Chapter 2. Prehension

224 THE PHASES OF PREHENSION
which a stimulus excites the receptor); receptor innervation density
(the number of receptive fields per unit area of skin); and adaptation
characteristics (whether the receptor potential and subsequent afferent
discharge decrease slowly (SA) or rapidly (FA) to a constant,
maintained stimulus).
Properties of these mechanoreceptive afferent units have been
extensively investigated using microneurography techniques, after
Vallbo and Hagbarth (1968). Tungsten microelectrodes are inserted in
peripheral (usually median) nerve fibers, and recordings are made.
Electrical stimulation is delivered through the same microelectrodes to
examine experienced sensations also. A summary of the results
obtained by Roland Johansson and colleagues is provided in Figure
6.7. Sixty eight per cent of mechanoreceptive units arise from
receptors superfically located in the dermal papillae (Type I units).
The FA1 and SAI units have small and well-defined cutaneous
receptive fields (about 3-8 mmz), and high innervation densities.
Innervation densities are greatest distal to the whorl of the papillary
ridges of the finger tips. There is also a density step at the
metacarpophalangeal joint, with decreased innervation density in the
palm (Johansson, 1978; Johansson & Vallbo, 1983). Their receptive
fields usually span 4 - 10 papillary ridges, and Type I units are
responsible for providing spatial acuity or high resolution information.
Meissner corpuscles (giving rise to 43% of all mechanoreceptive
afferent units) give rise to FA1 fibers. Merkel's receptors or disks
(giving rise to 25% of afferent units) give rise to SAI fibers. The FAII
and SAII units have large, obscure borders of their receptive fields,
innervate deeper receptors and are evenly distributed from the
fingertips to the wrist. Pacinian corpuscles and Golgi-Mazzoni bodies
give rise to FA11 fibers (13% of all afferents). Ruffini endings or
corpuscles give rise to SAII fibers (19% of all afferents). The two
slowly adapting mechanoreceptors (SA), exhibit a static response
related to the strength of the maintained skin deformation. The two
rapidly adapting mechanoreceptors (FA) respond only to changes in
skin deformation. These characteristics make the receptors unique for
providing information about skin stretch, skin deformation (hysteresis
friction), changes in deformation, and other mechanical transients
relevant to the finger flexions and force applications at the interface
between the grasping hand and the object.
The experienced sensations when individual mechanoreceptive
units were stimulated (with a train of .2-2 microA) were of light
mechanical deformations within the receptive field of the particular
afferent unit. For SAI units, a light uniform pressure sensation and