Calibre spectra of some fibre tracts in the feline central nervous system during postnatal development

Temporal Course of Transient Direct Corticomotoneuronal Connections during Development in Rodents

We previously observed in rodents that during the 2nd postnatal week corticospinal axons make monosynaptic connections with motoneurons. Prior to that finding, it had been believed that such contacts only occur in higher primates. Although an in vitro electrophysiological study is prerequisite for studying the developmental time course of synaptic connections, the technical difficulty of reliably recording synaptic responses from spinal motoneurons in animals over 2 weeks old hampered the study. Instead, we used retrograde transsynaptic labeling with a genetically modified rabies virus to confirm the presence of direct corticomotoneuronal connections at an early developmental stage and to show that these connections were subsequently eliminated. However, determination of an accurate elimination time course and quantitative evaluation of synaptic connectivity cannot be achieved through viral-tracing experiments. For the present study, we improved the slice preparation procedure and maintenance of slice viability, which enabled us to record postsynaptic responses using the whole cell patch-clamp technique from retrogradely labeled forearm motoneurons up until postnatal week 7. We examined the extent of corticomotoneuronal monosynaptic connections and studied the time course of their accumulation and loss. Positive ratios of monosynaptic corticomotoneuronal EPSCs increased from P6 to P8 and then plateaued (P8-P13: 65%). Thereafter, the monosynaptic connections declined until P21, at which time they were no longer detected. The time course of the falling phase and elimination was confirmed by experiments using optogenetic stimulation. The timing of the elimination fell within the same range (P18-22) estimated in our earlier study using retrograde transsynaptic labeling.

The formation of paranodal spirals at the ends of CNS myelin sheaths requires the planar polarity protein Vangl2

During axonal ensheathment, noncompact myelin channels formed at lateral edges of the myelinating process become arranged into tight paranodal spirals that resemble loops when cut in cross section. These adhere to the axon, concentrating voltage-dependent sodium channels at nodes of Ranvier and patterning the surrounding axon into distinct molecular domains. The signals responsible for forming and maintaining the complex structure of paranodal myelin are poorly understood. Here, we test the hypothesis that the planar cell polarity determinant Vangl2 organizes paranodal myelin. We show that Vangl2 is concentrated at paranodes and that, following conditional knockout of Vangl2 in oligodendrocytes, the paranodal spiral loosens, accompanied by disruption to the microtubule cytoskeleton and mislocalization of autotypic adhesion molecules between loops within the spiral. Adhesion of the spiral to the axon is unaffected. This results in disruptions to axonal patterning at nodes of Ranvier, paranodal axon diameter and conduction velocity. When taken together with our previous work showing that loss of the apico-basal polarity protein Scribble has the opposite phenotype-loss of axonal adhesion but no effect on loop-loop autotypic adhesion-our results identify a novel mechanism by which polarity proteins control the shape of nodes of Ranvier and regulate conduction in the CNS.

Assessment of axonal recruitment using model-guided preclinical spinal cord stimulation in the ex vivo adult mouse spinal cord

Spinal cord stimulation (SCS) is used clinically to limit chronic pain, but fundamental questions remain on the identity of axonal populations recruited. We developed an ex vivo adult mouse spinal cord preparation to assess recruitment following delivery of clinically analogous stimuli determined by downscaling a finite element model of clinical SCS. Analogous electric field distributions were generated with 300-µm × 300-µm electrodes positioned 200 µm above the dorsal column (DC) with stimulation between 50 and 200 µA. We compared axonal recruitment using electrodes of comparable size and stimulus amplitudes when contacting the caudal thoracic DC and at 200 or 600 μm above. Antidromic responses recorded distally from the DC, the adjacent Lissauer tract (LT), and in dorsal roots (DRs) were found to be amplitude and site dependent. Responses in the DC included a unique component not seen in DRs, having the lowest SCS recruitment amplitude and fastest conduction velocity. At 200 μm above, mean cathodic SCS recruitment threshold for axons in DRs and LT were 2.6 and 4.4 times higher, respectively, than DC threshold. SCS recruited primary afferents in all (up to 8) caudal segments sampled. Whereas A and C fibers could be recruited at nearby segments, only A fiber recruitment and synaptically mediated dorsal root reflexes were observed in more distant (lumbar) segments. In sum, clinically analogous SCS led to multisegmental recruitment of several somatosensory-encoding axonal populations. Most striking is the possibility that the lowest threshold recruitment of a nonprimary afferent population in the DC are postsynaptic dorsal column tract cells (PSDCs) projecting to gracile nuclei.NEW & NOTEWORTHY Spinal cord stimulation (SCS) is used clinically to control pain. To identify axonal populations recruited, finite element modeling identified scaling parameters to deliver clinically analogous SCS in an ex vivo adult mouse spinal cord preparation. Results showed that SCS first recruited an axonal population in the dorsal column at a threshold severalfold lower than primary afferents. These putative postsynaptic dorsal column tract cells may represent a previously unconsidered population responsible for SCS-induced paresthesias necessary for analgesia.

