Observations on the innervation of the blood vessels in skeletal muscle

Self-reported neck pain is associated with migraine but not with tension-type headache in adolescents

Aim

The aim of the present analysis is to confirm or refute the association of neck pain to migraine or tension-type headache and to assess whether this association is independent of other risk factors for headache.

Methods

Secondary school students were invited to complete a questionnaire on headache and lifestyle factors in a cross-sectional study. Neck pain was assessed via (a) a screening question concerning neck pain and (b) denoting affected areas in schematic drawings of the human body.

Results

Absolute increment in prevalence of headache with pain in the shoulder-neck region was between 7.5% and 9.6%. Gender, grade, stress and lifestyle factors were assessed as potential confounding factors. Nearly all factors were associated with shoulder-neck pain and most with headache. After adjustment for confounders, the association of neck pain with headache was almost completely confined to migraine (OR 2.39; 95% CI 1.48–3.85) and migraine + tension-type headache (OR 2.12; 95% CI 1.50–2.99), whereas the association with isolated tension-type headache was negligible (OR 1.22, 95% CI 0.87–1.69).

Conclusion

Neck pain is associated with migraine but not with tension-type headache. A possible link between migraine and neck pain may be the cervico-trigeminal convergence of neck and meningeal sensory afferents or a disturbed descending inhibition in migraine.

The role of the neurovascular scalp structures in migraine

AIM

To review reports suggesting a role for neurovascular scalp structures in migraine.

MAIN DATA REPORTED

(A) Scalp periarterial nervous fibres contain all the main peptides and receptors involved in pain. (B) It is possible to interrupt or alleviate migraine pain with a prolonged compression of the main scalp arteries, which decreases blood flow through the pain-sensitized vessels and probably induces a temporary conduction block of periarterial nociceptive fibres. (C) Painful points are present on the scalp arteries of a considerable percentage of migraine sufferers. (D) It is possible to stop or alleviate pain by intervening on nociceptive periarterial fibres, as for example with the injection of lidocaine or 3-5 ml saline, and with percutaneous application of a capsaicin cream.

CONCLUSION

The data reported suggest a role for neurovascular scalp structures in at least some patients with migraine. It would be of interest to find a clinical distinction between patients according to the prevalence of an intracranial or extracranial peripheral pain mechanism. This could lead to more efficacious treatments.

Structure and function in the conceptual development of mammalian neuroendocrinology between 1920 and 1965

With the growing realization in the 1930s that the brain played a crucial role in regulating the secretions of the pituitary gland, neuroendocrinology as we now know it developed from two rather separate directions. One approach relied heavily on morphological techniques to define neurosecretion; a novel, but for many years flawed model that was originally developed to explain the presence of gland-like cells in the diencephalon. During its first 20 years neurosecretion, as a concept, made no significant contribution to our understanding of how the pituitary was controlled. Then, following the identification by Sanford Palay and Wolfgang Bargmann of a continuous neurosecretory pathway from the hypothalamus to the neural lobe, neurosecretion became incorporated into a more broadly based concept of pituitary function, particularly regarding the neural lobe. The second approach integrated structural and functional methods to investigate neural regulation of the pituitary. This work eventually explained how the pituitary was controlled by the brain. It led directly to our understanding of the control of vasopressin and oxytocin release by neuroendocrine terminals in the neural lobe, the neurohumoral control of the pars distalis, and eventually to a detailed description of the neural networks that control pituitary function. As increasingly sophisticated morphological, neurophysiological, and eventually molecular biological techniques were applied to the problem, the original notion of the diencephalic gland and neurosecretion became unsustainable. The gland-nerve cells of the 1930s became the neurosecretory cells of the 1940s and 1950s, and then finally neuroendocrine neurons in the 1960s. From then on neuroendocrinology developed into the more unified discipline we know today. The chronology of these two approaches will be examined here using examples from research that occurred approximately between 1920 and 1965. The goal is not to give a comprehensive history of pituitary function or neuroendocrinology. Instead, the focus will be to compare the rationales and effectiveness of two contrasting experimental approaches: predominantly structural analyses as opposed to more integrated approaches.

