Somatofugal events in Wallerian degeneration: a conceptual overview

The neuroimmunology of degeneration and regeneration in the peripheral nervous system

Peripheral nerves regenerate following injury due to the effective activation of the intrinsic growth capacity of the neurons and the formation of a permissive pathway for outgrowth due to Wallerian degeneration (WD). WD and subsequent regeneration are significantly influenced by various immune cells and the cytokines they secrete. Although macrophages have long been known to play a vital role in the degenerative process, recent work has pointed to their importance in influencing the regenerative capacity of peripheral neurons. In this review, we focus on the various immune cells, cytokines, and chemokines that make regeneration possible in the peripheral nervous system, with specific attention placed on the role macrophages play in this process.

A critical role for macrophages near axotomized neuronal cell bodies in stimulating nerve regeneration

Macrophages have been implicated in peripheral nerve regeneration for some time, supposedly through their involvement in Wallerian degeneration, the process by which the distal nerve degenerates after axotomy and is cleared by phagocytosis. Thus, in several studies in which macrophage accumulation in the distal nerve was reduced and Wallerian degeneration inhibited, regeneration was delayed. However, this interpretation ignores the more recent findings that macrophages also accumulate around axotomized cell bodies. The function of macrophage action at this second site has not been clear. In two mutant strains of mice, the slow Wallerian degeneration (Wld(s)) mouse and the chemokine receptor CCR2 knock-out mouse, we report that macrophage accumulation after axotomy was abolished in both the dorsal root ganglion (DRG) and the distal sciatic nerve. To measure neurite outgrowth, DRG neurons were given a conditioning lesion, and outgrowth was measured in vitro 7 d later in the absence of the distal nerve segment. The increased growth normally seen after a conditioning lesion did not occur or was reduced in Wld(s) or CCR2(-/-) mice. In the superior cervical ganglion (SCG), particularly in Wld(s) mice, macrophage accumulation was reduced but not abolished after axotomy. In SCG neurons from Wld(s) mice, the conditioning lesion response was unchanged; however, in CCR2(-/-) mice in which the effect on macrophage accumulation was greater, SCG neurite outgrowth was significantly reduced. These results indicate that macrophages affect neurite outgrowth by acting at the level of peripheral ganglia in addition to any effects they might produce by facilitation of Wallerian degeneration.

The spatiotemporal localization of JAM-C following sciatic nerve crush in adult rats

JAM-C is a junctional adhesion molecule, enriched at tight junctions on endothelial and epithelial cells, and also localized to Schwann cells at junctions between adjoining myelin end loops. The role of JAM-C following peripheral nerve injury (PNI) is currently unknown. We examined the localization of JAM-C after sciatic nerve crush injury in adult rats. JAM-C immunoreactivity was present in paranodes and incisures in sham surgery control nerve, but distal to the crush injury significantly decreased at three and 14 days. JAM-C was re-expressed at 28 days and, by 56 days, was significantly increased in the distal nerve compared to controls. In a 7-mm length of sciatic nerve sampled distal to the crush site, the densities of JAM-C immunoreactive paranodes increased in the distal direction. Conversely, the densities of JAM-C immunoreactive incisures were highest immediately distal to the crush site and decreased in the more distal direction. Further analysis revealed a strong correlation between JAM-C localization and remyelination. Fifty-six days after crush injury, greater densities of JAM-C paranodes were seen compared to the nodal marker jacalin, suggesting that paranodal JAM-C precedes node formation. Our data are the first to demonstrate a potential role of JAM-C in remyelination after PNI.

The progressive nature of Wallerian degeneration in wild-type and slow Wallerian degeneration (WldS) nerves

BACKGROUND

The progressive nature of Wallerian degeneration has long been controversial. Conflicting reports that distal stumps of injured axons degenerate anterogradely, retrogradely, or simultaneously are based on statistical observations at discontinuous locations within the nerve, without observing any single axon at two distant points. As axon degeneration is asynchronous, there are clear advantages to longitudinal studies of individual degenerating axons. We recently validated the study of Wallerian degeneration using yellow fluorescent protein (YFP) in a small, representative population of axons, which greatly improves longitudinal imaging. Here, we apply this method to study the progressive nature of Wallerian degeneration in both wild-type and slow Wallerian degeneration (WldS) mutant mice.

RESULTS

In wild-type nerves, we directly observed partially fragmented axons (average 5.3%) among a majority of fully intact or degenerated axons 37-42 h after transection and 40-44 h after crush injury. Axons exist in this state only transiently, probably for less than one hour. Surprisingly, axons degenerated anterogradely after transection but retrogradely after a crush, but in both cases a sharp boundary separated intact and fragmented regions of individual axons, indicating that Wallerian degeneration progresses as a wave sequentially affecting adjacent regions of the axon. In contrast, most or all WldS axons were partially fragmented 15-25 days after nerve lesion, WldS axons degenerated anterogradely independent of lesion type, and signs of degeneration increased gradually along the nerve instead of abruptly. Furthermore, the first signs of degeneration were short constrictions, not complete breaks.

CONCLUSIONS

We conclude that Wallerian degeneration progresses rapidly along individual wild-type axons after a heterogeneous latent phase. The speed of progression and its ability to travel in either direction challenges earlier models in which clearance of trophic or regulatory factors by axonal transport triggers degeneration. WldS axons, once they finally degenerate, do so by a fundamentally different mechanism, indicated by differences in the rate, direction and abruptness of progression, and by different early morphological signs of degeneration. These observations suggest that WldS axons undergo a slow anterograde decay as axonal components are gradually depleted, and do not simply follow the degeneration pathway of wild-type axons at a slower rate.

Watery and dark axons in Wallerian degeneration of the opossum's optic nerve: different patterns of cytoskeletal breakdown?

In this paper we report a qualitative morphological analysis of Wallerian degeneration in a marsupial. Right optic nerves of opossums Didelphis marsupialis were crushed with a fine forceps and after 24, 48, 72, 96 and 168 hours the animals were anaesthetized and perfused with fixative. The optic nerves were immersed in fixative and processed for routine transmission electron microscopy. Among the early alterations typical of axonal degeneration, we observed nerve fibers with focal degeneration of the axoplasmic cytoskeleton, watery degeneration and dark degeneration, the latter being prevalent at 168 hours after crush. Our results point to a gradual disintegration of the axoplasmic cytoskeleton, opposed to the previous view of an "all-or-nothing" process (Griffin et al 1995). We also report that, due to an unknown mechanism, fibers show either a dark or watery pattern of axonal degeneration, as observed in axon profiles. We also observed fibers undergoing early myelin breakdown in the absence of axonal alterations.

