Somatosensory & motor research
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Psychophysical measurements of itch and itchy skin ("alloknesis"--itch produced by innocuous mechanical stimulation) were obtained in human volunteers following intracutaneous or subcutaneous injections of histamine or papain into the volar forearm. Histamine and papain were given in doses of 0.1, 1, or 10 micrograms in 10 microliters of saline. The effects of the depth of injection and of skin temperature on the latency, magnitude, and duration of itch were examined. ⋯ However, the magnitude and duration of alloknesis were dependent on dose. The mean maximum areas of alloknesis produced by intracutaneous injections of 0.1, 1, and 10 micrograms of histamine were 28.3, 47.2, and 43.8 cm2, respectively. Alloknesis was present at 2 min after injection, increased to a maximum area without 10 min, and then gradually decreased during the next 25-40 min.(ABSTRACT TRUNCATED AT 250 WORDS)
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Prior studies have documented a trigeminal (V) mandibular primary afferent projection to the dorsomedial portion of the contralateral medullary and cervical dorsal horns in cat, hamster, and rat. We now report the existence of a much more substantial V ophthalmic primary afferent projection to the ventrolateral portion of contralateral medullary and cervical dorsal horns in rat. Horseradish peroxidase (HRP) injections into the V ganglion or V brainstem complex anterogradely labeled a fascicle of primary afferent axons that exited the caudal ventrolateral V spinal tract to form a rostrocaudally continuous, transversely oriented, V primary afferent decussation. ⋯ All of these findings suggest that nociceptive-specific ganglion cells are not a source of the crossed ophthalmic primary afferent projection. Unilateral transection of the infraorbital nerve on the day of birth did not alter the crossed primary afferent projection to the partially deafferented side of the brainstem. This is further evidence of an absence of central sprouting in spared V primary afferents following neonatal V deafferentation.
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We have used single-unit recording techniques to map the spatial distribution of the primary somatosensory (SI) cortical influences on thalamic somatosensory relay nuclei in the rat. A total of 193 microelectrode penetrations were made to record single neurons in tracks through the medial and lateral ventroposterior (VPL and VPM), ventrolateral (VL), posterior (Po), and reticular (nRt) thalamic nuclei. Single units were classified according to their (1) location within the nuclei, (2) receptive fields, and (3) response to standardized microstimulation in deep layers of the SI cortical forepaw areas. ⋯ Considerable rostrocaudal variation was observed, however. Whereas neurons in the rostral VPL (near VL) were strongly inhibited (-69%), neurons in the middle and caudal VPL exhibited facilitations at long and short C-T intervals, respectively. This study establishes a specific projection system from the forepaw region of SI cortex to different subregions of the VPL thalamus, producing specific temporal patterns of sensory modulation.
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It was recently shown that the cutaneous sensitivity to airpuffs is decreased by a low-frequency vibrotactile masker in the hairy skin, and by a low-frequency but especially by a high-frequency masker in the glabrous skin. In the current study, the spatial features of this masking effect were determined in four healthy human subjects, using a reaction time paradigm. The masking effect decreased monotonically with increasing interstimulus distance, and identically in longitudinal and transverse (i.e., lateral) directions in the palm or dorsal surface of the hand. ⋯ Different peripheral neural mechanisms underlie the airpuff-elicited sensations in the hairy and glabrous skin. The afferent inhibitory mechanisms are stronger for signals coming from the glabrous skin of the fingers than for signals coming from the hairy skin. Furthermore, the peripheral innervation density and size of the cortical representational areas may be of importance in determining the magnitude of the masking effect.
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Prior intracellular recording and labeling experiments have documented local-circuit and projection neurons in the spinal trigeminal (V) nucleus with axons that arborize in more rostral and caudal spinal trigeminal subnuclei and nucleus principalis. Anterograde tracing studies were therefore carried out to assess the origin, extent, distribution, and morphology of such intersubnuclear axons in the rat trigeminal brainstem nuclear complex (TBNC). Phaseolus vulgaris leucoagglutinin (PHA-L) was used as the anterograde marker because of its high sensitivity and the morphological detail provided. ⋯ Such collateral projections were rare in nucleus principalis and more superficial laminae of subnucleus caudalis. TBNC cells labeled by cerebellar injections were not double-labeled by tracer injections into the thalamus, colliculus, or TBNC. These findings lend generality to currently available data obtained with intracellular recording and HRP labeling methods, and suggest that most intersubnuclear axons originate in TBNC local-circuit neurons, though some originate in cells that project to midbrain and/or diencephalon.