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

Effects of acute selective pudendal nerve electrical stimulation after simulated childbirth injury

During childbirth, a combinatorial injury occurs and can result in stress urinary incontinence (SUI). Simulated childbirth injury, consisting of vaginal distension (VD) and pudendal nerve crush (PNC), results in slowed recovery of continence, as well as decreased expression of brain-derived neurotrophic factor (BDNF), a regenerative cytokine. Electrical stimulation has been shown to upregulate BDNF in motor neurons and facilitate axon regrowth through the increase of β(II)-tubulin expression after injury. In this study, female rats underwent selective pudendal nerve motor branch (PNMB) stimulation after simulated childbirth injury or sham injury to determine whether such stimulation affects bladder and anal function after injury and whether the stimulation increases BDNF expression in Onuf's nucleus after injury. Rats received 4 h of VD followed by bilateral PNC and 1 h of subthreshold electrical stimulation of the left PNMB and sham stimulation of the right PNMB. Rats underwent filling cystometry and anal pressure recording before, during, and after the stimulation. Bladder and anal contractile function were partially disrupted after injury. PNMB stimulation temporarily inhibited bladder contraction after injury. Two days and 1 wk after injury, BDNF expression in Onuf's nucleus of the stimulated side was significantly increased compared with the sham-stimulated side, whereas β(II)-tubulin expression in Onuf's nucleus of the stimulated side was significantly increased only 1 wk after injury. Acute electrical stimulation of the pudendal nerve proximal to the crush site upregulates BDNF and β(II)-tubulin in Onuf's nucleus after simulated childbirth injury, which could be a potential preventive option for SUI after childbirth injury.

Neural control of the female urethral and anal rhabdosphincters and pelvic floor muscles

The urethral rhabdosphincter and pelvic floor muscles are important in maintenance of urinary continence and in preventing descent of pelvic organs [i.e., pelvic organ prolapse (POP)]. Despite its clinical importance and complexity, a comprehensive review of neural control of the rhabdosphincter and pelvic floor muscles is lacking. The present review places historical and recent basic science findings on neural control into the context of functional anatomy of the pelvic muscles and their coordination with visceral function and correlates basic science findings with clinical findings when possible. This review briefly describes the striated muscles of the pelvis and then provides details on the peripheral innervation and, in particular, the contributions of the pudendal and levator ani nerves to the function of the various pelvic muscles. The locations and unique phenotypic characteristics of rhabdosphincter motor neurons located in Onuf's nucleus, and levator ani motor neurons located diffusely in the sacral ventral horn, are provided along with the locations and phenotypes of primary afferent neurons that convey sensory information from these muscles. Spinal and supraspinal pathways mediating excitatory and inhibitory inputs to the motor neurons are described; the relative contributions of the nerves to urethral function and their involvement in POP and incontinence are discussed. Finally, a detailed summary of the neurochemical anatomy of Onuf's nucleus and the pharmacological control of the rhabdosphincter are provided.

Spinal mechanisms contributing to urethral striated sphincter control during continence and micturition: "how good things might go bad"

The external urethral sphincter motoneurons in the sacral ventral horn control the striated external urethral sphincter muscles that circle the urethra. Activity in these motoneurons and muscle normally contribute to continence but during micturition, when urine must pass through the urethra, the motoneurons and striated muscle must be silenced. Following injury to descending pathways in the spinal cord, the ability to inhibit sphincter activity is disrupted or lost, resulting in bladder-sphincter dyssynergia and functional obstruction of the urethra during voiding. This chapter will first review the various reflex pathways and neuronal properties that contribute to continence, and which must be modulated during micturition in the spinal intact animal. A discussion about how the dyssynergia seen with spinal cord injury may be produced will then be presented.

