Magnetic stimulation of muscle evokes cerebral potentials

Somatosensory evoked potentials (SEPs) were recorded from the scalp in man to magnetic stimulation of various skeletal muscles. The potentials consisted of several components, the earliest of which decreased in latency as the stimulated site moved rostrally, ranging from 46 msec for stimulation of the gastrocnemius, to 14 msec for stimulation of the deltoid. Experiments were performed to distinguish the mechanisms by which magnetic stimulation of the muscle was effective in evoking these cerebral potentials. For the gastrocnemius, the intensity of the magnetic stimulus needed for evoking cerebral potentials was less than that required for activating mixed or sensory nerves in proximity to the muscle belly (eg, posterior tibial nerve in the popliteal fossa, sural nerve at the ankle). Vibration of the muscle or passive lengthening of the muscle, procedures which activate muscle spindles, were accompanied by a significant attenuation of the potentials evoked by magnetic stimulation of the muscle. Anesthesia of the skin underlying the stimulating coil had no effect on the latency or amplitude of the early components of the magnetically evoked potentials, whereas electrically evoked potentials from skin electrodes were abolished. Thus, the cerebral potentials accompanying magnetic stimulation of the muscle appear to be due to activation of muscle afferents. We suggest that magnetic stimulation of muscle can provide a relatively simple method for quantifying the function of muscle afferents originating from a wide variety of skeletal musculature.     

Organization of the efferent vestibular nuclei and nerves of the toadfish, Opsanus tau

The efferent vestibular nuclei and nerves were studied in the toadfish, Opsanus tau, with morphological and electrophysiological techniques. The origin and course of the efferent vestibular nerves was extensively documented. One major morphological observation was that the efferent nerves comprise a peripheral network that is anatomically distinct, and separable by dissection from the primary afferents innervated by each end organ. These anatomically distinct nerves are likely to be a major asset in physiological studies of efferent vestibular function. The retrograde transport of horseradish peroxidase (HRP) from each of the nerves innervating the vestibular and lateral line organs was used to delineate the subgroups of efferent neurons projecting to these end organs. The efferent vestibular nuclei are located in the posterior medulla in and around the median longitudinal fasciculi (MLF). We divided the nuclei cytoarchitecturally into lateral, medial, and dorsal subdivisions. The lateral cells had bilateral dendritic trees while the dorsal cells had ipsilateral, unilateral dendritic trees. There was a higher proportion of lateral cells that innervated the canal organs and the utricle while the dorsal cells tended to innervate the other organs. The total number of cells obtained by summing those from separate nerve label was twice the total cell count present in the nuclei. Indirectly, this indicates that some cells project to more than one end organ. Efferent neurons were penetrated with glass microelectrodes, and their end organs and patterns of connectivity with other end organs were investigated by stimulating various vestibular nerves. Posterior semicircular canal efferent cells are electrically coupled to each other and could be activated electrically or chemically by stimulating other ipsilateral or contralateral vestibular nerves. It is suggested that electrical coupling might be responsible for the uniform behavior of these cells under certain conditions. Morphological and physiological experiments suggested that the semicircular canals are innervated by their own, exclusive populations of efferent neurons while other end organs may share efferent innervation. Single cells were injected intracellularly with HRP and their morphology was studied and characterized by light microscopy. Intracellular label confirmed the morphological features demonstrated by retrograde transport of HRP and also revealed that some cells had central axon collaterals that terminated within the MLF. These morphological and physiological results provide a basis for understanding the behavior of efferent vestibular neurons in the alert animal.     

Effects of electrical and chemical stimulation of nucleus raphe magnus on responses to renal nerve stimulation

Electrical stimulation of the nucleus raphe magnus (NRM) inhibits some somatic and visceral input at the spinal level. This study was designed to examine the effects of electrical and chemical stimulation of NRM on neuronal responses to afferent renal nerve (ARN) stimulation. In chloralose-anesthetized rats, electrical stimulation of ARN elicited predominantly excitatory responses in spinal gray neurons. In 10 neurons studied, electrical stimulation of the NRM elicited an inhibition of spontaneous activity of 8 neurons and inhibited evoked responses to ARN stimulation in 6 neurons. Microinjection of glutamate (5-10 nmol in 0.5-1 microliter) into the NRM elicited an inhibition of spontaneous activity in 9 neurons, a facilitation in 6 neurons and no response in 8 neurons receiving ARN input. Responses evoked by ARN stimulation were inhibited in 12 neurons, facilitated in 4 neurons and not affected in 8 neurons. We conclude that renal input can be modulated at the spinal level by activation of the NRM and adjacent tissue. Furthermore, the inhibition of spinal gray neuronal responses elicited by stimulation of the NRM appears to be due, at least in part, to activation of fibers of passage since non-selective electrical stimulation is more efficacious than selective chemical stimulation of neuronal soma and dendrites.     

