[Mechanisms of divergent-convergent connections in the spinal cord].
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In acute experiments on anesthetized and on spinal cats, three major mechanisms of divergent-convergent interrelationships in the spinal cord responsible for the formation of electrical responses in the efferent and afferent links of the spinal reflex apparatus, were electrophysiologically established: 1) afferent-efferent, 2) afferent-interneuron-efferent, and 3) afferent--interneuron-afferent mechanisms each having special features of its structural and functional organization. The first two mechanisms are actualized through mono- and poly-synaptic discharges in the ventral roots, resp., and the third one--through electrotonic potentials and reflexes of the dorsal roots. Intracellular recording of potentials revealed that single interneurons participated in the actualization of the second and third mechanisms of the divergent-convergent interrelationships.Keywords:
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Two pairs of wire electrodes were used to record single afferent action potentials from ventral roots and single efferent action potentials from dorsal roots of dogs and humans. A human lower sacral ventral root contained about 20 to 30% afferents among fibres with a diameter larger than 5 microns; a comparable ventral root of a dog contained about 1% afferents. Human S3, S4 and S5 dorsal roots contained 3, 18, and 20 to 30% efferent fibres respectively; a comparable dorsal root of the dog contained less than 1% efferent fibres. Primary and secondary muscle spindle afferents, Golgi tendon organ afferents, and afferents from the mechanoreceptors of the urinary bladder and anal canal mucosa were activated in a dog ventral root by pulling bladder and anal catheters. Their peak group conduction velocities were 82, 57, 71 and 18 m/s at 34 degrees C respectively. The dog afferents conducted more than 30% faster than did comparable human nerve fibres. By strongly pulling the bladder catheter, the static human dorsal root gamma 21-motoneurons increased their activity for about 7 s which in turn strongly increased the dorsal root spindle afferent activity for more than 10 min; the human static intrafusal gamma-motoneurons seemed to show cumulative properties.
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In acute experiments on anesthetized and on spinal cats, three major mechanisms of divergent-convergent interrelationships in the spinal cord responsible for the formation of electrical responses in the efferent and afferent links of the spinal reflex apparatus, were electrophysiologically established: 1) afferent-efferent, 2) afferent-interneuron-efferent, and 3) afferent--interneuron-afferent mechanisms each having special features of its structural and functional organization. The first two mechanisms are actualized through mono- and poly-synaptic discharges in the ventral roots, resp., and the third one--through electrotonic potentials and reflexes of the dorsal roots. Intracellular recording of potentials revealed that single interneurons participated in the actualization of the second and third mechanisms of the divergent-convergent interrelationships.
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THE firing characteristics of individual neurons of the mammalian nervous system have been studied in varying structures with a wide range of stimuli, anesthetics, and surgical procedures, while attention has not been focused upon comparing spontaneous activity in normal and abnormal states. Previous studies of activity in the cortical epileptic focus by Ward et al1-4have elucidated two attributes of "epileptic" neurons: autonomous high-frequency bursts and loss of synaptic drive which has been attributed to the absence of dendritic spines.5On the basis of these investigations, it has been postulated that the "epileptic" cell is in a state of partial functional deafferentation and that the abnormal firing pattern is a consequence of this deafferentation. Studies utilizing cortical neurons cannot divorce afferent from efferent activity because of the complex geometry of the cortical feltwork but the anatomy of the spinal cord permits deafferentation without manipulation of efferent systems.
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1. Stimulation of a loop of small intestine by pressure, heat, mechanical injury or electrical stimulation of its afferent nerve causes an inhibition of the whole intestine. It is a reflex response and is independent of the vagus and of the adrenal gland. 2. The reflex centre lies in the lower part of the thoracic and upper part of the lumbar cord. Afferent fibres enter the cord via dorsal roots from the seventh thoracic to the first lumbar segments. Efferent fibres leave the cord via ventral roots from the eighth thoracic to the first lumbar segments. 3. The fibre connections in the spinal cord was also investigated. It is found that most of the afferent fibres do not cross to the opposite side, nor ascend or descend in the cord, but make connections with the efferent neurones in the same side and same segment of the cord.
