Page 227 - Control Theory in Biomedical Engineering

208 Control theory in biomedical engineering

2.1 Tactile feedback
Tactile feedback such as pressure, stretch, vibration, and combined stimuli is
recorded by the human body through sensory organs called mechanorecep-
tors, which are located in our whole skin, being in greater density on the
fingers and palm of our hands ( Johnson, 2001). These mechanoreceptors
are the Meissner’s corpuscles, Merkel’s disks, Ruffini endings and Pacinian
corpuscles (PCs), which are connected to the peripheral nervous system by
peripheral nerve bundles along with the human dermis.
Mechanoreceptors have temporal and spatial resolution ranges. Hence
they are categorized by types and adaptation due to a stimulus. Mechanore-
ceptors type I have small receptive fields, while those of type II have large
receptive fields. In addition, mechanoreceptors can be PC rapid adaptors
(RAs), whose response ranges cannot register static stimuli signals, or they
can be slow adaptors (SAs), which can register transient stimuli. The two
types of SAs are type I and type II.
Type I SAs are responsible for roughness and shape recording, while type
II SAs sense skin stretches with a large receptive field. Otherwise, RAs sense
the slip and flutter on the skin with a small receptive field. PCs, unlike RAs,
respond to a dynamic stimulus but have large receptive fields (see the char-
acteristics of skin mechanoreceptors in Caldwell et al. (1997) for further
insight). In effect, the mechanoreceptors’ features must be considered
carefully when developing wearable haptic devices, as they are key for
stimulating amputees’ residual limbs to generate the illusion of movement
as well as to improve the adherence of prostheses.

2.2 Kinesthetic feedback
Kinesthetic feedback is characterized by the sense of movement and strength
in our limbs. The receiving organs that allow this sensation are the muscular
spindles and Golgi tendon organs (Proske and Gandevia, 2012). When these
are stimulated, the individual can perceive an illusion of movement
(Culbertson et al., 2018). The Golgi tendon organ is a capsule in the con-
nection between tendon and muscle fibers. In this capsule, there are several
sensory nerve fibers called Ib afferents that transport movement sensation to
the nervous system. Some experiments of tactile stimulation were reported
in the 1970s, for instance, one study induced vibration at 100Hz on the
muscles and tendons that resulted in an illusion of movement or a change
in limb position (Goodwin et al., 1972; Eklund, 1972).