Early myelin breakdown following sural nerve crush: a freeze-fracture study

In this study we describe the early changes of the myelin sheath following surgical nerve crush. We used the freeze-fracture technique to better evaluate myelin alterations during an early stage of Wallerian degeneration. Rat sural nerves were experimentally crushed and animals were sacrificed by transcardiac perfusion 30 h after surgery. Segments of the nerves were processed for routine transmission electron microscopy and freeze-fracture techniques. Our results show that 30 h after the lesion there was asynchrony in the pattern of Wallerian degeneration, with different nerve fibers exhibiting variable degrees of axon disruption. This was observed by both techniques. Careful examination of several replicas revealed early changes in myelin membranes represented by vacuolization and splitting of consecutive lamellae, rearrangement of intramembranous particles and disappearance of paranodal transverse bands associated or not with retraction of paranodal myelin terminal loops from the axolemma. These alterations are compatible with a direct injury to the myelin sheath following nerve crush. The results are discussed in terms of a similar mechanism underlying both axon and myelin breakdown.

Role of thyroid hormones and their receptors in peripheral nerve regeneration

After peripheral nerve injury in adult mammals, reestablishment of functional connections depends on several parameters including neurotrophic factors, the extracellular matrix, and hormones. However, little is known about the contribution of hormones to peripheral nerve regeneration. Thyroid hormones, which are required for the development and maturation of the central nervous system, are also important for the development of peripheral nerves. The action of triiodothyronine (T3) on responsive cells is mediated through nuclear thyroid hormone receptors (TRs) which modulate the expression of specific genes in target cells. Thus, to study the effect of T3, it is first necessary to know whether the target tissues possess TRs. The fact that sciatic nerve cells possess functional TRs suggests that these cells can respond to T3 and, as a consequence, that thyroid hormone may be involved in peripheral nerve regeneration. The silicone nerve guide model provides an excellent system to study the action of local administration of T3. Evidence from such studies demonstrate that animals treated locally with T3 at the level of transection have more complete regeneration of sciatic nerve and better functional recovery. Among the possible regulatory mechanisms by which T3 enhances peripheral nerve regeneration is rapid action on both axotomized neurons and Schwann cells which, in turn, produce a lasting and stimulatory effect on peripheral nerve regeneration. It is probable that T3 up- or down-regulates gene expression of one or more growth factors, extracellular matrix, or cell adhesion molecules, all of which stimulate peripheral nerve regeneration. This could explain the greater effect of T3 on nerve regeneration compared with the effect of any one growth factor or adhesion molecule.

Cyclosporin A retards the wallerian degeneration of peripheral mammalian axons

The distal (anucleate) segments of mammalian peripheral axons typically undergo complete Wallerian degeneration within 1-3 days after severance from their cell bodies, unlike invertebrates and lower vertebrates, where anucleate axons do not degenerate for weeks to months. This rapid Wallerian degeneration in mammals could be due to a more efficient immune system and/or to differences in calcium-dependent pathways relative to invertebrates and lower vertebrates. To suppress the immune system and to inhibit calcium-dependent pathways in axons, we gave daily subcutaneous injections of cyclosporin A (CsA: 10 mg/kg) to Sprague-Dawley rats for 7 days before, and 5 days after, severing their right ventral tail nerves. To confirm that CsA suppressed the immune system, white blood cell density was measured in CsA-treated and in non-treated rats. Our data showed that the number of surviving anucleate myelinated axons at 5 postoperative days in CsA-treated rats was significantly higher than the number in non-treated rats. Anucleate unmyelinated axons in the ventral tail nerve also exhibited better survival in CsA-treated rats than in nontreated rats. These results are consistent with the hypothesis that the immune response and/or calcium-dependent pathways play important roles in the rapid Wallerian degeneration of anucleate mammalian axons.

Mechanism of calcium entry during axon injury and degeneration

Axon degeneration is a hallmark consequence of chemical neurotoxicant exposure (e.g., acrylamide), mechanical trauma (e.g., nerve transection, spinal cord contusion), deficient perfusion (e.g., ischemia, hypoxia), and inherited neuropathies (e.g., infantile neuroaxonal dystrophy). Regardless of the initiating event, degeneration in the PNS and CNS progresses according to a characteristic sequence of morphological changes. These shared neuropathologic features suggest that subsequent degeneration, although induced by different injury modalities, might evolve via a common mechanism. Studies conducted over the past two decades indicate that Ca2+ accumulation in injured axons has significant neuropathic implications and is a potentially unifying mechanistic event. However, the route of Ca2+ entry and the involvement of other relevant ions (Na+, K+) have not been adequately defined. In this overview, we discuss evidence for reverse operation of the Na+-Ca2+ exchanger as a primary route of Ca2+ entry during axon injury. We propose that diverse injury processes (e.g., axotomy, ischemia, trauma) which culminate in axon degeneration cause an increase in intraaxonal Na+ in conjunction with a loss of K+ and axolemmal depolarization. These conditions favor reverse Na+-Ca2+ exchange operation which promotes damaging extraaxonal Ca2+ entry and subsequent Ca2+-mediated axon degeneration. Deciphering the route of axonal Ca2+ entry is a fundamental step in understanding the pathophysiologic processes induced by chemical neurotoxicants and other types of nerve damage. Moreover, the molecular mechanism of Ca2+ entry can be used as a target for the development of efficacious pharmacotherapies that might be useful in preventing or limiting irreversible axon injury.

Anomalous cross-modal plasticity following posterior fossa surgery: some speculations on gaze-evoked tinnitus

A unique and intriguing form of subjective tinnitus evoked by eye gaze is reviewed. A new perspective is presented because this condition is sufficiently different from other forms of subjective tinnitus and its manifestation cannot be adequately explained by existing models or conceptual frameworks. Our examination of this topic considers pathophysiologic changes in the central nervous system in the context of deafferentation-induced plasticity. Potential neuroanatomical areas contributing to this effect include a number of distributed and functionally diverse areas in the brainstem and neocortex involved in the auditory control of eye movements. We also consider contemporary psychophysical methods to evaluate the perceptual correlates of this phenomenon and tools for the development of objective tinnitus measurements. Although theoretical and speculative in nature, this article is intended to stimulate interest in, advance knowledge of, and provide a better understanding about this condition.

