Palisade Endings Have an Exocytotic Machinery But Lack Acetylcholine Receptors and Distinct Acetylcholinesterase Activity

Characterization, number, and spatial organization of nerve fibers in the human cervical vagus nerve and its superior cardiac branch

BACKGROUND

Electrical stimulation of the vagus nerve (VN) is a therapy for epilepsy, obesity, depression, and heart diseases. However, whole nerve stimulation leads to side effects. We examined the neuroanatomy of the mid-cervical segment of the human VN and its superior cardiac branch to gain insight into the side effects of VN stimulation and aid in developing targeted stimulation strategies.

METHODS

Nerve specimens were harvested from eight human body donors, then subjected to immunofluorescence and semiautomated quantification to determine the signature, quantity, and spatial distribution of different axonal categories.

RESULTS

The right and left cervical VN (cVN) contained a total of 25,489 ± 2781 and 23,286 ± 3164 fibers, respectively. Two-thirds of the fibers were unmyelinated and one-third were myelinated. About three-quarters of the fibers in the right and left cVN were sensory (73.9 ± 7.5 % versus 72.4 ± 5.6 %), while 13.2 ± 1.8 % versus 13.3 ± 3.0 % were special visceromotor and parasympathetic, and 13 ± 5.9 % versus 14.3 ± 4.0 % were sympathetic. Special visceromotor and parasympathetic fibers formed clusters. The superior cardiac branches comprised parasympathetic, vagal sensory, and sympathetic fibers with the left cardiac branch containing more sympathetic fibers than the right (62.7 ± 5.4 % versus 19.8 ± 13.3 %), and 50 % of the left branch contained sensory and sympathetic fibers only.

CONCLUSION

The study indicates that selective stimulation of vagal sensory and motor fibers is possible. However, it also highlights the potential risk of activating sympathetic fibers in the superior cardiac branch, especially on the left side.

Proprioceptors in extraocular muscles

Proprioception is the sense that lets us perceive the location, movement and action of the body parts. The proprioceptive apparatus includes specialized sense organs (proprioceptors) which are embedded in the skeletal muscles. The eyeballs are moved by six pairs of eye muscles and binocular vision depends on fine-tuned coordination of the optical axes of both eyes. Although experimental studies indicate that the brain has access to eye position information, both classical proprioceptors (muscle spindles and Golgi tendon organ) are absent in the extraocular muscles of most mammalian species. This paradox of monitoring extraocular muscle activity in the absence of typical proprioceptors seemed to be resolved when a particular nerve specialization (the palisade ending) was detected in the extraocular muscles of mammals. In fact, for decades there was consensus that palisade endings were sensory structures that provide eye position information. The sensory function was called into question when recent studies revealed the molecular phenotype and the origin of palisade endings. Today we are faced with the fact that palisade endings exhibit sensory as well as motor features. This review aims to evaluate the literature on extraocular muscle proprioceptors and palisade endings and to reconsider current knowledge of their structure and function.

Autonomic Nerve Fibers Aberrantly Reinnervate Denervated Facial Muscles and Alter Muscle Fiber Population

