Morphometry and histoenzymology of the hamster tenuissimus and its muscle spindles

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.

Evaluating Sexual Dimorphism of the Muscle Spindles and Intrafusal Muscle Fibers in the Medial Gastrocnemius of Male and Female Rats

This study sought to investigate the sexual dimorphism of muscle spindles in rat medial gastrocnemius muscle. The muscles were cut transversely into 5–10 and 20 μm thick serial sections and the number, density, and morphometric properties of the muscle spindles were determined. There was no significant difference (p > 0.05) in the number of muscle spindles of male (14.45 ± 2.77) and female (15.00 ± 3.13) rats. Muscle mass was 38.89% higher in males (1.08 vs. 0.66 g in females), making the density of these receptors significantly higher (p < 0.01) in females (approximately one spindle per 51.14 mg muscle mass vs. one per 79.91 mg in males). There were no significant differences between the morphometric properties of intrafusal muscle fibers or muscle spindles in male and female rats (p > 0.05): 5.16 ± 2.43 and 5.37 ± 2.27 μm for male and female intrafusal muscle fiber diameter, respectively; 5.57 ± 2.20 and 5.60 ± 2.16 μm for male and female intrafusal muscle fiber number, respectively; 25.85 ± 10.04 and 25.30 ± 9.96 μm for male and female shorter muscle spindle diameter, respectively; and 48.99 ± 20.73 and 43.97 ± 16.96 μm for male and female longer muscle spindle diameter, respectively. These findings suggest that sexual dimorphism in the muscle spindles of rat medial gastrocnemius is limited to density, which contrasts previous findings reporting differences in extrafusal fibers diameter.

Density of muscle spindles in prosimian shoulder muscles reflects locomotor adaptation

We examined the correlation between the density of muscle spindles in shoulder muscles and the locomotor mode in three species of prosimian primates: the slow loris (Nycticebus coucang), Garnett's galago (Otolemur garnettii), and the ring-tailed lemur (Lemur catta). The shoulder muscles (supraspinatus, infraspinatus, teres major, teres minor, and subscapularis) were embedded in celloidin and cut into transverse serial thin sections (40 microm); then, every tenth section was stained using the Azan staining technique. The relative muscle weights and the density of the muscle spindles were determined. The slow loris muscles were heavier and had sparser muscle spindles, as compared to Garnett's galago. These features suggest that the shoulder muscles of the slow loris are more adapted to generating propulsive force and stabilizing the shoulder joint during locomotion and play a less controlling role in forelimb movements. In contrast, Garnett's galago possessed smaller shoulder muscles with denser spindles that are suitable for the control of more rapid locomotor movements. The mean relative weight and the mean spindle density in the shoulder muscles of the ring-tailed lemur were between those of the other primates, suggesting that the spindle density is not simply a consequence of taxonomic status.