Timecourse for receptive field plasticity following spinal cord hemisection.

Previously, we demonstrated that between 5 days and 3 months following a partial spinal cord hemisection, proximal hindlimb receptive fields of neurons in the ipsilateral L7 dorsal horn of cats become enlarged. In this study, we used somatotopic mapping procedures, applied bilaterally, to demonstrate that this change in receptive field size occurs between 10 and 14 days postoperatively.

Dye and electrical coupling between frog motoneurons.

Coupling between lumbar motoneurons in the isolated frog spinal cord was studied by using intracellular recordings and intracellular injections of Lucifer Yellow CH. Physiological studies revealed intra- and intersegmental, short latency electrical interactions between many motoneurons. Injections of Lucifer Yellow into motoneurons that were found to be coupled electrically revealed intra- and intersegmental dye-coupling.

Increased receptive field size of dorsal horn neurons following chronic spinal cord hemisections in cats.

The somatotopic organization of the 17 dorsal horn was studied using extra-cellular recordings in normal cats, and in cats with acute or chronic spinal cord hemisection at T13, sparing the dorsal columns. Based on data concerning recovery of function and collateral sprouting of afferents following hemisections, we predicted that the lesion would result in increases in receptive field size and decreases in the specificity of the somatotopic map. In normal animals, the usual mediolateral, rostrocaudal and dorsoventral somatotopic sequences were found. Following acute hemisections (6 h-5 days), there were changes in spontaneous and evoked activity, but receptive field sizes and somatotopic organization remained unchanged. Following chronic hemisections (88-174 days), proximal hindlimb receptive fields in the lateral dorsal horn ipsilateral to the lesion increased dramatically in size and were significantly larger than similar receptive fields on the contralateral side. The largest of these fields extended from the dorsal midline to the middle of the foot. Receptive field sizes elsewhere in the dorsal horn remained unchanged, as did somatotopic organization in general. These findings indicate that hemisections result in a complex series of changes consisting of an early stage of anatomically generalized changes in excitability and a later stage of highly localized changes in receptive field size. Possible mechanisms for these changes, as well as their relationship to recovery of function, are discussed.

Increased response to sural nerve input in the dorsal horn following chronic spinal cord hemisection.

Monosynaptic input from sural nerve afferents to dorsal horn neurons was mapped bilaterally using electrical stimulation in normal cats and cats with spinal cord hemisections. Animals hemisected 6 h-5 days previously did not differ significantly from normals and the sides of the cord did not differ in either group. In animals hemisected 88-182 days previously there were significantly more sites responsive to sural nerve input ipsilateral to the hemisection, than contralateral to it.

Cutaneous mechanoreceptor distribution and its relationship to behavioral specializations in squirrels.

The relationship between sensory specializations and behavioral specializations in two ecologically distinct species of squirrels was examined. It was predicted that the relative density of receptors in the glabrous forepaw skin of tree squirrels (Sciurus niger), which are skilled climbers and manipulate food items extensively, would be higher than that in ground squirrels (Spermophilus tridecemlineatus), which dig underground burrows. In addition to testing this prediction, several other aspects of the distribution of receptors were quantitatively examined in silver-stained material. As predicted, the relative density of receptors in the glabrous forepaw skin of tree squirrels is significantly higher than that in ground squirrels. Receptors are randomly dispersed and different classes of receptors (corpuscular vs. non-corpuscular) are intermingled in the palmar skin of both species. The proportions of the different classes of receptors do not differ among species.

Control of food handling by cutaneous receptor input in squirrels.

In a complementary neuroanatomical study by Brenowitz in 1980, it was shown that tree squirrels (Sciurus niger) have a higher relative density of mechanoreceptors in their glabrous forepaw skin than do ground squirrels (Spermophilus tridecemlineatus). The main purpose of this sudy was to test the prediction that tree squirrels would depend upon somatic sensory (cutaneous) input from their forepaws to a greater extent than would ground squirrels in food handling behavior. In addition, a series of more general questions about the sensory control of food handling was examined. First, using different sized food items, it was shown that food handling (rate of manipulation) is subject to sensory control, in general. Secondly, comparision of sham-operated groups with groups receiving median nerve (innervating the palmar surface) lesions showed that cutaneous input from the volar surface of the forepaw contributes to the sensory control in both species of squirrels. Thirdly, comparison of lesion effects in the two species showed that, as predicted, tree squirrels depend upon cutaneous input from their volar forepaw to a greater extent than do ground squirrels. Fourthly, by reanalyzing the above data it was shown that there is continued sensory feedback from food items rather than only an initial evaluation of them.