Stability switches, oscillatory multistability, and spatio-temporal patterns of nonlinear oscillations in recurrently delay coupled neural networks

A model of time-delay recurrently coupled spatially segregated neural assemblies is here proposed. We show that it operates like some of the hierarchical architectures of the brain. Each assembly is a neural network with no delay in the local couplings between the units. The delay appears in the long range feedforward and feedback inter-assemblies communications. Bifurcation analysis of a simple four-units system in the autonomous case shows the richness of the dynamical behaviors in a biophysically plausible parameter region. We find oscillatory multistability, hysteresis, and stability switches of the rest state provoked by the time delay. Then we investigate the spatio-temporal patterns of bifurcating periodic solutions by using the symmetric local Hopf bifurcation theory of delay differential equations and derive the equation describing the flow on the center manifold that enables us determining the direction of Hopf bifurcations and stability of the bifurcating periodic orbits. We also discuss computational properties of the system due to the delay when an external drive of the network mimicks external sensory input.

Comparative MR imaging study of brain maturation in kittens with T1, T2, and the trace of the diffusion tensor

PURPOSE

To assess the time-course of the relaxation times and the orientationally averaged water diffusion coefficient Doav in postnatal brain development.

MATERIALS AND METHODS

Multisection maps of T1, T2, and the trace of the diffusion tensor (Trace[D] = 3 x Doav) were obtained in four kittens at eight time points.

RESULTS

In the adult, Doav was about 700 micron 2/sec in both white and gray matter. In the newborn, Doav was 1,100-1,350 micron 2/sec in white matter and 1,000 micron 2/sec in gray matter. For all anatomic regions and time points, the correlation between Doav and 1/T2 was high (R2 = 0.87, P << .001). T1 showed a lower correlation with Doav and a higher sensitivity to myelinization than did T2.

CONCLUSION

Although Doav shows dramatic changes in the maturing brain, the high correlation between Doav and T2 indicates that little additional information can be obtained by measuring this diffusion parameter during normal brain development. This contrasts with previous findings in brain ischemia, where Doav and T2 appear to be uncorrelated. After including the authors' data and published iontophoretic measurements in a simple model of diffusion in tissues, the authors suggest that the underlying mechanisms of Doav reduction in brain maturation and ischemia are different. Doav changes during development are mainly affected by events occurring in the cellular compartment, while changes in extracellular volume fraction and tortuosity, which are thought to determine the reduction in Doav during ischemia, are probably of secondary importance.

Cellular and extracellular components at nodes of Ranvier in rat white matter

Rat CNS nodes of Ranvier were investigated by electron microscopy and immunohistochemistry. Nodes along thin callosal axons possess tiny node gaps containing few or no astrocytic processes. Nodes along thick spinal axons exhibit spatious node gaps containing relatively few irregularly arranged astrocytic processes. Antibodies against HNK-1, chondroitin sulfate, tenascin or NSP-4 do not label small nodes but stain large nodes. We conclude that rat CNS fibers do not exhibit a strict relation between nodal complexity and fiber size comparable to that found in rat PNS fibers.

