A light and electron microscopic analysis of the sacral parasympathetic nucleus after labelling primary afferent and efferent elements with HRP

Comparative localization of colorectal sensory afferent central projections in the mouse spinal cord dorsal horn and caudal medulla dorsal vagal complex

The distal colon and rectum (colorectum) are innervated by spinal and vagal afferent pathways. The central circuits into which vagal and spinal afferents relay colorectal nociceptive information remain to be comparatively assessed. To address this, regional colorectal retrograde tracing and colorectal distension (CRD)-evoked neuronal activation were used to compare the circuits within the dorsal vagal complex (DVC) and dorsal horn (thoracolumbar [TL] and lumbosacral [LS] spinal levels) into which vagal and spinal colorectal afferents project. Vagal afferent projections were observed in the nucleus tractus solitarius (NTS), area postrema (AP), and dorsal motor nucleus of the vagus (DMV), labeled from the rostral colorectum. In the NTS, projections were opposed to catecholamine and pontine parabrachial nuclei (PbN)-projecting neurons. Spinal afferent projections were labeled from rostral through to caudal aspects of the colorectum. In the dorsal horn, the number of neurons activated by CRD was linked to pressure intensity, unlike in the DVC. In the NTS, 13% ± 0.6% of CRD-activated neurons projected to the PbN. In the dorsal horn, at the TL spinal level, afferent input was associated with PbN-projecting neurons in lamina I (LI), with 63% ± 3.15% of CRD-activated neurons in LI projecting to the PbN. On the other hand, at the LS spinal level, only 18% ± 0.6% of CRD-activated neurons in LI projected to the PbN. The collective data identify differences in the central neuroanatomy that support the disparate roles of vagal and spinal afferent signaling in the facilitation and modulation of colorectal nociceptive responses.

Pitx2 cholinergic interneurons are the source of C bouton synapses on brainstem motor neurons

Cholinergic neuromodulation has been described throughout the brain and has been implicated in various functions including attention, food intake and response to stress. Cholinergic modulation is also thought to be important for regulating motor systems, as revealed by studies of large cholinergic synapses on spinal motor neurons, called C boutons, which seem to control motor neuron excitability in a task-dependent manner. C boutons on spinal motor neurons stem from spinal interneurons that express the transcription factor Pitx2. C boutons have also been identified on the motor neurons of specific cranial nuclei. However, the source and roles of cranial C boutons are less clear. Previous studies suggest that they originate from Pitx2+ and Pitx2- neurons, in contrast to spinal cord C boutons that originate solely from Pitx2 neurons. Here, we address this controversy using mouse genetics, and demonstrate that brainstem C boutons are Pitx2+ derived. We also identify new Pitx2 populations and map the cholinergic Pitx2 neurons of the mouse brain. Taken together, our data present important new information about the anatomical organization of cholinergic systems which impact motor systems of the brainstem. These findings will enable further analyses of the specific roles of cholinergic modulation in motor control.

Swimming against the tide: investigations of the C-bouton synapse

C-boutons are important cholinergic modulatory loci for state-dependent alterations in motoneuron firing rate. m2 receptors are concentrated postsynaptic to C-boutons, and m2 receptor activation increases motoneuron excitability by reducing the action potential afterhyperpolarization. Here, using an intensive review of the current literature as well as data from our laboratory, we illustrate that C-bouton postsynaptic sites comprise a unique structural/functional domain containing appropriate cellular machinery (a “signaling ensemble”) for cholinergic regulation of outward K⁺ currents. Moreover, synaptic reorganization at these critical sites has been observed in a variety of pathologic states. Yet despite recent advances, there are still great challenges for understanding the role of C-bouton regulation and dysregulation in human health and disease. The development of new therapeutic interventions for devastating neurological conditions will rely on a complete understanding of the molecular mechanisms that underlie these complex synapses. Therefore, to close this review, we propose a comprehensive hypothetical mechanism for the cholinergic modification of α-MN excitability at C-bouton synapses, based on findings in several well-characterized neuronal systems.

Differential synaptic inputs to the cell body and proximal dendrites of preganglionic parasympathetic neurons in the rat conus medullaris

Preganglionic parasympathetic neurons (PPNs) reside in the intermediolateral (IML) nucleus of the rat lumbosacral spinal cord and contribute to the autonomic control of visceral pelvic organs. PPNs provide the final common pathway for efferent parasympathetic information originating in the spinal cord. We examined the detailed ultrastructure of the type and organization of synaptic inputs to the cell body and proximal dendrites of PPNs in the rat conus medullaris. The PPNs were retrogradely labeled by a systemic administration of the B subunit of cholera toxin conjugated to horseradish peroxidase. We demonstrate four distinct types of synaptic boutons in apposition with PPN somata and proximal dendrites: S-type boutons show clear, spheroid vesicles; F-type boutons show flattened vesicles; dense-cored vesicle-type (DCV-type) boutons show a mixture of clear and dense-cored vesicles; L-type boutons were rare, but large, exhibited clear spheroid vesicles, and were only encountered in apposition with the PPN dendrites in our sample. The membrane surface covered by apposed boutons was markedly higher for the proximal dendrites of PPNs, compared with their somata. The inhibitory synaptic influence was markedly higher over the PPN somata compared with their proximal dendrites, as suggested by the higher proportion of putative inhibitory F-type boutons in apposition with the soma and a higher frequency of S-type boutons per membrane length for the proximal dendrites. Our studies suggest that the synaptic input to PPNs originates from multiple distinct sources and is differentially distributed and integrated over the cell membrane surface.

Spinal neurons activated in response to pudendal or pelvic nerve stimulation in female rats

The overlapping distribution of spinal neurons activated with either pudendal sensory nerve or pelvic nerve stimulation was examined in the female rat using c-fos immunohistochemistry. Pudendal sensory nerve stimulation resulted in a significant increase in fos-positive cells in the ipsilateral dorsal horn and bilaterally in the medial, lateral and intermediate gray of L5-S1. Pelvic nerve stimulation resulted in significant increases of c-fos immunoreactive nuclei in the ipsilateral dorsal horn, lateral and intermediate gray and bilaterally in the medial gray of L5-S1. Co-distribution of fos immunoreactive nuclei with the vesicular glutamate transporters (VGlut2 and VGlut3) and neurokinin I receptors were found in distinct regions of the dorsal horn, medial and lateral gray. Specific areas in the medial dorsal horn, dorsal gray commissure, laminae VI and X and dorsal lateral gray were activated after stimulation of the pudendal sensory and pelvic nerves, suggesting these areas contain spinal neurons that receive both somatomotor and visceral inputs and are part of the intraspinal circuit that regulates sexual and voiding function.

Prenatal development of transient receptor potential vanilloid 1-expressing primary sensory projections to sacral autonomic preganglionic neurons

The visceral reflexes of the pelvic organs are mediated by connections between primary afferents innervating the pelvic organs and parasympathetic preganglionic neurons in the intermediolateral column of the sacral spinal cord. The present immunohistochemical study revealed many varicosities expressing transient receptor potential vanilloid 1 (TRPV1) that were closely apposed to the preganglionic neuronal perikarya at embryonic day 16 in mice. Many, but not all, varicosities expressing TRPV1 in the intermediolateral column were also immunopositive for calcitonin gene-related peptide. In contrast, no nerve fibers expressing TRPV1 projected to the sympathetic preganglionic cell column in the lumbar spinal cord in prenatal stages. The results of the present study raised the possibility that the primary afferents transmit signals elicited by the activation of TRPV1 receptors to the sacral parasympathetic preganglionic neurons. Thus, the functional circuit for pelvic spinal reflexes, such as micturition induced by urine influx, might develop in the prenatal stages in mice.

