MODULE SUPPLEMENT: NEUROLOGICAL SYSTEM
Changes with Age

A number of changes have already been alluded to in relation to the neuron, several of which we will discuss in more detail further on in the module. First, however, it's helpful to get an overall idea of the changes that occur in the brain with age that need to be considered when caring for an older adult.

The brain gets smaller as we age (atrophy) and experiences a decrease in weight, changes that are generally attributed to loss of neurons. Note that it is important to differentiate between neurons and neuroglia (definitions). Neurons are differentiated and generally non-replaceable, although some recent data have called this latter belief into question. In contrast, neuroglia can undergo cell division and proliferate, a point we'll come back to.

While loss of neurons occurs, it is localized to specific layers and regions of the brain and shows considerable individual variability (Timeras, 1994). Thus data are often conflicting, a problem that is exacerbated by the difficulties inherent in studying changes in humans rather than animals.

Losses in the locus ceruleus, a major source of catacholaminergic neurons, and the substantia nigra, which is part of the basal ganglia and a major source of dopaminergic neurons, have been reported and may be a factor in age changes in stability, gait, homeostasis, and sleep as well as cognitive functioning (Reeves, et al., 2002; Timeras, 1994). On the other hand, loses in the nucleus basalis of Meynert (an area of the Amygdala), which is a major source of cholinergic neurons, occur mainly in persons with Alzheimer's type dementia, with relatively minor changes occurring in non-demented elders (Timeras, 1994).

In general, changes tend to affect the white matter (myelinated neurons) to a greater extent than the gray matter (Mrak, et al., 1997). This may be consistent with the the idea that retrogenesis occurs with age and may underpin a number of neurodegenerative conditions (Braak, et al., 1999; Reisberg, et al., 1999). Retrogenesis is defined as "the process by which degenerative mechanisms reverse the order of acquisition in normal development" (Reisberg, et al., 1999). According to this hypothesis, the parts of the brain that undergo late myelination are first to undergo demyelination and the development of pathologic intra- and extra- cellular changes. While more data are needed to support this process, it has been linked to the stages of development of Alzheimer's disease and related clinical findings (Braak, et al., 1999; Reisberg, et al., 1999).