Conduction velocity distribution of afferent fibers innervating the rat urinary bladder

The conduction velocities of individual afferent fibers innervating the rat urinary bladder were determined by the antidromic stimulation of dorsal roots while recording from bladder postganglionic nerves. Conduction velocities ranged from 0.5 to 21.0 m/s; 70% of the velocities were less than 2.5 m/s. The distribution within the dorsal roots was ipsilateral with 84% in L₆ and 16% in S₁. Neuroanatomical tracing with horseradish peroxidase applied to individual bladder postganglionic nerves resulted in ipsilateral labeling of dorsal root ganglion cells with 77% in L₆, 20% in S₁, and 3% in L₁–L₂. Ultrastructural examination of bladder postganglionic nerves revealed some myelinated fibers (average diameter: 2.6 μm) and many unmyelinated fibers. Therefore, in the rat, most of the bladder afferent fibers appear to be unmyelinated, although a population of myelinated afferent fibers is also present. Bladder afferent fibers enter the spinal cord mainly in segment L₆ with a minor fraction entering in S₁.     

The central distribution pattern of primary afferent fibers innervating the thigh muscle posterior iliotibialis in the chicken.

The anatomical relationship between motoneuron dendrites and primary afferent fibers innervating the same muscle were examined by using a modified cholera toxin tracing method. The cholera toxin B subunit conjugated with latex beads was injected into the posterior iliotibial muscle (PIT) of chicken. Both motoneuron dendrites and primary afferent fibers were intensely labeled. Labeled primary afferent fibers innervating the PIT were mainly located in laminae I (ventral part), V (lateral part), VI, VII (except ventromedial part) and lateral motor column (LMC, lamina IX) of the lumbar spinal cord. Numerous labeled primary afferent fibers were observed in proximity to the motoneuron pool of the PIT located in the dorsolateral region of the LMC; primary afferent fibers were closely apposed to somatic profiles of the PIT motoneurons.     

Aging and the myelinated fibers in prefrontal cortex and corpus callosum of the monkey.

In the rhesus monkey, the myelin sheaths of nerve fibers in area 46 of prefrontal cortex and in splenium of the corpus callosum show age-related alterations in their structure. The alterations are of four basic types. Most common is splitting of the dense line of myelin sheaths to accommodate electron dense cytoplasm derived from the oligodendroglia. Less common are splits of the intraperiod line to form balloons or blisters that appear to contain fluid, the occurrence of sheaths with redundant myelin, and thick sheaths that are almost completely split so that one set of compact lamellae is surrounded by another set. But despite these alterations in the sheaths, few nerve fibers show axonal degeneration. To quantify the frequency of the age-related alterations in myelin, transversely sectioned nerve fibers from the splenium of the corpus callosum and from the vertical bundles of nerve fibers within area 46 were examined in electron photomicrographs. The material was taken from 19 monkeys, ranging between 5 and 35 years of age. It was found that the frequency of alterations in myelin sheaths from both locations correlates significantly with age. In area 46, the age-related alterations also significantly correlate (P < 0.001) with an overall assessment of impairment in cognition, i.e., the cognitive impairment index, displayed by individual monkeys. The correlation is also significant when only the old monkeys are considered as a group. A similar result was obtained previously in our examination of the effects of age on the myelin sheaths of nerve fibers in primary visual cortex (Peters et al. [2000] J Comp Neurol. 419:364-376). However, in the corpus callosum the myelin alterations correlate significantly with only one component of the cognitive impairment index, namely the delayed nonmatching to sample task with a 2-minute delay. It is proposed that age-related myelin alterations are ubiquitous and that the correlations between their frequency and impairments in cognition occur because the conduction velocity along the affected nerve fibers is reduced, so that the normal timing sequences within neuronal circuits break down.     

