Table 2.

Classification of NAUs based on a predominance of somatosensory receptors.

Type	Definition and characteristics	Related acupoints
Muscle-spindle-rich NAUs	When 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 NAUs	Relatively 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 NAUs	Tendon 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).