Quantity and Distribution of Muscle Spindles in Animal and Human Muscles

Abstract

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.

Keywords: connective tissue; density; distribution; fascia; muscle spindle; proprioception.

Figure 1

Basic structure of MS; nuclear bag fiber is thicker than chain fiber, with nuclear bag 2 fiber being the thickest overall. Afferent sensory neurons of type Ia located in the area of equatorial region contribute to innervating all fibers. Afferent sensory neurons of type II fibers located to the side of Ia, innervate the nuclear bag 2 fibers and chain fibers. Signals generated from mechanical force sense turn into electro-neural signals and ascend into the brain via the spinal cord, contributing to proprioception and motor control.

Figure 2

Flow diagram of search strategy for MSs distribution.

Figure 3

Flow diagram of MSs quantity/density.

Figure 4

Staining methods used in MS studies. Silver and H&E were the main techniques. The bars in blue present the animal studies and orange bars stand for human studies. PAS: Periodic Acid-Schiff stain.

Figure 5

Number and density of MSs in human muscles and animal muscles; density of MSs presented in “n°/ gram”. Some data of animal muscles and human muscles were not presented due to space issue. For more details, please check the Supplementary Tables S2 and S3. The number in front of muscle name indicates the number of studies reporting the finding.

Figure 5

Number and density of MSs in human muscles and animal muscles; density of MSs presented in “n°/ gram”. Some data of animal muscles and human muscles were not presented due to space issue. For more details, please check the Supplementary Tables S2 and S3. The number in front of muscle name indicates the number of studies reporting the finding.

Figure 6

The main distribution area of MSs, a piece of muscle cut laterally was used for presentation. NEPs were the main related anatomical structure followed by vessels, IMCT and type I muscle fiber. From the perspective of the anatomical position of the proximal and distal region of muscle, the middle region owned the largest number of MSs in animals while proximal region owned the largest number in humans.