Axonal damage in severe traumatic brain injury: an experimental study in cat

Summary Based upon recent clinical findings, evidence exists that severe traumatic brain injury causes widespread axonal damage. In the clinical setting, it has been assumed that such axonal damage is the immediate consequence of traumatically induced tearing. However, in laboratory studies of minor head injury, evidence for primary traumatically induced axonal tearing has not been found. Rather the traumatic event has been linked to the onset of subtle axonal abnormalities, which become progressively severe over time (i.e., 12–24 h). In the light of these discrepant findings, we investigated, in the present study, whether progressive axonal change other than immediate tearing occurs with severe traumatic brain injury. Anesthetized cats were subjected to high intensity fluid-percussion brain injury. Prior to injury all animals received cortical implants of horseradish peroxidase (HRP) conjugated to what germ agglutinin to anterogradely label the major motor efferent pathways. Such an approach provided a sensitive probe for detecting traumatically induced axonal abnormality via both light microscopy (LM) and transmission electron microscopy (TEM). The animals were followed over a 1- to 6-h posttraumatic course, and processed for the LM and TEM visualization of HRP. Through such an approach no evidence of frank traumatically induced tearing was found. Rather, with LM, an initial intra-axonal peroxidase pooling was observed. With time, unilobular HRP-containing pools increased in size and progressed to bi- or multilobulated profiles. Ultimately, these lobulated configurations separated. Ultrastructurally, the initial unilobular pool was associated with organelle accumulation and focal axolemmal distention without frank disruption. Over time, such organelle accumulations increased in size and sequestered into multiple pools reminiscent of the bi- and multilobulated structures seen with LM. Ultimately, these organelle accumulations became detached, resulting in physically separated proximal and distal organelle-laden swellings surrounded by a distended axolemma and thinned myelin sheath. The findings reject the hypothesis that axons are immediately torn upon impact.Supported by NIH grants NS-20193, NS-12587 and NS-07288     

弥漫性轴突损伤早期超微结构改变

目的通过观察弥漫性轴突损伤(DAI)患者伤后早期轴突的超微结构变化以探索DAI的发生机理.方法对12例DAI患者的14份活体脑组织标本进行透射电镜检查.结果 DAI患者在伤后早期可发生多方面的轴突改变,包括(1)轴突的细胞骨架破坏;(2)轴膜改变;(3)膜性细胞器的变化;(4)髓鞘的改变;(5)轴突出现肿胀和离断,轴突近侧断端呈现球状.结论在DAI的发生中,可能有多种机理参与.推测,在受到足够强的外力作用时,一些管径较细的轴突可能会立即断裂;其它受损轴突则会出现进行性的延迟性轴突断裂.在此演化过程中,细胞骨架破坏和轴膜受损继而通透性改变可能是造成轴突局灶性轴浆转运障碍最终离断的最重要的因素.     

An immediate light microscopic response of neuronal somata,dendrites and axons to contusing concussive head injury in the rat

Summary Thirty-four rats were killed by transcardial perfusion fixation 1 min after a contusing concussive head injury, and 17 rats 1 day later. From the results obtained with a new silver method demonstrating traumatically damaged neuronal somata, dendrites and axons the following conclusions were drawn: (1) outside the contused territories all features of traumatically induced neuronal argyrophilia are similar to those found in non-contusing concussive head injury, as reported in an accompanying paper; (2) within contused territories the neuronal argyrophilia is abolished by some substance released either from damaged blood vessels or from damaged parenchymal cells, while the neuronal damage otherwise underlying the induction of argyrophilia is present; (3) different phenotypes of neurons are vulnerable to different values of the parameters of the intracranial pressure wave generated by the trauma; (4) some of the neurons may recover from the traumatically induced argyrophilic damage; (5) traumatically induced inundation of neurons with extracellular tracers, as reported by other authors, and somato-dendritic argyrophilia may be different manifestations of one and the same phenomenon; and (6) diffuse primary traumatic axonal injury in human neuropathology may be closely correlated to axonal argyrophilia.     

