Biomedical Engineering Reference
In-Depth Information
to the nerve fiber, we would then be able to generate in the subjects the
same somatic sensations as would be evoked by stimulation of the actual
corresponding mechanoreceptor unit.
We have been using the micro-stimulation (microneurographic) technique
[184,185] in order to evoke the desired sensation by electrical stimulation to
the sensory nerve fiber, and we have been analyzing the relationship between
the properties of the electrical stimulation and those of the evoked sensation
generated by stimulating only one sensory nerve fiber coming from a single
mechanoreceptor unit.
Microneurography involves percutaneous and direct insertion of a very
thin tungsten microelectrode (needle-type) into a peripheral nerve, so that
the signal of the nerve fiber attached to the tip of the electrode can be
measured: it is also possible to stimulate nerve fibers which are attached to
the tip of the electrode (microstimulation) using this technique. Details of
the technique of microneurography are provided in Sect. 4.7.3.
In our experiments, we inserted a microneurographic electrode into the
median nerve of each subject. The position of the electrode was adjusted and
fixed so that the nerve signal from a single mechanoreceptor unit could be
recorded (Fig. 4.56), and the type and receptive area of the involved mech-
anoreceptor unit were determined from the response of the receptor to the
mechanical stimuli [184,185]. (In a set of our experiments, SA I type mech-
anoreceptors are basically selected as the object for electrical stimulation.)
Then, we gave electrical stimulation to the nerve fiber in a series through the
same microelectrodes by changing the circuit from the measurement mode
to the stimulation mode using a switching box. Subsequently, we investiga-
ted the properties of the evoked sensation and the relationship between the
area and the subjective intensity of the evoked sensation and the frequency,
amplitude, and temporal profiles of the electrical stimulation. The relations-
hip between the intensity of the evoked sensation (pressure sensation) and
the frequency or amplitude of the electrical stimulation was quantitatively
evaluated using the so-called “magnitude estimation method” (we used a
positive square-wave pulse for 250
s; the frequency was fixed at 20 Hz).
μ
Note 1:
Six kinds of mechanoreceptors (or endings of the nerve fiber) have
been reported to exist on the skin [185-187] (Fig. 4.57): Merkel disks; Pinkus
corpuscles; Ru
ni endings; Meissner corpuscles; Vater-Pacini corpuscles;
and hair follicle receptors. The response of these receptors is classified into
four patterns: SA I (slowly adapting type I), SA II (slowly adapting type II),
RA (or FA I - rapidly adapting type), and PC (or FA II - Pacinian type)
according to their properties with respect to the adaptation of the response
and the area of the receptive field (refer to Fig. 4.58).
The slowly adapting types correspond to the signals from Merkel disks
or Ru
ni endings, which detect deviations of the skin; these two receptors
are suitable for detecting continuous pressure or touch.
The rapidly adapting type corresponds to the signals from Meissner
corpuscles or hair follicle receptors, which detect the velocity of the skin
