The organization of serotonin fibers in the anterior column of the mammalian spinal cord. An immunohistochemical study

Detailed comparative analysis of the organization of serotonin fibers in the anterior column of the mammalian spinal cord (rat, guinea pig, cat, dog and monkey) was carried out by use of the indirect antibody peroxidase-antiperoxidase (PAP) method. The plexus formation of serotonin-containing varicose fibers around the alpha-motoneurons in the monkey anterior horn was in much closer apposition to the cell bodies in comparison with the spinal cords of the rodents and carnivores. The results may suggest that anterior horn motoneurons in the simian spinal cord are intimately innervated by serotonin fibers in a manner different from that of rodents and carnivores. Furthermore, the small cell groups endowed with particularly dense networks of serotonin fibers were demonstrable in the anterior horn of L1-L2 segments of rats, and L3-L4 of guinea pigs and monkeys; however, in the lumbar levels of the carnivores this was not the case. Hence it seems doubtless that there exists in the lumbar anterior horn of the rodent and primate spinal cords a cell group with an unknown specialized function.     

The organization of serotonin fibers in the mammalian superior colliculus

Summary The distribution of serotonin immunoreactivity in the superior colliculus (SC) of the rat, hamster, chipmunk, cat, and monkey was studied using a sensitive immunohistochemical method. In all of these animals, serotonin immunoreactivity formed a dense network of varicose fibers throughout the SC. These fibers had a characteristic arrangement corresponding to the laminar structures of the SC. Except in the chipmunk, serotonergic fibers were more dense in the stratum griseum superficiale than in the other layers. In the SC of the chipmunk, these fibers appeared evenly distributed.To explore the degree of scrotonergic innervation in each layer, a semi-quantitative assay of serotonin immunoreactive varicosities was conducted in the rat, chipmunk, cat, and monkey. Peaks in varicose density were seen in the stratum griseum superficiale, the stratum griseum intermedium and the stratum griseum profundum. In the rat, cat, and monkey, the highest density of these varicosities was in the stratum griseum superficiale. On the other hand, the stratum griseum intermedium of the chipmunk SC received the greatest innervation of serotonergic varicose fibers.Supported by a grant from the Ministry of Education, Science, and Culture of Japan (No. 57214028)     

Sexual differences in the topographical distribution of serotonergic fibers in the anterior column of rat lumbar spinal cord

Summary Sexual dimorphism in the topographical organization of immunoreactive serotonergic fibers has been shown for the first time in the anterior column of the rat lumbar cord. A characteristic preferential arrangement of serotonergic fibers on the small cell column composed of anterior horn motoneurons, which have been proved to send their axons to the cremaster muscle, was demonstrated at the lumbar segments (L1–L2) of male rats, using the immunoperoxidase technique with antiserum against serotonin. A similar finding was also observed in female rats, but was less prominent than in males. Comparative analysis of the cell numbers and the size of neuronal somata of the aforementioned nucleus, done using retrograde transport of fluorescent dye (DAPI) via axonal fibers coursing down the genitofemoral nerve to the cremaster muscle, gave significantly larger values in males.The sexual difference in the serotonergic innervation pattern was, in consequence, surmised to be caused by the cytoarchitectonic contrast ascertained in the lumbar anterior column. Furthermore, there may be a striated muscle endowed with some active functions homologous to those of the male cremaster muscle in the female rat.     

The pattern of distribution of serotoninergic fibers in the anterior horn of the chick spinal cord

Summary The pattern of distribution of serotonin positive fibers in the motor nuclei of the chick spinal cord was examined immunohistochemically by using an antiserum against serotonin. A dense aggregation of serotoninergic fibers was located around anterior horn cells in the cervical spinal cord. In the brachial spinal cord, serotoninergic fibers were densely aggregated in the medial motor column and in the parts of the lateral motor column. There were two regions of serotonin immunoreactivity in the lateral motor column of the brachial spinal cord; one located in the ventromedial regions where a dense aggregation of serotoninergic fibers was found, and the reminder of the lateral motor column where only a few serotoninergic fibers were observed. The region containing a dense cluster of serotoninergic fibres around profiles of motoneuron somata and proximal dendrites appears to correspond to motor neuron pools of flexor muscles. In the thoracic spinal cord a high density of serotoninergic fibers was found in the motor nucleus. In the lumbosacral spinal cord (segments LS1–LS8) serotoninergic fibers were not observed in the medial motor column. However, there were five regions in the lateral motor column, where a high density of serotoninergic fibers was found. These very likely correspond to motor neuron pools of muscles which extend the hip joint.     

