Processing of trigeminocervical nociceptive afferent input by neuronal circuity in the upper cervical lamina I

Noise or signal? Spontaneous activity of dorsal horn neurons: patterns and function in health and disease

Spontaneous activity refers to the firing of action potentials by neurons in the absence of external stimulation. Initially considered an artifact or "noise" in the nervous system, it is now recognized as a potential feature of neural function. Spontaneous activity has been observed in various brain areas, in experimental preparations from different animal species, and in live animals and humans using non-invasive imaging techniques. In this review, we specifically focus on the spontaneous activity of dorsal horn neurons of the spinal cord. We use a historical perspective to set the basis for a novel classification of the different patterns of spontaneous activity exhibited by dorsal horn neurons. Then we examine the origins of this activity and propose a model circuit to explain how the activity is generated and transmitted to the dorsal horn. Finally, we discuss possible roles of this activity during development and during signal processing under physiological conditions and pain states. By analyzing recent studies on the spontaneous activity of dorsal horn neurons, we aim to shed light on its significance in sensory processing. Understanding the different patterns of activity, the origins of this activity, and the potential roles it may play, will contribute to our knowledge of sensory mechanisms, including pain, to facilitate the modeling of spinal circuits and hopefully to explore novel strategies for pain treatment.

Novel aspects of signal processing in lamina I

The most superficial layer of the spinal dorsal horn, lamina I, is a key element of the nociceptive processing system. It contains different types of projection neurons (PNs) and local-circuit neurons (LCNs) whose functional roles in the signal processing are poorly understood. This article reviews recent progress in elucidating novel anatomical features and physiological properties of lamina I PNs and LCNs revealed by whole-cell recordings in ex vivo spinal cord. This article is part of the Special Issue on "Ukrainian Neuroscience".

Neuralgia in the occipital region associated with ipsilateral trigeminal herpes zoster: Three case reports

BACKGROUND

Nerve fibers related to pain and temperature sensation in the trigeminal nerve territory converge with the upper cervical spinal nerves from the level of the lower medulla oblongata to the upper cervical cord. This structure is called the trigemino-cervical complex and may cause referred pain in the territory of the trigeminal or upper cervical spinal nerves.

CASE SERIES

Here, we report three cases of paroxysmal neuralgia in the occipital region with mild conjunctivitis or a few reddish spots in the ipsilateral trigeminal nerve territory. The patients exhibited gradual progression of these reddish spots evolving into vesicles over the course of several days, despite the absence of a rash in the occipital region. The patients were diagnosed with trigeminal herpes zoster and subsequently received antiherpetic therapy. Remarkably, the neuralgia in the occipital region showed gradual amelioration or complete resolution before the treatment, with no sequelae reported in the occipital region.

DISCUSSION

The trigemino-cervical complex has the potential to cause neuralgia in the occipital region, as referred pain, caused by trigeminal herpes zoster. These cases suggest that, even if conjunctivitis or reddish spots appear to be trivial in the trigeminal nerve territory, trigeminal herpes zoster should be considered when neuralgia occurs in the ipsilateral occipital region.

Utilizing directional preference in the management of cervicogenic headache: a case series

BACKGROUND/PURPOSE

Headaches are among the most common complaints requiring medical care, and annual expenditures for this condition are estimated to be 14 billion US dollars. The International Headache Society (IHS) describes cervicogenic headache (CGH) as a secondary type of headache emanating from the cervical spine which may be referred to one or more regions of the head and/or face. Mechanical Diagnosis and Therapy (MDT) is an approach shown to be effective in the management of spinal musculoskeletal disorders; however, there is limited evidence as to its efficacy in the management of CGH. The purpose of this case series was to examine the MDT approach in the assessment, classification, and management of a sample of patients experiencing cervicogenic headache.

CASE DESCRIPTION

This study was a prospective case series. Following IRB approval, 15 patients meeting the study inclusion criteria were recruited from a hospital-based outpatient physical therapy clinic. All subjects received a physical therapy examination by a Diploma trained MDT clinician which included but was not limited to patient self-report forms and the testing of repeated end range movements. The Numerical Pain Rating Scale (NPRS), Neck Disability Index (NDI), Headache Disability Index (HDI), Yellow Flag Risk Form (YFRF), Cervical Flexion Rotation Test (CFRT), and the Craniocervical Flexion Test (CCFT) were administered at the initial visit, 5th visit, and 10th visit or discharge, whichever occurred first. The NPRS, NDI, and HDI were re-administered at a 3 month follow up. Following the initial examination, patients were classified into the MDT categories of derangement, dysfunction, postural, or 'other' and then received intervention based on directional preference.

