Distribution of somatic and visceral primary afferent fibres within the thoracic spinal cord of the cat

VIP terminals, axons, and neurons: distribution throughout the length of monkey and cat spinal cord

The distribution of vasoactive intestinal polypeptide (VIP) was mapped by peroxidase immunocytochemistry in the spinal cords of seven Macaca fascicularis monkeys and two cats. The animals were perfusion fixed with different chemicals. Those that were perfused with either a Zamboni fixative or 5% acrolein had significantly greater immunoreactivity outside the sacral cords; those fixed with 4% paraformaldehyde had little in nonsacral regions. VIP-like immunoreactive (VIP) axons and terminals were found in the superficial dorsal horn, reticular nucleus of lamina V, intermediomedial nucleus, and lamina X at all levels from C2 to S4; a few axons and terminals were also seen in the ventral horn. Axons were found in Lissauer's tract at all levels, and axons appeared in the dorsolateral and ventrolateral white matter at midthoracic levels; in the lumbosacral cord the number and extent of axons in the lateral and ventral white matter increased progressively in a caudal direction. VIP neurons were identified in thoracic intermediate gray lateral to the central canal and in the intercalatus (IC) and intermediolateral (IML) nuclei. Electron microscopy of the VIP terminals in laminae I and II of the cervical cord revealed they contain small round vesicles and many large granular vesicles; some are glomerular terminals and most form asymmetrical synaptic contacts onto dendrites. These results indicate VIP is much more widely distributed in the spinal cord than previously thought; VIP may be associated with both visceral thoracic and lumbosacral afferents, and with other afferents in the cervical cord; VIP neurons are present in the thoracic intermediate gray; and VIP axons in the ventral and lateral white matter indicate that the spinal cord is supplied in part by VIP sources other than primary afferents.

Spinal termination of nociceptive afferent fibres from deep tissues in the cat

In anaesthetized cats, the impulse activity in afferent fibres from the deep tissues of the lower hindlimb was recorded at the dorsal root entry zone with micropipettes filled with horseradish peroxidase solution. After identification of the fibres as nociceptive with the use of mechanical stimuli and ascertainment of the deep location of the receptive field (in fascia, muscle, tendon, ligament or joint capsule) the enzyme was injected iontophoretically into the axon. Histochemical processing of the tissue with diaminobenzidine was employed to visualize the spinal terminal arborizations of the units. The fibres conducted at 18.3-40.0 m/s; they showed two different patterns of termination: (1) units terminating exclusively in lamina I of the dorsal horn; (2) units possessing their main projection area in laminae IV/V with only a few collaterals and boutons in lamina I. Thus, the nociceptive fibres from deep tissues terminated in those laminae of the dorsal horn which are known to contain cells of origin of ascending nociceptive tracts.

The distribution of muscle primary afferents from the masseter nerve to the trigeminal sensory nuclei

Transganglionic transport of horseradish peroxidase--wheat germ agglutinin conjugate was used to study the pattern of termination of somatic afferent fibers innervating the masseter muscle within the trigeminal sensory nuclear complex (TSNC) of the cat. The central processes of the masseteric nerve terminated in the caudal third of the pars interpolaris, and laminae I/V through the caudal two-thirds of caudalis and rostral parts of the C1 spinal cord segment. The functional significance of the masseteric afferent projections to the TSNC with a preferential pattern was discussed, particularly with respect to muscle pain.

The pattern of spinal and medullary projections from a cutaneous nerve and a muscle nerve of the forelimb of the cat: a study using the transganglionic transport of HRP

The transport of HRP into the spinal cord and medulla in the cat has been examined from a forelimb cutaneous nerve, the lateral superficial radial nerve (LSR), and from the muscle nerves supplying both heads of the forelimb muscle, extensor carpi radialis (ECR). HRP transported by the LSR was widely distributed in the spinal cord throughout laminae I-IV in the vicinity of the root entry zone and from spinal segments T1 to C5. HRP was also transported from the LSR to the medulla where there was intense patchy, discontinuous labelling in the main cuneate nucleus. The pattern of labelling in the cuneate nucleus did not follow any simple somatotopic plan. Exposure of the muscle nerve to HRP led to labelling in the spinal dorsal horn in lamina I, in the deep dorsal horn on the lamina V/VI border, and in lateral and medial lamina VI at sites that contain cells of origin of spinocerebellar tracts. The medial lamina VI label was contiguous with a deposit that extended medially to the central canal. The label in lateral lamina VI was patchy and formed a discontinuous column from T1 to C5. HRP transported by the muscle nerve also produced label in the more ventral regions of the cuneate nucleus where it had a lacy appearance, in part due to its extensive distribution around dendrites. A relatively dense, patchy, and discontinuous deposit of reaction product was also present in the external cuneate nucleus after muscle nerve exposure. This deposit was most intense on the dorsomedial surface of this nucleus, but another, less intense, deposit was also present ventrally.