Spinal nerve root distribution of the myelinated and unmyelinated nerve fibers of a cat cutaneous nerve]

The distribution of myelinated and nomyelinated nerve fibres from n. saphenus in the dorsal and ventral roots of the cat spinal cord was investigated, using methods improving the signal-to-noise ratio in the neurogram of the nerve evoked response. Nerve fibres from n. saphneus enter the spinal cord through roots of segments L4-6. In the dorsal roots of these segments the nerve fibres have conduction velocities from 80 to 0.38 m/s. In the ventral roots four groups of the nerve fibres with conduction velocities 80--60, 40--30, 12.0--3.0 and 1.1--0.51 m/s are found that are likely to be afferents. The conditions for low amplitude potentials detection in the spinal cord roots as well as the possible functional significance of the nerve fibres in the ventral roots are discussed.     

Axoplasmic incorporation of amino acids in myelinated fibers of the cat

Incorporation of amino acids into the axoplasm of nerve fibers of the cat was studied with radioautography. The dorsal funiculus of the spinal cord, or the sciatic nerve, was exposed to tritiated leucine, lysine, and proline; the tissue was processed 1 h later or the following day. In all instances, a radioautographic response of widely varying intensity was observed over the axoplasm; this response could not be adscribed to emissions originating from axonal mitochondria or myelin. The axoplasmic response ranged to as much as 56% compared with that over nearby glial cytoplasm and was often conspicuously greater than that over the surrounding myelin. We interpret our findings as axoplasmic synthesis of proteins. This finding has implications in the maintenance of the neuron.     

Ganglionic distribution of afferent neurons innervating the canine heart and cardiopulmonary nerves

The ganglionic distribution of the perikarya of afferent axons in cardiopulmonary nerves or the heart was studied in 64 dogs by injecting horseradish peroxidase into physiologically identified cardiopulmonary nerves or different regions of the heart. In 6 additional dogs, horseradish peroxidase was injected into the aortic arch, pericardial sac, left ventricular cavity or the skin. After injections into cardiopulmonary nerves, retrogradely labeled perikarya were found in the ipsilateral nodose ganglion and the ipsilateral C7-T7 dorsal root ganglia. After injections into different regions of the heart, retrogradely labeled neurons were found in the nodose ganglia bilaterally and in the C6-T6 dorsal root ganglia bilaterally. Many more retrogradely labeled neurons were found in the nodose ganglia in comparison to the dorsal root ganglia. The largest numbers of retrogradely labeled perikarya in the dorsal root ganglia occurred in the T 2-4 ganglia following nerve or heart injections. Following injections into specific regions of the heart or individual physiologically identified cardiopulmonary nerves, regional distributions of labeled neurons could not be identified within or among ganglia with respect to the structures injected. Perikarya in dorsal root ganglia which were labeled after heart injections ranged in area from 436-3280 microns 2 (X = 1279 +/- 51 S.E.M.) while after skin injections labeled perikarya ranged in area from 224-5701 microns 2 (X = 1631 +/- 104 S.E.M.). The results show that the afferent innervation of the canine heart is provided by neurons located throughout the nodose ganglia and to a lesser degree in the C6-T6 dorsal root ganglia bilaterally. The bilateral distribution of cardiac afferent neurons raises questions regarding mechanisms underlying unilateral symptoms frequently associated with heart disease.     

Spinal termination of functionally identified primary afferent neurons with slowly conducting myelinated fibers.

