Post‐natal development of pyramidal tract neurones in kittens.

Abstract

SUMMARY1. The post-natal development of pyramidal tract neurones (p.t.n.s) was investigated in twenty-one barbiturate-anaesthetized kittens from birth to 28 days of age using a combination of electrophysiological and anatomical techniques.2. P.t.n. responses were recorded intracellularly as well as extracellularly with glass micropipettes filled with horseradish peroxidase (HRP) on stimulation of the medullary pyramid and cerebellar nuclei.3. Latency histograms of antidromic responses of p.t.n.s were compared at various ages. In the neonate, p.t.n.s were divided into two groups which were presumed to be analogous with fast and slow p.t.n.s in adult animals. During the first post-natal week, latency shortening was not conspicuous, but by the end of the second post-natal week, the faster group showed a marked decrease of latencies (up to around 10 ms at 14 days of age), while those of the slower group did not change so much. The slower group increased their conduction velocity during the third post-natal week (latencies up to around 18 ms). At the end of the fourth post-natal week, the distribution of antidromic latencies was in a narrower range, but the values were still longer than those reported in adult animals.4. Intracellular HRP staining revealed that apical dendrites of p.t.n.s spread fully to the pial surface even at birth. The somata of these neurones were characteristically covered with somatic appendages and development of the basal dendritic tree was immature in 0-1-day-old kittens. Basal dendrites developed nearly completely by 7 days, but somata were still covered with appendages. During the fourth post-natal week, these appendages disappeared almost completely. The sizes of the dendritic field, especially of apical dendrites, became larger in parallel with the development of cortical layers.5. From the morphological point of view, differentiation of fast and slow p.t.n.s was not clear until 28 days of age except in somatic volumes, which were already different in the first post-natal week. At the end of the fourth post-natal week, p.t.n.s with short antidromic latencies had a tendency to bear spines more sparsely over the secondary and tertiary dendritic surface in comparison with p.t.n.s with longer latencies.6. Intracortical axonal trajectories developed fairly well in the immature cerebral cortex, and the general pattern of ramification changed little during the first month * To whom reprint requests and correspondence should be addressed. 16PHY 363

Development of Functional Topography in the Corticorubral Projection: AnIn VivoAssessment Using Synaptic Potentials Recorded from Fetal and Newborn Cats

“…Immature axons, however, have low conduction velocities and thus long conduction times (Song et al, 1995b;Olivier et al, 1997), making it difficult to judge whether observed synaptic potentials are monosynaptic. The issue is f urther complicated by the fact that there are considerable variations in conduction velocity, even among axons in a single group of neurons during development (Oka et al, 1985;Song et al, 1995b). Nevertheless, the results presented here together support the view that the rising phase of the cortically evoked potentials was composed of monosynaptic EPSPs.…”

“…The latencies of all EPSPs are plotted as a function of age in Figure 3D. These latencies are comparable to the conduction times of kitten cortical axons from the cortex to the trapezoid body, at corresponding ages (Oka et al, 1985). Because the trapezoid body is further away from the cortex than is the RN, these results support the view that the EPSPs are monosynaptic.…”

Synapse Elimination in the Corticospinal Projection During the Early Postnatal Period

“…The CV values estimated in our study (0.85-1.21 m/s) were 1/10th that of adult (Alstermark et al 2004;Bannister and Porter 1967;McComas and Wilson 1968;Mediratta and Nicoll 1983;Porter and Lemon 1993;Stewart et al 1990). In fact, neonatal CVs of the CS tract of postnatal monkeys and cats are 10 times slower than those of adult animals (Oka et al 1985;Olivier et al 1997). The CV values we estimated in early postnatal rat are consistent with the decreased CV in other species.…”

Postnatal Development of Corticospinal Postsynaptic Action

“…After week 8, skill is noticeably greater; this is that age that we can begin to train kittens to reach (unpublished observations; Martin et al 2000). Clearly this improvement is not due to CS myelination, which is well developed by week 4 (Oka et al 1985). Presynaptic corticospinal site development has not yet been examined in the primate.…”