Origin of pain in migraine: evidence for peripheral sensitisation

Migraine is the most common neurological disorder, and much has been learned about its mechanisms in recent years. However, the origin of painful impulses in the trigeminal nerve is still uncertain. Despite the attention paid recently to the role of central sensitisation in migraine pathophysiology, in our view, neuronal hyperexcitability depends on activation of peripheral nociceptors. Although the onset of a migraine attack might take place in deep-brain structures, some evidence indicates that the headache phase depends on nociceptive input from perivascular sensory nerve terminals. The input from arteries is probably more important than the input from veins. Several studies provide evidence for input from extracranial, dural, and pial arteries but, likewise, there is also evidence against all three of these locations. On balance, afferents are most probably excited in all three territories or the importance of individual territories varies from patient to patient. We suggest that migraine can be explained to patients as a disorder of the brain, and that the headache originates in the sensory fibres that convey pain signals from intracranial and extracranial blood vessels.

Cortical representation of venous nociception in humans

Painful sensations can be evoked by application of thermal, mechanical, and chemical stimuli to the blood vessels. The cortical substrates of these sensations are unknown. We therefore used whole-head magnetoencephalography to record cortical responses to painful laser stimuli applied cutaneously and intravenously to the dorsum of the hand in healthy human subjects. Similar to the cutaneous stimuli, venous stimulation nearly simultaneously activated the contralateral primary and the bilateral secondary somatosensory cortices. In the venous stimulation condition, all activation peaks were about 50 ms earlier than in the cutaneous stimulation condition. Locations of responses to both stimuli did not differ. These results show that the afferent volley from the veins reaches the cerebral cortex significantly earlier than that from the skin. This might be due to differences in peripheral conduction velocity. Apart from this, these findings demonstrate that venous nociception shares the cortical representation of cutaneous nociception in human somatosensory cortices. Thus the cortical representation of nociceptive processing from tissues of mesodermal and ectodermal origin appears to be similar.

Localization of sympathetic postganglionic neurons innervating the femoral-saphenous vein in cats

Physiological and histochemical studies have suggested that the limb veins are innervated by sympathetic adrenergic fibers. In the present experiment, horseradish peroxidase (HRP) was used as a retrograde tracer to identify and localize the sympathetic postganglionic neurons that innervate the femoral-saphenous vein in cats. In anesthetized cats, HRP was applied perivascularly on a femoral and a saphenous vein segment (4-8 mm in length for each segment) to allow uptake into the nerve endings. The sympathetic chains on both sides were dissected after the animal was sacrificed and fixed 60 h following the HRP application. Histological examination on serial section was done to count the HRP-labeled neurons in each sympathetic ganglion from L1 to S1. In 10 cats, the total number of HRP neurons amounted to 8569. Most neurons arose from L3 (47%) and then L4 (31%). The number of neurons became progressively decreasing towards both ends of the sympathetic chain. Few neurons (less than 2% of the total) were discovered in the contralateral sympathetic ganglia. In each ganglion, the distribution of HRP neurons appeared to be scattering. Our findings provide anatomical evidence to support that the femoral-saphenous vein of the cat was innervated by the sympathetic efferent fibers. The main origins of these neurons are the third and fourth lumbar sympathetic ganglia.

Responses of spinal cord neurons following stimulation of A beta femoral-saphenous venous afferent fibers

