Somatic motor axons can innervate autonomic neurones in the frog heart

The effects of sympathetic and parasympathetic stimulation on the heart rate in frogs were tested after hearts were reinnervated with a somatic motor nerve. When frogs were vagotomized and hypoglossal axons were redirected to the heart for 8 or more weeks, stimulating the redirected hypoglossus nerve produced a parasympathetic-like inhibition of the heart. Stimulating sympathetic rami of the anastomosed hypoglossus nerve produced cardiac acceleration. Individual parasympathetic neurones received synaptic input from hypoglossal terminals. The excitatory post-synaptic potentials evoked by hypoglossal stimulation were much smaller than those evoked by vagal stimulation in control or vagal-reinnervated ganglia. However, hypoglossal axons innervated most (71%) of the ganglion cells and this level of innervation persisted for at least 60 weeks. Hypoglossal axons formed networks of varicose terminals within cardiac ganglia and established axo-axonic synapses with parasympathetic neurones. Hypoglossal terminals did not reinnervate the neuronal perikarya, in contrast to vagal axons in control or vagal-reinnervated ganglia. Axo-axonic synapses from redirected hypoglossal axons were identified in cardiac ganglia by bathing isolated hearts in horseradish peroxidase (HRP) and stimulating the redirected nerve. Electron micrographs showed that axo-axonic synapses contained HRP-labelled presynaptic vesicles. The source of foreign innervation in experimental cardiac ganglia was confirmed to be hypoglossal motoneurones (a), by comparing the conduction velocity of the redirected presynaptic axons (1.32 m/sec) with regenerating vagal preganglionic fibres ( < 0.3 m/sec), and (b), by retrograde HRP-labelling of large motoneurones in the hypoglossal nucleus after applying peroxidase to the axons which had grown into the heart.