Dept. of Ophthalmology, Univ. of Minnesota, Minneapolis, MN 55455, USA.
J Appl Physiol (1985). 2011 Oct;111(4):1178-89. doi: 10.1152/japplphysiol.00368.2011. Epub 2011 Jul 21.
Extraocular muscle (EOM) myofibers do not fit the traditional fiber typing classifications normally used in noncranial skeletal muscle, in part, due to the complexity of their individual myofibers. With single skinned myofibers isolated from rectus muscles of normal adult rabbits, force and shortening velocity were determined for 220 fibers. Each fiber was examined for myosin heavy chain (MyHC) isoform composition by densitometric analysis of electrophoresis gels. Rectus muscle serial sections were examined for coexpression of eight MyHC isoforms. A continuum was seen in single myofiber shortening velocities as well as force generation, both in absolute force (g) and specific tension (kN/m(2)). Shortening velocity correlated with MyHCIIB, IIA, and I content, the more abundant MyHC isoforms expressed within individual myofibers. Importantly, single fibers with similar or identical shortening velocities expressed significantly different ratios of MyHC isoforms. The vast majority of myofibers in both the orbital and global layers expressed more than one MyHC isoform, with up to six isoforms in single fiber segments. MyHC expression varied significantly and unpredictably along the length of single myofibers. Thus EOM myofibers represent a continuum in their histological and physiological characteristics. This continuum would facilitate fine motor control of eye position, speed, and direction of movement in all positions of gaze and with all types of eye movements-from slow vergence movements to fast saccades. To fully understand how the brain controls eye position and movements, it is critical that this significant EOM myofiber heterogeneity be integrated into hypotheses of oculomotor control.
眼外肌（EOM）肌纤维不符合通常用于非颅面骨骼肌肉的传统纤维分型分类，部分原因是其单个肌纤维的复杂性。通过从正常成年兔的直肌中分离出单层肌纤维，测定了 220 根纤维的力和缩短速度。通过电泳凝胶的密度分析，对每根纤维的肌球蛋白重链（MyHC）同工型组成进行了检查。对直肌连续切片进行了 8 种 MyHC 同工型的共表达检查。在单个肌纤维的缩短速度以及力产生方面，都可以看到一个连续体，无论是在绝对力（g）还是比张力（kN/m(2)）方面。缩短速度与 MyHCIIB、IIA 和 I 含量相关，这些是单个肌纤维中表达更为丰富的 MyHC 同工型。重要的是，具有相似或相同缩短速度的单个纤维表达了明显不同的 MyHC 同工型比例。在眼眶层和全局层中的绝大多数肌纤维都表达了一种以上的 MyHC 同工型，在单个纤维段中最多有六种同工型。MyHC 的表达沿单个肌纤维的长度显著且不可预测地变化。因此，EOM 肌纤维在其组织学和生理学特征上代表了一个连续体。这种连续性将有助于精细控制眼球位置、速度和运动方向，无论是在注视的所有位置还是进行各种眼球运动——从缓慢的聚散运动到快速的扫视运动。为了全面理解大脑如何控制眼球位置和运动，将 EOM 肌纤维的这种显著异质性纳入眼运动控制的假设中是至关重要的。