Mechanisms underlying the recovery of lower urinary tract function following spinal cord injury

The lower urinary tract has two main functions, the storage and periodic expulsion of urine, which are regulated by a complex neural control system in the brain and lumbosacral spinal cord. This neural system coordinates the activity of two functional units in the lower urinary tract: (1) a reservoir (the urinary bladder) and (2) an outlet (consisting of bladder neck, urethra and striated muscles of the pelvic floor). During urine storage the outlet is closed and the bladder is quiescent, thereby maintaining a low intravesical pressure over a wide range of bladder volumes. During micturition the outlet relaxes and the bladder contracts to promote the release of urine. This reciprocal relationship between bladder and outlet is generated by visceral reflex circuits, some of which are under voluntary control. Experimental studies in animals indicate that the micturition reflex is mediated by a spinobulbospinal pathway passing through a coordination center (the pontine micturition center) located in the rostral brainstem. This reflex pathway is in turn modulated by higher centers in the cerebral cortex that are presumably involved in the voluntary control of micturition. Spinal cord injury at cervical or thoracic levels disrupts voluntary control of voiding as well as the normal reflex pathways that coordinate bladder and sphincter functions. Following spinal cord injury, the bladder is initially areflexic but then becomes hyperreflexic due to the emergence of a spinal micturition reflex pathway. Studies in animals indicate that the recovery of bladder function after spinal cord injury is dependent in part on plasticity of bladder afferent pathways and the unmasking of reflexes triggered by capsaicin-sensitive C-fiber bladder afferent neurons. The plasticity is associated with changes in the properties of ion channels and electrical excitability of afferent neurons, and appears to be mediated in part by neurotrophic factors released in the spinal cord and the peripheral target organs.

Alterations in eliminative and sexual reflexes after spinal cord injury: defecatory function and development of spasticity in pelvic floor musculature

Spinal cord injury often results in loss of normal eliminative and sexual functions. This chapter is focused on defecatory function, although aspects of micturition and erectile function will be covered as well due to the overlap in anatomical organization and response to injury. These systems have both autonomic and somatic components, and are organized in the thoracolumbar (sympathetic), lumbosacral (somatic), and sacral (parasympathetic) spinal cord. Loss of supraspinal descending control and plasticity-mediated alterations at the level of the spinal cord, result in loss of voluntary control and in abnormal functioning of these systems including the development of dyssynergies and spasticity. There are several useful models of spinal cord injury in rodents that exhibit many of the autonomic dysfunctions observed after spinal cord injury in humans. Numerous studies involving these animal models have demonstrated development of abnormalities in bladder, external anal sphincter, and erectile function, such as detrusor-sphincter-dyssynergia and external anal sphincter hyperreflexia. Here we review many of these studies and show some of the anatomical alterations that develop within the spinal cord during the development of these hyperreflexias. Furthermore, we show that spasticity develops in other pelvic floor musculature as well, such as the bulbospongiosus muscle, which results in increased duration and magnitude of pressures developed during erectile events and increased duration of micturition. Advances and continued improvement in the use of current animal models of spinal cord injury should encourage and increase the laboratory work devoted to this relatively neglected area of experimental spinal cord injury.

A compartmental model of an external urethral sphincter motoneuron of Onuf's nucleus

This article discusses a model of the electrical behavior of an external urethral sphincter motoneuron, based on morphological parameters like soma size, dendritic diameters and spatial dendritic configuration, and several electrical parameters. Because experimental data about the exact ion conductance mix of external urethral sphincter neurons is scarce, the gaps in knowledge about external urethral sphincter motoneurons were filled in with known data of alpha-motoneurons. The constructed compartmental model of motoneurons of Onuf's nucleus contains six voltage-dependent ionic conductances: a fast sodium and potassium conductance and an anomalous rectifier in the soma; a fast delayed rectifier type potassium conductance and a fast sodium conductance in the initial axon segment; an L-type calcium channel in the dendritic compartments. This paper considers the simulation of external urethral sphincter motoneuron responses to current injections that evoke bistable behavior. Simulations show self-sustained discharge following a depolarizing pulse through the microelectrode; the firing was subsequently terminated by a short hyperpolarizing pulse. This behavior is highly functional for neurons that have to exhibit prolonged activation during sphincter closure. In addition to these 'on' and 'off ' responses, we also observed a particular firing behavior in response to long-lasting triangular current pulses. When the depolarizing current was slowly increased and then decreased (triangular pulse) the firing frequency was higher during the descending phase than during the initial ascending phase.