Single primary afferent myelinated fibers from cutaneous receptors of cat and monkey were functionally identified by recording from the spinal cord with micropipettes filled with horseradish peroxidase (HRP). Relatively slowly conducting fibers (less than 40 m/sec) from high threshold mechanoreceptors (mechanical nociceptors) and two types of low threshold mechanoreceptor (D-hair and field) were selected for staining. Iontophoresis of the HRP and subsequent histochemical reaction stained the axons recorded from and their collaterals, including terminations, for several millimeters. The termination patterns in the two species proved essentially identical. Ipsilaterally, the mechanical nociceptor fibers terminated principally in the dorsal horn's marginal zone and in the ventral parts of the nucleus proprius (lamina V in the cat). Some of these nociceptors also had terminals in the midline just dorsal to the central canal, contralaterally in the marginal zone, and at the base of the opposite nucleus proprius. In contrast, the D-hair primary afferent axons terminated in the dorsal part of the nucleus proprius overlapping into the innermost portion of the substantia gelatinosa. The field receptor fibers terminated predominantly in the middle part of the nucleus proprius. These results suggest that there is a highly specialized central projection of primary afferent endings which is related to sensory function and not to fiber diameter. The marginal zone and most dorsal parts of the substantia gelatinosa receive direct projections from cutaneous nociceptors but do not have direct input from cutaneous receptors transmitting activity initiated by innocuous stimulation.     

The distribution of myelinated nerve fibers in the mature opossum esophagus.

Myelination of nerve fibers could be important in establishing normal esophageal peristalsis. We therefore examined the general distribution and the age of appearance of myelinated nerve fibers in the smooth-muscle part of the esophagus of the American opossum. Tissues stained with thionein and Sudan black B were examined by light microscopy. Other tissues were prepared for electron microscopy and examined in the light microscope in Toluidine blue stained sections as well as at electron microscopy. In mature animals (weight greater than 2.0 kg, age greater than or equal to 1 year), myelinated nerve fibers, oriented mainly craniocaudally, were most abundant at the striated muscle-smooth muscle junction, and declined in density distally along the organ. They were nearly absent at the esophagogastric junction. They were more abundant in the stomach just below the esophagogastric junction. The myelinated nerve fibers commonly lay within sheathed fascicles that had the appearance of peripheral nerves, like the shunt fascicles of the stomach and colon. In immature animals myelinated fibers did not appear until a weight of about 1 kg was reached, 50 days after weaning and about 150 days after birth. Since the younger animals are presumably swallowing normally, myelination of the extrinsic nerves is not essential for esophageal motor function.     

Funicular course of catecholamine fibers innervating the lumbar spinal cord of the cat

The purpose of this study was to determine the funicular location of descending catecholamine (CA) fibers innervating the lumbar spinal cord from the dorsolateral pons (DLP). The locations of catecholamine-containing cell bodies which project to the lumbar spinal cord were determined by combining the use of the retrogradely transported fluorescent dye, Evans Blue (EB), with the glyoxylic acid histofluorescence technique. Lumbar injections of Evans Blue were combined with thoracic lesions of the dorsolateral funiculi (DLF) or ventrolateral funiculi (VLF) in order to retrogradely label those CA-containing or non-CA-containing cell bodies whose axons descend within the spared hemispinal cord. By this technique it was determined that descending CA fibers innervating the lumbar spinal cord of the cat project through both the DLF and the VLF. The nucleus subcoeruleus, the Kolliker-Fuse nucleus and the CA cell bodies in the area of A5 each contain a significant number of CA-containing cells whose fibers descend both within the DLF and the VLF, while the nucleus locus coeruleus projects to the lumbar cord primarily through the VLF. Catecholamine cells of the DLP innervate the lumbar spinal cord bilaterally, although there is an ipsilateral predominance. The CA-containing cells of the DLP which innervate the contralateral spinal cord were shown by ipsilateral or contralateral thoracic hemisection to decussate both above and below the thoracic lesion. Non-CA-containing cells from the DLP also crossed at all levels of the spinal cord; however, cells from the caudal pons had a larger number of cells which crossed above the thoracic lesion while cells of the more rostral pons had a larger number of cells which crossed below the lesion.     

Central distribution of afferent pathways from the uterus of the cat.

