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PubMed1985

Occipital cortical neurons with transient pyramidal tract axons extend and maintain collaterals to subcortical but not intracortical targets.

During the early postnatal development of the rat large numbers of pyramidal tract neurons are present in layer V of the occipital cortex, but by the end of the third postnatal week the distribution of pyramidal tract neurons becomes restricted to the more rostral cortical areas. This restriction is brought about by selective collateral elimination rather than by cell death. We have found, by using retrogradely transported fluorescent dyes as either short-term or long-term markers, that occipital cortical neurons which had transiently extended pyramidal tract axons maintain subcortical axonal connections to either the superior colliculus or the pons, and, at least in the case of the corticotectal projection, that the maintained collateral is present prior to the elimination of the transient pyramidal tract collateral. Further, it appears that at no time during postnatal development do the occipital pyramidal tract neurons form either callosal or ipsilateral cortico-cortical collaterals. Thus in the early postnatal occipital cortex the neurons which project through the pyramidal tract constitute a population of cells which is separate from neurons which make cortico-cortical connections, but which largely overlaps with the population of corticotectal and corticopontine neurons.

Amidines↗

PubMed1975

Projections of pyramidal tract cells to alpha-motoneurones innervating hind-limb muscles in the monkey.

1. We have investigated the spatial organization of monosynaptic corticospinal projections to hind-limb motoneurones, using near threshold stimulation of the surface of the precentral gyrus to activate pyramidal tract (PT) cells and intracellular recording from motoneurones to detect the resulting e.p.s.p.s. 2. Monosynaptic e.p.s.p.s. of cortical origin were seen in all motoneurone species investigated, those of distal as well as of proximal hind-limb muscles. The proportion of motoneurones in which the e.s.p.s. were evoked and the amplitudes of the latter indicated a more extensive cortical projection to motor nuclei for distal than for proximal muscles, as previously found for forelimb motoneurones. 3. Cortical areas from which monosynaptic e.p.s.p.s. were evoked in individual motoneurones were remarkably large, most often between 3 and 7 mm2. Several motoneurones appeared to have two or three separate areas within the hind-limb division of the motor cortex. 4. Areas of location of pyramidal tract cells projecting to various motoneurones innervating one muscle were usually not identical. They overlapped often only partially or did not overlap at all. 5. Areas of location of pyramidal tract cells projecting to motor nuclei for different muscles often showed an extensive overlap. When it occurred, various motoneurones of a given motor nucleus had common cortical projection areas with motoneurones of other motor nuclei, either to synergistic or to antagonistic muscles. Our results give further evidence for overlapping of areas of cortical projections to motoneurones and speak against a mosaic-like organization of pyramidal tract cells projecting to different motor nuclei. 6. The rise times of cortically evoked e.p.s.p.s. indicate that the corticospinal tract fibres terminate on motoneurones at approximately similar distances from the soma as group Ia afferents. The small amplitudes of the majority of e.p.s.p.s. evoked by near threshold cortical stimulation therefore suggest that unitary e.p.s.p.s of cortical origin are small and that the density of pyramidal tract cells projecting to individual motoneurones is usually low, even in the centrum of projection areas. 7. Effects of intracortical stimulation depended on the stimulus strength. With currents of 2-3 muA, e.p.s.p.s were usually evoked in one motoneurone species or in close synergists. With currents of 5-10 muA, largest e.p.s.p.s a number of other motoneurones. Latencies of descending volleys in the lumbar corticospinal tract indicated that intracortical stimuli activated pyramidal tract cells indirectly; the effects of these stimuli could thus not be used to indicate the location of pyramidal tract cells responsible for them.

Animals↗

PubMed1991

Outgrowth of the pyramidal tract in the rat cervical spinal cord: growth cone ultrastructure and guidance.

