Projections of group Ia afferents to motoneurons of thigh muscles in man

Summary Changes in the firing probability of single motor units in response to electrical stimulation of muscle nerves and to tendon taps were used to derive the projections of large muscle afferents to the motoneurons of various thigh muscles in man. Homonymous facilitation was demonstrated to virtually all of the sampled motor units of biceps (BI), semitendinosus (ST), vastus lateralis (VL) and vastus medialis (VM). Heteronymous facilitation was readily demonstrated between VM and VL but was never obtained from ST to BI and never unequivocally obtained from BI to ST. Reciprocal inhibition was demonstrated from femoral nerve afferents to all of the sampled units of BI, and ST but reciprocal inhibition of VM or VL was never obtained from BI afferents and infrequently from ST afferents. These projections of group I afferents in man show certain specific differences from those of the cat and baboon that may reflect the normal function of the limb.     

A selective effect of naloxone on heterosynaptic C-fibre-mediated inhibitions in the rat dorsal horn

The effect of naloxone on C-primary afferent-mediated inhibitions of C-fibre-evoked activity in deep dorsal horn neurones has been examined in decerebrate-spinal rats. The same C-afferents that evoke activity in a given neurone can inhibit that C-evoked activity (homosynaptic inhibition), and C-afferent input can also inhibit the activity evoked in dorsal horn neurones by other C-afferents (heterosynaptic inhibition). Naloxone was found to selectively reverse heterosynaptic C-mediated inhibitions without affecting homosynaptic inhibitions. In several neurones the heterosynaptic inhibitions were completely abolished by naloxone. These results show that homo- and heterosynaptic C-mediated inhibitions operate by different mechanisms and that, at least in some neurones, endogenous opioids are likely to be the major inhibitory transmitters involved in producing the heterosynaptic inhibition of the activity evoked by one C-input by another C-input.     

Vagal mediation of the cholecystokinin satiety effect in rats

Central (intracerebroventricular) and peripheral (intraperitoneal) injections of the octapeptide of cholecystokinin (CCK-8) were compared to determine the most effective route of administration to elicit satiety for food intake in the rat. Subdiaphragmatic bilateral vagotomy and spinal cordotomy (T2-T3) were also performed to investigate the importance of visceral nerves for the satiety effect. CCK-8 suppressed feeding and elicited satiety resting behavior when injected peripherally but it was less effective when injected centrally. The satiety effect of CCK-8 or CCK-33 following peripheral injections was blocked by vagotomy whereas spinal cordotomy had no effect. The results indicate that some component of the vagus is required to mediate the peripherally induced cholecystokinin satiety effect, but the splanchnic nerves are not necessary. The weak effect of CCK-8 following ventricular administration is additional evidence suggesting that cholecystokinin of intestinal origin acts in the periphery rather than directly on the brain to elicit its typically rapid satiety effect in rats.     

Separation of temperature sensitive and temperature insensitive components of the postsynaptic potentials in the frog motoneurons

Intracellular measurements were made in the in situ spinal cord of the frog at temperatures below 5 degrees C. Responses to volleys in the sciatic nerve, in the descending fibres and in the motor axons were studied. About 30% of the motoneurons responded to sciatic volleys with 1-3 ms segmental latency, which was short enough to assume electrotonic mediation of these responses. Another group of motoneurons responded with 6-8 ms latency, i.e. with the expected delay at chemical synapses at low temperature. Latency distribution of the sciatic-evoked postsynaptic potentials was clearly bimodal in contrast with that found at higher temperatures. Postsynaptic discharges occurred with rather long latency and they were attributed to chemically-mediated excitation. Some of the postsynaptic potentials to descending volleys also occurred with quite short latency, indicating possible electrotonic transmission from supraspinal centres to motoneurons. Latency distribution of the action potentials evoked from the motor axons was bimodal, corresponding to the different, i.e. antidromic and recurrent facilitatory, mechanism of these spikes. Calculated Q10 ratios for the sciatic-evoked reflex discharges and the afferent fibre volleys were about 2.3 and 1.8, respectively. We concluded that cooling helps to separate postsynaptic potentials according to their electrotonic and chemical mediation and that electrotonic excitation does not seem to have a primary role in the generation of postsynaptic discharges initiated by dorsal root volleys in the frog.     

