Spinothalamic lumbosacral lamina I cells responsive to skin and muscle stimulation in the cat.

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Abstract
The response characteristics of lamina I neurons recorded extracellularly in the lumbrosacral enlargement of chloralose-anesthetized cats were examined with peripheral nerve electrical stimulation, adequate mechanical and thermal stimulation of hind-limb skin, and algesic mechanical and chemical stimulation of musculotendinous structures, particularly the gastrocnemius-soleus (g.s.) muscle. Antidromic activation from an electrode array that spanned the contralateral thalamus was used to identify lamina I spinothalamic tract (lam.I-s.t.t.) neurons. Recordings were made from a total of 218 lumbosacral lam.I-s.t.t. neurons. Their mean central conduction latency was 90.1 ms (range 20-300 ms), corresponding to a mean conduction velocity of 3.7 m/s (range 1.1-16.7). Neurons responsive only to peripheral A.delta. fiber stimulation had significantly shorter central condution latencies (mean = 62.8 ms) than those with both A.delta. and C fiber input (mean = 81.9 ms) and those with only C fiber input (mean = 134.6 ms). Of these 218 neurons, 103 (47%) projected only to medial thalamus, 41 (19%) only to lateral thalamus, and 56 (26%) to both; 18 (8%) were classified as mid-thalamic projecting cells. About 10% of all cells had ongoing activity when 1st isolated. Ninety-three lam.I-s.t.t. neurons responded to stimulation of the sciatic nerve. Twenty-eight of the lam.I-s.t.t. neurons tested with the complete of stimuli responded specifically to either cutaneous noxious (n = 19), cutaneous innocuous cold (n = 6) or algesic musculo-tendinous (n =3) stimulation. Two response categories had distinct thalamic projection patterns. The majority of cold-specific neurons projected only to medial thalamus. Almost all multireceptive cold-sensitive neurons projected to both medial and lateral thalamus. Lam.I-s.t.t. neurons were distinct from non-s.t.t. lam. I neurons in that wide dynamic range neurons were regularly observed in the latter population, but not in the former, and lam.I-s.t.t. neurons that received g.s. input responded only to A.delta. g.s. afferent fibers, while most non-s.t.t. g.s.-responsive neurons responded to both A.delta. and C g.s. fibers. The lumboscacral lam.I-s.t.t. projection in the cat comprises several physiologically distinct classes that generally maintain the functional specificity of small-diameter primary afferent fibers, and that distribute their axonal projections to both medial and lateral thalamus in a functionally selective manner. Lam.I-s.t.t. neurons may be involved in the central representation of both nociception and thermoreception as specific sensory modalities. Yet the identification of multireceptive neurons and underscores the question of the involvement of the lam.I-s.t.t. projection in other functions.