In order to examine the mode of outgrowth of the pyramidal tract in the rat, the ultrastructure of its pathway in the dorsal funiculus of the spinal cord was analysed. The analysis was performed by means of serial sections of the third cervical segment before and during the arrival of pyramidal tract axons, and focussed on the morphology and microenvironment of the growth cones. Growth cones appear as elongated terminal enlargements without side branches. Two zones could be discerned: the distal, usually lamellipodial fine granular zone, containing no organelles, except for an occasional clear vesicle; and the proximal organelle-rich zone, which contains various organelles, such as agranular reticulum and vesicular structures. In addition, the proximal organelle-rich zone contains round or elliptic structures, limited by two concentric membranes, that enclose reticular and vesicular elements. The electron density of these structures varied from as low as the surrounding growth cone matrix to as dark as lysosomal structures, suggesting their involvement in turnover processes. At embryonic day 20, the most ventral part of the dorsal funiculus, where the first pyramidal tract axons are due to arrive within two days, is populated by axons that are relatively small compared to those in the rest of the dorsal funiculus. At birth, the arrival of the first pyramidal tract axons is marked by the presence of numerous large growth cone profiles in between small axons in the most ventral part of the dorsal funiculus; no circumscript bundle separated from the ascending sensory fiber tracts is present yet. The growth cones descend, club-shaped and 1 to 2 microns in diameter, without lamellipodia or filopodia. Within the same area a second growth cone type is present, which contains dense-core vesicles and has spread-out lamellipodia. Most of these growth cones are ascending and they probably belong to primary afferent or propriospinal fibers. At postnatal day 2, the pyramidal tract can be readily delineated from the adjacent fasciculus cuneatus where myelination has already started, but no glial boundary is present. The abundant growth cones are 1-2 microns wide and extend single unbranched lamellipodia, up to 15 microns long, which often enfold parallel axons or other growth cones. At postnatal day 4, growth cones are scarce in the tract. They measure 1 micron or less in diameter and each extends a single, straight lamellipodium or filopodium over 1 to 7 microns in the caudal direction.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1975

Variation in form of the pyramidal tract and its relationship to digital dexterity.

A morphometric analysis of the pyramidal tract's relation to digital dexterity was performed on data from 69 mammals. The results show that the variation in digital dexterity among mammals corresponds most closely to the variation in place of termination of pyramidal tract fibers within the spinal cord, corresponds less closely to the variation in the size of the tract itself and its constituent fibers, and does not correspond reliably with any other feature yet reported. Since the termination of pyramidal tract fibers on or very near spinal motor neurons is a prerequisite even for the peculiar kind of dexterity seen in some non-primates (e.g., raccoon, kinkajou), this one feature alone seems to be a critical factor.

Animals↗

PubMed1985

Fiber analysis of the pyramidal tract of the laboratory rat.

Light and electron microscopic study of the pyramidal tract of the laboratory rat at a midbulbar level revealed the total number of myelinated fibers on one side to be about 200,000. They ranged from 0.2 micron to more than 5 microns, but clustered strongly in the neighborhood of 1.0 micron (mode of 0.9 micron and mean of 1.2 micron), forming the highly skewed fiber spectrum so familiar for mammalian pyramidal tracts and other central fiber pathways. Numerous small clusters of unmyelinated axons were found scattered throughout the tract, adding another 100,000 axons to the estimated number. Not only were the fibers exceedingly small, but also the degree of myelination relative to axon diameter varied widely, suggesting that conduction speed within the tract is not optimal for all fibers. In fact, about half of the fibers in the pyramidal tract would, in theory, conduct faster if they had no myelin wrapping.

Animals↗

PubMed1985

Superposition of antidromic responses in pyramidal tract cell clusters.

Large-seeing-distance microelectrodes were used to record simultaneously the activity of several pyramidal tract neurons in cerebral cortex. When activated antidromically, these neurons responded simultaneously, forming a "stack" of superimposed spikes, rather than responding at different times within the 10-ms time interval during which they might be expected to respond. Using a variety of spike collision tests, we found that these individual spikes arose from separate sources and reflected the activity of individual, albeit neighboring, pyramidal tract neurons. The collaterals of neurons within a stack projected to different structures, further verifying that separate neurons were involved. Such synchrony of antidromic activity among neighboring pyramidal tract neurons is an exceedingly low-probability event, if neighboring fibers conduct independently of one another. Our results imply that fibers from small clusters of neurons in the cortex assemble to form synchronously conducting bundles of fibers within the pyramidal tract.

