Anatomical and physiological properties of ipsilaterally projecting spinothalamic neurons in the second cervical segment of the cat's spinal cord

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Abstract
Anatomical and electrophysiological methods were used to investigate the projections and response properties of neurons in the second cervical (C2) spinal segment of the cat giving origin to a previously undescribed projection to the ipsilateral thalamus.The method of retrograde axonal transport of horseradish peroxidase (HRP) was used to identify neurons in C2 giving rise to thalamic projections. Following large (3.0 μl) thalamic HRP injections, a large numbers of labeled neurons was observed in lateral laminae VII–VIII of C2 ipsilateral to the injections. They occurred as small clusters of cells along the longitudinal axis of C2. Labeled neurons were also observed contralaterally in the lateral cervical nucleus, dorsal horn (especially medial lamina VI), and loosely distributed in the ventral horn. The ipsilaterally projecting neurons were also labeled following small (0.2–0.5 μl) HRP injections restricted to individual spinothalamic terminal zones (intralaminar nuclei, ventrobasal complex‐nucleus ventralis lateralis border zone, medial division of the posterior nuclei), indicating that as a group they project widely throughout the thalamus.Single unit recording methods were used to obtain complementary information on the functional properties of these neurons. The antidromic stimulation method was applied to identify units in C2 projecting to the ipsilateral thalamus in anesthetized, paralyzed cats. Three categories of ipsilaterally projecting C2 units were identified: (i) units not driven by any type of natural stimulation; (2) units having large cutaneous receptive fields (RFs) and wide dynamic response ranges (“widefield”), and (3) units with smaller RFs and varied properties (“other”).Widefield units with bilaterally symmetrical and asymmetrical RFs were observed. Co‐stimulation of different portions of an excitatory RF produced summation of the unit response. Inhibitory RF components were identified in one third of the widefield units. Unit recordings after spinal tract lesions revealed that the afferent input passed via the ipsilateral lateral and/or ventral funiculi. Widefield unit responses to somatosensory stimuli could be inhibited by dorsal column conditioning stimulation. Several “other” units resembled widefield units, while a second group had small RFs restricted to the C2 dermatome.Possible functional roles of the projecting C2 neurons in somatosensory and non‐specific systems are discussed.