Abstract
Although the occurrence of muscle spindles (MS) is quite high in most skeletal muscles of humans, few MS, or even absence, have been reported in digastric and mylohyoideus muscles. Even if this condition is generally accepted and quoted in many papers and books, observational studies are scarce and based on histological sections of a low number of specimens. The aim of the present study is to confirm previous data, assessing MS number in a sample of digastric and mylohyoideus muscles. We investigated 11 digastric and 6 mylohyoideus muscles from 13 donors. Muscle samples were embedded in paraffin wax, cross-sectioned in a rostrocaudal direction, and stained using haematoxylin-eosin. A mean of 5.1 ± 1.1 (range 3–7) MS was found in digastric muscles and mean of 0.5 ± 0.8 (range 0–2) in mylohyoideus muscles. A significant difference (P < 0.001) was found with the control sample, confirming the correctness of the histological procedure. Our results support general belief that the absolute number of spindles is sparse in digastric and mylohyoideus muscles. External forces, such as food resistance during chewing or gravity, do not counteract jaw-opening muscles. It is conceivable that this condition gives them a limited proprioceptive importance and a reduced need for having specific receptors as MS.
Highlights
Muscle spindles (MS) were recognized as specialized entities of the skeletal musculature of vertebrates since the early 1860s [1]
All MS were present in the anterior belly of digastric muscle, whereas none was found in the posterior belly
The current study examined numbers and density of spindle cells in a cadaveric sample of digastric and mylohyoideus muscles
Summary
Muscle spindles (MS) were recognized as specialized entities of the skeletal (somatic) musculature of vertebrates since the early 1860s [1]. MS lie in parallel to the muscle-fibers and in series to the elastic elements, contributing to the complex and functionally partitioned muscle’s architecture. Spindle deformations, related to the variation of muscle’s length, are monitored by specialized sensory fibers (types Ia and II), which contact the intrafusal fibers only in the multinucleated (and noncontractile) region under the equatorial region of the capsule [5]. There are several evidences that spindles in skeletal muscles are part of a complex functional system. They possess multiple roles such as generating antigravity thrust during quiet upright stance, timing of locomotor phases, correcting for muscle nonlinearities, compensating for muscle fatigue, determining synergy formation, and modulating plasticity and motor learning [5, 6]
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