Modulation of cutaneous responses in the cuneate nucleus of macaques during active movement

To achieve stable and precise movement execution, the sensorimotor system integrates exafferent sensory signals originating from interactions with the external world and reafferent signals caused by our own movements. This barrage of sensory information is regulated such that behaviorally relevant signals are boosted at the expense of irrelevant ones. For example, sensitivity to touch is reduced during movement - when cutaneous signals caused by skin stretch are expected and uninteresting - a phenomenon reflected in a decreased cutaneous responsiveness in thalamus and cortex. Some evidence suggests that movement gating of touch may originate from the cuneate nucleus (CN), the first recipient of signals from tactile nerve fibers along the dorsal columns medial lemniscal pathway. To test this possibility, we intermittently delivered mechanical pulses to the receptive fields (RFs) of identified cutaneous CN neurons as monkeys performed a reach-to-grasp task. As predicted, we found that the cutaneous responses of individual CN neurons were reduced during movement. However, this movement gating of cutaneous signals was observed for CN neurons with RFs on the arm but not those with RFs on the hand. We conclude that sensory gating occurs in the first processing stage along the somatosensory neuraxis and sculpts incoming signals according to their task relevance.

Myelopathy associated with intrathecal methotrexate

A 21-year-old man developed progressive and bilateral lower limb numbness, gait impairment and urinary incontinence over 10 days. He had received intrathecal methotrexate 20 days previously for acute lymphoblastic B-cell leukaemia, following 7 months of systemic chemotherapy. MR scan of the spinal cord showed bilateral symmetric and extensive T2/fluid attenuated inversion recovery (FLAIR) increased signal involving the dorsal columns in the thoracic cord. His serum folate concentration was at the lower end of the normal range. We stopped the intrathecal chemotherapy and gave folate; after a few days, he progressively improved. Myelopathy is an important adverse effect of intrathecal methotrexate, which may cause clinical and imaging features resembling subacute combined degeneration of the spinal cord. CNS infiltration must be excluded, intrathecal chemotherapy stopped and deficiency of folate or vitamin B12 treated as appropriate.

THE PLASTICITY OF NERVE FIBRES: THE PROLONGED EFFECTS OF POLARIZATION OF AFFERENT FIBRES

The review surveys various aspects of the plasticity of nerve fibres, in particular the prolonged increase in their excitability evoked by polarization, focusing on a long-lasting increase in the excitability of myelinated afferent fibres traversing the dorsal columns of the spinal cord. We review the evidence that increased axonal excitability (i) follows epidurally applied direct current as well as relatively short (5 or 10 ms) current pulses and synaptically evoked intrinsic field potentials; (ii) critically depends on the polarization of branching regions of afferent fibres at the sites where they bifurcate and give off axon collaterals entering the spinal grey matter in conjunction with actions of extrasynaptic GABAA membrane receptors; and (iii) shares the feature of being activity-independent with the short-lasting effects of polarization of peripheral nerve fibres. A comparison between the polarization evoked sustained increase in the excitability of dorsal column fibres and spinal motoneurons (plateau potentials) indicates the possibility that they are mediated by partly similar membrane channels (including non-inactivating type L Cav++ 1.3 but not Na+ channels) and partly different mechanisms. We finally consider under which conditions trans-spinally applied DC (tsDCS) might reproduce the effects of epidural polarization on dorsal column fibres and the possible advantages of increased excitability of afferent fibres for the rehabilitation of motor and sensory functions after spinal cord injuries.

Wallerian degeneration as a mimic of recurrence of myelitis

A middle-aged woman with idiopathic longitudinally extensive myelitis underwent repeat MR scan of cervical spine at 5-month follow-up, which showed new non-enhancing T2 hyperintensities, initially reported as myelitis recurrence. However, the hyperintensities involved both lateral corticospinal tracts caudal to the initial lesion and both dorsal columns rostral to the initial lesion and were therefore compatible with Wallerian degeneration. This radiological mimic should be considered in the differential of recurrence of myelitis.

Nitrous Oxide Abuse‐Induced Subacute Combined Degeneration: Classic Neuroimaging Findings on MRI

Subacute combined degeneration is uncommonly a result of nitrous oxide abuse and presents with high signal in the dorsal columns of the spinal cord on T2-weighted MR imaging. We present a case of subacute combined degeneration in a young patient who abused nitrous oxide, which is an uncommon cause and infrequently seen in this patient population. Symptoms are often reversible with treatment of vitamin B12, and radiologists should be aware of these findings to avoid delay in treatment.

