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Serotonin (5-HT) exerts a variety of effects on the excitability of motoneurons, interneurons, and ascending tract cells. Spinocerebellar-tract cells in the dorsal horn receive synaptic connections from serotoninergic axons, but little is known about the relationships between serotoninergic axons and dorsal spinocerebellar tract (DSCT) cells in Clarke's column. We studied these relationships by using a combination of immunohistochemical localization of 5-HT-immunoreactive boutons and intracellular staining with horseradish peroxidase (HRP) or neurobiotin of identified DSCT cells in vivo. In the adult cat, Clarke's column displayed a lower density of 5-HT-immunoreactive axons and boutons than adjacent regions of the spinal gray matter. Eleven intracellularly stained DSCT cells were analyzed with light microscopy, and six of these cells were entirely reconstructed in three dimensions. A total of 3,739 close appositions (340+/-101 per postsynaptic neuron: mean +/- SD) were observed on the labeled DSCT cells. The majority (97%) of the appositions were formed on dendrites, including proximal and distal branches, with an average density of about 1.4 appositions per 1,000 microm2 of dendritic membrane. These results indicate that the bulbo-spinal serotoninergic system(s) provide direct innervation of Clarke's column-DSCT cells in the upper lumbar spinal cord and that the inputs are spread widely over all regions of the target neurons' soma-dendritic membrane.

Neurons in the lumbosacral, superficial spinal dorsal horn in the cat were recorded extra- and intracellularly, using dorsal root stimulation as a search stimulus. Isolated neurons were tested for antidromic activation from the contra- and ipsilateral parabrachial region. Seventy-one nociceptive-specific neurons, 11 innocuous cooling neurons, and 8 multireceptive neurons were antidromically activated from the lateral parabrachial region. The receptive fields and response properties were typical of other lamina I and lamina II neurons, in that the receptive fields were usually discrete and relatively small, and the responses ranged from sluggish and decrementing to brisk and augmenting with afterdischarge. The conduction velocity to the parabrachial region averaged 3.7 m/sec for the nociceptive-specific neurons, 3.9 m/sec for the innocuous cooling neurons, and 13.5 m/sec for the multireceptive neurons. Intracellularly labeled neurons were mostly medium to large Waldeyer-like neurons in lamina I. Some had axon collaterals that distributed varicosities in laminae I, II, and V. These data indicate that a slowly conducting nociceptive-specific and thermoreceptive pathway exists between the superficial dorsal horn and the parabrachial region at the pontine-midbrain junction.

The objective of the present study was to determine in vivo the effects of opioids applied locally via microiontophoresis on physiologically characterized laminae I and II dorsal horn neurons in the cat spinal cord. Experiments were performed on pentobarbital-anesthetized or decerebrate, spinalized cats. The effects of morphine (MOR), [D-Ala2,methyl-Phe4, Gly-ol5]enkephalin (DAGO) and naloxone (NALO) on spontaneous- and D,L-homocysteic acid-evoked unit activity were examined for 94 laminae I and II dorsal horn neurons. MOR, DAGO and NALO produced mixed effects (i.e., excitation or inhibition) on unit activity; however, the majority of cells examined (67%) were inhibited. Whether MOR, DAGO or NALO exerted excitatory or inhibitory influences on unit activity did not depend on the modality of the neuron (66% of high threshold units, 56% of multireceptive units and 69% of low threshold units were inhibited; some cells of each modality also were excited). NALO also was found rarely to be neutral; 75% of the cells examined were inhibited and 16% excited by NALO. Since NALO was found to itself have significant effects on laminae I and II dorsal horn unit activity, the ability to use NALO as an opioid antagonist to MOR and DAGO was confounded.

In the present study we examined action potentials recorded from somata in the cat L7 and S1 dorsal root ganglia. We found (i) that there is a characteristic spike duration for each receptor type (e.g. Golgi tendon organ, D-hair) and (ii) that, for a given peripheral fiber conduction velocity, somata whose axons supply low-threshold cutaneous receptors exhibit spikes with shorter durations than those innervating high-threshold cutaneous receptors. These results suggest that there may be significant differences in somal membrane composition which are specific to the physiological receptor types innervated.

