Synaptic Organization of Afferent Fibers from Viscera in the Spinal Cord

Observations have been made upon a typical flexor reflex with the aim of disclosing the changes in amount, latency, and temporal configuration of reflex discharge that take place as afferent input is varied from zero to maximal for the band of cutaneous myelinated afferent fibers that extends upward from approximately 6 µ in diameter (group II fibers). Reflex threshold is reached at 6 to 12 per cent maximal afferent input. From threshold to maximal input the relation between input and amount of output is essentially linear, latency on the average decreases, the shorter central paths in general gain preference, but the known minimum pathway, one of three neurons, does not transmit unless aided by convergent activity. Flexor reflex discharge may occur in several bursts suggesting the existence of closed chain connections in the internuncial pools of the spinal cord. At any given input there is, in successively elicited reflexes, little correlation between latency and amount of discharge, at first sight a surprising result for each variable can be taken as a measure of excitability status of the motoneuron population. However, latency of discharge indicates excitability at the beginning of the reflex event whereas amount of discharge is an expression of excitability over the entire period of discharge. Given a constantly and rapidly fluctuating excitability absence of correlation between these variables would be an anticipated result.

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Synaptic Organization of the Intermediolateral Nucleus of the Thoracic Spinal Cord: Monoamine Histochemistry and Peptide Immunohistochemistry

Histochemistry and Cell Biology of Autonomic Neurons and Paraganglia ◽

10.1007/978-3-642-72749-8_28 ◽

1987 ◽

pp. 157-162

Author(s):

T. Chiba ◽

S. Masuko

Keyword(s):

Spinal Cord ◽

Synaptic Organization ◽

Thoracic Spinal Cord ◽

Thoracic Spinal ◽

Intermediolateral Nucleus

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Response and receptive-field properties of spinomesencephalic tract cells in the cat

Journal of Neurophysiology ◽

10.1152/jn.1986.55.1.76 ◽

1986 ◽

Vol 55 (1) ◽

pp. 76-96 ◽

Cited By ~ 41

Author(s):

R. P. Yezierski ◽

R. H. Schwartz

Keyword(s):

Spinal Cord ◽

Receptive Field ◽

Dynamic Range ◽

Receptive Fields ◽

The Body ◽

Noxious Stimulation ◽

Primary Afferent ◽

Receptive Field Properties ◽

Afferent Fibers ◽

Primary Afferent Fibers

Recordings were made from 90 identified spinomesencephalic tract (SMT) cells in the lumbosacral spinal cord of cats anesthetized with alpha-chloralose and pentobarbital sodium. Recording sites were located in laminae I-VIII. Antidromic stimulation sites were located in different regions of the rostral and caudal midbrain including the periaqueductal gray, midbrain reticular formation, and the deep layers of the superior colliculus. Twelve SMT cells were antidromically activated from more than one midbrain level or from sites in the medial thalamus. The mean conduction velocity for the population of cells sampled was 45.2 +/- 21.4 m/s. Cells were categorized based on their responses to graded intensities of mechanical stimuli and the location of excitatory and/or inhibitory receptive fields. Four major categories of cells were encountered: wide dynamic range (WDR); high threshold (HT); deep/tap; and nonresponsive. WDR and HT cells had excitatory and/or inhibitory receptive fields restricted to the ipsilateral hindlimb or extending to other parts of the body including the tail, forelimbs, and face. Some cells had long afterdischarges following noxious stimulation, whereas others had high rates of background activity that was depressed by nonnoxious and noxious stimuli. Deep/tap cells received convergent input from muscle, joint, or visceral primary afferent fibers. The placement of mechanical lesions at different rostrocaudal levels of the cervical spinal cord provided information related to the spinal trajectory of SMT axons. Six axons were located contralateral to the recording electrode in the ventrolateral/medial or lateral funiculi while two were located in the ventrolateral funiculus of the ipsilateral spinal cord. Stimulation at sites used to antidromically activate SMT cells resulted in the inhibition of background and evoked responses for 22 of 25 cells tested. Inhibitory effects were observed on responses evoked by low/high intensity cutaneous stimuli and by the activation of joint or muscle primary afferent fibers. Based on the response and receptive-field properties of SMT cells it is suggested that the SMT may have an important role in somatosensory mechanisms, particularly those related to nociception.

