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

Research of human muscle reflexes strives to distinguish the dynamic properties
of the different parts of the reflex loop. For this purpose, the research group
of Prof. van der Helm is using since a few years mathematical physiological
models of the neuromusculoskeletal system of the ankle, wrist, shoulder and
other joints. These models are validated by applying mechanical stimuli to
subjects, using robotic manipulators and measuring muscle force, position and
EMG. Besides for fundamental physiological research, this method has been found
useful for the acquisition of patient data. For instance, it has been found
that healthy subjects dynamically adapt their reflexes to the task, but that
neurological disorders decrease this capability for reflex modulation. The
physiological mechanisms of reflex modulations are not well understood. Reflex
strength could be modified via the pyramidal tract, by pre-synaptic inhibition,
but also by the fusimotor system, which regulates the sensitivity of muscle
spindles via the efferent gamma fibers. The current method can not discriminate
between the effects of muscle mechanoreceptors (as non-linear sensors) and the
central nervous system. This requires direct measurement of the
mechanoreceptors signals, which can only be done using microneurography,
inserting a micro-electrode into a nerve fascicle.
The common 'single-unit' microneurographic technique records the signal from a
single axon. This method has major practical drawbacks. It is hard to find the
appropriate axon type; and even the smallest movements of the needle can cause
signal loss, such that one often has to be content with 5 minute recordings.
Furthermore, a single-unit recording gives only a very limited subset of all
afferent information that reaches the central nervous system. These practical
aspects seriously limit the applicability of single-unit microneurography.
We hypothesize that multi-unit microneurography, using a bigger electrode
pick-up area, will reduce these problems. This technique measures the activity
of multiple nerve fibers simultaneously. It is to be expected that this will
relax the requirements on electrode position, requiring less time to find an
electrode position and giving the opportunity of lengthier registrations.
Increased stability would be a great benefit, especially when studying subjects
during natural tasks. In the long term, we hope to use this method also for
investigations in subjects with neurological disorders.
Simultaneous contributions from various afferent (muscle spindle, Golgi tendon
organ, cutaneous) and efferent (alpha motor neuron, autonomic) nerves are to be
expected in the recordings. It is new and innovative in the proposed research
project to separate these signals using advanced system identification
techniques, taking advantage of the robot manipulator to accurately generate
and measure a variety of carefully designed movement and force patterns.

In this research project, we will investigate the feasibility of multi-unit
microneurography for the research of human muscle mechanoreceptors. We will
therefore answer the following questions:
1) How hard is it to find and maintain an effective electrode site? In terms of
success rate, search and measurement times; these are aspects where single unit
microneurography is notorious?
2) Which types of nerve fibers contribute to a multi-unit recording, and can we
discriminate between the various afferent and efferent signal sources?
3) What is the quality of the obtained signals, expressed as signal-to-noise
ratio and reproducibility?

We will use successfully obtained recordings for modeling of multi-fiber
(muscle spindle) responses and to optimize the applied technique.

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

The subjects are asked for a measurement sessions 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. This is known as a safe
technique. There is a chance (< 10%) of mild
aftereffects. Such effects normally dissolve spontaneously within two weeks.
Searching an electrode position can be uncomfortable for the subject. Searching
time is limited to 45 minutes. Decreased
searching times (as compared to single-unit microneurography) is one of the
expected advantages of multi-unit
microneurography that we want to research.