Why Are Extraocular Muscles So Resilient to Aging?
Most of us put little thought into the muscles that control the movement of the eye. They just work. Researchers here ask the interesting question: why are these extraocular muscles so resilient? Why does their function decline so little with age, when other muscles throughout the body lose strength and mass, leading ultimately to sarcopenia? There is no complete answer to this question, but it is suggested here that this resilience might have something to do with the fact that the extraocular muscles are much more heavily innervated than other muscles in the body. That in turn might direct a greater focus towards the effects of aging on neuromuscular junctions and consequent loss of innervation in muscles elsewhere in the body. This loss of innervation has been suggested as a contributing cause of sarcopenia.
The extraocular muscles (EOMs) are unique in several aspects: They represent the fastest and most fatigue-resistant muscles within the human body. Extraocular muscles (EOMs) predominantly exhibit impairment in conditions such as myasthenia gravis and mitochondrial myopathies, yet, remarkably, they are spared from various muscular dystrophies, including Duchenne, Becker, limb-girdle, and congenital muscular dystrophies, as well as aging. Furthermore, EOMs demonstrate particular resistance to amyotrophic lateral sclerosis (ALS).
he complexity of the actions performed by the extraocular muscles (EOMs) is reflected in their anatomical and physiological characteristics. Morphologically and in terms of their molecular composition, they significantly differ from the muscle fibers (MFs) of other skeletal muscles. The gene expression profile of the EOMs is distinct from that of limb muscles, with differences encompassing over 330 genes involved in metabolic pathways, structural components, development markers, and regenerative processes. Unlike skeletal muscles, the EOMs predominantly utilize an aerobic pathway for carbohydrate metabolism and relies directly on the glucose influx from the blood. This metabolic strategy enables them to be among the fastest muscles in the body while also being exceptionally resistant to fatigue.
Notably, EOM fibers express a diverse array of myosin heavy-chain isoforms, retaining embryonic forms into adulthood. Moreover, their motor innervation is characterized by a high ratio of nerve fibers to muscle fibers and the presence of unique neuromuscular junctions. These features contribute to the specialized functions of EOMs, including rapid and precise eye movements. Understanding the mechanisms behind the resilience of EOMs to disease and aging may offer insights into potential therapeutic strategies for treating muscular dystrophies and myopathies affecting other skeletal muscles.
A typical part of the aging process is Presbyopia (also known as age-related farsightedness), one of the causes of which is an age-related decrease in the strength of the ciliary muscle. This phenomenon can be considered the earliest manifestation of senile sarcopenia. It is surprising that the body did not protect this muscle from age-related decrease in strength; moreover, it is one of the first (by the age of 45 - 50) to suffer from sarcopenia.