Nerve endings in bronchi of the dog that react with antibodies against neurofilament protein

Tree-like nerve endings in the smooth muscle layer of bronchi of the dog were examined by immunohistochemical staining with antibodies against neurofilament protein (NFP). The endings were revealed as ramified axon terminals, with arborisation at their termini. The endings were 100-300 microns in maximal length and 50-100 microns in minimal length. Most of the endings were arranged parallel to the smooth muscle strands. The endings were densely distributed in the proximal region but their density decreased towards the alveoli. In the histological sections, the endings were seen between smooth muscle cells. Terminal Schwann cells, which reacted with antibodies against glial fibrillary acidic protein and S-100 protein, and putative 'septal cells' with vimentin-like immunoreactivity were distributed near the endings. In addition, the nerve endings with NFP-like immunoreactivity were surrounded dense connective tissue that contained large amounts of fine elastic fibres. These findings indicate the nerve endings with NFP-like immunoreactivity are similar to other slowly adapting receptors (i.e. Golgi tendon organs, Ruffini endings). Some degenerated endings, which found in the unilaterally vagotomised dog, suggest the endings in the bronchi are originated from vagal nerves.