Morphological heterogeneity of rat oligodendrocytes: electron microscopic studies on serial sections

The microanatomy of ensheathing and early myelinating rat oligodendrocytes was analyzed through electron microscopic examination of serial sections. The study included cells in the spinal cord (SC) ventral funiculus and the corpus callosum (CC), containing early myelinating, prospective large axons and late myelinating, prospective small axons, respectively. The results show that ensheathment commences fetal day (F) 19 in the SC and 12 days postnatally (P12) in the CC. By then, multipolar SC and CC oligodendrocytes provide axons with uncompacted cytoplasmic sheaths. The average number of axons ensheathed by each such cell was 7 in the SC and 13 in the CC. The mean diameter of the ensheathed axons was 0.69 micron in the SC and 0.36 micron in the CC. The formation of compact myelin had clearly been initiated at birth in the SC and at P17 in the CC. At that stage, the mean number of myelinated axons per analyzed oligodendrocyte was 3 in the SC and 15 in the CC. The mean diameter of the myelinated axons was 1.02 micron in the SC and 0.54 micron in the CC. These observations show that myelin-related rat oligodendrocytes are morphologically heterogeneous. It also seems that this heterogeneity is related to time of onset of myelination and prospective axon diameter. Further, the data suggest that some oligodendrocytes reduce the number of sheaths initially elaborated before formation of compact myelin.

[Postnatal development of central motor pathways. An electrophysiological study]

The percutaneous electrical stimulation of the brain and spinal cord has been used to study the central motor pathways in 19 healthy full-term newborns and in 19 infants. The evoked compound muscle action potential were recorded by bipolar surface electrodes fixed on the skin overlying the thenar eminence muscles and the tibialis anterior muscle. In full-term newborns, the responses of lower limb muscles to cortical stimulation are more difficult to obtain that those of upper limb muscles. At birth, the conduction velocity of central motor fibres along the spinal cord are around 10 m/s, 4 or 5 times lower that the lowest values published for adult subjects. Thus, as has been demonstrated in animals, there seems to exist in man a very clear dissociation between myelination of central motor pathways and that of peripheral motor fibres.

Postnatal development of cat hind limb motoneurons supplying the intrinsic muscles of the foot sole

The postnatal development of dendrite anatomy in alpha-motoneurons intracellularly labeled with horseradish peroxidase (HRP), innervating the intrinsic muscles of the sole of the foot (IFS MNs) in the cat, was investigated. The number of dendrites per neuron was about 11 and did not change from birth to adult. The number of branches per dendrite decreased during the same period by 20-25%. The net elimination of dendritic branches appeared to occur at distal branching points, as revealed by topological analysis. The dendritic branching pattern tended to be asymmetric at birth and the net decrease in dendritic branching postnatally did not alter this pattern. The length of preterminal branches (PTB) increased by a factor of 2, while terminal branch (TB) length increased by a factor of 3.3 postnatally. The large increase in TB length was attributed to both longitudinal growth and an apparent lengthening caused by resorption of distal branches during development. Dendritic length in the transverse spinal cord plane increased in parallel with the overall growth of the parent spinal cord segment, while dendritic growth along the rostro-caudal axis exceeded, by about one order of magnitude, dendritic growth in the transverse plane. Average branch diameter doubled from birth to adult. The decrease in branch diameter across branching points did not obey satisfactorily to the 'power rule' of Rall. However, the 1.5 power ratio of daughters-to-parents branch dropped from 1.18 to 1.08 between 3 weeks of age and adult. Tapering was evident in both PTBs and TBs. The rate of taper did not change postnatally. From birth onwards, 'local' branch diameter correlated closely with amount of membrane area and combined length of the dendritic branches located distal to the 'supporting' parent branch. These relations were similar in all age groups and are suggested to be properties intrinsic to the IFS MNs. The local branch diameter also co-varied with the number of distal dendritic branches, but in this case there was a systematic shift in the relationship with increasing postnatal age. It appears that the local diameter in IFS MN dendrites is a key indicator of the size of the distal dendritic arborization.

