Chapter 2. Prehension

Small, sharp borders
Large, obscure borders
ADAPTATION
Fast,
no static response
l-----L
--HHc----c
Edge sensitivity
(43%)
FA1 Meissner
FA11 p(aliz)a
Golgi-Mazzoni
Slow,
#latic response preseni
J-----L *
-
Irregular
Edge sensitivity
(25%)
SAI Merkel
_/--7_
Regular
Sensitive to
lateral skin stretct
(1 9%)
SAII Ruffini
Figure 6.7. Types of mechanoreceptive afferent units in the glabrous skin of the human hand
and some of their distinguishing characteristics. The graphs in the middle schematically show
the impulse discharge (lower traces) to perpendicular ramp indentations of the skin (upper
traces) for each unit type. The black patches and the dashed areas of the left drawings of the
hand show the extent of typical receptive fields for type I and type I1 units, respectively. Each
dot in the drawings represents a single sensory unit. In addition, the relative frequency of
occurrence and the probable morphological correlate are indicated for each unit type
respectively. It should be noted that the terminology of the tactile afferent units in the
present account follows Johansson and Vallbo (1983) (from Johansson & Westling, 1990;
adapted by permission).