Subacute combined degeneration of the cord secondary to nitrous oxide misuse: No laughing matter

Nitrous oxide (N2O) has several traditional uses as a surgical and dental anaesthetic, as well as in aerosol spray propellants. It is the combination of analgesic and euphoric qualities coupled with accessibility as an over-the-counter household item that lends N2O to recreational use. Despite increasing evidence that prolonged use of N2O both medically and as a drug of abuse can cause disabling neurological side-effects, it remains widely used. We present the case of an 18-year-old male who was diagnosed with subacute combined degeneration of the cord (SCDC) secondary to acute, heavy recreational use of N2O. The patient presented with progressive paraesthesia affecting his hands and feet associated with distal weakness. MRI of the cervical spine revealed symmetric bilateral high T2 signal within the dorsal columns extending from the level of C2 to T2 with the inverted ‘V’ sign on axial T2-weighted slices indicative of SCDC. Although vitamin B12 levels were within normal range, marked elevation of methylmalonic acid and homocysteine support the diagnosis of B12 inactivation and functional B12 deficiency, which fully resolved with treatment.

Branching points of primary afferent fibers are vital for the modulation of fiber excitability by epidural DC polarization and by GABA in the rat spinal cord

Polarization of sensory fibers traversing dorsal columns of the spinal cord may considerably increase the excitability of these fibers. We show that this involves the effects of current at branching points of afferent fibers and depends on extrasynaptic of GABA. These results contribute to our understanding of the mechanism underlying plasticity of activation of nerve fibers and may be used to increase the effectiveness of epidural stimulation in humans and recovery of spinal functions.

Neurobiology of Sensory Motor Systems and Internal Environment

Control of motor function—our activities like walking, lifting an object, writing, etc.—is accomplished through integrated and coordinated action of motor neurons. Motor neurons can be classified into two broad categories: upper motor neurons and lower motor neurons. Upper motor neurons have cell bodies in the motor cortex of brain and carry the impulses from cortex to spinal cord segment. Cell bodies of lower motor neurons are located in the anterior horn of spinal cord. Axons of lower motor neuron end in neuromuscular junction, and excitation of muscle fibres leads contraction of muscle. Different types of receptors carry different sensations like touch, pain, position, and vibration through sensory nerve fibres. Fibers mediating fine touch and proprioception ascend in the dorsal columns to the medulla, where they synapse in the gracile and cuneate nuclei. This chapter explores the neurobiology of sensory motor systems and the internal environment.

Importance of Laterality in Cervical Spinal Cord Stimulation for Facial Pain: Case Report and Anatomic Review

BACKGROUND AND IMPORTANCE Chronic neuropathic facial pain is a debilitating disease that can be approached with multiple different treatment modalities. Cervical spinal cord stimulation has been demonstrated to be effective for patients suffering from neuropathic facial pain. Consensus does not exist in the literature regarding technique for placement. CLINICAL PRESENTATION A 49-yr-old female presented with chronic intractable neuropathic facial pain. She underwent a successful percutaneous spinal cord stimulation trial, followed by placement of a paddle electrode for permanent implantation. The paddle electrode failed to duplicate the pain relief of her trial. Measurement of the width of the paddle demonstrated that it was 2 mm smaller than the separation of the percutaneous trial electrodes. Electrodes with wider interelectrode distance were then placed with satisfactory pain relief. CONCLUSION Although conventional spinal cord stimulation targets the dorsal columns, cervical spinal cord stimulation for facial pain targets the spinal trigeminal nucleus. The effectiveness of stimulation may be increased with a more laterally positioned electrode in order to recruit the more laterally positioned spinal trigeminal nucleus. This case report illustrates the importance of this anatomic consideration.

Pelvic/Visceral Cancer Pain

The management of medically refractory cancer pain is a complex, multi-disciplinary effort. When optimal medical management has failed, neuroablative and neuromodulatory efforts can be used. While neuromodulation is an attractive option due its minimally invasive nature, neuroablation offers the advantage of high efficacy and the lack of maintenance or upkeep requirements. Punctate midline myelotomy is an ablative procedure targeting the ascending visceral pain pathway in the dorsal columns. The procedure can be performed through an open approach creating a mechanical lesion, or percutaneously via either a mechanical lesion or radiofrequency ablation. Careful attention to the spinal cord midline during lesion creation and attention to surgical technique, including an excellent fascial closure, can minimize potential complications.