Dendritic organization of phrenic motoneurons in the adult rat

The dendritic organization of the phrenic nucleus as a whole was studied after injections of the B-subunit of cholera toxin conjugated to horseradish peroxidase were made into the diaphragm of adult rats. Transverse, sagittal, and horizontal sections through the phrenic nucleus (C3-C5) were incubated according to a modified tetramethylbenzidine HRP technique. The conjugated form of HRP used in this study has a special affinity for the GM1 ganglioside receptors on neuronal cell surfaces. As a result, extensive labeling of the terminal dendritic fields of a large number of phrenic motoneurons occurred simultaneously. The results showed that the majority of the dendrites of phrenic motoneurons were tightly organized rostrocaudally and confined to the boundaries of the column made up of the phrenic cell bodies. In addition, analysis of transverse and horizontal sections revealed dendritic bundles radiating at right angles to the long axis of the cell column in the following directions: dorsolateral into the dorsal half of the lateral funiculus, lateral into the lateral funiculus, ventromedial into the lateral half of the anterior funiculus, ventrolateral into the ventral half of the lateral funiculus, and dorsal into the intermediate gray matter. Some dendritic bundles were measured as far as 900 microns from phrenic cell bodies into the white matter. The horizontal sections also showed that there was a periodicity in the arrangement of the dendritic fascicles in that they were separated by distances ranging from 180 to 250 microns. From the analysis of phrenic dendritic distribution the present results suggest that the majority of synaptic input to phrenic motoneurons occurs within the column of the phrenic cell bodies. In addition, there is evidence to suggest that a synaptic input may also occur directly on distal phrenic dendrites in the lateral and ventral funiculi of the spinal cord white matter.

Effect of cervical spinal cord hemisection and hemidiaphragm paralysis on arterial blood gases, pH, and respiratory rate in the adult rat

We documented the changes that occur in arterial blood gases, pH, and respiratory rate in rats subjected to a C2 spinal cord hemisection which results in paralysis of the ipsilateral hemidiaphragm. The results indicated that rats compensate for hemidiaphragm paralysis by maintaining arterial pO2 through an increased rate of respiration. This observation is of interest because it implies a form of functional recovery from spinal cord injury which has heretofore not been demonstrated.

Origin and distribution of phrenic primary afferent nerve fibers in the spinal cord of the adult rat

Previous studies from this laboratory have localized and morphologically characterized phrenic motor neurons in the rat spinal cord at light and electron microscopic levels. The present investigation used a modification of the TMB method for the retrograde transport of horseradish peroxidase (HRP) to describe at light microscope levels the origin and distribution of phrenic primary afferent axons in the adult rat spinal cord. Dry HRP crystals were applied to the central stump of the transected phrenic nerve in the neck to label spinal ganglion cell bodies and thus determine the levels of origin of afferent axons in the phrenic nerve. Camera lucida drawings were then made from serial sections through the appropriate spinal cord levels to determine the specific distribution of transganglionically labeled phrenic central axonal processes within the spinal cord. HRP-labeled phrenic primary neurons were observed in the C3 to C7 spinal ganglia. The camera lucida studies indicated that the transganglionically labeled central processes of phrenic primary afferent axons distributed into the dorsal horn at the C4 and C5 levels of the spinal cord. Furthermore, central processes distributing to C5 were more numerous than those that distributed to C4. Afferent axons were never seen in the dorsal horn at C3, C6, or C7. As spinal ganglion cells were labeled at C3 above and C6 and C7 below, it follows that central processes of phrenic afferent fibers descend and ascend in the dorsal columns of the spinal cord before distributing into the dorsal horn. Specifically, the labeled primary afferent axons and their collateral branches were found in the fasciculus cuneatus, and in laminae I, II, III, and IV of the dorsomedial aspect of the dorsal horn. The function of these central axonal processes is unknown, but based on a comparison of our morphologic data with previous physiological and anatomical studies, we suggest that phrenic afferent fibers may arise from proprioceptors (muscle spindles and Golgi tendon organs), nociceptors, or rapidly adapting mechanoreceptors (Pacinian corpuscles) within the diaphragm.

