New Advances in Single Fiber Electromyography

New Advances in Single Fiber Electromyography

Javier Rodriguez-Falces (Public University of Navarra, Spain)
DOI: 10.4018/978-1-4666-6090-8.ch002
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Abstract

The aim of this chapter is to present a general perspective of SFEMG together with a description of the anatomical, physiological, and technical aspects that are involved in the recording of single fibre action potentials (SFAPs). First, a simulation model that relates analytically the intracellular action potential (IAP) and SFAP mathematical expressions is described. Second, the most recent findings regarding the shape features of human SFAPs are outlined. Third, a description of how different types of needle electrodes affect the characteristics of the recorded potential is detailed. Fourth, an explanation of the most important effects of filtering on the SFAP characteristics is provided. Finally, a description of the principles of jitter estimation together with the most important sources of errors is presented.
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1. Introduction

The advent of single fibre electromyography (SFEMG) allowed investigators to record potentials produced by single muscle fibres (i.e., the so-called SFAPs). Characterization of the shape peculiarities of the SFAP (i.e., the SFAP morphologic features) is essential as it enables to extract information about the characteristics of human intracellular action potentials (IAPs). This is highly valuable as knowledge about the characteristics of human IAPs is still incomplete (Rodriguez-Falces et al., 2012a, 2012b).

Over the years, SFEMG has been developed to study the microphysiology of the motor unit, such as the propagation velocity of individual muscle fibres (Stålberg, 1966), the distribution of muscle fibres within individual motor units (Sanders & Stålberg, 1996), and, most remarkably, the neuromuscular jitter (Stålberg & Trontelj, 1979). Nowadays, estimation of the neuromuscular jitter is the most reliable test to evaluate the functioning of neuromuscular transmission in vivo and SFEMG has become a useful technique for the diagnosis of a great variety of neuromuscular disorders.

Buchthal was the first to analyze extensively the characteristics of extracellular potentials produced by the activation of human skeletal muscle fibres (Buchthal & Pinelli, 1953; Buchthal et al., 1954a, 1954b). However, the large recording surface of the electrodes he used prevented him to record electrical activity from individual muscle fibres. It was not until the advent of SFEMG, promoted by Ekstedt (1964) and Stålberg (1966), that the identification of SFAPs was possible. Therefore, the feature that makes the SFEMG technique unique is its high selectivity, which is provided by the small recording surface of the single-fiber (SF) electrode, approximately 25 μm in diameter (Stålberg & Trontelj, 1979). This selectivity is further enhanced by using a high-pass filter with a typical cut-off frequency of 500 Hz.

As established by Ekstedt (1964), the conditions for recording an SFAP in a voluntarily activated muscle are:

  • 1.

    That the fibre is close to the electrode, and

  • 2.

    That the other fibres of the motor unit that have coincident action potentials are remote enough from the electrode to make their contribution small.

Based on these conditions, Ekstedt (1964) established the criteria to select true SFAPs:

  • 1.

    Have a clean and smooth biphasic spike and

  • 2.

    Present identical shape at consecutive discharges when the recording system has a time resolution of 10 μs (these days much shorter acquisition times are possible).

In the context of clinical neurophysiology studies, the most appreciated feature of SFAPs is the fact that they show a constant shape at consecutive discharges (Ekstedt, 1964) and so they are ideal “time events” with which to estimate jitter. The study of the pure morphologic features of SFAPs has received little attention from investigators. There are several reasons for the relative lack of research in this direction:

  • 1.

    The extremely high sensitivity of the SFAP characteristics to minor changes in the position of the electrode,

  • 2.

    The difficulty to establish when, and to what extent, the time-course of an SFAP is contaminated by distant electrical activity from the same motor unit, and

  • 3.

    Other technical problems related to SFEMG (inappropriate filter settings, contribution from the needle cannula, baseline fluctuation, physical noise, etc) (Dumitru et al., 1994).

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