Friday, 30 March 2012

Characteristics of Denervated Muscle

The denervated muscle is very different to that of an innervated muscle. When innervation is lost the muscle undergoes a progressive decay, where many morphological and physiological changes occur. These are principally (Robinson & Snyder-Mackler, 2008):
o Loss of voluntary and reflex activity
o Muscle Atrophy
§ Decrease in muscle weight, and muscle sarcoplasm
§ Decrease in myofibril (muscle contractile protein)
§ Decrease in the number of muscle fibres
o Replacement of muscle by fibrous and adipose tissue
o Changes in muscle excitability
§ Fibrillation: spontaneous contraction of muscle fibres
§ Oscillations in resting membrane potential
Figure 2. Comparison of (A) healthy muscle biopsy and a biopsy of a (E) denervated muscle as a result of a LMN lession (Kern et al., 2010)
These progressive internal changes to the muscle make denervated muscle tissue less excitable than healthy normal muscle, requiring a greater electric charge to cause a contraction. Electrical stimulation to these muscles produces a slow worm-like contraction (Low & Reed, 2000). The electrical charge slowly spreads through the muscle with a diminished rate of contraction compared with innervated muscle (Low & Reed, 2000). Studies have shown that using electrical stimulation to produce isometric contractions of denervated muscle can retard the atrophy process. As such, electrical stimulation treatment of denervated muscle should commence as soon as possible after the event of denervation (but not before axonal sprouting) to decrease the amount of atrophy that will ensue (Kern et al., 2010).  The best results are seen with vigorous isometric stimulation to the point of fatigue (three sessions per day for at least 10 minutes in duration) (Low & Reed, 2000). However, this treatment has shown that it cannot completely prevent the degenerative process (Low & Reed, 2000). Further studies have used large surface electrodes with high intensity, long-duration impulses to directly elicit contraction with the aim to reverse and treat denervated muscle that have the longstanding severe atrophy (Kern et al., 2010).

Robinson, A. J.  & Synder-Mackler, L. , 2008. Electrotherapy and electrophysiological testing. Clinical Electrophysiology 3rd Edition.

Kern, H., Carraro, U., Adami N., Biral D., Hofer C., Frostner C., Modlin M., Vogelauer M., Pond A., Boncompagni S., Paolini C., Mayr W., Protasi F., & Zampieri S., (2010), Home-based functional electrical stimulation rescues permanently denervated muscles in paraplegic patients with complete lower motor neuron lesion. Neurorebilitation and Neural Repair, 24(8), 709 – 721.

Low, J., & Reed, A. (2000). Electrotherapy explained: principles and practice. Oxford: Butterworth-Heinemann

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