A population of large (A beta) afferents is known to have endings in the wall of the femoral-saphenous vein. These afferents project to the lower lumbar spinal cord. The purpose of the present study was to identify, localize, and characterize spinal neurons that receive inputs from such afferents. Responses of 50 neurons in the L6 spinal cord segment of decerebrate-spinal cats or intact cats anesthetized using alpha-chloralose were recorded following electrical stimulation of these afferents. Observations were also made on the convergence of muscle and cutaneous afferent inputs onto neurons driven by stimulation of afferents terminating in the femoral-saphenous vein. All recording sites were marked either by intracellularly staining the element characterized with HRP or by extracellularly iontophoresing a small quantity of this tracer. The cells were driven for long durations (mean of 51.5 ms, S.E.M. of 10.0) by single-shock stimulation of femoral-saphenous venous afferents. The recording sites were located in Rexed's laminae IV-VIII and X. Eight of the 50 neurons were activated by venous afferent stimulation at latencies equal to or shorter than that of the first negative wave of the cord dorsum potential; these units were driven at a mean latency of 1.4 ms (S.E.M. of 0.25) following the arrival of the afferent volley at the cord and were assumed to receive monosynaptic, or at least relatively direct, inputs from the primary afferents. Most of these cells (6 of 8) were located in lamina V. The majority of the neurons studied (37 of 50) were activated at latencies longer than 3 ms following the arrival of the afferent volley at the cord; about half (19 of 37) of those activated at longer latencies were located in lamina VII, and the rest were scattered among the other laminae. Twenty-eight of 40 venous afferent-driven cells tested could also be activated by electrical stimulation of either the posterior tibial or sural nerve. In general, the stimulation intensities necessary to activate the neurons were only sufficient to excite large (A alpha or A beta) muscle and cutaneous afferents. Neurons receiving the shortest latency inputs from the femoral-saphenous vein were less likely to receive convergent inputs from muscle or skin than were neurons activated by venous afferents at longer latency.

Tracing of afferent pathways from the femoral-saphenous vein to the dorsal root ganglia using transport of horseradish peroxidase

The retrograde transport of horseradish peroxidase (HRP) was used to trace afferents from the femoral-saphenous vein to the dorsal root ganglia in the cat. Afferents arising along the entire length of the vein projected to very localized spinal levels; 63% of the labeled cells counted were located in the L6 dorsal root ganglion, 37% were located in the L5 ganglion and less than 1% were located at other levels. Most of the cell bodies labeled by the application of HRP to the femoral-saphenous vein were small in size (diameter less than 35 microns). However, some large cell bodies (diameter greater than 50 microns) were also noted. It was estimated that over two-thirds of the femoral-saphenous venous afferents were C fibers; at least 15% were estimated to be A fibers. The largest venous afferents were predicted to conduct action potentials at approximately 60 m/s.

Properties of femoral venous afferent inputs and lumbosacral distribution of spinal evoked activity

The present studies were done to determine details of the anatomical and physiological characteristics of femoral-saphenous venous afferent input to the lumbar spinal cord. Gross anatomical examination revealed that afferent bundles could be seen coursing from the saphenous nerve to the femoral-saphenous vein. Compound action potentials elicited by femoral-saphenous venous afferent stimulation were recorded from the femoral nerve and in dorsal rootlets of the 6th lumbar cord segment. The compound action potentials included activity correlated with that of fibers conducting impulses at the rate of 31 to 61 m/s. Lumbar cord dorsum potentials elicited by femoral-saphenous venous afferent stimulation were abolished by rhizotomy of the most caudal rootlets of the 6th lumbar cord segment. L6 was also the cord segment from which the largest amplitude cord dorsum negative potentials were recorded, while action potentials with large late positive waves were recorded from more caudal cord segments. These observations suggested that the L6 segment contained the largest number of spinal neurons responding to primary femoral-saphenous venous afferent input, and that input reached the more caudal segments via intersegmental connections. A proposed physiological role of these afferents is briefly described.

Activation of spinal cord interneurons which process inputs from the femoral-saphenous vein

Recordings were made from single spinal cord interneurons which could be activated by electrical stimulation of afferents terminating in the wall of the femoral-saphenous vein. Interneurons were either excited or both excited and inhibited by venous afferent stimulation. Most of the venous afferent-driven interneurons could also be driven by electrical activation of A-alpha beta muscle and cutaneous afferents. Stimulation of several different muscle nerves drove single interneurons.

Properties of spinal cord processing of femoral venous afferent input revealed by analysis of evoked potentials

Two characteristics of spinal cord interneurons which receive inputs from femoral-saphenous venous afferents were examined. The shortest pathway between primary femoral-saphenous venous afferents and alpha-motoneurons was shown to be a di- or tri-synaptic circuit. In addition, the largest focal synaptic field potentials elicited by venous afferent stimulation at short latency were recorded from Rexed's lamina V. It was thus concluded that most of the first interneurons excited by venous afferents are found in this lamina.

