INTRODUCTION: There are numerous sex differences concerning male and female skeletal muscles, including muscle mass, the number and diameter of muscle fibers, and number of motor units; however, there is no data concerning the number and density of muscle spindles, which are the most important muscle proprioceptors. AIM(S): The experiments aimed to determine the number of gamma motoneurons, number and density of muscle spindles, as well as, their morphometric properties for rat medial gastrocnemius. METHOD(S): The motoneurons were stained with the horseradish peroxidase and neurons exceeding 27.5 µm were accepted as gamma size. Muscle spindles were counted in muscle cut into 5‑10 µm sections. The rat medial gastrocnemius was innervated by a similar number of gamma motoneurons (27.6±3.9 and 28.7±6.6, p>0.05) for female and males, respectively. RESULTS: However, size of gamma motoneurons was higher in males (23.5±2.9 µm) than in females (21.6±2.9 µm) (p<0.01). The number of muscle spindles amounted to 13.16±1.25 and 14.0±2.71 for female and males, respectively (p>0.05). The diameter of intrafusal muscle fibers was similar in males (5.16±2.43 µm) and in females (5.37±2.27 µm) (p>0.05), whereas the number of intrafusal muscle fibers was smaller in females (4.76±1.23) than in males (5.2±1.89, p<0.05). The mass of studied muscle was 61% higher in males (1.08 g vs. 0.66 g in females); therefore, the muscle mass per one spindle was 77 mg in males and 50 mg in females. CONCLUSIONS: The number of muscle spindles as well as their motor innervation by gamma motoneurons is similar in male and female rats, but the density of muscle spindles is considerably higher in females. FINANCIAL SUPPORT: Supported by grant 2018/31/ B/NZ7/01028.
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