High-threshold mechanoreceptors (mechanical nociceptors) with myelinated axons were electrophysiologically identified in hairy skin of the cat as described by Burgess and Perl ('67). Such elements possess receptive fields consisting of a number of punctate areas from which maximal firing can be elicited by intense (skin-damaging) mechanical stimuli. The spots of the receptive field are separated from each other by unresponsive regions, i.e., by skin areas from which responses cannot be evoked by stimuli effective at the spots. Fine steel pins were inserted to bracket closely a number of the spotlike responsive areas for each of several units. After aldehyde perfusion of the animal, osmification of the tissue and embedding in plastic, the marked skin zones were examined in semithin and ultrathin sections at the light and electron microscopic level. Near each delineated area, a thinly myelinated axon was found that could be traced to the papillary layer where it loses its myelin sheath. Unmyelinated axons accompanied by thin Schwann cell processes were then traced and found to penetrate the epidermal basal lamina in one of the papillae. At the epidermal penetration site, the axons contained both clear round, and large, dense core vesicles; at this level, the surrounding Schwann cell cytoplasm exhibited numerous pinocytotic vesicles. The zone of penetration may constitute the receptive apparatus. Some of these axons have been traced within the basal epidermal layer where they become surrounded by keratinocytes, lose their Schwann sheath, and apparently terminate. This overall morphological pattern was consistently present in the demarked areas of focal responsiveness, and was rare in the surrounding skin; this and its difference from other cutaneous neural endings suggest that the intraepidermal axon-Schwann cel complex constitutes the receptive structure for myelinated mechanical nociceptors. It is suggested that such complexes are the sense organs responsible for initiating the sensation of pricking pain produced by localized mechanical injury of the skin.

The rostral projection of A delta and C primary afferents in Lissauer's tract (LT) was studied by assessing how far, rostrally, axons entering at a given level could be activated by electrical stimulation. Sural A delta primary afferents in LT exhibited a rostral projection at least as far as the L3/L4 border, 4 segments rostral to their segment of entry (S1 dorsal root). C-fibers from the sural nerve projected in LT less than 2 segments rostral to their segment of entry. It is concluded that small afferent fibers can project appreciable distances along the spinal cord to bring them into regions not normally responsive to natural stimulation of the receptive field of such fibers.

Twenty-five physiologically identified spinocervical tract (SCT) neurons in the sixth lumbar segment of the cat were filled with HRP by intracellular injection. All were reconstructed from sagittal sections using the camera lucida, and a subset (n = 18) was also reconstructed using a computer reconstruction system. Thirteen cells were in intact preparations, nine were in spared root preparations (L5, L6, S1, S2 cut; L7 spared), and three were in preparations with L5 through S2 cut. Analysis of the dendritic tree of these neurons revealed little change in gross morphology after partial deafferentation despite increased proportions sensitive to nociceptive input (Sedivec et al., 1983). The dendrites still largely respected the lamina II-III border, and relatively few dendrites were directed ventrally from the cell body, although the ratio of ventral to dorsal dendrites was greater than normal. The major change was an increase in surface area and volume caused by changes in diameter (but not length) of the dendrites. Larger-than-usual maximum branch order of individual dendritic trees of some cells was also observed after chronic deafferentation. Thus, SCT cells in deafferented segments do not undergo atrophy, but show, rather, limited signs of growth and the possibility of dendritic reorganization. We have also computed correlations between different parameters of these cells (cell body size, number and size of primary dendrites, total area and length of individual dendrites) and have found that, as in motoneurons, diameter of the primary dendrite measured 30 micron from the soma is significantly correlated with total dendritic surface area and length. SCT neurons tend to have more dendrites than spinal alpha-motoneurons, but total surface area is smaller for a given diameter of a proximal dendrite.

Recordings from antidromically identified spinocervical tract (SCT) cells in the partially deafferented spinal cord of chloralose-anesthetized cats have revealed reorganization of their input from peripheral receptors in the skin. Immediately after dorsal rhizotomy of segments L5, L6, S1, and S2, (sparing L7), units in L6 had a lower than normal probability of responding to moderate pressure, mechanical nociceptive, noxious heat, and cooling inputs. This persisted for about 6 weeks, after which normal or even larger than normal proportions of SCT units could be driven by these inputs. The proportion of units driven by hair deflection remained large throughout this period. Responses in the L7 segment with intact L7 dorsal root were unchanged. We suggest that afferent fibers from different receptor types differ in the extent of their normal functional projection, accounting for the selective changes after acute deafferentation. The chronic changes may reflect a differential ability of surviving afferent fibers to undergo compensatory changes in their projection (e.g., sprouting) after chronic rhizotomy.