The second somatic sensory area (SmII) of opossum neocortex.

Organization of the neocortical second somatic sensory area (SmII) of anesthetized Virginia opossums has been examined utilizing micro-electrode recording techniques. SmII is situated between the first somatic sensory area (SmI) medially, and the rhinal fissure laterally. The head representation is located anteromedially within SmII, and the hindlimb representation posterolaterally, with the forelimb representation in between. Approximately 49% of SmII is devoted to representation of the head, 36% to forelimb representation, and 15% to trunk and hindlimb representation. All peripheral receptive fields (RF's) were either contralateral or bilateral. Approximately 63% of head RF's, 25% of forelimb RF's and 100% of hindlimb RF's were bilateral. For a given body locus, SmII RF's are larger than those for SmI. SmII is contained entirely within an area yielding evoked potentials responses to auditory click stimuli.

Somatosensory thalamocortical connections in the raccoon: an HRP study.

In the North American raccoon (Procyon lotor), representations of the glabrous surfaces of the hand digits are found within separate subnuclei of the thalamic ventrobasal complex (VB) and on separate subgyri of the somatosensory cortex (SmI). In the present study, the retrograde transport of horseradish peroxidase from SmI to VB was utilized to study relationships between physiologically identified cortical subgyri and somatotopically corresponding thalamic subnuclei. Single large or multiple small injections confined to a single gyral crown led to retrograde labeling of large groupings of cells filling the entire VB subnucleus for the appropriate digit. In the aggregate, the regions of label appeared as thin, wedge-shaped sheets extended in the dorsoventral and anteroposterior dimensions, but flattened mediolaterally, and curving to form a laterally directed convexity; these appear to correspond to the lamellae of monkey VB described by others. These large injections led to labeling of approximately 80% of all large (18-30-micron diameter) cells within the lamella. Single, small, focal injections of a gyral crown led to variable amounts of labeling, ranging from an entire digital lamella to only a small focal cluster of cells. No evidence was obtained for the existence of anteroposteriorly extending "rods" of cells, as reported in primates. Finally, there was a sparse, but consistent labeling of cells of the posterior nuclear group (Po) following gyral crown injections. These results are in agreement with expectations based on prior electrophysiological studies of raccoon VB and SmI, as well as prior anatomical studies of thalamocortical relationships.

Substance P-like and serotonin-like immunoreactivity in the lateral cervical nucleus of the raccoon.

The distribution of substance P and serotonin in the lateral cervical nucleus (LCN) of the raccoon was examined by light microscopic immunohistochemistry. Substance P-immunoreactive fibers were found to be clustered in the ventromedial part of the LCN, whereas only few such fibers appeared in the dorsolateral part of the nucleus. This organization is closely similar to that previously observed in the cat, and provides further evidence for an anatomic and functional segregation along the transverse axis of the LCN in carnivores. In some sections, substance P-positive fibers were found primarily in areas of the ventromedial LCN containing small neurons, indicating that such fibers may be involved in functions of the LCN associated with nociceptive projection neurons and/or local circuit neurons. The raccoon LCN also received a relatively sparse innervation of serotonin-positive fibers that were distributed throughout the nucleus, an organization similar to that previously observed in the cat. The functional role of the serotonergic fibers is unclear. However, their presence suggests that descending influences on transmission in the spinocervicothalamic pathway, in addition to the well-documented descending control of spinocervical tract neurons, may be present also at the level of the LCN.

Opossum somatic sensory cortex: a microelectrode mapping study.