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Abstract Using Golgi‐stained serial sections obtained at the sacro‐caudal levels of the cat spinal cord, it was possible to reconstruct the trajectory of primary afferents. They were classified into two groups: reliable primary afferents directly traced from the dorsal root and probable primary afferents traced from the dorsal funiculus or Lissauer's tract. The diameters of the reliable primary afferents vary from 0.88–1.88 μm. According to their courses, reliable primary afferents as well as probable primary afferents were classified into three groups: the first is distributed to both medial and lateral halves of the dorsal horn, the second to the medial half, and the third to the lateral half. Commissural fibers were also observed among the probable primary afferents. The rostro‐caudal and medio‐lateral extents of reliable primary afferents are found to be between 250 and 950 μm and 270 and 700 μm respectively, while those of the probable primary afferents were between 125 and 670 μm and 270 and 1,640 μm respectively. These primary afferent fibers are connected with at least two or more laminae of the dorsal horn gray matter.
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The different subdivisions along the mediolateral extent of the superficial dorsal horn of the spinal cord are generally regarded as identical structures that execute the function of sensory information processing without any significant communication with other regions of the spinal gray matter. In contrast to this standing, here we endeavor to show that neural assemblies along the mediolateral extent of laminae I-IV cannot be regarded as identical structures. After injecting Phaseolus vulgaris leucoagglutinin and biotinylated dextran amine into various areas of the superficial dorsal horn (laminae I-IV) at the level of the lumbar spinal cord in rats, we have demonstrated that the medial and lateral areas of the superficial dorsal horn show the following distinct features in their propriospinal afferent and efferent connections: 1) A 300- to 400-microm-long section of the medial aspects of laminae I-IV projects to and receives afferent fibers from a three segment long compartment of the spinal dorsal gray matter, whereas the same length of the lateral aspects of laminae I-IV projects to and receives afferent fibers from the entire rostrocaudal extent of the lumbar spinal cord. 2) The medial aspects of laminae I-IV project extensively to the lateral areas of the superficial dorsal horn. In contrast to this, the lateral areas of laminae I-IV, with the exception of a few fibers at the segmental level, do not project back to the medial territories. 3) There is a substantial direct commissural connection between the lateral aspects of laminae I-IV on the two sides of the lumbar spinal cord. The medial part of laminae I-IV, however, does not establish any direct connection with the gray matter on the opposite side. 4) The lateral aspects of laminae I-IV appear to be the primary source of fibers projecting to the ipsi- and contralateral ventral horns and supraspinal brain centers. Projecting fibers arise from the medial subdivision of laminae I-IV in a substantially lower number. The findings indicate that the medial and lateral areas of the superficial spinal dorsal horn of rats may play different roles in sensory information processing.
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Abstract Central distribution of afferent and efferent components of the pudendal nerve was examined in the cat by the HRP method after applying HRP to the central cut end of the pudendal nerve. Retrogradely labeled neuronal cell bodies were located primarily in the feline homologue of the Onuf's X nucleus, constituting a slender longitudinal cell column in the ventral horn of the S1 and S2 cord segments. The Onuf's nucleus was present constantly from middle S1 to high S2 cord segments, and occasionally extended rostrally to high S1 or low L7, and caudally to middle S2, low S2, or high S3 cord segments. No sex differences were observed in the distribution pattern, number, and soma size of labeled neurons in the Onuf's nucleus. Transganglionically labeled dorsal root fibers were found to terminate ipsilaterally in the lamina I of the dorsal horn at levels of lower lumbar, sacral, and higher coccygeal cord segments and the gracile nucleus, and bilaterally with an ipsilateral predominance in the dorsal commissural gray and laminae III, IV, V, and VI of the dorsal horn at levels of lower lumbar, sacral, and higher coccygeal cord segments. Some labeled dorsal root fibers appeared to end ipsilaterally in the regions where the sacral parasympathetic preganglionic neurons have been shown to be located.
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