Postnatal development of descending motor pathways studied in man by percutaneous stimulation of the motor cortex and the spinal cord

Using the percutaneous electrical stimulation of the brain and spinal cord, we have determined central motor pathway conduction velocities in a group of 19 healthy fullterm newborns (average post-conceptional age 39.8 weeks) and in 19 infants between 2 months and 8 years of age. The newborns were examined during the first postnatal week. The percutaneous stimulation of the motor cortex and of the cervical and lumbar enlargements of the spinal cord was made by means of bipolar electrodes. The evoked compound muscle action potentials were recorded by bipolar surface electrodes fixed on the skin overlying the thenar eminence muscles and the tibialis anterior muscle. In fullterm newborns, the responses of lower limb muscles to cortical stimulation were more difficult to obtain than those of upper limb muscles. Conduction velocities of central motor fibres along the spinal cord (between vertebrae C7 and L4) were around 10 m/sec in fullterm newborns and 38 m/sec at the age of 4 years. These values are considerably lower than those described for adult man (48-60 m/sec). The adult values are established around the age of 8 years.

Relation between axons and oligodendroglial cells during initial myelination. I. The glial unit

The morphology of oligodendroglial-axon units was examined by electron microscopy during ensheathment and initial myelination in developing feline spinal cord and corpus callosum white matter. In addition to a qualitative examination of single sections from many stages of development, a morphological analysis of spinal cord and corpus callosum units was made on the basis of serial sections from a few stages. The results show that myelination commences around embryonic/fetal day 40 and the 20th postnatal day in the spinal cord and corpus callosum areas, respectively. In both areas immature glial cells, lacking the cytological features of typical oligodendrocytes, initially associate with several axons and provide them with cytoplasmic sheaths. Serial section analysis of units, which have begun formation of compact myelin, indicates that individual cells are associated with single myelin sheaths in the spinal cord area, in a way principally similar to the Schwann cell-myelin units in developing peripheral nerves. This suggests the possibility that early spinal cord oligodendrocytes might shift from a polyaxonal to a monoaxonal association after initial ensheathment and before formation of compact myelin. In the corpus callosum area the examined serially-sectioned cells were found to be connected to several myelin sheaths through long thin processes. The myelin sheaths related to one cell are relatively uniform in terms of number of myelin lamellae and axon diameter, but the clockwise/counter-clockwise course of the myelin spiral varies randomly. Units containing both homogeneously uncompacted (cytoplasmic) and fully compacted (myelin) sheaths have not been found. In both areas the ensheathing cells achieve an oligodendrocyte-like cytology during formation of the first layers of compact myelin. These observations support the view that oligodendrocytes are structurally heterogeneous: those myelinating prospective large axons seems to differ from those myelinating axons destined to remain small. The possible functional and pathophysiological implications of this heterogeneity remain to be elucidated.

Demyelination in spinal cord injury

Morphological and physiological studies demonstrate that demyelination constitutes a significant component of the pathology in compressive spinal cord injury. In many cases of spinal cord injury, a rim of demyelinated axons surrounds a central core of hemorrhagic necrosis. This provides a pathophysiological basis for "discomplete" spinal cord injuries, characterized by apparently complete transection as judged by clinical criteria, but with neurophysiological evidence of conduction through the level of damage. Recovery of conduction in demyelinated axons may permit recovery of function, and can be mediated by several mechanisms, including remyelination by oligodendrocytes or Schwann cells. Alternatively, conduction of action potentials can occur in the absence of remyelination, but this requires plasticity of the demyelinated axon. The biophysics of conduction favors recovery of electrogenesis after demyelination of small diameter axons. This may account, in part, for the observation that functional recovery is more common after demyelination of visual, compared to spinal, axons. Restoration or modification of conduction in demyelinated fibers represents an important strategy for promoting functional recovery in spinal cord injury.