Effect of nor-trimebutine on neuronal activation induced by a noxious stimulus or an acute colonic inflammation in the rat

Nor-trimebutine is the main metabolite of trimebutine that is used in the treatment of patients with irritable bowel syndrome. Nor-trimebutine has a blocking activity on sodium channels and a potent local anesthetic effect. These properties were used to investigate the effect of nor-trimebutine on spinal neuronal activation induced by models of noxious somato-visceral stimulus and acute colonic inflammation. Nor-trimebutine was administered in rats either subcutaneously 30 min before intraperitoneal administration of acetic acid or intracolonically 30 min before intracolonic infusion of trinitrobenzenesulfonic acid. Abdominal contractions were counted for 1 h as a marker of abdominal pain. c-fos expression was used as a marker of neuronal activation and revealed by immunohistochemistry 1h after intraperitoneal acetic acid injection and 2 h after colonic inflammation. Nor-trimebutine decreased Fos expression in the thoraco-lumbar (peritoneal irritation) and lumbo-sacral (colonic inflammation) spinal cord in laminae I, IIo V, VII and X. This effect was also observed in the sacral parasympathetic nucleus after colonic inflammation. Nor-trimebutine induced a significant decrease of abdominal contractions following intraperitoneal acetic acid injection. These data may explain the effectiveness of trimebutine in the therapy of abdominal pain in the irritable bowel syndrome.

Structural mechanisms to produce differential dendritic gains

The axons of sacral parasympathetic preganglionic neurons (PGNs) originate on a primary dendrite between 10 and 110 mum from the soma. Therefore, it was hypothesized that the location of the axon origin would impact the relative efficacy of ipsilateral and contralateral synaptic inputs. The morphology of two PGNs was reconstructed, and the transfer impedance was used to quantify the influence of synaptic inputs on the transmembrane potential at the axon initial segment. The ratio of ipsilateral transfer impedance to contralateral transfer impedance (termed the relative gain) was increased by 14-29% for axons originating from the dendrite vs. axons originating from the soma. The addition of 50 synchronized "gating" synapses on the proximal dendrites increased the relative gain by 17-38% when the axon originated from the dendrite, but only by 11-15% when the axon originated from the soma. The efficacy of synaptic inputs and the ability of proximal gating synapses to regulate synaptic efficacy were strongly influenced by the site of origin of the axon. The position of axon origin is an effective structural mechanism to regulate the relative efficacy of synaptic inputs arriving at different locations on the dendritic tree.

Central projections of thoracic splanchnic and somatic nerves and the location of sympathetic preganglionic neurons in Xenopus laevis

The central and peripheral organization of thoracic visceral and somatic nervous elements was studied by applying dextran amines to the proximal cut ends of the thoracic splanchnic and somatic nerves in Xenopus laevis. Many labeled dorsal root ganglion cells of visceral afferents, and all somatic afferents, were located in a single ganglion of one spinal segment, and the two types of cells were distributed topographically within the ganglion. The labeled sympathetic preganglionic neurons were located predominantly in the same area of the thoracic spinal gray as in other frogs and in mammals. The labeled visceral afferents projected to Lissauer's tract and the dorsal funiculus. The visceral fibers of the tract ascended to the level of the subcerebellar area, supplying collateral branches to the lateral one-third of the dorsal horn and to the area of brainstem nuclei, including lateral cervical and descending trigeminal nucleus, and descended to the filum terminale. The visceral fibers of the dorsal funiculus were distributed to the dorsal column nucleus and the solitary tract. A similar longitudinal projection was also seen in the somatic afferents. The dual central pathway of thoracic primary afferents in the anuran spinal cord is a property held in common with mammals, but the widespread rostrocaudal projection through Lissauer's tract may be a characteristic of the anuran central nervous system. In frogs, the direct transmission of primary afferent information to an extremely wide area of the central nervous system may be important for prompt assessment of environmental factors and control of body functions.

Quantitative analysis of the dendrites of sacral preganglionic neurons in the cat

Quantitative analyses were performed on the dendrites and somata of 25 electrophysiologically identified preganglionic neurons (PGN) obtained from the sacral spinal cord of the cat by intracellular injection of Neurobiotin or horseradish peroxidase. Total dendritic length and surface area were measured for each dendrite. The sizes of the stem dendrites measured at their base were positively correlated with the sizes of the entire tree and numbers of end branches. Total surface area of somata and dendrites averaged 39,138 microm(2); 90.7% of that was from the dendrites. To obtain measurements of the relative contributions of PGN dendrites to specific regions of the spinal cord, the percentage of each dendrite occupying eight spinal cord regions was recorded. Sixty-three percent of the dendrites projected dorsal to their somata, whereas an average of 33.3% of dendrites were located in the white matter, most of them in the lateral and dorsolateral funiculi. The neurons within this sample formed a continuum with some neurons having a large percentage of dendrites in lamina I but little in the white matter, whereas at the other end of the continuum were cells with the reverse configuration. The intermediate neurons had dendrites in both locations. Taken together, these data indicate a heterogeneous population of PGN in the lateral band of the sacral parasympathetic nucleus.

Role of the target in the pathfinding of facial visceral motor axons

Axon navigation depends, in part, on guidance cues emanating from the target. We have investigated the possible role of the target in the pathfinding of visceral motor axons to cranial parasympathetic ganglia. Mice homozygous for a tau-LacZ transgene targeted in the Phox2a locus lack the sphenopalatine ganglion, which is the normal target of visceral motor axons of the facial nerve. We found that in these mutants, facial visceral motor axon pathfinding was disrupted, and some axons were misrouted to an alternative parasympathetic ganglion. Moreover, the absence of correct facial visceral motor pathways was concomitant with defects in the pathfinding of rostrally-projecting sympathetic axons.

Increased c-fos expression in spinal lumbosacral projection neurons and preganglionic neurons after irritation of the lower urinary tract in the rat

Chemical irritation of the lower urinary tract (LUT) induces c-fos expression in neurons in the lumbosacral (L(6) and S(1)) spinal cord. This study used axonal tracing with fluorescent dyes to identify the types of spinal neurons expressing Fos immunoreactivity (IR) after LUT irritation in the rat. Fos-IR was detected in lateral and medial superficial dorsal horn, the sacral parasympathetic nucleus (SPN) and lamina X around the central canal. Fos-IR was detected in spinal neurons projecting to supraspinal sites (brainstem and hypothalamus), in preganglionic neurons (PGN) and in unlabeled segmental interneurons. A substantial percentage (20%) of dye labeled PGN exhibited Fos-IR after LUT irritation; and a larger percentage (36%) exhibited Fos-IR after electrical stimulation of the pelvic nerve which contains afferent pathways from all of the pelvic organs. The majority (average 55%) of Fos-positive neurons projecting to supraspinal sites were also located in the region of the SPN. A selective distribution of different types of neurons was detected in this region: PGN were located ventral to the spinal projection neurons which in turn were located ventral to the majority of unidentified Fos-positive neurons. The distribution of Fos-positive PGN and projection neurons was similar in spinal intact and spinal transected animals indicating that c-fos expression was mediated by monosynaptic afferent input or input from segmental interneurons and was not due to activation of supraspinal micturition reflex pathways.

