Distribution of myelinated and unmyelinated nerve fibers and their possible role in blood flow control in crotaline snake infrared receptor organs

Abstract

We used transmission electron microscopic montages to examine the composition of nerve bundles serving the infrared pit organs of two species of crotaline snakes, Agkistrodon blomhoffii and A. brevicaudus. In the three main bundles, the myelinated fibers totaled 2,200-3,700, and unmyelinated fibers 2,400. We also discovered for the first time two accessory bundles composed almost entirely of unmyelinated fibers running alongside the main bundles, containing an average total of 3,300 unmyelinated fibers vs. an average of 10 myelinated fibers. Thus, the average total of unmyelinated fibers was nearly twice that of myelinated fibers. To study the nature of the unmyelinated fibers, we did double staining immunohistochemistry with antibodies for substance P (SP) and vasoactive intestinal polypeptide (VIP) in combination with and without capsaicin pretreatment. SP and VIP immunoreactive varicose fibers ran straight toward the center of the pit membrane in parallel with arterioles and venules, and also formed a dense network around the periphery of the membrane. There were three types of fibers: fibers containing only SP, fibers containing only VIP, and fibers containing both peptides. SP-only fibers were distributed singly throughout the pit membrane and in small bundles around the periphery. SP+VIP fibers were distributed sparsely in the pit membrane and around its periphery. VIP-only fibers were distributed throughout the pit membrane and were of smaller diameter than SP and SP+VIP fibers. After treatment with capsaicin, most of the three types of varicose fibers disappeared from the central part of the pit membrane, but those around the periphery remained unaffected. The capsaicin-sensitive fibers may be unmyelinated sensory types, and the unaffected ones may be autonomic nerve fibers.

Organotopic organization of the primary Infrared Sensitive Nucleus (LTTD) in the western diamondback rattlesnake (Crotalus atrox)

“…Even though TNMs in the epithelium are formed by different nerve branches, the diameters of all TNMs are very similar, compatible with the measurements of earlier reports (Bullock and Fox, ; Terashima et al, ). Furthermore, the number of labeled TNMs in our study of ∼5,000 for a pit area of ∼6 mm 2 was comparable to the number of TNMs (Hisajima et al, ) and the number of infrared‐sensitive cell bodies in the trigeminal ganglia reported for Gloydius brevicaudus (Kishida et al, ) as well as for Crotalus horridus (Bullock and Fox, ) after correction for noninfrared‐sensitive C‐fibers in the latter study. Although the peripheral innervation of the pit membrane by V2s and V2d is separated, there is considerable intermingling of the cell bodies in the maxillomandibular ganglionic complex, compatible with the inference from previous studies (Kishida et al, ; Terashima and Liang, ).…”

Snake Infrared Receptors Respond to Dimethylsulfoxide in the Blood Stream

“…It is also possible that DMSO affects the IR response via the pit vasculature in crotaline snakes. However, the potent vasodilator CGRP and substance P (McCulloch et al, 1986) do not affect the IR response in this snake (Terashima and Moon, 2003), although the coexistence of CGRP and substance P immunoreactivity has been reported in autonomic/c-fiber nerves in the pit membrane (Kadota et al, 1997;Hisajima et al, 2002). It is interesting, therefore, that all the effective vasodilators (EtOH, DMSO, and NO) increased the IR response, while a vasoconstrictor (ET-1) decreased it.…”

Thermoconforming rays of the star-nosed mole

“…The enhanced visual acuity gained from maintaining the eye at an elevated temperature provides a distinct reaction time advantage over their highly active, but ectothermic prey. On the other hand, consider an ectothermic predator that senses heat; the pit organ of the pit viper is a highly evolved infrared sensing tissue, consisting of a thin membranous tissue dense in mitochondria, myelinated and unmyelinated nerves, and a high vascularity (Goris et al, 2007; Hisajima et al, 2002). The latter has been argued to aid in providing rapid blood flow to help reduce after images formed during heat sensing (Goris et al, 2007).…”