SHOULDER REGION OF THE RAT - ANATOMY AND FIBER COMPOSITION OF SOME SUPRASCAPULAR NERVE BRANCHES

Background: The pathophysiology of chronic supraspinatus tendinitis is not fully understood. This may be due to the scarcity of experimental studies on this issue. Methods: In search for a system suitable for e xperimental analysis, the present study describes the relevant gross a natomy of the rat shoulder region (dissection), and examines the fiber composition of relevant suprascapular nerve branches (electron micros copy, selective denervations). Results: The rat shoulder region is sim ilar to the human shoulder in terms of gross anatomy. The average supr ascapular nerve (SSC) is derived mainly from the spinal cord segment C 5 and contains 3,435 axons, 74% of which are unmyelinated. The suprasp inatus branch (SSP) contains 627 fibers. Of the SSP fibers, 52% are my elinated, including 32% motor and 20% sensory axons. Of the C-fibers i n the SSP 16% are sympathetic efferents and 32% are sensory. Many of t he latter disappear after neonatal capsaicin treatment. The SSC emits a subacromial articular branch (ART), with some 260 axons, about 90% o f which are unmyelinated. The myelinated ART fibers are sensory, and o f the unmyelinated ones about 24% are sympathetic efferents and 66% ar e afferents. The latter resist neonatal capsaicin treatment. Conclusio ns: In view of the anatomy of the supraspinatus muscle, of the subacro mial space, and of relevant nerves, the rat shoulder should be appropr iate for experimental studies on inflammatory conditions in the subacr omial space. (C) 1994 Wiley-Liss, Inc.