Non-linear membrane properties of sacral sphincter motoneurones in the decerebrate cat

1. Responses to pudendal afferent stimulation and depolarizing intracellular current injection were examined in sacral sphincter motoneurones in decerebrate cats. 2. In 16 animals examined, 2-10 s trains of electrical stimulation of pudendal afferents evoked sustained sphincter motoneurone activity lasting from 5 to >50 s after stimulation. The sustained response was observed in: 11 animals in the absence of any drugs; two animals after the intravenous administration of 5-hydroxytryptophan (5-HTP; <= 20 mg kg-1); one animal in which methoxamine was perfused onto the ventral surface of the exposed spinal cord; and two animals following the administration of intravenous noradrenergic agonists. 3. Extracellular and intracellular recordings from sphincter motoneurones revealed that the persistent firing evoked by afferent stimulation could be terminated by motoneurone membrane hyperpolarization during micturition or by intracellular current injection. 4. Intracellular recordings revealed that 22/40 sphincter motoneurones examined displayed a non-linear, steep increase in the membrane potential in response to depolarizing ramp current injection. The mean voltage threshold for this non-linear membrane response was -43 +/- 3 mV. Five of the 22 cells displaying the non-linear membrane response were recorded prior to the administration of 5-HTP; 17 after the intravenous administration of 5-HTP (<= 20 mg kg-1). 5. It is concluded that sphincter motoneurones have a voltage-sensitive, non-linear membrane response to depolarization that could contribute to sustained sphincter motoneurone firing during continence.

Nucleus retroambiguus projections to lumbosacral motoneuronal cell groups in the male cat

Recently, in the female cat, nucleus retroambiguus (NRA) projections have been described as distinct motoneuronal cell groups in the lumbar enlargement, possibly involved in lordosis behavior. The present study deals with the NRA-lumbosacral pathway in the male cat. Lumbosacral injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were made to localize and quantify retrogradely labeled neurons in the caudal medulla. These injections were preceded by spinal hemisections to distinguish between neurons with ipsi- and contralaterally descending axons. The NRA-lumbosacral fibers descended almost exclusively contralaterally, but neurons in areas surrounding the NRA projected mainly ipsilaterally. Injections of WGA-HRP were made in the region of the NRA to determine its targets in the lumbosacral cord. To distinguish between the contralateral NRA pathways and the ipsilateral projections from neurons in the adjoining lateral tegmentum, the injections were preceded by ipsilateral hemisections in C2. A new scaling method was used to compare the results of the different cases, despite variations in size and segmental organization. The results show that the distribution pattern of anterogradely labeled fibers in the lumbosacral cord matched precisely the location of certain motoneuronal cell groups. The NRA projected densely to the abdominal wall and pelvic floor motoneurons in Onuf's nucleus, moderately to adductor longus, semimembranosus, and biceps femoris anterior motoneuronal cell groups, and only sparsely to iliopsoas and semitendinosus motoneuronal cell groups. Compared with the findings in the female, the NRA in the male cat projects more heavily to the biceps anterior and adductor longus and only sparsely to the iliopsoas and semitendinosus motoneuronal cell groups. These male-female differences are discussed.

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.

Neurophysiology of micturition and continence

This article reviews the neuroanatomy, neurophysiology, and neuropharmacology involved in micturition and continence. Knowledge of these topics helps the clinician diagnose and treat voiding disorders that are caused by disease, trauma, drugs, and aging.

