Differentiation of lamina I spinomedullary and spinothalamic neurons in the cat

Abstract

We characterized spinomedullary neurons that project to the ventrolateral portion of the medulla that receives lamina I terminations in two sets of experiments in the cat. First, their distribution was examined using single unilateral iontophoretic injections of cholera toxin subunit B. The injection sites were characterized by microelectrode recordings from nociceptive- and thermoreceptive-specific units, indicative of lamina I input. The spinomedullary neurons were symmetrically distributed bilaterally, predominantly (63-69%) in lamina I but also in laminae V-VIII and the thoracic lateral horn (intermediolateral cell column). In horizontal sections, spinomedullary lamina I neurons included all three main morphological types described earlier. Second, spinomedullary and spinothalamic neurons were compared in retrograde double-labeling experiments. Different combinations of tracers were injected in the right thalamus and the left or right ventrolateral medulla (guided by recordings). The numbers of spinomedullary and spinothalamic neurons on the left side were comparable, and the segmental and laminar distributions were similar, except that a greater proportion of spinomedullary neurons originated from thoracic segments. However, the proportion of double-labeled neurons was consistently approximately 1%, indicating that spinomedullary and spinothalamic pathways arise from separate subpopulations. Spinomedullary neurons were more ventrally located within lamina I than spinothalamic neurons. A significantly greater proportion of spinomedullary neurons had fusiform somata (49% vs. 36%). These observations indicate that lamina I is the major source of spinal input to this portion of the ventrolateral medulla, that the projection includes several morphological types of inputs, and that this projection is distinct from the spinothalamic projection. These findings are consistent with the concept that lamina I projections constitute an ascending homeostatic afferent pathway relating the physiological condition of the body.

A Trigeminoreticular Pathway: Implications in Pain

“…Their projection to the cmVLM provides another route by which noxious stimuli may exit the dorsal horn for ascension to areas important for either the appreciation or modulation of pain. This projection to the cmVLM may be analogous to those previously described from lamina I neurons in the spinal cord responsive to noxious stimulation to the cmVLM [35], [74], [75]. Numerous TCC neurons in laminae I and II were labeled retrogradely herein after injection of a retrograde tracer into the cmVLM.…”

Morphology and neurokinin 1 receptor expression of spinothalamic lamina I neurons in the rat spinal cord

“…Such cells could also project only to other supraspinal targets such as the parabrachial nucleus (Todd et al, 2000). Interestingly, Andrew et al (2003) found that spinomedullary and spinothalamic lamina I neurons in the cat belonged to separate populations. It is also interesting to note that fusiform cells with or without ventrally directed dendrites could be differentiated on the basis of their response to glutamate receptor agonists using cobalt uptake (Nagy et al, 1994).…”

TrkB expression and phospho-ERK activation by brain-derived neurotrophic factor in rat spinothalamic tract neurons

“…Spinomedullary neurons are as numerous as STT cells and have a similar distribution (Andrew et al, 2003), as do spinosolitary tract projection neurons, which are thought to integrate somatic and visceral afferent inputs (Menetrey and Basbaum, 1987). Therefore, because these projection neurons are distributed in the same laminae as STT cells and have similar morphology to them, they are likely to contain TrkB and respond to BDNF released from primary afferents following nocicep- tive stimulation of various origins (cutaneous, muscular, and/or visceral).…”