Robert Guerring (1) & Osvaldo Delbono (1,2,3)

1: Department of Physiology and Pharmacology, 2: Department of Internal Medicine, Section on Gerontology and 3: Neuroscience Program, Wake Forest University School of Medicine, Winston-Salem, NC, USA

https://doi.org/10.36866/pn.61.30

As a human reaches old age it is common to observe a general decrease in muscle mass and a proportionate decrease in muscle force and power. Both of these effects can lead to an impairment of daily living activities, morbidity, disability, and mortality and decreased life expectancy. The mechanisms by which these losses occur are only partially understood and are the subject of much scientific investigation.

Muscle weakness in ageing mammals may result from factors affecting the neural control of muscle structure and function, directly from changes in the intrinsic properties of the muscle, or a combination of both (for a review see Delbono, 2003). One neural factor, denervation, has been shown to increase with age. Denervation is the loss of muscle innervation and results in atrophy and loss of muscle fibres. The graduated disconnection of muscle fibres from their spinal cord motoneuron ultimately affects the entire muscle, causing a decreased force-­generating capacity and a smaller muscle mass.

Studies of spinal cord motoneuron and muscle fibre counting, together with electrophysiological motor unit and muscle functional recordings, indicate that denervation slowly evolves over decades. The process of muscle denervation occurs concurrently with reinnervation, whereby some denervated fibres become reinnervated by axonal sprouting of neighbouring healthy and slow-type motor units. Reinnervation apparently remodels motor units and is characterized by a resulting higher proportion of slow-type fibres (Larsson et al. 1993). The effect of these innervation changes is a concomitant increase or decrease in the number of fibres in muscle motor units. As this dynamic process progresses a reduction in muscle strength takes place when muscle fibers are partially denervated, or when denervation outpaces reinnervation and the absolute number of contracting fibres decreases (Fig. 1). A decline in muscle specific force occurs before a significant decline in muscle fibre number and suggests that impaired motoneuron function may have a deleterious effect on the intrinsic fibre capacity to develop force. Innervation, therefore, is vital to the survival of muscle fibres. Due to the aforementioned consequences of denervation it is important to know the extent and quality of this process in ageing muscle.

Several studies have reported skeletal muscle denervation and reinnervation, as well as motor unit remodeling in ageing rodents or humans (for a review see Payne & Delbono, 2004). Accurate measurement of denervation has remained elusive due to technical difficulties in functionally and histochemically assessing the expression of molecules that appear with denervation.

To investigate this issue, Wang and coworkers explored the expression of the sodium channel Nav 1.5 as an index of denervation (Wang et al. 2005). Electrophysiological and immunohistochemical assays were performed in a short muscle of the mouse paw, flexor digitorum brevis (FDB), which provides an excellent model preparation for functional recordings (Fig. 2). The use of the potent and specific toxin tetrodotoxin (TTX) allowed the researchers to discriminate between cells expressing Nav 1.5 (TTX-resistant, denervated) or Nav 1.4 (TTX-sensitive, innervated) channels. Three populations of fibres were found in old mice:

(a) innervated, corresponding to 50%of all the fibres recorded and exhibiting high sensitivity to TTX;
(b) denervated, representing 13% of the fibres and exhibiting a significant resistance to TTX; and
(c) partially denervated, composed of 35% of the fibres that exhibited a sensitivity to TTX intermediate between the previous two groups.

These experiments were complemented by the detection of denervated fibres in histological sections of the FDB muscle exposed to specific and purified antibody against Nav 1.5. About half of the fibres from old mice were positive for the Nav1.5 antibody, compared to less than 2% of fibres from young animals, providing further evidence for greater denervation in ageing muscle than was previously estimated. In summary, this study detected the presence of a significant fraction of partially denervated fibres that, together with the small fraction of fully denervated fibres, add up almost 50% of the FDB fibres recorded at late stages of life. Taken together, partially and fully denervated fibres, represent about half of all fibres tested in the FDB muscle of old mice in this study. Literature values have ranged between 25 and 50% loss in human spinal cord motoneuron and motor units with age (for a review see (Lexell, 1997)).

Whether differences in magnitude or extent of denervation among species and muscle subtypes exist is not known at the present time. The neural cell adhesion molecule (NCAM) has been used as a marker of denervation in other studies. Approximately 10% of extensor digitorum longus muscle fibres of aging rats have been reported denervated using immunohistochemistry for NCAM (Urbanchek et al. 2001). This number likely corresponds to the highest denervated/most TTX-resistant fibres observed in the study reported above.

Two important questions remain:

(1) Are the fibres which exhibit intermediate sensitivity to TTX capable of developing force? and
(2) Is the FDB muscle more susceptible to denervation than other hindlimb muscles?

Though a fertile area for continued investigation, we can expect that fibres depicting an intermediate response to TTX are at different stages of denervation in ageing rodents. A significant fraction of those fibres may be electrically excitable both directly and indirectly, through the nerve, and still contribute somewhat to the loss in muscle force with ageing. The location of the FDB in the plantar aspect of the paw can render this muscle more susceptible to trauma than the remaining hindlimb muscles. Muscle and nerve mechanical trauma, in association with impaired repair during ageing, could be the propitious territory for a relentless denervation process.

Acknowledgements

The preparation of this manuscript and the studies from laboratory reported here were supported by grants from the National Instituted of Health/National Institute on Ageing (AG18755, AG13934 and AG15820) to Osvaldo Delbono. We are greatful to Gregory Piccola for editing this manuscript.

References

Delbono O (2003). Neural control of aging skeletal muscle. Aging Cell 2, 21-29.

Larsson L, Biral D, Campione M & Schiaffino S (1993). An age­related type IIB to IIX myosin heavy chain switching in rat skeletal muscle. Acta Physiol Scand 147, 227-234.

Lexell J (1997). Evidence for nervous system degeneration with advancing age. J Nutr 127, 1011S-1013S.
Payne AM & Delbono O (2004). Neurogenesis of excitation­contraction uncoupling in aging skeletal muscle. Exerc Sport Sci Rev 32, 36-40.

Urbanchek MG, Picken EB, Kalliainen LK . & Kuzon WM Jr (2001). Specific force deficit in skeletal muscles of old rats is partially explained by the existence of denervated muscle fibers. J Gerontol A Biol Sci Med Sci 56, B191-197.

Wang ZM, Zheng Z, Messi ML & Delbono O (2005). Extension and magnitude of denervation in skeletal muscle from ageing mice. J Physiol 565, 757-764.