Molecular adaptations to endurance exercise and skeletal muscle fibre plasticity
Adam P. Sharples, James P. Morton, Henning Wackerhage in Molecular Exercise Physiology, 2022
A motor unit is defined as a motor neurone and all of the muscle fibres it innervates (Figure 9.4). There are different types of motor units which are characterised by their α-motor neurone and muscle fibre type: S (slow) motor units comprise small α-motor neurones with a low excitation threshold and small axons that innervate a small number of slow type I fibres.FR (fatigue-resistant) motor units comprise moderate-sized α-motor neurones that innervate intermediate type IIa fibres. The α-motor neurone properties are in between those of Fast Fatiguing (FF – see below) and S α-motor neurones.FF (fast fatiguing) motor units comprise large, hard to excite but fast-conducting α-motor neurons that innervate many large fibres that contract rapidly but fatigue easily (type IIb and IIx fibres);
The movement systems: skeletal and muscular
Nick Draper, Helen Marshall in Exercise Physiology, 2014
A motor unit describes a motor neuron and all the muscle fibres it is responsible for innervating. Stimulation of a motor neuron, therefore, results in contraction of all the muscle fibres it innervates in an all-or-none fashion so they work together as a single unit. The size of motor units varies between muscles depending on the degree of control required by that muscle. Typically a nerve can stimulate anything from 6–10 individual muscle fibres for fine-control movements (such as muscles controlling the movement of the eye), to a thousand or more, for gross movements such as walking (for example, the gastrocnemius in the lower leg). Refer to Chapters 3 and 4 for detail on resting membrane potential, action potential initiation and propagation, respectively.
Biochemical Contributors to Exercise Fatigue
Peter M. Tiidus, Rebecca E. K. MacPherson, Paul J. LeBlanc, Andrea R. Josse in The Routledge Handbook on Biochemistry of Exercise, 2020
Muscle contraction is linked by a series of events, and fatigue can potentially be attributed to failure at any of the cellular sites involved in the activation and contraction of the muscle fibres (Figure 5.4). The activation of skeletal muscle contraction starts at the central nervous system, thereby activating α-motor neurons, and action potentials propagate along the axons of these neurons out to the muscle fibres. The smallest functional unit of the central motor system, the motor unit, consists of one α-motor neuron and the muscle fibres it activates. The number of muscle fibres in a motor unit varies from muscle to muscle. For instance, in the hand an α-motor neuron generally activates fewer than 100 muscle fibres, whereas for the lower leg a single motor unit may contain up to 1,000 muscle fibres (12). Generally, the larger the number of muscle fibres in a motor unit, the less precise the associated movements.
Blood perfusion changes during sacral nerve root stimulation versus surface gluteus electrical stimulation on in seated spinal cord injury
Published in Assistive Technology, 2019
Liang Qin Liu, Martin Ferguson-Pell
In theory, all muscles consist of a number of motor units, and the fibers belonging to a motor unit are dispersed and interlink among fibers of other units. A motor unit normally consists of one motor neuron and all of the muscle fibers it stimulates. The muscle fibers belonging to one motor unit can be spread throughout a part, or most of the entire muscle, depending on the number of fibers and size of the muscle. When a motor neuron is activated, all of the muscle fibers innervated by the motor neuron are stimulated and contracted. The activation of single motor neuron results in a weak distributed muscle contraction (twitch contraction). In contrast, the activation of more motor neurons will result in more muscle fibers being activated, and therefore, a stronger muscle contraction (tetanic contraction) was produced. The higher the recruitment of motor unit, the stronger the muscle contraction will be. The activation of more motor neurons will result in more muscle fibers being activated, and therefore, a stronger muscle contraction (Guyton & Hall, 0000). In comparison, between sacral nerve root stimulation versus traditional surface FES of gluteal muscles, the larger numbers of motor neurons recruitment in sacral nerve roots stimulation may produce stronger contraction than surface FES. Therefore, it can activate gluteus muscles more efficiently. Sacral nerve root stimulation can efficiently activate all motor neurons that innervate gluteal maximus, whereas surface FES of gluteus maximus may be limited by the size of electrodes and the depth of electrical signal to reach the muscle motor points.
Proteomic serum biomarkers for neuromuscular diseases
Published in Expert Review of Proteomics, 2018
Sandra Murphy, Margit Zweyer, Rustam R. Mundegar, Dieter Swandulla, Kay Ohlendieck
In mature skeletal muscles, the extrafusal fiber population of a distinct motor unit are usually innervated by the axon branches of a single-motor neuron, which represents the basic functional division of muscle contraction. Neuromuscular connections are distributed over a wide area within a muscle ensuring an even spread of contractile force of the motor unit under normal conditions [41]. Motor units join forces for highly coordinated contractions of an individual skeletal muscle and all motor units within a single skeletal muscle are classified as a motor pool. However, this rigid physiological control system and tight cellular arrangement is vulnerable to tissue damage by neuronal degeneration, nerve crush, or traumatic denervation [42]. Long-term denervation triggers progressive muscular atrophy within all affected motor units. This includes the loss of skeletal muscle mass and voluntary function, the collapse of sarcomeric organization, and finally severe fiber degeneration and replacement by connective and fatty tissue [43]. Counteractive to muscular atrophy is the proliferation and activation of satellite cells, and the initiation of neo-myogenesis within the denervated motor unit. A key pathological feature of muscular atrophy is slow-to-fast fiber-type shifting and this is observed to a varying degree in the diverse group of motor neuron diseases. In these neurological conditions, motor neurons are selectively affected and cause amyotrophic lateral sclerosis, spinal muscular atrophies, hereditary spastic paraplegia, primary lateral sclerosis, or progressive pseudo/bulbar palsy [44].
The role of arm volumes evaluation in the functional outcome and patient satisfaction following surgical repair of the brachial plexus traumatic injuries
Published in Neurological Research, 2020
Lukas Rasulić, Vesna Simić, Andrija Savić, Milan Lepić, Vojin Kovačević, Vladimir Puzović, Jovan Grujić, Stefan Mandić-Rajčević, Miroslav Samardžić
It is well known that the skeletal muscle is composed of different types of muscle fibers grouped into motor units. Motor units are always formed from the same types of muscle fibers – 1, 2A and 2X [46], which differ according to myosin heavy chain (MyHC) composing them [47] and are the major determinant of contractile performance [8]. Although many muscle fibers are composed of a single MyHCs, coexistence of different MyHCs in them is considered to be normal [48]. During the denervation of the muscle, certain physiological and morphological changes occur, as presented in the review of older studies by Gutmann and Zelena [8,49], and in more recent work published by Blaauw et al. [8,49]. As they summarized, according to the studies performed on rodents, the fastest muscle mass loss is at the early stage after injury, and it drops up to 50% during the first 2 weeks, after which the speed of muscle mass losing declines. In our study, patients were examined at least 3 years after surgery, which is a period considered to be long enough for all possible processes of the functional outcome to get into a stable phase.
Related Knowledge Centers
- Axon Terminal
- Excitatory Postsynaptic Potential
- Inhibitory Postsynaptic Potential
- Motor Neuron
- Muscle Contraction
- Neuromuscular Junction
- Thigh
- Skeletal Muscle
- Extraocular Muscles
- Motor Unit Recruitment