Are there motor fibers in the sural nerve?

Mother and sibling interactions during the preweaning period influence myelination and impulse propagation of the sensory sural nerve in the adult rat

To investigate whether mother and sibling interactions during the preweaning period influence the histological and electrophysiological characteristics of the sensory sural nerve (SUn) in the adult rat, litters composed of 1, 3, 6, 9, and 12 male pups (P) were formed and the pups routinely weighed until postnatal day 60 (PND60). At PND9, 3P and 6P litters showed greater body weight than pups without siblings or from 9P or 12P litters, and such differences in weight were maintained until adulthood. Analysis of maternal licking at PND8 and 9 showed that pups from large litters received fewer licks than pups from small size litters. At PND60, SUn of rats from 6P and 9P litters had greater compound action potential (CAP) amplitude and a higher proportion of axons with large myelin thickness than nerves from rats of 1P, 3P, or 12P litters. SUn of heaviest rats from 9P and 12P litters had greater CAP area and myelination than the lightest rats from the same litters. We propose that a complex interplay of sensory, social, and nutritional factors arising from mother and littermate interactions during the preweaning period influence myelination and the propagation of action potentials in the SUn of adult rats.

The Internal Superior Laryngeal Nerve in Humans: Evidence for Pure Sensory Function

OBJECTIVES

To determine if the internal branch of the superior laryngeal nerve (iSLN) provides direct motor innervation to the interarytenoid muscle, a laryngeal adductor critical for airway protection. We studied the iSLN-evoked motor response in the interarytenoid and other laryngeal muscles. If the iSLN is purely sensory, there will be no detectable short latency motor response upon supramaximal stimulation, indicating the absence of a direct efferent conduction path.

STUDY DESIGN

Intraoperative case series.

METHODS

In seven anesthetized patients undergoing laryngectomy for unilateral laryngeal carcinoma, the iSLN of the unaffected side was electrically stimulated intraoperatively with 0.1-ms pulses of progressive intensities until supramaximal stimulation was reached. Electromyographic responses were measured in the ipsilateral interarytenoid, thyroarytenoid, and cricothyroid muscles.

RESULTS

None of the subjects exhibited short-latency interarytenoid motor responses to iSLN stimulation. Supramaximal electrical stimulation of the intact iSLN evoked ipsilateral motor responses with long latencies: 18.7-38.5 ms in the interarytenoid (n = 6) and 17.8-24.9 ms in the thyroarytenoid (n = 5). Supramaximal stimulation of the recurrent laryngeal nerve evoked ipsilateral motor responses with short latencies: 1.6-3.9 ms in the interarytenoid (n = 6) and 1.6-2.7 ms in the thyroarytenoid (n = 6).

CONCLUSION

The iSLN provides no functional efferent motor innervation to the interarytenoid muscles. The iSLN exclusively evokes an interarytenoid motor response via afferent activation of central neural circuits that mediate the laryngeal reflex arc. These findings suggest that the role of the iSLN in vital laryngopharyngeal functions, such as normal swallowing and protection of the airway from aspiration, is purely sensory.

LEVEL OF EVIDENCE

4 Laryngoscope, 131:E207-E211, 2021.

Early postnatal development of electrophysiological and histological properties of sensory sural nerves in male rats that were maternally deprived and artificially reared: Role of tactile stimulation

Early adverse experiences disrupt brain development and behavior, but little is known about how such experiences impact on the development of the peripheral nervous system. Recently, we found alterations in the electrophysiological and histological characteristics of the sensory sural (SU) nerve in maternally deprived, artificially reared (AR) adult male rats, as compared with maternally reared (MR) control rats. In the present study, our aim was to characterize the ontogeny of these alterations. Thus, male pups of four postnatal days (PND) were (1) AR group, (2) AR and received daily tactile stimulation to the body and anogenital region (AR-Tactile group); or (3) reared by their mother (MR group). At PND 7, 14, or 21, electrophysiological properties and histological characteristics of the SU nerves were assessed. At PND 7, the electrophysiological properties and most histological parameters of the SU nerve did not differ among MR, AR, and AR-Tactile groups. By contrast, at PND 14 and/or 21, the SU nerve of AR rats showed a lower CAP amplitude and area, and a significant reduction in myelin area and myelin thickness, which were accompanied by a reduction in axon area (day 21 only) compared to the nerves of MR rats. Tactile stimulation (AR-Tactile group) partially prevented most of these alterations. These results suggest that sensory cues from the mother and/or littermates during the first 7-14 PND are relevant for the proper development and function of the adult SU nerve. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 351-362, 2018.

