Figure 1.
Deciphering projections from the superficial MDH to the caudalmost ventrolateral medulla.
Brightfield photomicrographs of brain sections reacted for BDA after its injection into the MDH in three cases. These injections (marked by X in A, E, and I) suggested a projection from neurons in the superficial MDH (laminae I–II) to the cmVLM, including the caudal pressor area (CPA; encircled in A, B, F, J, K). This projection becomes more apparent as the injection center moves from ventrolateral (A) to intermediate (E) to dorsomedial parts of the MDH. Terminal-like label in the cmVLM in B, F, and J is seen in higher magnification in C, G, and K, respectively; note that many small varicosities are associated with these labeled fibers. When injections were centered superficial to the II–III laminar border (white line in A, E), the cmVLM was marked with terminal label (C, G,) but was relatively void of such label (K) with an injection deeper (I). The absence of label in laminae III–IV is seen in D, H, and L, but some large neurons in lamina V were encrusted by boutons (inserts in D, H, L). Encircled area in K marks the CPA. Abbreviation: LRt, lateral reticular nucleus; I, II, III, IV, V, lamina I through V of the dorsal horn. See text for other abbreviations.
Figure 2.
The caudalmost ventrolateral medulla receives projections from neurons in laminae I and II of the TCC.
Line drawings and photomicrographs of sections from cases injected with a retrograde tracer in the cmVLM. An injection of fluorogold into the cmVLM (A) resulted in numerous retrogradely-labeled neurons in laminae I and IIo of the ipsilateral MDH (A, B, C), the contralateral cmVLM, including the CPA (A, B, C), and the reticular formation. Numerous retrogradely-labeled neurons in the MDH (E–J) were intensely-stained and small, with dendrites oriented around the curvature of the dorsal horn (yellow arrowheads). Fewer were larger, more lightly-stained, and near the border between lamina II–III (E, J, arrow; H, yellow arrowheads). An injection of fluorescent microspheres, which are not incorporated into fibers of passage, showed similar results (G). Contours of these neurons (H) was used for morphometric analysis. Each dot in A–D represents a single labeled neuron. Abbreviations: Amb, nucleus ambiguus; AP, area postrema; Cu, cuneate nucleus; IO, inferior olivary nucleus; py, pyramidal tract; sp5, spinal tract of the trigeminal nerve; 10, dorsal motor nucleus of the vagus nerve; 12, hypoglossal motor nucleus. See text and previous figure for other abbreviations.
Figure 3.
The lateral medulla is implicated in a pathway from the TCC through the caudalmost ventrolateral medulla.
Line drawings showing immunoreactivity in the caudal brainstem to c-Fos after a 20 µl injection of 0.05% capsaicin into the temporalis muscle (A, B, C), projections from the injection of BDA into the lateral medulla (D, E, F), and the location of retrogradely-labeled neurons after injections of FG into the PBil (G, H, I). Dorsomedial parts of the MDH were labeled with Fos after temporalis injections of capsaicin (A), as was the CPA in the cmVLM (A, arrow). Since numerous neurons in the LRF were labeled after temporalis injections of capsaicin (B, C; arrows), injections of BDA were made here (E; arrow). Labeled fibers with varicosities spread to the cmVLM caudally (D; arrow) and more rostrally in the LRT (F). Numerous retrogradely-labeled neurons were found in the LRF (H, I) and CPA in the cmVLM (G) after injections of FG into the PBil (see Fig. 5L). Abbreviations: ECu, external cuneate nucleus; Gr, gracile nucleus; Sol, nucleus of the solitary tract; Sp5I, nucleus of the spinal tract of the trigeminal nerve, interpolar part; Sp5O, nucleus of the spinal tract of the trigeminal nerve, oral part; 7, facial motor nucleus; icp, inferior cerebellar peduncle. See text and previous figures for other abbreviations.
Figure 4.
Neurons in the caudalmost ventrolateral medulla and lateral reticular formation are suspected relays of the trigeminoreticulothalamic tract.
