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Skeletal Muscle
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
A motor unit consists of a motoneuron and the muscle fibers it innervates in a given skeletal muscle. The motoneuron is referred to as an alpha-motoneuron (α-motoneuron), or a lower motoneuron, to distinguish it from other types of motoneurons discussed later (Section 9.4.2.1 and Section 11.2.1.1). Figure 9.8 illustrates two motor units in a skeletal muscle. The muscle fibers of a motor unit are referred to as the muscle unit. A muscle fiber is normally innervated, through a neuromuscular junction (Section 5.1), by one – and only one – α-motoneuron, and a given α-motoneuron only innervates the muscle fibers of a single motor unit. All the motoneurons that innervate a single muscle are collectively referred to as the motoneuron pool for that muscle.
Brainstem Organization of Swallowing and Its Interaction With Respiration
Published in Alan D. Miller, Armand L. Bianchi, Beverly P. Bishop, Neural Control of the Respiratory Muscles, 2019
André Jean, Alexandre Car, Jean-Pierre Kessler
With regard to motoneurons, in addition to separate pools of motoneurons, results indicate that ambigual or hypoglossal motoneurons can fire both in phase with respiration or during deglutition42,56,65 (Figure 3B). Thus, common motoneurons likely exist for these two functions (Figure 2B). By driving muscles which can be active in both functions, these motoneurons very likely participate in the fine-tuning of the muscular activity during the motor behavior. It may be supposed that these motoneurons receive a drive from two separate CPGs and that their activity depends on synaptic interactions between the two networks.
Pathophysiology of Spasticity
Published in Anand D. Pandyan, Hermie J. Hermens, Bernard A. Conway, Neurological Rehabilitation, 2018
Jens Bo Nielsen, Maria Willerslev-Olsen, Jakob Lorentzen
It may seem self-evident to point out that the task of a spinal motoneuron is to activate the muscle fibres that it innervates and thereby contribute to posture and movement. However, this simple notion provides an intuitive way of understanding the adaptive changes that occur in the spinal networks in response to lesion of descending pathways. When descending excitation of the motoneuron is lost, regulatory mechanisms that aim to restore an efficient output to the muscle set in. A simplified version of this point is illustrated in Figure 2.1. In the normal situation, sensory input and descending drive both contribute to the activation of the motoneurons. Following lesion of central motor fibres, the descending drive is reduced and the sensory input is insufficient to maintain a normal output from the spinal cord to the muscles. As spasticity develops plastic mechanisms that could be considered compensatory set in. These involve increased descending drive in surviving motor tract fibres and alteration of intrinsic properties in spinal motoneurons. These changes help to increase the output from the spinal cord to the muscles, albeit not necessarily to the level before lesion, and they do not necessarily normalise motor control.
Clinical features and diagnostic tools in idiopathic inflammatory myopathies
Published in Critical Reviews in Clinical Laboratory Sciences, 2022
Konstantinos I. Tsamis, Constantinos Boutsoras, Evripidis Kaltsonoudis, Eleftherios Pelechas, Ilias P. Nikas, Yannis V. Simos, Paraskevi V. Voulgari, Ioannis Sarmas
In early stages of the disease, due to findings of atrophy and weakness and before the tendon reflexes are diminished, the clinician may consider motor neuron disease as a possible diagnosis. EMG, although needed to orientate the investigation toward muscle pathology, shows nonspecific results. As most cases are diagnosed approximately 5 years from onset, EMG results are often consistent with a chronic myopathy presenting both neuropathic and myopathic findings. Thus, needle EMG examination may be difficult to interpret as both small-short and large-long motor unit action potentials (MUAPs) can be found even in the same muscle. In rare cases, the large-long MUAP may be even more prominent. The recruitment pattern can also be doubtful. Evidence of denervation such as fibrillation potentials and positive sharp waves are present. These cases are even more complicated, as in one-third to one-half of the patients, the nerve conduction studies highlight a mild sensory or sensorimotor neuropathy that is often associated with comorbidities of older age. For other patients with IBM, the EMG findings are typical of IIMs, including small-short and polyphasic MUAPs that often are associated with proximal weakness. It is noteworthy that the disease has an asymmetric pattern and that not all muscles are affected equally [112].
Mechanisms of Modulation of Automatic Scapulothoracic Muscle Contraction Timings
Published in Journal of Motor Behavior, 2021
Samuele Contemori, Roberto Panichi, Andrea Biscarini
Our results imply the presence of at least one neural pathway that is capable to modulate the automatic timing of contraction of the scapulothoracic muscles. These muscles receive extensive projections from the extrapyramidal tracts, which are responsible for innate automatic postural/stabilization motor responses (Kandel et al., 2013). The descending cortical projections responsible for the delivering of voluntary motor commands to the spinal motoneurons and interneurons, via the pyramidal tracts, make also connections with the midbrain nuclei from which to the extrapyramidal tracts originate (Noback et al., 2005). Therefore, the midbrain nuclei of the extrapyramidal tracts might represent a candidate hub where the volitional motor commands can modulate the contraction onset time of the scapulothoracic muscles. However, we are mindful that other supraspinal or spinal circuits cannot be ruled out.
In vitro models of neuromuscular junctions and their potential for novel drug discovery and development
Published in Expert Opinion on Drug Discovery, 2020
Olaia F Vila, Yihuai Qu, Gordana Vunjak-Novakovic
Several investigators have explored alternative, optically based methods to create controllable NMJs. Electrical stimulation has classically been used for the controlled stimulation of excitable cell types but cannot easily target only the specific cell types in culture – such as motoneurons – without affecting muscle cells. An elegant solution to this problem can be achieved through the use of optogenetics, defined as the creation of transgenic cell lines expressing light-actuated proteins. ChR2 is a light-activated ion channel that opens upon stimulation with blue light, allowing for the precise spatial-temporal control of neural activation in neurons expressing this protein [58]. With the aim to achieve a controllable NMJ model, Steinbeck et al. combined human ESC-derived motoneurons expressing the optogenetic protein channelrhodopsin-2 (ChR2) with non-optogenetic skeletal myotubes in a co-culture system. The resulting muscle contractions in response to photostimulation of the motoneurons provided concrete evidence of controllable NMJ function. The authors used their system to model MG and showed the ability to detect a reduction in light responsiveness in their disease samples. However, due to the disorganized nature of their 2D co-culture, this quantification was based on the recording of random optical fields, hindering reproducibility and automatization [59] (Figure 1, bottom panel).