Characterization of two ganglion cell populations in avian ciliary ganglia

Abstract

The cellular organization of the avian ciliary ganglion was investigated with electrophysiological, histological and pharmacological techniques. Nerve connections in the pigeon ciliary ganglion were studied. The two cell populations which had been described anatomically were found to distribute their axons separately in the choroid and ciliary nerves. There was no connection between the two groups of cells. These findings also apply to the chick ciliary ganglion. The ciliary population has 2953±683 (mean ± S.D.) ganglion cellswith single myelinated axons 5.4 ± 2.1 μm (median ± A.D.) in diameter. The choroid group is composed of 3084 ± 640 (mean ± S.D.) cells with 2.1 ± 1.0 μm (median ± A.D.) diameter myelinated axons. The conduction velocity of the presynaptic fibers which innervate the low threshold ciliary cells is 6.9 ± 3.04 m/sec (mean ± S.D.). The conduction velocity of the presynaptic fibers innervating the choroid cells is 3.4 ± 0.7 m/sec (mean ± S.D.) at 22–24°C. Similarly, the latency of the orthodromically initiated ciliary chemical response was 1.8 ± 0.5 msec (mean ± S.D.) shorter than that of the choroid response. The cells which send axons to the ciliary nerves posses a dual, chemical and electrical, mechanism of synaptic transmission. The cells sending axons to the choroid nerves only have chemical transmission. After sectioning of the ciliary nerves, leaving choroid nerves intact, retrograde changes occur only in the cells possessing myelin lamellae, i.e., the cliary cells. No changes were found in the choroid cells. Thus, electrical coupling is associated with the myelinated cells. Although chemical synaptic transmission is cholinergic for both cell group, the choroid cells are more suscetible to block by hexamethonium than are the ciliary cells. Conversely, the ciliary neurons are more sensitive to d-tubocurarine than are the choroid. A given concentration of dihydro-β-erythroidine reduced transmission about equally in the choroid and ciliary pathways. Such a pattern of selectivity by different blockers suggests that the properties of the cholinergic receptors of the two ganglion cell populations may differ qualitatively. Adrenergic blocking agents did not modify transmission in the avian ciliary ganglion.