Functional history of two motor neurons and the morphometry of their neuromuscular junctions in the gill of Aplysia: evidence for differential aging.

Because the viability of the gill withdrawal reflex is dependent on age in Aplysia, we examined physiologic and morphometric properties of two motor neurons, L7 and LDG1, involved in the reflex in three postmetamorphic age groups: young, mature, and old. L7's capability to elicit pinnule contraction, a major component of the reflex, was reduced markedly in old gills; facilitation at old L7 terminals, upon which contraction is dependent, was significantly reduced. The morphology of the pinnule neuromuscular junctions changed with increased age. In contrast, LDG1's capability to elicit efferent vessel contraction, a major component of respiratory movements, was not significantly altered by age; facilitation also was relatively unaffected; and morphologic changes at neuromuscular junctions were poorly correlated with age. Aging occurs differentially in two motor neurons innervating the gill. The dissimilarity in function and in frequency of activation of L7 and LDG1 may help explain the greater vulnerability of L7 to the effects of aging. The possibility that disuse is involved in the aging process is discussed.