Photomicrographs of immunolabeling of neurons in the cmVLM to c-Fos after temporalis injections of capsaicin (A; CPA encircled), after BDA injections into the LRF (E; dashed outline) and FG into the PBil injections (G; CPA encircled). The medullary LRF was labeled with Fos immunoreactivity after temporalis injections of capsaicin (B, C; dashed white outline) which mimicked the distribution labeled neurons after the PBil injection of FG (H, I; dashed white outline). The injection of BDA into the LRF also labeled rostral facial motoneurons and the superior salivatory nucleus (F). Abbreviations: Li, linear nucleus of the medulla; SSN, superior salivary nucleus. See text and previous figures for other abbreviations.
Figure 5.
Pontine neurons implicated in pain pathways are labeled with c-Fos, BDA and FG in their respective experiments.
Photomicrographs and line drawings through the rostral medulla and pons showing immunoreactivity to c-Fos after capsaicin injection into the temporalis muscle (A–D), fibers and varicosities after an injection of BDA into the LRF (E–F), and retrogradely-labeled neurons after an injection of FG into the PBil (I–L). Note that the PBil is labeled both with c- Fos (B, arrow; D, encircled) and BDA (F, arrow; H, encircled). Labeling with each marker was in the A5 area (A, C, E, G, I, K), while dorsomedial parts of subnucleus oralis were labeled after PBil injections of FG (I, K). Abbreviations: LVe, lateral vestibular nucleus; Me5, mesencephalic trigeminal nucleus; Mo5, motor trigeminal nucleus; MVe, medial vestibular nucleus; PN, pontine nuclei; PnR, pontine raphe nucleus; RtTg, reticulotegmental nucleus; SO, superior olivary nucleus; Tz, trapezoid nucleus; VC, ventral cochlear nucleus; VLL, ventral nucleus of the lateral lemniscus; bc, brachium conjunctivum; g7, genu of facial nerve; mcp, middle cerebellar peduncle; 7n, facial nerve root. See text and previous figures for other abbreviations.
Figure 6.
Other areas implicated in pain were labeled after capsaicin injections into the temporalis muscle.
Numerous nuclei were immunohistochemically labeled with Fos in lamina I (B; yellow arrowheads) and lamina V of the TCC near the spinomedullary junction (A, B). Similar labeling was seen in the dorsomedial MDH near levels of the calamus scriptorius, but also included neurons in lamina II and III (C; D, arrow). Nuclei were labeled in dorsal subnucleus interpolaris (E, arrow; F, encircled), but their function is unknown. C-Fos profiles were found in the RVM bilaterally (G, H, arrow) dorsal and lateral to the pyramidal tract. The superior salivatory nucleus also was labeled with FOS after temporalis injections bilaterally (I), as were neurons in the subcoeruleus area (J) and A7 area (K). Abbreviations: RVM, rostral ventromedial medulla; SubC, subcoeruleus nucleus. See text and previous figures for other abbreviations.
Figure 7.
Sparse c-Fos labeling was seen in control rats.
Line drawings of brainstem sections showing nuclei immunoreactive to c-Fos after an injection of saline into the temporalis muscle of a rat. Note the minimal c-Fos label in dorsomedial parts of the MDH (A, B, C), the CPA in the cmVLM (A, B, C), lateral medulla (D, E), RVM (F), A5 area (H), and the external lateral, dorsal lateral and internal lateral subnuclei of the parabrachial complex (I, J) from this control animal. Compare to figures 3 and 4. Abbreviations: SpVe, spinal vestibular nucleus; SuVe, superior vestibular nucleus; ml, medial lemniscus. See text and previous figures for other abbreviations.
Figure 8.
The caudalmost ventrolateral medulla exhibits numerous peptides associated with pain pathways.
Photomicrographs through the cmVLM showing immunoreactivity to multiple peptides (white oval encircles the CPA). All these peptides have been implicated in the processing of painful information. See text for discussion and abbreviations.
Figure 9.
A trigeminoreticular pathway as a route for ascending noxious information.
Summary diagram illustrating the presumptive pain pathway from the TCC to the cmVLM and continuing through the LRF and PBil. We propose this is the initial part of a paleo-reticulo-thalamic pathway of the trigeminal system and suggest it may be important for transmitting deep pain from head and neck regions.