The innervation of glomus cells, ganglion cells and blood vessels in the rat carotid body: A quantitative ultrastructural analysis

We studied the ultrastructure of the rat carotid body and found that glomus cells (Type I cells) are of two types (A and B) based on the size of their dense-cored vesicles. Dense-cored vesicles in type A cells have a mean diameter nearly 30% larger than those in type B cells. Although we seldom found nerve endings on type B cells, at least two types of nerves end on type A cells. Axonal degeneration studies showed that more than 95% of these nerves are afferent axons which leave the carotid body in the carotid sinus nerve and have their cell bodies in the sensory (petrosal) ganglion of the glossopharyngeal nerve. Less than 5% are preganglionic efferent axons from the cervical sympathetic trunk which enter the carotid body with axons from the superior cervical sympathetic ganglion. We found no efferent axons from the glossopharyngeal nerve which end on glomus cells, although some do end on ganglion cells. Afferent and efferent nerve endings can be distinguished morphologically, although both types contain many synaptic vesicles and few large dense-cored vesicles. Synaptic vesicles in afferent nerve endings are 15% larger but 60% less numerous than those in efferent nerve endings. Large densecored vesicles in afferent nerve endings are similar in size but 80% less numerous than those in efferent nerve endings. Some regions of afferent nerve endings are presynaptic to glomus cells, some are postsynaptic, and some form reciprocal synapses. Efferent nerve endings are presynaptic to glomus cells but not in synaptic contact with afferent nerve endings. Blood vessels in the carotid body have both a parasympathetic and a sympathetic innervation. Most parasympathetic vasomotor nerves arise within the carotid body from ganglion cells whose preganglionic innervation is from the glossopharyngeal nerve. Terminals of these vasomotor nerves contain clear-cored synaptic vesicles. Sympathetic vasomotor nerves, most of which come from ganglion cells in the superior cervical ganglion (and from a few ganglion cells in the carotid body) have dense-cored synaptic vesicles. We postulate that (I) afferent nerve endings, which are interconnected with glomus cells by reciprocal synapses, are chemoreceptors; (2) glomus cells are dopaminergic interneurons which modulate the sensitivity of chemoreceptive nerve endings; (3) glomus cells and afferent nerves interact through reciprocal synapses which form an inhibitory feedback loop: sensory nerves release an excitatory transmitter when stimulated, the transmitter causes glomus cells to release dopamine, and dopamine inhibits the sensory nerves; (4) the feedback loop may contribute to the hyperbolic nature of the curve described by the relationship between arterial oxygen pressure and the rate of chemo-receptor firing; (5) by enhancing dopamine release from some glomus cells, preganglionic sympathetic nerves decrease chemoreceptor activity, an effect opposite from that of vasoconstriction produced by postganglionic sympathetic nerves on blood vessels j (6) synaptic interconnections enable glomus cells to influence one another. We cannot exclude the possibility that glomus cells, like afferent nerve endings, are chemoreceptors sensitive to hypoxia and hypercapnia or that glomus cells, in addition to their other functions, secrete a polypeptide hormone.