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 and evaluation of treatment methods
and neuromuscular disorders. However, it can not discriminate between the
effects of muscle mechanoreceptors (muscle spindles and Golgi Tendon organs)
and the central nervous system. Notably the role of non-linear mechanoreceptor
behavior and of fusimotor activity (efferents dynamically change muscle spindle
sensitivity) in the muscle reflexes is not yet well understood.
To investigate these effects, the mechanoreceptors signals must be measured
directly. This can only be done using microneurography, inserting a
micro-electrode into a nerve fascicle and carefully positioning it to find the
signal of a single mechanoreceptor. This 'single-unit' technique can give very
detailed information, but 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. 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.
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.

Our goal is to acquire a practical, maybe even clinically useful
microneurographic technique. In this research project, we will investigate the
usefulness of multi-unit microneurography for the research of human muscle
mechanoreceptors, by answering two questions:
1) Practical feasibility, especially regarding aspects for which single-unit
microneurography is notorious. How hard is it to find and keep an effective
electrode position?
2) Contributing nerve fibers. Can we discriminate between the various afferent
and efferent signal sources? What fibers contribute to a multi-unit recording?
Secondary goals are the comparison of multi-fiber microneurograms with
published single-unit results for various movements, velocities and forces, and
the optimization of the microneurographic technique and signal processing.

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.