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The Spinal Cord and the Spinal Canal
Published in Bernard J. Dalens, Jean-Pierre Monnet, Yves Harmand, Pediatric Regional Anesthesia, 2019
Bernard J. Dalens, Jean-Pierre Monnet, Yves Harmand
Basically, these neurons consist of alpha and gamma motor neurons. Another type of neuron, the Renshaw cells, have been (physiologically) identified in the ventral horns: these synapse with alpha motor neurons, from which they receive some recurrent axonal collaterals.
The nervous system
Published in Laurie K. McCorry, Martin M. Zdanowicz, Cynthia Y. Gonnella, Essentials of Human Physiology and Pathophysiology for Pharmacy and Allied Health, 2019
Laurie K. McCorry, Martin M. Zdanowicz, Cynthia Y. Gonnella
The cell bodies of somatic motor neurons are found in the ventral horn. The axons of these neurons exit the CNS through the ventral root of the spinal nerve and innervate skeletal muscles. There are two types of motor neurons located in the ventral horn: Alpha motor neurons: innervate skeletal muscle fibers to cause contractionGamma motor neurons: innervate intrafusal fibers of the muscle spindle to cause contraction, which allows the muscle spindle to remain sensitive to changes in muscle length
Botulinum toxins: Pharmacology, immunology, and current developments
Published in Anthony V. Benedetto, Botulinum Toxins in Clinical Aesthetic Practice, 2017
The SNARE-mediated mechanism inhibiting acetylcholine release occurs not only at alpha motor neurons, which innervate extrafusal muscle fibers, but also at gamma motor neurons, which innervate intrafusal muscle fibers. Intrafusal fibers make up muscle spindles (Figure 2.6)—the proprioceptive organs that are sensitive to stretch and are important in setting the resting tone and reflex sensitivity of muscle. Inhibition of gamma motor neurons decreases activation of muscle spindles, which effectively changes the sensory afferent system by reducing the Ia afferent traffic. However, this mechanism likely does not occur in facial muscles as they are reported to lack muscle spindles.56,57
Dry needling equilibration theory: A mechanistic explanation for enhancing sensorimotor function in individuals with chronic ankle instability
Published in Physiotherapy Theory and Practice, 2021
Jennifer F. Mullins, Arthur J. Nitz, Matthew C. Hoch
Muscle spindles can detect changes in static and dynamic muscle length through the gamma motor neuron. As a muscle shortens, the gamma motor neuron increases its firing rate to maintain an appropriate calibration with the extrafusal muscle fibers to ensure accurate information regarding muscle length. The need to increase input to the gamma motor neuron is conveyed to the spinal cord via Iα and type II (dynamic and static changes, respectively) afferent fibers from the muscle spindle. These fibers work in concert to create a monosynaptic reflex arc with the alpha motor neuron innervating the corresponding muscle as noted in the patellar tendon reflex. Additionally, this reciprocal relationship between the alpha and gamma motor neurons provides continuous feedback and normal and necessary tone to support joint stability in healthy systems (Riemann and Lephart, 2002b). A loss of calibration of the muscle spindle would result in misinformation to the spinal cord and supraspinal levels. Advances in neuroplasticity illustrate that motor output is directly influenced by sensory input. Faulty afferent information will ultimately result in dysfunctional motor output, creating or perpetuating chronic dysfunction (Needle, Lepley, and Grooms, 2017).
Four-week training involving ankle mobilization with movement versus static muscle stretching in patients with chronic stroke: a randomized controlled trial
Published in Topics in Stroke Rehabilitation, 2019
Donghwan Park, Ji-Hyun Lee, Tae-Woo Kang, Heon-Seock Cynn
With treatment, SBA and BBS scores significantly improved, by 227.8% and 266.7% respectively, in the MWM group relative to the SMS group. Additionally, in the MWM group, SBA significantly improved by a mean difference of −0.59 (95% CI: −0.28 to −0.9) after treatment compared with baseline. In both groups, BBS significantly increased after treatment compared with baseline, increasing by a mean difference of 2.4 (95% CI: 3.37–1.43) in the SMS group and a mean difference of 8.8 (95% CI: 11.39–6.21) in the MWM group. These results are consistent with previous studies.10,24 Mecagni et al.24 reported a relationship between balance measures and ankle ROM in community-dwelling women and found that decreased balance performance measures associated with limited ankle motion may be due to noncontractile tissues such as capsule, ligaments, or bone. Further, An and Jo10 examined effects of joint mobilization on patients with chronic stroke. The findings showed that joint mobilization resulted in better ankle strength, mobility, and WB ability test outcomes compared to control treatments after 5 weeks. Joint mobilization improves mechanoreceptor activity as the stretching is performed in the capsule and ligaments of the ankle. This improves their sensory output as gamma motor neurons are activated with tissue traction, Golgi tendon organs, and other proprioceptors, to aid in maintaining balance.25,26 Each patient was also asked to perform a WB-lunge while bending the knee joint during MWM. In these procedures, forward shifting of the body weight during MWM may improve plantar flexor strength.10,11 As a result, improved mechanoreceptor activity and plantar flexor strength in the affected lower limb contribute to improvements in balance. Thus, our results indicate that MWM effectively improves SBA and BBS and may be more effective than SMS.
Interlimb Responses to Perturbations of Bilateral Movements are Asymmetric
Published in Journal of Motor Behavior, 2021
Jacob E. Schaffer, Robert L. Sainburg
Whereas stretch reflexes are typically elicited during a postural task, the ipsilateral EMG findings here are consistent with the proposition that the rapid feedback responses reported here were mediated by stretch reflexes in our movement task. The R1 response reflects spinal circuits and has a latency of about 20-25 ms in the lower arm muscles in humans (Shemmel et al., 2010). Given the short latency of the R1 component, it is reasonable to conclude that a perturbation that elicits significant response during this short latency would result from activation of the same spinal circuits. As described in the results, the effect of the perturbation was to arrest forward motion of the hand, resisting ongoing extension at the elbow. The predictions for induced reflex responses at the elbow and shoulder, however, are not straightforward. This is because of two factors: 1) During voluntary movements, gamma motor neurons are activated along with alpha motor neurons (Prochazka, 1981). Thus, resisting ongoing movement should result in continued activation of gamma motor neurons, without ongoing shortening of the muscle. This should result in increased activation of the spindle afferents. 2) During multijoint movements, heteronymous pathways link the actions of muscles spanning multiple joints (Manning & Bawa, 2011), and can result in activation of muscles that are not directly stretched by the stimulus. These heteronymous responses can occur in muscles proximal and distal to the stretched muscle, as well as in short and long-latency intervals (Manning & Bawa, 2011). We expect that our stimulus that arrests forward motion of the arm should result in stimulation of stretch reflexes in the triceps brachii and anterior deltoid, due to continued contraction of the muscle, against resistance. However, due to multi-joint effects, the posterior deltoid and biceps can be stimulated through heterogenous pathways that appear to stabilize the limb against inertial interactions produced by motion of connected segments (Shemmel et al., 2010). Stretch reflexes have also been shown to elicit short latency coactivation of muscles under conditions that warrant impedance responses in the limb (Lacquaniti et al, 1991), which is similar to our results.