6.6: Age Changes in Sensory Functioning and Consequences

Age changes that affect the sensory neurons are important because by providing monitoring and communication, these neurons initiate reflexes and start or influence many voluntary actions, memories, thoughts, and emotions. Therefore, alterations in sensory functioning can affect homeostasis and the quality of life.

Aging causes a gradual decline in sensory functioning as a result of a reduction in the numbers of several types of sensory neurons, a decline in the functioning of the remaining sensory neurons, and changes within the CNS. The following section concentrates on changes in PNS sensory neurons other than those involved in vision, hearing, and other inner ear functions.

Skin Receptors

In the skin there is little change in either the number of sensory neurons for touch that are associated with hairs or the number of pain receptors. However, touch receptors called Meissner's corpuscles, which are not associated with hairs, and pressure receptors called pacinian corpuscles decrease in number and become structurally distorted. In addition, the capsule in each pacinian corpuscle becomes thicker. Further reductions in sensations from the skin seem to result from a weakening of the action potentials that conduct impulses to the CNS. Alterations in action potentials may be due to age changes in neuron cell membranes or thickening of the myelin that surrounds many sensory neurons.

The age changes in Meissner's corpuscles and pacinian corpuscles lead to a decreased ability to notice that something is touching or pressing on the skin; identify the place where touch or pressure is occurring; distinguish between being touched by one object and being touched by more than one at the same time; and identify objects by touching them. In addition, some skin sensory neurons require more time to respond to stimuli; this may contribute to the declining ability to feel vibrations, particularly those with higher frequencies. An age‑related increase in impulse speed in some sensory neurons may partially compensate for these changes.

In addition to the effects of age changes on sensory neurons, the monitoring of conditions in and on the skin may be altered by changes in the thickness of the skin and the subcutaneous layer; the quality and distribution of hair; the ability of the CNS gray matter to respond to and interpret impulses from sensory neurons; and psychological status. Because of these factors, the effects of aging on the perception of temperature and pain are ambiguous.

Decreases in the ability to detect, locate, and identify objects touching or pressing on the skin result in decreases in the ability to respond to those objects. As a consequence, harmful objects may be encountered more frequently, more severely, and for longer periods. There is also a decline in the ability to perform precise actions that depend on good sensory input, such as moving the lips when forming words and manipulating small objects with the fingers. Reductions in skills may lead to problems in certain professions and loss of satisfaction with hobbies. Furthermore, reduced sensation means reduced pleasure from favorable physical contact, and this can have psychological and social consequences. Since sensory neurons associated with pain release substances that promote wound healing, age-related decreases in these neurons or in processing impulses from them may contribute to the age-related slowing of healing.

Sense of Smell

Aging causes decreases in the number of sensory neurons for smell. These neurons are called olfactory neurons and are high in the nasal cavities. Aging also causes deterioration of the pathways that carry olfactory impulses through the brain. All these changes cause a decline in the ability to detect and to identify aromas. The degree of change is difficult to measure, however, because of the influence of changes in other brain functions (e.g., memory, emotional state) and of previous experiences. Furthermore, the degree of change seems to be highly variable among individuals.

Since much of what is commonly referred to as flavor is actually aroma, age changes in the sense of smell reduce the pleasure derived from eating and can contribute to malnutrition. Reduced olfaction also means a reduced ability to detect harmful aromas such as toxic fumes and dangerous gases. Finally, a declining ability to notice offensive odors can lead to socially embarrassing situations.

Sense of Taste

The sense of taste accounts for only four of the sensations that many people call flavors; all other flavors are due to the sense of smell. The four taste flavors are salt, sweet, sour, and bitter. Aging seems to cause slight decreases only in the ability to detect salty and bitter substances. The amount of change is highly variable among individuals, and the ability to detect salt declines the most.

Even in the oldest individuals, the threshold levels for these four taste sensations are well below the levels in ordinary foods. The threshold for a stimulus is the lowest level of that stimulus which causes a response. If the threshold for tastes approaches the values found in foods, adding more of the ingredient that produces the flavor can compensate for this age change. Therefore, unlike the sense of smell, age changes in the sensory neurons for taste normally do not have a significant effect on food selection or diet. Of course, this may not be true for persons with medical problems such as high blood pressure because these individuals may be on restricted diets that prohibit the use of flavorings such as salt. It may also be untrue for individuals who smoke because smoking greatly reduces taste sensations.

A main reason for the small age change in the sense of taste may be the lifelong ability of these sensory neurons to reproduce rapidly and thus replace taste receptors lost to aging or injury (e.g., from hot foods).

Other Sensory Neurons

Other types of sensory neurons that seem to have reduced functioning because of aging include those which monitor blood pressure in arteries; materials in the throat; thirst; amount of urine in the urinary bladder; amount of material in the rectum (the end of the large intestine); and positions, tensions, and lengths of the joint structures, muscles, and tendons. Additional decrements in these sensory functions may derive from changes in the ability of the organs being monitored to stretch and from alterations in the ability of the CNS to respond to sensory impulses.

Corresponding outcomes from these decreases in sensory functioning include high blood pressure; dehydration; swallowing and choking problems; urinary incontinence; constipation or bowel incontinence; and reduced control and coordination of voluntary movements.