钾通道阻滞剂可以有效修复受伤轴突的冲动传导(英文)

在脊椎中枢神经系统中,少突胶质细胞能形成轴突的髓鞘。髓鞘对轴突具有保护作用,使轴突具有电绝缘的特性,其独特的节段状结构使髓鞘化的神经轴突能快速、跳跃式地传导神经冲动。髓鞘损伤常见于脊髓损伤和一些慢性神经退行性疾病,由其引起的轴突传导阻滞被认为是引起损伤相关的神经并发症的主要原因。钾离子通道在发生于脊髓损伤和多发性硬化征的轴突传导阻滞中扮演重要角色。髓鞘损伤后会暴露钾离子通道,引起钾离子泄漏,从而阻断神经传导。将钾离子通道阻滞后,离子泄漏得到抑制,进而能促进神经传导。本综述主要详细介绍了修复轴突神经传导功能技术的研究进展和脱髓鞘轴突的神经功能。最近的研究表明,4-氨基吡啶能有效治疗多发性硬化征。此外,转化型研究也筛选出了一些能有效修复轴突神经传导的新型的钾离子通道阻滞剂。     

头部创伤后延迟性神经症状恶化：可能是创伤性轴索损伤

We report a patient who presented with neurological deterioration 26 days after a motor vehicle accident. A 25-year-old man crashed a car onto farmland from a height of approximately 3 meters. Neurological deterioration including ataxia and visual disturbance became apparent 26 days after the accident. Brain magnetic resonance imaging did not reveal any abnormality, but brain single photon emission computed tomography showed mild hypoperfusion in the left frontotemporal lobe. An ophthalmic examination revealed no specific abnormality, but visual-evoked potentials revealed prolonged latencies in both eyes. We propose that this neurological deterioration might have resulted from traumatic axonal injury. As such, future studies should examine preventive strategies against delayed deterioration in patients with head trauma.     

Traumatically induced reactive change as visualized through the use of monoclonal antibodies targeted to neurofilament subunits.

Reactive axonal change has long been recognized as a feature of traumatic brain injury. To date, the histological methods used to identify reactive axons have been of limited utility, and they have not provided insight into the initial intraaxonal event that triggers reactive change. In this investigation, monoclonal antibodies to the 68, 150, and 200 kilodalton (kD) neurofilament subunits have been used to follow the progression of reactive axonal change. Anesthetized rats and cats were subjected to moderate traumatic brain injury. One to 72 hours (h) postinjury, their brains were processed for the light (LM) and electron (EM) microscopic immunocytochemical visualization of the various neurofilament subunits. Although all of the chosen antibodies revealed some degree of immunoreactivity within the reactive axon, the 68 kD antibody revealed a dramatic increase in immunoreactivity following injury. Within one h of injury, intensely 68 kD-immunoreactive axonal segments were observed with LM, and parallel EM microscopic analyses demonstrated that this increased immunoreactivity was associated with an increased number of 68 kD-immunoreactive neurofilaments, the majority of which coursed in an axis parallel to the axon's course. Over 2-6 h postinjury, these 68 kD-immunoreactive filaments demonstrated increasingly disordered alignment in relation to the axon's long axis, withdrawing from the focus of injury while becoming encompassed by an expanding organelle cap. It is posited that this increased 68 kD immunoreactivity is associated with a traumatically-induced increase in subunit exchange which contributes to cytoskeletal dysfunction leading to organelle accumulation, focal swelling and ultimate axonal detachment.     

Ubiquitin marks the reactive swellings of diffuse axonal injury

Summary Ubiquitin is a protein that targets proteins for non-lysosomal degradation. It has been found to be present in a number of inclusions characteristic of neurodegenerative diseases. Using the fluid percussion model of diffuse axonal injury (DAI), we now report that the reactive axonal swellings and the retraction balls produced in this model stain positively with antiubiquitin immunohistochemistry. Furthermore, the affected axons become ubiquitin positive quickly (with-in the first 6 h after injury). Anti-ubiquitin immunohistochemistry compares well with the recently reported ability of antibodies to low molecular weight neurofilament proteins to demonstrate reactive axonal change in DAI, and it could provide additional clues to the pathogenesis of axonal transection.Supported by a grant from the NINDS (NS01230) and by a grant from the Baptist Memorial Health Care Foundation     

Traumatic axonal injury in the perisomatic domain triggers ultrarapid secondary axotomy and Wallerian degeneration