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Synergistic Enhancement of Glutamate-Mediated Responses by Serotonin and Forskolin in Adult Mouse Spinal Dorsal Horn Neurons

Journal of Neurophysiology ◽

10.1152/jn.00423.2001 ◽

2002 ◽

Vol 87 (2) ◽

pp. 732-739 ◽

Cited By ~ 24

Author(s):

Guo-Du Wang ◽

Min Zhuo

Keyword(s):

Spinal Cord ◽

Nmda Receptors ◽

Dorsal Horn ◽

Spinal Dorsal Horn ◽

Adult Mouse ◽

Dorsal Horn Neurons ◽

Spinal Dorsal ◽

Afferent Fibers ◽

Primary Afferent Fibers ◽

Spinal Dorsal Horn Neurons

Glutamate is the major excitatory amino acid neurotransmitter in the CNS, including the neocortex, hippocampus, and spinal cord. Normal synaptic transmission is mainly mediated by glutamate AMPA and/or kainate receptors. Glutamate N-methyl-d-aspartate (NMDA) receptors are normally inactive and only activated when a sufficient postsynaptic depolarization is induced by the activity. Here we show that in sensory synapses of adult mouse, some synaptic responses (26.3% of a total of 38 experiments) between primary afferent fibers and dorsal horn neurons are almost completely mediated by NMDA receptors. Dorsal root stimulation did not elicit any detectable AMPA/kainate receptor-mediated responses in these synapses. Unlike young spinal cord, serotonin alone did not produce any long-lasting synaptic enhancement in adult spinal dorsal horn neurons. However, co-application of the adenylyl cyclase activator forskolin and serotonin (5-HT) produced long-lasting enhancement, including the recruitment of functional AMPA receptor-mediated responses. Calcium-sensitive, calmodulin-regulated adenylyl cyclases (AC1, AC8) are required for the enhancement. Furthermore the thresholds for generating action potential responses were decreased, and, in many cases, co-application of forskolin and 5-HT led to the generation of action potentials by previously subthreshold stimulation of primary afferent fibers in the presence of the NMDA receptor blocker 2-amino-5-phosphonovaleric acid. Our results suggest that pure NMDA synapses exist on sensory neurons in adult spinal cord and that they may contribute to functional sensory transmission. The synergistic recruitment of functional AMPA responses by 5-HT and forskolin provides a new cellular mechanism for glutamatergic synapses in mammalian spinal cord.

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Intraaxonal injection of neurobiotin reveals the long-ranging projections of A beta-hair follicle afferent fibers to the cat dorsal horn

Journal of Neurophysiology ◽

10.1152/jn.1996.76.1.242 ◽

1996 ◽

Vol 76 (1) ◽

pp. 242-254 ◽

Cited By ~ 11

Author(s):

P. Wilson ◽

P. D. Kitchener ◽

P. J. Snow

Keyword(s):

Spinal Cord ◽

Horseradish Peroxidase ◽

Hair Follicle ◽

Dorsal Horn ◽

Receptive Fields ◽

Dorsal Horn Neurons ◽

Somatotopic Organization ◽

Afferent Fibers ◽

Synaptic Boutons

1. The morphology and somatotopic organization of the spinal arborizations of identified A beta-hair follicle afferent fibers (HFAs) with receptive fields (RFs) on the digits have been investigated in the cat by the use of intraaxonal injection of the tracer n-(2 aminoethyl) biotinamide. 2. In three cats, the long-ranging projections of six HFAs were examined by selectively injecting afferents with RFs on digit 2, 4, or 5, directly over the digit 3 representation, and examining their collateral morphology in transverse sections of the spinal cord. The rostral and caudal boundaries of the digit 3 representation were determined by mapping the RFs of identified spinocervical tract (SCT) neurons. 3. In two more cats, three HFAs were injected at random rostrocaudal positions and their morphology was examined in parasagittal sections. In one animal (2 HFAs), the somatotopy of the digit representation was again determined by mapping the RFs of SCT neurons. In the remaining cat (1 HFA), the somatotopy of the dorsal horn was mapped from the RFs of unidentified dorsal horn neurons. 4. Hair follicle afferents emitted many more collaterals, over much greater rostrocaudal distances, than indicated by previous horseradish peroxidase studies, and all collaterals gave rise to synaptic boutons. 5. HFAs that have RFs confined to a small part of a digit give rise to bouton-bearing axonal branches throughout the entire rostrocaudal extent of the hindpaw representation.

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