The central projections of primary afferent neurons of greater splanchnic and intercostal nerves in the rat. A horseradish peroxidase study

The central projections of primary afferent fibers of the greater splanchnic nerve of the rat were investigated using the transganglionic horseradish peroxidase transport technique. In addition, the corresponding spinal ganglion cells and the preganglionic sympathetic neurons were demonstrated. For comparing visceral and somatic afferents, intercostal nerve afferents were labelled by the same technique. Splanchnic afferent dorsal root ganglion cells were found at segments T3 to T13 ipsilaterally, with the greatest density at T8 to T12. Labelled cells represented about 10%-15% of all neurons in the ganglia at maximal projection levels. They were randomly distributed within individual ganglia. The great majority were medium to small sized and round to slightly oval in shape. In the spinal cord, labelled visceral afferent axons were found maximally at T8 to T11, but could be detected in decreasing density up to T1 and down to L1. They were distributed over Lissauer's tract and the dorsal funiculus to a medial and lateral collateral pathway (MCP and LCP, respectively). The MCP, somewhat more prominent than the LCP, was destined primarily to clustered presumptive terminal fields in medial lamina I and outermost lamina IIa. Only a few axons continued further to laminae V and X. Splanchnic afferent axons, most likely derived from the MCP, formed a longitudinal bundle ventral to the central canal. The LCP consisted of more or less well-defined axon bundles emanating from the lateral Lissauer's tract and curving round the lateral edge of the dorsal horn and through the dorsolateral funiculus. Presumptive terminal sites of LCP axons are the lateral laminae I and IIa, the nucleus of the dorsolateral funiculus and the dorsal part of lamina V. A few LCP axons were seen in the vicinity of lateral dendrites of preganglionic sympathetic axons. Visceroafferent terminals were absent from laminae IIb-IV and VII. The possible consequences of the MCP/LCP duality for the central connections of splanchnic afferents are discussed. Some splanchnic afferents ascended to the gracile and cuneate nuclei, and rarely to the spinal trigeminal nucleus. These results fit into the general concept of visceroafferent terminal organization that has emerged during the last few years. Differences to other reports in the detailed arrangement of fibers and terminals are discussed. Somatoafferent cell bodies represented the vast majority of neurons in the respective spinal ganglia. Cell sizes encompassed the whole range from very small to very large without a clear predominance of one particular size class.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphology of sympathetic preganglionic neurons in the thoracic spinal cord of the cat: an intracellular horseradish peroxidase study

Horseradish peroxidase was intracellularly injected into sympathetic preganglionic neurons (SPN) of the third thoracic segment in cats. Seven neurons were reconstructed from serial horizontal or parasagittal sections of the spinal cord. The cell bodies of all neurons were located in the n. intermediolateralis pars principalis (ILp). They were spindle-shaped with the long axis in craniocaudal direction or large and multipolar or small and oval in shape. Preferentially on the cranial and caudal pole of the cell body, five to eight primary dendrites arose from the cell body. Dendritic branches were traced to their terminations at distances up to 1,330 microns from the cell body. The dendritic fields of all SPNs were strictly oriented in the longitudinal direction with a total length of 1,500-2,540 microns. The cranial and caudal dendritic fields were about equal in length but, with one exception, the degree of branching was always greater in the cranial than in the caudal dendritic field. The dendritic fields of all SPNs were primarily restricted to the ILp. In the mediolateral direction it extended from 130 to 360 microns and in the dorsoventral direction from 50 to 180 microns. Only rarely, a higher-order dendrite left the boundaries of the ILp and projected dorsolaterally or laterally into the white matter or ventromedially or medially into the adjacent n. intercalatus. All dendrites showed various forms of spines. At a distance of 132-437 microns from the cell body the axon arose as a direct extension of a process which closely resembled a primary or second-order dendrite. The axons projected ventrally and mostly caudally along the lateral border of the gray matter until they turned laterally at the end of the ventral horn. No axon collaterals were observed.