Behavioral analysis of the postnatal development of micturition in kittens

Micturition in neonatal mammals of various species can be induced by a somatovesical reflex that is triggered by the mother licking the perineal region of the neonate. The present study was conducted to ascertain whether neonatal kittens can also urinate via a vesicovesical reflex that is elicited by bladder distension. Initial experiments consisted of isolating kittens from their mothers and observing urine release. Kittens less than 3 weeks of age did not urinate despite the presence of large volumes of urine in their bladders. Kittens older than 3 weeks of age did urinate, completely emptying their bladders. The volumes of urine that induced micturition in the older kittens were generally less than those that were ineffective in inducing micturition at younger ages. Cystometrograms in unanesthetized kittens less than 3 weeks of age revealed that distension of the bladder by saline infusion did not evoke bladder contractions. However, saline infusion in older kittens did produce bladder contractions that were accompanied by squatting and the release of urine from around the bladder cannula. Surprisingly, bladder contractions, which were typically abolished by spinal transection, were observed in kittens less than 3 weeks of age when the kittens were anesthetized with either ketamine or chloralose. This latter finding indicates that the vesicovesical reflex is present in neonatal kittens, but it is being suppressed by anesthetic-sensitive mechanisms. Thus, micturition in neonatal kittens is normally mediated entirely by the somatovesical reflex.

The use of somatosensory evoked potentials in infants and children

Somatosensory evoked potentials (SSEPs) are a useful, reliable means of assessing function of the somatosensory system. Complex maturational changes of the CNS such as synaptogenesis and myelination, as well as body growth, complicate interpretation of SSEPs. An understanding of these factors enhances clinical interpretation in infants and children.

Postnatal development of cat hind limb motoneurons. II: In vivo morphology of dendritic growth cones and the maturation of dendrite morphology

The maturation of dendrite morphology was studied by light and electron microscopy in cat spinal alpha-motoneurons intracellularly labeled with horseradish peroxidase. Alpha-motoneurons supplying the triceps surae (TS) and the intrinsic foot sole (SP) muscles were investigated in kittens from birth to 44-46 days of postnatal (d.p.n.) age. At birth, a large number of dendritic branches displayed growth cones, filopodia, and fusiform processes. The growth cones were of lamellipodial and filopodial types, but intermediate forms also occurred. The growth cones shared several morphological features with the neuritic growth cones studied in vitro. It was suggested that the occurrence of different types of growth cones--even in the same dendrite--may reflect their transformation from one type to the other and the level of growth activity could be inferred from the number and form of the growth cones. About 50-70% of the terminal branches in the dendrites of newborn kittens possessed growth cones, filopodia, and/or fusiform processes. The corresponding figure for preterminal branches was 20-30%, with a clear decrease in incidence when approaching the soma. During the period under study, most of these growth-associated processes disappeared from the dendrites so that at 44-46 d.p.n. of age only about 10% of the terminal and less than 1% of the preterminal branches had growth-associated processes. Analysis of the three-dimensional distribution of dendritic branches with such processes disclosed that they were relatively more frequent in the medial, rostral, and caudal dendritic territories. It was concluded that the pattern of distribution and disappearance of growth cones, filopodia, and fusiform processes coincided with postnatal longitudinal dendritic growth and the development of the adult dendritic territories described in a preceding paper (Ulfhake et al., '88). Dendritic growth, with respect to length and caliber, also occurred in the absence of growth cones and filopodia. It is suggested that the important role of these processes may be to act as a steering device in establishing the adult distribution and synaptology of the dendrites. Comparison of TS and SP alpha-motoneuron dendrite morphology at birth and at 22-24 d.p.n. age showed that the SP neurons lagged in the maturation process. Light and electron microscopic observations indicated that postnatally direct contacts might exist between dendrites and fine blood vessels in the neuropil without any interposing glial sheath. The number of such suspected contacts diminished during the period under study, indicating that the glial ensheathment of the blood vessel takes place, in part, postnatally.

Postnatal development of cat hind limb motoneurons. I: Changes in length, branching structure, and spatial distribution of dendrites of cat triceps surae motoneurons

