Multi-unit microneurography and modeling of afferent responses of human muscle mechanorecpetors

In the research into the physiology and pathophysiology of human muscle
reflexes, it is attempted to discern the effects of motoneurones, muscles,
mechanoreceptors and the central nervous system. For this purpose, the research
group of Prof. van der Helm is using since a few years a mathematical
physiological model of the neuromusculoskeletal system of the ankle, wrist,
shoulder and other joints. These models are validated by applying force
perturbations using robotic manipulators, and measuring muscle force, position
and EMG. Besides physiological research, these models and methods can also be
used to acquire patient data. It is not possible with this method to
discriminate between the effects of muscle mechanoreceptors (muscle spindles
and Golgi Tendon organs) and the central nervous system. The muscle
mechanoreceptors deliver signals that can only be directly measured using
microneurography. These signals are the input of the central nervous system for
reflex tasks.
Microneurography involves the insertion of a micro-electrode into a nerve
fascicle, and delicate manipulation of this needle until the signal of a single
mechanoreceptor is recognized. This 'single-unit' technique can give very
detailed information, but it has major practical problems. It is hard to find a
axon, statistics and luck determine the type of nerve fiber that is found, and
even the smallest movements of the needle can cause signal loss, and one often
has to be content with recordings of only 5 minutes. Furthermore, a single-unit
recording gives only a very limited subset of the information that reaches the
central nervous system, namely the combined signal of many receptors with
different working ranges, sensitivity and dynamic response. All these problems
are related to the minute active area of the micro-electrode.
Multi-unit microneurography, using a bigger active electrode area, might reduce
these problems. This technique, which is usual for recording sympathetic nerve
activity, measures the activity of multiple nerve fibers simultaneously. It is
to be expected that this will relax the requirements on electrode position and
stability, giving the opportunity of lengthier registrations, that will be less
determined by luck, and more by statistics, en possible be more representative
for the combined signal going to the central nervous system. Many applications
of this technique are conceivable in physiological research and diagnostics of
neurological patients, where the advantages are important and the absolute
selectivity of the single-unit method is not required.

The goal of this research is to determine whether multi-unit microneurography
can be a practically useful technique for recording human muscle-afferent
signals, and to determine which fiber types are present in the recorded signal
to and from the central nervous system (type Ia and II afferents from muscle
spindles, type Ib afferents from Golgi tendon organs and alpha motor
efferents). While doing this, the technique will be further optimized.
A secondary goal is to compare the multi-fiber microneurograms with published
single-unit results for various movements, velocities and forces.

Therefore, in an observational study setup, we will make multi-unit
microneurograms during a variety of active and passive movements of the wrist
joint.

The burden for the subject is small: a measurement session in a seating
posture, with a maximum duration of 3 hours, with passive and active movements
of the wrist joint, with limited amplitude and force. For the microneurography,
a 0.2mm needle electrode will be inserted in the radial nerve. The chance of
(mild) aftereffects is small (< 10%), and if any aftereffects occur, the
usually dissolve within two weeks.