Cadherins in the central nervous system

The central nervous system (CNS) is divided into diverse embryological and functional compartments. The early embryonic CNS consists of a series of transverse subdivisions (neuromeres) and longitudinal domains. These embryonic subdivisions represent histogenetic fields in which neurons are born and aggregate in distinct cell groups (brain nuclei and layers). Different subsets of these aggregates become selectively connected by nerve fiber tracts and, finally, by synapses, thus forming the neural circuits of the functional systems in the CNS. Recent work has shown that 30 or more members of the cadherin family of morphoregulatory molecules are differentially expressed in the developing and mature brain at almost all stages of development. In a regionally specific fashion, most cadherins studied to date are expressed by the embryonic subdivisions of the early embryonic brain, by developing brain nuclei, cortical layers and regions, and by fiber tracts, neural circuits and synapses. Each cadherin shows a unique expression pattern that is distinct from that of other cadherins. Experimental evidence suggests that cadherins contribute to CNS regionalization, morphogenesis and fiber tract formation, possibly by conferring preferentially homotypic adhesiveness (or other types of interactions) between the diverse structural elements of the CNS. Cadherin-mediated adhesive specificity may thus provide a molecular code for early embryonic CNS regionalization as well as for the development and maintenance of functional structures in the CNS, from embryonic subdivisions to brain nuclei, cortical layers and neural circuits, down to the level of individual synapses.     

Effect of maximal arm exercise on skin blood flux in the paralyzed lower limbs in persons with spinal cord injury

 The purpose of the present study was to examine the effect of maximal arm exercise on the skin blood circulation of the paralyzed lower limbs in persons with spinal cord injury (PSCI). Eight male PSCI with complete lesions located between T3 and L1 performed graded maximal arm-cranking exercise (MACE) to exhaustion. The skin blood flux at the thigh (SBFT) and that at the calf (SBFC) were monitored using laser-Doppler flowmeter at rest and for 15 s immediately after the MACE. The subject's mean peak oxygen uptake and peak heart rate was 1.41 ± 0.22 l · min^–1 and 171.6 ± 19.2 beats · min^–1, respectively. No PSCI showed any increase in either SBFT or SBFC after the MACE, when compared with the values at rest. These results suggest that the blood circulation of the skin in the paralyzed lower limbs in PSCI is unaffected by the MACE. Accepted: 12 September 1996     

Anaplastic Ependymoma of the Spinal Cord in Childhood A Case Report

We report a 6-year-old girl with anaplastic ependymoma probably originating in the region of the conus medullaris and probably spreading retrogradely to the region of the interventricular foramen (Monro) through the cere-brospinal fluid (CSF). Since ependymoma of the spinal cord rarely occurs in children, and retrograde spreading is extremely rare, the histological features and mechanism of metastasis of the tumor are discussed.     

The origin of the reticulo-olivary projection in the rat: a retrograde horseradish peroxidase study

Electrophoretic injections of horseradish peroxidase were made in different parts of the rat inferior olivary complex using a ventral approach. Data from these injections provide anatomical evidence for the existence of a projection to the inferior olive which takes origin from reticular nuclei in the brainstem. The majority of reticulo-olivary neurons are located in the nucleus raphe obscurus and nucleus raphe pallidus. Other reticular nuclei which contribute to this projection include the nucleus reticularis ventralis and nucleus reticularis gigantocellularis. Analysis of injections confined to specific parts of the olivary complex reveals a topographical pattern in the reticulo-olivary projection. Caudal parts of the complex receive input primarily from the nucleus reticularis ventralis. As more rostral and medial parts of the inferior olive are included in the injection, there is concomitant shifting of labeled neurons to the nucleus reticularis gigantocellularis and the raphe nuclei. The reticulo-olivary neurons may serve several non-mutually exclusive roles in olivary circuitry. They may be the source of serotonin and/or substance P to the nucleus. Physiologically, they may provide the inhibitory input observed in the nucleus. Finally, some of these neurons may be the brainstem relay of the lateral funiculus and dorsolateral funiculus spino-olivo-cerebellar pathway proposed by Larson and his co-workers (J. Physiol., Lond. 203, 611-640, 641-649).     