The surgical redirection of efferent neural input to a denervated muscle via a nerve transfer can reestablish neuromuscular control after nerve injuries. The role of autonomic nerve fibers during the process of muscular reinnervation remains largely unknown. Here, we investigated the neurobiological mechanisms behind the spontaneous functional recovery of denervated facial muscles in male rodents. Recovered facial muscles demonstrated an abundance of cholinergic axonal endings establishing functional neuromuscular junctions. The parasympathetic source of the neuronal input was confirmed to be in the pterygopalatine ganglion. Furthermore, the autonomically reinnervated facial muscles underwent a muscle fiber change to a purely intermediate muscle fiber population myosin heavy chain type IIa. Finally, electrophysiological tests revealed that the postganglionic parasympathetic fibers travel to the facial muscles via the sensory infraorbital nerve. Our findings demonstrated expanded neuromuscular plasticity of denervated striated muscles enabling functional recovery via alien autonomic fibers. These findings may further explain the underlying mechanisms of sensory protection implemented to prevent atrophy of a denervated muscle.SIGNIFICANCE STATEMENT Nerve injuries represent significant morbidity and disability for patients. Rewiring motor nerve fibers to other target muscles has shown to be a successful approach in the restoration of motor function. This demonstrates the remarkable capacity of the CNS to adapt to the needs of the neuromuscular system. Yet, the capability of skeletal muscles being reinnervated by nonmotor axons remains largely unknown. Here, we show that under deprivation of original efferent input, the neuromuscular system can undergo functional and morphologic remodeling via autonomic nerve fibers. This may explain neurobiological mechanisms of the sensory protection phenomenon, which is because of parasympathetic reinnervation.

Extraocular Motoneurons and Neurotrophism

Extraocular motoneurons are located in three brainstem nuclei: the abducens, trochlear and oculomotor. They control all types of eye movements by innervating three pairs of agonistic/antagonistic extraocular muscles. They exhibit a tonic-phasic discharge pattern, demonstrating sensitivity to eye position and sensitivity to eye velocity. According to their innervation pattern, extraocular muscle fibers can be classified as singly innervated muscle fiber (SIF), or the peculiar multiply innervated muscle fiber (MIF). SIF motoneurons show anatomical and physiological differences with MIF motoneurons. The latter are smaller and display lower eye position and velocity sensitivities as compared with SIF motoneurons.

MIF versus SIF Motoneurons, What Are Their Respective Contribution in the Oculomotor Medial Rectus Pool?

Multiply-innervated muscle fibers (MIFs) are peculiar to the extraocular muscles as they are non-twitch but produce a slow build up in tension on repetitive stimulation. The motoneurons innervating MIFs establish en grappe terminals along the entire length of the fiber, instead of the typical en plaque terminals that singly-innervated muscle fibers (SIFs) motoneurons establish around the muscle belly. MIF motoneurons have been proposed to participate only in gaze holding and slow eye movements. We aimed to discern the function of MIF motoneurons by recording medial rectus motoneurons of the oculomotor nucleus. Single-unit recordings in awake cats demonstrated that electrophysiologically-identified medial rectus MIF motoneurons participated in different types of eye movements, including fixations, rapid eye movements or saccades, convergences, and the slow and fast phases of the vestibulo-ocular nystagmus, the same as SIF motoneurons did. However, MIF medial rectus motoneurons presented lower firing frequencies, were recruited earlier and showed lower eye position (EP) and eye velocity (EV) sensitivities than SIF motoneurons. MIF medial rectus motoneurons were also smaller, had longer antidromic latencies and a lower synaptic coverage than SIF motoneurons. Peristimulus time histograms (PSTHs) revealed that electrical stimulation to the myotendinous junction, where palisade endings are located, did not recurrently affect the firing probability of medial rectus motoneurons. Therefore, we conclude there is no division of labor between MIF and SIF motoneurons based on the type of eye movement they subserve.SIGNIFICANCE STATEMENT In addition to the common singly-innervated muscle fiber (SIF), extraocular muscles also contain multiply-innervated muscle fibers (MIFs), which are non-twitch and slow in contraction. MIF motoneurons have been proposed to participate only in gaze holding and slow eye movements. In the present work, by single-unit extracellular recordings in awake cats, we demonstrate, however, that both SIF and MIF motoneurons, electrophysiologically-identified, participate in the different types of eye movements. However, MIF motoneurons showed lower firing rates (FRs), recruitment thresholds, and eye-related sensitivities, and could thus contribute to the fine adjustment of eye movements. Electrical stimulation of the myotendinous junction activates antidromically MIF motoneurons but neither MIF nor SIF motoneurons receive a synaptic reafferentation that modifies their discharge probability.