Multifunctional ventral respiratory group: bulbospinal expiratory neurons play a role in pudendal discharge during vomiting

Pudendal motoneurons are activated in phasic bursts during the retching and expulsion phases of vomiting. The resulting contraction of the anal and urethral sphincters serves to maintain continence during the large increase in abdominal pressure that occurs during vomiting. We evaluated the contribution of bulbospinal expiratory neurons located in the portion of the ventral respiratory group (VRG) caudal to the obex (nucleus retroambigualis) to the control of pudendal motoneurons during fictive vomiting in decerebrate, paralyzed cats. Pudendal nerve discharge is abolished by cutting the axons of caudal VRG expiratory neurons as they cross the midline between the obex and C1 before descending in the spinal cord. All caudal VRG expiratory neurons that were antidromically activated from the sacral spinal cord, where the pudendal motor pool (nucleus of Onuf) is located, discharged strongly during the end of the expulsion phase of vomiting. However, only a small proportion of these neurons was active in phase with pudendal discharge during the retching phase. The apparent involvement of caudal VRG expiratory neurons in the control of pudendal motoneurons during vomiting is another example of the multifunctional role that can be played by respiratory-related neurons in the mammalian nervous system.

Long-lasting facilitation and depression of periurethral skeletal muscle following acupuncture-like stimulation in anesthetized rats

The effects of acupuncture-like stimulation on the tone of the partially filled bladder and on the periurethral electromyogram (EMG) were examined in urethane-anesthetized rats. Acupuncture-like stimuli were usually applied to the skin and underlying muscles (or other structures), either separately or together, for a period of 1 min; the effects were studied in spinal cord intact and in spinalized animals. Maps have been constructed showing the effects of acupuncture-like stimulation at different sites on the body surface and of similar stimulation applied to individual muscles, the urethra and the testis. When acupuncture-like stimuli were applied to the skin and underlying structures, in the rostral half of the body and the hindpaw, testis or urethra, these stimuli usually induced excitation of periurethral EMG activity. Depression of EMG activity was seen predominantly during stimulation of structures close to the urethra, but not opposed to it. When acupuncture-like stimuli were applied only to structure beneath the skin, depression of EMG activity usually occurred. Acupuncture-like stimulation of the bulbocavernosus, which partly overlies the proximal urethra produced depression of EMG activity in 50% of trials, but the incidence of similar effects from the more distant pubococcygeus, or the dorsal or ventral sacrococcygeal muscles was about 90-100%. Acupuncture-like stimulation for 1 min could produce either excitation or depression of periurethral EMG activity lasting about 5 or 6 min, depending on the site of insertion and rotation of the acupuncture needles. Excitation of short duration (less than 3 min) was consistently observed from areas of the body distant to the bladder, i.e. the nose, forepaw, forelimb, chest, abdominal wall and hindpaw. Longer lasting excitation of EMG activity was often seen from the penile urethra, perineal area and hindlimb. Depression of EMG activity with a duration of more than 3 min was consistently seen from the muscles at the base of the tail (sacrococcygeus) and perineal area (pubococcygeus and bulbocavernosus). The bladder was partially filled in these experiments, so that micturition contractions were never seen; acupuncture-like stimulation of the perineal area induced some increase in bladder tone in 40% of trials. In spinalized animals, the pattern of activity induced by acupuncture-like stimulation was similar to that seen in spinal cord intact animals and the durations of the effects were not significantly different in these two groups. The distribution of sites from which acupuncture-like stimuli can influence the activity of the lower urinary tract is discussed.

Caudal medullary pathways to lumbosacral motoneuronal cell groups in the cat: evidence for direct projections possibly representing the final common pathway for lordosis

The nucleus retroambiguus (NRA) projects to distinct brainstem and cervical and thoracic cord motoneuronal cell groups. The present paper describes NRA projections to distinct motoneuronal cell groups in the lumbar enlargement. Lumbosacral injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were made to localize and quantify the retrogradely labeled neurons in the caudal medullary lateral tegmentum. These injections were combined with spinal hemisections to distinguish between neurons having ipsi-or contralaterally descending axons. The NRA-lumbosacral fibers descend almost exclusively contralaterally, but neurons in areas surrounding the NRA project mainly ipsilaterally. In an anterograde tracing study, injections of WGA-HRP or tritiated leucine were made in the region of the NRA to determine the NRA targets in the lumbosarcral cord. Hemisections in C2 made it possible to distinguish between NRA projections and projections from neurons in the adjoining lateral tegmentum. The results show delicate NRA projections to distinct lumbosacral motoneuronal cell groups innervating specific hindlimb muscles (iliopsoas, adductors, and hamstrings) as well as axial muscles (medial longissimus and proximal tail muscles). The projection is bilateral, with a contralateral predominance. Ipsilaterally terminating fibers are derived from NRA neurons whose axons cross the midline at the level of the obex, descend through the contralateral spinal white matter, and recross at the level of termination. A conceptual description is presented in which the periaqueductal gray-NRA-lumbosacral projections form the final common pathway for lordosis in the cat.