Endogenous lectin (RL-29) expression of the autonomic preganglionic neurons in the rat spinal cord

Rat spinal neurons expressing lectin RL-29 are visualized immunohistochemically. RL-29 immunoreactive (RL-29 IR) neurons are found in the lateral parts of laminae V-VII, designated as the intermediolateral cell column (IML) in the thoracic cord, the sacral parasympathetic nucleus (SPN) in the lumbosacral cord, and the dorsomedial nucleus (DMN) of the ventral horn. The majority of RL-29 IR neurons in the SPN are also labeled by a retrograde tracer DAPI applied to the cut L6-S1 ventral roots. These data indicate that the majority of RL-29 IR neurons in the SPN are autonomic preganglionic neurons, thus suggesting that RL-29 can be a useful tool in marking this subpopulation of neurons. In addition, the presence of previously described RL-29 IR primary afferent fibers and terminals in the dorsal parts of the cord are confirmed.

Comparative analysis of nitric oxide synthase immunoreactivity in the sacral spinal cord of the cat, macaque and human

Nitric oxide synthase immunoreactivity (bNOS-ir) was examined in the sacral spinal cord of the cat, macaque monkey and human using an antibody to the c-terminal region of neuronal NOS. In S2 of all 3 species NOS-ir was identified in both dorsal and ventral horns. In cat, monkey and human, bNOS-r occurred in sensory neurons of superficial laminae and the base of the dorsal horn, in autonomic neurons around the central canal and in the intermediolateral sacral spinal nucleus. In all 3 species, a large proportion of somatic motor nuclei in the ventromedial (VM), ventrolateral (VL) nuclei, and Onuf's nucleus (ON) showed high bNOS-ir, while others exhibited markedly lower immunoreactivity. Validatory experiments showed separate cellular localisation for bNOS, inducible NOS (iNOS), and endothelial NOS (eNOS) with only bNOS being localised to neuronal perikarya. Comparative morphometric analyses of the relative proportions and diameters of motor neurons in the VL, VM and ON exhibiting high and low levels of bNOS-ir revealed statistically significant differences in proportions in individual nuclei, and differences in size were generally not statistically significant. Finally, a comparison between cat sacral and thoracic spinal cord showed bNOS-ir in motor neurons of S2 was subject to less animal and rostrocaudal segment variation than in T10.

Ultrastructural evidence for a direct pathway from the pontine micturition center to the parasympathetic preganglionic motoneurons of the bladder of the cat

Light microscopy tracing studies have provided evidence that the pontine micturition center (PMC) projects to the area of the intermediolateral cell column of the sacral spinal cord. Although this region contains parasympathetic preganglionic motoneurons of the bladder and colon, it also contains many local interneurons and neurons projecting to supraspinal levels. The present study demonstrates that neurons in the PMC indeed project to preganglionic bladder motoneurons. Wheat germ agglutinin horseradish peroxidase was injected in the PMC and cholera toxin B subunit was injected in the bladder wall. Many anterogradely labeled fibers from the PMC were found to terminate on somata and dendrites of the retrogradely labeled preganglionic bladder motoneurons. The terminals were filled with many round vesicles and possessed an asymmetric synaptic cleft, suggesting an excitatory function.

Ganglionic axons in motor roots and pia mater

In addition to motor axons and preganglionic axons, ventral roots contain unmyelinated or thin myelinated sensory axons and postganglionic sympathetic axons. It has been said that ventral roots channel sensory axons to the CNS. However, it now seems that these axons end blindly, shift to the pia or loop and return towards the periphery and that these units reach the CNS via dorsal roots. Sensory ventral root axons project from a variety of somatic or visceral receptors; some of them are third branches of dorsal root afferents and some seem to lack a CNS projection. Many ventral root afferents contain substance P (SP) and/or calcitonin gene-related peptide (CGRP). These fibres are not affected by neonatal capsaicin treatment and they cannot induce radicular or pial extravasation. Some thin ventral root axons are sympathetic and relate to blood vessels. Afferents containing SP and/or CGRP and sympathetic axons also occur in the spinal pia mater. The sensory axons mediate pain. They might also have vasomotor, tissue-regulatory and/or mechanoreceptive functions. The motor roots of cranial nerves IV, VI and XI contain unmyelinated axons arranged like in ventral roots outside the autonomic outflow. However, the motor root of cranial nerve V channels some unmyelinated axons into the CNS. The occurrence of thin axons in ventral roots and pia mater changes during development and ageing. After peripheral nerve injury, ipsilateral ventral roots and pia are invaded by new sensory and postganglionic sympathetic axons.

Quantitative electromicroscope study of the oculomotor parasympathetic neurons projecting to the ciliary ganglion in cats: comparison of the synaptic (axon-somatic and axo-proximal dendritic) organization of anterior-dorsal and ventral cell groups

The synaptic organization of the oculomotor parasympathetic preganglionic neurons (OPNs), labeled retrogradely after a horseradish peroxidase (HRP) injection into the ciliary ganglion, was studied in cats by electron microscopy. We divided the OPNs into two groups, anterior-dorsal (ADG) and ventral (VG) cell groups, based upon physiological studies in cats suggesting that accomodation-related OPNs are predominantly located anterior and dorsal to the somatic nuclei of the oculomotor nuclear complex (i.e., the anteromedian and Edinger-Westphal nuclei, and the ventral central gray area), while pupillo-constriction-related OPNs are predominantly located ventral to the somatic nuclei (i.e., the ventral tegmental area). The synaptic organization of these two groups was quantitatively compared, using a nested analysis of variance to determine statistical significance (P < 0.05). Partial reconstructions of the labeled somata and proximal dendrites were made from tracings of electron micrographs of every 2nd section in serial ultrathin sections that included the nucleolus or were adjacent to sections that included the nucleolus. The mean number of boutons of apposition on a reconstructed labeled soma of VG was significantly greater than that of ADG (mean +/- SD; ADG, 5.3 +/- 3.3; VG, 8.6 +/- 3.2). The mean synaptic density on a VG soma was significantly greater than on an ADG soma (mean +/- SD; ADG, 3.74 +/- 2.11 counts/100 microns2; VG, 6.30 +/- 1.99 counts/100 microns2). The mean synaptic covering ratio on a VG soma was significantly greater than on an ADG soma (mean +/- SD; ADG, 5.21 +/- 2.91%; VG, 10.14 +/- 3.76%). The mean estimated number of boutons of apposition on a VG soma was significantly greater than on an ADG soma (mean +/- SD: ADG, 53 +/- 36; VG, 100 +/- 48). Boutons were classified on the basis of the shape of their synaptic vesicles as S-type (containing spherical clear synaptic vesicles) or P-type (containing both flattened and spherical clear synaptic vesicles). The significantly greater than on an ADG soma (mean +/- SD; ADG, 0.31 +/- 0.20; VG, 0.67 +/- 0.18). The differences demonstrated in this study reinforce, morphologically, the assumption of functional localization of OPNs, and further allow us to estimate the relative characteristics of the synaptic organization of accommodation-related OPNs and pupillo-constriction-related OPNs.