The postnatal development of length, branching structure, and spatial distribution of dendrites of triceps surae motoneurons, intracellularly stained with horseradish peroxidase, was studied from birth up to 44-46 days of postnatal (d.p.n.) age in kittens and compared with corresponding data from adult cats. The number of dendrites of a triceps surae motoneuron was about 12, and the arborization of each dendrite generated an average of 12-15 terminal branches. There was no net change in the number of dendrites of a neuron or in the degree of branching of the dendrites despite the occurrence of both a transient remodeling of the dendritic branching structure and changes of the spatial distribution of the dendritic branches during postnatal development. The perisomatic territory in the transverse plane occupied by the dendritic branches of a motoneuron increased in parallel with the overall growth of the spinal cord. Thus, the relative size of the dendritic territory in this plane was kept almost constant, whereas dendritic branches projecting in the rostrocaudal direction grew much faster than the spinal cord and also became more numerous. At birth the rostro-caudal dendritic span of individual motoneurons bridged 1:6 to 1:5 of the L7 spinal cord segment length; this figure was 1:3 at 22-24 d.p.n. Hence, in this direction, the growing dendritic branches invaded novel dendritic territories. The change in dendritic branch length from birth to 6 weeks of age corresponded to an average growth rate of 2 to 4 microns per dendritic branch and day, which implies that the total increase in length of the dendrites of a neuron could amount to 1 mm/day. The increase in branch length did not occur in a uniform or random manner; instead, it followed a spatiotemporal pattern with three phases: From birth to 22-24 d.p.n., growth was particularly prominent in greater than or equal to 3rd order preterminal and 2nd through 6th order terminal branches. From 22-24 to 44-46 d.p.n., a large increase in branch length confined to terminal branches of greater than or equal to 3rd branch orders was observed. As indicated by topological analysis, this length increase was probably due in part to a resorption of peripheral dendritic branches during this stage of development. From 44-46 d.p.n. to maturity, the increase of dendritic branch length was restricted to preterminal branches of low (less than or equal to 4th) branch order.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of lumbar spinal cord and cortical evoked potentials after tibial nerve stimulation in the pre-term newborns: effects of gestational age and other factors

Pre-term neonates are at increased risk for neurological dysfunction. Several investigators have found scalp recorded somatosensory evoked potential studies (SSEPs) after median nerve stimulation useful in the evaluation of newborn infants with asphyxiation and the effects of other adverse prenatal and perinatal factors. In order to evaluate the entire developing neuraxis, we undertook SSEPs after tibial nerve stimulation (PTN-SSEP) in pre-term neonates. Using bilateral simultaneous stimulation, potentials were recorded from the following sites: PF-spT6 (N5), spL1-spT6 (N16), spC7-Fpz (N27), Cz' (1 cm behind the vertex)-Fpz (P55). In all newborns studied, the N5 and N16 were reliably recorded. The N5 appeared relatively independent of the length of the newborns. The N16 correlated inversely with length. The N27 and P55 were recorded in 52% and 65% of the newborns, respectively. N27 inversely correlated modestly with length. The P55 was independent of most factors and probably reflects variable rates of cerebral myelination, neuronogenesis, varying states of alertness, and possibly subclinical encephalopathies. These results demonstrate the feasibility of obtaining such data in pre-term newborns.

Developmental assessment of spinal cord and cortical evoked potentials after tibial nerve stimulation: effects of age and stature on normative data during childhood

Somesthetic information from lower extremities is processed by cerebral cortex after traversing the sensory pathways of peripheral nerve, spinal cord, brain-stem and thalamus. Clinical utility of somatosensory evoked potentials (SSEPs) during human development requires systematic analysis of normative data acquired during various stages of body growth and nervous system maturation. Accordingly, SSEPs after tibial nerve stimulation were studied in 32 normal awake children (1-8 years old) and compared with values obtained in young adults (18-40 years old). Potentials were recorded from the tibial nerve (N5), first lumbar spinous process (N14), seventh cervical spinous process (N20) and from the scalp, 2 cm behind the vertex (P28). In all children studied, the N5, N14 and N20 latencies were positively correlated with age and height yielding a predictive nomogram. An extremely variable electropositive cortical SSEP was recorded from Cz' which did not show a highly predictable linear relationship in association with a relatively poor correlation coefficient for height and age. It may be concluded that between 1 and 8 years of normal postnatal development, latencies reflecting peripheral nerve and lumbar spinal cord vary directly with height and age and can be represented by a simple cable model of a lengthening myelinated pathway. In contrast, the latency of the cortical SSEP reflects asynchronous maturation of elongating polysynaptic pathways and apparently requires a more complex model for prediction in order to enhance its clinical utility.

