Effects of Aging on the PNS (Peripheral Nervous System)

Effects of Aging on the PNS

The majority of skeletal muscles are derived from paired blocks of mesoderm, also called somites. These are distributed in segments down the posteromedial aspect of the developing embryo. The spinal nerves branch from the spinal cord as well as the adjacent neural crest. They exit between the vertebrae. Each spinal nerve becomes associated with its adjacent mus-cle mass. The spinal nerves supply sensory and motor fibers to the developing muscles, helping to guide their maturing process. The cranial nerves innervate the head muscles in a similar way.

The cutaneous nerves are distributed to the skin in a related pattern, with the trigeminal nerves inner-vating most of the facial skin and scalp. Spinal nerves supply cutaneous branches to certain adjacent der-matomes, which eventually become dermal segments.

Distribution and growth of the spinal nerves is related to the segmented plan of the body. This is established by the fourth week of embryonic development. Limb growth and unequal growth of other areas of the body develop an adult pattern of dermatomes. These have unequal sizes and shapes and overlap in various degrees. Since embryonic muscle cells migrate to a large degree, much of the early segmented pattern is lost.

With aging, sensory receptors atrophy. The mus-cle tone of the face and neck decreases. There is slower conduction of impulses, decreased sensation, slower reflexes, and often, clumsiness. Nerve conduction slows when the myelin sheaths surrounding the nerves degenerate. Neurons are lost and there are fewer syn-apses per neuron. Blood flow also decreases with aging. Nearby bones overgrow and may put pressure on the nerves. The peripheral nerves themselves remain viable and retain normal function throughout life. However, age-related functional changes are more noticeable when nerves are injured by conditions such as diabetes. The response of the PNS to injury becomes reduced. Self-repair of axons is slower and incomplete in older people, making them more vulnerable to injury and disease. When ischemia affects the PNS, the most com-mon symptoms are regional tingling or numbness.