Sensory nerve endings in the anterior cruciate ligament (Lig. cruciatum anterius) of sheep

This study examines the structure of sensory nerve endings in the sheep ant erior cruciate ligament (ACL). Three types of nerve endings are found: free nerve endings (FNE), Ruffini corpuscles, and lamellated corpuscles. The FNE (more than 100) are found subsynovially. The afferent nerve fibres are either thin myelinated axons (A delta) or C fibres with diameters of 1- 2 pm. FNE have been reported to function as thermoreceptors and polymodal n ociceptors. In addition, FNE are also seen between fascicles of collagen fi bres, often close to blood vessels. Part of this group may be efferent auto nomic fibres controlling local blood flow. The corpuscles are seen subsynovially and between fascicles of connective t issue close to the attachment points of the ACL. A ligament contains about 20 Ruffini corpuscles, which are mainly located in the subsynovial connecti ve tissue. They consist of cylinders formed from perineural cells surroundi ng the afferent myelinated axons (diameters 4-5 mu m) with enlarged nerve t erminals anchored between collagen fibres. These enter in bundles from the surrounding connective tissue at one open pole, pass through the length of the cylinder, and leave at the other pole. Functionally, Ruffini corpuscles have been described as slowly adapting stretch receptors. Lamellated corpuscles (usually between 5 and 15) are found in the subsynovi al connective tissue. The afferent myelinated axon has a diameter of 4-6 mu m, and the nerve terminal is located in the centre of numerous layers form ed by lamellated terminal glial cells and by a perineural capsule. They are known to function as rapidly adapting pressure receptors. The most important function of the ACL is its mechanical function, but addi tional sensory functions must be considered triggering reflex mechanisms in case of extreme positioning or overload. Anat Rec 254:13-21, 1999. (C) 199 9 Wiley-Liss, Inc.