Segmental somatotopic organization of cutaneous afferent fibers in the lumbar spinal cord dorsal horn in rats

In the present study, we investigated the central representation of segmental cutaneous afferent fiber projection fields in the horizontal plane of the spinal cord dorsal horn in adult rats. The neurotracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil) was applied to spinal nerves T12-S2 and cutaneous ventrodorsal axial lines T13-S1. The Dil fluorescent zones in transverse sections of the dorsal horn were observed microscopically. Mediolateral locations of Dil fluorescent zones were measured, followed by reorganization on the horizontal plane through lamina I-I111. Rostral and caudal boundary lines of the central projection fields of spinal nerves T12-S2 formed 'waves' in the horizontal plane of the dorsal horn, pitching rostrocaudally about one spinal cord segment. The rostral and caudal apexes of the waves could be linked with those of adjacent segments, suggesting that the wave pattern is continuous rostrocaudally in the dorsal horn. The waves were markedly transformed in the central projection fields of the hindlimb and genital regions, in the L5 and L6 spinal cord segments.     

The organization of spinal motor neurons in a monotreme is consistent with a six‐region schema of the mammalian spinal cord

The motor neurons in the spinal cord of an echidna (Tachyglossus aculeatus) have been mapped in Nissl‐stained sections from spinal cord segments defined by spinal nerve anatomy. A medial motor column of motor neurons is found at all spinal cord levels, and a hypaxial column is found at most levels. The organization of the motor neuron clusters in the lateral motor column of the brachial (C5 to T3) and crural (L2 to S3) limb enlargements is very similar to the pattern previously revealed by retrograde tracing in placental mammals, and the motor neuron clusters have been tentatively identified according to the muscle groups they are likely to supply. The region separating the two limb enlargements (T4 to L1) contains preganglionic motor neurons that appear to represent the spinal sympathetic outflow. Immediately caudal to the crural limb enlargement is a short column of preganglionic motor neurons (S3 to S4), which it is believed represents the pelvic parasympathetic outflow. The rostral and caudal ends of the spinal cord contain neither a lateral motor column nor a preganglionic column. Branchial motor neurons (which are believed to supply the sternomastoid and trapezius muscles) are present at the lateral margin of the ventral horn in rostral cervical segments (C2–C4). These same segments contain the phrenic nucleus, which belongs to the hypaxial column. The presence or absence of the main spinal motor neuron columns in the different regions echidna spinal cord (and also in that of other amniote vertebrates) provides a basis for dividing the spinal cord into six main regions – prebrachial, brachial, postbrachial, crural, postcrural and caudal. The considerable biological and functional significance of this subdivision pattern is supported by recent studies on spinal cord hox gene expression in chicks and mice. On the other hand, the familiar ‘segments’ of the spinal cord are defined only by the anatomy of adjacent vertebrae, and are not demarcated by intrinsic gene expression. The recognition of segments defined by vertebrae (somites) is obviously of great value in defining topography, but the emphasis on such segments obscures the underlying evolutionary reality of a spinal cord comprised of six genetically defined regions. The six‐region system can be usefully applied to the spinal cord of any amniote (and probably most anurans), independent of the number of vertebral segments in each part of the spinal column.     

Neurotrophins and synaptic plasticity in the mammalian spinal cord

The pathway mediating the monosynaptic stretch reflex has served as an important model system for studies of plasticity in the spinal cord. Its usefulness is extended by evidence that neurotrophins, particularly neurotrophin-3 (NT-3), which has been shown to promote spinal axon elongation, can modulate the efficacy of the muscle spindle-motoneurone connection both after peripheral nerve injury and during development. The findings summarized here emphasize the potential for neurotrophins to modify function of both damaged and undamaged neurones. It is important to recognize that these effects may be functionally detrimental as well as beneficial.     

