Functional organization of the tectothalamic section of the transcollicular visual afferent channel

On the basis of the results of electrical stimulation of the optic tract and upper layers of the superior colliculus in cats anesthetized with pentobarbital maps were compiled of the distribution of evoked potentials of the posterior thalamic nuclear complex, including the pulvinar and n. lateralis posterior, posterior, suprageniculatus, and lateralis dorsalis. Functional projections of the superior colliculus and optic tract to the posterior thalamus were shown to differ from each other. In response to stimulation of the superior colliculus the distribution of projections was more regular than to stimulation of the tract. Fibers running from the optic tract occupy a smaller territory than fibers from the superior colliculus. It is suggested that the transcollicular afferent channel of the visual system is not reduced in the course of evolution but, on the contrary, it acquires connections with the younger thalamic formations of the brain and assume more complex functions.Brain Institute, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 10, No. 4, pp. 355–359, July–August, 1978.     

Visceral nociception: peripheral and central aspects of visceral nociceptive systems

Discomfort and pain are the sensations most commonly evoked from viscera. Most nociceptive signals that originate from visceral organs reach the central nervous system (c.n.s.) via afferent fibres in sympathetic nerves, whereas parasympathetic nerves contain mainly those visceral afferent fibres concerned with the non-sensory aspects of visceral afferent function. Noxious stimulation of viscera activates a variety of specific and non-specific receptors, the vast majority of which are connected to unmyelinated afferent fibres. Studies on the mechanisms of visceral sensation can thus provide information on the more general functions of unmyelinated afferent fibres. Specific visceral nociceptors have been found in the heart, lungs, testes and biliary system, whereas noxious stimulation of the gastro-intestinal tract appears to be detected mainly by non-specific visceral receptors that use an intensity-encoding mechanism. Visceral nociceptive messages are conveyed to the spinal cord by relatively few visceral afferent fibres which activate many central neurons by extensive functional divergence through polysynaptic pathways. Impulses in visceral afferent fibres excite spinal cord neurons also driven by somatic inputs from the corresponding dermatome (viscero-somatic neurons). Noxious intensities of visceral stimulation are needed to activate viscero-somatic neurons, most of which can also be excited by noxious stimulation of their somatic receptive fields. The visceral input to some viscero-somatic neurons in the spinal cord can be mediated via long supraspinal loops. Pathways of projection of viscero-somatic neurons include the spino-reticular and spino-thalamic tracts. All these findings give experimental support to the 'convergence-projection' theory of referred visceral pain. Visceral pain is the consequence of the diffuse activation of somato-sensory nociceptive systems in a manner that prevents accurate spatial discrimination or localization of the stimuli. Noxious stimulation of visceral receptors triggers general reactions of alertness and arousal and evokes unpleasant and poorly localized sensory experiences. This type of response may be a feature of sensory systems dominated by unmyelinated afferent inputs.     

Afferent and efferent components of the facial nerve in the bullfrog (Rana catesbeiana).

The afferent and efferent components of the facial nerve were traced within the brain stem of Rana catesbeiana, using three different neuroanatomical techniques. Primary afferent fibers could be traced to the spinal tract of trigeminal nerve and to fasciculus solitarius as far caudally as the first or second spinal segment, using silver degeneration methods. Cobalt filling of of the entire nerve showed the same distribution of afferent fibers, as well as the filling of the cells within the mesencephalic nucleus of trigeminal, indicating the origin of a proprioceptive component of the facial nerve. Cobalt iontophoresis and horseradish perioxidase experiments showed that the motor nucleus of the facial nerve was located just ventral to the fourth ventricle, and caudal to the motor nucleus of trigeminal. The distribution of afferent fibers to fasciculus solitarius and the spinal tract of trigeminal is similar in some respects to the distribution of afferent fibers from the trigeminal and vagal nerves in the bullfrog. The afferent fibers from the three cranial nerves are found as far caudally in the brain stem as the second spinal segment.     

Projection linkage from spinal neurons to both lateral cervical nucleus and solitary tract nucleus in the cat

Intracellular recordings from the lumbosacral dorsal horn were made to identify the axonal projection and the afferent innervation of the lateral cervical nucleus (LCN) and solitary tract nucleus (STN) on the spinal neurons of chloralose-anesthetized cats. A total of 49 neurons from laminae III-V in the spinal dorsal horn responded to stimulation of both the LCN and STN. Of these, 28 and 21 neurons responded antidromically and orthodromically to stimulation of the LCN and STN, respectively. Seven of the 28 antidromically activated neurons were followed by one or more responses synaptically driven from the LCN and/or STN. The diameter of these ascending or descending fibers was in the range of A delta fibers. The results indicate that (1) some spinal neurons, namely spinocervical tract-spinosolitary tract (SCT-SST) neurons, issue branched axons of A delta-fibers and dually project to both LCN and STN; (2) some SCT-SST neurons receive innervation from both the LCN and STN; (3) some spinal neurons and interneurons are dually innervated by descending fibers originating from both the LCN and STN, and (4) the convergence and integration between somatic and visceral sensory inputs might occur in the SCT-SST neurons.     

