Fibre composition in the interosseous nerve of the pigeon.

The interosseous nerve of birds innervates a string of Herbst corpuscles located near the interosseous membrane between the tibia and fibula. Fibre composition of this nerve was assessed including both myelinated and unmyelinated axons. The diameter of the whole nerve is approximately 100 microm. Complete data were obtained for 3 nerves. The mean total number of myelinated fibres and unmyelinated axons was 2872 +/- 53. The mean number of myelinated fibres was 280 +/- 20 and that for unmyelinated axons was 2600 +/- 47. There was a broad distribution of diameters for myelinated fibres ranging from approximately 2 microm to 10 microm with a distinct peak at approximately 3-5 microm and a less prominent second peak at 6-8 microm. Similarly, myelin sheath thickness distribution showed 2 peaks, one at 0.6-0.8 microm and another at 1.4-1.6 microm. It is suggested that the group represented by the second peak innervates the Herbst corpuscles. The group of smaller myelinated fibres and the unmyelinated axons are assumed to innervate other types of receptors, some of which may be nociceptors.