The loss of regenerated host axons in nerve allografts after stopping immunosuppression with cyclosporin A is related to immune effects on allogeneic Schwann cells.

After immunosuppressive therapy with Cyclosporin A (Cy-A) is stopped, nerve allograft rejection occurs. In addition to the loss of allogeneic perineurial, vascular, and Schwann cells, host axons that regenerate into the allograft disappear despite the fact that the axons are not foreign tissue. The present experiment was performed to correlate immune events and allogeneic cell and host axonal loss in nerve allografts after terminating Cy-A treatment. Nerve grafts (4 cm long) were taken from American Cancer Institute (ACI) rats and joined to the peroneal nerves of Fischer (FR) or ACI rats that received a daily dose of Cy-A (10 mg/kg, intraperitoneally). After one week, Cy-A therapy was stopped and the grafts were examined 2-6 weeks postoperatively by light and electron microscopy. No immune reaction nor destruction of perineural, vascular, or Schwann cells was found in 2- or 3-week-old allografts (i.e., ACI to FR grafts). These grafts underwent Wallerian degeneration and were invaded proximally by regenerating host axons, some of which were thinly myelinated. At 4 weeks, the perineurium of each allograft became infiltrated by mononuclear cells and was destroyed. Many of the endoneurial blood vessels of these grafts were occluded and their endothelial cells were degenerating or missing. Despite the immune reaction, allogeneic Schwann cells remained and continued to myelinate or ensheath host axons that had now grown up to 3 cm into the grafts.(ABSTRACT TRUNCATED AT 250 WORDS)

Observations on the blood and perineurial permeability barriers of surviving nerve allografts in immunodeficient and immunosuppressed rats.

The authors investigate whether there are any permeability changes in the endoneurial blood-nerve barrier and the perineurium-nerve barrier of surviving nerve allografts. In a normal nerve, the blood-nerve barrier regulates the passage of substances from endoneurial blood vessels into the endoneurium, whereas the perineurium-nerve barrier protects the endoneurium from agents that escape from permeable epineurial vessels and accumulate around the nerve. Nerves from ACI rats were transplanted into immunologically deficient nude rats or normal Fischer rats immunosuppressed with cyclosporin A. None of the nerve allografts was rejected. The blood-nerve barrier of nerve allografts at 2 and 6 weeks postoperatively was permeable to intravenously injected horseradish peroxidase, which spread into endoneurial tissue. Electron microscopy revealed that horseradish peroxidase escaped from endoneurial vessels through intercellular junctions between endothelial cells. At 24 weeks, the blood-nerve barrier of nerve allografts had recovered and the endoneurial vessels, like those in normal nerves, were impermeable to horseradish peroxidase. The perineurium-nerve barrier of nerve allografts remained impermeable to horseradish peroxidase at all times. Axons were grouped into numerous minifascicles at nerve anastomosis zones at 24 weeks. Each nerve fascicle was surrounded by an impermeable perineurium. These results demonstrate that regenerated axons in long-term surviving nerve allografts and at anastomosis zones are protected by permeability barriers. It is concluded that permeability barriers of nerve allografts are not permanently altered by a foreign environment (grafts to nude rats) even when immunosuppression with cyclosporin A is required to prevent allograft rejection (grafts to Fischer rats).

The fate of cryopreserved nerve isografts and allografts in normal and immunosuppressed rats.