Distribution and morphology of sacral spinal cord neurons innervating pelvic structures in Xenopus laevis

Relatively little is known about the organization of neural input to pelvic viscera in amphibia. In this study, sacral spinal efferent neurons were labeled in Xenopus laevis frogs by application of horseradish peroxidase (HRP) to the tenth spinal nerve, to pelvic musculature, or to the pelvic nerve. DiI was applied to the pelvic nerve with similar results. Labeled spinal neurons were located in the intermediate gray or in the ventral horn. Neurons in the tenth dorsal root ganglion, but not in the spinal cord, were labeled after application of HRP or DiI to the pudendal nerve. The labeled neurons in the spinal cord intermediate gray were in a position comparable to that of the mammalian sacral parasympathetic nucleus (SPN). Two apparent subdivisions included 1) a medial cluster of cells with mediolaterally oriented dendrites and 2) a lateral group with dorsoventrally oriented dendrites. An intermediate group, not clearly classed with the other two, was also identifiable. In some cases, labeled tenth nerve primary afferents were seen in contact with efferent neurons of the intermediate gray. Labeled neurons in the ventral horn medial to the lateral motor column were small, with dendrites oriented mediolaterally, in a position comparable to that of the mammalian Onuf's nucleus. The peripheral targets of DiI-labeled pelvic nerve axons were the compressor cloaca muscle, cloaca, and bladder. DiI-labeled pudendal nerve axons distributed peripherally to cloacal lip and medial thigh integument. These data suggest that the pudendal nerve in amphibians is purely sensory and that both somatic and autonomic motor axons traverse the pelvic nerve.

Serotoninergic, peptidergic and GABAergic innervation of the ventrolateral and dorsolateral motor nuclei in the cat S1/S2 segments: an immunofluorescence study

Indirect single- and double-staining immunofluorescence techniques were used to study the serotoninergic, peptidergic and GABAergic innervation of the ventrolateral (Onuf's nucleus) and dorsolateral (innervating intrinsic foot sole muscles) nuclei, located in the S1/S2 segments of the cat spinal cord. The relative density of 5-hydroxytryptamine-, thyrotropin-releasing hormone-, substance P- and gamma-aminobutyric acid-immunoreactive axonal varicosities was similar in both nuclei. The highest relative density was recorded for varicosities immunoreactive to gamma-aminobutyric acid, while those immunoreactive to 5-hydroxytryptamine or thyrotropin-releasing hormone yielded the lowest values. The density of enkephalin-immunoreactive varicosities was higher in the ventrolateral than in the dorsolateral nucleus. Calcitonin gene-related peptide-like immunoreactivity could be seen in neurons of the ventrolateral and dorsolateral nuclei. Occasionally, calcitonin gene-related peptide-immunoreactive axonal fibers were also encountered in these nuclei. Virtually all thyrotropin-releasing hormone-immunoreactive varicosities in the ventrolateral and dorsolateral nuclei also contained 5-hydroxytryptamine-like immunoreactivity, while a somewhat smaller number of them were co-localized with substance P. About 5-10% of the 5-hydroxytryptamine-immunoreactive varicosities were devoid of peptide-like immunoreactivity, and the number of 5-hydroxytryptamine-immunoreactive varicosities lacking thyrotropin-releasing hormone-like immunoreactivity was higher in the dorsolateral than in the ventrolateral nucleus. Finally, the free fraction of substance P-immunoreactive varicosities, i.e., those lacking both 5-hydroxytryptamine and thyrotropin-releasing hormone, was about 39% in the ventrolateral and 26% in the dorsolateral nucleus. Spinal cord transection at the lower thoracic level induced a depletion of 5-hydroxytryptamine and thyrotropin-releasing hormone-immunoreactive fibers from the ventrolateral and dorsolateral nuclei, indicating an exclusive supraspinal origin for these fibers. A reduction in substance P-like immunoreactivity following spinal cord transection alone or spinal cord transection combined with unilateral dorsal rhizotomy was also detected in both nuclei, suggesting a dual origin for substance P-immunoreactive fibers, i.e., both supra- and intraspinal. The decrease in number of substance P-immunoreactive fibers was however smaller than expected from the analysis of the fraction of substance P-immunoreactive fibers co-localized with 5-hydroxytryptamine, indicating thus that the experimental lesions may have triggered a sprouting of substance P-immunoreactive axons originating from spinal cord sources. The distribution of gamma-aminobutyric acid in the ventrolateral and dorsolateral nuclei was not affected by the different lesion paradigms. It is therefore assumed that these inputs are intrinsic to the spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinal projections of pelvic visceral afferents of the rat: a calcitonin gene-related peptide (CGRP) immunohistochemical study

Little information is available concerning the pelvic visceral afferent system, in view of its terminal location in the spinal cord and its associated transmitter substances in the rat. By utilizing an immunostaining method to examine the transneuronal neurotransmitter depletion resulting from peripheral sensory nerve injury, the spinal projections of primary afferent fibers containing calcitonin gene-related peptide (CGRP) and originating from pelvic viscera were studied in the lumbosacral spinal cord of the rat. After unilateral or bilateral pelvic nerve section, CGRP immunoreactivity in the lumbosacral spinal cord was decreased greatly in the sacral parasympathetic nucleus (SPN), the dorsolateral fasiculus, the medial border of the dorsal horn, the dorsal gray commissure (DGC), and the intermediate gray connecting the SPN and DGC. Fine structural analysis showed that the CGRP-immunoreactive terminals made synaptic contact with dendrites and, rarely, with somata. Although there was some incidence of a synaptic contact between a CGRP-IR terminal and a vesicle-containing profile, definite evidence of axo-axonal synapse has not been confirmed. These data indicate that CGRP-containing pelvic visceral primary afferent fibers project to autonomic areas of the lumbosacral spinal cord by way of the pelvic nerve and make synaptic contact with dendrites and somata.

Spinal and hindbrain structures involved in visceroception and visceronociception as revealed by the expression of Fos, Jun and Krox-24 proteins

We have used the evoked expression of the immediate early gene-encoded proteins (Krox-24, c-Fos, Fos B, Jun D, Jun B, c-Jun) to monitor visceral processing in both the spinal cord and hindbrain structures of rats undergoing either mechanical colorectal or chemical intraperitoneal stimulation. Experiments were conducted under controlled volatile anaesthesia to suppress affective reactions that visceral stimulations may induce. The results refer to the effects of anaesthesia alone, and of both innocuous and noxious stimulations. Non-nociceptive and nociceptive stimulation but not anaesthesia were effective in evoking c-Fos, c-Jun, Jun B and Krox-24 expressions in the spinal cord. Intraperitoneal injections labelled cells mostly at the thoracolumbar junction levels, while colorectal distension labelled cells mostly at the lumbrosacral junction levels. Labelling was widely distributed throughout the gray matter including superficial layers, deep dorsal horn, lamina X and sacral parasympathetic columns. Krox-24- and, to a lesser degree, c-Jun-labelled cells were quite numerous in the superficial layers of the dorsal horn; Jun B, and especially c-Fos, were very effective in demonstrating inputs to all parts of the spinal cord. Both anaesthesia and noxious visceral stimulation were effective in evoking c-Fos, Krox-24 and Jun B expressions in discrete hindbrain subregions. The structures which are primarily labelled under anaesthesia are the rostral ventrolateral medulla, the external medial and lateral nuclei of the parabrachial area, the medial and dorsal subnuclei of the nucleus of the solitary tract, the area postrema, the central gray including pars alpha and nucleus O, the nucleus beta of the inferior olive, the locus coeruleus, and the inferior colliculi and adjacent parts of central gray. The structures which are primarily labelled following noxious visceral stimulation are the caudal intermediate reticular nucleus as part of the caudalmost ventrolateral medulla and the superior lateral nucleus of the rostrolateral parabrachial area. Labelling in the caudal intermediate reticular nucleus was maximal for colorectal distension. Labelling in the superior lateral nucleus was specific to peritoneal inflammation. The Edinger-Westphal nucleus is a structure in which noxious-evoked labelling was superposed onto the anaesthesia-evoked labelling. Nociception-evoked overexpression in this nucleus was maximal for intraperitoneal inflammation. The present work demonstrates that the central effects induced by either anaesthesia or visceroception including pain can be effectively monitored through the induction of an array of immediate early genes.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular injection of neurobiotin or horseradish peroxidase reveals separate types of preganglionic neurons in the sacral parasympathetic nucleus of the cat