Traumatic axonal injury (TAI) arising from diffuse brain injury (DBI) results in focally impaired axonal transport with progressive swelling and delayed disconnection over several hours within brainstem axons. Neocortical DBI-mediated perisomatic axotomy does not result in neuronal death, suggesting that a comparably delayed axotomy progression was responsible for this unanticipated response. To evaluate delayed perisomatic axotomy, the current study was initiated. Rats received intracerebroventricular 10-kDa dextran followed by moderate midline/central fluid percussion injury (FPI) or FPI alone. At 15, 30, 60, and 180 min post-injury, light and transmission electron microscopy identified impaired axonal transport via antibodies targeting amyloid precursor protein (APP), while double-label fluorescent microscopy explored concomitant focal axolemmal alterations via dextran-APP co-localization. At 15 min post-injury, perisomatic TAI was identified with LM within dorsolateral and ventral posterior thalamic nuclei. Using TEM, many sustaining somata and related proximal/distal axonal segments revealed normal ultrastructural detail that was continuous with focal axonal swellings characterized by cytoskeletal and organelle pathology. In other cases, axotomy was confirmed by loss of axonal continuity distal to the swelling. By 30 min post-injury, perisomatic axotomy predominated. By 60-180 min, somatic, proximal axonal segment, and swelling ultrastructure were comparable to earlier time points although swelling diameter increased. Distal axonal segment ultrastructure now revealed the initial stages of Wallerian degeneration. The site of perisomatic axotomy did not internalize dextran, suggesting that its pathogenesis occurred independent of altered axolemmal permeability. Collectively, this DBI-mediated ultrarapid perisomatic axotomy and its sequelae further illustrate the varied axonal responses to trauma.     

Mechanically-induced membrane poration causes axonal beading and localized cytoskeletal damage

Diffuse axonal injury (DAI), a major component of traumatic brain injury, is a manifestation of microstructural cellular trauma and various ensuing neurochemical reactions that leads to secondary neuronal death. DAI is suggested to result from the initial increase in the membrane permeability caused by the mechanical forces acting on the axons. Permeability increases disturb ion balance and lead to cytoskeletal disruption resulting in the impairment of axonal transport. We present an in vitro model that reproduces important features of in vivo DAI such as membrane permeability changes, focal disruption of microtubules, impaired axonal transport, and focal accumulation of organelles. We induced fluid shear stress injury (FSSI) on cultured primary chick forebrain neurons and characterized the resulting structural and morphological changes. In addition, we tested the effect of Poloxamer 188 (P188), a tri-block co-polymer that is known to promote resealing membrane pores. We found that FSSI induces mechanoporation that leads to axonal bead formation, the “hallmark” morphology of DAI. Beads contained accumulated mitochondria and co-localized with focal microtubule disruptions, also a characteristic of DAI. Post-injury P188 treatment prevented FSSI-induced membrane permeability changes and reduced axonal beading to control levels. These results indicate that acute mechanoporation of axons in response to injury is a necessary condition for subsequent axonal pathology, suggesting that membrane integrity is a potential target for therapeutic interventions. P188 provides neuroprotection via resealing the plasma membrane following injury and prevents focal disruption of microtubules and axonal bead formation.     

An intrathecal bolus of cyclosporin A before injury preserves mitochondrial integrity and attenuates axonal disruption in traumatic brain injury.

Traumatic brain injury evokes multiple axonal pathologies that contribute to the ultimate disconnection of injured axons. In severe traumatic brain injury, the axolemma is perturbed focally, presumably allowing for the influx of Ca2+ and initiation of Ca2+ -sensitive, proaxotomy processes. Mitochondria in foci of axolemmal failure may act as Ca2+ sinks that sequester Ca2+ to preserve low cytoplasmic calcium concentrations. This Ca2+ load within mitochondria, however, may cause colloid osmotic swelling and loss of function by a Ca2+ -induced opening of the permeability transition pore. Local failure of mitochondria, in turn, can decrease production of high-energy phosphates necessary to maintain membrane pumps and restore ionic balance in foci of axolemmal permeability change. The authors evaluated the ability of the permeability transition pore inhibitor cyclosporin A (CsA) to prevent mitochondrial swelling in injured axonal segments demonstrating altered axolemmal permeability after impact acceleration injury in rat. At the electron microscopic level, statistically fewer abnormal mitochondria were seen in traumatically injured axons from CsA-pretreated injured animals. Further, this mitochondrial protection translated into axonal protection in a second group of injured rats, whose brains were reacted with antibodies against amyloid precursor protein, a known marker of injured axons. Pretreatment with CsA significantly reduced the number of axons undergoing delayed axotomy, as evidenced by a decrease in the density of amyloid precursor protein-immunoreactive axons. Collectively, these studies demonstrate that CsA protects both mitochondria and the related axonal shaft, suggesting that this agent may be of therapeutic use in traumatic brain injury.     

