Morphology of the rat carotid sinus nerve. II. Number and size of axons.

Ten carotid sinus nerves from five rats were examined by electron microscopy at a level of 0.5 mm from the glossopharyngeal nerve (nerve IX). The sinus nerves were found to contain from 455 to 757 (mean 625) axons per nerve, of which an average of 86.3% were unmyelinated. The unmyelinated axons had a size distribution that fitted a Gaussian distribution with a mean diameter of 0.78 micron and a variance of 0.013 micron. Such axons ranged in size from 0.17 to 1.7 microns. The myelinated axons had a unimodal size distribution skewed to the right, with a median total fibre diameter of 2.49 microns. Although total diameter of myelinated fibres ranged from 1.5 to 5.3 microns, 96% of such fibres were smaller than 4 microns. Axon diameter of myelinated fibres averaged 64% of the total diameter, but this proportion tended to increase with the size of the axon. Some 68% of myelinated fibres had axons with a diameter within the range of sizes of unmyelinated axons. The number of axons varied along the length of the sinus nerve, but no consistent pattern of change was found among different rats. The two nerves examined at 0.1 and 0.5 mm from nerve IX had 8-10 more myelinated axons at the more distal level, and the number of unmyelinated axons increased by four in one nerve but decreased by 26 in the other nerve. In three nerves examined at 0.5 and 2.0 mm from nerve IX, the number of unmyelinated axons increased from proximal to distal by 11 (2%) to 220 (43%), whereas the number of myelinated axons increased by 20 (48%) in one nerve but decreased by 7-10 (13-21%) in the others. One day after nerve IX was cut distal to the petrosal ganglion, most myelinated axons in the sinus nerve were degenerating and only 109 unmyelinated axons were still present. By four days all but two myelinated axons were gone and the normal complement of unmyelinated axons was replaced by more than 1800 rounded profiles, most of which probably were pseudopodia of reactive Schwann cells. Transection of nerve IX central to the petrosal ganglion did not produce such ultrastructural changes in Schwann cells, nor did it reduce the number of axons in the sinus nerve to a degree sufficient to be detected by the counting procedure. Although these results indicate that most axons in the sinus nerve are sensory, some nonsensory axons undoubtedly are present too. The sensory and nonsensory axons in the nerve apparently are closely associated with one another and in some cases might be enveloped by the same Schwann cells.