Cellular morphology of chronic spinal cord injury in the cat: analysis of myelinated axons by line-sampling

A systematic line-sampling method is described for counting and mapping myelinated axons in transverse sections of the spinal cord. Its advantages over random sampling of small areas are considered. The technique was applied to quantifying experimental weight-drop contusion injuries of cat spinal cord, from several months to more than a year after injury. Contusion of the mid-thoracic cord with a 20 g weight dropped 20 cm was usually sufficient to produce chronic hindlimb paralysis whilst allowing the survival of significant numbers (40,000-110,000) of myelinated axons passing through the lesion site. The axons which survived were concentrated towards the pial surface. There was a proportionally greater loss of larger diameter axons, but this was independent of distance from the pia, indicating that at least two independent factors contribute to selective axonal death following injury, one related to depth within the cord, the other to axon diameter. Myelin sheath thickness was decreased from normal and this deficit also increased with depth. There was overlap in all these quantitative morphological characteristics between animals showing some recovery of hindlimb locomotion and those with maintained spastic paralysis at more than six months after injury. Effective locomotion was found to recover in some cases with the maintenance of a small proportion (5-10%) of the original axonal population, largely concentrated in a rim only 200-300 microns thick. Morphological correlates of paralysis in chronic injuries included severe reduction of axonal number, selective elimination of large fibers, and sustained dysmyelination. Any one or combination of these may be responsible for chronic paralysis in individual animals.

Postnatal development of the discharge pattern of phrenic motor units in the kitten

The postnatal change of the mean frequency (F), the maximal frequency (FM) and the onset frequency (FO) of discharge of kitten phrenic motor units was studied and compared to adult values. The latency (recruitment time) and duration of discharge of phrenic units were also analyzed. In kittens less than 3 weeks old, there were relatively few early units (latency less than or equal to 10% of phrenic discharge duration), TI). The duration of discharge of early and late units, expressed in percentage of TI, was the same in kittens and adult cats, and the duration of discharge of the early units was greater than that of the late units. In kittens, F, FM and FO of the early and late units were always greater than in adult cats. In adult cat, as in Kittens, F, FM and FO of early units were not significantly greater than those of late units. In conclusion, the relatively small number of early units in kittens may reflect either a small number of active early bulbo-spinal neurons or may be linked to the electrical and morphological properties of phrenic motoneurons in the kitten.

2-Deoxyglucose uptake in the cat spinal cord during sustained and habituated activity in the plantar cushion reflex pathway

[14C]2-deoxy-D-glucose (2-DG) was administered intravenously to anesthetized cats during electrical stimulation of the plantar cushion (central foot pad). Afferent volleys and the efferent reflex were monitored by recording from the tibial nerve at the ankle. Plantar cushion stimulation at 3 HZ, 5 x threshold for 45 minutes led to a discrete region of increased 2-DG uptake dorsomedially in the dorsal horn, predominantly in Rexed's laminae III and IV, ipsilateral to the stimulation. A less marked increase in labeling was also sometimes observed in the medial portion of lamina V. No labeling specific to stimulation was observed in the ventral horns. A control preparation, to determine the effects of surgical manipulations alone, confirmed that the labeling was indeed specific to the plantar cushion stimulation. A pattern of labeling identical to that seen during 3 Hz, 5 x threshold stimulation occurred when 2-DG was administered during 10 Hz, 5 x threshold stimulation, after the reflex elicited by plantar cushion stimulation had completely habituated. The most straightforward interpretation of our results suggests that the increases in 2-DG labeling produced by stimulation of the plantar cushion probably occurred in regions with heavy concentrations of axon collaterals of the primary afferent A alpha beta fibers from the plantar cushion, at or near their sites of termination in the dorsal horn.