Intracerebral injection of AMPA causes axonal damage in vivo

Brain injury following acute and chronic neurological conditions can involve both neuronal perikaryal and axonal damage, yet considerably less is known about the mechanisms of axonal damage. Oligodendrocytes and myelin are highly vulnerable to AMPA receptor-mediated excitotoxicity. In vitro studies using isolated white matter preparations have shown that AMPA receptor-mediated excitotoxicity results in axonal damage. The effect of AMPA on axons in vivo remains to be determined. We established an in vivo model to determine if axons were vulnerable to AMPA-mediated toxicity, and furthermore, to examine if axonal damage occurred through an AMPA receptor-mediated mechanism. Adult rats received stereotaxic injection of AMPA (2.5 or 25 nmol) or vehicle (PBS) into the external capsule. Axonal damage was detected in the external capsule and cortex in sections immunostained for cytoskeletal components microtubule associated protein-5 (MAP 5), the 200 kDa neurofilament subunit (NF 200) and non-phosphorylated neurofilament-H (SMI 32). Quantification of axonal damage in the external capsule of MAP 5-immunostained sections showed that AMPA caused a significant, dose-dependent increase in axonal damage compared to the vehicle-treated controls. AMPA also induced a dose-dependent increase in myelin and neuronal perikaryal damage. Systemic administration of the AMPA receptor antagonist SPD 502 significantly reduced the amount of AMPA-induced axonal, myelin and neuronal damage. These data suggest that AMPA induces structural damage to the cytoskeleton of axons in vivo, as well as neuronal and myelin damage, and that this occurs through AMPA receptor-mediated mechanisms. AMPA receptor antagonism may have therapeutic potential to salvage both axons and neuronal perikarya in a number of neurological disorders.     

Mild traumatic brain injury to the infant mouse causes robust white matter axonal degeneration which precedes apoptotic death of cortical and thalamic neurons

The immature brain in the first several years of childhood is very vulnerable to trauma. Traumatic brain injury (TBI) during this critical period often leads to neuropathological and cognitive impairment. Previous experimental studies in rodent models of infant TBI were mostly concentrated on neuronal degeneration, while axonal injury and its relationship to cell death have attracted much less attention. To address this, we developed a closed controlled head injury model in infant (P7) mice and characterized the temporospatial pattern of axonal degeneration and neuronal cell death in the brain following mild injury. Using amyloid precursor protein (APP) as marker of axonal injury we found that mild head trauma causes robust axonal degeneration in the cingulum/external capsule as early as 30 min post-impact. These levels of axonal injury persisted throughout a 24 h period, but significantly declined by 48 h. During the first 24 h injured axons underwent significant and rapid pathomorphological changes. Initial small axonal swellings evolved into larger spheroids and club-like swellings indicating the early disconnection of axons. Ultrastructural analysis revealed compaction of organelles, axolemmal and cytoskeletal defects. Axonal degeneration was followed by profound apoptotic cell death in the posterior cingulate and retrosplenial cortex and anterior thalamus which peaked between 16 and 24 h post-injury. At early stages post-injury no evidence of excitotoxic neuronal death at the impact site was found. At 48 h apoptotic cell death was reduced and paralleled with the reduction in the number of APP-labeled axonal profiles. Our data suggest that early degenerative response to injury in axons of the cingulum and external capsule may cause disconnection between cortical and thalamic neurons, and lead to their delayed apoptotic death.     

Light microscopic response of neuronal somata,dendrites and axons to post-mortem concussive head injury

Summary Forty anesthetized rats were cooled below 3°C by 30-min transcardial perfusion of chilled physiological saline before a concussive head injury. The animals were then perfusion-fixed with a buffered formaldehyde-glutaraldehyde solution. Another forty rats were fixed by 30-min transcardial perfusion of the same fixative before a similar concussive head injury. In brain sections of both groups of animals a new silver method stained, in a Golgi-like fashion, a number of neurons and long axonal segments scattered among unstained ones. The similarity between these findings and those obtained following in vivo concussive head injuries described in accompanying papers suggests that the formation of traumatically induced argyrophilic neuronal damage is independent of metabolic processes, i.e., it may be a primary morphopathological process.     