Vagal Intramuscular Arrays: The Specialized Mechanoreceptor Arbors That Innervate the Smooth Muscle Layers of the Stomach Examined in the Rat

The fundamental roles that the stomach plays in ingestion and digestion notwithstanding, little morphological information is available on vagal intramuscular arrays (IMAs), the afferents that innervate gastric smooth muscle. To characterize IMAs better, rats were given injections of dextran biotin in the nodose ganglia, and, after tracer transport, stomach whole mounts were collected. Specimens were processed for avidin–biotin permanent labeling, and subsets of the whole mounts were immunohistochemically processed for c‐Kit or stained with cuprolinic blue. IMAs (n = 184) were digitized for morphometry and mapping. Throughout the gastric muscle wall, IMAs possessed common phenotypic features. Each IMA was generated by a parent neurite arborizing extensively, forming an array of multiple (mean = 212) branches averaging 193 µm in length. These branches paralleled, and coursed in apposition with, bundles of muscle fibers and interstitial cells of Cajal. Individual arrays averaged 4.3 mm in length and innervated volumes of muscle sheet, presumptive receptive fields, averaging 0.1 mm³. Evaluated by region and by muscle sheet, IMAs displayed architectural adaptations to the different loci. A subset (32%) of circular muscle IMAs issued specialized polymorphic collaterals to myenteric ganglia, and a subset (41%) of antral longitudinal muscle IMAs formed specialized net endings associated with the serosal boundary. IMAs were concentrated in regional patterns that correlated with the unique biomechanical adaptations of the stomach, specifically proximal stomach reservoir functions and antral emptying operations. Overall, the structural adaptations and distributions of the IMAs were consonant with the hypothesized stretch receptor roles of the afferents. J. Comp. Neurol. 524:713–737, 2016. © 2015 Wiley Periodicals, Inc.     

Interoception in Autism Spectrum Disorder: A review

PurposeThis review article summarizes original scientific research published to date on interoception in individuals with Autism Spectrum Disorder (ASD). Sensory processing has been shown to be atypical in ASD, yet physiological processing and subjective experience of internal sensation processing, namely interoception, has not been reported sufficiently in research or clinical settings.BackgroundThere is a small but growing body of scientific research on interoception in ASD, which is relevant to understanding the behavioral and cognitive characteristics inherent in this condition, and may provide a foundation for clinical interventions such as biofeedback, pain management, and brain stimulation techniques.MethodsA literature review of original research was performed using major scientific databases.ResultsInteroception, which occurs due to multisensory connections and integration of internal afferents in cortical and subcortical areas, is atypical in ASD, but the degree and directionality of this abnormality is not yet clear due to the heterogeneity of the condition. Between-group interoceptive differences in individuals with and without ASD have been repeatedly demonstrated, with a slight tendency towards hyporeactivity in interoceptive awareness in individuals with ASD.SignificanceMultidimensional research combining neuroimaging with psychophysiological and self-report measures guided by a clear theoretical model is necessary to understand how interoceptive differences link to the behavioral and cognitive characteristics of ASD. Sensory processing models and autism theory should also be updated to incorporate these recent findings.     

Increased short latency afferent inhibition after anodal transcranial direct current stimulation

Transcranial direct current stimulation (tDCS), a technique for central neuromodulation, has been recently proposed as possible treatment in several neurological and psychiatric diseases. Although shifts on focal brain excitability have been proposed to explain the clinical effects of tDCS, how tDCS-induced functional changes influence cortical interneurones is still largely unknown. The assessment of short latency afferent inhibition (SLAI) of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS), provides the opportunity to test non-invasively interneuronal cholinergic circuits in the human motor cortex. The aim of the present study was to assess whether anodal tDCS can modulate interneuronal circuits involved in SLAI. Resting motor threshold (RMT), amplitude of unconditioned MEPs and SLAI were assessed in the dominant hemisphere of 12 healthy subjects (aged 21-37) before and after anodal tDCS (primary motor cortex, 13min, 1mA). SLAI was assessed delivering electrical conditioning stimuli to the median nerve at the wrist prior to test TMS given at the interstimulus interval (ISI) of 2ms. Whereas RMT and the amplitude of unconditioned MEPs did not change after anodal tDCS, SLAI significantly increased. In conclusion, anodal tDCS-induced effects depend also on the modulation of cortical interneuronal circuits. The enhancement of cortical cholinergic activity assessed by SLAI could be an important mechanism explaining anodal tDCS action in several pathological conditions.