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Skeletal Muscle
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
The intrafusal fibers are connected in parallel with the extrafusal fibers, so when the latter are stretched, the former will also be stretched and will fire accordingly. But when the extrafusal fibers contract, the intrafusal fibers will be relieved of stretch and the muscle spindles may go silent, so they will not signal any information about muscle length or speed of contraction. Activation of the fusimotor fibers will cause contraction of the intrafusal muscle fibers, so they will send the appropriate signals under these conditions. The role that this fusimotor drive plays in the control of movement is discussed in Section 13.5.1.
Muscle Spindles, Golgi Tendon Organs and Spinal Reflexes
Published in Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal, Principles of Physiology for the Anaesthetist, 2020
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal
There are two types of intrafusal muscle fibres: Nuclear bag fibres which are longer and thicker with nuclei aggregated in the central region, andNuclear chain fires which are shorter and thinner with the nuclei arranged in a chain (row) along the centre of the fibre. Typically, a muscle spindle contains one to three nuclear bag fibres and three to nine nuclear chain fibres (Figure 13.2).
Fascial Anatomy
Published in David Lesondak, Angeli Maun Akey, Fascia, Function, and Medical Applications, 2020
Another important neural element in connection with the fascial system are the muscle spindles. They are located in the periphery of the muscle in direct connection with the muscle connective tissue. Muscle spindles are part of the perimysium.25 And are composed of intrafusal fibers and noncontractile connective tissue. The intrafusal fibers provide tension to the spindle in order to maintain tautness and are therefore sensitive to stretch across a wide range of muscle. Motor unit contraction can be activated via efferent gamma impulses resulting from the stretch reflex mechanism (gamma loop).
An overview of the pharmacotherapeutics for dystonia: advances over the past decade
Published in Expert Opinion on Pharmacotherapy, 2022
O. Abu-hadid, J. Jimenez-Shahed
One of the most interesting hypotheses is that dystonia is a disorder of sensorimotor processing [142]. This concept has gained popularity due to dystonia exacerbation when performing tasks in a ‘particular’ fashion and significant reduction when performing a similar task in a ‘different’ fashion. Further support relates to the frequent presence of certain sensory tricks that can alleviate motor symptoms. Using this concept, pharmacotherapeutics that influence neural plasticity by modulating long-term depression and/or potentiation should be considered. Understanding of the alterations in various sensory modalities that are seen in dystonia has led to evaluating the role of intrafusal muscle spindles, their sensory afferents, and the gamma motor neuron that innervates them. Interestingly, botulinum toxin acts on the neuromuscular junctions of the gamma motor neurons in a similar fashion to how it acts on the alpha motor neurons, implying that botulinum toxin could have a role beyond weakening neuromuscular transmission of extrafusal muscle fibers. Indeed, intramuscular botulinum toxin has been shown in several human studies to alter central sensorimotor processing thereby possibly correcting maladaptive plastic changes [143]. This is postulated to occur either by altering peripheral sensory input to the central nervous system, or from retrograde transport of toxin [142,144].
Regenerative replacement of neural cells for treatment of spinal cord injury
Published in Expert Opinion on Biological Therapy, 2021
William Brett McIntyre, Katarzyna Pieczonka, Mohamad Khazaei, Michael G. Fehlings
Motor neurons (MNs) are detrimentally affected in SCI, where synaptic connections regulating coordinated movement are disrupted. In the healthy cord, functionally and molecularly diverse spinal MN subtypes exhibit distinct profiles of activation and patterns of connectivity. Alpha MNs (α-MNs; Fox3+/Err3-) innervate force-generating extrafusal muscle fibers that control skeletal movement through muscle contractile forces. Gamma MNs (γ-MNs; Fox3-/Err3+) are abundant in the spinal cord, where they connect to intrafusal muscle fibers in muscle spindles. They modulate the sensitivity of muscle spindles to stretch [96], as well as regulate proprioceptive afferent feedback to α-MNs [97]. In several models of degenerative MN diseases, the excitatory afferent feedback present only in α-MNs is implicated in their rapid death following disease onset [97]. Interestingly, this phenomenon is not observed in spinal cord transection, as both α-MNs and γ-MNs exhibit a higher proportion of inhibitory:excitatory inputs, which can be correlated to poor bipedal stepping [98]. This could effectively explain failed attempts to restore α-MN circuitry after spinal transection [99], where it is likely that a diverse group of MN-pools require restoration following spinalization.
Effects of Botulinum Toxin A Injection on Ambulation Capacity in Patients with Cerebral Palsy
Published in Developmental Neurorehabilitation, 2019
Sibel Çağlar Okur, Mahir Uğur, Kazım Şenel
The mode of action of botulinum toxin includes extracellular binding to glycoprotein structures on cholinergic nerve terminals and intracellular blockade of the acetylcholine secretion. Thus, it prevents the release of acetylcholine at the neuromuscular junction, causing presynaptic neuromuscular blockade. BT affects the spinal stretch reflex by blockade of intrafusal muscle fibers with consecutive reduction of Ia/II afferent signals and muscle tone without affecting muscle strength (reflex inhibition).7 Thus, it allows muscles to become paralyzed for 3–6 months. Although its lethal dose is rather low, no significant side effect has been observed in the treated patients. Sometimes, it can cause temporary weakness in adjacent muscle groups, and local pain and tenderness may occur at the injection site. It is a reliable drug except for these side effects.8