Axonal projection patterns of ventrolateral medullospinal sympathoexcitatory neurons

Abstract

We studied the following properties of cat ventrolateral medullary (VLM) neurons that projected to the thoracic spinal cord: the relationship between their spontaneous activity and that in the inferior cardiac postganglionic sympathetic nerve, their responses to baroreceptor-reflex activation, their axonal conduction velocities, the funicular trajectories of their axons, the likely sites of termination of their axons, and their axonal branching patterns. Microstimulation in the second thoracic spinal segment (T2) antidromically activated 67 VLM neurons (as determined with time-controlled collision of spontaneous and evoked action potentials), whose activity was correlated to inferior cardiac sympathetic nerve discharge (as determined with spike-triggered averaging). We tested the effect of baroreceptor-reflex activation on the firing rate of 20 of these VLM-spinal neurons. Because the firing rate decreased in each instance, these neurons apparently subserved a sympathoexcitatory function. The axonal branching patterns of 51 VLM-spinal sympathoexcitatory neurons were studied. Thirty-four neurons were antidromically activated by stimulation in the T2 gray matter and in more caudal thoracic spinal segments (T11 and/or T6). In each case, the antidromic response evoked by stimulation in the T2 gray matter was due to activation of an axonal branch rather than the main axon (via current spread to the white matter). This was demonstrated with tests that included time-controlled collision of the action potentials initiated by stimulation in T2 and a more caudal thoracic spinal segment. Some VLM-spinal axons that projected to T11 branched in T6 as well as in T2. These data indicate that some VLM-spinal neurons exerted widespread excitatory influences on sympathetic outflow. Seventeen VLM sympathoexcitatory neurons that innervated the T2 gray matter could not be antidromically activated by stimulation in T5, T6, and T11 despite an extensive search at each level. Thus the axonal projections of some VLM-spinal neurons were restricted to upper thoracic segments. Antidromic mapping in T2 revealed that the axons of VLM sympathoexcitatory neurons coursed through the dorsolateral or ventrolateral funiculus to innervate the region of the intermediolateral nucleus. Mean axonal conduction velocity was 3.5 +/- 0.3 m/s. Those VLM-spinal axons restricted to upper thoracic segments generally were located dorsally and/or medially to those that innervated widely separated thoracic segments. The discharges of 35 other VLM neurons that were antidromically activated by T2 stimulation were not related to sympathetic nerve activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Medullary substrate and differential cardiovascular responses during stimulation of specific acupoints

“…The rVLM contains many barosensitive reticulospinal neurons that project to the thoracic IML and receives sensory inputs from somatic sensory and visceral afferents (4,25) and serves an important role in somatosympathetic responses (17). This medullary region is located caudal and ventromedial to the facial nucleus, medial and inferior to the retrofacial nucleus, and includes the medial and caudal parts of nucleus paragigantocellularis lateralis (41).…”

“…7). These recording sites are located within the area of the rVLM as described by several authors (4,5,8,9,20,24,38). Five neurons not responsive to convergent inputs of splanchnic and median nerves are indicated with open circles.…”

Central respiratory modulation of subretrofacial bulbospinal neurones in the cat.

“…These actions are potent, specific and well localized. Thirdly, anatomical methods have shown that this nucleus is formed by a compact group of cells that project to the spinal cord (Amendt, Czachurski, Dembowsky & Seller, 1978;Goodchild, Cao & Dampney, 1986) (Dampney, Czachurski, Dembowsky & Seller, 1985 (Barman & Gebber, 1985;McAllen, 1986cMcAllen, , 1987 (Numao & Gilbey, 1987) (Preiss & Polosa, 1977) was not found for s.r.f. cells, although the latter did show greater respiratory modulation with increasing end-tidal CO2.…”

Rhythmic activity of neurons in the rostral ventrolateral medulla of conscious cats: effect of removal of vestibular inputs

“…These findings raise the prospect that CR activity of RVLM neurons, as well as higher-frequency rhythmic discharges of these cells, could differ in awake animals from responses identified in anesthetized and decerebrate animals (3,5,7). Both the CR and 10-Hz rhythms in SNA are mediated, at least partly, by the RVLM (4,5,7), and in anesthetized animals, bulbospinal RVLM neurons are capable of expressing both CR and 10-Hz activity (7).…”