Compensatory mechanisms in the aging motor system

Abstract

Motor functions decline with age due to a number of factors. There is interest in whether these changes are reflected in the organisation of the cerebral motor system in older subjects and whether such changes might be in some way compensatory. Most studies in humans have used functional brain imaging techniques to compare motor system activation in younger and older subjects. Interpretation of these results is made more difficult by potential neurovascular changes in older subjects. However, in general, there appears to be greater motor task-related brain activity in a wider network of brain regions in older compared to younger subjects. The evidence that these changes are compensatory in nature is less clear. Incorporation of behavioural and anatomical data will be required in order to fully interpret the functional imaging results.

Introduction

Motor function declines with increasing age (Jebsen et al., 1969, Potvin et al., 1980, Hackel et al., 1992, Kolb et al., 1998). The factors leading to this decline are complex and multifactorial. They include changes in musculoskeletal architecture (Lexell, 1995, Lexell, 1997, Dutta et al., 1997) peripheral and central nerve conduction (Dorfman and Bosley, 1979), proprioception (Kaplan et al., 1985), neuromuscular coupling (Lexell, 1997, Delbono, 2003). In addition, although it is often widely assumed that there is a progressive loss of cortical and striatal neurons in normal aging, the evidence is against this (Winblad et al., 1985, Sawle et al., 1990, Haug and Eggers, 1991). In particular, there is no significant age-related neural loss in primary motor cortex (Haug and Eggers, 1991). There is evidence, however, of disruption of white matter integrity with age which will affect cortico-cortical and corticospinal connectivity (Madden et al., 2004). The effects of these changes are widespread from simple motor tasks such as repetitive finger tapping (Shimoyama et al., 1990), to more demanding timed tasks and visually guided hand movements (Houx and Jolles, 1993, Kauranen and Vanharanta, 1996, Smith et al., 1999), and the somatosensory and visual control of dexterity (Cole, 1991, Cole et al., 1999).

However, aging is not just the steady loss of these component parts. With increasing age comes an increase in experience which appears to have a marked effect on the central nervous system morphology, at least in animal models, many of which are compensatory (Kolb et al., 1998). Consequently, there is increasing interest in determining whether compensatory changes can be seen in the aging human nervous system.