Organization of opossum somatic sensory cortex has been investigated utilizing closely spaced microelectrode penetrations (0.25-0.5 mm apart) and delicate mechanical stimulation of body surfaces including the facial vibrissae. Results may be summarized as follows: (1) the general organization of somatic sensory cortex, as originally defined by Lende ('63a) has been confirmed; (2) a double representation of the contralateral mystacial vibrissae and rhinarium, implicit in Lende's original data, was revealed in detail, the two representations being orderly, adjacent, mirror-images of each other; (3) units at a given cortical locus responded to deflection of between one and five mystacial vibrissae, about half responding to movement of a single vibrissa only; (4) about 40% of mystacial vibrissa units showed a directional specificity to the extent that they responded to deflections in only one or two cardinal directions; (5) units located in the medial vibrissa area showed a greater directional specificity than did units located in the lateral vibrissa area; (6) the surface area of rhinarial receptive fields was about ten times the area of first-order rhinarial unit receptive fields (B. Pubols et al., '73); (7) representation of the contralateral forelimb, especially the ventral surface of the forepaw, is extensive, orderly, and precise; (8) representation of the contralateral hindlimb, foot, and tail is minimal, and is confined to the midline convexity; (9) the presence of a small region of bilateral representation, lateral to the regions of contralateral representation, was confirmed. It is suggested that the region of contralateral postcranial representation plus the medial rhinarium and mystacial vibrissa areas are the homologue of SmI in placental mammals, and the region of bilateral representation is homologous to SmII of placental mammals, but that the lateral vibrissa and rhinarium areas are a specialization of somatic sensory cortex unique to the Virginia opossum.

Correlation of peripheral receptive field area and rostrocaudal locus of neurons within the raccoon cuneate nucleus.

There is a significant positive correlation between peripheral receptive field (RF) area and neuron locus within the rostrocaudal extent of the raccoon cuneate nucleus for units having tactile RFs on the glabrous surfaces of the forepaw digits, larger RFs being associated with more rostrally located neurons, smaller ones with caudally situated neurons, but not for units having tactile RFs on the glabrous surfaces of the palm. For digits and palm, RF areas of neurons of the cuneate nucleus are approximately 40 and 100 times larger, respectively, than RF areas of primary afferent fibers of the cuneate fasciculus.

Slowly adapting type I mechanoreceptor discharge as a function of dynamic force versus dynamic displacement of glabrous skin of raccoon and squirrel monkey hand.

The effects of dynamic force and dynamic displacement on single unit discharge rate during ramp stimulation were examined in 10 raccoon and 8 squirrel monkey slowly adapting Type I (SAI) mechanoreceptive afferent fibers, all having receptive fields on glabrous skin of the hand. In all 18 cases, power function exponents were higher for effects of dynamic displacement than for effects of dynamic force on discharge frequency. Thus, these SAI mechanoreceptors are more sensitive to variations in dynamic displacement than to variations in dynamic force. This differential sensitivity may be explained by the fact that the relationship between dynamic force and dynamic displacement is, itself, nonlinear, dynamic displacement being a power function of dynamic force, with exponents less than 1.0.

The raccoon lateral cervical nucleus: mediolateral organization of GABA-positive and GABA-negative neurons and fibers.

In the lateral cervical nucleus (LCN) of the cat, GABA-immunoreactive neurons and substance P-immunoreactive fibers are concentrated in the medial part of the nucleus, whereas in the monkey LCN no preferential locations have been identified. In raccoons, substance P-immunoreactive fibers display a distribution pattern similar to that in cats. However, the presence and distribution of GABA-immunoreactive neurons in the raccoon LCN has not been examined, and it is therefore not known whether raccoons are similar to cats or primates in this respect. Thus, in the present study, the raccoon LCN was examined for the presence and distribution of GABA-immunoreactive cells with respect to their numbers, locations, and sizes. The distribution of GABA-positive fibers and varicosities within the LCN was also investigated. The results of measurements of cross-sectional areas of LCN neurons indicate a trend toward decreasing cell size along the dorsolateral to medial axis of the raccoon LCN. Compared to neurons of the centrally located ventromedial division, neurons are statistically significantly larger in the dorsolateral division and smaller in the medial division of the nucleus. Cell counts in post-embedding-stained semithin sections through the nucleus revealed an average of 8,700 neurons per LCN. Approximately 4% of LCN neurons are GABA-immunoreactive. These neurons are small and most (80%) of them are located in the medial third of the LCN. In contrast, GABA-immunoreactive fibers and varicosities are present in about equal density throughout the raccoon LCN. Thus, the distributions of GABA-immunoreactive neurons and neuron sizes in the raccoon LCN conform closely to those in cats. Together with previous observations in cats and raccoons, the present findings support the notion that these small GABA-immunoreactive neurons may be local circuit inhibitory neurons and indicate the presence of a mediolateral segregation that may be of fundamental importance for the functional organization of the carnivore LCN.