Animals↗

PubMed1991

[MR evaluation of wallerian degeneration of the pyramidal tract].

This study is based on 135 magnetic resonance (MR) exams of 110 patients with wallerian degeneration of the pyramidal tract shown on MR images acquired on a mid field imaging scanner. The MR findings of wallerian degeneration were abnormal signal band along the course of the pyramidal tract and ipsilateral brain stem shrinkage. In all 110 cases an abnormal signal band was seen on T2-weighted spin-echo images, that is, a hypointense band in four exams between 30 days and 116 days after onset of symptoms, and hyperintense bands in 122 exams. The hyperintense signal on T2-weighted images was shown in most cases after 200 days from the onset. In one case a signal of the pyramidal tract showed a hyperintense band at 7 days, hypointense at 30 days, and hyperintense again at 123 days after onset. Sequential MR exams of another case showed gradual narrowing of the hyperintense signal band and progression of the ipsilateral brainstem shrinkage. The narrowing of the pyramidal tract and the ipsilateral brain stem shrinkage tended to be shown after 6 months from onset.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗

PubMed1979

Prolonged changes in excitability of pyramidal tract neurones in the cat: a post-synaptic mechanism.

1. Prolonged changes in the excitability of cortical neurones can be produced by altering their firing rates for brief periods. In the anaesthetized cat, increased firing of pyramidal tract cells induced by trains of antidromic conditioning shocks led to increases in cell excitability, as measured by the size of the mass response at the medullary pyramid to test shocks applied to the cortical surface. We have shown in two ways that post-synaptic mechanisms could be responsible. 2. In one experimental design, MgCl2 solution (1 mole/l.) was applied to the cortical surface in order to block synaptic activity throughout the cortical depth. Following antidromic conditioning trains, cell excitability was increased; the size of the mass response was up to 30% larger than the control values. This persisted undiminished for up to 3 hr. 3. In the second experimental design, synaptic activity was not blocked, but we compared the effects of antidromic plus synaptic activation of pyramidal tract cells with the effects of synaptic activation alone. Antidromic plus synaptic activation was obtained by applying conditioning trains to the pyramidal tract at the medulla ipsilateral to the cortical test shock; prolonged increases in the ipsilateral response to the test shock were produced. Synaptic activation alone was obtained by the same conditioning trains, but in those cells whose axons projected into the contralateral pyramidal tract; prolonged increases in the contralateral response to the cortical test shock were never seen. In many instances prolonged decreases in excitability were found. 4. We conclude that prolonged increases in excitability of pyramidal tract cells can occur in the absence of any synaptic input, demonstrating that the underlying mechanism is post-synaptic; this does not preclude the action of synaptic mechanisms when synaptic transmission is not blocked.

Animals↗

PubMed1989

Retrograde pyramidal tract degeneration in a patient with cervical haematomyelia.

Retrograde pyramidal tract degeneration has been described only very rarely in the human central nervous system. In most of these cases the thoracic or cervical corticospinal tracts were shown to have degenerated following long-standing, lower spinal cord lesions. In a 67 year old man, who lived 2 years following the rupture of a mid-cervical cavernous angioma, we observed such degeneration which reached as high as the pons. This axonal dissolution was much less manifest above the ponto-medullary junction. Large pyramidal cells of Betz were not identified in the precentral gyrus, suggesting that the parental soma of the damaged axons had undergone atrophic changes. Furthermore, the involvement of the so-called aberrant pyramidal tract in the pontine medial lemniscus indicated that retrograde degeneration had occurred there as well.

Aged↗

PubMed1989

B-50/GAP43 is localized at the cytoplasmic side of the plasma membrane in developing and adult rat pyramidal tract.

