Fine structure of the recptors at the myotendinous junction of human extraocular muscles

Abstract

The myotendinous junction of the human extraocular muscles was studied by electron microscopy. Some peculiar receptorial structures have been found in the majority of the samples examined. These structures are very small and consist of 1) the terminal portion of one muscle fibre, 2) the tendon into which it inserts and y), within the tendon, a rich nerve arborization, whose branches are always very close to the rnuscle component. Only one discontinuous layer, made up of tlat cells. which lack a basa1 lamina and often show pinocytotic vesicles, encapsules every musculo-tendinous complex. The tendinous component consists of amorphous ground substance of different electron density. of collagen and elastic fibres and is divided in compartments by ramified cells, which make an inner capsular-like covering to the nerve fibres. Three types of afferent nerve endings can be identified. One type is usually more frequent than the others, possesses a large number of neurotubules and neurofilaments and few mitochondria and is always surrounded by a Schwann cell which forms finger-like processes penetrating into the axoplasm. The second type is only partially enveloped by the Schwann cell. The axoplasm is devoid of neurotubules and contains few neurofilarnents, several mitochondria and groups of small clear vesicles placed in the areas uncovered by the glial sheath. The third one is completely surrounded by the Schwann cell, but is devoid of neurotubules and neurofilaments and full of mitochondria. These rnorphological features correspond well with the probable role of these receptorial structures, which is to ensure very exact and precise ocular movements.