Afferent pathways from the uterus of the cat were labeled by injections of horseradish peroxidase (HRP), wheat germ agglutinin-HRP, or fluorescent dyes into the uterine cervix and uterine horns. Afferent input to the uterus arises from small to medium size neurons (average size 31 x 28 microns) in dorsal root ganglia at many levels of the spinal cord (T12-S3). The segmental origin correlates with the location of the afferent terminal field in the uterus. Eighty-seven percent of the dorsal root ganglion cells (average, 822 on one side) innervating the cervix are located in sacral ganglia, whereas 97% of the cells innervating the uterine horn (average 479 on one side) are located in lumbar ganglia. Double dye labeling experiments indicate that a small percentage (average 15%) of lumbar neurons innervating the uterine cervix also innervate the uterine horn. The majority (70-80%) of afferent input to the uterine cervix passes through the pelvic nerve and the remainder through the pudendal nerve, whereas afferent input to the uterine horn must travel in sympathetic nerves. Ovariectomy (10-14 days) did not change significantly the number, sizes, or segmental distribution of uterine afferent neurons. In some cats (25%) injections of WGA-HRP into the uterine cervix labeled neurons (90-125 per animal) in lamina VII in the S2 spinal segment in the region of the sacral parasympathetic nucleus. Central projections of uterine horn afferent neurons were not labeled; however, afferent projections from the cervix were detected in the sacral spinal cord. The most prominent labeling was present in Lissauer's tract and in lamina I and outer lamina II on the lateral edge of the dorsal horn. From this region some labeled axons extended through lamina V into the dorsal gray commissure. Very few afferents were labeled on the medial side of the dorsal horn. These results are discussed in regard to the physiological function of uterine afferents and the possible transmitter role of vasoactive intestinal polypeptide, which is present in a large percentage (70%) of cervical afferent neurons.     

Segmental distribution and central projectionsof renal afferent fibers in the cat studied by transganglionic transport of horseradish peroxidase

The segmental and central distributions of renal nerve afferents in adult cats and kittens were studied by using retrograde and transganglionic transport of horseradish peroxidase (HRP). Transport of HRP from the central cut ends of the left renal nerves labeled afferent axons in the ipsilateral minor splanchnic nerves and sensory perikarya in the dorsal root ganglia from T12 to L4. The majority of labeled cells (85%) were located between L1 and L3. A few neurons in the contralateral dorsal root ganglia were also labeled. Labeled cells were not confined to any particular region within a dorsal root ganglion. Some examples of bifurcation of the peripheral and central processes within the ganglion were noted. A small number of preganglionic neurons, concentrated in the intermediolateral nucleus, were also identified in some experiments. In addition, many sympathetic postganglionic neurons were labeled in the renal nerve ganglia, the superior mesenteric ganglion, and the ipsilateral paravertebral ganglia from T12 to L3 Transganglionic transport of HRP labeled renal afferent projections to the spinal cord of kittens from T1 1 to L6, with the greatest concentrations between Ll and L3. These afferents extended rostrocaudally in Lissauer's tract and sent collaterals into lamina I. In the transverse plane, a major lateral projection and a minor medial projection were observed along the outer and inner margins of the dorsal horn, respectively. From the lateral projection many fibers extended medially in laminae V and VI forming dorsal and ventral bundles around Clarke's nucleus. The dorsal bundle was joined by collaterals from the medial afferent projection and crossed to the contralateral side. The ventral bundle extended into lamina VII along the lateroventral border of Clarke's nucleus. Some afferents in the lateral projection could be followed ventrally into the dorsolateral portion of lamina VII in the vicinity of the intermediolateral nucleus. In the contralateral spinal cord, labeled afferent fibers were mainly seen in laminae V and VI These results provide the first anatomical evidence for sites of central termination of renal afferent axons. Renal inputs to regions (laminae I, V, and VI) containing spinoreticular and spinothajamic tract neurons may be important in the mediation of supraspinal cardiovascular reflexes as well as in the transmission of activity from nociceptors in the kidney. In addition, the identification of a bilateral renal afferent projection in close proximity to the thoracolumbar autonomic nuclei is consistent with the demonstration in physiological experiments of a spinal pathway for the renorenal sympathetic reflexes.     