Sacral preganglionic neurons are essential to the neural control of the excretory and sex organs. Previously employed multi-cell tracing methods have certain limitations in the precise morphological analysis of the neural pathways that control these organs. These limitations were overcome by the intracellular injection of neurobiotin or horseradish peroxidase into single preganglionic neurons in the lateral sacral parasympathetic nucleus of the cat. Following light microscopic examination, these neurons, as a group, were found to have an average of five stem dendrites, which divided into 15 dendritic end-branches that were distributed among eight dendritic terminal fields. These dendrites had a major transverse orientation and were quite long, many of them reaching well into the dorsal and ventral horns and into the dorsal gray commissure. These dendrites also exhibited a major longitudinal orientation, extending an average of 869 microns (combined length of rostral and caudal dendrites) within the nucleus. Two groups of cells emerged on the basis of different dendritic patterns. Cells classed as Type I had dendrites in lamina I and in the ventral horn but lacked a significant projection into the lateral funiculus. Cells classed as Type II had major dendritic projections into the lateral funiculus but lacked dendrites in lamina I. The diverse dendritic patterns of these two cell types indicate dissimilar afferent control mechanisms and suggest that these preganglionic neurons may innervate different target organs.

Presynaptic modulation of sensory afferents in the invertebrate and vertebrate nervous system

1. Ultrastructural examination of the central terminals of sensory afferent neurons in both invertebrates and vertebrates demonstrates that the synapses that form the substrate for presynaptic inhibition and facilitation are almost universally present. 2. Presynaptic modulation of afferent input acts in many ways which tailor the inflow of sensory information to the behaviour of the animal, effectively providing a means of turning this on and off, or of combining information of the same or different modalities to refine responsiveness or clarify ambiguity. 3. Presynaptic modulation may act in several different roles on the same afferent. 4. A comparison of the mechanisms of presynaptic inhibition in different animals demonstrates the likelihood of a variety of common mechanisms, several of which may act simultaneously on the same terminal. These include changes in the conductance of the afferent membrane to Cl-, K+ and Ca2+ ions, in addition to less well understood mechanisms that directly affect transmitter release. 5. A single transmitter can produce several effects on a terminal through the same or different receptors. 6. Ultrastructural studies of afferent terminals reveal that only a proportion of boutons on a given afferent may receive presynaptic input and that this may depend on the region of the nervous system in which these are found or on the identity of the postsynaptic neurons contacted. 7. The synaptic relationships of afferent terminals can be complex. In invertebrates different types of presynaptic neuron may interact synaptically, as may postsynaptic dendrites in vertebrates. 8. Axons presynaptic to afferent terminals in vertebrates frequently synapse also with dendrites postsynaptic to the afferents. 9. In both invertebrates and vertebrates reciprocal interactions between afferents and postsynaptic neurons are seen. 10. Ultrastructural immunocytochemistry reveals the likely dominance of GABA as an agent of presynaptic inhibition but also demonstrates the possible presence of other transmitters some of whose roles are less completely understood.

Fine structure of olfactory epithelia of gastropod molluscs

Among gastropod molluscs the chemical senses are most important for location of distant objects. They are used in food finding, locating mates, avoiding predators, trail following, and homing. Chemoreceptors are commonly associated with the oral area, the tentacles, and the osphradium, which lies in the mantle cavity. Most chemosensory neurons are primary sensory neurons, although secondary sensory cells have been reported in the osphradium of some prosobranch gastropods. Most chemosensory organs contain sensory cells with ciliated sensory endings that are in contact with the external environment. Some sensory endings have only microvilli or have no surface elaborations. Cilia on sensory endings are commonly of the conventional type, but some species have modified cilia; some lack rootlets, some have an abnormal microtubular content, and some have paddle-shaped endings. The perikarya of sensory neurons may be within the sensory epithelium, below it, or in ganglia near the sensory surface. In some groups of gastropods there are peripheral ganglia in the olfactory pathway; in others chemosensory axons appear to pass directly to the CNS. Olfactory epithelia of terrestrial pulmonates have modified brush borders with long branching plasmatic processes and a spongy layer of cytoplasmic tubules which extend from the epithelial cells. Sensory endings of the olfactory receptors are entirely within this spongy layer. Aquatic pulmonates may have a similar spongy layer in their olfactory epithelia, but the cilia of sensory endings, as well as motile cilia of epithelial cells, extend well beyond the spongy layer.

Ultrastructure of pre-synaptic input to motor neurons in Onuf's nucleus: controls and motor neuron disease

Ultrastructural analyses of sphincteric motoneurons in Onuf's nucleus at S2 were undertaken in human spinal cord obtained 3-6 h post-mortem from three subjects with no neurological disease ('controls') and five in which death was due to motor neuron disease (MND). Neurons in specified locations within Onuf's nucleus of control subjects ranged between 17.8 and 71.7 microns diameter (mean 38.6 microns). Analyses of synaptology revealed five ultrastructural classes of presynaptic terminal synapsing with the neuronal surface membrane. When classified by size, vesicle morphology, and synaptic site structure these conformed to the S, F, T, M and C-terminals defining somatic motoneurons. No terminals characteristic of autonomic motoneurons were found. In MND subjects, neurons in Onuf's nucleus at S2 were preserved despite a paucity of neurons in medial and lateral motor nuclei and were of similar size range to those in control subjects. The morphological classes of pre-synaptic terminal found in controls, also characterized sphincteric motoneurons in MND subjects, including the C-type terminal. The presence of C-terminals indicates (i) that sphincteric motoneurons are somatic alpha-motoneurons, and (ii) that hypotheses explaining the survival of sphincteric motoneurons in MND on the basis of Onuf's nucleus being an extension of the pre-ganglionic parasympathetic nucleus, or having intrinsic autonomic properties are incorrect.