Characterization of a prolonged regenerative attempt by diffusely injured axons following traumatic brain injury in adult cat: a light and electron microscopic immunocytochemical study

Traumatic brain injury in animals and humans is well known to cause axonal damage diffusely scattered throughout the brain without evidence of other brain parenchymal change. This observation has prompted some to posit that such damaged axons are well positioned to mount a regenerative attempt. The present study uses an immunocytochemical marker specific for regenerating neurites to explore this issue. Further, in an attempt to expedite and enhance any potential regenerative effort, this study evaluates the efficacy of intrathecally applied nerve growth factor. Three sets of experiments were performed in adult cats. One group of animals was subjected to moderate fluid percussion brain injury and followed for 7 or 14 days post injury, with the continuous intraventricular infusion of nerve growth factor delivered by implanted osmotic pumps. These animals were compared to a second group of time-matched, sham-operated animals receiving artificial cerebrospinal fluid infusion. To assess axonal damage immunohistochemical staining for the low molecular weight neurofilament subunit (NF-L) was carried out using an NR4 monoclonal antibody. To localize axons exhibiting a regenerative response immunohistochemical staining for the growth associated protein GAP43 was employed. In sham controls, at the light microscopic level NF-L-immunoreactive axonal swellings were numerous at 7 days, but by 14 days post injury their frequency declined markedly. In contrast, GAP43-immunoreactive, disconnected reactive axonal swellings were rarely observed at 7 days but were numerous at 14 days. Ultrastructural analysis at 14 days post injury of carefully matched sections revealed reactive axons demonstrating sprouting consistent with a regenerative effort. Analysis of tissue from animals of 14 days of survival indicated that supplementation with nerve growth factor did not appear to enhance the capacity of damaged brain axons to mount a regenerative attempt. Rather, it appears that regenerative efforts seen reflect a spontaneous response. A third group of adult cats, subjected to the same injury but not subjected to osmotic pump implantation, was allowed to survive for 22–28 days. Animals in this group also demonstrated GAP43 immunoreactivity in reactive axonal swellings in the brain stem. This study demonstrates that diffusely injured axons can mount a sustained regenerative attempt that is associated with a reorganization of their cytoskeleton and accompanied by an up-regulation of growth-associated proteins. Received: 3 March 1997 / Revised, accepted: 1 April 1997     

Expression and activation of EphA4 in the human brain after traumatic injury

Glial scars that consist predominantly of reactive astrocytes create a major barrier to neuronal regeneration after traumatic brain injury (TBI). In experimental TBI, Eph receptors and their ephrin ligands are upregulated on reactive astrocytes at injury sites and inhibit axonal regeneration, but very little is known about Eph receptors in the human brain after TBI. A better understanding of the functions of glial cells and their interactions with inflammatory cells and injured axons will allow the development of treatment strategies that may promote regeneration. We analyzed EphA4 expression and activation in postmortem brain tissue from 19 patients who died after acute closed head injury and had evidence of diffuse axonal injury and 8 controls. We also examined downstream pathways that are mediated by EphA4 in human astrocyte cell cultures. Our results indicate that, after TBI in humans, EphA4 expression is upregulated and is associated with reactive astrocytes. The expression was increased shortly after the injury and remained activated for several days. EphA4 activation induced under inflammatory conditions in vitro was inhibited using unclustered EphA4 ligand. These results suggest that blocking EphA4 activation may represent a therapeutic approach for TBI and other types of brain injuries in humans.     

Does β-amyloid plaque formation cause structural injury to neuronal processes?

The precise role of beta-amyloid plaque formation in the cascade of brain cell changes that lead to neurodegeneration and dementia in Alzheimer's disease has been unclear. Studies have indicated that neuronal processes surrounding and within plaques undergo a series of biochemical and morphological alterations. Morphological alterations include reactive, degenerative and sprouting-related 'dystrophic' neuritic structures, derived principally from axons, which involve specific changes in cytoskeletal proteins such as tau and NF triplet proteins. More compact and fibrous plaques are associated with more extensive neuritic pathology than non-fibrillar, diffuse beta-amyloid deposits. Cortical apical dendritic processes are either 'clipped' by plaque formation or are bent around more compact plaques. Examination of cases of 'pathological' brain ageing, which may represent a preclinical form of Alzheimer's disease, demonstrated that the earliest neuritic pathology associated with plaques was similar to the reactive changes that follow structural injury to axons. In vivo and in vitro experimental models of structural injury to axons produce identical reactive changes that subsequently lead to an attempt at regenerative sprouting by damaged axons. Thus, beta-amyloid plaque formation may cause structural injury to axons that is subsequently followed by an aberrant sprouting response that presages neurodegeneration and dementia. Identification of the key neuronal alterations underlying the pathology of Alzheimer's disease may provide new avenues for therapeutic intervention.     

