Age-related changes in motor cortical properties and voluntary activation of skeletal muscle

Abstract

Aging is associated with dramatic reductions in muscle strength and motor control, and many of these agerelated changes in muscle function result from adaptations in the central nervous system. Aging is associated with widespread qualitative and quantitative changes of the motor cortex. For example, advancing age has been suggested to result in cortical atrophy, reduced cortical excitability, reduced cortical plasticity, as well as neurochemical abnormalities.The associated functional effects of these changes likely influence numerous aspects of muscle performance such as muscle strength and motor control. For example, there is evidence to suggest that the muscle weakness associated with aging is partially due to impairments in the nervous system's ability to fully activate motor neurons- particularly in the larger proximal muscle groups. In this review article we discuss age-related changes in the motor cortex, as well as the abilityor lack thereof- of older adults to voluntarily activate skeletal muscle. We also provide perspectives on scientific and clinical questions that need to be addressed in the near future.

Fig. (1). Age-associated atrophy of the precentral gyrus

These data represent thickness values of the precentral gyrus obtained via high-resolution MRI from over 100 individuals ranging in age from 18–93 years. Overall there was significant thinning of the precentral cortex, and regression analyses indicated that age explained 34% of the between-subject variability in precentral gyrus thickness. Reprinted with permission from Salt, D.H., R.L. Buckner, A.Z. Snyder, D.N. Greve, R.S.R. Desikan, E. Busa, J.C. Morris, A.M. Dale and B. Fischl. Thinning of the cerebral cortex in aging. Cerebral Cortex. 14: 721–730, 2004.

Fig. (2). Age-related changes in motor cortex excitability as assessed using paired-pulse transcranial magnetic stimulation

Older adults (70.9±1.8 yrs) exhibited less intracortical facilitation than younger adults, and more short-interval intracortical inhibition under resting conditions when compared to younger adults (21.4±0.8 yr) (*P<0.05). Reprinted with permission from McGinley, M.P., Hoffman, R.L., Russ, D.W., Thomas, J.S., and Clark, B.C. Older adults exhibit more intracortical inhibition and less intracortical facilitation than young adults. Experimental Gerontology. (In Press).

Fig. (3). Age-related changes in voluntary activation (a measure of how much of a muscle’s possible force is produced by a voluntary contraction) is muscle group specific

Studies quantifying age-related differences in voluntary activation of the knee extensors (A), elbow flexors (B), and ankle dorsiflexors (C) during isometric contractions. Selected studies have been highlighted with an arrow pointing to them (see main text for further discussion on these specific papers). Data points in A correspond to the following articles: Filled circles: Stevens et al. 2003, Filled squares: Roos et al. 1999, Filled diamonds: Callahan et al. 2009, Filled triangle: Harridge et al. 1999, Open circles: Wilder and Cannon 2009, Open Squares: Cannon et al. 2007, Open diamonds: Knight and Kamen 2001.Data points in B correspond to the following articles: Filled circles: DeSerres and Enoka 1998, Filled squares: Bilodeau et al. 2001, Filled diamonds: Klein et al. 2001, Filled triangle: Jakobi and Rice 2002, Open circles: Yue et al. 1999, Open Squares: Hunter et al. 2008, Open diamonds: Yoon et al. 2007.Data points in C correspond to the following articles: Filled circles: Kent-Braun and Ng 1999, Filled squares: Conley et al. 1999,Filled diamonds: Lanza et al. 2004, Filled triangle: Klass et al. 2005, Open circles: Simoneau et al. 2005, Open Squares: Chung et al. 2007.