Axons of Passage and Inputs to Superior Cervical Ganglion in Rat

Wheat germ agglutinin-horseradish peroxidase was injected into the entire (0.8 μL) or partial (rostral or caudal, 0.1–0.3 μL) superior cervical ganglion (SCG) of the rat (male Sprague–Dawley, N = 35) to examine the distribution of neurons in the middle (MCG) and inferior (ICG) cervical ganglion that send axons bypass the SCG. Whole-mounts of the SCG, cervical sympathetic trunk (CST), MCG, ICG, and sections of the brainstem and spinal cord were prepared. With entire SCG tracer injection, neurons were labeled evenly in the MCG (left: 258, right: 121), ICG (left: 848, right: 681), and CST (up to 770). Some neurons grouped in a single bulge just rostral to the MCG, which we termed as the “premiddle cervical ganglion” (pMCG). The left pMCG (120) is larger and has more neurons than the right pMCG (82). Centrally, neurons were labeled in lamina IX of cervical segments (C1: 18%, C2: 46%, C3: 33%, C4: 3%), intermediate zone of thoracic segments (T1: 31%, T2: 35%, T3: 27%, T4: 7%), and intermediate reticular nuclei (96%) and perifacial zone (4%) of brainstem. The rostral and caudal SCG injection selectively labeled neurons mainly in brainstem, C1-C2 and in T1-T2, respectively. Before projecting to their peripheral targets, many neurons in pMCG, MCG and ICG run rostrally within the CST rather than segmentally through the closest rami, from the level of SCG or above. Neurons in pMCG and MCG may have similar or complementary function and those in brainstem may be involved in the vestibulo-autonomic interaction. Anat Rec, 301:1906–1916, 2018.