Immunoreactive dynorphin B in sacral primary afferent fibers of the cat

Immunocytochemical analysis of the distribution of dynorphin B terminals in the sacral spinal cord of the cat revealed a pattern of staining very similar to that produced with antisera directed against the primary afferent derived, putative neurotransmitter, vasoactive intestinal polypeptide. Labeled axons and terminals were concentrated in lamina I and V and there was dense fiber staining in the tract of Lissauer. Of particular interest was the presence of immunoreactive axons in attached dorsal rootlets. To specifically focus on the possibility that some of the sacral primary afferent fibers are dynorphin-immunoreactive, we first tried to increase perikaryal labeling in the sacral dorsal root ganglia by topical treatment with colchicine. This did not produce immunoreactive labeling of cell bodies in the ganglia. Unilateral multiple dorsal rhizotomy (L5 to coccygeal 1), however, significantly decreased the staining of dynorphin-immunoreactive axons and terminals in the tract of Lissauer and in the dorsal horn of sacral segments ipsilateral to the deafferentation. No changes were detected in the lumbar cord. Finally, radioimmunoassay of caudal lumbar and sacral dorsal root ganglia was performed. Measurable immunoreactivity was found in all ganglia assayed, but, consistent with the histochemical analysis, sacral ganglia contained the highest concentration of immunoreactive dynorphin B. These data indicate that a significant component of the sacral spinal cord dynorphin terminal immunoreactivity derives from primary afferent fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of aging on myelinated nerve fibers in monkey primary visual cortex

In monkeys, myelin sheaths of the axons in the vertical bundles of nerve fibers passing through the deeper layers of primary visual cortex show age-related alterations in their structure. These alterations have been examined by comparing the myelin sheaths in young monkeys, 5-10 years old, with those in old monkeys, between 25 and 33 years of age. The age-related alterations are of four basic types. In some sheaths, there is local splitting of the major dense line to accommodate dense cytoplasm derived from the oligodendrocytes. Other sheaths balloon out, and in these locations, the intraperiod line in that part of the sheath opens up to surround a fluid-filled space. Other alterations are the formation of redundant myelin so that a sheath is too large for the enclosed axon and the formation of double sheaths in which one layer of compact myelin is surrounded by another one. These alterations in myelin increase in frequency with the ages of the monkeys, and there is a significant correlation between the breakdown of the myelin and the impairments in cognition exhibited by individual monkeys. This correlation also holds even when the old monkeys, 25 to 33 years of age, are considered as a group. It is suggested that the correlation between the breakdown of myelin in the old monkeys and their impairments in cognition has not to do specifically with visual function but to the role of myelin in axonal conduction throughout the brain. The breakdown of myelin could impair cognition by leading to a change in the conduction rates along axons, resulting in a loss of synchrony in cortical neuronal circuits.     

Response properties of hypogastric afferent fibers supplying the uterus in the cat

The present study was undertaken to examine the sensory function of uterine afferent fibers in cats at unknown stages of the estrous cycle. Single unit activity was recorded from strands of the hypogastric nerve of the anesthetized cat. Once a unit was found, the conduction velocity was determined and the mechanical receptive field localized on the uterus. The response properties of the unit to mechanical stimuli applied to the receptive field and to chemical stimuli applied by intra-arterial injection of algesic chemicals (bradykinin, KCl and capsaicin) into the uterine artery were studied. Single unit activity from a total of 52 units was examined in this study. Based on the conduction velocities, about2/3 of these fibers were found to be unmyelinated C fibers and the remaining1/3 were thinly myelinated Aδ fibers. The receptive fields of most of these fibers were located at different parts of the uterine horn and body while a few were at the uterine cervix. Mechanical thresholds, as determined by von Frey filaments applied to the external surface of the uterus, varied more than 150-fold among mechanically sensitive units, ranging from extremely low to high thresholds. In addition, most of these afferents were activated by intra-arterially injected algesic chemicals, often by more than one chemical. The data in the present study suggest that a large portion of the cat uterus is innervated by the hypogastric nerve and that these afferents originate from sensory receptors that have potentially a wide range of functions. Their potential functions as low threshold mechanoreceptors and nociceptors are discussed.     

