Unmyelinated axon ratios in cat motor, cutaneous and articular nerves

Somatoautonomic reflexes in acupuncture therapy: A review

Oriental therapies such as acupuncture, moxibustion, or Anma, have been used to treat visceral disorders since ancient times. In each of these therapies, stimulation of the skin or underlying muscles leads to excitation of afferent nerves. The sensory information is carried to the central nervous system, where it is transferred to autonomic efferents, thus affecting visceral functions. This neuronal pathway, known as the "somatoautonomic reflex", has been systematically studied by Sato and his colleagues for over a half century. Nearly all their studies were conducted in anesthetized animals, whereas human patients are conscious. Responses in patients or the events following therapeutic somatic stimulation may differ from those observed in anesthetized animals. In fact, it is increasingly apparent that the responses in patients and animals are not always coincident, and the differences have been difficult for clinicians to reconcile. We review the mechanism of the "somatoautonomic reflex" as described in anesthetized animals and then discuss how it can be applied clinically.

Characterization of cutaneous and articular sensory neurons

BACKGROUND

A wide range of stimuli can activate sensory neurons and neurons innervating specific tissues often have distinct properties. Here, we used retrograde tracing to identify sensory neurons innervating the hind paw skin (cutaneous) and ankle/knee joints (articular), and combined immunohistochemistry and electrophysiology analysis to determine the neurochemical phenotype of cutaneous and articular neurons, as well as their electrical and chemical excitability.

RESULTS

Immunohistochemistry analysis using RetroBeads as a retrograde tracer confirmed previous data that cutaneous and articular neurons are a mixture of myelinated and unmyelinated neurons, and the majority of both populations are peptidergic. In whole-cell patch-clamp recordings from cultured dorsal root ganglion neurons, voltage-gated inward currents and action potential parameters were largely similar between articular and cutaneous neurons, although cutaneous neuron action potentials had a longer half-peak duration (HPD). An assessment of chemical sensitivity showed that all neurons responded to a pH 5.0 solution, but that acid-sensing ion channel (ASIC) currents, determined by inhibition with the nonselective acid-sensing ion channel antagonist benzamil, were of a greater magnitude in cutaneous compared to articular neurons. Forty to fifty percent of cutaneous and articular neurons responded to capsaicin, cinnamaldehyde, and menthol, indicating similar expression levels of transient receptor potential vanilloid 1 (TRPV1), transient receptor potential ankyrin 1 (TRPA1), and transient receptor potential melastatin 8 (TRPM8), respectively. By contrast, significantly more articular neurons responded to ATP than cutaneous neurons.

CONCLUSION

This work makes a detailed characterization of cutaneous and articular sensory neurons and highlights the importance of making recordings from identified neuronal populations: sensory neurons innervating different tissues have subtly different properties, possibly reflecting different functions.

Peripheral mu-opioid receptors attenuate the augmented exercise pressor reflex in rats with chronic femoral artery occlusion

Recently, opioid receptors have been shown to be expressed on group III and IV afferents, which comprise the sensory arm of the exercise pressor reflex. Although the stimulation of opioid receptors in the central nervous system has been shown to attenuate the exercise pressor reflex, the effect on the reflex of their stimulation in the periphery is unknown. We therefore tested the hypothesis that the activation of peripheral mu-opioid receptors attenuates the exercise pressor reflex. The pressor responses to static contraction were compared before and after the injection of the mu-opioid receptor agonist [d-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO; 1 microg) into the abdominal aorta of decerebrated rats in which one femoral artery had been occluded 72 h previously (n = 10) and in control rats whose femoral arteries were freely perfused (n = 8). DAMGO attenuated the peak pressor response to contraction in rats whose femoral arteries had been occluded (before: increase of 34 + or - 3 mmHg and after: increase of 22 + or - 2 mmHg, P = 0.008); the inhibitory effect of DAMGO was prevented by the injection of naloxone (100 microg) into the abdominal aorta (before: increase of 29 + or - 5 mmHg and after: increase of 29 + or - 5 mmHg, P = 0.646, n = 7). An intravenous injection of DAMGO (1 microg, n = 6) had no effect on the peak pressor response to contraction in both groups of rats. DAMGO had no effect on the peak pressor response to contraction in rats whose femoral arteries were freely perfused (before: Delta 23 + or - 4 mmHg, after: Delta 23 + or - 3 mmHg, n = 6) but appeared to have a small effect on topography of the response. DAMGO had no effect on the peak pressor response to tendon stretch in both groups of rats (both P > 0.05). We conclude that the stimulation of peripheral mu-opioid receptors attenuates the exercise pressor reflex in rats whose femoral arteries have been ligated for 72 h.