Sensory innervation of the rat portal vein and the hepatic artery

Several neuroanatomical and neurophysiological experiments suggest that the hepatic portal vein is not only richly innervated with sympathetic efferents, but also it is an important source of afferent information. By combining retrograde tracing (with True Blue as a marker) and immunological techniques, the cell bodies for the substance P-containing nerves that surround the portal vein and the hepatic artery of the rat have been localized to the spinal sensory ganglia (T8-T13). Since dorsal root rhizotomy abolished all substance P immunoreactive material from nerve fibres that surround these blood vessels, and since no double-labelled cells were detected in the nodose ganglia, an exclusive spinal origin for the substance P-containing sensory nerves is suggested.

Lumbar spinal cord responses to limb vein distention

The purpose of this study was to determine if central neural responses were elicited by distention of limb veins, and to compare the pattern of these response to those produced in previous studies using electrical stimulation to excite limb venous afferent fibers. Spinal evoked potentials were measured in response to stretch of the wall of a segment of the femoral-saphenous vein by perfusion-distention or by mechanical stretch. These studies revealed that spinal cord evoked potentials were elicited by these procedures, and that the activated venous afferent fibers coursed through the saphenous nerve and entered the sixth lumber spinal cord segment. The minimum stretches which were required to elicit spinal evoked potentials were produced by perfusion pressures starting at 2-3 mm Hg, or by mechanical stretch of the wall of 5 micron/mm. A vein wall proprioceptor hypothesis is proposed and discussed in the light of these findings. In addition to the cord dorsum evoked potentials, distention or stretch of the vein wall elicited ventral root potentials (excitatory postsynaptic population potentials) which are known to be produced by excitatory inputs to motoneurons. A venous afferent mediated muscle-tonus venopressor mechanism hypothesis is proposed and discussed in the light of these and previous findings.

Interactions between femoral venous afferents and lumbar spinal reflex pathways

This study reports findings of spinal reflex connections of afferent fibers electrically excited in the wall of the femoral vein. Condition-test experiments, and EMG recordings revealed that the femoral venous afferents have facilitatory connections to flexor and extensor motoneurons of both the proximal and the distal hindlimb muscles. Femoral venous afferent stimulation which produced facilitation, also produced inhibition of the test reflexes following the facilitation. Because the inhibition was enhanced by diazepam injection and because the inhibitory time-course correlated closely to the time-courses of both dorsal root potentials and individual tests of primary afferent depolarization, the inhibition was suggested to be produced by presynaptic inhibition. The potentially significant role of the venous afferent connections in a reflex-elicited skeletal muscle pump or in an increase in intramuscular venous counterpressure is discussed.

Substance P-like immunoreactivity in nerves associated with the vascular system of guinea-pigs

Substance P-like immunoreactivity was localized by an indirect immunohistochemical technique in whole mounts and sections of blood vessels from the guinea-pig. There was a widespread association of nerve fibres that had substance P-like immunoreactivity with blood vessels, extending into all vascular beds. The relative densities of supply of different vessels were assessed visually and a rating scale used to compare them. Large elastic arteries close to the heart had dense networks of immunoreactive nerves associated with them. The density decreased as more peripheral beds were approached, except that there was a particularly dense network of nerves with arteries of the splanchnic beds. Arteries to myocardial, central nervous system, renal, reproductive and skeletal muscle beds all had substance P-immunoreactive nerves associated with them to varying extents. The venae cavae near the heart were densely supplied, but there were few fibres with their more peripheral extensions. Some large veins (e.g. pulmonary, hepatic portal and superior mesenteric) had a few fibres with them, but veins of peripheral vascular beds had very few or no immunoreactive nerve fibres. Substance P-like immunoreactivity in vascular nerves was markedly reduced in guinea-pigs that were injected with capsaicin but was unaffected by the injection of 6-hydroxydopamine. It is concluded that the vascular substance P-immunoreactive nerves are likely to be of sensory origin.