A morphological study of the axons and recurrent axon collaterals of cat sciatic alpha-motoneurons after intracellular staining with horseradish peroxidase

Utilizing the centrifugal neuronal transport of intracellularly injected horseradish peroxidase (HRP), we have performed a light microscopic (LM) investigation of the intramedullary parts of the axons and axon collaterals of sciatic alpha-motoneurons in the adult cat. The intramedullary parts of the alpha-motor axons had comparatively short internodes (down to 75 microns) and were thinner than reported in earlier studies on the ventral root. Positive correlations were obtained when relating nodal diameters (2.8-7.8 micron) or the mean diameters of the motor axons in the white matter (4.4-9.0 micron) to the diameters of the initial axonal segments (2.3-4.9 micron). Eighty percent of the motor axons gave off one to five collaterals. There was no correlation between the numbers of collaterals and the lengths of the parent motor axons in the gray matter. The branching patterns of the axon collaterals showed considerable variation and the number of end branches from a single collateral ranged between 1 and 39. The rostro-caudal distribution of the collateral end branches was arranged symmetrically within a narrow space (+/- 300 micron) around the origins of the first order collaterals. Outbulgings of the motor axon collaterals, interpreted as synaptic terminals, were found along (59%) or at the ends (41%) of the collateral branches, and were located 200-700 micron away from the origin of the first order collateral. No characteristic LM feature of the outbulgings was distinguished.

Age-related changes in occurrence of Marchi-positive granules and Marchi-positive myelinoid bodies in postnatally developing feline white matter

The occurrence of clusters of Marchi-positive granules and of Marchi-positive myelinoid bodies at different postnatal developmental stages was examined lightmicroscopically in Vibratome sections from the cervical lateral funiculus of the cat, after perfusion-fixation with glutaraldehyde. The findings show that clusters of Marchi-positive granules are most common at birth and rapidly decrease in number with development, being largely absent in animals older than 1 month. The pattern of change resembles the postnatal changes in content of esterified cholesterol in the cervical lateral funiculus and is compatible with the view that the clusters of Marchi-positive granules may result from spontaneous myelin sheath disintegration occurring early postnatally. The incidence of Marchi-positive myelinoid bodies increases 7 times during the first 4 months after birth to a peak and declines about 40% during late maturation. The size spectrum of the Marchi-positve bodies shifts markedly towards larger sizes with development and presents a close to mature picture from 120 days on. Comparisons between the Marchi-positive myelinoid bodies and the myelin sheaths in the same region, with respect to postnatal change in occurrence and size spectrum, suggest that the Marchi-positive bodies are related to myelin sheaths of large fibres or fibres destined to become large.

Ascending collaterals of cutaneous neurons in the fasciculus gracilis of the cat

Primary sensory neurons with myelinated axons in the sural nerve of the cat were found to be divisible into 3 systems on the basis of the length of their central collaterals in the dorsal columns. The short system consists of neurons that ascend only a segment or two in the fasciculus gracilis above their level of entry into the spinal cord. It is composed of all neurons with peripheral conduction velocities in the Adelta range and thus includes both D hair and nociceptive neurons. Approximately 35% of the Aalpha neurons join the intermediate system and ascend 4-12 segments before leaving the forsal columns. This system is composed of all sural type I neurons, as well as about 40% of the G2 hair, 40% of the intermediate field, and 50% of the F2 field neurons in the nerve. Those nociceptive neurons conducting at Aalpha velocities also contribute to the intermediate system. The remaining G2 hair, intermediate field, and F2 field neurons, together with almost all the sural type II, G1 hair, intermediate hair and F1 field neurons, join the long system and ascend to the nucleus gracilis. Fibers in the intermediate system showed a relatively abrupt decrease in conduction velocity usually of 50% or more (median 71%) a few millimeters rostral to their entry into the spinal cord. Members of the long system also decreased in conduction velocity at this point, but the magnitude of the changes was typically less than 50% of the peripheral velocity (median 36%). In addition, the ascending collaterals of the long system underwent a second reduction in conduction velocity near the cervical enlargement.

Caliber spectra of fibers in the fasciculus gracilis of the cat cervical spinal cord: a quantitative electron microscopic study

In order to obtain a better understanding of the microscopic structure of the cat fasciculus gracilis, entire cross-sections of the fasciculus were examined with the electron microscope. Twenty-five thousand, two hundred and eighty-four fibers were encountered in one fasciculus. The fiber caliber spectra obtained from the study show that the fasciculus gracilis at cervical level has a unique fiber distribution pattern. The fiber diameters range from less than 1 mu to 15 mu, however, 97 percent of fibers have diameters less than 8 mu; and the majority of the fibers are in the 2-5 mu range.