Cutaneous receptive fields of somatic and viscerosomatic neurones in the thoracic spinal cord of the cat

Extracellular single-unit recordings were made from 121 neurones in the thoracic spinal cord of the cat. All neurones could be driven by electrical stimulation of dorsal root afferent fibres. The neurones were classified, according to the absence or presence of inputs from the ipsilateral splanchnic nerve, as "somatic" or "viscerosomatic", respectively. Cutaneous receptive fields were identified for 75 of the neurones: 31 were somatic and 44 viscerosomatic. Only two of the somatic cells received cutaneous nociceptive inputs, compared with 33 of the viscerosomatic cells. Sixty-four percent of the whole sample of neurones had receptive fields which included three or more dermatomes. Viscerosomatic cells tended to have larger receptive fields than the somatic neurones, and six of them had fields which did not include the corresponding (T11) dermatome. Neurones with receptive fields in the dorsal one-third of the dermatome tended to be located in the lateral one-third of the dorsal horn, but those with receptive fields in the ventral two-thirds of the dermatome showed no differential distribution within the gray matter. This is discussed with respect to the results of anatomical studies on the dorsal horn projections of cutaneous afferent fibres from different regions of the dermatome. Preliminary results from intracellular staining with horseradish peroxidase reveal extensive branching of primary afferents in the dorsal horn, and large dendritic fields of dorsal horn neurones. Our physiological and morphological results indicate that the somatotopic organisation of the thoracic spinal cord is less well defined than that of the lumbosacral region.

Supraspinal loops that mediate visceral inputs to thoracic spinal cord neurones in the cat: involvement of descending pathways from raphe and reticular formation

Single unit electrical activity has been recorded from 29 viscero-somatic neurones in the T11 spinal cord segment of chloralose anaesthetized cats. Twenty-six of these neurones showed changes in their responses to electrical stimulation of the splanchnic nerve after reversible spinalization of the animals by cold block: 14 showed increased responses whereas 12 presented reduced or abolished responses during the spinal block. The majority of neurones in the first group were located in laminae IV, V and VII and were inhibited by electrical stimulation of the nucleus raphe magnus (NRM) and the reticular formation (Ret.F). Most neurones of the second type were located in the ventral horn, and the majority were excited by electrical stimulation of the NRM and the Ret.F. This second type of neurone may play a role in the maintenance of the excitation in the central nervous system which follows visceral noxious stimulation.

Visceral nociception: peripheral and central aspects of visceral nociceptive systems

Discomfort and pain are the sensations most commonly evoked from viscera. Most nociceptive signals that originate from visceral organs reach the central nervous system (c.n.s.) via afferent fibres in sympathetic nerves, whereas parasympathetic nerves contain mainly those visceral afferent fibres concerned with the non-sensory aspects of visceral afferent function. Noxious stimulation of viscera activates a variety of specific and non-specific receptors, the vast majority of which are connected to unmyelinated afferent fibres. Studies on the mechanisms of visceral sensation can thus provide information on the more general functions of unmyelinated afferent fibres. Specific visceral nociceptors have been found in the heart, lungs, testes and biliary system, whereas noxious stimulation of the gastro-intestinal tract appears to be detected mainly by non-specific visceral receptors that use an intensity-encoding mechanism. Visceral nociceptive messages are conveyed to the spinal cord by relatively few visceral afferent fibres which activate many central neurons by extensive functional divergence through polysynaptic pathways. Impulses in visceral afferent fibres excite spinal cord neurons also driven by somatic inputs from the corresponding dermatome (viscero-somatic neurons). Noxious intensities of visceral stimulation are needed to activate viscero-somatic neurons, most of which can also be excited by noxious stimulation of their somatic receptive fields. The visceral input to some viscero-somatic neurons in the spinal cord can be mediated via long supraspinal loops. Pathways of projection of viscero-somatic neurons include the spino-reticular and spino-thalamic tracts. All these findings give experimental support to the 'convergence-projection' theory of referred visceral pain. Visceral pain is the consequence of the diffuse activation of somato-sensory nociceptive systems in a manner that prevents accurate spatial discrimination or localization of the stimuli. Noxious stimulation of visceral receptors triggers general reactions of alertness and arousal and evokes unpleasant and poorly localized sensory experiences. This type of response may be a feature of sensory systems dominated by unmyelinated afferent inputs.