Donor Schwann cells, perineurial cells, and vasculature are known to survive in grafts of peripheral nerve. In the present study, we attempted to cryopreserve nerve to determine whether these cellular components of nerve would survive after transplantation and support host axonal regeneration through the graft. Four-centimeter lengths of peroneal nerves were removed from inbred adult American Cancer Institute (ACI) rats and placed into vials that contained a cryoprotective mixture of dimethyl sulfoxide and formamide (DF) at room temperature. Each vial with nerves in DF was cooled at a rate of 1-1.5 degrees C/minute down to -40 degrees C at which point the vials were plunged into liquid nitrogen at -196 degrees C. After 5 weeks of storage, the nerves were thawed and DF removed. Some of the cryopreserved-thawed ACI nerves were transplanted as isografts into the legs of ACI rats. Other ACI nerves were used as allografts and inserted into immunologically normal Fischer (FR) rats that were untreated or were immunosuppressed with the drug Cyclosporin A (Cy-A). At surgery, only one end of the nerve graft was joined to the cut proximal end of the peroneal nerve of the host. The cellular elements of ACI grafts were present at 5 weeks in grafts removed from ACI rats and FR rats treated with Cy-A. Non-immunosuppressed FR rats rejected ACI nerves as did FR rats in whom Cy-A was stopped after 5 weeks of treatment. All surviving ACI grafts underwent Wallerian degeneration and consisted of columns of Schwann cells, which in their proximal portion were associated with regenerating host axons. The donor perineurial sheath and vasculature were also present in surviving grafts. ACI isografts only were examined 20 weeks postoperatively. All normal tissue components survived in these older grafts and contained regenerated and myelinated host axons throughout their 4 cm lengths. These results demonstrated that the cellular elements of nerve can be cryopreserved, and after transplantation, survive and function. Because nerves survived after prolonged cryopreservation, it seems feasible to establish a nerve bank from which grafts can be withdrawn to repair gaps in injured nerves. However, cryopreserved nerves used as allografts remain immunogenic and require immunosuppression for their survival.

Regenerating axons are not required to induce the formation of a Schwann cell cable in a silicone chamber.

After suture of proximal and distal nerve stumps into the ends of a silicone chamber, a tissue cable forms inside the chamber through which axons regenerate. Schwann cells are a critical cellular component of the cable because in their absence axons fail to regenerate into the cable. In this study, we sought to determine whether axons were needed to induce the formation of a Schwann cell-containing cable. Transected stumps of sciatic nerves of adult rats were sutured into the ends of silicone chambers prefilled with phosphate-buffered saline or dialyzed plasma, leaving a 10-mm interstump gap. In order to eliminate any axonal influence in the chamber, the proximal sciatic nerve was further transected, ligated, and reflected, leaving a 4-mm piece of denervated nerve in the proximal chamber. A tissue cable formed at 4 weeks only in those chambers prefilled with dialyzed plasma. Light and electron microscopy revealed a central core of Schwann cells and fibroblasts within the cable that were collectively surrounded by a circumferential layer of fibroblasts and collagen. Blood vessels were randomly located throughout the cable. The Schwann cells extended numerous processes that were confined within a basal lamina-like membrane. Many of these processes contained microtubules and resembled unmyelinated axons. The ultrastructure of the processes, however, differed from that of axons in that some of the processes were in direct contact with the basal lamina of the Schwann cells and not surrounded by any other cell extensions. However, since these processes neither stained with silver nor disappeared after transection of the nerves entering or leaving the chamber, we conclude that they are not axons but in fact Schwann cell processes. In other animals bearing 4-week cables, the reflected nerve stump was reattached to the nerve piece in the proximal end of the chamber. Four weeks later, all the cables and varying lengths of the distal nerve trunks were filled with numerous myelinated and unmyelinated axons. The Schwann cell cable that forms within a dialyzed plasma prefilled chamber presents a useful system for basic research concerning the molecular mechanisms of Schwann cell or Schwann cell-axonal interactions and for applied research involving the clinical repair of human peripheral nerve injuries. Since a cable formed by our surgical method supports axonal regeneration, it has the potential to eliminate the need for a nerve graft to repair a gap in a nerve that requires delayed surgical intervention.

Nerve cables formed in silicone chambers reconstitute a perineurial but not a vascular endoneurial permeability barrier.

The passage of molecules into the endoneurial environment of the axons of normal peripheral nerve is regulated by two permeability barriers, the perineurial-nerve barrier and the endoneurial blood-nerve barrier. These barriers exist because of the presence of tight junctions between adjacent perineurial cells and adjacent endothelial cells. In the present study we investigated whether permeability barriers form in nerve cables, which develop inside silicone chambers. The sciatic nerves of adult rats were cut, and the proximal and distal ends sutured into opposite ends of silicone chambers that were filled with dialyzed plasma. The presence of barriers was determined with the tracer horseradish peroxidase (HRP), which was injected intravenously and detected histochemically in tissues by light and electron microscopy. At four weeks, a regenerated nerve cable extended across the 10 mm length of each chamber. However, no permeability barriers were present since the reaction product for HRP was visible throughout the cable. At twenty-six weeks, all the axons in cables were gathered into minifascicles. Each minifascicle of axons was surrounded by perineurial cells. Blood vessels were excluded from the minifascicles by the perineurial cells and the vessels were permeable to HRP, thus indicating that their endothelial cells had not formed tight junctions. Despite the leakage of HRP from the excluded vessels, the tracer did not reach the axons because the perineurial cells encircling the minifascicles developed tight junctions. In some animals, the chambers were removed at four weeks to determine whether the chamber influenced barrier development. This manipulation had no effect since cables, with or without chambers, exhibited similar findings at twenty-six weeks. Our results indicate that nerve cables regenerate a perineurial but not an endoneurial permeability barrier. We conclude that axons in long-term cables are protected by only a perineurial permeability barrier.

