Using cats anesthetized with pentobarbital, the activity of high-threshold nociceptive neurons that responded to injection of 5% NaCl solution (hypertonic solution) in the splenius muscle (i.e., chemical stimulation using a muscle pain-producing substance) was investigated in the caudal medulla oblongata and thalamus to thoroughly evaluate the functional properties of muscle pain neurons, to clarify modification of muscle pain neurons in the central nervous system, and to ascertain the physiological significance of these findings.
The extracellular activity of single neurons was obtained from the reticular formation in the caudal medulla oblongata (BRF), as well as the nucleus parafascicularis (Pf) and the nucleus centralis lateralis (CL) belonging to the intralaminar nuclei (IL) in the thalamus.
Neurons detected from the BRF display peripheral receptive fields spreading to the maxillofacial or trunk areas and respond to nociceptive mechanical stimuli applied to deep subcutaneous tissue. Neurons detected from the Pf and CL showed broad peripheral receptive fields like neurons in the BRF and responded to nociceptive mechanical stimulation to the deep tissue of the trunk.
The above results clarify that afferent fibers relaying pain information of the splenius muscle project to the BRF and then ascend to the IL. In addition, the results for convergence of neurons in the relay nucleus are useful in explaining referred pain involving the splenius muscle.