The neuron-specific phosphoprotein B-50/GAP43 has been implicated in axonal outgrowth, since high levels of B-50/GAP43 are found in growth cones and during development of the nervous system. In adult brain, the B-50 levels are decreased. B-50 is primarily found in axons and presynaptic terminals. It is phosphorylated by protein kinase C, and this process has been implicated in the modulation of membrane signal transduction. During the outgrowth of the pyramidal tract, high levels of B-50 have been reported, whereas a low amount of B-50 persists into the adult stage. By immunoelectron microscopy, using immunogold labeling on cryosections and pre-embedding peroxidase labeling, we examined the distribution of B-50 in the pyramidal tract at the third cervical segment in developing 2-d-old and adult 90-d-old rats. B-50 immunoreactivity was found in axons and growth cones of the outgrowing tract. In the adult pyramidal tract, both unmyelinated and myelinated axons contained B-50 immunoreactivity. The immunogold label was predominantly located at the plasma membrane. Since the peroxidase reaction product was observed exclusively intracellularly, we conclude that the B-50 immunoreactivity is predominantly located at the cytoplasmic side of the plasma membrane of axons and growth cones. The high immunoreactivity in growth cones and axons of the outgrowing pyramidal tract further supports the hypothesis that B-50 plays a role in neurite outgrowth. The presence of B-50 in the adult pyramidal tract cannot merely be attributed to transport to the synapse. Therefore, it is suggested that B-50 plays, in addition, a local, growth-associated role in the adult tract.

Aging↗

PubMed1984

Morphological and electrophysiological characteristics of pyramidal tract neurons in the rat.

Responses evoked in neurons of rat sensorimotor cortex upon stimulation of the pyramidal tract and ipsilateral cerebral peduncle were analysed using intracellular recording. Neurons responding antidromically to pyramidal tract stimulation (PT cells) and neurons failing to respond antidromically but exhibiting orthodromic responses were both stained by intracellular injection of horseradish peroxidase (HRP). Layer V pyramidal neurons, including those responding antidromically, exhibited prominent long lasting membrane hyperpolarizations and inhibitions of action potentials following pyramidal tract or cerebral peduncle stimulation. Upon passage of polarizing intracellular current two components were identified within the hyperpolarizing potential. A short duration initial component readily reversed with hyperpolarizing current. Frequently this earlier component overlapped a period of early excitation consisting of action potentials arising from recurrent EPSPs or large slow depolarizing potentials (SDPs). The second, much longer duration hyperpolarizing component did not reverse with passage of hyperpolarizing current and was often followed by a rebound period of depolarization and action potential generation. Both the excitatory and the inhibitory portions of these responses could be demonstrated in animals with acute thalamic transections severing the ascending lemniscal pathway to cortex. Following intracellular staining with HRP, two types of PT cells were identified by their different intracortical axonal arborizations. Most of the injected neurons had local axonal fields extending widely in layers V and VI, but with few or no collaterals extending radially toward the more superficial layers. A second type of PT cell had axon collaterals limited to a narrow zone around the dendritic field but extending radially as far as layer I. Cells of both types were observed to send axon collaterals into neostriatum. Both types of neurons exhibited morphological and physiological characteristics of slow PT cells, and we could find no cells comparable to the fast conducting PT cells observed in other species.

Animals↗

PubMed1975

Modulation of cortical and pyramidal tract induced motor responses by electrical stimulation of the basal ganglia.

