Motor Unit Presentation
- Physiological introduction to the Motor Unit
- Experimental Procedure
- Motor Unit Action Potential Train Analysis
The Motor Unit
Muscle fibers are innervated by neurons whose cell bodies are located in spinal cord. The nerve fibers, or axons, of these motor neurons leave the spinal cord and are distributed to the motor nerves. Each motor axon branches several times and innervates many muscle fibers.
The combination of a single motor neuron and all the muscle fibers it innervates is called a motor unit. Although the muscle fibers of a given motor unit tend to be located near one another, motor units have overlapping territories.
In response to an action potential from the neuron, a muscle fiber depolarizes as the signal propagates along its surface and the fiber twitches (contracts). This depolarization generates an electric field in the vicinity of the muscle fibers which can be detected by a skin surface electrode located near this field, or by a quadrifilar electrode inserted in the muscle. The resulting signal is called the muscle fiber action potential. The combination of the muscle fiber action potentials from all the muscle fibers of a single motor unit is the motor unit action potential (MUAP). All of the muscle fibers in a motor unit are fired each time a motor unit fires. The repetitive firing of a motor unit creates a train of impulses known as the motor unit action potential train (MUAPT). The summation of electrical activity created by each active motor unit is the myoelectrical signal (ME) (4).
To sustain muscle contraction, the motor units must be repeatedly activated (2). As the firing rates of motor units active in a contraction increase, the twitches associated with each firing will eventually fuse to yield large forces.
Most of the experiments launched at the Motor Unit Lab have been done on the first dorsal interosseous (FDI) and the tibialis anterior (TA). Here is the procedure which is followed:
The hand (or the leg) of each subject is placed in a specially designed device that constrained the muscle to contract isometrically. This apparatus substantially isolates the force generated by the muscle. The maximal voluntary contraction (MVC) level is measured for each subject. Then the subject is instructed to contract his muscle so as to generate a force-time course which tracked trajectory displayed on a monitor. The trajectory consisted of a trapezoidal shape, with a sustained contraction between 20 and 50% MVC during 30 seconds.
A special quadrifilar electrode is inserted in the muscle. Myoelectric (ME) signals are obtained from three differential combinations of the four wires (75 mm in diameter) exposed in cross section at a side port on the cannula of the needle, as well as from the cannula itself. The signals from the side port wires are amplified with a bandwidth of 1 kHz – 10 kHz and are digitized at a rate of 50 kHz. The force signal is amplified with a bandwidth of 10Hz to 1 kHz and is digitized at a rate of 2000 Hz.
The ME signals are decomposed into their constituent MUAPT’s by the Precision Decomposition technique (Mambrito and De Luca , Le Fever and De Luca ) to obtain the map of MU firings. The Precision Decomposition technique is a template matching technique which arrives at decisions for identifying the shape of individuals MU’s by a weighted combination of probability of occurrence and the least-squared signal space distance between the MUAP and an established template. The technique also continuously update the templates if the shape of the MUAP’s is modified slowly.
A few tools
The output of the EMG Precision Decomposition algorithm is a set of impulse trains. Each impulse corresponds to a firing of the appropriate motor unit. Several patterns are useful to understand the control properties of individual motor unit:
- The Bar Plot shows the location of all the MUAPs of each motor unit. This plot may be used to determine the recruitment and derecruitment threshold of a motor unit.
- The Dot Plot shows the duration of all the inter firing interval of each motor unit. A dot represents the time between a firing and the next one. This plot may be used to study the behavior of the motor unit firings.
- The Instantaneous Firing Rate (IFR) is computed by inversing the time between a firing and the next one.
The time varying Mean Firing Rate for each Motor Unit is estimated by passing the impulse train showed in the IFI plot through a Hanning filter, with symmetric unit area impulse response. From empirical observation, it was found that a filter with a window 400-800 ms long provides an acceptable compromise between firing rate estimation bias and estimation stability. This type of plots is very useful for studying relationships among different Motor Units and the Force.
A few results
Number of studies have shown interesting results to understand the neuromuscular system. The most important is the common drive of motor units:
- Motor units have been found to modulate their firing rates in unison and simultaneously.
- The firing rate of motor units is not constant, even during constant force contractions; it fluctuates.
- The firing rates of earlier recruited motor units are greater than those of later recruited motor units at any given force value. At a force reversal, the firing rates of high threshold motor units reduce their firing rates before the low threshold motor units.
- The fluctuations in a force output of a muscle during a constant-force contraction are caused by the fluctuations in the firing rates of the motor units.
- The firing rates of the motor units decrease during a constant-force isometric contraction.
These phenomena suggests the following implications:
- The motor units have a net excitation which acts through a common input. The most likely location of this common input is the anterior horn cell.
- The control to the muscle is not designed to generate constant-force contractions.