Two behavioral paradigms for study of rapid changes in functional grouping of neurons.

Recent progress in extracellular recording technology and in analytic methodology for the resulting spike trains are making practical the simultaneous registration of twenty or more neurons. This begins to make possible the direct experimental observation of functional neuronal assemblies, particularly as they dynamically change in membership or properties during a behavioral task. Behavioral paradigms appropriate for such experiments have very stringent requirements in order, with high likelihood, to produce changes in neuronal assembly structure during the time that stable recording can be maintained. We describe two motor system paradigms that seem to be appropriate, the first with crayfish claw, the second with monkey paw. The crayfish task involves a rapid learning of claw position. The monkey task involves a preset state which determines the responses to a subsequent somatosensory discrimination.

On- versus off-responses of raccoon glabrous skin rapidly adapting cutaneous mechanoreceptors.

1. The on- and off-responses of 50 raccoon median nerve fibers associated with rapidly adapting cutaneous mechanoreceptors in glabrous skin were examined under experimental conditions designed to allow comparable opportunities for on- and off-responses to occur. Trapezoidal stimuli were utilized, providing for equal stimulus indentation and retraction velocities and equal static displacement times and intertrial intervals. Principal findings were as follows: 2. At stimulus levels well above displacement and velocity thresholds for on-responses, 80% of units yielded a more vigorous on-response than off-response (as measured by the total number of ramp impulses); in 6%, the reverse was true; while in the remaining 14%, the off-discharge was absent. 3. On and off displacement thresholds were approximately equal (on median, 43 micron; off median, 42 micron). However, on velocity thresholds were significantly lower than off velocity thresholds (on median, 1.0 micron/ms; off median, 3.8 micron/ms). 4. Exponents (b) of power functions relating discharge rate to ramp velocity (frequency = a x velocityb) were consistently greater for on-responses than for off-responses, but intercept constants (a) were consistently greater for off-responses than for on-responses. 5. Previous findings that mammalian rapidly adapting (RA) mechanoreceptors possess a "linear directionality" generally favoring on-responses were confirmed. 6. Results are discussed in relation to the role of viscoelastic properties of RA mechanoreceptors and neighboring skin. It is suggested that, when considerations is also given to the mechanical properties of surrounding tissues, the Loewenstein and Skalak (18) analysis of the mode of operation of Pacinian corpuscles might also apply, at least qualitatively, to the simple dermal (rapidly adapting) corpuscle of raccoon glabrous skin.

Effect of mechanical stimulus spread across glabrous skin of raccoon and squirrel monkey hand on tactile primary afferent fiber discharge.

The role of spread of skin deformation in activating cutaneous mechanoreceptors at a distance from their threshold receptive fields (RFs) was examined in glabrous skin of the North American raccoon and the squirrel monkey. One feedback-controlled mechanical stimulus probe was used to indent the skin to a controlled depth at a constant velocity, at varying distances from a second probe, which was used to monitor vertical displacement depth and velocity at this distant site. In many instances, the monitor probe was positioned over the RF of a cutaneous mechanoreceptor, and single-unit action potentials were simultaneously recorded from individual fibers of the median or ulnar nerve. With distance from the site of stimulation, there was a systematic, monotonic decline in indentation depth and velocity; velocity fell off with distance more rapidly than depth. The degree of diminution with distance varied with the size, shape, and curvature of the digital or palm pad stimulated. Spread of indentation was more restricted on digital than on palm pads, and was more restricted across monkey skin than across raccoon skin. Spread was less with higher-velocity than with lower-velocity indentations, but was seemingly unaffected by indentation depth. As expected from the findings noted above, the number of spikes discharged by slowly adapting mechanoreceptive afferent fibers declined more rapidly with distance between stimulus site and RF for digital than for palmar RFs, in squirrel monkey than in raccoon skin, and with higher-velocity than with lower-velocity stimuli. Furthermore, the number of spikes occurring during either ramp or early static indentation phases of stimulation dropped to zero more rapidly with distance than did either vertical indentation depth or velocity. Decreases with distance in both indentation depth and velocity acted to restrict the size of suprathreshold RFs. For most units, horizontal components of mechanical stimulation subtracted from the effects of vertical components. It is suggested, on the basis of this and other studies, that many neural and perceptual phenomena usually attributed to central mechanisms of afferent inhibition may be attributable, at least in part, to mechanical properties of the skin. In addition, the present data suggest that regional variations in the two-point limen may be associated with variations in spread of mechanical deformation. The conclusion that glabrous skin and subjacent soft tissues act as a low-pass filter system provides a mechanical basis for the relative efficacy of high-frequency vibratory stimuli in tactile pattern perception.(ABSTRACT TRUNCATED AT 400 WORDS)

