Chapter 2. Prehension

Chapter 6 - During Contact 255
Table 6.3 Summary of the activity of the four mechanoreceptive
tactile afferent types (probable receptor morphology) during the
grasp, lift and replace task of Johansson and Westling.
phase/ FA1 SAI FA11 SAII
event (Meissner) (Merkel) (Pacini) (Ruffini)
preloading/
contact
-at contact
-30-200
imp/s
-friction
affects
fresUenCY
at contact at contact
weaker than
FAI
no response
terminate noresponse no distinct no response
loading/ response bursts,
object starts even from
to move wrist and
palm fields
holding no response some, noresponse regular
irregular impulses.
firing directiOMl
replacement no response no distinct
sensitivity
to shearing
forces
no response
ends/
object stops
response bursts,
even from
moving wrist and
palm fields
R1tX.W distinct burst 112 showed distinct no response
distinct burst,
burst less reliable
slips distinct burst distinct distinct no response
(see below) burst burst
grasp. Recall that Babu and Devanandan (1991) found a high density
of Paciniform corpuscles (FAII) in the ventral parts of the joint cap-
sules of the phalangeal joints. The SAII units show three dimensional
force sensitivity during holding. Johansson 8z Westling (1990)
suggested that during manipulation, the population of SAII units may
play a role in registering the magnitudes and directions of load forces
and the balance between the grip forces and other manipulative forces.
Responses from isotonic finger movements between the lifts, about 1
cm at the fingertips, were seen only in Type I1 units. The