The cells of origin of the spinohypothalamic tract in cats

Various cutaneous and visceral stimuli alter the discharge rates of neurons in the hypothalamus. Changes in the activity of hypothalamic neurons are thought to play important roles in eliciting neuroendocrine, autonomic, and affective responses to somatosensory and visceral stimuli. Information from peripheral structures has been considered generally to reach the hypothalamus via multisynaptic ascending pathways. Recently, a direct projection from the spinal cord to the hypothalamus was demonstrated in rats. The goal of this study was to determine whether a similar projection exists in cats. Either wheat germ agglutinin conjugated to horseradish peroxidase, a mixture of this tracer and the B subunit of cholera toxin conjugated to horseradish peroxidase, or fast blue was injected into the hypothalamus of cats. Injections were centered in the hypothalamus in seven cats and did not spread to the thalamus, zona incerta or midbrain. After these injections, retrogradely labeled neurons were observed bilaterally in each of the 17 spinal segments that were examined. A total of approximately 400-500 labeled neurons was observed in alternate sections through these segments in the most effective cases. Roughly 70% of the labeled neurons were located contralaterally. Labeled neurons were found predominantly in the deep dorsal horn, the intermediate zone/ventral horn and in the area surrounding the central canal. A few were also noted in the superficial dorsal horn. The first and second sacral segments contained the largest numbers of retrogradely labeled neurons in the spinal cord. The number of spinohypothalamic tract neurons observed in this study in cats was roughly an order of magnitude smaller than that previously reported for rats. This finding suggested either that the spinohypothalamic tract is relatively small in cats or that our tracing techniques did not label many spinohypothalamic tract neurons in cats. To test the sensitivity of one of our tracing techniques, control injections of wheat germ agglutinin conjugated to horseradish peroxidase that filled the ventrobasal thalamus were made in two cats. In both cases, thousands of spinal cord neurons were labeled. In summary, our results indicate that a spinohypothalamic tract exists in cats. However, our findings also suggest that the total number of spinohypothalamic tract neurons in cats may be an order of magnitude smaller than it is in rats.

Central distribution of afferent pathways from the uterus of the cat

Afferent pathways from the uterus of the cat were labeled by injections of horseradish peroxidase (HRP), wheat germ agglutinin-HRP, or fluorescent dyes into the uterine cervix and uterine horns. Afferent input to the uterus arises from small to medium size neurons (average size 31 x 28 microns) in dorsal root ganglia at many levels of the spinal cord (T12-S3). The segmental origin correlates with the location of the afferent terminal field in the uterus. Eighty-seven percent of the dorsal root ganglion cells (average, 822 on one side) innervating the cervix are located in sacral ganglia, whereas 97% of the cells innervating the uterine horn (average 479 on one side) are located in lumbar ganglia. Double dye labeling experiments indicate that a small percentage (average 15%) of lumbar neurons innervating the uterine cervix also innervate the uterine horn. The majority (70-80%) of afferent input to the uterine cervix passes through the pelvic nerve and the remainder through the pudendal nerve, whereas afferent input to the uterine horn must travel in sympathetic nerves. Ovariectomy (10-14 days) did not change significantly the number, sizes, or segmental distribution of uterine afferent neurons. In some cats (25%) injections of WGA-HRP into the uterine cervix labeled neurons (90-125 per animal) in lamina VII in the S2 spinal segment in the region of the sacral parasympathetic nucleus. Central projections of uterine horn afferent neurons were not labeled; however, afferent projections from the cervix were detected in the sacral spinal cord. The most prominent labeling was present in Lissauer's tract and in lamina I and outer lamina II on the lateral edge of the dorsal horn. From this region some labeled axons extended through lamina V into the dorsal gray commissure. Very few afferents were labeled on the medial side of the dorsal horn. These results are discussed in regard to the physiological function of uterine afferents and the possible transmitter role of vasoactive intestinal polypeptide, which is present in a large percentage (70%) of cervical afferent neurons.

Spinal block reveals roles for brain and spinal cord in the mediation of reflexive penile erections in rats

Five experiments were addressed at two questions: (1) is the influence of the brain on spinal control of reflexive erection exclusively inhibitory; and (2) are the effects of copulation on erectile potential mediated by the brain, the spinal cord, or both? After various amounts of antecedent copulation, spinal anesthesia was induced in male rats by tetracaine (TET) injected through chronically implanted cannulae into the thoracic or lumbar spinal subarachnoid space, and the animals were then tested for reflexive penile erections (supine position, penile sheath retracted, no phasic stimulation applied). In sexually rested rats, TET injected at either T4 or T10 reduced reflex latency but also reduced the number and intensity of responses. Penile erection was inhibited by TET injected at L5, the region of the cord receiving sensory and motor projections from the genitalia. In previous studies surgical transection of the thoracic cord facilitated erection latency and production. In the present experiments the divergent effects of intrathecal TET on these variables suggested that they are controlled by separate systems within the spinal cord and that in the sexually rested rat the net influence of the brain is to inhibit the latency system and excite the production system. After rats copulated to sexual satiety, thoracic spinal block did not reverse the complete abolition of reflexive erections, establishing for the first time that copulation has direct inhibitory effects on the spinal cord's intrinsic system. Fewer antecedent ejaculations had less inhibitory effects on reflexive erection, and TET then acted, as in rested males, to reduce the erection latency and the number of erections, indicating that copulation may act on the brain's descending influences on spinal systems. The overall pattern of results suggested that the brain exerts both excitatory and inhibitory influences on separate, interacting spinal mechanisms that regulate the latency to reflexive erection and the number and intensity of erections displayed.

Astrocyte-polymer implants promote regeneration of dorsal root fibers into the adult mammalian spinal cord

To overcome obstacles to the regeneration of crushed dorsal root fibers at the dorsal root entry zone, we have employed specially designed Millipore implants coated with embryonic astrocytes to serve as a substrate for axonal growth. This strategy was successful in promoting the growth of crushed dorsal root axons into the grey matter of the adult mammalian spinal cord in a small number of animals. Fiber ingrowth into the spinal cord was closely associated with the surface of the polymer implant. In addition, unique terminal arbor malformations, not normally present, were seen in several animals. A consistent finding was the presence of a limited inflammatory response in regions immediately adjacent to the implant where axons penetrate the spinal cord. Our findings suggest that providing the dorsal root entry zone with an embryonic milieu can stimulate a limited amount of axonal regeneration into the adult mammalian spinal cord.

Motoneurons innervating the external anal and urethral sphincters of the female cat have different patterns of dendritic arborization

The locations and dendritic arbors of sacral motoneurons (MNs) innervating the striated external anal and urethral sphincters (EAS and EUS, respectively) of the cat were investigated using muscle injections of cholera toxin-conjugated HRP (CT-HRP). Serial reconstructions showed that all cells labelled after EAS injections were located in the dorsomedial (DM) subdivision of Onuf's nucleus, whereas all cells labelled from the EUS were located in the ventrolateral (VL) subdivision. The dendritic arbors of EAS and EUS MNs were very different, suggesting differences in afferent control. In addition, prominent bundles of dendrites extended preferentially into the regions occupied by functionally appropriate preganglionic neurons (PGNs) in the sacral parasympathetic nucleus (SPN) which innervate the colon and bladder.