Spinal tanycytes in the adult rat: a correlative Golgi gold-toning study

In Golgi impregnated transverse sections through the cervical spinal cord of the 7-12-week-old adult rat, numerous tanycytes were observed radiating from the ependyma into the gray matter that surrounds the central canal. The tail processes of these tanycytes terminated as foot processes in association with blood vessels. Spinal tanycytes were classified into ependymal (E) and subependymal (S) types on the basis of the shape and position of the soma. The soma of the E tanycyte was shaped as a column and was entirely located within the ependyma. In contrast, the soma of the S tanycyte was flask shaped, with the widest portion of the flask located subependymally and the elongated portion extending through the ependyma ultimately reaching the luminal surface. Selected Golgi impregnated sections were gold toned and deimpregnated for direct correlative analysis at the ultrastructural level. Gold-toned tanycytes contained the fine clusters of gold particles underlying the plasma membrane of the cell body and coarse clusters of gold particles throughout the tail and foot processes. The apical surface of tanycytes was characterized by numerous microvilli and large cytoplasmic protrusions that evaginated from the apical surface into the lumen of the central canal. At the luminal surface, adjacent tanycytes were joined laterally by junctional complexes with punctate tight junctions and zonulae adhaerentes associated with fibrils and microtubules. In contrast, gap junctions, hemidesmosomes, and puncta adhaerentia were found between adjacent tail processes of tanycytes. The foot processes interdigitated with one another and abutted the basal lamina around the perivascular space of blood vessels. The basal lamina was continuous around the lateral walls of foot processes and filled the spaces between membranous infoldings of the lateral walls. These basal membrane labyrinths were continuous with the basal lamina of the blood vessel and may provide an extensive surface relation between the perivascular space and the neighboring extracellular compartment. The findings of the present study support the contention that tanycytes may modify the composition of substances moving between the perivascular and extracellular spaces.

The ultrastructure and synaptic architecture of phrenic motor neurons in the spinal cord of the adult rat

Although light microscopic studies have analysed phrenic motor neurons in several different species, there has never been an ultrastructural investigation of identified phrenic motor neurons. In addition, electrophysiological studies have raised questions relating to the function of phrenic motor neurons which may be answered only by direct electron microscopic investigation. Thus, the present study was carried out to provide a detailed ultrastructural analysis of identified phrenic motor neurons. Phrenic motor neurons in the spinal cord of the rat were labelled by retrogradely transported horseradish peroxidase (HRP) after transecting the phrenic nerve in the neck and applying the enzyme directly to the central stump of the transected nerve. The results showed that the general ultrastructural characteristics of phrenic motor neurons were similar to those previously reported for other spinal motor neurons. However, phrenic primary dendrites appeared to be isolated from all other dendritic profiles in the neuropil. Primary dendrites were not fasciculated. Fasciculation occurred only among the more distal secondary and tertiary phrenic dendritic branches. Direct dendrodendritic or dendrosomatic apposition was rarely seen; gap junctions between directly apposing phrenic neuronal membranes were not observed. The membranes of adjacent phrenic neuronal profiles were most frequently separated by intervening sheaths of astroglial processes. Myelinated phrenic axons and a phrenic axon collateral were identified. The initial portion of the phrenic axon collateral was cone-shaped, lacked myelin, and thus resembled a miniature axon hillock. In one instance, a large accumulation of polyribosomes was observed within the hillock-like structure of a phrenic axon collateral. Eight morphological types of synaptic boutons, M, P, NFs, S, NFf, F, G and C were classified according to criteria used by previous investigators. Most of these endings (M, NFs, NFf, S and F) made synaptic contact with profiles of labelled phrenic somata and dendrites. F, NFf, and S boutons also terminated on phrenic axon hillocks. C and G boutons contacted exclusively phrenic somata and small calibre dendrites, respectively. P boutons established axo-axonic synaptic contacts with the M and NFs bouton. The morphological findings of the present study provide new data that may be related to phrenic synchronized output and presynaptic inhibition of primary afferents terminating on phrenic motor neurons.

