[Contribution of ultrastructural analysis to the knowledge of the neuromuscular relationships in the nodal and conduction tissue].

The ultrastructure of the atrio-ventricular junction in the rat heart fixed in situ was studied on serial fine sections. The relationships between the specialized tissue and the nervous system were studied after labelling adrenergic endings with 5-OH-dopamine. Among the various types of nerve endings a distinction may be made between: 1) adrenergic or cholinergic free endings; 2) large "cluster" endings of the cholinergic types, seen mainly in the equatorial regions of nodal cells; 3) small diameter cholinergic before the development of heart failure or significant card synaptic images; 4) afferent neurons, whose endings, rich in glycogen and mitochondria supply the conduction tissue fibres, getting into or through the muscle fibre. The intermembrane space is then 20 nm. The analogy between the images observed and the ultrastructural aspects of intrafusorial fibres in skeletal muscle is discussed.