Muscle spindle distribution in the masticatory muscle of the tree shrew

Quantity and Distribution of Muscle Spindles in Animal and Human Muscles

Muscle spindles have unique anatomical characteristics that can be directly affected by the surrounding tissues under physiological and pathological conditions. Understanding their spatial distribution and density in different muscles is imperative to unravel the complexity of motor function. In the present study, the distribution and number/density of muscle spindles in human and animal muscles were reviewed. We identified 56 articles focusing on muscle spindle distribution; 13 articles focused on human muscles and 43 focused on animal muscles. The results demonstrate that spindles are located at the nerve entry points and along distributed vessels and they relate to the intramuscular connective tissue. Muscles' deep layers and middle segments are the main topographic distribution areas. Eleven articles on humans and thirty-three articles on animals (totaling forty-four articles) focusing on muscle spindle quantity and density were identified. Hand and head muscles, such as the pronator teres/medial pterygoid muscle/masseter/flexor digitorum, were most commonly studied in the human studies. For animals, whole-body musculature was studied. The present study summarized the spindle quantity in 77 human and 189 animal muscles. We identified well-studied muscles and any as-yet unfound data. The current data fail to clarify the relationship between quantity/density and muscle characteristics. The intricate distribution of the muscle spindles and their density and quantity throughout the body present some unique patterns or correlations, according to the current data. However, it remains unclear whether muscles with fine motor control have more muscle spindles since the study standards are inconsistent and data on numerous muscles are missing. This study provides a comprehensive and exhaustive approach for clinicians and researchers to determine muscle spindle status.

Jaw-tongue reflex: afferents, central pathways, and synaptic potentials in hypoglossal motoneurons in the cat

The tongue position is reflexively controlled by the jaw position (the jaw-tongue reflex). The purpose of this study was to clarify the mechanism of this reflex in terms of afferents, central pathways, and synaptic potentials in hypoglossal motoneurons in the cat. Intracellular recordings from hypoglossal motoneurons revealed that electrical stimulation of the temporalis muscle nerve evoked excitatory and inhibitory post-synaptic potentials in hypoglossal motoneurons. The threshold of temporalis muscle nerve stimulation for evoking the synaptic potentials was higher than 2.0 times the nerve threshold. The amplitude of the potentials increased with stimulus intensity up to 5.0 times the nerve threshold. Punctate light pressure applied to the temporalis muscle induced a tonic depolarizing potential in hypoglossal motoneurons on which action potentials as well as depolarizing synaptic activation noise were superimposed. On the other hand, electrical stimulation of the temporalis muscle during jaw-opening could slightly inhibit the electromyographic activities in the genioglossus and styloglossus muscles. Lesions including the Probst's tract at the level caudal to the trigeminal motor nucleus abolished both excitation and inhibition in hypoglossal motoneurons induced by tonic depression of the lower jaw, but exerted no effects on either the tonic stretch reflex or the trigemino-hypoglossal reflex. In contrast, lesions including the trigeminal spinal tract produced no changes in either excitation or inhibition of hypoglossal motoneurons induced by temporalis muscle afferents, whereas the excitation of hypoglossal motoneurons was abolished by the lesions. We conclude that the group II muscle spindle afferents from the temporalis muscle are primarily responsible for evoking the jaw-tongue reflex.

The occurrence of an unusual tubular organelle in surface epithelial cells of the mouse ascending colon after injection of diazo-oxo-norleucine

Tubular structures were observed in surface epithelial cells of mice that had been injected with high dosages of diazo-oxo-norleucine (DON), a glutamine antagonist. The tubules often occurred in bundles which contained a variable number of tubules, often as many as one hundred being present. Within the bundles, the tubules were oriented either randomly or parallel to one another. They measured 25 to 35 nm in diameter with angular or circular profiles and were as long as 1 to 2 micron. In the center of each tubule, a small tubule-like component was evident that measured 5 to 7 nm in diameter. With the exception of endoplasmic reticulum, often with attached ribosomes, organelles were excluded from the bundles. Since the tubules and the endoplasmic reticulum occasionally were observed to be continuous, it is suggested that the tubules may originate from this organelle.

The absence of proprioceptive nerve endings in the human periodontal ligament: the role of periodontal mechanoreceptors in the reflex control of mastication

A review of the literature was conducted to determine the presence or absence of proprioceptive nerve endings in the human periodontal ligament. A histologic review of the periodontal ligament innervation concluded that nerve endings found were those mediating pain, pressure, or touch and that there is no histologic evidence of any "classic" proprioceptive nerve ending in the periodontal ligament. A summary is given concerning the precise role of nerve endings in the periodontal membrane, their afferent pathways, and the role of masticatory muscle proprioception, jaw reflexes, and the temporomandibular joint in the coordinated control of mastication and mandibular proprioception.

Muscle spindle supply to the human jaw muscle

Histological study of the human jaw muscle revealed that the temporal muscle displayed 342 muscle spindles, 208 in the horizontal and 134 in the vertical portion; the masseter muscle contained 114 spindles, 91 in the superficial and 23 in the profound portion; the medial pterygoid muscle had 59; and the lateral pterygoid muscle contained 6, four in the upper head and two in the lower head. These data suggest that the extensive mobility of the temporomandibular joint, and the maintenance of mandibular posture during mastication and speech are strongly influenced by proprioceptive mechanisms.

Proprioceptive innervation of the masticatory muscles in Pinché

This study of the masticatory muscles of the primate showed that the temporal muscle contained 107 muscle spindles, 45 in the horizontal portion and 62 in the vertical portion; the masseter muscle contained 70, 58 in the profundus portion and 12 in the superficial portion; the medial pterygoid muscle contained 15; the lateral pterygoid muscle contained 6; and the zygomaticomandibular muscle contained 9. The muscle spindles were located around the coronoid process and mandibular ramus.