Different rates of axonal degeneration in the crossed and uncrossed retinofugal pathways of Monodelphis domestica

The uncrossed retinofugal fibres in the marsupial Monodelphis domestica form a separate bundle as they pass through the optic chiasm. The uncrossed fibres segregate from the crossed fibres a short distance before they reach the chiasm, gathering as an essentially exclusive bundle in the ventral part of the optic nerve. This bundle then passes laterally through the optic chiasm and into the optic tract. The distinctive position of the uncrossed fibres has allowed us to recognise that, surprisingly, the uncrossed fibres degenerate more rapidly than the rest. Seven days after a monocular enucleation approximately 60-80% of the fibres of the crossed component in the main part of the optic nerve near the chiasm have a normal cross sectional appearance in electron micrographs whereas less than 20% of the fibres in the uncrossed bundle look normal. The rapid degeneration of the uncrossed fibres cannot be related to any morphological parameter of the axons. Their fibre diameters are mainly medium to thick, lying within the range of axon diameters found in the rest of the nerve. The axon-myelin ratios of the uncrossed fibres are also no different from those of the crossed optic fibres. There are no structural peculiarities identifiable with light or electron microscopical methods in either the axons or in the glia of the uncrossed bundle that might account for the more rapid degeneration. There is evidence that the degenerative change in the main part of the optic nerve progresses from the lesion towards the chiasm, and that for the crossed fibres it may progress slightly faster for the thicker than for the thinner fibres. The degeneration in the uncrossed bundle does not fit any of the rules that have been proposed for relating rate of degeneration to fibre diameter. We conclude that the rate of Wallerian degeneration is determined by factors that yet remain to be defined.

Immunohistochemical studies on neurofilamentous hypertrophy in degenerating retinal terminals of the olivary pretectal nucleus in the rat

Following section of the optic nerve, degenerating retinal terminals reveal an accumulation of neurofilaments (neurofilamentous hypertrophy) as demonstrated by silver impregnation techniques or electron microscopy. The present study examined degenerating retinal terminals by means of immunohistochemistry and antibodies specific for the triplet of neurofilament proteins of low (NF-L), medium (NF-M), and high (NF-H) molecular weight class. Following unilateral optic nerve section in the rat and survival of 1, 2, 4, 8, and 21 days, brains were perfused with aldehyde fixative, sliced on a vibratome and stained for neurofilaments by using the peroxidase-antiperoxidase technique. Other brains were frozen, cut in the native state, and slide-mounted sections were fixed by acetone. Side comparisons in visual pathways were made in frontal sections, taking advantage of the near complete crossing of retinal fibers in the rat. Anterograde degeneration of axons occurred in the optic tract and branchium colliculi. Changes of terminals were investigated in the olivary pretectal nucleus, which contains a dense aggregation of retinal terminals in the core region. The optic tract and branchium colliculi showed a reduction in immunostaining for neurofilament proteins following axotomy. Within the core region of the olivary pretectal nucleus, strong increases of immunoreactivity of NF-L and NF-M were detected beginning at 2 days postlesion and persisting at 8 days. No changes in NF-H proteins were found in the terminal regions with three different antibody probes. The increase in immunostaining reflects the accumulation of neurofilament proteins in the degenerating retinal terminals, i.e., neurofilamentous hypertrophy.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged survival of transected nerve fibres in C57BL/Ola mice is an intrinsic characteristic of the axon

Transected axons in C57BL/Ola mice survive for extraordinary lengths of time as compared to those of normal rodents. The biological difference in the substrain that confers the phenotype of prolonged axonal survival is unknown. Previous studies suggest that 'defect' to be a property of the nervous system itself, rather than one of haematogenous cells. Neuronal or non-neuronal elements could be responsible for this phenotype. This study was undertaken to determine whether Schwann cells, the most numerous of the non-neuronal cells intrinsic to the peripheral nerve, are responsible for delayed degeneration of transected axons. We created sciatic nerve chimeras by transplanting nerve segments between standard C57BL/6 and C57BL/Ola mice, allowing regeneration of host axons through the grafts containing donor Schwann cells. These nerves were then transected and the time course of axonal degeneration was observed. The results show that fast or slow degeneration is a property conferred by the host, and therefore cannot be ascribed to the Schwann cells. Similarly, transected C57BL/Ola axons in explanted dorsal root ganglia cultures survived longer than transected axons from standard mice. Taken together these results indicate that the responsible abnormality is intrinsic to the C57BL/Ola axon.

Changes in nuclear 3,5,3'-triiodothyronine receptor expression in rat dorsal root ganglia and sciatic nerve during development: comparison with regeneration

The action of the thyroid hormones on responsive cells in the peripheral nervous system requires the presence of nuclear triiodothyronine receptors (NT3R). These nuclear receptors, including both the alpha and beta subtypes of NT3R, were visualized by immunocytochemistry with the specific 2B3 monoclonal antibody. In the dorsal root ganglia (DRG) of rat embryos, NT3R immunoreactivity was first discretely revealed in a few neurons at embryonic day 14 (E14), then strongly expressed by all neurons at E17 and during the first postnatal week; all DRG neurons continued to possess clear NT3R immunostaining, which faded slightly with age. The peripheral glial cells in the DRG displayed a short-lived NT3R immunoreaction, starting at E17 and disappearing from the satellite and Schwann cells by postnatal days 3 and 7 respectively. In the developing sciatic nerve, Schwann cells also exhibited transient NT3R immunoreactivity restricted to a short period ranging from E17 to postnatal day 10; the NT3R immunostaining of the Schwann cells vanished proximodistally along the sciatic nerve, so that the Schwann cells rapidly became free of detectable NT3R immunostaining. However, after the transection or crushing of an adult sciatic nerve, the NT3R immunoreactivity reappeared in the Schwann cells adjacent to the lesion by 2 days, then along the distal segment in which the axons were degenerating, and finally disappeared by 45 days, when the regenerating axons were allowed to re-occupy the distal segment.(ABSTRACT TRUNCATED AT 250 WORDS)

