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.
Discussions (D)
Terence R. Anthoney in Neuroanatomy and the Neurologic Exam, 2017
Since publication of the works cited above, attempts to assign differential functional characteristics to nerve fibers in the various categories of the “A, B, C” and “I, II, III, IV” classifications have continued. In some cases, the association between a given functional class of fibers and a certain diameter/conduction rate category has become so strong that the alphanumeric name of the category is commonly used as a synonym for the class. The best-known afferent examples are probably “group la fibers,” defined as those arising from primary endings of muscle spindles, “group lb fibers,” defined as those arising from Golgi tendon organs, and “group II fibers,” defined as those arising from secondary endings of muscle spindles; and the best-known efferent examples are probably “the (α) fibers of α motoneurons,” defined as those innervating extrafusal muscle fibers, and “the (γ) fibers of γ motoneurons,” defined as those innervating intrafusal muscle fibers (e.g., B&K, p. 39 [including Fig. 3–3]; C&S, p. 165–171 |including figures|, 179; W&W, p. 848 [Fig. 7.26A], 857, 859).
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).
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.
No Relationship Between Joint Position Sense and Force Sense at the Shoulder
Published in Journal of Motor Behavior, 2018
David Phillips, Andrew Karduna
The lack of relationship in normalized RMS error between the two protocols may also be due to variations in feedback from the peripheral mechanoreceptors during isometric and concentric contractions. When contracting a muscle both alpha and gamma (fusimotor) motor neurons fire simultaneously, which is referred to as alpha-gamma coactivation (Vallbo, 1970). This prevents a muscle spindle from becoming slack and unable to respond to the muscle lengthening. During isometric contractions, the agonist muscle's muscle spindles firing increases at an inconsistent rate (Vallbo, 1974). In this case the alpha-gamma coactivation may be attempting to shorten intrafusal muscle fibers that are remaining at the same length and applying tension on the muscle spindle increasing its firing rate. This would be the only signal from muscle spindles since during an isometric contraction the antagonist muscle will also remain at a constant length. However, it is argued that these fusimotor induced signals from muscle spindles during an isometric contraction are filtered out because they do not induce an illusion of movement at the joint (McCloskey, Gandevia, Potter, & Colebatch, 1983).
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