Functional development of the cervical sympathetic pathway in the neonatal rat.
Maturation of the cervical sympathetic innervation of the levator palpebrae smooth muscle was studied physiologically in vivo in the rat. Tonic pre- and postganglionic nerve activity and responses to centrally acting stimuli, asphyxia and hypoglycemia, were recorded. Development of functional capacity of the sympathetic nerve terminal-smooth muscle complex was assessed by measuring contractions evoked by directly acting noradrenergic agonists, endogenous nerve terminal norepinephrine released by tyramine, and electrical stimulation of preganglionic axons. Tonic preganglionic activity and responses to asphyxia and hypoglycemia were fully developed by the second postnatal day. Ganglionic neurotransmission, however, is not established until the end of the first postnatal week so that sympathetic outflow from the central nervous system (CNS) does not affect the nerve terminal-smooth muscle complex before this time. Functional capacity of the complex develops in a stepwise fashion with intervening plateau periods. There is an initial rapid increase before onset of ganglionic neurotransmission, and development to the mature level is attenuated only modestly by decentralization of the ganglion on the first postnatal day. It is concluded that conduction of impulses to the postganglionic neuron is not essential to initiate maturation of this nerve terminal-smooth muscle complex, that a significant degree of maturation occurs before ganglionic transmission is established, and that maturation to the adult level is usually largely independent of neural connections between the CNS and the postganglionic neuron.
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