Environmental Engineering Reference
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signals steadily change as a function of age. Multiple authors demonstrate physical changes
in brain tissues (Andersen, Gundersen, & Pakkenberg, 2003; Kuo & Lipsitz, 2004; Raz &
Rodrigue, 2006), changes in excitability of the corticospinal tract and anterior horn cells
(Rossini, Desiato, & Caramia, 1992), and changes in neurotransmitter systems. There is a
general loss of neural substrate, including grey and white matter. This has been
demonstrated in both the cerebral cortex (Raz & Rodrigue, 2006) and the cerebellum
(Andersen et al., 2003). These tissue changes then result in a myriad of functional changes
within the central nervous system (CNS). There is a general deterioration of motor planning
capabilities (Sterr & Dean, 2008;Yan, Thomas, & Stelmach, 1998) and feed-forward
anticipatory control (Hwang et al., 2008) with aging. Along with this decrease in planning
ability, there also appears to be slowing of central processing (Chaput & Proteau, 1996; Inui,
1997; Light, 1990; Shields et al., 2005). This change in processing is partially due to
neurophysiologic changes within the CNS resulting in a decrease in the available resources
of the processing pool (Craik & McDowd, 1987; Schut, 1998). Loss of attentional resources
also contributes to this slowing of central processing (Goble et al., 2008; Kluger et al., 1997;
Sparrow, Begg, & Parker, 2006). This in itself results from a multifactorial process relating to
neurophysiologic changes in the CNS and degradation of afferent information arriving from
compromised peripheral receptors (Chaput & Proteau, 1996; Goble et al., 2008). The result of
these attentional and processing changes is a decline in the ability to integrate multiple
sensory modalities causing a relative decrease in the use of proprioceptive feedback and an
increased use of vision (Adamo, Martin, & Brown, 2007; Chaput & Proteau, 1996; Goble et
al., 2008; Lemay, Bertram, & Stelmach, 2004). This shift to the use of visual resources is due
to the tendency of the CNS to re-weight sensory information when one source of feedback is
compromised (Horak & Hlavacka, 2001), as well as a general systems neuroplasticity effect
(Heuninckx, Wenderoth, & Swinnen, 2008; Romero et al., 2003). These compensatory
neuroplastic changes are the end manifestation of the normal aging process within the CNS.
The peripheral nervous system (PNS) undergoes concordant neurophysiologic changes as
well (Chaput & Proteau, 1996; Goble et al., 2008; Roos, Rice, & Vandervoort, 1997). These
changes occur in both the afferent and efferent pathways. Studies have shown both a
decrease in number and density of proprioceptors (Goble et al., 2008), as well as a slowing of
sensory receptors in general (Light, 1990). In the efferent systems, research demonstrates a
loss of motor units and a decrease in firing rate and increased discharge variability of intact
motor units (Roos et al., 1997). The available literature also demonstrates a loss of larger
motor neurons resulting in a net decrease of alpha motor neurons, a slowing in the
conduction velocity of remaining motor neurons, and changes in the excitability of alpha
motor neurons (Leonard et al., 1997; Roos et al., 1997).
The changes in the CNS and PNS with age are accompanied by changes in the muscular
system as well. In the aging adult, research shows a loss of muscle fibers and a decrease in
size of remaining fibers resulting in a net loss of muscle mass (Roos et al., 1997). Changes in
motor units in the PNS result in fiber type changes, causing a loss of fast-twitch fibers and a
proportional increase of slow-twitch fibers.
Transformations in the sensorimotor system have a resultant detrimental effect on motor
performance in daily life. This decrease has a physiologic basis in aging and is amplified by
disuse and dysfunction. In general, aging adults demonstrate decreases in movement speed
(Light, 1990; Mankovsky, Mints, & Lisenyuk, 1982; Poston et al., 2008; Yan et al., 1998),
