EFFECTS OF AGING ON THE SENSES
As a person ages, both the general and the special sensory
func-tions gradually decline. Among the general senses, free nerve endings and
hair follicle receptors in the skin remain largely unchanged. However, the
numbers of Meissner corpuscles and pacinian corpuscles decrease with age, and
those that remain are often structurally distorted and less functional. As a
result, elderly people are less conscious of something touching or pressing the
skin, which increases the risk of skin injuries. The sense of two-point
discrimination decreases, and the elderlyhave a more difficult time identifying
objects by touch. A loss of pacinian corpuscles also decreases their awareness
of limb and joint positions, which can affect balance and coordination. The
functions of receptors for proprioception also decline with age, which
decreases information on the position, tension, and length of tendons and
muscles. This can further reduce coordination and control of movements.
Among the special senses,
elderly people experience only a slight loss in the ability to detect odors.
However, their ability to correctly identify specific odors decreases,
especially in men over age 70.
In general, the sense of
taste decreases as people age. The number of sensory receptors decreases, and
the brain’s ability to interpret taste sensations declines.
The lenses of the eyes lose
flexibility as a person ages because the connective tissue of the lenses
becomes more rigid. Consequently, the lenses’ ability to change shape initially
declines and eventually is lost. This condition, called presbyopia, is the most common age-related change in the eyes. In
addition, the number of cones decreases, especially in the fovea centralis,
resulting in a gradual decline in visual acuity and color perception.
The most common visual
problem in older people requiring medical treatment, such as surgery, is the
development of cataracts. Following cataracts in frequency are macular
degeneration, glau-coma, and diabetic retinopathy, in that order (see the
Diseases and Disorders table earlier).
As people age, the number of
hair cells in the cochlea decreases, resulting in age-related sensorineural
hearing loss, called presbya cusis. This
decline doesn’t occur equally in both ears. Therefore,because direction is
determined by comparing sounds coming into each ear, elderly people may
experience a decreased ability to local-ize the origin of certain sounds. This
may lead to a general sense of disorientation. In addition, CNS defects in the
auditory pathways can lead to difficulty understanding sounds when echoes or
back-ground noises are present. This deficit makes it difficult for elderly
people to understand rapid or broken speech.
With age, the number of hair
cells in the saccule, utricle, and ampullae decreases. The number of otoliths
also declines. As a result, elderly people experience a decreased sensitivity
to gravity, acceleration, and rotation, which may lead to disequilibrium
(insta-bility) and vertigo (a feeling of spinning). Some elderly people feel
that they can’t maintain posture and are prone to fall.