Morphological analysis of external urethral and external anal sphincter motoneurones of cat

A previous electrophysiological study suggested that there is a common correlation among axonal conduction velocity, input resistance, and size of motoneurones regardless of the muscle type innervated by a motoneurone and that the dendritic arborization is less developed in sphincter than in hindlimb motoneurones. To verify the previous suggestions, cell bodies, axons, and dendrites of external urethral sphincter (EUS) and external anal sphincter (EAS) motoneurones were studied after intracellular labelling with horseradish peroxidase in cats anaesthetised with pentobarbitone. All the cell bodies were located in Onuf's nucleus. The soma diameter (19.7-50.0 microns) was positively correlated with the axonal conduction velocity, and the plots of these two variables were extrapolations from similar plots obtained for hindlimb alpha-motoneurones, supporting the suggestion of the preceding report. Half of motoneurones had recurrent axon collaterals, which terminated within Onuf's nucleus and the ventral border of lamina VIII. The diameter of the cell body with collaterals was significantly larger than that without collaterals. Dendrites extended in five directions: dorsal, medial, lateral, rostral, and caudal. Dorsally directed dendrites of both EUS and EAS motoneurones coursed in lamina VII toward the intermediolateral nucleus, and dendrites of EAS motoneurones further extended toward the intermediomedial nucleus. Long dendrites directed rostrally or caudally within Onuf's nucleus, more prominently in EUS motoneurones. EAS motoneurones had longer end branches and relatively larger number of end branches and summed length and surface area of a dendrite compared with EUS. Among relations, the dendritic surface area and the dendritic volume were tightly correlated with the diameter of the first-order dendrite. The latter relation was almost identical between EUS, EAS, and hindlimb alpha- and gamma-motoneurones. The large values for the dendritic-to-soma surface area ratio (31 in EUS, 50 in EAS) indicated the well-developed dendritic arborization comparable to hindlimb motoneurones. The ratio of sum sigma (daughter diameter)3/2 to the 3/2 power of the parent diameter was 0.99 in EUS and 1.08 in EAS motoneurones, indicating that the 3/2 power constraint at branching points is well satisfied, and Rall's equivalent-cylinder model is applicable to sphincter motoneurones.

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.

Neonatal nerve injury causes long-term changes in growth and distribution of motoneuron dendrites in the rat

Disruption of neuromuscular contact by nerve-crush during the early postnatal period results in the death of a large proportion of affected motoneurons. Increased activity and abnormal reflex responses are evident in those that survive. We have studied the aberrant dendritic morphology of surviving cells and have attempted to correlate the observed alterations in morphology with the above experimental findings. Motoneurons supplying the extensor hallucis longus muscles of the rat were retrogradely labelled with cholera toxin subunit-B conjugated to horseradish peroxidase. The dendritic tree of labelled cells was analysed in adult animals having undergone unilateral sciatic nerve-crush at birth. Unoperated control animals were also examined. Following nerve-crush at birth, total visible dendritic length was more than 30% smaller than control cells in the transverse plane. This decrease was confined largely to the medially directed segments of the dendritic field and appeared to be due to a reduction in dendritic branching combined with a failure to achieve the correct branch length. There was no overall change in total visible dendritic length in the longitudinal plane, but a reorientation of dendrites in favour of rostrodorsal regions was observed. There was no alteration in dendritic length in cells contralateral to the nerve injury. These results show that nerve injury during early postnatal development produces lasting changes in the distribution of motoneuron dendrites. The localized nature of these changes may explain the altered activity and induced death of motoneurons seen after neonatal nerve-crush.