The phrenic nucleus of th albino rat: a correlative HRP and Golgi study

The phrenic nucleus of the adult albino rat was studied by utilizing the O-dianisidine method for the retrograde transport of horseradish peroxidase in conjunction with the zinc chromate modification of the Golgi technique. Application of HRP to the transected phrenic nerve in the neck labeled a column of phrenic motor neurons from C3 to C5 in the ipsilateral spinal cord. However, when HRP was applied to the phrenic nerve intrathoracically, labeled neurons were found from C3 to C6. The long axis of the column of phrenic neurons was oriented tangentially from rostral to caudal poles. There was a gradual shift of the column from posterior to anterior and from lateral to medial positions in the ventral horn. The peroxidase material was also used to localize impregnated phrenic motor neurons in the Golgi sections and to provide quantitative data on phrenic motor neurons. In Golgi-impregnated material two types of phrenic neurons were distinguished on the basis of dendritic morphology and orientation. These neurons were designated (1) large neurons with smooth, radially oriented dendrites, and (2) smaller neurons with varicose, tangentially oriented dendrites. Both types of neurons had a small number of spines and bulbous appendages issuing from the dendritic trunks and branches. The dendritic fields of adjacent phrenic neurons overlapped extensively with one another and with dendrites of more distally placed ventral horn motor neurons. In peroxidase-labeled sagittal sections the dendrites of phrenic neurons were primarily oriented in the rostrocaudal plane. The mean total number of peroxidase-labeled neurons in the phrenic nucleus was 415.75 +/- 18.36 cells. In sagittal sections the mean long axis diameter of phrenic cell bodies was 34.5 micrometers. In frontal sections the mean long axis diameter of phrenic cell bodies was 22.5 micrometers. Thus, from direct measurement, the phrenic neurons were 34% longer in the sagittal plane than in the frontal plane. In the present study each phrenic nucleus contributed fibers only to the ipsilateral phrenic nerve, and no evidence for peripheral crossing of fibers was found.

The interaction of nerves of opposite regenerating polarity in fused newt forelimbs

Previous studies involving nerve interactions and limb regenerative processes were carried out on adult newts after their forelimbs were amputated through the distal radius and ulna and fused end-to-end. On the basis of limb regeneration results at the junction of the fused limbs, it was postulated that regenerating nerves from each limb (i.e., nerves of opposite polarity) would not invade the foreign territory of the contralateral limb if it were already normally innervated. A direct study of this nerve interaction, however, was not made in this earlier study. The present investigation was designed to obtain direct histological and electrophysiological evidence for the interaction of nerves of opposite regenerating polarity in fused newt forelimbs. The primary objective was to determine how the regenerating nerves would interact in the establishment of innervation territories-first, at the fusion zone, which represents the junction of the normal innervation territories of the nerves of each limb; and secondly, half way up one of the limbs, where interaction would occur in a territory normally innervated by only one of the regenerating nerves. The results showed that when nerves of opposite regenerating polarity approached one another at the junction of the fused limbs a discontinuation of axonal growth occurred; no indication of overlap of nerves into foreign territory was seen. When the nerves were allowed to interact within one of the fused limbs, however, an overlap of nerve fibers and a functional "double innervation" of that limb was demonstrated. These results are discussed in terms of possible mechanisms for the establishment of innervation territories in salamander limbs. The question of nerve-muscle reinnervation specificity is also raised.

The regeneration of brachial nerves of contralateral origin into benervated fused newt forelimbs

Previous studies involving the end-to-end fusion of the forelimbs of the adult newt have demonstrated that new limbs can regenerate from the transected ends of proximo-distally reversed limb segments. The limb regeneration could only have been initiated by nerve fibers of contralateral origin. The purpose of the present study is to describe histologically the manner in which nerve fibers of contralateral origin regenerate through the junction of fused limbs into the opposite limb. The first sign of nerve regeneration into the opposite limb was observed at eight days post fusion. The nerves crossed over into the opposite, originally denervated limb in a highly dispersed manner. These nerve fibers eventually aggregated, however, either under the skin or within persisting nerve trunks. By 19 days post fusion the nerve fibers had reached the elbow region of the originally denervated limb and by 25 days they were seen at the most proximal extent of the limb. The diameters of the axons seemed smaller than the diameters of regenerating axons observed in non-fused newt forelimbs.