Turnover of Axonally Transported Phospholipids in Nerve Endings of Retinal Ganglion Cells

We have investigated the metabolic turnover of axonally transported phospholipids in myelinated axons (optic tract) and nerve endings (superior colliculus) of retinal ganglion cells. One week following intraocular injection of [2-3H]glycerol, turnover rates for individual phospholipid classes in the retina (which contains a number of other cell types in addition to the ganglion cells) were all very similar to each other, with apparent half-lives of approximately 7 days. Apparent half-lives of labeled phospholipids in superior colliculus (presumably primarily in retinal ganglion cell nerve endings) were 10 days for both choline and inositol phosphoglycerides and 13 days for both serine and diacylethanolamine phosphoglycerides. Subcellular fractionation data obtained from superior colliculus at various times after injection suggested that apparent turnover rates determined for nerve ending phospholipids probably were not significantly affected by transfer of axonally transported 3H lipids into myelin. Apparent half-lives for phospholipids in optic tract were somewhat longer than in superior colliculus, ranging from 11 to 18 days. The slower turnover rates in optic tract may, in part, reflect the transfer of some axonal lipids to the more metabolically stable pool of lipids in the myelin ensheathing the retinal ganglion cell axons. In both optic tract and superior colliculus, apparent half-lives for axonally transported phospholipids labeled with [32P]phosphate were only slightly longer than for [2-3H]glycerol, while those for [14C]choline and [3H]acetate were markedly longer, indicating differing degrees of metabolic conservation or reutilization of these precursors relative to glycerol.     

Application of the glycine labelling method to the cerebellum, hippocampus and spinal cord

3H-glycine was applied to the cat cerebellar cortex under resting conditions and during inferior olive stimulation which activated the climbing fiber system on a restricted area. Electric recording was made. The autoradiograms showed, that under resting condition labelled glycine was incorporated mainly in granule, Golgi and basket cells and only a few Purkinje and stellate cells were active. Also cerebellar glomeruli remained without labelling. On climbing fiber stimulation Purkinje cells became activated singly and grouped, also Golgi and stellate cells increased in number. Granule cells were totally inhibited. 3H-glycine, when applied to the rat hippocampus, the dentate gyrus, CA1 and CA4 fields showed labelling at low frequency stimulation. When 400 Hz high frequency stimulation periods were interposed, long-term potentiation ensued. The overall labelling of each hippocampal region was intensified significantly, indicating that glycine incorporation may be a sign not only of excitation but also of long-term potentiation. 3H-glycine was applied to frog spinal cord during rest and dorsal root stimulation. Interneurons and motor neurons excited by the afferent fibers showed intensive glycine uptake. It is concluded that the glycine labelling method is suitable for detecting neural excitation in the structures dealt with in this paper.     

Directed early axonal outgrowth from retinal transplants into host rat brains

Axons from retinae transplanted to the brain stem of neonatal rats exhibit two patterns of outgrowth that can be experimentally uncoupled from each other depending upon the location of the graft. Retinae placed close to the surface of the rostral brain stem (as much as 5 mm from the tectum) emit axons that project toward the superior colliculus along the subpial margin of the rostral brain stem. In contrast, axons from grafts embedded deep within the midbrain parenchyma project through the neuropil directly to the overlying superior colliculus, as long as the retina is within about 1 mm of the tectal surface. The present study shows that, as long as the retina is located outside the superior colliculus, and regardless of whether the axons derive from grafts in subpial or intraparenchymal locations, the earliest projections are oriented towards the superior colliculus. We have also found, however, that axons from retinae transplanted directly onto the superior colliculus can form projections that extend along the subpial margin away from the tectum. There are several major conclusions that may be drawn from these observations. First, the final tectopetal, transplant-derived projection does not result from the reorganization of an initially random outgrowth but is directed from the start toward an appropriate region of termination. Second, it appears that the interaction of retinal axons with a primary target alters the ability of the growth cone to respond to directional cues along the optic tract. Thus, although adding support to the proposal that optic axons attain the superior colliculus through an interaction involving substrates distributed along the optic tract and diffusible factors originating in the target region, it is increasingly clear that such interactions are likely to be complex and hierarchical.     

