Table 2: Classification of NAUs based on a predominance of somatosensory receptors.

TypeDefinition and characteristicsRelated acupoints

Muscle-spindle-rich NAUsWhen an acupuncture needle is inserted, a large portion of the inserted needle body is surrounded by muscle fibers. Muscle spindles are the major neural components in this type of NAUs. There are about 210–2,860 muscle spindles/cm3 in muscle tissues in the back of the Bladder-Meridian [225].Nearly 60% acupoints located on thick muscle areas contain this type of NAUs, for example, Zu-San-Li (ST36), He-Gu (LI4), and Huan-Tiao (GB30). Most acupoints can be performed with large-scale manual techniques.

Cutaneous-receptor-rich NAUsRelatively dense and concentrated cutaneous receptors dominate in NAUs. About 100–240 encapsulated cutaneous receptors/mm2 and 300 free nerve endings/mm2 are distributed in the cutaneous tissues of the finger pads [226].Most acupoints containing this type of NAUs are located on the finger pads, palms, plantar areas, and the surrounding of the lips, for example, Shao-Shang (LU11), Lao-Gong (PC12), and Ren-Zhong (GV26). Only prick and shallow needling can generally be performed on these acupoints.

Tendon-organ-rich NAUsTendon organs, Ruffini and Pacinian corpuscles dominate in NAUs.Most acupoints containing this type of NAUs are located around the elbow, wrist, knee, and ankle joints, for example, Chi-Ze (LU5), Da-Ling (PC7), Du-Bi (ST35), and Jie-Xi (ST41).