A comparison of lectin binding in rat and human peripheral nerve.

Eleven different fluorescein- or peroxidase-conjugated lectins with different sugar-binding affinities were employed to analyze and compare glycoconjugates of rat and human peripheral nerves at the light microscopic level. A majority of lectins showed a distinct binding pattern in different structures of the nerve. Lectin binding was similar but not identical in rat and human nerves. Limulus polyhemus agglutinin did not stain any structures in rat or human nerves. In both species, all other lectins bound to the perineurium. Perineurial staining was intense with Canavalia ensiformis (Con A), Triticum vulgaris (WGA), Maclura pomifera (MPA); moderate with Glycine max (SBA), Griffonia simplicifolia-I (GS-I) and GS-II; weak with Ulex europaeus (UEA), Dolichos biflorus (DBA), and Ricinus communis (RCA). In the endoneurium of both species, ConA staining was intense, MPA and WGA moderate, SBA, GS-II, PNA, and RCA weak, and UEA and DBA absent. Interestingly, GS-I stained rat but not human endoneurium. Most lectins bound to blood vessels. GS-I bound to rat but not human, whereas UEA bound to human but not rat vessels. The results show that lectins can be used to reveal heterogeneity in sugar residues of glycoconjugates within neural and vascular components of nerves. They may therefore be potentially useful in detecting changes in glycoconjugates during nerve degeneration and subsequent regeneration after trauma or in pathological states.

Ultracytochemical localization of ouabain-sensitive K+-dependent, p-nitrophenyl phosphatase in myelin.

Ouabain-sensitive, K+-dependent p-nitrophenyl phosphatase (K-NPPase) activity was demonstrated ultracytochemically in the myelin of nerve fibers in peripheral and central white matter. Enzyme activity was more prominent in paranodal than compact myelin, and it was absent from nodal and interparanodal axolemma. Since K-NPPase is part of the Na-KATPase complex, we consider myelin as an important site of the sodium pump and believe that myelin participates in cationic regulation of the nervous tissue.

An immunofluorescent analysis of lectin binding to normal and regenerating skeletal muscle of rat.

Ten different fluorescein-conjugated lectins of various known sugar specificities were used to study cell surface glycoconjugates of normal and regenerating rat skeletal muscle. In normal muscle, Canavalia ensiformis agglutinin, Triticum vulgaris agglutinin (wheat germ agglutinin, WGA), Ricinus communis agglutinin-I, and Maclura pomifera agglutinin bound strongly to the endomysial region of the myofibers. No binding was observed in the cytoplasm of the myofibers. Other lectins (Dolichos biflorus agglutinin, Griffonia simplifolia agglutinin I and II, Ulex europaeus agglutinin, Arachis hypogaea agglutinin, Glycine max agglutinin) bound very poorly or not at all in the normal muscle. Skeletal muscle degeneration and regeneration was induced by autotransplantation of the extensor digitorum longus muscle. In the transplanted muscles, the endomysial lectin binding of the degenerating myofibers became weak and lacked continuity, indicating a breakdown of the endomysium. Of all the lectins tested, only WGA revealed intense binding in the myogenic zone of regenerating muscle. As the regeneration progressed, this WGA binding became restricted to the new endomysium. In completely regenerated muscle, binding of all lectins was similar to that seen in normal muscle. The specific binding of WGA to the myogenic zone may be important in identifying factors or glycoconjugates, which constitute a favorable environment for skeletal muscle regeneration.

Basement membrane component changes in nerve allografts and isografts.