A new somatostatinergic system in the mammalian spinal cord

In addition to the nerve endings originating from primary afferent neurons and terminating in the substantia gelatinosa of the spinal cord, a somatostatinergic network surrounding the central canal (lamina X) and extending into the zona intermedia including the nucleus intermediomedalis and the columna lateralis (laminae VI and VII) has been detected. somatostatin-immunoreactive axons have also been seen in many other regions of the grey matter in the spinal cord. These results indicate that the somatostatinergic system is more developed in the spinal cord as has yet been suggested.     

Topographic organization of monosynaptic reflexes in the cat spinal cord

The topographic organization of monosynaptic reflexes in the cat spinal cord has been studied by comparing the amplitude of reflex discharges recorded from ventral roots consequent to stimulation of dorsal roots entering the cord at different spinal segments. The results indicate that up to 80% of the potentiated monosynaptic reflex discharge recorded from a ventral root can be attributed to afferent input entering the spinal cord at the same segmental level. Moreover, within the same segment, afferents with a more rostral cord entry level exert a stronger synaptic effect on the more rostral portion of the corresponding ventral root.     

The specificity of strychnine as a glycine antagonist in the mammalian spinal cord

Summary An investigation was made of the influence of strychnine on the depression of the firing of spinal interneurones and Renshaw cells by glycine, GABA, nor-adrenaline and 3-hydroxytyramine. Administered electrophoretically or intravenously, strychnine blocks the effect of glycine more readily than that of the other depressants. Such specific antagonism of glycine action by relatively low concentrations of strychnine may be competitive in nature, but technical difficulties precluded a full assessment of the type of antagonism. The effects of relatively high concentrations of strychnine on the action of the other depressants probably result from interference with membrane permeability changes. The findings are considered to support previous proposals that glycine is the transmitter at spinal strychnine-sensitive inhibitory synapses.     

The organization of primary afferent depolarization in the isolated spinal cord of the frog

1. The organization of primary afferent depolarization (PAD) produced by excitation of peripheral sensory and motor nerves was studied in the frog cord isolated with hind limb nerves.2. Dorsal root potentials from sensory fibres (DR-DRPs) were evoked on stimulation of most sensory nerves, but were largest from cutaneous, joint and flexor muscle afferents. With single shock stimulation the largest cutaneous and joint afferent fibres gave DR-DRPs, but potentials from muscle nerves resulted from activation of sensory fibres with thresholds to electrical stimulation higher than 1.2-1.5 times the threshold of the most excitable fibres in the nerve. This suggests that PAD from muscle afferents is probably due to excitation of extrafusal receptors.3. Dorsal root potentials produced by antidromic activation of motor fibres (VR-DRPs) were larger from extensor muscles and smaller or absent from flexor muscles. The VR-DRPs were produced by activation of the lowest threshold motor fibres.4. Three types of interactions were found between test and conditioning DRPs from the same or different nerves. With maximal responses occlusion was usually pronounced. At submaximal levels linear summation occurred. Near threshold the conditioning stimulus frequently resulted in a large facilitation of the test DRP. All three types of interactions were found with two DR-DRPs, two VR-DRPs or one DR-DRP and one VR-DRP.5. The excitability of sensory nerve terminals from most peripheral nerves was increased during the DR-DRP. The magnitude of the excitability increase varied roughly with the magnitude of the DR-DRP evoked by the conditioning stimulus.6. There was a marked excitability increase of cutaneous and extensor muscle afferent terminals during the VR-DRP. Flexor muscle afferent terminals often showed no excitability changes to ventral root stimulation. In those experiments where afferent terminals from flexor muscles did show an excitability increase, the effects were smaller than those of cutaneous and extensor terminals.7. The VR-DRPs appear to reflect activity of a negative feed-back loop from extensor motoneurones on to sensory fibres from cutaneous and extensor muscles. This system may have a role in modulating the ballistic movement of the frog. DR-DRPs, on the contrary, are widespread in origin and distribution. PAD from sensory fibres may function to sharpen contrast between incoming afferent information.     