Two general mechanisms based on anatomical studies are possible for modulation of motor activity by the caudate nucleus and globus pallidus. These mechanisms are: (1) modulation of the output of cortical neurons that exert motor influences; and (2) modulation of subcortical neurons that exert motor influences. Differentiation between these two mechanisms was accomplished in the present study by two experimental approaches, both of which employed the conditioning-test paradigm. The first approach was an investigation of caudate nucleus or globus pallidus modulation (conditioning stimulus) of flexor responses of the anterior tibialis muscle elicited by electrical stimulation of the sensorimotor cortex (test stimulus) or pyramidal tract (test stimulus). These investigations were carried out in the intact and in decorticate cats. The second approach was an analysis of modulation or cortically induced pyramidal tract responses (direct and indirect, D-I potentials) by conditioning shock trains delivered to various loci within the caudate nucleus or globus pallidus. Both approaches were designed to determine whixh inhibitory and facilitatory motor influences of the basal ganglia occurred at a cortical or subcortical level. Simultaneous stimulation of a locus within the caudate nucleus and the sensorimotor cortex evoked either an enhancement, reduction or no alteration of the cortically induced increase in flexor responses (measured by Ia afferent activity, EMG, myogram). In contrast, no inhibitory influences occurred from caudate nucleus stimulation upon pyramidal tract induced flexor responses in either the intact or decorticate preparation. Inhibitory loci were distributed toward the rostral portion of the caudate nucleus, whereas facilitatory loci were distributed throughout; this distribution was statistically significant (chi2; P less than 0.01). Only enhancement or no influence upon cortical induced or pyramidal tract induced responses were obtained by conditioning stimuli to the globus pallidus. In the unanesthetized but immobilized cat, trains of shocks delivered to the caudate nucleus enhanced, reduced or had no influence upon the cortically evoked direct (D) and indirect (I) potentials recorded in the bulbar pyramidal tract. The distribution of facilitatory and inhibitory loci was organized in a similar fashion as in theanesthetized preparation. From these observations, a model was proposed in which the output of the caudate nucleus exerts both facilitatory or inhibitory modulation of the tonically active globus pallidus cells. The latter in turn predominantly or exclusively facilitate output of pyramidal tract neurons as well as the output of subcortical structures; both effects facilitate motor responses at the spinal level.

Animals↗

PubMed1987

The physiological identification of pyramidal tract neurons within transplants in the rostral cortex taken from the occipital cortex during development.

Axons from neurons in the occipital cortex transiently extend to the pyramidal tract (PT) during the early postnatal development of rats. Normally, these axons are eliminated by the end of the third postnatal week. However, if a portion of fetal occipital cortex is transplanted to the parietofrontal region in newborn hosts then some neurons in the transplant will extend pyramidal tract axons and maintain them. Intracortical microstimulation and electrophysiological recording techniques were used to identify the physiological characteristics of the transplanted pyramidal tract cells and to determine if motor effects could be elicited from the occipital transplant. Microstimulation of the transplant did not reliably evoke movement but the low density and disarray of PT cells within the transplant might account for this. Recording from within the transplant revealed that the overall cell activity was depressed. We were able to identify neurons within the transplant which responded antidromically to stimulation of the pyramidal tract, indicating that their axons have the capacity to conduct impulses and are therefore likely to have developed some viable connections. The functional significance of such projections remains uncertain.

Animals↗

PubMed1985

Pattern of myelination in the pyramidal tract of the rat.

The size and myelination of midbulbar pyramidal tract axons were measured by electron microscopy in the rat. We found that myelin thickness did not increase linearly with fiber size; rather, it took on certain preferred thicknesses almost independently of fiber size. This pattern of growth and development is fundamentally different from that of peripheral nerve and may be important for the physiology of the pyramidal tract.

Animals↗

PubMed1986

Elevated synthesis of an axonally transported protein correlates with axon outgrowth in normal and injured pyramidal tracts.

Axons of the adult mammalian CNS typically fail to regenerate after injury. Among the hypotheses to account for this failure is the proposition that certain axonal proteins necessary for axon growth are expressed in much greater abundance in developing than in mature neurons, and that these proteins are not reinduced after injury to mature axons (Skene and Willard, 1981b). In the present experiments, we have found that hamster pyramidal tract neurons synthesize an acidic, 43K protein that is transported into growing axons during the first 2 weeks of postnatal development, and then declines at least an order of magnitude by the fourth postnatal week. The decline in synthesis of the 43K protein coincides with the cessation of pyramidal tract axon elongation. This protein resembles a "growth-associated protein," GAP-43, which is induced during regeneration of CNS axons in lower vertebrates. The 43K protein in hamster pyramidal tract neurons is not reinduced after axotomy in adult animals, which correlates with the failure of the injured axons to regenerate. Injury to neonatal pyramidal tract axons does not reverse or delay the decline in 43K protein synthesis. This is consistent with previous findings (Kalil and Reh, 1982) that pyramidal tract axons regrow for only a brief period after neonatal injury. Taken together, these results lend support to the hypothesis that synthesis of GAP-43 is important for axon growth in development and regeneration.

Animals↗

PubMed1985

A detailed morphometrical analysis of the pyramidal tract of the rat.

