Somatosensory research最新文献

Surgical removal of the third forepaw digit in raccoons causes both long-term and short-term changes in functional organization within the digit 3 primary somatosensory (SmI) cortex. Previous studies have shown that 36-52 weeks following amputation in infant raccoons, neurons within the digit 3 cortical territory had become responsive to cutaneous stimulation of "new" forepaw regions adjoining the digit stump (Carson et al., 1981; Kelahan et al., 1980, 1981); the "novel" receptive fields (RFs) were often larger than normal and revealed no orderly somatotopic organization. In the present study, the cortical effects of digit 3 removal were examined in adult raccoons. Within 36 weeks after amputation, the digit 3 zone was also found to be reactivated by "novel" inputs from the forepaw, with no strictly topographic representation of the "new" skin fields. The basic features of cortical reactivation were very similar in animals amputated as adults and as infants, except that the former typically had larger neuronal RFs than the latter. Short-term cortical changes were studied in adult raccoons within 1 day and between 1 and 4 weeks after amputation: Significant time-dependent differences were found in the reactivated digit 3 territory. Within 1 hr following amputation, some cells in the digit 3 zone began to respond to low-intensity cutaneous stimulation of "new" forepaw regions, limited almost exclusively to digits 2 and 4. Neuronal RFs tended to be larger than normal and showed no strictly topographic organization. One to 4 weeks following amputation, the condition of the digit 3 zone differed dramatically from that found immediately and long after amputation--the majority of responsive neurons could be excited only by high-intensity stimulation of small RFs on the digit 3 stump; relatively few cells were sensitive to low-intensity stimulation of adjacent, intact skin regions. Again, no true somatotopic organization was evident. The combined results of these experiments indicate that within 36 weeks following removal of a digit in raccoons, the deprived SmI cortical sector undergoes a dynamic sequence of changes in functional organization: Neurons that are normally excited by stimulation of digit 3 first become responsive primarily to stimulation of digits 2 and 4 (within 1 day after amputation), then to the digit 3 stump (from at least 1-4 weeks after amputation), and finally again to digits 2 and 4 (within at least 36 weeks after amputation).(ABSTRACT TRUNCATED AT 400 WORDS)

The pars tensa of the rat tympanic membrane (TM) consists largely of a lamina propria of specialized unbanded collagen bounded on the outer surface by an unusually thin epidermal layer and on the inner surface by a flat, single-cell mucosal layer. The mucosal layer is innervated solely by unmyelinated (C) axons, whereas the cutaneous layer is supplied by both myelinated and C axons. The outer surface differs from general body skin, lacking dermal papillae, hairs, sweat glands, and distinctive dermal corpuscular structures. Epidermal innervation includes distinctive terminals in the basal layer, unassociated with Merkel cells, and deeper intraepidermal smaller endings containing accumulations of mitochondria and vesicles. The sensory nature of these endings can be inferred by their extensive, but not total, elimination following neonatal capsaicin treatment (a potent neurotoxin for thin sensory fibers) and their preservation following surgical or neurotoxin sympathectomy. The thin mucosal epithelium displays capillaries and beaded axons close to the free surface of the middle ear. The unmyelinated terminals contain predominantly large, dense-core vesicles (LDCVs). Capsaicin treatment results in extensive elimination of terminals containing LDCVs in surface epithelia. A possible small trophic influence of sensory thin-fiber supply was noted on the development of the epidermal layers. The sensory modalities elicited by natural stimulation of the TM is considered in relation to the pattern of innervation.

The ability to resolve two closely spaced cutaneous stimuli presumably depends upon the degree of overlap between the two populations of responding neurons. The degree of overlap is determined by receptive field (RF) geometry and location, and by interactive factors such as lateral inhibition. In this paper, we first consider some aspects of RFs that would be expected to influence two-point acuity. In some somatosensory brain regions, relatively few RFs overlap the body midline. As would be expected, discrimination is enhanced for two points straddling the backbone. This does not simply reflect a mediolateral gradient of acuity, as we found higher acuity laterally. On the limbs, where RFs are elongated along the length of the limb, transverse two-point acuity was greater than longitudinal acuity. However, on the back, where RFs are fairly round, there was an even larger orientation effect, with two-point acuity greater for stimuli longitudinal than for stimuli transverse to the spine. Thus, the substantial variation of two-point acuity with stimulus orientation on the back cannot be explained by RF geometry alone. We discuss the possibility that differences in lateral inhibition and degree of similarity in dermatomal composition contribute to the observed stimulus orientation effects.