Myelin sheath survival after guanethidine-induced axonal degeneration

Membrane-membrane interactions between axons and Schwann cells are required for initial myelin formation in the peripheral nervous system. However, recent studies of double myelination in sympathetic nerve have indicated that myelin sheaths continue to exist after complete loss of axonal contact (Kidd, G. J., and J. W. Heath. 1988. J. Neurocytol. 17:245-261). This suggests that myelin maintenance may be regulated either by diffusible axonal factors or by nonaxonal mechanisms. To test these hypotheses, axons involved in double myelination in the rat superior cervical ganglion were destroyed by chronic guanethidine treatment. Guanethidine-induced sympathectomy resulted in a Wallerian-like pattern of myelin degeneration within 10 d. In doubly myelinated configurations the axon, inner myelin sheath (which lies in contact with the axon), and approximately 75% of outer myelin sheaths broke down by this time. Degenerating outer sheaths were not found at later periods. It is probably that outer sheaths that degenerated were only partially displaced from the axon at the commencement of guanethidine treatment. In contrast, analysis of serial sections showed that completely displaced outer internodes remained ultrastructurally intact. These internodes survived degeneration of the axon and inner sheath, and during the later time points (2-6 wk) they enclosed only connective tissue elements and reorganized Schwann cells/processes. Axonal regeneration was not observed within surviving outer internodes. We therefore conclude that myelin maintenance in the superior cervical ganglion is not dependent on direct axonal contact or diffusible axonal factors. In addition, physical association of Schwann cells with the degenerating axon may be an important factor in precipitating myelin breakdown during Wallerian degeneration.

Sequential events of degeneration and synaptic remodelling in the viper optic tectum following retinal ablation. A degeneration, radioautographic and immunocytochemical study

The ultrastructural changes taking place in the retino-recipient layers of the viper optic tectum were examined between 5 and 122 days after retinal ablation. The initial degeneration of retinotectal terminals proceeds at widely different rates and is characterized by a marked degree of polymorphism in which a number of different patterns can be discerned. In the final stages of degeneration, either both the degenerating bouton and the distal portion of the postsynaptic element are engulfed by reactive glia, or, more frequently, only the degenerating terminal is eliminated and the postsynaptic differentiation remains. The free postsynaptic differentiations are reoccupied predominantly by boutons containing pleiomorphic vesicles and which are for the most part gamma-aminobutyric acid (GABA)ergic, thus forming heterologous synapses; less frequently these sites are occupied by boutons of the ipsilateral visual contingent to form homologous synapses. These two processes, both of which depend on terminal axonal sprouting, take place within the first 3 postoperative months. They are followed by a decrease in the number of heterologous synapses and a concurrent increase in the number of homologous synapses newly formed by optic boutons generated by collateral preterminal sprouting of ipsilateral retinotectal fibres. The data suggest that partial deafferentation of the optic tectum induces a transitory GABAergic innervation of free postsynaptic sites prior to the restoration of new retinal synaptic contacts.

Freeze-fracture analysis of myelin membrane changes in Wallerian degeneration

Transection of mouse sciatic nerves produced microscopic changes in the myelin sheaths distal to the transection. Studied with freeze-fracture, these microscopic changes were correlated with alterations in the macromolecular organization of nerve membranes. In control mice, sciatic nerve myelin contained randomly distributed intramembranous particles. In the early stages of myelin breakdown the lamellae split and large areas of myelin membrane lacked intramembranous particles. The remaining particles clustered with a greater than normal density. Degenerating myelin was found within Schwann cells which still had an outer mesaxon and a normal distribution of intramembranous particles on the cell outer membrane. As the degeneration proceeded, myelin ovoids formed which completely lacked intramembranous particles. The findings suggest that during Wallerian degeneration there is a progression of myelin changes leading to the eventual loss of myelin intramembranous particles. These observations are morphological evidence that Schwann cells remove components from selective portions of their membrane during Wallerian degeneration.

Cold blockade of axonal transport activates premitotic activity of Schwann cells and wallerian degeneration

Between 3 and 4 days after transection of cat sciatic nerve, Schwann cell-associated premitotic activity spreads anterogradely along degenerating distal nerve stumps at a rate of approximately 200 mm/day. We investigated whether fast anterograde axonal transport contributes to the initiation of this component of Wallerian degeneration. Axonal transport was blocked in intact and transected cat sciatic nerves by focally chilling a proximal segment to temperatures below 11 degrees C for 24 hr. Incorporation of [3H]thymidine (a marker of premitotic DNA synthesis) was then measured 3 and 4 days posttransection in cold blocked- and control-degenerating nerves. Effects of cold block prior to and concomitant with nerve transection were studied. Results failed to support the hypothesis that Schwann-cell premitotic activity after axotomy is associated with entry into the axon of mitogenic substances and their anterograde fast transport along the distal stump. Instead, data suggested that progressive anterograde failure of fast anterograde transport distal to transection serves to effect the Schwann-cell premitotic response to axotomy.

Morphological and physiological survival of goldfish Mauthner axons isolated from their somata by spinal cord crush

Axon segments isolated from their somata degenerate within days or months depending on species and neuronal type. To better understand the time course of morphological and physiological changes associated with degeneration of axon segments of vertebrate central neurons, we have studied the goldfish Mauthner axon (M-axon) when it has been separated from its soma by spinal cord crush. M-axon segments survive morphologically for at least 77 days at 14 degrees C. Cross-sectional areas of isolated M-axon segments (measured 25-30 mm caudal to the wound site at postoperative days 64 and 77) were greater than those of control axons at the same level. Sheath areas did not change. Electron microscopic observations at the same spinal cord location indicated no clear changes in the configuration or number of neurofilaments or any other organelle. M-axon segments studied morphologically after 87 postoperative days had all degenerated. Mauthner axon segments were capable of conducting action potentials and eliciting ipsilateral EMG responses. Repetitive firing of the M-axon segments elicited EMG responses that fatigued more easily and remained fatigued over a longer interval than did those of control axons. The long duration of M-axon segment survival is unusual in a vertebrate and may be due to the low temperature at which the experiments were conducted (14 degrees C) and/or temperature-independent factors. The increased susceptibility to synaptic depression, which has not reported previously, may represent an early sign of the degenerative process.

Structural properties of spinal nerve roots: biomechanics

The biomechanics of spinal nerve roots obtained from normal and nerve-crushed mice were evaluated. Photographs and longitudinal force measurements were taken as nerve roots were elongated through mechanical failure. Proportional limit stress and strain as well as the apparent modulus were calculated from photographic and force measurements to characterize nerve root strength, elasticity, and stiffness, respectively. Resulting mechanical data were indicative of an extremely weak material. Comparisons of nerve and nerve root mechanical properties revealed major differences. While nerve root elasticity was comparable to nerve, nerve root strength was only 10% that of nerve and root stiffness was only 20% of nerve values. Differences in nerve and root mechanics are attributed to the large discrepancies in relative amounts of connective tissue. Also in sharp contrast with peripheral nerve, unilateral nerve crush produced no significant alterations in root mechanics. Comparisons of nerve and nerve root strengths suggested possible pathways for dissipation of peripherally applied forces through epineurial and dural structures.

