The weight of the brain declines with age. By the age of 50, the weight has been reduced to 97% and by age 70, to 92%.
Brain size and weight reduction occur mainly in the area of the cerebral cortex. Brains of the elderly have narrower gyri (ridges) and a wider sulci (depressions near the gyri), with the subarachnoid space becoming larger as well.
Brain shrinkage has been linked to a loss of cortical neurons, although evidence exists that neuron loss does not occur to the same degree in the brain stem nuclei.
Blood flow reduction to the brain occurs as fatty deposits gradually accumulate in blood vessels (arteriosclerosis). Although the reduction in blood flow does not cause a cerebral crisis, it does increase the risk of stroke.
Changes in synoptic organization of the brain occur. The number of dendrite branches and interconnections are lost, and the rate of neurotransmitter production also declines, and synaptic connections are also lost as a result.
Many brain neurons begin accumulating such abnormal intracellular deposits as pigments or proteins that have no known function, causing problems only when excessive amounts have accumulated.
The general lack of replacement neurons in the nervous system leads to the inevitable decline in sensory function. Part of this decline is sometimes compensated with an increase in stimuli or concentration, but the loss of axons to conduct sensory action cannot be increased in the same manner.
The sensory systems (hearing, vision, balance, smell, taste, etc) become less acute. Lights may be brighter, sounds louder, smells stronger, etc., before they are perceived.
Reaction times, coupled with delayed perceptions, can be slowed to the point of danger. Reflexes become weaker or disappear altogether. Motor skills are not as precise, taking longer to perform a task.
There is a decrease in the number and sensitivity of sensory receptors, dermatomes, and neurons, resulting in dulling of pain, touch, and tactile sensation.
There is also some decline in the function of cranial nerves mediating taste and smell. Decreased autonomic nerve activity supplying the eyes causes changes in pupil size and pupillary reactivity.
Olfactory (smell) receptor cells are not like other neurons. They are regularly replaced by the division of basal stem cells in the olfactory epithelium. Nevertheless, the numbers produced do decline with age, causing the remaining receptors to become less sensitive. As a result, detecting odors declines with age.
Gustatory (taste) sensitivity declines as a result of the thinning of the numbers of mucous membranes, as well as a reduction in the numbers and sensitivity of taste buds. At the beginning of life, there are roughly 10,000 taste buds. That number is drastically reduced by the age of 50. As a result, the elderly often find food bland and tasteless, contributing to dietary problems. Children, on the other hand, will find the same food too spicy.
In the ear, there is often cumulative damage to hair cells in the organ of Corti after the age of 60. Older adults lose the ability to hear high-pitched sounds and the consonants ch, f, g, s, sh, t, th, and z.
However, hearing is less affected by aging than other senses despite popular opinion. The tympanum does lose some of its elasticity, causing high-pitched sounds to be heard less often. The progressive loss of hearing is called presbycusis.
Vision diminishes by age 70, primarily because of a decrease in the amount of light that reaches the retina and impaired focusing of the light on the retina. The muscles of the iris become less efficient so the pupils remain somewhat constricted most of the time. The lacrimal glands become less active and the eyes become dry and more susceptible to bacterial infection and irritation.
Various visual disorders are common with advancing age:
- The lens loses its elasticity and begins to stiffen, resulting in presbyopia (difficulty in seeing objects up close). In children, the inner limit of clear vision changes from 7-9 cm; in young adults, from 15-20 cm; by age 60, that distance has increased to 83 cm.
- Cataracts are often formed.
- There is a gradual loss in the number of rods, which explains why individuals after the age of 60 often require twice as much light as they did in their 40s.
- Macular degeneration often occurs and is the leading cause of blindness in those over the age of 50. It is typically associated with the growth and proliferation of blood vessels in the retina. The leakage of blood and plasma from these abnormal vessels causes retinal scarring and a loss of photoreceptors. The vascular growth begins in the macula lutea (the area of the retina responsible for acute vision). Color is affected as the cones deteriorate.
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