1: From Neurons to Acupoints: Basic Neuroanatomy of Acupoints

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CHAPTER 1 From Neurons to Acupoints: Basic Neuroanatomy of Acupoints

INTRODUCTION

The efficiency of acupuncture therapy is based on the selection of effective acupoints. There are 361 classic meridian acupoints, and more than 2000 new extrameridian acupoints have been recorded in the Chinese literature on acupuncture. Anatomic examination of the 361 classic meridian acupoints shows that most acupoints are associated with certain anatomic structures of the peripheral nervous system (PNS). Researchers have indicated that 309 acupoints lie on or near the nerves and that 286 acupoints are situated next to major blood vessels, which are surrounded by small nerve bundles.¹

Modern neuromuscular medicine named some of these points trigger points, motor points, and dermopoints. Dr. Ronald Melzack ² found that more than 70% of the classic meridian acupoints correspond to commonly used trigger points. Whatever terms are used, all these points are characterized by the ability to become painful or tender or to create other physical discomforts as a result of sensitized nerves. It is extremely important to understand that although more than 70% of the classic acupoints share the same characteristics as trigger points, acupoints and trigger points are not the same.

In their book Muscle Pain, Drs. Siegfried Mense and David G. Simons define trigger points as follows: “(1) A central trigger point is a tender, localized hardening in a skeletal muscle. Clinical characteristics include circumscribed spot tenderness in a nodule that is part of a palpably tense band of muscle fibers, patient recognition of the pain evoked by pressure on the tender spot as being familiar, pain referred in the pattern characteristic of trigger points in that muscle, a local twitch response, painful limitation of the stretch range of motion, and some weakness of that muscle. Diagnostic criteria of active trigger points are circumscribed spot tenderness in a nodule of a palpable taut band and patient recognition of the pain, evoked by pressure on the tender spot, as being familiar. Latent trigger points cause no complaint of clinical pain. (2) Attachment trigger points have tenderness in the region of muscle attachment caused by enthesitis or enthesopathy induced by the persistent tension of the taut band muscle fibers.”³

These trigger points as defined clearly refer to tender spots only on the skeletal muscles. The classic acupoints are found not only in the skeletal muscles but also in other soft tissue structures such as in tendons and fascias. Thus trigger points share some but not all characteristics of acupoints, whereas acupoints include all the characteristics of trigger points. This means that trigger points have some inclusive but not exclusive parameters of the classic acupoints.

In laboratory and clinical experiments, acupuncture stops being effective in a region supplied by a sensory nerve when that nerve is blocked by local anesthesia, cut by surgery, numbed by ice (low temperature), or bound by a band.⁴ These experiments prove that sensory nerves are vital anatomic components of acupoints. Sensory nerves are distributed all over the human body and are absent on only a few structures, such as nails and hair. Thus where there are sensory nerves there will be acupoints.

About 2500 years ago, the Yellow Emperor’s Canon of Internal Medicine (Huang Di Nei Jing) described about 135 bilateral acupoints. In the next 2000 years, the masters of traditional Chinese medicine (TCM) gradually discovered more and more acupoints and organized them into a meridian system according to Taoist principles.

The Chinese doctors of ancient times noticed that, in addition to meridian acupoints, some nonmeridian acupoints are as effective as classic meridian points. The two types of nonmeridian acupoints are Ashi acupoints and extrameridian acupoints. Ashi acupoints appear unpredictably in relation to individual symptoms. Extrameridian acupoints are not located on any of the 14 meridians but have fixed locations on the body. Gradually more and more extrameridian acupoints were recorded and incorporated into the classic acupuncture system. The recent Chinese acupuncture literature has recorded more than 2500 “new” acupoints.

In the last three decades, an impressive number of modern studies show that acupuncture efficacy does not always depend on the precise location of acupoints as indicated on meridian charts.⁵ Scientific data suggest that there is no statistical significance between needling either “sham” or “true” acupoints in treating patients with chronic pain.⁶ Well-known clinician Dr. Felix Mann, after many years of observation, has concluded that acupoints, in the classic understanding, do not exist at all.⁷

From a neuroanatomic point of view, acupoints can be defined by combining the classic concepts with the results of modern research.

Tender acupoints appear on the bodies of both healthy persons and pain patients alike. Let us look at a healthy person first. Dr. H.C. Dung discovered that tender acupoints appear in a generally consistent and predictable pattern. Dr. Dung provided an anatomic explanation for why acupoints appear in such a predictable pattern (see Chapter 2). This discovery is regarded as “a turning point in acupuncture’s long history.”⁸

When we examine patients who have similar pain symptoms, certain tender or painful points share the same anatomic location in these patients, even though each patient may carry some different tender acupoints that reflect his or her personal symptoms. For example, almost all patients with low back pain have similar tender points located at the iliac crest and 2 inches lateral to the spinous process of the second lumbar vertebra (refer to acupoints H14 and H15 on the figure in the inside cover). The area (size) of each tender acupoint changes dynamically. In the patient with acute pain, the tender acupoints are smaller and fit into the locations defined by the classic acupoint chart, whereas in a patient with severe or chronic pain, the tender areas are significantly increased and the whole muscle mass may become tender and painful, showing that the size of acupoints will grow depending on the chronicity of the pain symptoms. Acupoints are pathophysiologically dynamic structures.