Degenerative and regenerative changes in the trochlear nerve of goldfish

The features of unlesioned and lesioned trochlear nerves of goldfish have been examined electron microscopically. Lesioned nerves were studied between 1 and 107 days after cutting or crushing the nerve. Unlesioned nerves contained, on average, 77 myelinated axons and 19 unmyelinated axons. The latter were found in 1-2 fascicles per nerve. A basal lamina surrounded each myelinated axon and fascicle of unmyelinated axons. The numbers of myelinated axons, fascicles of unmyelinated axons and basal laminae varied by less than 5% over the intraorbital extramuscular segment of the nerve. Following interruption of the nerve, by either cutting or crushing, all of the axons and their myelin sheaths began to degenerate by 4 days in the distal nerve-stump. Both abnormally electron-dense and electron-lucent axons were observed. Both Schwann cells and macrophages appeared to phagocytose the myelin sheaths. Following a lesion, the Schwann cells and their basal laminae persisted in the distal nerve-stump. In crushed nerves, the basal laminae surrounding myelinated axons formed 97%, on average, of the Schwann tubes in the distal stump. The perimeters of the basal laminae were of similar size to those in the proximal stump, at least for the first 8 days after crush. In crushed nerves, single myelinated axons in the proximal nerve-stump gave rise to multiple sprouts, some of which reached the site of crush by 2 days, the distal stump by 4 days and the superior oblique muscle by 8 days. The regeneration of the unmyelinated axons was not examined. In both crushed and transected nerves, nearly all of the sprouts in the proximal and distal stumps were found within the basal laminae of Schwann cells, even though the spouts were disorganized in the transected region where there were no basal laminae. The growth cones of the regenerating axons were always found apposed to the inner surface of the basal laminae, which may have provided an adhesive substrate that directed their growth. Terminal sprouts from the ends of myelinated axons in the proximal stump accounted for the majority of the regenerating axons in the distal stump, as only a few collateral sprouts were found in the proximal stump, and only a small amount of axonal branching was found within the distal stump itself. The largest axons in the distal stump were remyelinated first, and the number of remyelinated axons increased progressively between 8 and 31 days after crush, at which time there were about twice as many as in unlesioned nerves.(ABSTRACT TRUNCATED AT 400 WORDS)

Prenatal alcohol exposure alters adult expression of sexually dimorphic behavior in the rat

Saccharin preference and performance in a Lashley III maze were found to be altered in adult male and female rats that had been exposed to alcohol during gestation. Specifically, the sexual dimorphism normally observed in both behaviors was absent in fetal alcohol-exposed animals. The lack of sexual dimorphism appeared to result from a masculinization of the exposed females and a feminization of the exposed males.

Myelin breakdown and elimination in the posterior funiculus of the adult cat after dorsal rhizotomy: a light and electron microscopic qualitative and quantitative study

Adult cats were subjected to unilateral dorsal L6, L7, and S1 rhizotomy. After survival times of 1-1,552 days the degeneration of axons and myelin sheaths and the elimination of degenerating myelin was studied qualitatively and quantitatively with light and electron microscopy and in Marchi-stained sections in the posterior funiculus in T12-L2. Degeneration was first observed as swollen or shrunken nerve fibers. Somewhat later there was an increased occurrence of collapsed myelin sheaths. The latter lost their myelin periods and appeared to be transformed into myelin bodies. The occurrence of myelin bodies coincided temporally with the presence of many Marchi-positive bodies. Later, an increasing number of intracellularly located lipid droplets occurred, paralleled by the occurrence of a great number of Marchi-positive granules and crystalline structures. Profiles of collapsed myelin sheaths, myelin bodies, and lipid droplets were frequently seen in the cytoplasm of microglial cells. Later, astrocytes and perivascular cells became filled with numerous lipid droplets. The findings suggest that microglial cells take up collapsed myelin sheaths and within these cells the sheaths become transformed into myelin bodies and subsequently into lipid droplets. These two products of myelin disintegration appear to correspond to the Marchi-positive structures seen during the degeneration process. The lipid droplets appear to be transported to astrocytes and perivascular cells.

Alterations in the mechanical properties of peripheral nerve following crush injury

The mechanical properties of injured nerves have been studied. At specific times following unilateral nerve crush, the sciatic nerves of mice were tested mechanically. Photographs and longitudinal force measurements were obtained as nerve segments were elongated to mechanical failure. Stress and strain at the proportional limit and apparent elastic modulus were used as indicators of strength, elasticity and stiffness. Injury led to time-dependent increases in strength and stiffness and decreases in elasticity. These changes were apparent in both damaged and contralateral, undamaged nerves. Many of the changes appear to be related to the epineurium. Some mechanical changes in nerve could have important consequences for the integrity and function of nerves and mechanically interfaced structures.

Comparison of ultrastructural changes in proximal and distal segments of transected giant fibers of the cockroach Periplaneta americana

When the giant axons of the cockroach Periplaneta americana are transected the proximal segment (the part connected to the soma) regenerates by tip sprouting and the distal segment degenerates. The initial ultrastructural response (24-48 h post-transection) occurring in the cut ends of the proximal and distal segments are similar. This response includes the disappearance of neurotubules; appearance of amorphous material in the axoplasm and a gradual accumulation of large numbers of small mitochondria, vesicles of various sizes and smooth endoplasmic reticulum. The axolemma in the region of organelle accumulation invaginates and glial processes are present in the invagination. The similarity of the changes that occur in the cut ends of the proximal and distal segments indicates that the primary reaction to axotomy is of a local nature and does not depend on the soma. Two to four days after transection, the cut end of the distal axonal segment reveals signs of degeneration. These include the appearance of swollen mitochondria, lysosomes, myelinated bodies and shrinking of the axon. In addition there is a massive proliferation of glial processes around the degenerating axons. Sprouting from the tip of the proximal segment starts 5--7 days post axotomy. Sprouts were identified as profiles containing few neurotubules, many vesicles and abundant smooth endoplasmic reticulum. 'Growth cone-like' structures were identified. The ultrastructural reorganization of the cut end of the proximal segment is discussed in relation to changes in membrane properties of the regenerating tip, as previously described by us.

