In Aplysia of different ages, three well defined behavioral responses and their substrates were examined. 2. Two of the behaviors and their substrates are age-sensitive, the gill withdrawal reflex, and osmoregulation. They are sensory-dependent for their activation. The third, the gill respiratory pumping movements, GPM, and its substrates is age-invariant. It is initiated by a network in the CNS, and modulated by sensory input. 3. Age-sensitivity of a neuron appears dependent on its pathway: the pathways mediating sensory-initiated behavior are more vulnerable to aging than the pathway mediating CNS-initiated behavior. 4. a) The age-sensitive, GWR, gill withdrawal reflex, in unrestrained animals and in reduced preparations conforms to the age-dependent reduced functioning of L7, the major mediator of the reflex. b) Whereas, the age-invariant GPM conforms to the age-independent functioning of LDG1, a major contributor to GPMs. c) The age-related slowing of osmoregulation is consistent with the lack of response in the putative water balance neuron, R15, to stimulation of osmoreceptors by dilute seawater. 5. Age-sensitivity of L7 is defined by its reduced function, decreased facilitation at its terminals, reduced input resistance, and remodeling of junctions in the muscles it innervates. This is in contrast to the age-invariance of these properties in LDG1 and of the junctions in muscles it innervates. Thus far, the age-sensitivity of R15 is revealed by its reduced responsiveness to synaptic input, reduced input resistance, and little response to osphradial stimulation. 6. Age-sensitivity of the GWR, osmoregulation and of their substrates is not necessarily maladaptive.(ABSTRACT TRUNCATED AT 250 WORDS)