Unravelling the relationship between age, nociception and joint destruction in naturally occurring osteoarthritis of Dunkin Hartley guinea pigs

Osteoarthritis (OA) is a debilitating and painful disease, the incidence of which increases with advancing age. One of the confounding aspects of OA is that there is a disconnect between the severity of joint degeneration and the intensity of pain reported. This study examined the relationship between age, joint nociception, and joint pathology in an animal model of naturally occurring OA. Dunkin Hartley guinea pigs were grouped according to age: young (2-5 months) and senescent (17-37 months). Joint nociception was objectively measured in these animals by recording electrophysiologically from knee joint primary afferents in response to non-noxious and noxious movements of the knee. Joint pathology in the same knees was then determined by histomorphology and micro-computerized tomography (micro-CT). A principal components analysis was carried out on the data to determine if any correlation exists between each of the measured variables. In aged guinea pigs, 33% of joint mechanosensory nerves were spontaneously active, whereas young animals showed no such neural activity at rest. The frequency of afferent firing evoked by noxious movements was greater in old guinea pigs. Micro-CT and histopathological determination of OA positively correlated with age; however, there was no significant correlation between the severity of joint degeneration and nociception. In the Dunkin Hartley model of inveterate OA, the level of joint pathology correlates well with increasing age. This study also provides the first objective evidence that there is no correlation between joint nociception and articular damage, thereby corroborating the clinical observation that pain is a poor predictor of OA severity.

Arthritis and pain. Neurogenic origin of joint pain

Arthritis pain affects millions of people worldwide yet we still have only a limited understanding of what makes our joints ache. This review examines the sensory innervation of diarthroidal joints and discusses the neurophysiological processes that lead to the generation of painful sensation. During inflammation, joint nerves become sensitized to mechanical stimuli through the actions of neuropeptides, eicosanoids, proteinase-activated receptors and ion channel ligands. The contribution of immunocytes to arthritis pain is also reviewed. Finally, the existence of an endogenous analgesic system in joints is considered and the reasons for its inability to control pain are postulated.

Biomechanics of musculoskeletal pain: dynamics of the neuromatrix

Pain, due to mechanical stimuli, is a normal, indeed healthy, response of animals to potential or actual damage to tissues. Mammals in general, and humans in particular, have evolved a highly sophisticated system of pain perception, which is characterized in humans by complementary but distinct neural processing of the intensity and location of a noxious stimulus, and a motivational/emotional or affective response to the stimulus. The peripheral and central neurons that comprise this system, which has been called the 'neuromatrix', dynamically (temporally) respond and adapt to noxious biomechanical stimuli. However, phenotypic variability of the neuromatrix can be large, which can result in a host of musculoskeletal conditions that are characterized by altered pain perception, which can and often does alter the course of the condition. This neural plasticity has been well recognized in the central nervous system, but it has only more recently become known that peripheral nociceptors also adapt to their altered extracellular matrix environment. This work reviews the biomechanics of pain focusing on the relevant stimulus that initiates responses by nociceptors to the cognitive perception of pain.

Abrogation of alpha-adrenergic vasoactivity in chronically inflamed rat knee joints

It has previously been shown that chronic inflammation causes a reduction in sympathetic nerve-mediated vasoconstriction in rat knees. To determine whether this phenomenon is due to an alteration in smooth muscle adrenoceptor function, the present study compared the alpha-adrenoceptor profile of blood vessels supplying the anteromedial capsule of normal and chronically inflamed rat knee joints. While the rats were under urethan anesthesia, the alpha(1)-adrenoceptor agonists methoxamine and phenylephrine and the alpha(2)-adrenoceptor agonist clonidine (0.1-ml bolus; dose range 10(-12)-10(-7) mol) were applied to exposed normal rat knees, resulting in a dose-dependent fall in capsular perfusion. Comparison of drug potencies indicated that alpha(2)-adrenergic effects > alpha(1)-vasoactivity. One week after intra-articular injection of Freund's complete adjuvant to induce chronic joint inflammation, the vasoconstrictor effects of methoxamine, phenylephrine, and clonidine were all significantly attenuated compared with normal controls. These findings show that the preponderance of sympathetic adrenergic vasoconstriction in the anteromedial capsule of the rat is carried out by postjunctional alpha(2)-adrenoceptors. Chronic joint inflammation compromises alpha(1)- and alpha(2)-adrenoceptor function, and this change in alpha-adrenergic responsiveness may help explain the perfusion changes commonly associated with inflammatory arthritis.

