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Define the terms skeletal muscle fiber motor unit skeltal muscle twitch

Bertorini MD, in Neuromuscular Case Studies2008 Motor Unit Action Potentials Motor unit action potentials MUAPs are produced by the summation of electrical potentials of individual muscle fibers innervated by the same motor neuron and are activated by voluntary muscle contractions.

The recorded MUAP represents potentials of about 8 to 10 fibers that are close to the needle electrode Fig. Normally, the motor units discharge at an initial rate of about 4 Hz, and this rate increases as the intensity of contractions increases Fig. Smaller units are recruited first, and, according to experimental studies, they belong to type I motor neurons. With the force of increasing contractions, more motor units are recruited until they fill the oscilloscope. This is called the interference pattern IP.

The characteristics of MUAP can be analyzed qualitatively or quantitively with signal delays, triggers, and averaging; or by the use of automatic methods. Analysis of MUAPs includes determination of their duration from onset to end or from departing to returning to the baseline. The phases represent spikes that cross the baseline. The MUAP amplitude is determined by the size and number of muscle fibers within 0. The phases are caused by temporal dispersion of potentials of individual muscle fibers contributing to the MUAP.

Individual muscle fiber potentials that appear later and separate from the main MUAP are called late potentials, or satellites Figs.

These can be seen in neuropathies, motor diseases, and myopathies, from slow conducting axons or muscle fibers. An important characteristic of the MUAPs analyzed during electromyography is their ability to recruit other motor units upon increasing the strength of muscle contraction.

Motor unit

The recruitment of a second motor unit usually occurs when a MUAP is firing at less than 10 Hz, and more motor units are recruited at faster rates. The recruitment rate is the rate of firing of the motor unit when a second motor unit is recruited. The recruitment ratio is calculated by dividing the firing rate by the number of motor units seen in the oscilloscope.

For example, if 3 motor units are firing at 15 Hz then the recruitment ratio would be 5. Normally, the recruitment ratio is less than 5. This is usually reduced in neurogenic conditions, causing a dropout of motor neurons or their axons; thus, fewer motor units are recruited that discharge at a faster rate. The IP may be reduced with lack of effort and in upper motor neuron disorders, but in these the MUAPs fire at slower rates.

MUAP stability represents lack of variability in amplitude and duration. Increased variability is caused by desynchronization of individual fiber potentials in disorders of neuromuscular transmission. This also occurs in neurogenic disorders such as ALS.

Types of Muscles Essay

Normal MUAPs that discharge spontaneously and not during voluntary contractions are seen in stiff-personsyndrome. These disappear with the use of nerve blocks, general anesthesia, and intravenous diazepam.

  • Because of this high firing rate, small motor units must be relatively resistant to fatigue;
  • The rest is composed of type B and type C fibers.

These smaller potentials are caused by a decreased number of functional muscle fibers that belong to a motor unit Fig. In myopathies there is rapid recruitment of motor units, and the IPs are usually full and show an increased number of turns with decreased amplitudes during an automatic analysis Figs. The conventional use of the term myopathic motor units is not accurate, because small polyphasic motor units can also be seen in disorders of neuromuscular transmission and in some neuropathies without complete reinnervation, causing the so-called nascent MUAPs that are small and polyphasic but show decreased recruitments Fig.

In neurogenic disorders, the MUAPs are larger because reinnervation produces an increased number of muscle fibers that belong to the same motor units close to the needle electrode Fig. The number and recruitment of MUAPs are usually decreased because of the reduced number of motor neurons and axons Figs. The MUAP can also be polyphasic from desynchronization of the individual potentials due to variation in conduction of individual axons.

A Muscle’s Force Depends on the Recruitment Patterns of Its Fibers

These could be duplets or triplets or quadruplets. Multiplets fire two or more times at very short intervals at a rate of about 2 to 20 Hz, and they maintain the same relationship to one another.