Primary afferent sprouting in the spinal cord was evaluated by comparing the central projection of horseradish peroxidase (HRP)-labeled sciatic nerve afferent axons in nonlesioned control rats, and in rats subjected to acute or chronic partial spinal hemisections as adults. The lesions were performed at various levels from T10 to L3, and removed supraspinal and varying amounts of descending propriospinal afferents to lumbar segments receiving the maximal sciatic projection. The hemisections typically involved all but the dorsal column, although in some cases a portion of the dorsal column, including the corticospinal tract, was also transected. The distribution pattern and density of spinal HRP reaction product was not significantly different in experimental and control preparations in any segment below the lesion, regardless of the quantity of denervation, or the density of the normal sciatic projection in a given terminal region. These results, together with our previous finding concerning an absence of primary afferent sprouting following long-term dorsal root ganglionectomies, suggest that current concepts concerning collateral sprouting as a factor in functional plasticity in the mature mammalian spinal cord warrant re-evaluation.

Axonally transported protein labeled many trigeminal nerve endings in subepithelial regions of the anterior hard palate of the rat. Sensory endings were most numerous in the lamina propria near the tips of the palatal rugae where large connective tissue and epithelial papillae interdigitated. Two kinds of sensory ending were found there: "free" endings, and a variety of corpuscular endings. The "free" sensory endings consisted of bundles of unmyelinated axons separated from the connective tissue by relatively unspecialized Schwann cells covering part or all of their surface and a completely continuous basal lamina; they were commonly found running parallel to the epithelium or near corpuscular endings. The corpuscular sensory endings all had a specialized nerve form, specialized Schwann cells, and axonal fingers projecting into the corpuscular basal lamina or connective tissue. There were at least four distinct types of corpuscular ending: Ruffini-like endings were found among dense collagen bundles, and they had a flattened nerve ending with a flattened Schwann lamella on either side. Meissner endings had an ordered stack of flattened nerve terminals with flattened Schwann cells and much basal lamina within and around the corpuscle. Simple corpuscles were single nerve endings surrounded by several layers of concentric lamellar Schwann processes. Glomerular endings were found in lamina propria papillae or encircling epithelial papillae; they were a tangle of varied neural forms each of which had apposed flattened Schwann cells, and a layer of basal lamina of varied thickness. Fibroblasts often formed incomplete partitions around Meissner and simple corpuscles. The axoplasm of all kinds of subepithelial sensory endings contained numerous mitochondria and vesicles, as well as occasional multivesicular bodies and lysosomes; the axoplasm of all endings was pale with few microtubules and neurofilaments. The specialized lamellar Schwann cells had much pinocytotic activity. Four kinds of junctions were found between the corpuscular sensory endings and the lamellar Schwann cells: (1) symmetric densities that resemble desmosomes; (2) asymmetric densities with either the neuronal or glial membrane more dense; (3) neural membrane densities adjacent to Schwann parallel inner and outer membrane densities; and (4) sites of apparent Schwann endocytosis associated with neural blebs. The "free" sensory endings only made occasional desmosome-like junctions with their Schwann cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Plots were made of multiunit activity versus ankle joint position for receptors in each of the 12 muscles crossing the cat ankle joint, except peroneus tertius, by recording from populations of afferent fibers in muscle nerves. The discharge was measured 15 or 30 sec after terminating the movements that altered the position of the joint. These recordings were dominated by large-spike activity that would be expected to originate mainly from primary spindle endings. Seven of the 12 muscles also cross other joints. Their responses at a given ankle joint position were so altered by changes in the position of the knee or toe joints that they could not reliably signal the position of the ankle joint. As judged from multiunit recording, receptors in each of the five muscles specific to the ankle joint were influenced by more than one axis of ankle joint displacement. Single-unit recording from dorsal root filaments was used to determine whether primary or secondary spindle receptors in soleus and tibialis anterior could selectively signal one axis of ankle joint rotation. Individual soleus receptors were tested both on the flexion-extension axis and with a combined adduction-eversion movement. For 38 of the 70 soleus receptors examined (54%), firm adduction-eversion produced a level of activity greater than that caused by 10 degrees of flexion, and for 77% the level of activity was greater than that caused by 5 degrees of flexion. For 168 of the 184 tibialis anterior receptors studied (91%), firm abduction-inversion produced a level of activity greater than that caused by 10 degrees of extension. Thus few receptors were found that responded exclusively to one axis of rotation. One way in which the position of the ankle joint could be specified in the face of multiaxial receptor activity is by examining the receptor discharge from more than one muscle. A suggestion for how the nervous system might do this is given in the discussion.

