Fine structure of the neuromuscular junction in the small intestine of a teleostean fish: tench (Tinca tinca L.).

Innervation of the midgut musculature has been studied in the tench (Tinca tinca) by light and electron microscopic methods. Smooth and striated muscle layers of the tunica muscularis of the gut are innervated by the same nerve bundles. Thick nerve bundles are found in the connective tissue between the two muscle layers projecting branches into both the smooth and striated muscle layers. The nerve fibres are varicose, the varicosities contain various vesicle populations. Some of these comprise small clear vesicles (measuring 40 nm in diameter), and a few dense-core vesicles (120 nm in diameter), while in others the amount of dense-core vesicles is higher; furthermore there are varicosities containing granules of peptidergic character measuring 200 nm in diameter. Nerve fibres often form neuropil in which synapses have not been observed. Axon terminals establish parallel membrane contacts with the muscle cells without synaptic specialization in the smooth muscle layer. Innervation of the striated muscle layer seems to be poor. Motor end-plates could not be found on the muscle fibres of the midgut, while these are common in the oesophagus, where the occurrence of striated muscle fibres is dominant.