The structural relations between nerve fibres and muscle cells in the urinary bladder of the rat

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Abstract
Intramuscular nerve fibres in the bladder of adult female rats were investigated by means of serial sections. The following observations were made. (1) Upon penetrating into the musculature the nerve bundles branch repeatedly, and almost all turn into single fibres; their axons become varicose, the Schwann cell sheath is attenuated, incomplete or absent, and the separation between axonal membrane and muscle cell membrane is reduced to tens of nanometres. (2) All single axons, and some of those within bundles, are varicose, but the characteristic of being varicose is expressed by degrees, and is not an all-or-none state. (3) Varicosities contain vesicles (mostly of the agranular type), microtubules (with little connection with the axolemma or the vesicles), some neurofilaments (scarce or absent in the best developed varicosities), mitochondria (whose size is on average smaller than those of the perikaryon, and a minute amount of endoplasmic reticulum. (4) Terminal varicosities, the true anatomical ending of an axon, are often devoid of Schwann cell sheath, are packed with vesicles, rarely contain microtubules or neurofilaments, and lie close to a muscle cell: the gap is often reduced to ∼10 nm. (5) Schwann cells accompany the axons within the muscle strands. Unlike the area of the axonal profiles, the area of glial sheath changes little along the length of the nerve fibre, except towards its end. (6) The Schwann cell sheath around a varicosity is often incomplete; the area of the axolemma thus exposed is covered by the basal lamina, and is here referred to as a ‘window’. While some varicosities have a window only a few tens of nanometres in width, others have more than one window, and some are devoid of Schwann cell altogether, so that their entire axolemma is in contact with the basal lamina. The Schwann cell never extends beyond the axon, whereas very often (and possibly always) the axon extends beyond the Schwann cell. (7) Intervaricose segments vary in length and diameter, the narrowest ones accompanying the more clear-cut varicosities. Some intervaricose segments are as small as 50 nm in diameter, contain a single microtubule and lack a Schwann cell sheath. Others, sheathed by a Schwann cell, contain a single neurofilament or no organelles at all. (8) Specialized contacts between an axon and a muscle cell (neuro-muscular junctions) are abundant and are identified by four features: the axon is a varicosity packed with vesicles; the axolemma is exposed (presence of a window); the distance between the two membranes ranges between 10 and 100 nm, mostly 30–50 nm; and the intercellular gap excludes fibrils, such as collagen, but is occupied by a single basal lamina. Any of these parameters, however, can also occur ‘uncoupled’ (windows on intervaricose segments; varicosities without a window; exposed axolemma far from a muscle cell). (9) There are no direct contacts between axons. Even when they run close to each other within a bundle, they are always separated by a Schwann cell process. (10) The muscle cell membrane is concave beneath the varicosities; however, the muscle cell ultrastructural features in the region of the neuro-muscular junction are not different from those in other regions of the cell. (11) On average there is more than one neuro-muscular junction per muscle cell, and examples of muscle cells receiving multiple nerve endings from one or from two axons are picked up by the serial sections. (12) A striking feature of the bladder innervation is the variability of its ultrastructural parameters. The bladder innervation does not appear to be built on a rigid structural plan, and the notion of ‘loose-patterned innervation’ is presented.