Spinal Cord and Reflexes
Nassir H. Sabah in Neuromuscular Fundamentals, 2020
Although the stretch reflex is encountered in most skeletal muscle, it is particularly strong in antigravity muscles, where it would tend to automatically maintain posture, this being an important function of the stretch reflex. The antigravity muscles of the back and legs are extensor muscles because the joints involved will tend to flex, or buckle, under the influence of gravity. Any such movement will stretch the extensor muscles, thereby eliciting a stretch reflex that would restore posture. The antigravity muscles of the arm, however, are the flexors, not the extensors because gravity would tend to extend the arm joints. The monosynaptic component of the stretch reflex can play an important physiological role during fast movements or for a fast readjustment of posture upon loss of balance. For example, in jumping from a moderate height to land with both feet on the ground, if the stiffness of the knee is insufficient, the knee extensors would be rapidly stretched, and the monosynaptic component of the reflex would produce a fast contraction of the extensors that would prevent the buttocks from hitting the ground. The monosynaptic component of the stretch reflex is fast, having a response time of few tens of milliseconds because: (i) there is only one synaptic delay, (ii) the Ia fibers are the largest and hence the fastest conducting peripheral fibers, and (iii) the monosynaptic connections of the Ia fibers are on the larger α-motoneurons, which in turn have large, fast conducting axons.
Muscle Spindles, Golgi Tendon Organs and Spinal Reflexes
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal in Principles of Physiology for the Anaesthetist, 2020
Muscle spindles are the starting point of the stretch reflex, the simplest of all reflexes in mammals. Passive muscle stretching produces reflex contraction by exciting spindle primary afferents, which synapse directly onto motor neurons in the anterior horn of the spinal cord (Figure 13.5). This is the basis of the knee jerk, the only monosynaptic reflex in the body. Type Ia sensory fibres enter the spinal cord through the dorsal roots, and their branches terminate in the spinal cord near the entry level or ascend to the brain. The fibres (from the primary sensory fibres) that terminate in the spinal cord synapse directly with motor neurons in the ventral horn, which innervate the extrafusal fibres of the same muscle. This neural pathway forms the circuitry for the stretch reflex. The stretch reflex has two components: a dynamic component that is activated while the spindle is being stretched and a static component that occurs when the muscle reaches a new static length. An important function of the dynamic component of stretch reflex is its dampening effect on jerky or oscillatory movements. Clonus or unusual repetitive contractions of muscles occur when muscle spindle sensory function is abnormal.
Stretching effects on injury reduction and health
David G. Behm in The Science and Physiology of Flexibility and Stretching, 2018
Dr McHugh continued as follows: I think the ability of a contracted muscle to absorb sudden tensile loads is important in the mechanism of muscle strains. If an isometrically contracted muscle is stretched suddenly (even a little stretch) there is a large increase in force that is due to the mechanical resistance of the MTU (19,20). In many cases, due to the very brief onset, the stretch reflex or other neural changes cannot contribute. I think if the contractile component is able to resist the elongation and impart it to the tendon the muscle will be protected from potential strain injury. Stretching might impact this (albeit in a small way) by allowing greater myofilament overlap at longer muscle lengths than was achievable before the acute bout of stretching, thus allowing the muscle tendon unit elongation to be transferred to the non-contractile component.
Typical Development of Finger Position Sense From Late Childhood to Adolescence
Published in Journal of Motor Behavior, 2023
Jinseok Oh, Arash Mahnan, Jiapeng Xu, Hannah J. Block, Jürgen Konczak
Proprioception and motor control are intricately linked. Proprioceptive afferents are used by the nervous system to regulate muscle tone, spinal stretch reflexes, and postural reflexes. Moreover, cerebro-cerebellar and somatosensory motor cortical networks process proprioceptive signals for voluntary motor control. It is firmly established that the networks undergo structural and functional changes during development (Cadwell et al., 2019; Van Essen et al., 2018), yet the exact neural mechanisms underlying proprioceptive development in humans are still not fully delineated. It would be useful to know what features of neural development are consistent with behavioral markers of proprioceptive development such as those reported here. It is unlikely that morphological changes in proprioceptive mechanoreceptors can explain such perceptual development, because muscle spindles are not going through significant morphological changes after 3 years of age in humans (Osterlund et al., 2011). However, there is evidence that an age-related increased sensitivity of the muscle spindles as stretch reflex responses increase across age in children from 7 to 11 years in TD (Grosset et al., 2007). Improved muscle spindle sensitivity will result in afferent signals that allow the proprioceptive system to differentiate between smaller differences in finger position, which could translate in more precise perceptual judgments.
Effect of inhibitory kinesiotaping on spasticity in patients with chronic stroke: a randomized controlled pilot trial
Published in Topics in Stroke Rehabilitation, 2022
Mahdad Mehraein, Zahra Rojhani- Shirazi, Ahmad Zeinali Ghotrom, Nasrin Salehi Dehno
In a recent study by Puce et al. (2021), the effect of KT on knee extensor spasticity was investigated in para-swimmers.54 In line with our study, they reported a significant decrease in the amplitude of stretch reflex 48 hours after KT, but MAS score did not change following KT.54 Despite the similarity between our results, they examined athletes and measured spasticity by stretch reflex. The H-reflex and stretch reflex have the same spinal circuitry.55 However, for H-reflex, Ia afferent axons are excited electrically and stretch reflex is elicited by the mechanical stimulation of group Ia and II afferent axons.55 Hence, our study is the first to highlight the specific effect of KT on the reflex component of muscle tone, which is spasticity in patients with stroke.
The effectiveness of interventions targeting spasticity on functional clinical outcomes in patients with multiple sclerosis: a systematic review of clinical trials
Published in European Journal of Physiotherapy, 2022
Merziye Ergul, Afsun Nodehi Moghadam, Rachel Soh
Pharmacological agents and non-pharmacological interventions are used in management of spasticity. Pharmacological treatment includes anti-spasticity medications, such as baclofen, botulinum toxin, diazepam, dantrolene and tizanidine [6,7]. They have limited efficiency and systemic side effects, such as cognitive impairment and muscle weakness [8]. Physiotherapy interventions for spasticity management which are currently being used include a diverse range of interventions, such as exercise therapy, electrotherapy, electromagnetic therapy, massage and orthotics. As a non-pharmacological intervention, the primary aim of physiotherapy interventions for reducing spasticity is to reduce stretch reflex excitability through decreasing excessive alpha-motor neuron activity and hypersensitivity of muscle spindles. The secondary aims are prevention and reducing consequences of spasticity, such as muscle and soft tissue contractures, limb stiffness, decreased range of motion, pain, gait problems, functional limitations and decreased quality of life [9].
Related Knowledge Centers
- Action Potential
- Alpha Motor Neuron
- Gamma Motor Neuron
- Muscle Contraction
- Muscle Spindle
- Reflex
- Spinal Nerve
- Spinal Cord
- Skeletal Muscle
- Golgi Tendon Reflex