OUTCOMES

Fifteen subjects (mean age, 45.9 years; F = 11, M = 4; symptom duration, 44.3 months; average visits, 8.8) received an examination and intervention and completed follow-up outcome measures. Based on MDT classification criteria, all 15 subjects in this case series were classified as derangements. A non-parametric Friedman test of Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation differences among repeated measures was conducted on all outcome measures revealing statistically significant improvements in NPRS (p < .01), NDI(p < .01), and HDI (p < .01) scores at visit 10 and 3 month follow up. The mean change scores exceeded the minimal clinical important difference (MCID) for NPRS (4.2), NDI (7.6), and HDI (28.5). CCFT scores improved significantly from the initial examination to visit 5 (p < .01) and YFRF scores improved significantly between visits 5 and 10 (p < .01).

DISCUSSION/CONCLUSION

The diagnosis of CGH is difficult to determine based on pathoanatomical assessment. This case series suggests that the patient's response to repeated end range movements may indicate a directional preference for manual procedures and exercises which may be used in management of musculoskeletal conditions such as CGH.

Contralateral Afferent Input to Lumbar Lamina I Neurons as a Neural Substrate for Mirror-Image Pain

Mirror-image pain arises from pathologic alterations in the nociceptive processing network that controls functional lateralization of the primary afferent input. Although a number of clinical syndromes related to dysfunction of the lumbar afferent system are associated with the mirror-image pain, its morphophysiological substrate and mechanism of induction remain poorly understood. Therefore, we used ex vivo spinal cord preparation of young rats of both sexes to study organization and processing of the contralateral afferent input to the neurons in the major spinal nociceptive projection area Lamina I. We show that decussating primary afferent branches reach contralateral Lamina I, where 27% of neurons, including projection neurons, receive monosynaptic and/or polysynaptic excitatory drive from the contralateral Aδ-fibers and C-fibers. All these neurons also received ipsilateral input, implying their involvement in the bilateral information processing. Our data further show that the contralateral Aδ-fiber and C-fiber input is under diverse forms of inhibitory control. Attenuation of the afferent-driven presynaptic inhibition and/or disinhibition of the dorsal horn network increased the contralateral excitatory drive to Lamina I neurons and its ability to evoke action potentials. Furthermore, the contralateral Aβδ-fibers presynaptically control ipsilateral C-fiber input to Lamina I neurons. Thus, these results show that some lumbar Lamina I neurons are wired to the contralateral afferent system whose input, under normal conditions, is subject to inhibitory control. A pathologic disinhibition of the decussating pathways can open a gate controlling contralateral information flow to the nociceptive projection neurons and, thus, contribute to induction of hypersensitivity and mirror-image pain.SIGNIFICANCE STATEMENT We show that contralateral Aδ-afferents and C-afferents supply lumbar Lamina I neurons. The contralateral input is under diverse forms of inhibitory control and itself controls the ipsilateral input. Disinhibition of decussating pathways increases nociceptive drive to Lamina I neurons and may cause induction of contralateral hypersensitivity and mirror-image pain.

Presynaptic Interactions between Trigeminal and Cervical Nociceptive Afferents Supplying Upper Cervical Lamina I Neurons

Cervical and trigeminal afferents innervate neighboring cranial territories, and their convergence on upper cervical dorsal horn neurons provides a potential substrate for pain referral in primary headache syndromes. Lamina I neurons are central to this mechanism, as they relay convergent nociceptive input to supraspinal pain centers. Unfortunately, little is known about the interactions between trigeminal and cervical afferents supplying Lamina I neurons. Here, we used rats of both sexes to show that cervical and trigeminal afferents interact via presynaptic inhibition, where monosynaptic inputs to Lamina I neurons undergo unidirectional as well as reciprocal presynaptic control. This means that afferent-driven presynaptic inhibition shapes the way trigeminal and cervical Aδ-fiber and C-fiber input reaches Lamina I projection neurons (PNs) and local-circuit neurons (LCNs). We propose that this inhibition provides a feedforward control of excitatory drive to Lamina I neurons that regulates their convergent and cervical-specific or trigeminal-specific processing modes. As a consequence, disruption of the trigeminal and cervical afferent-driven presynaptic inhibition may contribute to development of primary headache syndromes.SIGNIFICANCE STATEMENT Cervical and trigeminal afferents innervate neighboring cranial territories, and their convergence on upper cervical dorsal horn neurons provides a potential substrate for pain referral in primary headache syndromes. Lamina I neurons are central to this mechanism as they relay convergent nociceptive input to supraspinal pain centers. Here, we show that cervical and trigeminal afferents interact via presynaptic inhibition, where inputs to Lamina I neurons undergo unidirectional as well as reciprocal control. The afferent-driven presynaptic inhibition shapes the trigeminocervical Aδ-fiber and C-fiber input to Lamina I neurons. This inhibition provides control of excitatory drive to Lamina I neurons that regulates their convergent and cervical-specific or trigeminal-specific processing modes. Disruption of this control may contribute to development of primary headache syndromes.