Fibre composition in the interosseous nerve of the pigeon

The interosseous nerve of birds innervates a string of Herbst corpuscles located near the interosseous membrane between the tibia and fibula. Fibre composition of this nerve was assessed including both myelinated and unmyelinated axons. The diameter of the whole nerve is approximately 100 microm. Complete data were obtained for 3 nerves. The mean total number of myelinated fibres and unmyelinated axons was 2872 +/- 53. The mean number of myelinated fibres was 280 +/- 20 and that for unmyelinated axons was 2600 +/- 47. There was a broad distribution of diameters for myelinated fibres ranging from approximately 2 microm to 10 microm with a distinct peak at approximately 3-5 microm and a less prominent second peak at 6-8 microm. Similarly, myelin sheath thickness distribution showed 2 peaks, one at 0.6-0.8 microm and another at 1.4-1.6 microm. It is suggested that the group represented by the second peak innervates the Herbst corpuscles. The group of smaller myelinated fibres and the unmyelinated axons are assumed to innervate other types of receptors, some of which may be nociceptors.

Morphological and immunohistochemical examination of nerves in normal and injured collateral ligaments of rat, rabbit, and human knee joints

BACKGROUND

Knee joints possess an abundant nerve supply that relays sensory and motor information on such aspects as proprioception, nociception, and vasoregulation. Although synovial innervation has been well documented, little is known of the nerves that supply the collateral ligaments.

METHODS

The morphology of rabbit and human collateral ligament nerves was examined by silver impregnation. Immunohistochemistry was performed on rabbit and rat collateral ligaments to determine the presence of peptidergic nerves in these tissues. A 6-week gap injury was performed on three rabbit medial collateral ligaments, and the localisation of peptidergic nerves in these tissues was determined.

RESULTS

Irrespective of species or type of ligament examined, the greatest density of nerve fibres was found in the epiligament. Nerve fibres commonly accompanied blood vessels along the long axis of the ligament and then entered the substance of the tissue before ramifying in the deeper layers. Substance P and calcitonin gene-related peptide-immunoreactive nerve fibres were found in the collateral ligaments of the rat and rabbit. Injured ligaments showed a higher than normal level of immunoreactivity in and around the healing zone; however, the nerve fibres appeared tangled and truncated.

CONCLUSIONS

Like other structures in knee joints, collateral ligaments possess a complex nerve supply. The presence of peptidergic nerves suggests that ligaments may be susceptible to neurogenic inflammation and may be centres of articular nociception.

Acute joint inflammation--mechanisms and mediators

1. This review discusses factors contributing to acute joint inflammation, particularly sensory neuropeptides. 2. Mediators known to contribute importantly to the inflammatory process include cytokines, eicosanoids, complement and the kinin systems, histamine and 5-hydroxytryptamine and sensory neuropeptides substance P (SP) and calcitonin gene-related peptide (CGRP). 3. The pro-inflammatory neurokinins, SP and CGRP, are present in nerves innervating joints and could significantly contribute to the increased vascular permeability and hyperaemia occurring in acute arthritis. 4. Although perhaps contributing to the pathogenesis of chronic inflammatory joint disease, there is little evidence for involvement of the sympathetic nervous system in acute models of inflammation.

Anatomy of the rat knee joint and fibre composition of a major articular nerve

Several recent reports discuss the role of joint nerves in arthritis. Many of these are based on studies in the rat. The aim of this study is to examine the anatomy of the rat knee joint, in search for a primary articular nerve, and to analyze the fibre composition of that nerve. The results show that the structure of the joint differs in some respects from the human knee. At the upper end of the bony patella a cartilaginous patella extends proximally, forming the anterior wall of the suprapatellar bursa. Distinct collateral ligaments are integrated in the joint capsule. The extensor digitorum longus muscle bridges the knee joint, originating from the lateral femoral epicondyle. The well-developed menisci contain pyramid-shaped ossicles. The cruciate ligaments are arranged like in the human knee. A large posterior (PAN) and a small medial (MAN) articular nerve can be identified. The PAN is composed of some 400 axons, about 80% of which are unmyelinated. All myelinated fibres are sensory. They present a unimodal size spectrum with a size range of 1-8 microns, and a predominance of small fibres. Specific denervations indicate that about 1/3 of the unmyelinated axons represent afferents, and some 2/3 are sympathetic efferents. Interestingly, neonatal capsaicin treatment did not influence the number of unmyelinated PAN axons. The functional significance of the numerous unmyelinated sympathetic and sensory PAN axons in the normal knee joint remains to be elucidated.

Origin of sympathetic innervation of the knee joint in the cat: a retrograde tracing study with horseradish peroxidase

In the cat the origin of sympathetic nerve fibers in the medial and posterior articular nerve (MAN and PAN) of the knee joint was studied using retrograde labeling with horseradish peroxidase (HRP). Three to 5 days after uptake of HRP by MAN per cat 404 +/- 136 labeled somata (mean +/- S.D., n = 4 cats) were found mainly in the paravertebral ganglia L4 and L5 of the ipsilateral sympathetic trunk, and uptake of HRP by PAN labeled per animal 532 +/- 155 cell bodies (mean +/- S.D., n = 4 cats) mainly in the paravertebral ganglia L5 and L6. The cross-sectional areas of the labeled somata ranged from 200 to 1300 microns 2 with a mean of about 620 microns 2.