Importance of pathway formation for nodal sprout production in partly denervated muscles

Experiments were carried out to investigate possible factors controlling nodal sprout growth in partly denervated mouse gluteal muscles. Pretreatment of the muscle with botulinum toxin for up to 20 days, which produces denervation-like change and elicits terminal and nodal sprouting, increased reinnervation by terminal sprouting after partial denervation but did not alter the rate of reinnervation by nodal sprouts. This implies that nodal sprout growth is not limited by the development of an adequate growth stimulus from denervated muscle. A disto-proximal gradient of degeneration was observed in denervated intramuscular nerves in the electron microscope, suggesting that nodal sprout growth may be modulated by the availability of endoneurial pathways sufficiently degenerated to permit reinnervation by nodal sprouts, although the initial outgrowths from nodes of Ranvier may appear in response to a growth stimulus from denervated muscle.

Double myelination of axons in the sympathetic nervous system

Relationships between axons and Schwann cells in myelinated fibres of the superior cervical (sympathetic) ganglion have been examined in normal adult rats. In cross-sections through the ganglion up to 4% of myelinated fibres were focally encircled by an additional myelinating Schwann cell, forming regions termed 'double myelination'. In these regions and elsewhere in the ganglion, the structure of the inner fibre (axons and myelinating Schwann cell) conformed to the relationships expected on the basis of numerous previous investigations on normal peripheral nerve. However, the outer Schwann cell and myelin sheath, which formed an annulus around the inner fibre, was remarkable in that it apparently made no direct contact either with the centrally enclosed axons or with an neighbouring axon, yet appeared largely if not completely intact. In addition, the increasing frequency of double myelination in older animals and the rarity of myelin degeneration in the same ganglia indicate that the outer Schwann cell, and in particular its myelin sheath, persist for some period in an isolated form. Double myelination was not located in non-sympathetic peripheral nerve samples from the same animals. Double myelination may result from the displacement of one myelin internode by the interposition of another Schwann cell rendering the original Schwann cell redundant. There was no involvement of haematogenous cells as occurs in some demyelinating conditions. While some parallels may be found with previous studies, this would appear to be the first report of apparent survival of myelin in a Schwann cell not making, as far as could be determined in the present study, at least partial direct axonal contact. These observations on sympathetic nerve may provide a new perspective on axon-Schwann cell signalling.

Synthesis of myelin, particulate, and soluble protein subfractions of rat sciatic nerve during the early stage of Wallerian degeneration: a comparison of metabolic studies using double and single isotope methods and recovery

The recovery, electrophoretic composition and synthesis of the myelin, particulate protein and soluble protein subfractions of rat sciatic nerve were compared in normal, sham-operated, and degenerating rat sciatic nerve at one, three and five days after neurotomy. Both single and double isotope methods were used to measure changes in synthesis in vitro and double isotope methods were used in vivo. The wet weights of nerves undergoing Wallerian degeneration for 5 days increased by 40 percent compared to normal and sham-operated nerves. The recovery, specific radioactivity, and synthesis of the myelin was reduced. The effect on myelin protein synthesis was similar in vitro and in vivo. The myelin loss was relatively constant in amount (30-40 microgram) regardless of differences in nerve sizes of young and old rats, consequently the percentage of myelin loss was inversely proportional to nerve size. The recovery of particulate protein increased, its rate of synthesis remained unchanged, and accordingly the specific radioactivity was decreased. The recovery, specific radioactivity, and the rate of synthesis of the soluble protein fraction were all elevated. The protein composition of the three fractions, as analyzed qualitatively by polyacrylamide disc gel electrophoresis, remained essentially unchanged through five days of degeneration. With regard to comparisons of the single and double isotope methods, results shows that the latter are more ideally suited to measuring changes in synthesis during the non-steady state conditions that are characteristics of rapid degeneration.

Long-term survival of centrally projecting axons in the optic nerve of the frog following destruction of the retina

A significant number of unmyelinated axons and their synaptic endings in the frog, Rana pipiens, were found to retain a normal morphology long after separation from their cell bodies. At the end of various survival periods following unilateral removal of the retina, horseradish peroxidase (HRP) was administered to the optic nerve stump by a fiber-filling method. In frogs maintained at 20 degrees C, unmyelinated optic nerve axons conducted HRP from the site of application in the orbit to layers A, C, and E of the contralateral optic tectum, even though their retinas had been removed up to 69 days earlier. Such fiber-filling was absent beyond 19 days in other frogs surviving at 35 degrees C. No labeled fibers were continuous with any intracerebral neurons. The HRP was always localized intraaxonally, and the marked axons and terminals were ultrastructurally normal. Counts of surviving axons from electron micrographs of the optic nerves showed that, at 20 degrees C, more than half of the normal complement of unmyelinated axons disappeared in the first 10 days. All the myelinated axons degenerated during the first 6 weeks survival. However, approximately 55,000 normal-appearing unmyelinated axons (12% of the unmyelinated fiber population) persisted in the optic nerve at 10 weeks following removal of the retina. The survival rate was lower at 35 degrees C. In other frogs, one eye was injected with 3H-leucine to initiate axonal transport into the retinal ganglion cell axons. That eye was removed 48 hours later. Autoradiographic analysis of brain sections of frog surviving an additional 31 to 61 days at 20 degrees C showed strong labeling of the optic tract and layers A, C, and E of the contralateral optic tectum. The absence of displaced ganglion cells that might exist within the optic nerve was verified by other observations. It is hypothesized that the potential shown by frog optic axons for long-term survival in the absence of the cell-body expresses a general property of vertebrate (and invertebrate) axons, rather than a special property of the frog optic nerve.

Electroneurography in the prognostication of Bell's palsy

Electroneurography with automatic signal analysis and EMG have been used in repeated examinations of 23 patients with Bell's palsy in order to evaluate the usefulness of electroneurography for prognostication. Electroneurographic data (amplitude and area asymmetry) discriminate between groups of patients recovering with different degrees of sequelae on day 4. Prognosis for recovery for the individual patient can be judged with relatively high accuracy on day 4 (70% of the patients). The method is well adaptable for automatic analysis for routine diagnostic practice, fast, reliable and gives useful clinical information at an early stage of the palsy. Different methods for facial nerve examination are reviewed in the article and the pathophysiology of nerve damage and recovery with different degree of nerve involvement is discussed.