This study describes immunocytochemical changes in laminin, which is an integral basement membrane (BM) component, during axonal regeneration through antigenic nerve allografts and nonantigenic nerve isografts. In normal rat nerve, laminin was localized in the BM of Schwann cells and the perineurium. During nerve allograft rejection, the perineurium and Schwann cells disappeared. However, the Schwann cell BMs persisted and became distorted and collapsed. In isografted nerves, the perineurium and Schwann cells were present, and only a few Schwann cell BMs appeared to be distorted; however, the staining for laminin was faint, indicating a possible BM breakdown. A new BM appeared as small rings around the Schwann cells after they had become associated with regenerated axons. Because only a limited axonal regeneration occurred in allografts as compared to isografts, it is concluded that the viable Schwann cells, and their BM architecture, are essential for regeneration through long nerve grafts.

Histochemical localization of gamma glutamyl transpeptidase in the paranodal region of peripheral nerve.

Gamma glutamyl transpeptidase (GGT), an amino acid transport enzyme, was investigated in normal and degenerated sciatic nerve of rat. The enzyme activity, which is considered to be a marker for cerebrovascular endothelium, was found to be absent in microvessels of normal and degenerated nerves. In the perineurium of normal nerve, GGT activity was faint, while in degenerated nerve, it increased. The most striking finding of this study was the observation of GGT activity at the paranode of each normal myelinated axon. It is interesting that after axotomy (8 weeks), no GGT activity was observed in the Schwann cells of degenerated nerve. Thus, Schwann cell plasmalemma contributed to GGT staining only when this cell was in contact with an axon mature enough to cause it to produce myelin. We conclude that, in peripheral nerve, transmembrane amino acid transport is apparently regional and associated with the paranodal region of myelinated nerve fibers.

Changes in the extracellular matrix components fibronectin and laminin during immune rejection of skeletal muscle.

Extracellular matrix is known to play an important role during development and maintenance of various tissues. In the present study, changes in two extracellular matrix glycoproteins, fibronectin and laminin, were investigated in skeletal muscle undergoing immune rejection. Purified antibodies against fibronectin and laminin were used to analyze the matrix by indirect immunofluorescence at various intervals after transplantation of extensor digitorum longus muscle in rats. Fibronectin and laminin were localized in the pericellular basement membrane zone of the normal myofibers; however, the cytoplasm was devoid of both glycoproteins. Transplanted muscle grafts underwent a process of degeneration and then an initial regeneration during the first 7 days. This regeneration effort ceased with the onset of muscle rejection in 14-day transplants. At this time, fibronectin was seen in the cytoplasmic region as well as the extracellular matrix of myofibers and myotubes. At later time intervals, an increased intensity of staining for fibronectin was seen throughout the rejected muscle. In muscle grafts undergoing regeneration but not rejection (i.e., nonantigenic grafts), such an increase in the presence of fibronectin was not seen ( Gulati et al., 1982). The distribution of laminin did not change during the rejection process and was localized in the basement membrane zone of myofibers and myotubes, although the overall configuration of the basement membranes was deformed and collapsed. It appears that the basement membranes are resistant to degradation, and staining for laminin persists in rejected muscle. These results show marked changes in the extracellular matrix of muscle undergoing rejection. The appearance of fibronectin during the initial stages of muscle rejection may have a causal relationship to the process of immune defence mechanism; however, the exact role of fibronectin remains elusive.

Survival of nerve allografts in sensitized rats treated with cyclosporin A.

This study examined the effect of immunosuppressive treatment with cyclosporin A (Cy-A) on the survival of nerve allografts in sensitized rats. Nerve- or skin-sensitized untreated rats rejected a second nerve allograft of the same genotype as the first in an accelerated manner. In this situation, only a few host axons grew into the proximal 1 cm of a 4 cm-long nerve allograft. However, if sensitized rats were given Cy-A (10 mg/kg daily), the second nerve allograft survived, and numerous host axons regenerated through the 4-cm length of the allograft. These results indicated that Cy-A was an effective immunosuppressive agent in sensitized rats. We conclude that, in rats, donor-specific sensitization is not a contraindication to the use of nerve allografts to aid in the repair of injured nerve when Cy-A is used for immunosuppression.

Failure of cyclosporin-A to induce immunological unresponsiveness to nerve allografts.

