INTRODUCTION: AGE-RELATED CHANGES IN THE NEUROLOGIC SYSTEM
Deborah A. Kegelmeyer, PT, DPT, MS
The neuromuscular system undergoes many age-related changes that can negatively impact function. Normal age-related changes occur at a rate of 1% per year, starting at age 30.1 Therapists need to understand what constitutes a normal age-related change as opposed to a pathological change in function. Abnormal physiologic or behavioral changes that fall outside of the range of normal function can then be assessed and treated appropriately. Changes occur in the brain, spinal cord, and peripheral nervous system. These changes also lead to changes in the musculature. In addition, the sensory system declines with age and this impacts motor function. The cases in this chapter provide examples of both normal age-related and pathologic changes to the neuromuscular system.
It is necessary for therapists to have an understanding of the anatomy and physiology of the aging neurologic system to successfully treat their clients. Anatomic changes include a decline in brain volume overall with the greatest age-related differences in the prefrontal and orbitofrontal cortices. These areas are critical for executive function and memory.2 The parietal cortex also shows more age-related differences in gray matter volume than either the temporal or occipital regions. Motor control is not only dependent on these areas but is also more dependent on these areas in the elderly than in young adults. Subcortical structures, including the cerebellum and basal ganglia, also exhibit reduced volume with aging. The cerebellum is important for movement timing and coordination, while the caudate nucleus of the basal ganglia is involved in skill acquisition, specifically motor planning. Declines in white matter volume begin later but continue at a more accelerated rate than gray matter changes. Changes in the corpus callosum, the largest white matter bundle, significantly impact interhemispheric communication that is critical for bimanual coordination. There is a decrease in myelin in the gracilis fasciculus and an associated decrement in vibration threshold, indicating that fibers conveying proprioception are most affected by aging, which may contribute to balance deficits due to the loss of long latency postural reactions.2 Fortunately, some brain areas do not exhibit age-related changes, including the cingulate gyrus (influential in linking behavioral outcomes to motivation) and the occipital cortex (vision).2
In addition to the anatomic changes there are decreased levels of critical neurotransmitters in the aging brain. Acetylcholine declines are noted in the hippocampus and associated with changes in memory function.3 Changes in dopamine are associated with declines in frontal lobe function, including executive function and working memory. In addition, the loss of dopamine is closely linked to impairments in gait, balance, and fine motor control.3 Alterations in serotonin with age correlate with declines in activity level and diminished balance in mice.3 Norepinephrine levels decrease in the cerebellum and are related to diminished motor learning with age.
In the peripheral nervous system there ...