive part of this receptive field are the hands. In a sense, the two
hands are to the touch system, what the two foveas are to the retinas of the visual
system.
Haptics : active exploratory touch strategies for acquiring information from an object.
Haptics includes not only touch information but also kinesthesis (information about
the movement and location of the limbs and digits)
Measuring touch sensitivity
Von Frey hairs: small hairs, like those found on a paint brush, of various diameter
are pressed against different parts of the body to see if it is felt. The thicker the
diameter hair needed to get a response, the less sensitive the area.
Not surprisingly different parts of the body vary in sensitivity to touch, lips and
hands are highly sensitive, back and buttocks, much less so. Furthermore, for any
area of body, females tend to be more sensitive than males.
Measuring touch sensitivity
Two point thresholds: a little different procedure, but same general results. A
compass-type instrument is used which has two adjustable points, points can be
set a different distances from each other. Sensitivity is measured by determining
how far apart points must be sent before sub. can detect that there are two points
stimulating skin, not one.
Physiology of touch
Skin receptors: at various depths
under the skin are the
mechanoreceptors, which start the
process of analyzing skin sensations
by responding to indentation or
pressure on the skin.
In order of depth, nearest to
surface to deepest:
a) Meisnner Corpuscles: give
strongest response to transient
stimulation such as a finger rubbing
over a surface. RA-P
b) Merkel Disks: give strongest
response to steady pressure by
small object. SA-P
c) Ruffini Endings: give greatest
response to fairly strong, steady
pressure. Are also quite sensitive to
movements which result in
stretching of skin. SA-D
d) Pacinian Corpuscles: respond
best to initiation and termination of
diffused pressure against skin. RA-D
Physiology of touch
Nerve Fibers: afferent fibers travelling from skin receptors to spin
and (for some) eventually brain. These fibers are of four distinct
categories.
a) Slowly Adapting: fibers which carry messages about steady
pressure on skin. Not surprisingly these fibers are connected to
Mekel disks and Ruffini Endings in skin.
b) Rapidly Adapting: carry message about transient pressure
changes on skin. connected to Miessner and Pacinian Corpuscles.
c) Punctate fibers: ones with distinct receptive fields (connect to
Miessner & Merkel)
d) Diffuse fibers: ones with less disctict receptive fields. (connect to
Ruffini & Pacinian).
Combination of these four types produce four types of nerve fibers.
Touch pathway
runs up dorsal
(back) of spinal
column. Some
connect with
interneurons and
motor neurons
and mediate
reflexive arcs. 2
main pathways:
Lemniscal (red;
newer) more
sophisticated
aspects of touch.
Spinothalamic
(older; blue) pain
and temperature.
Touch pathways
Somatosensory cortex
Major touch processing
area in cortex
Cortical magnification
4 maps on S-I (primary
somatosensory cortex)
“middle maps” (3b and 1)
respond greatest to light
touch. Receive inputs from
superficial punctate (SA and
RA) fibers. Outer maps (3a
and 2) respond best to
movements of joints,
tendons, and muscles
(kinesthetic). Inputs from
deep diffuse fibers.
Pain perception
Pain serves an important
adaptive function – it alerts
the organism to potential
tissue damage and compels
withdraw of affected area
from pain source.
Chronic pain, however, often
makes life miserable for
those afflicted.
Nociceptors: free nerve
endings that signal pain. Two
locations: skin surface –
temperature; Subcutaneous
fat layer: punctures.
Pain fibers
A_delta:
myelinated; fast
responding;
sharp, acute;
thermal pain
C-type: slow
responding;
building pain;
mechanical;
thermal;
chemical
Pain pathways
Same basic design
as all touch
pathways. Up spinal
column to thalamus;
then Sensory cortex;
but note presence
of descending
pathway running
along same route.
Gate control theory of pain
T cells send pain message. When only fast (A-beta; A-alpha) fibers
active; no pain. Inhibitory message send from SG to T cells. When both
fast and slow (C-fibers) respond, SG cells are inhibited from sending
their inhibitory message to T cells, T cells fire and pain message is sent.
Note also: T’s can be inhibited by top-down messages from cortex.
“Meaning” of pain relevant.
Phantom limbs
It is not uncommon for amputees to claim that they experience
pain in the missing limb, often this seems associated with cortical
re-organization in somatosensory cortex after loss of limb