Although some allografts bearing major and minor transplantation antigens can survive after the cessation of immunosuppression with cyclosporin-A (Cy-A), nerve allografts do not. In an attempt to induce immunological unresponsiveness to nerve allografts, we used grafts containing only minor transplantation antigens and varied the duration of Cy-A therapy from 2 to 12 weeks. Our results demonstrated that nerve allografts survived in rats during Cy-A therapy, but when the drug administration ceased, the allografts were rejected. Other factors besides the degree of histoincompatibility and duration of Cy-A treatment must be involved in determining whether or not unresponsiveness develops to allografts after Cy-A withdrawal. We conclude that nerve allograft immunosuppression generated by Cy-A requires regular administration of the drug.

Changes in the basement membrane zone components during skeletal muscle fiber degeneration and regeneration.

The basement membrane of skeletal muscle fibers is believed to persist unchanged during myofiber degeneration and act as a tubular structure within which the regeneration of new myofibers occurs. In the present study we describe macromolecular changes in the basement membrane zone during muscle degeneration and regeneration, as monitored by immunofluorescence using specific antibodies against types IV and V collagen, laminin, and heparan sulfate proteoglycan and by the binding of concanavalin A (Con A). Skeletal muscle regeneration was induced by autotransplantation of the extensor digitorum longus muscle in rats. After this procedure, the myofibers degenerate; this is followed by myosatellite cell activation, proliferation, and fusion, resulting in the formation of new myotubes that mature into myofibers. In normal muscle, the distribution of types IV and V collagen, laminin, heparan sulfate proteoglycan, and Con A binding was seen in the pericellular basement membrane region. In autotransplanted muscle, the various components of the basement membrane zone disappeared, leaving behind some unidentifiable component that still bound Con A. Around the regenerated myotubes a new basement membrane (zone) reappeared, which persisted during maturation of the regenerating muscle. The distribution of various basement membrane components in the regenerated myofibers was similar to that seen in the normal muscle. Based on our present and previous study (Gulati, A.K., A.H. Reddi, and A.A. Zalewski, 1982, Anat. Rec. 204:175-183), it appears that some of the original basement membrane zone components disappear during myofiber degeneration and initial regeneration. As a new basement membrane develops, its components reappear and persist in the mature myofibers. We conclude that skeletal muscle fiber basement membrane (zone) is not a static structure as previously thought, but rather that its components change quite rapidly during myofiber degeneration and regeneration.

An immunofluorescent study of the distribution of fibronectin and laminin during limb regeneration in the adult newt.

Fibronectin and laminin are two extracellular glycoproteins which are involved in various processes of cellular development and differentiation. The present investigation describes changes in their distribution during regeneration of the newt forelimb, as determined by indirect immunofluorescence. The distribution of fibronectin and laminin was similar in normal limb tissue components. These glycoproteins were localized in the pericellular region of the myofibers corresponding to its basement membrane; the perineurium and endoneurium of the nerves; and the basement membranes of blood vessels, skin epithelium, and dermal glands. The cytoplasm of myofibers, axons, skin epithelium, and bone matrix lacked fluorescence for both glycoproteins. After limb amputation in the regenerating blastema, extensive presence of fibronectin, but not laminin, was seen in and around the undifferentiated blastemal cells. Increased fluorescence for fibronectin was also seen during blastema growth, blastemal cell aggregation, and early stages of redifferentiation. As redifferentiation continued, staining for fibronectin slowly disappeared from the cartilage matrix and the myoblast fusion zone. Laminin was first observed around the regenerated myotubes; this was followed by the appearance of fibronectin suggesting a sequential formation of these two components of the new myotube basement membrane. In the regenerated limb, the distribution of laminin and fibronectin was similar to that seen in normal limb. Based on the distribution pattern of these glycoproteins, it is concluded that fibronectin may play an important role in blastemal cell aggregation, cell alignment, and initiation of redifferentiation. After redifferentiation, both laminin and fibronectin may be important in the determination of the architecture of the regenerated limb.

Distribution of fibronectin in normal and regenerating skeletal muscle.