Deep intracerebral (basal ganglia) haematomas in fatal non-missile head injury in man.

Deep intracerebral (basal ganglia) haematomas were found post mortem in 63 of 635 fatal non-missile head injuries. In patients with a basal ganglia haematoma, contusions were more severe, there was a reduced incidence of a lucid interval, and there was an increased incidence of road traffic accidents, gliding contusions and diffuse axonal injury than in patients without this type of haematoma. Intracranial haematoma is usually thought to be a secondary event, that is a complication of the original injury, but these results suggest that a deep intracerebral haematoma is a primary event. If a deep intracerebral haematoma is identified on an early CT scan it is likely that the patient has sustained severe diffuse brain damage at the time of injury. In the majority of head injuries damage to blood vessels or axons predominates. In patients with a traumatic deep intracerebral haematoma, it would appear that the deceleration/acceleration forces are such that both axons and blood vessels within the brain are damaged at the time of injury.     

Degeneration of neuronal cell bodies following axonal injury in Wld(S) mice

The phenotype of Wld(S) ("slow Wallerian degeneration") mice demonstrates prolonged survival of injured axons. However, whether the Wld(S) mutation delays degeneration of the neuronal cell body following axonal injury is unclear. We used a retrograde model of axonal transport failure in Wld(S) mice to test whether the mutant Wld(S) protein has any beneficial effect on the neuronal cell body. Retrograde axonal transport was physically blocked by optic nerve crush and confirmed by the absence of Fluoro-Gold labeling in wild-type and in Wld(S) mice. After this axonal injury, there was marked protection of axonal degeneration in the Wld(S) phenotype, as confirmed by immunohistochemistry and electron microscopy. However, the Wld(S) protein, localized in the nucleus of retinal ganglion cells, did not prevent or delay degeneration of the retinal ganglion cell body, confirmed by TUNEL staining and Fluoro-Gold labeling. These results imply that, after axonal injury, Wallerian degeneration of axons and degeneration of the neuronal cell body have different mechanisms, which are autonomous and independent of each other. Although the Wld(S) phenotype can be used to demonstrate stable enucleate axons, the mutation is unlikely to protect neurons in neurodegenerative diseases in which there is failure of retrograde transport.     

Computer-assisted analyses of barrel neuron axons and their putative synaptic contacts

The "barrels" in layer IV of rodent SmI neocortex receive inputs from individual whiskers on the contralateral face. Previous analyses of neuronal morphology in mouse and rat barrel cortex, as revealed by Golgi impregnations, have focused on the dendritic patterns of the stellate cells. The cells can be classified into two groups: Class I cells with spiny dendrites and Class II cells with smooth, beaded dendrites. These classes can be subdivided further according to somal position and spatial distribution of dendrites with respect to barrel cytoarchitectonic boundaries. In the present study the axons of these cells were examined and the locations of close appositions to dendrites of other impregnated neurons were mapped. All data are taken from Golgi-Cox preparations, cut parallel to layer IV at 140 microns, counterstained with Nissl to reveal the barrels, and measured with a computer-microscope. Axons which had extensive branching within the section (present on 10% of all impregnated cells) were chosen for measurement. The analysis of the axons revealed: (1) Class I axons are thin and directed to the white matter with recurrent collaterals in the barrels, while Class II axons are thick, frequency beaded, and directed toward the pia before cascading down into the barrels; (2) in layer IV, the axons of both cell classes tend to be as restricted to a barrel as the dendrites of the same cell are (i.e., most axons are confined to one barrel); (3) within layer IV, the Class II cell axons have a total length about three times that of Class I cell axons, and about four times as many branch points. The analysis of the appositions of these axons to impregnated dendrites of other cells revealed: (1) A majority of "contacts" tended to be made by terminal branches of the axonal trees. (2) For the Class I neurons, a greater number of appositions occur near the distal ends of complete dendritic segments. As measured from the "contacted" cell soma, appositions are more or less uniformly distributed along dendritic trees. (3) No striking patterns are found, such as an obvious propensity for axons of one cell type to prefer or avoid another cell type. These results show that the axons of barrel cells of each class are as consistent and distinctive as their dendritic trees. Specifically, the cells in each class can be distinguished by their axonal patterns on purely numerical bases.(ABSTRACT TRUNCATED AT 400 WORDS)