Spinal entry route for ventral root afferent fibers in the cat

Twelve anesthetized and paralyzed cats were used to study the spinal entry routes of ventral root afferent fibers. In all animals, the spinal cord was transected at two different levels, L5 and S2. The L5 through S2 dorsal roots were cut bilaterally, making spinal cord segments L5-S2 neurally isolated from the body except for the L5-S2 ventral roots. From this preparation, a powerful excitation of the discharge rate of motor neurons and dorsal horn cells within the isolated spinal segments was observed after intraarterial injection of bradykinin (50 micrograms in 0.5 ml saline). This excitation of the spinal neurons can be considered the most convincing evidence of the potential physiologic role of the ventral root afferent fibers entering the spinal cord directly through the ventral root, because the apparent route of neuronal input from the periphery is through the ventral roots. However, additional control experiments conducted in the present study showed that the excitation persisted even after cutting all ventral roots within the isolated spinal segments, indicating that excitation was not mediated by the ventral roots. Furthermore, direct application of bradykinin on the dorsal surface of the spinal cord also increased the motoneuronal discharge rate, suggesting that excitation of spinal neurons produced by intraarterial injection of bradykinin is due to a direct action of bradykinin on the spinal cord. Thus, we provided an alternate explanation for the most convincing evidence indicating that physiologically important ventral root afferent fibers enter the spinal cord directly through the ventral root. Based on existing experimental evidence, it is likely that the majority of physiologically active ventral root afferent fibers travel distally toward the dorsal root ganglion and then enter the spinal cord through the dorsal root.     

Central projection of nuchal group I muscle afferent fibers of the cat

To investigate the central projection of nuchal group I afferent fibers of nerves which supply the complexus (COM), biventer cervicis (BIV), splenius (SP), and occipitoscapularis (OCC) muscles, experiments were conducted on cats anesthetized with chloralose-urethane. A total of 246 neurons was found to respond to electrical stimulation of those nerves. Of these neurons, 99.2% were located in the ipsilateral external cuneate nucleus (ECN) and 0.8% in the ipsilateral main cuneate nucleus. Ninety-seven neurons responded only to the BIV, 88 neurons only to the SP, and 56 neurons to the OCC alone. The number of monosynaptically activated neurons from group I fibers were found to be 26 in 97 neurons which responded to the BIV, 20 in 88 neurons responded to the SP, and 8 in 56 neurons which responded to the OCC. The ECN contains these neurons organized in a somatotopic manner. Neurons from distal muscle (the OCC) project to medial parts and from proximal muscle (the BIV) to more lateral parts of the nucleus. Forty-three of the 143 neurons in the ECN (30.1%) were activated antidromically to electrical stimulation of the ipsilateral inferior peduncle and anterior lobules IV and V of the cerebellum. Thalamic responses from nuchal muscle afferent fibers were recorded in a very narrow region of nucleus ventralis posterolateralis (VPL) which is situated dorsolaterally to the forelimb muscle afferent (deep radial nerve) projection area. The cortical evoked potentials from these nerve stimulations were observed in the anterior suprasylvian gyrus, areas 2 or 5, which we regarded as a transitional area between the second somatosensory and association areas, and the postcruciate dimple (PCD) or area 3a. Cortical potentials in the PCD were reduced after lesion of the VPL, where the focal potentials evoked by nuchal muscle afferent stimulation were recorded.