Neuromuscular Physiology
Michael H. Stone, Timothy J. Suchomel, W. Guy Hornsby, John P. Wagle, Aaron J. Cunanan in Strength and Conditioning in Sports, 2023
Three proprioceptors (mechanoreceptors) that have a major impact on aspects of muscle function are the muscle spindle, Golgi tendon organ, and joint receptors. Muscle spindles (MS) are in parallel with muscle extrafusal fibers and are fluid-filled fusiform-shaped capsules approximately 2 to 20 nm long enclosing 5 to 12 specialized intrafusal muscle fibers (80, 109, 125). The MS capsule contains two types of intrafusal fibers based on the number and distribution of their nuclei. There are 8 to 12 nuclear chain fibers, which have nuclei distributed fairly evenly (chain-like) along their length. Typically, there are two to four nuclear bag fibers; in this type of fiber, most of the nuclei are located in the middle forming a bulge in the intrafusal fiber. The bag fibers are thicker and longer than the nuclear chain fibers. The bag fibers are innervated by γ-1-motor neurons, and the nuclear chain fibers by γ–2-motor neurons (109, 127). Group Ia sensory neurons innervate the central portions of both the bag and chain fibers, while group II sensory neurons innervate one end, typically opposite the MNs, of both fibers. The characteristics of the intrafusal fibers are shown in Table 1.4.
Anatomy of the Respiratory Neural Network
Susmita Chowdhuri, M Safwan Badr, James A Rowley in Control of Breathing during Sleep, 2022
Starting from the pump muscles, our path to the CNS follows the phrenic nerve, C3–C5. Starting from the airway resistance muscles there are four paths along cranial nerves (CNs) IX, X, XI, and XII. The phrenic motor neurons are located within a column spanning ventral horns at a minimum of three cervical segments: C3–C5 in humans. Phrenic motor neurons receive recurrent inhibition via Renshaw cells like most motor neurons innervating skeletal muscles but not Ia inhibitory feedback from stretch receptors of intrafusal muscle fibers (muscle spindles), which are sparse in the diaphragm. Most diaphragmatic motor units supply type 1 slow oxidative fibers, which aligns with the need to avoid fatigue in regular (eupneic) breathing (15, 16). The diaphragm contains non-oxidative fatigable, high force-generating motor units, but their respiratory roles are limited to episodic behaviors like expectoration, cough, or sneeze, but not eupnea (17). Phrenic motor neurons are recruited by Henneman's size principle, which allows for uniform recruitment from smallest soma size (highest input resistance) to largest soma size (lowest input resistance), a property in which somatic size maps to motor unit size and force production capability (18, 19).
Physiology of excitable cells
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal in Principles of Physiology for the Anaesthetist, 2015
Sensory receptors include exteroreceptors, interoreceptors and proprioceptors. In general, sensory transduction is accomplished by the production of a receptor potential and encode modality, spatial localization, intensity, duration and frequency of stimuli. Sensory receptors may show accommodation. Muscle spindles are sensory receptors in skeletal muscles that lie parallel to the regular extrafusal muscle fibres. They consist of nuclear bag and nuclear chain intrafusal fibres. Ia afferent fibres form primary nerve endings on both nuclear bag and chain fibres. Group II afferent fibres form a secondary ending, which is found chiefly on the nuclear chain fibres. Primary endings detect static (change in length) and dynamic (rate of change in muscle length) changes in muscle, whereas secondary endings detect only static responses. The γ efferent system controls the sensitivity of the muscle spindle. The muscle spindles also dampen jerky or oscillatory muscle contractions. The Golgi tendon organs, located in the tendons of the muscles, are arranged in series with the skeletal muscle. They are supplied by IIb afferent fibres and are stimulated by both stretch and contraction of the muscle. The stretch reflex includes a monosynaptic excitatory pathway from muscle spindle afferent (Ia and II) fibres to the α motor neurons to the same and synergistic muscle and a disynaptic inhibitory pathway to the motor neurons of the antagonist muscles.
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
Related Knowledge Centers
- Alpha Motor Neuron
- Extrafusal Muscle Fiber
- Muscle
- Muscle Spindle
- Proprioception
- Connective Tissue
- Sense
- Nuclear Bag Fiber
- Nuclear Chain Fiber
- Fibroblast