The spinocervical tract as a possible pathway for muscular nociception.

In decerebrate and chloralose-anaesthetized spinal cats, micro-electrode recordings were made from axons of the spinocervical tract (SCT) during intra-arterial injection of bradykinin, 5-hydroxytryptamine and potassium into the gastrocnemius-soleus (GS) muscle. The results suggest that muscular group IV and group III afferent units which respond to the above painful stimulation project into the SCT. The portion of SCT units that could be activated by injections of these algesic substances into the GS-muscle was different in spinal and decerebrate preparations, showing that the SCT neurones are subject to a descending influence. It is concluded that the spinocervical tract, in addition to its function as a cutaneous sensory pathway, may serve as a pathway for muscular nociception in the cat.     

Maternal care effects on the development of a sexually dimorphic motor system: The role of spinal oxytocin

Maternal licking in rats affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus that controls penile reflexes involved with copulation. Reduced maternal licking results in decreased motoneuron number, size, and dendritic length in the adult SNB, as well as deficits in adult male copulatory behavior. Our previous findings that licking-like tactile stimulation influences SNB dendritic development and upregulates Fos expression in the lumbosacral spinal cord suggest that afferent signaling is changed by differences in maternal stimulation. Oxytocin afferents from the hypothalamus are a possible candidate, given previous research that has shown oxytocin is released following sensory stimulation, oxytocin modulates excitability in the spinal cord, and is a pro-erectile modulator of male sex behavior. In this experiment, we used immunofluorescence and immediate early gene analysis to assess whether licking-like tactile stimulation of the perineum activated parvocellular oxytocinergic neurons in the hypothalamus in neonates. We also used enzyme immunoassay to determine whether this same stroking stimulation produced an increase in spinal oxytocin levels. We found that stroking increased Fos immunolabeling in small oxytocin-positive cells in the paraventricular nucleus of the hypothalamus, in comparison to unstroked or handled control pups. In addition, 60 s of licking-like perineal stimulation produced a transient 89% increase in oxytocin levels in the lumbosacral spinal cord. Together, these results suggest that oxytocin afferent activity may contribute to the effects of early maternal care on the masculinization of the SNB and resultant male copulatory behavior.     

Cortical localisation of magnetic fields evoked by oesophageal distension

Magnetoencephalographic source localisation techniques were used to measure oesophageal evoked magnetic fields from the cerebral cortex in 3 subjects. By using rapid balloon distension as a stimulus, a comparison of proximal and distal oesophageal cortical representation was made. The distal oesophagus was represented bilaterally in the insular cortex and SII as well as the inferior aspect of SI. The proximal oesophagus was represented unilaterally in superior and inferior SI, insular cortex and SII. Significantly, the superior portion of SI was consistently activated in subjects following stimulation of the proximal oesophagus, but similar activation was not found in response to distal stimulation. This may reflect the contribution from somatic afferent fibres in the striate muscle of the proximal segment. In conclusion, vagal afferents appear to contribute more to cortical activation following stimulation of the distal rather than the proximal oesophagus, while spinal afferents appear to be activated by both proximal and distal oesophageal stimulation.     

Orienting responses and vocalizations produced by microstimulation in the superior colliculus of the echolocating bat, Eptesicus fuscus

An echolocating bat actively controls the spatial acoustic information that drives its behavior by directing its head and ears and by modulating the spectro-temporal structure of its outgoing sonar emissions. The superior colliculus may function in the coordination of these orienting components of the bat's echolocation system. To test this hypothesis, chemical and electrical microstimulation experiments were carried out in the superior colliculus of the echolocating bat, Eptesicus fuscus, a species that uses frequency modulated sonar signals. Microstimulation elicited pinna and head movements, similar to those reported in other vertebrate species, and the direction of the evoked behaviors corresponded to the site of stimulation, yielding a map of orienting movements in the superior colliculus. Microstimulation of the bat superior colliculus also elicited sonar vocalizations, a motor behavior specific to the bat's acoustic orientation by echolocation. Electrical stimulation of the adjacent periaqueductal gray, shown to be involved in vocal production in other mammalian species, elicited vocal signals resembling acoustic communication calls of E. fuscus. The control of vocal signals in the bat is an integral part of its acoustic orienting system, and our findings suggest that the superior colliculus supports diverse and species-relevant sensorimotor behaviors, including those used for echolocation.     