Pattern of myelin breakdown during sciatic nerve Wallerian degeneration: reversal of the order of assembly

Myelin sheaths of rapidly growing rats were sequentially labeled with the 3H and 14C isotopes of leucine as precursors of protein synthesis. The two injections were separated by time intervals ranging from 2 to 12 d. Wallerian degeneration was initiated by sciatic nerve neurotomy at 2 or 10 d after the second injection of radioactivity. After 5 d of degeneration, myelin was purified and the ratio of isotopes was determined in the delipidated protein. Regardless of the order in which the two isotopes were administered, the relative recovery of radioactivity resultant from the second injection was greatly reduced in degenerating nerves compared with sham-operated controls. Radioactivity incorporated from the first injection was also reduced, but to a lesser extent. Consequently, the isotope ratio corresponding to the first/second injection was greater in degenerating nerves than in controls, and the ratio increased in proportion to the time interval separating the two injections. The magnitude of the effect of degeneration was only slightly greater when degeneration was initiated 2 d after the second injection than when initiated 10 d after the last injection. Consequently, myelin disintegration rather than diminished incorporation of radioactivity accounts for the losses of radioactivity. Furthermore, the pattern of myelin degeneration preferentially involves the last myelin to be formed.

Fate of axonally transported taurine and proteins in the developing rabbit visual system following optic nerve section

These experiments were performed to characterize the axonally transported taurine in the visual system of developing rabbits. [35S]Taurine, transported axonally after intravitreal injection, disappeared from the components of the visual system more rapidly after nerve section than it did with intact nerves. The decrease was most rapid in the youngest animals, and tended to be most pronounced in the elements nearest to the section (optic nerve, optic tract). 3H-labeled proteins present in the visual system changed less markedly than [35S]taurine after nerve section; only in the youngest rabbits was there a marked decrease. These results suggest that a greater proportion of the intraaxonal taurine is labile in young rabbits than in mature rabbits.

Lipid changes in the normal proximal and distal stumps of the rat sciatic nerve and during Wallerian degeneration

The lipid composition of normal proximal and distal stumps of the rat sciatic nerve was studied. The highest lipid content was found in the proximal stump. A parallel decrease of lipid phosphorus, free cholesterol, and galactose contents were observed in both stumps until the 30th day of degeneration. 60 days after nerve section, the investigated parameters increased again in the proximal stump, but remaining, however below the normal values. During the same period, no such increase was found in the distal stumps. Phospholipids decreased in the proximal stump until 30 days after nerve section and increased thereafter. In the distal stumps, the phospholipids decreased sharply at 15 days after nerve section and their levels remained very low thereafter.

Study of regeneration in the garfish olfactory nerve

Previous studies of the olfactory nerve, mainly in higher vertebrates, have indicated that axonal injury causes total degeneration of the mature neurons, followed by replacement of new neuronal cells arising from undifferentiated mucosal cells. A similar regeneration process was confirmed in the garfish olfactory system. Regeneration of the nerve, crushed 1.5 cm from the cell bodies, is found to produce three distinct populations of regenerating fibers. The first traverses the crush site 1 wk postoperative and progresses along the nerve at a rate of 5.8 +/- 0.3 mm/d for the leading fibers of the group. The second group of fibers traverses the crush site after 2 wk postcrush and advances at a rate of 2.1 +/- 0.1 mm/d for the leading fibers. The rate of growth of this group of fibers remains constant for 60 d but subsequently falls to 1.6 +/- 0.2 for the leading population of fibers. The leading fibers in the third group of regenerating axons traverse the crush site after 4 wk and advance at a constant rate of 0.8 +/- 0.2 mm/d. The multiple populations of regenerating fibers with differing rates of growth are discussed in the context of precursor cell maturity at the time of nerve injury and possible conditioning effects of the lesion upon these cells. Electron microscopy indicates that the number of axons decreases extensively after crush. The first two phases of regenerating axons represent a total of between 6 and 10% of the original axonal population and are typically characterized by small fascicles of axons surrounded by Schwann cells and large amounts of collagenous material. The third phase of fibers represents between 50 and 70% of the original axonal population.

Early influx of horseradish peroxidase into axons of the hypoglossal nerve during Wallerian degeneration

Horseradish peroxidase (HRP) was applied around mouse hypoglossal nerves which were damaged by a crush or a ligature. HRP was then visualized distal to the lesions by light- and electron microscopic histochemistry. At the injury the enzyme entered axons and could also be detected several millimetres down in the distal segment. By 24 h reaction product (r.p.) was either diffusely distributed in the axoplasm or present in various vesicular organelles. Our results indicate that there is a rapid influx of macromolecules into axons after a lesion to a nerve. A similar uptake of 'wound substances' into axons distal to an injury might well have some relation to the process by which axonal breakdown is initiated during Wallerian degeneration.

The morphological and physiological properties of a regenerating synapse in the C.N.S. of the leech

Regeneration of an electrical synapse between particular interneurons in the medicinal leech was traced physiologically and morphologically using intracellular recording the horseradish peroxidase (HRP) injection. The synapse between S-cell interneurons lies in the connective midway between segmental ganglia, so crushing near one ganglion severs only one S-cell's axon. The severed distal stump remains connected to the adjacent uninjured S-cell and continues for weeks to conduct impulses. The injured cell regenerates, while its uninjured "target" neuron in the next ganglion does not grow. After the sprouts of the regenerating neuron cross the crush, one or a few branches grow along the surviving distal stump toward the original synapse. After about one month when the region of original synapse has been reached, regenerating neurons form electrical junctions and stop growing. Thereafter electrical coupling improves in stages. Within two months the regenerated neuron attains full caliber, the stump degenerates and function is normal. In some instances within days or weeks of crushing, the regenerating neuron forms a basket of synapses upon its severed distal stump and then continues growing to synapse with the target. When this occurs, electrical coupling and subsequent impulse transmission between S-cells rapidly resumes. These experiments indicated that the regenerating neuron is guided to its proper synaptic target by recognizing and following its severed distal stump. Sometimes the distal stump itself becomes an intermediate synaptic target.

