Morphology of the rat carotid sinus nerve. I. Course, connections, dimensions and ultrastructure.

An analysis of the morphology of the carotid sinus nerve in 39 rats revealed that the nerve emerged from the glossopharyngeal nerve (nerve IX) 0.6-1.0 mm beyond the distal extent of the petrosal ganglion. In 68% of cases the nerve consisted of a single bundle of axons; two bundles were present in the other nerves. Near the rostral pole of the carotid body, the nerve divided into multiple bundles of axons. Most axon bundles entered the carotid body, but some instead joined sympathetic nerves from the superior cervical ganglion. Some of the latter group of sinus nerve axons innervated the wall of the carotid sinus. Other presumptive baroreceptor axons reached the carotid sinus from the sinus nerve by first traversing the carotid body. The sinus nerve had an average length of 2.0 +/- 0.13 mm (mean +/- S.E.M., range = 1.3-2.9 mm, N = 15) and was elliptical in cross-section (major axis = 77 microns, minor axis = 46 microns). The nerve contained an average of 625 axons, 86% of which were unmyelinated. The perineurial sheath that enveloped the circumference of the nerve was comprised of 3 or 4 layers of cells interconnected by gap junctions and tight junctions. Near the carotid body, the perineurium extended into the nerve where it compartmentalized fascicles of axons. The endoneurium consisted mainly of collagenous fibres (mean diameter 40 nm) that were only half the size of those outside the perineurium. Endoneurial connective tissue cells were sparse, being only 5% as numerous as Schwann cells. The one blood vessel in the nerve (mean luminal diameter 12.4 +/- 1.2 microns) usually arose from vessels in nerve IX and terminated in a venule at the rostral surface of the carotid body. Ganglion cells were located in nerve IX near the origin of the sinus nerve and along the length of the sinus nerve itself. All ganglion cells examined were postsynaptic to vesicle-containing nerve terminals, and therefore were presumed to be autonomic rather than sensory. Ganglion cells were more numerous in terminal branches of the sinus nerve on the ventral surface of the carotid body. Most other ganglion cells in the carotid body were located among axons of the ganglioglomerular nerve, although some were associated with a small branch of the vagus nerve. Paraganglia were located within the sinus nerve in 15% of cases, and in another 10% of cases they were found nearby in nerve IX. The paraganglia, which measured approximately 50 X 50 X 100 microns, were innervated by axons that resembled sensory nerves of the carotid body.