B-50 (GAP-43) in Onuf's nucleus of the adult cat

The nucleus of Onuf in the sacral spinal cord contains motoneurons that innervate the pelvic floor muscles and possess somatic and autonomic characteristics. We show in this study that in the intact adult cat, the immunocytochemical labelling of the nervous tissue-specific growth-associated protein, B-50 (GAP-43), which persists in Onuf's nucleus, differs markedly from that in the remaining 'purely somatic' motor nuclei of the sacral spinal cord. At the light microscopic level, an intense B-50 (GAP-43) immunoreactivity (B-50-IR) in the neuropil of Onuf's nucleus contrasts with a faint staining in the other spinal motor nuclei. Ultrastructurally, B-50-IR is found in Onuf's nucleus within some unmyelinated small diameter nerve fibres and numerous axon terminals on dendritic and somatic surfaces. Conversely, in all other motor nuclei only a few of these structures are stained. No other cellular profiles show B-50-IR in the tissue examined. According to the proposed functions of B-50 (GAP-43), its persistence in mature spinal axon terminals may indicate a latent capability of functional and structural remodeling, as well as an involvement in long-term enhancement in synaptic transmission. If so, these properties would be considerably more pronounced in Onuf's nucleus as compared to purely somatic motor nuclei.

A relay for input from group II muscle afferents in sacral segments of the cat spinal cord

1. A neuronal relay for input from group II afferents of hindlimb muscle nerves has been found in the previously little explored sacral segments of the cat spinal cord. 2. Electrical stimulation of group II muscle afferents of a number of nerves evoked negative potentials on the surface (cord dorsum potentials) and population postsynaptic potentials (field potentials) within the sacral segments. The largest potentials were evoked by stimulation of the posterior biceps-semitendinosus and triceps surae nerves which evoke much smaller potentials in other segments. Group II afferents of other nerves, notably those which have their main relay within the middle lumbar segments, were much less effective. 3. The sites at which cord dorsum and field potentials evoked by group II muscle afferents were recorded varied in relation to the external topography of the L7-S2 spinal segments but were consistent in their location relative to the pudendal motor nucleus (Onuf's nucleus). Potentials evoked by group II afferents of the posterior biceps and semitendinosus nerves peaked at a level corresponding to the rostral half of Onuf's nucleus and potentials evoked by afferents of the gastrocnemius nerves peaked just rostral to this nucleus. The largest field potentials (of 0.5-1.0 mV) were recorded within the dorsal horn. Field potentials in the intermediate zone were much smaller (< 0.3 mV) and were seen less frequently. 4. Evidence was obtained that the dorsal horn field potentials are to a great extent evoked monosynaptically by the fast conducting fraction of group II muscle afferents: (i) they were evoked at short latencies (2.4-2.7 ms from the stimulus; 1.3-1.7 ms from group I components of afferent volleys and 0.5-0.7 ms from group II components of these volleys), (ii) the conduction times of impulses in the fastest conducting fraction of group II afferents, between the sacral segments (where these impulses were induced by intraspinal stimuli) and the peripheral nerves, were only about 0.5 ms shorter than the latencies of field potentials recorded at the site of intraspinal stimulation and evoked by stimulation of the same peripheral nerves and, (iii) the field potentials followed repetitive stimuli without temporal facilitation. 5. Negative cord dorsum and field potentials were also evoked by small stretches of the semitendinosus and triceps surae muscles. Although they were smaller than potentials evoked by electrical stimulation of sensory fibres and appeared at longer latencies, their presence is consistent with a contribution of muscle spindle afferents to the actions of group II muscle afferents within the sacral segments.(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.