The distribution of fibronectin in normal and regenerating skeletal muscle (the latter caused by autotransplantation) was investigated by means of indirect immunofluorescent technique. Normal myofibers exhibited a thin, continuous pericellular (endomysium) fibronectin distribution; however, their sarcoplasm was devoid of fibronectin. After autotransplantation, the skeletal muscle fibers underwent a process of degeneration that was followed by regeneration from the premyogenic satellite cells. These cells multiplied, fused to form myotubes, and matured into new myofibers. A decrease and an eventual loss of endomysial fibronectin was seen in the degenerating myofibers. At the same time, fibronectin appeared in the sarcoplasm. No significant fibronectin was expressed in the myogenic zone until the formation of myotubes which possessed a complete, circular fibronectin ring. The sarcoplasm of the myotubes lacked fibronectin. Since fibronectin is a component of basement membrane of several tissues, its disappearance and reappearance can be used to follow the fate of basement membrane. We conclude that fibronectin may not be essential for early myogenesis and that regenerated myotubes form an entirely new or at least certain new molecular components of their basement membrane. The present muscle autotransplantation model can be used to further study the role of fibronectin during myogenesis and cell transformation in vivo.

Failure of host axons to regenerate through a once successful but later rejected long nerve allograft.

Host axons will regenerate through a long nerve allograft in an immunologically tolerant rat. However, if tolerance is abolished, rejection occurs and allogeneic cells (e.g., Schwann, vascular, perineurial, etc.) as well as regenerated host axons disappear from the allograft. Because following tolerance abolition host axons begin to regenerate into the connective tissue remnants of the rejected nerve allografts, the extent of this renewed axonal growth was investigated. It was found that in a tolerance-abolished rat, host axons only regrew in to the proximal 1 cm of a 4-cm allograft which in a fully tolerant recipient would have had numerous allogeneic Schwann cell-myelinated axons throughout its length. It is concluded that viable allogenic cells (i.e., Schwann, fibroblast, and vascular) together with their connective tissue matrix provide the best way to aid host nerve fiber regeneration through a long nerve allograft.

Evaluation of histocompatibility as a factor in the repair of nerve with a frozen nerve allograft.

Host axons in dogs can regenerate through a long nerve allograft provided that the allograft bears only minor transplantation antigens, and is frozen and thawed before transplantation. The authors have tried to confirm this important observation in rats. Host rats received a 4-cm fresh or frozen nerve isograft (that is, a non-antigenic nerve), or a fresh or frozen nerve allograft with cells containing only minor transplantation antigens. The results showed that after 2 and 9 months only a fresh isograft permitted many host axons to traverse its entire length. Only a few host axons grew into the proximal 1 to 2 cm of a frozen isograft or into an allograft (fresh or frozen). Because frozen grafts failed, the authors examined some specimens after 2 weeks and found that freezing killed most of the Schwann cells. on the other hand, many proliferating Schwann cells were found in 2-week fresh isografts. In addition, hosts that received a frozen nerve allograft underwent regrafting after 9 months with an isograft and allograft (of the same genotype as th original nerve allograft) of nodose ganglion. These rats accepted the isograft but rejected the allograft of ganglion. It is concluded that axonal regeneration through a long frozen nerve graft fails in rats because freezing destroys Schwann cells. Moreover, a frozen nerve allograft does not induce a state of immunological tolerance, as has been suggested, because these recipients reject a second allograft. Since the present data failed to confirm findings obtained in dogs, the clinical use of a frozen nerve allograft is not recommended.

Regeneration of taste buds after reinnervation of a denervated tongue papilla by a normally nongustatory nerve.

Taste buds degenerate and disappear after transection of their sensory nerve supply, and they differentiate anew from epithelial cells (e.g., lingual) following regeneration of sensory but not motor or autonomic axons. A controversy exists as to whether only gustatory sensory nerves can cause buds to reform or whether any sensory nerve can perform this function. This issue arose because the results of cross-innervation studies revealed a specificity whereas grafting data demonstrated a nonspecificity. A retest of specificity in the cross-reinnervation situation was performed by reinnervating the denervated vallate papilla of adult rat tongue with a sensory branch of the vagus nerve that is not normally gustatory. It was found that taste buds disappeared and remained lost from acutely and chronically denervated papilla. However, some buds were found 90-100 days after reinnervation by the normally nongustatory vagus nerve branch. Transection of the regenerated vagus nerve resulted in the loss of innervation and the degeneration of taste buds from reinnervated papilla indicating that this nerve had supported buds. These results show that a normally nongustatory nerve can induce the formation of taste buds after its axons grow into appropriate tissue. It appears that the ability to support taste buds is a nonspecific, rather than a specific, property of sensory nerve.