Peripheral nerve function is significantly affected by age, with functional deficits resulting from fiber loss, myelin sheath abnormalities, or connective tissue changes and nerve vascularization. Age also influences the ability of peripheral nerves to regenerate and reinnervate target organs, with different patterns for motor, sensory and autonomic fibers. Experimental studies on aging show that morphological and morphometric changes of peripheral nerves of rats are not linear during life. Some of the morphological observations with aging include an increase in the heterogeneity of the structure of myelin fibers: changes in the thickness and shape of the myelin sheath, abnormalities in the width of Schmith Lanterman incisures, separation of myelin sheaths lamellae (myelin splits) appearance of ovoid myelin figures inside the axon, images suggestive of demyelination and axonal degeneration. Foamy cells with myelin content in the cytoplasm, suggestive of active macrophages, have also been identified as well as the proportion of irregular myelin fibers increased progressively with age. Morphometric parameters also change with aging, mainly the decrease in the area and diameter of the myelinated fiber and myelin sheath, the decrease in the area and diameter of the axon, and the decrease in the Schwann cell nuclei number. Functional studies confirmed nerve alterations with aging, in humans and experimental animals, demonstrating a reduction in nerve conduction velocity, with differences for sensory and motor nerves, being sensory nerves affected first. This review lightens up chronologic literature support indicating that aging affects peripheral nerve morphology and morphometric parameters, impairing nerve function, either in experimental animals or humans.

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