Postnatal maturation of the dendritic fields of motoneuron pools supplying flexor and extensor muscles of the distal forelimb in the rat

In the rat cervical spinal cord the corticospinal projection on motoneurons either direct or indirect (via interneurons) comes about postnatally making it accessible for experimental research. Therefore, the postnatal developmental changes of motoneurons and in particular their dendritic fields were examined. Motoneurons innervating the two antagonistic muscles in the distal forepaw, the m. flexor digitorum profundus and the m. extensor digitorum communis, were retrogradely labelled by intramuscular injections of cholera toxin subunit B conjugated with horseradish peroxidase in rats of various postnatal ages. Following a 48–72 hour survival period the motoneurons and their dendritic fields were studied in the seventh and eighth cervical spinal cord segments. Both the number and the position of motoneurons were found to remain constant throughout postnatal development. Extensor motoneurons were positioned dorsolaterally in the ventral horn at the border of grey and white matter, flexor motoneurons were in general medial to extensor motoneurons. The results on the dendritic field demonstrate firstly, that during postnatal development the extension of the dendrites of both flexor and extensor motoneurons changes from spreading out in all directions at postnatal day 2 to spreading in only a few, specific directions from postnatal day 21 onwards, with the restriction that both motoneuron pools follow a different time scale to achieve this. Secondly, both pools have a temporal dendritic component extending into the white matter of the lateral funiculus.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinal distribution of extracellular field potentials generated by electrical stimulation of pudendal and perineal afferents in the cat

Electrical stimulation of sensory pudendal and superficial perineal nerves evokes focal synaptic potentials produced by activation of spinal neurons in the lumbosacral gray matter in chloralose anesthetized or decerebrate cats. The field potentials evoked by sensory pudendal nerve stimulation were located in medial parts of laminae V and VI, and lamina X in the S1 to S3 spinal segments. The superficial perineal cutaneous field potentials partially overlapped with those produced by the pudendal nerve, but in general were localized more laterally in laminae V and VI. The central latencies of the earliest portion of the field potentials evoked by either sensory pudendal or superficial perineal nerves were less than 0.9 ms suggesting that monosynaptic activation of neurons contributed to the potentials.

Structural changes of the soleus and the tibialis anterior motoneuron pool during development in the rat

The morphological development of motoneuron pools of two hindlimb muscles of the rat, soleus (SOL) and tibialis anterior (TA), was studied in rats ranging in age between 8 and 30 postnatal days (P8-P30). Motoneurons were retrogradely labelled by injecting a cholera toxin B subunit solution directly into the muscles. This resulted in extensive labelling of motoneurons as well as their dendritic trees. The distribution of cross sectional areas of neuronal somata was determined for both muscles at various ages. Somal size increased considerably between P8 and P12, whereas growth was moderate between P12 and P20. The size distribution of SOL motoneurons was bimodal from P20, whereas the size distribution of TA motoneurons remained largely unimodal. The morphological development of the dendritic tree was studied qualitatively. The development of dendritic arborization within the SOL and the TA motoneuron pool showed major differences. The arborization pattern of dendrites of TA motoneurons was basically multipolar at all ages. In contrast, dendrites of SOL neurons tended to line up with the rostro-caudal axis and became organized in longitudinal bundles from P16 onwards. The relatively late appearance of dendrite bundles in the soleus motoneuron pool suggests that they might be related to the fine-tuning of neuronal activity rather than patterning of motor activity. The occurrence of dendrite bundles in SOL and not in TA motoneuron pools suggests that they may be related to the different afferent organization of this postural muscle or to its tonic activation pattern.

Membrane electrical properties of external urethral and external anal sphincter somatic motoneurons in the decerebrate cat

Membrane electrical properties of motoneurons innervating the striated muscle of the external urethral and anal sphincters were examined in the decerebrate cat. Both populations of motoneurons had similar electrical properties (mean pooled values; conduction velocity 48 m/s, membrane time constant 3.3 ms, afterhyperpolarization (AHP) duration 97 ms, membrane input resistance 2.2 M omega, rheobase 3.3 nA, and threshold voltage 8.1 mV). Although a portion of cells in both subpopulations of sphincter motoneurons displayed subthreshold conductances (i.e. sag, anomalous rectification), the incidence was higher in the external urethral sphincter motoneurons. The sphincter motoneuron membrane properties were likened to 'slow' hindlimb motoneurons and the low rheobase values suggest that the sphincter motoneurons are easily recruited.