Modulation of spinal cord transmission by changes in extracellular K+ activity and extracellular volume

The neuronal activity in spinal cord in response to electrical or adequate stimulation of afferent fibres increases extracellular K+ activity. The increase during a stimulation can reach 9-10 mM (so-called ceiling level) and persists for some time even when a stimulation is discontinued. The activation of a neuronal Na-K pump is a limiting factor in stimulation-evoked increase in extracellular K+ activity and in the time course of its recovery to the resting level. Drugs that affect either the neuronal activity (picrotoxin, strychnine, GABA, 5-HT) or activity of Na-K ATPase (oubain, naloxone, morphine, enkephalins) substantially change the K+ transience. Repetitive electrical stimulation of low threshold cutaneous afferents at frequency 1-100 Hz induced transient shrinkage of extracellular space in spinal dorsal horns by 5-75%. The increase in extracellular K+ activity depolarizes the membranes of neurones, glial cells, and primary afferent fibres and may eventually lead to either facilitation or inhibition of synaptic transmission. It is also suggested that the transient poststimulation changes in extracellular volume may alter synaptic potency in spinal cord.     

Visual corticopontine and tectopontine projections in the macaque.

We studied projections from extrastriate visual areas and the superior colliculus to the pontine nuclei of monkeys using degeneration staining and transport of wheatgerm agglutinin horseradish peroxidase, and 3H amino acids. The superior colliculus and the extrastriate cortical visual areas both project to the ipsilateral dorsolateral region of the pontine nuclei. The projections from extrastriate visual cortex occupy a much larger territory within the pontine nuclei than those from the superior colliculus. The superficial laminae of the superior colliculus project only to the ipsilateral pontine nuclei. The projection to the contralateral nucleus reticularis tegmenti pontis arises from cells in deeper laminae within the superior colliculus.     

Organization of the afferent input in the rabbit superior colliculus

The organization of the afferent input into the superior colliculus was investigated in unanesthetized curarized rabbits. The afferent signal reaches the rabbit superior colliculus via at least two groups of fibers with mean conduction velocities of 3 and 6 m/sec. The components C₁ and C₂ of the evoked potential reflect postsynaptic unit activity arising after the arrival of impulses along these groups of fibers. This is confirmed by investigation of the shape of the evoked potential after stimulation of different points of the afferent pathway, data on conduction velocities, and comparison of single unit activity with the phases of evoked potential. The presence of components of this potential with short latent periods is evidence of predominance of the direct retinotectal input in the rabbit, a fact which may be connected with the lissencephalic type of brain development.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 9, No. 3, pp. 281–289, May–June, 1977.     

Interactions between spinal cord stimulation and activity blockade in the regulation of synaptogenesis and motoneuron survival in the chick embryo

The present study investigated the effects of spinal cord stimulation, neuromuscular blockade, or a combination of the two on neuromuscular development both during and after the period of naturally occurring motoneuron death in the chick embryo. Electrical stimulation of the spinal cord was without effect on motoneuron survival, synaptogenesis, or muscle properties. By contrast, activity blockade rescued motoneurons from cell death and altered synaptogenesis. A combination of spinal cord stimulation and activity blockade resulted in a marked increase in motoneuron death, and also altered synaptogenesis similar to that seen with activity blockade alone. Perturbation of normal nerve–muscle interactions by activity blockade may increase the vulnerability of developing motoneurons to excessive excitatory afferent input (spinal cord stimulation) resulting in excitotoxic-induced cell death. © 1993 John Wiley & Sons, Inc.     

c-Fos expression in the central nervous system elicited by phrenic nerve stimulation.

Phrenic nerve afferents (PNa) have been shown to activate neurons in the spinal cord, brain stem, and forebrain regions. The c-Fos technique has been widely used as a method to identify neuronal regions activated by afferent stimulation. This technique was used to identify central neural areas activated by PNa. The right phrenic nerve of urethane-anesthetized rats was stimulated in the thorax. The spinal cord and brain were sectioned and stained for c-Fos expression. Labeled neurons were found in the dorsal horn laminae I and II of the C3-C5 spinal cord ipsilateral to the site of PNa stimulation. c-Fos-labeled neurons were found bilaterally in the medial subnuclei of the nucleus of the solitary tract, rostral ventral respiratory group, and ventrolateral medullary reticular formation. c-Fos-labeled neurons were found bilaterally in the paraventricular and supraoptic hypothalamic nuclei, in the paraventricular thalamic nucleus, and in the central nucleus of the amygdala. The presence of c-Fos suggests that these neurons are involved in PNa information processing and a component of the central mechanisms regulating respiratory function.     

