Mechanosensitive afferent units in the shoulder and adjacent tissues

Electrophysiologic experiments were done to identify the somatosensory afferent units in the shoulder and adjacent tissues of the rabbit. Conduction velocity and mechanical threshold of the units were examined to determine whether the units have a nociceptive function or proprioceptive function. Eighty mechanosensitive units were identified in the shoulder and adjacent tissues. Forty-six of these units were found in the rotator cuff area, 26 in the supraspinatus muscle, and eight in the infraspinatus muscle. These tissues contained low to high threshold Group II and Group III units. This study showed there are nociceptors that receive pain sensation and proprioceptors that receive position sense with motion of the shoulder in the rotator cuff and the supraspinatus and infraspinatus muscles. The high density of nociceptors found in the rotator cuff and infraspinatus muscles near their insertion to the shoulder suggests these tissues are more sensitive to noxious stimuli.