The proportions of sympathetic postganglionic and unmyelinated afferent axons in normal and regenerated cat sural nerves

Electrophysiological experiments have been carried out to see if the proportions of sympathetic postganglionic and unmyelinated afferent axons in a cutaneous nerve were changed after injury and regeneration. It seemed possible that an alteration in the relative numbers of the two groups of axons could contribute to the aetiology of reflex sympathetic dystrophy, but the experiments provided no evidence for such a change. There were, however, signs of a decrease in axon numbers in the regenerated nerves.

Long-lasting neuronal activity in rat dorsal horn evoked by impulses in cutaneous C fibres during noxious mechanical stimulation

The responses of 44 nociceptive neurones in the lumbar dorsal horn evoked by controlled mechanical stimulation of the skin, with or without conduction block in myelinated afferent fibres, were studied in the halothane-anaesthetized rat, in order to evaluate the effects of impulses in cutaneous nociceptive C fibres on dorsal horn neurones. Continuous non-noxious pinch of the skin evoked a short-latency discharge (mean latency 15 ms) in all the 13 class 2 neurones (i.e. neurones responding to both non-noxious and noxious stimulation of the skin) tested. The short-latency discharge was followed by weak prolonged activity in 6 neurones. Following noxious pinch of the skin a prominent late discharge (peak latency 150 ms-2 s) was evoked, which in all but two class 2 neurones outlasted the stimulation period (5-10 s). The discharge evoked by noxious pinch in class 3 neurones (i.e. neurones responding to noxious stimulation only) did not usually outlast the stimulation period. In all but two nociceptive neurones tested (n = 26) the late activity evoked by noxious pinch remained, albeit at a lower frequency in some neurones, during a conduction block in A fibres2,3. Hence this late discharge is probably mainly generated by impulses in nociceptive C fibers. It is concluded that nociceptive C fibres have an important role in sustaining long-lasting activation of class 2 neurones during noxious stimulation of the skin and that long-lasting discharges in these neurones indicates tissue damage to their receptive fields.

Climbing fibres projecting to cat cerebellar anterior lobe activated by cutaneous A and C fibres

1. Climbing fibre responses evoked on stimulation of the ipsilateral superficial radial nerve were examined in the forelimb area of the C3 zone in the barbiturate-anaesthetized cat. Climbing fibre responses were recorded in sixty-five Purkinje cells and as field potentials from the surface of the cerebellum. 2. In addition to the previously described A beta-fibre-evoked climbing fibre response, late climbing fibre responses were consistently evoked in all Purkinje cells studied when C fibres were stimulated. The latencies of the A beta- and C-fibre-evoked climbing fibre responses were 11-20 ms and 110-220 ms, respectively. In most experiments climbing fibre responses with an intermediate latency (20-30 ms) were evoked. It was demonstrated that this response depended on A delta fibres. 3. The long-latency climbing fibre response generated by electrical stimulation at C-fibre strength was evoked also during selective anodal block of conduction in A fibres (Brown & Hamman, 1972). Hence, impulses in C fibres were sufficient for generation of climbing fibre responses. 4. The distribution within the forelimb area of the C3 zone of the A beta- and C-fibre-evoked climbing fibre field potential was similar. No climbing fibre response was evoked in this area of the C3 zone by stimulation of A and C fibres in the contralateral superficial radial nerve or in the plantar nerves of the hind limbs. 5. It can be concluded that climbing fibres projecting to the forelimb area of the C3 zone in the cerebellum receive a somatotopically organized input from both A beta and C fibres.

Electrical stimulation reveals relatively ineffective sural nerve projections to dorsal horn neurons in the cat

Electrical stimulation of the sural nerve (SN) revealed input from sural nerve afferents to L6 and L7 dorsal horn neurons that were not apparent using natural mechanical stimuli, especially in cells with variable latency responses to SN stimulation. Nearly all (31/32) cells that had reliable, fixed latency responses to SN stimulation also had an excitatory receptive field (RF) in the region of skin innervated by the sural nerve (SN region). About one-third (20/57) of the cells with variable latency responses to SN stimulation, however, had an RF outside the SN region. Most (130/146) cells with no response to SN stimulation had RFs outside the SN region. There were no obvious differences between variable latency cells with RFs in the SN region vs those with RFs outside it in latency of response to SN stimulation, recording depth, RF sizes or modality properties. In a subsample of 31 postsynaptic dorsal column neurons all cells responding to SN stimulation also had an RF in the SN region. Strengthening of relatively ineffective projections from the sural nerve by lesions might be expected to lead to an increase in the proportion of cells responding with impulses to natural stimulation of the skin innervated by the sural nerve, and, hence, to an increase in average RF size.