Central neural control of esophageal motility: a review

We review recent studies on the central neural control of esophageal motility, emphasizing the anatomy and chemical coding of esophageal pathways in the spinal cord and medulla. Sympathetic innervation of the proximal esophagus is derived primarily from cervical and upper thoracic paravertebral ganglia, whereas that of the lower esophageal sphincter and proximal stomach is derived from the celiac ganglion. In addition to noradrenaline, many sympathetic fibers in the esophagus contain neuropeptide Y (NPY), and both noradrenaline and NPY appear to decrease blood flow and motility. Preganglionic neurons innervating the cervical and upper thoracic ganglia are located at lower cervical and upper thoracic spinal levels. The preganglionic innervation of the celiac ganglion arises from lower thoracic spinal levels. Both acetylcholine (ACh) and enkephalin (ENK) have been localized in sympathetic preganglionic neurons, and it has been suggested that ENK acts to pre-synaptically inhibit ganglionic transmission. Spinal afferents from the esophagus are few, but have been described in lower cervical and thoracic dorsal root ganglia. A significant percentage contain calcitonin gene-related peptide (CGRP) and substance P (SP). The central distribution of spinal afferents, as well as their subsequent processing within the spinal cord, have not been addressed. Medullary afferents arise from the nodose ganglion and terminate peripherally both in myenteric ganglia, where they have been postulated to act as tension receptors, and, to a lesser extent, in more superficial layers. Centrally, these afferents appear to end in a discrete part of the nucleus of the solitary tract (NTS) termed the central subnucleus. The transmitter specificity of the majority of these afferents remains unknown. The central subnucleus, in turn, sends a dense and topographically discrete projection to esophageal motor neurons in the rostral portion of the nucleus ambiguous (NA). Both somatostatin-(SS) and ENK-related peptides have been localized in this pathway. Finally, motor neurons from the rostral NA innervate striated portions of the esophagus. In addition to ACh, these esophageal motor neurons contain CGRP, galanin (GAL), N-acetylaspartylglutamate (NAAG), and brain natriuretic peptide (BNP). The physiological effect of these peptides on esophageal motility remains unclear. Medullary control of smooth muscle portions of the esophagus have not been thoroughly investigated.

Interaction between cholinergic neurons and substance P or calcitonin gene-related peptide terminals of the rat sacral intermediolateral nucleus: double immunostaining at the light and electron microscopic levels

The relationships both between cholinergic neurons and substance P (SP) and between cholinergic neurons and calcitonin gene-related peptide (CGRP) terminals were examined in the rat sacral intermediolateral nucleus at the light and electron microscopic levels by means of double-immunostaining methods. Cholinergic neurons were labeled by a monoclonal antibody to choline acetyltransferase (CAT) with the avidin-biotin technique and stained bluish-green by indolyl-beta-galactoside reaction products with beta-galactosidase as a marker. On the same sections, SP or CGRP fibers were labeled by polyclonal antisera to SP or CGRP after application of the peroxidase-antiperoxidase (PAP) method and stained brown by the p-dimethylaminoazobenzene (DAB) reaction. After embedding in Epon, light and electron microscopic sections were examined. At the light microscopic level, CGRP-like immunoreactive (CGRP-I) fibers and SP-like immunoreactive (SP-I) fibers were found to pass through the lateral edge of the dorsal horn and then into the dorsal region of the sacral intermediolateral nucleus. In addition, SP-I fibers also extend from the dorsolateral funiculus into the entire sacral intermediolateral region. At the electron microscopic level, many axosomatic and axodendritic synapses were found between CAT-I structures and SP-I terminals in the intermediolateral nucleus, whereas most of the CGRP-I terminals in this area made axodendritic synapses with CAT-I dendrites. These results indicate that cholinergic neurons in the sacral intermediolateral nucleus receive direct synaptic input from SP-I and CGRP-I terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

Anatomical specificity of regenerated muscle sensory afferents in the spinal cord of the bullfrog

The specificity of central projections made by regenerated muscle sensory fibers in the brachial spinal cord was studied with anatomical tracing methods. Sensory fibers were interrupted by freezing dorsal roots in postmetamorphic bullfrogs. After several months, regenerated sensory fibers were labeled with horseradish peroxidase applied to the triceps brachii muscle nerve, and their arborizations within the spinal cord were reconstructed from serial cross sections. Most of the regenerated projections from triceps muscle sensory afferents ended in or near their normal terminal field. A few branched and appeared to terminate more dorsally than normal, however, sometimes within the region where cutaneous afferents normally terminate. In contrast to the normal pathway followed by muscle afferents within the spinal cord, many regenerated afferents grew along the circumference of the spinal cord, just under the pial surface, and then turned abruptly toward the midline and into their appropriate terminal region. This suggests that regenerating afferents may actively seek out their appropriate targets and are not simply passively guided to them.

Preganglionic sympathetic neurones innervating the rat adrenal medulla: immunocytochemical evidence of synaptic input from nerve terminals containing substance P, GABA or 5-hydroxytryptamine

Sympathetic preganglionic neurones that innervate the adrenal medulla were identified for subsequent light and electron microscopic study by the retrograde transport of horseradish peroxidase (HRP) or a conjugate of HRP and cholera B-chain. Most labelled neurones were found in the intermediolateral column, but some occurred in the intercalated nucleus and in the lateral funiculus of the thoracic spinal cord. Three morphologically distinct types of neurone were retrogradely labelled, two of which had dendrites that extended medially towards the central canal and laterally across the entire lateral funiculus. A combination of retrograde labelling with pre-embedding immunocytochemistry allowed us to demonstrate synaptic contacts between boutons immunoreactive for substance P or 5-hydroxytryptamine (5-HT) and the cell bodies or proximal dendrites of sympathoadrenal neurones. The 5-HT-immunoreactive boutons appeared to be of two morphologically distinct types. Postembedding immunocytochemistry enabled us to show that sympathoadrenal neurones receive a heavy synaptic innervation from GABA-immunoreactive boutons: 32% of a random series of boutons in synaptic contact with cell bodies were GABA-immunoreactive. Proximal dendrites and also distal dendrites within the white matter were ensheathed in synaptic boutons, 37% of which were GABA-immunoreactive. It is concluded that sympathoadrenal neurones receive at least 4 distinct types of afferent synaptic input: from neurones containing substance P, or GABA and from two types of neurones containing 5-HT. The presence of synaptic inputs on distal dendrites that extend across the white matter adds further complexities to the control of the activity of sympathetic preganglionic neurones.

Quantitative synaptology of feline motoneurones to external anal sphincter muscle

Motoneurones innervating the cat external anal sphincter muscle were labelled retrogradely following intramuscular injections with horseradish peroxidase (HRP). Labelled motoneurones were examined by correlative light and electron microscopy (LM and EM) with special regard to a qualitative and morphometric analysis of the axon terminals resident on the neuronal membrane. By LM, labelled motoneurones were (1) ipsilateral to the injections; (2) all in S1-S2; (3) found only in the superior dorsomedial region of Onuf's nucleus; and (4) exhibited a broad spectrum of diameters (25-72 micron, mean 47.4 +/- 11.3 micron). By EM, axon terminals on the neuronal membrane when classified according to size, vesicle shape, and synaptic complex ultrastructure conformed to the S-, F-, T-, M-, and C-type terminals previously described for cat lumbosacral motoneurones. C-terminals confirmed these sphincteric motoneurones to be skeletomotor. Pooled data from midnuclear sections through 15 random labelled motoneurones (20-64-micron diameter) revealed that S- and F-type terminals predominated, with numerically few M and C types. Notwithstanding their low frequency (0.3/100 micron membrane) C-terminals contributed 1% of the mean areal coverage by terminals, which implies a potentially larger synaptic influence relative to other terminal types. Linear relationships occurred between terminal frequency (or cover) and motoneurone diameter. While motoneurones greater than 40 micron in diameter exhibited all five terminal types, labelled motoneurones less than or equal to 30 micron generally possessed only S-, F-, and occasional T-type terminals, and in this respect resembled gamma motoneurones.