Electrophysiologic response recorded in the first dorsal interosseous muscle with stimulation of the tibial and deep fibular nerves

Foot intrinsic muscle innervation may demonstrate some variability. The first dorsal interosseous muscle (FDI) is innervated by the deep branch of the lateral plantar nerve (LPN) from the main trunk of the tibial nerve. Contribution from the deep fibular nerve (DFN) may also play a role in the supply of the FDI. Thirty healthy adult volunteers were studied to determine the presence and type of response in the FDI with stimulation of the tibial nerve/deep branch of the LPN and DFN. Both nerves were stimulated at the ankle and knee with a surface and needle recording from the FDI. Latency, amplitude, and conduction values were recorded for each nerve. The incidence of DFN supply to the FDI was 16.6% with a mean ankle amplitude of 152 microV. The incidence of tibial nerve/deep branch of the LPN supply to the FDI was 100%, with a mean ankle amplitude of 5.11 mV. The superficial branch of the LPN is most often studied when evaluating for tarsal tunnel syndrome because the standard recording site is the abductor digiti minimi (ADM). Recording from the ADM, however, frequently produces a less than desirable waveform, and the technical challenges encountered with this site make tarsal tunnel syndrome assessment difficult. It is also possible that selective involvement of the deep branch of the LPN may occur, and if so, recording from the FDI may prove valuable.

Sural-tibial nerve communications in humans

We investigated the occurrence of a communication between the sural and tibial nerves in 49 legs of 28 Japanese cadavers. In front of the calcanean tendon, we found the communication in 7 legs (14.3%) or in 5 cadavers (18.9%). The sural nerve gave rise to a number of medial and lateral branches, including the lateral calcanean branch at the lateral side of the ankle. The communicating branch with the tibial nerve arose from the first medial branch and pierced the deep fascia of the leg. In 4 cases, the U-shaped communication was formed between the sural and tibial nerves, and in 3 cases, the Y-shaped communication. Electrophysiological evidence of an anomalous motor function of the sural nerve has been reported recently. We consider that the U-shaped communication between the sural and tibial nerves gives a morphological basis to the motor function of the sural nerve.

Effect of perinatal food deficiencies on the compound action potential evoked in sensory nerves of developing rats

The aim of this study was to analyze the possible alterations produced by inadequate perinatal food intake, in quantity (undernutrition) or quality (malnutrition), on the generation and propagation of the compound action potential (CAP) evoked in sensory sural nerves, during the postnatal development of the rat. Low intensity stimulation (2-3 times the threshold of the most excitable nerve fibers; xT) of the sural nerve evoked an early potential (CAP-A component) which is due to activation of low-threshold, fast-conducting myelinated group A afferent fibers. Meanwhile, at higher stimulus intensity (20-30T) it produced a second, long-lasting potential (CAP-C component) probably due to activation of high-threshold, slow-conducting group Adelta or C afferent fibers. Compared to control nerves, the CAP-A component, but not the CAP-C component of undernourished and malnourished nerves showed significant changes in amplitude, area, electrical threshold and conduction velocity (except absolute refractory period) at several postnatal ages. Our results may suggest that a relative large number of myelinated group A afferent fibers in the sural nerve of undernourished and malnourished animals suffer severe alterations on their electrophysiological properties of generation and propagation of the action potential during the postnatal development of the rat.

Reinnervation of denervated skeletal muscles by grafted dorsal root ganglion

We examined whether or not the cervical dorsal root ganglion (DRG) of the rat, when isografted and connected to the distal stump of the severed common peroneal nerve, could survive, project axons to the denervated leg muscles, and exert beneficial influences to delay the degeneration of the denervated muscles. Rats in which the muscles were similarly denervated but no DRG was grafted served as the control. After a postoperative period of 72 to 286 days, histological study showed that nerve cells at the superficial part of the grafted DRG survived. Indirect electrical stimulation via the distal stump of the common peroneal nerve produced no contraction of the muscles, indicating that no functional neuromuscular contacts had been reestablished. Direct stimulation of the denervated muscles did elicit contraction, and the isometric twitch and tetanic tensions were significantly much higher in the experimental rats with a grafted DRG than in the control rats. Cholinesterase-silver staining indicated the presence of axons in the denervated muscles, but the axons did not terminate on endplates. Compared with the control muscles, the experimental muscles had significantly more axons, and had atrophied less as indicated by muscle wet weight and histological appearance. These results indicate that the sensory axons can delay the weakening and atrophy of muscles after denervation. We suggest that the sensory axons may exert certain trophic influence on the denervated muscle fibers, though the actual mechanism is unknown.