The present study is aimed at a quantitative analysis and comparison of the fibers of the pyramidal tract of the rat at two levels: the pyramis medullae and the second cervical segment. For this purpose both levels of the pyramidal tracts of 5 rats have been studied morphometrically at the ultrastructural level. Fiber numbers as well as fiber diameter distributions are presented for both myelinated and unmyelinated axons. At the level of the pyramis medullae the number of unmyelinated fibers (140,000 +/- 7000) exceeds the number of myelinated axons (103,000 +/- 6000). In contrast at the level of the second cervical segment the numbers of fibers of the axon populations studied are not significantly different, viz. 43,000 +/- 2000 myelinated and 35,000 +/- 8000 unmyelinated fibers. However, these numbers mean a significant decrease of myelinated axons (48%) compared with the pyramis medullae level and an even much larger decrease (75%) in the numbers of unmyelinated fibers. Diameter distributions, obtained from the minimal fiber diameter, show a similar, monomodal shape for all axon and myelin profiles. For unmyelinated axons the minimal diameter ranges from 0.05 to 1.21 micron with a mean of 0.18 +/- 0.03 micron. For myelinated axons a mean minimal diameter of 0.72 +/- 0.12 micron has been found (range 0.13-4.92 micron), whereas their diameter measured with myelin sheath measure 1.08 +/- 0.13 micron (range 0.25-6.03 micron). The average thickness of the myelin sheath is 0.2 micron and is strongly correlated to axon diameter. Furthermore, g-ratio has been computed at a mean of 0.65 and is fairly constant throughout the diameter spectrum. Several possibilities concerning the functional and anatomical qualities of the numerically important unmyelinated axon population in the pyramidal tract of the rat are discussed.

Animals↗

PubMed1985

Cerebral neocortical neurons in the aged rat: spontaneous activity, properties of pyramidal tract neurons and effect of acetylcholine and cholinergic drugs.

The properties of cortical cerebral neurons have been studied and compared in 2, 22 and 26 month-old Sprague-Dawley rats, using electrophysiological techniques. The mean spontaneous activity of the neurons in old animals (unidentified as well as pyramidal tract neurons) was not different from that of young adult rats. In contrast the mean latency of the antidromic response of pyramidal tract neurons to pyramidal tract stimulation was significantly longer in 26 month-old animals. No difference was observed in the effects of the excitatory amino acid glutamate applied by iontophoresis. The percentage of cortical neurons excited by the iontophoretic application of acetylcholine was similar in young and old animals. Neither the laminar distribution, nor the individual sensitivity of these neurons to acetylcholine were found to be modified. The pharmacological properties of the acetylcholine-induced excitations were unchanged, exhibiting muscarinic as well as nicotinic properties. These results are consistent with the suggestion that the impairment of the cholinergic system with aging is for a large part presynaptic. They also emphasize the fact that several physiological and pharmacological properties of the cerebral cortical neurons show little change with age in Sprague-Dawley rats.

Acetylcholine↗

PubMed1976

Further study on the excitation of pyramidal tract cells by intracortical microstimulation.

The effective spread of stimulating current for pyramidal tract (PT) cells and fibers was studied using a method of cancelling the shock artifacts and the following results were obtained: 1. The excitability of PT axon collaterals was as high as that of PT cells. 2. These axon collaterals extended as far as 1.0 mm horizontally from the PT cells. 3. The low threshold area for activation of a given PT cell was as wide as 3--4 mm2 on the surface of the cortex. 4. Intracortical microstimulation (ICMS) delivered to the PT cell layer produced direct (D) and indirect (I) descending volleys in the pyramidal tract, but ICMS to the superficial layer (III) produced only I-waves. 5. These I-waves grew significantly larger after 15--20 msec from the start of the train of stimuli. 6. It is concluded that either surface stimulation, or short train of ICMS is inadequate for delineating fine localization of motor function within the cortex. Longer train (30--40 msec) with high frequency pulses (300--400 cy/sec) can produce muscle contraction with much smaller currents, increasing the accuracy of measuring the localization of motor function.

Animals↗