We have studied the sprouting of intact high-threshold mechanosensory nerves into adjacent denervated trunk skin in adult rats behaviorally, histologically, and electrophysiologically. In the anesthetized animal, stimulation of high-threshold endings in back skin by localized pinching elicits a bilateral reflex excitation of the underlying skeletal muscle, the cutaneous trunci muscle (CTM), visible as a twitch-like puckering of the skin. The reflex was also evoked by electrical excitation of A delta and of C fibers in the dorsal cutaneous nerves (DCNs), with characteristic latencies of 7-20 msec and 40-60 msec, respectively; excitation of low-threshold (A alpha) fibers was ineffective. After cutting selected DCNs, the deprived skin became insensible, but pinch responsiveness gradually recovered over the following 2 weeks. Regeneration of cut axons was not responsible for this recovery; when neighboring intact DCNs were cut, however, all responses were abolished in the recovered skin that had been initially denervated. By 3-5 days after denervation, axons in the dermis were all histologically absent or degenerating; when pinch sensitivity was restored to such skin, silver-stainable axons reappeared in the formerly empty Schwann tubes. During the work we noticed that the periodic examination by pinching, used to follow the time course of recovery of function in individual animals, led to an earlier development of this recovery than in animals that were examined only once at a specified time after denervation. This apparent acceleration in the redevelopment of pinch sensitivity was correlated with the appearance of axons in the recovered skin, and was shown to be due to the impulse activity evoked in the remaining nerves by the periodic pinching; it did not occur when the nerves were blocked by tetrodotoxin (TTX), and it was mimicked by a brief (10-min) period of electrical excitation of the A delta fibers in a remaining nerve carried out at the time when the denervation of skin was done. The time course of the phenomenon suggested that the principal effect of the impulses was to shorten the latency to the onset of sprouting in the activated A delta axons; that is, they induced precocious sprouting. The impulses needed to be conducted centrally for the effect to occur, and precocious sprouting failed to occur if the impulses were allowed to proceed only distally toward the skin. It seems that a brief conditioning burst of impulses in A delta axons sensitizes the neurons to the influence of a sprouting stimulus that appears when skin is denervated.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological experiments have been carried out on rabbits and cats to find out whether there is a relationship between the dorsal rootlet by which a particular saphenous nerve fiber passes to the spinal cord and the part of the saphenous nerve territory it supplies. Preliminary experiments were carried out to determine the extent of the saphenous nerve field in both animals and to determine the dorsal rootlets by which saphenous nerve fibers reach the cord in rabbits. Then, in experiments on rabbits and cats in which recordings were made from fiber bundles dissected from dorsal rootlets, it was found that saphenous nerve fibers in any one dorsal rootlet supplied only a limited portion of the saphenous nerve field; that there was overlap in the areas of the saphenous nerve field supplied by adjacent rootlets; and that moving caudo-rostrally through the rootlets containing saphenous fibers, the areas of skin supplied gradually moved from distal parts of the field to proximal ones. The results show that the saphenous nerve input to the spinal cord is somatotopically organized.