Proportion and location of spinal neurons receiving ventral root afferent inputs in the cat

Spinal neurons receiving ventral root afferent inputs were investigated in anesthetized and paralyzed cats. We were concerned with the afferent fibers in the ventral root that travel distally and then enter the spinal cord through the dorsal root. The questions to be answered included the proportion and distribution of spinal neurons receiving ventral root afferent inputs and their peripheral input characteristics. The 1.7 ventral root was cut near the spinal cord and the distal stump was stimulated while making a systematic search for neurons in the entire gray matter of the ipsilateral spinal cord that responded to the stimulation. The following conclusions were made: (i) the afferent fibers in the cat ventral root enter the spinal cord through the dorsal root and evoke a variety of responses (excitation, inhibition, or mixed) in a large proportion of spinal neurons (about 20%): (ii) these responses seem to be mediated largely by spinal mechanisms: (iii) spinal neurons receiving ventral root afferent inputs are situated in a wide region of the ventral spinal cord: (iv) ventral root fibers in a single root enter the spinal cord and exert their responses over a large region of the spinal cord (at least two spinal segments rostrally and caudally): (v) some of the spinal neurons that responded to ventral root stimulation were found to be ascending tract cells, suggesting that ventral root afferent inputs can reach supraspinal structures: (vi) ventral root afferent fibers converge onto spinal neurons that have a variety of peripheral receptive field characteristics: and (vii) with some exceptions, most neurons receiving ventral root inputs were excited best by mechanical and/or thermal noxious stimuli applied to the periphery.

Differences in synaptic inputs to preganglionic neurons in the dorsal and lateral band subdivisions of the cat sacral parasympathetic nucleus

In the cat, preganglionic neurons (PGNs) found in the dorsal portion of the sacral parasympathetic nucleus (dorsal band or DB cells) participate in bowel control, while those found along the lateral edge (lateral band or LB cells) influence bladder function. In order to determine whether differences in the synaptic inputs exist between these two populations, HRP was applied to the sacral ventral rootlets of cats, and the S2 cord segment was prepared for sequential light and electron microscopy. When measured with light microscope, the LB somata had greater cross-sectional areas than did the DB cells. Ultrastructurally, the LB cells had a significantly greater percentage of their membrane apposed by synaptic active sites than did the DB cells. Also, the proximal dendrities of the labelled neurons received greater synaptic input than did the somata. No difference was found in the proportion of terminals containing dense cored vesicles (DCVs) when comparing LB and DB somata; however, the LB proximal dendrites had a higher proportion of their surface contacted by DCV-containing terminals than did the DB dendrites. These ultrastructural results offer evidence that these two populations of preganglionic neurons differ with respect to their synaptic input as well as their peripheral targets.

Central distribution of efferent and afferent components of the pudendal nerve in rat

Central distribution of efferent and afferent components of the pudendal nerve was examined in the rat by the horseradish peroxidase (HRP) method after HRP application to the central cut end of the pudendal nerve. The pudendal motoneurons were located in the dorsolateral, dorsomedial and lateral groups at L5 and L6. Each of the dorsolateral and dorsomedial groups constituted a slender longitudinal cell column. Pudendal motoneurons in the lateral group were scattered at L5, rostrodorsally to the dorsolateral group. The neurons in the dorsolateral and lateral groups were labelled with HRP applied to the nerve branch innervating the ischiocavernosus and sphincter urethrae muscles. The neurons in the dorsomedial group were labelled with HRP applied to the branch supplying the sphincter ani externus and bulbospongiosus muscles. Some dendrites of pudendal motoneurons in the dorsomedial group extended to the contralateral dorsomedial group. These crossing dendrites were observed not only in male rats but also in female. The average number of the pudendal motoneurons in the dorsolateral and dorsomedial groups were larger in male rats than in female. A few neurons of the intermediolateral nucleus at upper L6 were also labelled with HRP applied to the dorsalis penis (clitoridis) nerve. Axon terminals of the pudendal nerve were distributed, bilaterally with an ipsilateral predominance, to the gracile nucleus, as well as to the dorsal horn and dorsal commissural gray from L4 to S2. A few labelled axons were seen in the intermediolateral nucleus at L6 and S1. Axon terminals from the dorsalis penis nerve were distributed more medially in the dorsal horn than those from the perinealis nerve.

Testosterone-induced plasticity of synaptic inputs to adult mammalian motoneurons

The effects of testosterone administration on penile reflexes, and on the motoneurons of the spinal nucleus of the bulbocavernosus which innervate perineal muscles involved in these reflexes, were investigated in castrated male rats. Penile reflexes were restored following 48 h of testosterone administration initiated 6 weeks after castration. The amount of synaptic input to the identified motoneurons was increased following short term testosterone treatment, compared to that seen in animals receiving no testosterone, albeit to a lesser extent than that seen in animals receiving long term testosterone treatment. This increase in synaptic inputs in the short term testosterone group occurred despite the lack of an increase in somatic area. Thus, plasticity of the synaptic input to these neurons, as well as recovery of penile reflexes, occurred as a result of alterations in the hormonal state of the animal, and such changes occurred relatively rapidly.

Many ventral root afferent fibers in the cat are third branches of dorsal root ganglion cells

The arrangement of the ventral root afferent fibers was investigated in anesthetized and paralyzed cats. Single unit activity was recorded from a fascicle of the distal stump of the cut S1 dorsal root. Activity was elicited by stimulating the distal stump of the cut S1 ventral root. Attempts were then made to collide this activity with that elicited by stimulation of the S1 spinal nerve. Single unit activity elicited by ventral root stimulation was recorded from a total of 33 axons. In 17 of these, the activity collided with that elicited by peripheral stimulation. These results indicate that more than half the sampled population of ventral root afferent fibers are branches of dorsal root ganglion cells that have at least 3 processes: one in the dorsal root, one in the ventral root and one in a peripheral nerve. In 10 of these units, the conduction velocity of each of 3 processes was determined using the collision technique. The conduction velocities differed in the processes of a given ganglion cell, with conduction in the ventral root process generally being the slowest. The change in conduction velocity along the length of the ventral root was examined by comparing latency differences for the unit activity elicited by ventral root stimulation at different sites in the same root separated by known distances. The conduction velocity was found not to be uniform along the course of the ventral root. In many cases, the conduction velocity slowed down as the fiber approached the spinal cord. We conclude from the present study that many ventral root afferent fibers are the third branches of dorsal root ganglion cells that also have processes in the dorsal root and in a peripheral nerve. The sizes of each of these 3 processes of the dorsal root ganglion cell may differ; the ventral root process tends to be the smallest and is usually unmyelinated. Furthermore, many of the ventral root afferent fibers may taper as they approach the spinal cord.

Distribution and ultrastructure of ventral root afferents to lamina I of the cat sacral spinal cord

Following ventral root injury-filling with horseradish peroxidase at sacral spinal levels S1, S2, and S3, small fascicles of axons can be observed to enter the ventral portion of lamina I and arborize primarily in the dorsolateral region just medial to the tract of Lissauer (TL). Some axons enter the TL and turn in a rostrocaudal direction. Labeled axons studied by electron microscopy are both myelinated and unmyelinated, and terminals in lamina I contain round clear, and dense cored vesicles and contact smaller diameter dendrites and vesicle-containing elements. The concentration of these putative ventral root afferents appears to be largest in the S2 segment, with smaller numbers found within S1 and S3.