Peripheral and Spinal Inputs to Physiologically Identified Thalamic and Nonthalamic Relay Neurons in Cat Cuneate Nucleus

A single-unit population study of the feline cuneate nucleus was carried out to identify principal neuron types, their distribution within the nucleus, pattern of peripheral activation, and receptive field characteristics. Units were also tested for response to isolated dorsal column or dorsolateral funicular electrical stimulation. The nucleus was explored in a uniform pattern, and sample size was optimized by applying the search stimulus shocks to the dorsal spinal cord. Single units were defined as spinal afferents, cuneothalamic-relay (CTR) neurons, and non-cuneothalamic-relay (non-CTR) neurons. The following features were observed: (1) The distribution within the nucleus of specific cell types agreed with cytoarchitectural studies: Spinal afferent fibers were superficial and caudal; 22% of neurons were CTR neurons; CTR neurons were most dense in the middle of the nucleus and were largely separate from non-CTR neurons. (2) Of the 58 neurons tested for response to isolated dorsal column and dorsolateral funicular stimulation, 24% were activated from both tracts. (3) Convergent input from the off-focus periphery (defined as other than the ipsilateral forelimb) was detected in both CTR and non-CTR neurons, most commonly from the contralateral forepaw. Several neurons were activated from three limbs. (4) Thirty-seven percent of units were unresponsive to hair movement, touch, muscle palpation, or movement of joints. Compared to spinal fibers and non-CTR neurons, CTR neurons were most likely to have an identifiable input.     

Tectal and Related Target Areas of Spinal and Dorsal Column Nuclear Projections in Hedgehog Tenrecs

The terminal distributions of spinal and dorsal column nuclear projections to tectum, pretectum, and central gray of hedgehog tenrecs (Echinops telfairi and Setifer setosus) were investigated using anterograde axonal flow and various tracer substances. In the inferior colliculus, the densest and most extensive mesencephalic projections were found within the pericentral regions. One target area, referred to as the external portion of the inferior colliculus, was represented as a semicircle of grain patches lateral and caudal to the central nucleus. This region received somesthetic afferents from the dorsal column nuclei and from spinal segments at various levels. In contrast, after high cervical injections, the pericentral portion dorsomedial to the rostral half of the central nucleus was labeled almost exclusively. This area of labeling was distinct from the labeling in the central gray and might be best compared with the intercollicular zone in other species. The superior colliculus received projections predominantly from the high cervical cord; minor projections also arose from lumbar spinal segments and the dorsal column nuclei. The terminal field covered roughly the caudal half of the colliculus and involved the stratum griseum intermediale in a patch-like fashion. Some labeling was also found in the stratum griseum profundum and in the stratum griseum superficiale. Other than in the colliculi, weak pretectal projections were observed following dorsal column nuclear injections, while the nucleus of Darkschewitsch was labeled best following lumbosacral injections. All mesencephalic target areas were labeled consistently on the contralateral side, while their ipsilateral side was involved to a varying degree: The relatively most prominent ipsilateral labeling was seen in the central gray, being roughly similar on both sides; scarcely any labeling was noted in the ipsilateral superior colliculus. Tectal injections of retrograde tracer, in addition, revealed a considerable number of labeled neurons in a relatively cell-poor region immediately ventral to the high cervial dorsal horn. This region might correspond to the lateral cervical nucleus, an aggregation of neurons that so far has only been demonstrated in higher mammals.     

Tachykinins as modulators of the micturition reflex in the central and peripheral nervous system

In the normal urinary bladder, tachykinins (TKs) are expressed in a population of bladder nociceptors that is sensitive to the excitatory and desensitizing effects of capsaicin (i.e., capsaicin-sensitive primary afferent neurons (CSPANs)). Several endobiotics or xenobiotics excite CSPANs and release TKs and other mediators at both the peripheral and spinal cord level. The peripheral release of TKs determines a set of responses (known as neurogenic inflammation) that includes vasodilatation, plasma protein extravasation, smooth muscle contraction and stimulation of afferent nerves. Following chronic inflammation, both immune cells and capsaicin-resistant sensory neurons can de novo express TKs: whether these pools of TKs are releasable and contribute to inflammatory processes is presently unsettled. At the spinal cord level, the release of TKs contributes in determining an altered pattern of vesicourethral reflexes in response to nociceptive stimulation of the bladder by conveying: (a) the afferent transmission to supraspinal sites, and (b) descending or sensory inputs to the sacral parasympathetic nucleus (SPN). Recent evidence also attribute a synergetic role of TKs in the supraspinal modulation of the sensory arm of the micturition reflex.The overall available information suggests that TK receptor antagonists may affect bladder motility/reflexes which occur during different pathological states, while having little influence on the normal motor bladder function.