Neural acupuncture unit: a new concept for interpreting effects and mechanisms of acupuncture

Abstract

When an acupuncture needle is inserted into a designated point on the body and mechanical or electrical stimulation is delivered, various neural and neuroactive components are activated. The collection of the activated neural and neuroactive components distributed in the skin, muscle, and connective tissues surrounding the inserted needle is defined as a neural acupuncture unit (NAU). The traditionally defined acupoints represent an anatomical landmark system that indicates local sites where NAUs may contain relatively dense and concentrated neural and neuroactive components, upon which acupuncture stimulation would elicit a more efficient therapeutic response. The NAU-based local mechanisms of biochemical and biophysical reactions play an important role in acupuncture-induced analgesia. Different properties of NAUs are associated with different components of needling sensation. There exist several central pathways to convey NAU-induced acupuncture signals, Electroacupuncture (EA) frequency-specific neurochemical effects are related to different peripheral and central pathways transmitting afferent signals from different frequency of NAU stimulation. More widespread and intense neuroimaging responses of brain regions to acupuncture may be a consequence of more efficient NAU stimulation modes. The introduction of the conception of NAU provides a new theoretical approach to interpreting effects and mechanisms of acupuncture in modern biomedical knowledge framework.

Figure 1

A representative muscle-spindle-rich NAU in the response to manual twists of acupuncture stimulation. The NAU with the related neural and neuroactive components is illustrated as the dotted line-defined vase-like pattern, which is principally determined by twist-produced different distant effects on mechanoreceptors located in cutaneous and muscle tissues (see Section 5.1).

Figure 2

Schematic illustration of major nonneuronal neuroactive mediators and their corresponding receptors involved in the modulation of NAU afferent impulses. Molecules in red, green, and violet color represent stimulatory, inhibitory, and both effects on afferent fiber excitability, respectively. Autoreceptors to be identified are indicated with question symbols (?). A₁, adenosine A₁ receptor; ACh, acetylcholine; Auto-R, autoreceptor; B_1/2, bradykinin receptors 1 and 2; BK, bradykinin; CGRP, calcitonin-gene-related peptide; β-END, β-endorphin; EP, prostaglandin E receptor; GABA, γ-aminobutyric acid; Glu, glutamate; H₁/H₂, histamine H₁/H₂ receptors; His, histamine; 5-HT, 5-hydroxytryptamine; IL, interleukin; M₂, muscarinic M₂ receptor; NA, noradrenaline; NO, nitric oxide; PG, prostaglandins; P2X/P2Y, purinergic receptors P2X and P2Y; α ₂-R, α ₂ adrenoceptor; SP, substance P; SS, somatostatin; SSR, somatostatin receptor; TNF-α, tumor necrosis factor-α.

Figure 3

Acupuncture-induced robust axon reflex (a) and its involvement in the propagated sensation along meridians (PSM) (b). In (a), hyperemia (flare) was induced by acupuncture needling in acupoint areas of Bladder Meridian Foot Taiyang in the back. (b) illustrates putative communication between adjacent branches of nerves from different spinal segments via neuroactive mediators released by acupuncture stimulation from neural and non-neuronal tissues. (b) was reproduced based on the work done by Professor Zhao's research group with his generous permission (also see [20, 21, 224]).

Figure 4

The distant effects of different manual techniques of acupuncture in activating cutaneous superficial mechanoreceptors (a), deep pressure-detected receptors (b), and muscle stretch receptors (c) located in the posterior aspect of the leg of Bladder Meridian Foot Taiyang in rabbits. Percent of the number of the activated receptors in each defined area surrounding needling point was calculated from the total number of the activated receptors. The plots were produced based on the data reported in [58, 59].

Figure 5

The distant effects of different intensities of electroacupuncture (EA) in activating cutaneous superficial mechanoreceptors (a), deep pressure-detected receptors (b), and muscle stretch receptors (c) located in rabbit triceps surae muscle areas of Bladder Meridian Foot Taiyang. Percent of the number of the activated receptors in each defined area surrounding needle point was calculated from the total number of the activated receptors. The plots were produced based on the data reported in [60, 61].

Figure 6

Schematic illustration of multiple central neural pathways transmitting NAU afferent impulses from different parts of the body. The brain areas commonly observed in neuroimaging response to acupuncture stimulation are indicated with gray shadow. DCEAS: dense cranial electroacupuncture stimulation.