An electron-microscopic analysis of axonal alterations following blunt contusion of the spinal cord of the rhesus monkey (Macaca mulatta)

Following contusion (500 g-cm) at upper thoracic levels, sections from the spinal cords of 13 rhesus monkeys were examined with the electron microscope. Survival times ranged from 4 hr to 10 weeks. Samples were taken from the lesion site, from areas 3 and 10 mm rostral and caudal to the lesion center, and from the lumbosacral cord. Four hours postoperatively, several small axons located close to the grey matter at the lesion site exhibit abnormal accumulations of organelles including mitochondria, dense bodies, vesicular structures, and multivesicular bodies. By 12 hr postoperatively many axons at the lesion site appear to be swollen with organelles and exhibit thinning of their myelin sheath. Some organelle-rich profiles lack a myelin sheath altogether. At this time dark axons are present, and myelin sheaths which appear to be empty or to contain small amounts of flocculent material. By 18 hr the first signs of axonal changes appear in the tissue taken 3 mm from the center of the lesion, both swollen and pyknotic axons being present. The axonal pathology spreads from the central part of the cord to the periphery at the impact site, and from the impact site rostrally and caudally, beginning at 18 hr and continuing for the duration of the study. Small fibers degenerate first and large fibers later. The axonal changes observed appear to be comparable to those reported for the central and peripheral nervous systems in other species.

Histological studies of trophic dependencies in crayfish giant axons

Medial giant (MGA) and lateral giant (LGA) axons of crayfish were doubly cut in order to selectively isolate axonal segments from perikaryal and transsynaptic sources of trophic input. Isolated MGA segments remained morphologically intact for over 43 days, whereas isolated LGA segments usually degenerated within one week. The glial sheaths around isolated MGA segments had significantly increased in thickness within one week, but severed LGA segments showed no increase in sheath thickness at any time after lesioning. These data suggest that cells of the surrounding glial sheath can provide trophic support to isolated MGA segments but not to isolated LGA segments. Extent of glial hypertrophy seems dependent upon specific spatiotemporal parameters. The diameters of isolated MGA segments decreased more rapidly than the diameters of singly cut MGA segments. These data suggest that the MGA also receives some trophic support from pre- or postsynaptic sources. Conversely, some singly cut LGA segments completely degenerated within one week, whereas other singly cut LGA segments remained intact for at least 43 days after lesioning. Such results suggest that the LGA receives a significant trophic input from pre- or postsynaptic structures.

Structural alterations of peripheral nerve induced by the calcium ionophore A23187

Desheathed segments of rat peripheral nerve were incubated at 37 degrees C in oxygenated Ringer's solution with and without the addition of calcium ionophore, A23187, 10 microgram/ml. Nerve fibers incubated in the presence of both ionophore and calcium revealed extensive granular disintegration of their axonal microtubules and neurofilaments after 30 and 60 min incubation intervals. These changes were not seen following control incubations in Ringer's solution without ionophore or in calcium-free Ringer's solution containing ionophore and EGTA, 1 mmole/1. Ionophore-induced alterations were also noted in Schwann cell cytoplasm. The granular degradative alteration of axoplasm caused by exposure of nerve fibers to ionophore and calcium were believed to be due to an ionophore-mediated influx of calcium into the axoplasm with resultant elevation of intra-axoplasmic calcium concentration. These axoplasmic changes were indistinguishable from the axoplasmic alteration occurring in the distal portions of transpected neurites during Wallerian degeneration. The findings support the view that abnormal calcium influxes are determinants in the degeneration of peripheral nerve.

Early course of Wallerian degeneration in myelinated fibres of the rat phrenic nerve

A quantitative study of Wallerian degeneration was carried out on teased fibres. The breakdown into ovoids was used as the criterion of degeneration. The fragmentation of fibres begins near the point of nerve interrruption and spreads along the unbranched parts of axons at velocities correlated with fibre diameter and internodal length. The latent period, preceding the onset of fragmentation, lasts from 25.6 h in thin fibres to 45.0 h in the thickest fibres. The speed of the subsequent advance along the nerve varies correspondingly from 250 to 46 mm/day. In each internode the first ovoids appear in the middle, the ends of the internodal segment being initially spared. The spatiotemporal pattern of degeneration is consistent with the hypothesis that a neuronal trophic substance, normally ensuring the integrity of the axon, exerts transcellularly an inhibitory influence on the Schwann cell. This influence disappears when the amount or concentration of migrating trophic substances falls below a critical level in a stretch of axon. The overlying Schwann cells become mobile and exhibit intense metabolic activity, leading eventually to axonal disruption.

Distribution of the tract of Lissauer and the dorsal root fibers in the primate spinal cord

The tract of Lissauer receives small caliber dorsal root fibers in addition to axons arising from dorsal horn neurons. The termination of Lissauer's tract and dorsal root fibers was examined in the C7 segment of the rhesus monkey spinal cord. The distribution of normal dorsal root afferents was mapped by labelling the C7 dorsal root ganglion with tritiated amino acids, and then compared with the degeneration of C7 dorsal root fibers following an intradural dorsal rhizotomy. To focus on the distribution of the small afferents, the degeneration following a Lissauer tractotomy was compared with the degeneration following dorsal rhizotomy and following selected lesions involving the large afferents. The survival times following the lesions and rhizotomies were varied to facilitate identification of groups of fibers and terminals which might degenerate at different rates. Both large and small diameter dorsal root afferents were found to exhibit the same rostro-caudal topography within the dorsal horn. The C7 root axons and terminals distribute throughout the mid-C7 dorsal horn grey. Proceeding rostrally through C6, the majority of the C7 root fibers ending in laminae I-IV shift to a lateral position. Proceeding caudally through C8, the C7 root fibers shift medially. Few of the small diameter C7 afferents entering via Lissauer's tract extend above C6 or below C8. Large diameter C7 afferents, arising as dorsal column collaterals, can extend several segments above and below C7. Autoradiography revealed label in all dorsal horn laminae, the heaviest always occurring in the substantia gelatinosa. After one day, label was absent over dorsal column and Lissauer's tract axons, suggesting that the label was mainly associated with fine axonal branches or possibly terminals. After six to ten days many axons were labelled and could be traced into the dorsal and ventral horn. Degeneration from the rhizotomies and lesions, as demonstrated with Fink-Heimer and Nauta methods, depended on the survival time. No degeneration products were present before three days. The large afferents begin to degenerate within the dorsal horn after three to four days and mainly terminate in laminae IV-VI; by 12 days they can also be traced into the intermediate and ventral grey. The small afferents, which include those serving pain and temperature sensibility, arise from the tract of Lissauer and distribute to laminae I, II and III. The tract of Lissauer consists of two populations, each containing small afferents. One population degenerates at three to five days and distributes mainly to laminae II and III (substantia gelatinosa); the other degenerates around 12 days and distributes mainly to lamina I and the outer zone of II. It is suggested that the exclusive termination of the small afferents to laminae I, II and III may be correlated with certain unique histochemical properties (e.g., high substance P and high opiate receptor binding levels) of these same dorsal horn areas...