Retinal ganglion cells that project to the superior colliculus and pretectum in the macaque monkey

Horseradish peroxidase was injected into the superior colliculus or pretectum or both in order to label, by retrograde axoplasmic transport, the retinal ganglion whose cells axons innervate the dorsal midbrain. The dendrites of ganglion cells were sufficiently well-labelled to reveal their overall morphological characteristics. It was therefore possible to compare the number and form of ganglion cells projecting to the midbrain with the total population of ganglion cells as revealed by optic nerve injections, and with ganglion cells labelled by injections in the lateral geniculate nucleus. We found that not more than 10% of all retinal ganglion cells project to the superior colliculus in the macaque monkey. This percentage varies little over the retina, being about 6% of all ganglion cells near the fovea and increasing slightly with eccentricity. The superior colliculus does not receive a projection from P beta cells and only a few P alpha cells terminate there. The majority of cells which project to the superior colliculus have a small- to medium-sized cell body and sparsely branched dendritic tree. We have called them P gamma and P epsilon cells by analogy with the gamma cells and epsilon cells in the cat's retina. Anatomically the P gamma and P epsilon cells are heterogeneous, which would be consistent with the physiological heterogeneity found for ganglion cells which project to the midbrain in monkeys.     

Extracellular ionic and volume changes: The role in glia—Neuron interaction

Activity-related changes in extracellular K⁺ concentration ([K⁺]_e), pH (pH_e) and extracellular volume were studied by means ofion-selective microelectrodes in the adult rat spinal cord in vivo and in neonatal rat spinal cords isolated from pups 3–14 days of age (P3–P14). Concomitantly with the ionic changes, the extracellular space (ECS) volume fraction (α), ECS tortuosity (λ) and non-specific uptake (k′), three parameters affecting the diffusion of substances in nervous tissue, were studied in the rat spinal cord gray matter. In adult rats, repetitive electrical nerve stimulation (10–100 Hz) elicited increases in [K⁺]_e of about 2.0–3.5 mm, followed by a post-stimulation K⁺-undershoot and triphasic alkaline-acid-alkaline changes in pH_e with a dominating acid shift. The ECS volume in the adult rat occupies about 20% of the tissue, α = 0.20 ± 0.003, λ = 1.62 ± 0.02 and k′ = 4.6 ± 0.4 × 10⁻³s⁻¹ (n = 39). In contrast, in pups at P3–P6, the [K⁺]_e increased by as much as 6.5 mm at a stimulation frequency of 10 Hz, i.e. K⁺ ceiling level was elevated, and there was a dominating alkaline shift. An increase in [K⁺]_e as large as 1.3–2.5 mm accompanied by an alkaline shift was evoked by a single electrical stimulus. The K⁺ ceiling level and alkaline shifts decreased with age, while an acid shift, which was preceded by a small initial alkaline shift, appeared in the second postnatal week. In pups at P1–P2, the spinal cord was X-irradiated to block gliogenesis. The typical decrease in [K⁺]_e ceiling level and the development of the acid shift in pH_e at P10–P14 were blocked by X-irradiation. Concomitantly, continuous development of glial fibrillary acidic protein positive reaction was disrupted and densely stained astrocytes in gray matter at P10–P14 revealed astrogliosis.The alkaline, but not the acid, shift was blocked by Mg²⁺ and picrotoxin (10⁻⁶m). Acetazolamide enhanced the alkaline but blocked the acid shift. Furthermore, the acid shift was blocked, and the alkaline shift enhanced, by Ba²⁺, amiloride and SITS. Application of glutamate or gamma-aminobutyric acid evoked an alkaline shift in the pH_e baseline at P3–P14 as well as after X-irradiation. The results suggest that the activity-related acid shifts in pH_e are related to membrane transport processes in mature glia, while the alkaline shifts have a postsynaptic origin and are due to activation of ligand-gated ion channels.At P4–P6, the ECS volume was almost double that in adult rats, α = 0.37 ± 0.01 (n = 17), the ECS tortuosity was significantly higher, λ = 1.78 ± 0.02, while the non-specific uptake was not significantly different, k′ = 3.61 ± 0.56 × 10⁻³ s⁻¹. The α gradually decreased to about 24% at P12. In adult rats, electrical or adequate stimulation evoked a shrinkage of the extracellular space by 20–50%, while no significant changes in ECS volume were found in P3–P6. We conclude that the [K⁺]_e ceiling level, character of the pH_e transients, the size of the ECS volume and the activity-related ECS shrinkage are closely related to gliogenesis.