Spinocervical tract neurons responsive to light mechanical stimulation of the raccoon forepaw.

1. The extracellular activity of 45 antidromically identified spinocervical tract (SCT) neurons responsive to light mechanical stimulation of the glabrous surfaces of the forepaw was examined in raccoons anesthetized with pentobarbital sodium. An additional seven neurons had peripheral receptive fields (RFs) located on hairy skin of the forelimb, and three had deep RFs. 2. All recording sites were histologically verified as falling within Rexed's laminae III and IV in spinal cord segments C6-T1. Antidromic conduction velocities of the 55 neurons ranged between 8.3 and 64.2 m/s. 3. Units with glabrous skin RFs were classified according to their response to a maintained mechanical stimulus as either rapidly adapting (n = 39) or slowly adapting (n = 6). Of 11 cells tested, 2 displayed enhanced responses to noxious stimuli and were classed as multireceptive. 4. RF areas were significantly smaller on digits (range = 0.4-45.0 mm2) than on palm pads (range = 5.6-76.0 mm2), and comparable in size to RF areas previously reported in raccoon cuneate nuclear cells (32). 5. RA neurons fell into three distinct categories with respect to the relationship between instantaneous spike frequency during displacement ramp stimulation, and ramp velocity, steep functions (as defined by the value of power function exponents), flat functions, and discontinuous functions; SA neurons fell into two categories, continuous, and discontinuous. 6. The results, in conjunction with those of previous studies, lead to two major conclusions: 1) raccoon and primate spinocervicothalamic systems are more similar to each other than either is to that of the cat and 2) the ability of the raccoon SCT to convey information from the glabrous skin of the forepaw regarding characteristics of light mechanical stimuli is at least as precise as that of neurons of the dorsal column-medial lemniscal system.

The raccoon spinocervical and spinothalamic tracts: a horseradish peroxidase study.

The locations of the cells of origin of the spinocervical tract (SCT) and spinothalamic tract (STT) were examined in relation to the somatotopic and laminar organization of the cervical enlargement of the raccoon dorsal horn (DH). In different animals, either the lateral cervical nucleus or the lateral thalamus was injected with a 2% solution of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Following 24 h or 4 days, respectively, the animals were sacrificed and both injection and target sites (spinal cord segments C6-T2) were processed using the TMB method. All labelled cells were counted in every fifth 50 microns section. Following injection of WGA-HRP into the lateral cervical nucleus, all labelled SCT cells were located ipsilateral to the injection sites. Most (84%) were in laminae III and IV, laminae known from other studies to contain cells preferentially responsive to light tactile stimulation, with very few (3%) in lamina I. Nearly 50% of labelled cells were located in the medial 1/3 of the DH, the region of representation of the glabrous surfaces of the raccoon forepaw. The mean number of labelled SCT cells per section was 4.19. After tracer injections of the lateral thalamus, more than 75% of STT cells were located contralateral to the injection sites. Forty-three percent were located in lamina I and 24% were in lamina V, laminae whose cells have been shown to be responsive to more intense forms of stimulation, as well as to light touch. Only 22% were located in the medial 1/3 of the DH. The mean number of labelled STT cells per section was 0.83. The results suggest that the SCT may play a more critical role in relaying discriminative light tactile and nociceptive information from the glabrous surfaces of the forepaw, but that there may be a greater role for the STT in relaying nociceptive information from the forelimb as a whole.