脊髓内去甲肾上腺素能纤维的来源

目的：研究脊髓内去甲肾上腺素(NA)能纤维的来源。方法：选用Wistar大鼠，应用神经递质NA特异性的标志酶——多巴胺β羟化酶(DBH)抗体注入其脊髓单侧，分别于注射后1h-7天灌注大鼠，取出脊髓和脑组织作免疫组化染色。结果：ADBH注入脊髓后48h内依次在腰、胸、颈髓同侧观察到大量DBH阳性纤维，对侧仅少量阳性纤维出现：注射后3—4天，双侧上橄榄核、LC、SC、KF核、臂旁核中出现大量阳性神经元，在延髓内仅见大量阳性纤维，未观察到阳性神经元胞体出现；注射后7天，上述阳性神经元和阳性纤维消失。结论：脊髓内的NA能纤维及终末几乎全部来源于脑桥NA能细胞群。     

Afferent connections of posterior column nuclei of the spinal cord

Brain Institute, All-Union Mental Health Research Center, Academy of Medical Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR D. S. Sarkisov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 110, No. 9, pp. 229–231, September, 1990.     

Depolarizing afterpotentials in myelinated axons of mammalian spinal cord

Microelectrode recordings were made from 5-10 micron dia axons of adult rat spinal cord in vitro. Action potentials in response to electrical stimulation were recorded intracellularly and electrical characteristics of the axons were examined by injecting current pulses through a bridge circuit. All action potentials larger in amplitude than 80 mV were followed by depolarizing afterpotentials, similar to those recorded in peripheral axons [Barrett and Barrett (1982) J. Physiol., Lond. 323, 117-144]. The afterpotential could be described as the sum of three exponential components, the time constants of which (tau 1, tau 2 and tau 3) were 25.2 +/- 5.6, 3.1 +/- 0.8 and 0.8 +/0 0.3 ms, respectively, at 25 degrees C and a membrane potential of -80 mV. The maximal amplitudes of the afterpotential components, obtained by extrapolating to the peak of the action potential, were 3.8 +/- 1.0, 6.4 +/- 5.2 and 21.7 +/- 9.8 mV, for action potential amplitudes of 102 +/- 11 mV. The amplitude of the longest component of the afterpotential decreased with depolarization and increased with hyperpolarization at the recording site. The amplitude decreased markedly with increase of temperature to physiological levels, in conjunction with the expected decrease in action potential duration. Similar afterpotential components were present in the response of the axon to injected hyperpolarizing current pulses. The observations are consistent with the suggestion [Barrett and Barrett (1982) J. Physiol., Lond. 323, 117-144] that the afterpotential results from charging of the axolemmal capacitance by current passing through the myelin sheath during the action potential. They are inconsistent with a number of calculations of electrical characteristics of peripheral axons derived from voltage clamp experiments in isolated fibers. It is argued that the electrical resistance of the myelin lamellae is relatively low, though within the range calculated for other glial membranes. This suggestion is found more compatible with the available morphological data than the alternative proposal that a leakage pathway under the myelin sheath might be responsible for the afterpotential [Barrett and Barrett (1982) J. Physiol., Lond. 323, 117-144]. The significance of this organization for the function of myelinated axons and the electrical basis of the afterpotential are examined further in the accompanying paper [Blight (1985) Neuroscience 15, 13-31].     

Motor coordination without action potentials in the mammalian spinal cord

Coordination of neuronal activity to produce movement is generally thought to depend on spike activity in premotor interneuronal networks. Here we show that even without such activity, the neonatal rat spinal cord could produce a stable motor rhythm mediated by the synchronization of motor neuron oscillations across gap junctions. These rhythms, however, were not coordinated between motor pools in different parts of the spinal cord. We further showed that neural coordination through gap junctions contributed to the fundamental organization and function of spinal motor systems. These results suggest that neural coordination across gap junctions is important in motor-pattern generation in the mammalian spinal cord.