BASIC NEUROANATOMY FOR DEFINING ACUPOINTS

Anatomically the human nervous system consists of two subdivisions: the central nervous system (CNS) and the PNS. This book focuses on the PNS because it is the most important structure affecting both diagnosis and treatment in acupuncture therapy.

Functionally the nervous system is divided into two systems: the somatic nervous system (SNS) and the autonomic (visceral) nervous system (ANS). The SNS controls the muscles and carries information to the brain; the ANS operates without conscious control as the caretaker of the body. For example, the ANS controls heartbeat and respiration without our consciousness.

Neurons, or nerve cells, are the basic morphologic and functional units of the nervous system. Here we discuss these complicated structures only in relation to acupuncture medicine.

Neuron (Nerve Cell)

A neuron, or nerve cell, is the anatomic and physiologic unit of the nervous system. Neurons vary in size, shape, and complexity (Figure 1-1). All neurons share common structural and functional features. Typically a neuron has one or more processes, called dendrites, radiating from the cell body. The endings of the dendrites are called receptors.

Figure 1-1 Diagram of two types of neurons. The common structure of all the neurons are dendrites, a cell body, and an axon. When dendrites are stimulated, the electrical impulses always travel unidirectionally from dendrites to presynaptic terminals. A, The typical form of a sensory nerve. The action potentials (AP) generated by stimulation or disturbance are conducted from dendrites in the skin and muscle to the spinal cord. The action potentials usually bypass the cell body. When the intervertebral disk is herniated and presses the dorsal root ganglion, the cell bodies of the sensory neurons also generate impulses that cause pain. B, The most prevalent form of neurons in the spinal cord and brain. Extensive branching of the dendrites emanates from the cell body and an axon emerges from the opposite end of the dendrites.

The dendrite receptors are morphologically specialized for particular function. The neuron is a dynamically polarized cell that serves as the major signaling unit of the nervous system.

Acupuncture needles directly stimulate and activate the dendrite receptors of the sensory neurons in the skin, muscles, and other soft tissues. Figure 1-2 shows some of the sensory nerve endings (receptors), such as free nerve endings and encapsulated nerve endings.

Figure 1-2 Examples of specialized sensory endings that innervate the skin: free ending (most for pain and temperature), Ruffini endings (shearing stress), follicular endings (when the hairs are being bent).

(From FitzGerald M, Folan-Curran J: Clinical neuroanatomy and related neuroscience, ed 4, Philadelphia, 2002, WB Saunders.)

Another single process, called an axon, extends from the cell body (see Figure 1-1). An axon of a sensory neuron sends the impulses from the cell body to the next neuron, or an axon of a motoneuron brings the impulses to the muscles. Stimulation of the receptors by acupuncture needling generates electrical signals, which travel from receptors along the dendrites to the cell body and then to the axon. Stimulation below the threshold will not generate incoming signals to the brain. When stimulation to the receptors is strong enough, it will initiate in the cell body a transient electrical signal called an action potential, which will propagate from the cell body along the axon to the axon terminal. The axon terminal passes the signals to the dendrites or cell body or axon of another neuron through a special structure called a synapse. Thus signals generated by receptors are transmitted from one neuron to another neuron and finally reach the brain (Figure 1-3).

Figure 1-3 An ultrastructural diagram of an axon synapsing with a dendrite of another neuron. The synaptic vesicles contain the chemicals that will activate or deactivate the dendrite of another neuron.

(From FitzGerald M, Folan-Curran J: Clinical neuroanatomy and related neuroscience, ed 4, Philadelphia, 2002, WB Saunders.)

Within the brain, the signals are processed, either suppressed or strengthened, and may continue to travel to different neurons.

The Central Nervous System

A neuron is a major functional unit in the nervous system. The CNS consists of the brain and the spinal cord. Electrophysiologic ⁹ and neurochemical research shows that both the brain and the spinal cord actively react to acupuncture stimulation. Today, after more than three decades of research, we know more about the involvement of the CNS in acupuncture therapy, but the exact mechanism of this correlation is still unclear. We briefly discuss this mechanism in Chapters 3 and 4.

The Peripheral Nervous System

The importance of PNS in clinical acupuncture cannot be overstated. The PNS is an extension of the CNS. Acupuncture needles directly stimulate and activate the PNS to achieve the desirable therapeutic results because all the nervous structures outside the brain and the spinal cord belong to PNS. The PNS is complex, and we discuss only the parts of PNS that are directly related to clinical acupuncture.

The PNS consists of the cranial nerves from the brain and the spinal nerves from the spinal cord, a total of 12 pairs of cranial nerves and 33 pairs of spinal nerves. Traditionally Roman numerals are used to designate the cranial nerves. Among them, the trigeminal nerve (V), facial nerve (VII), and spinal accessory nerve (XI) are the most important in acupuncture therapy (Figure 1-4).

Figure 1-4 The cranial nerves accessible by acupuncture needling in the head and neck: trigeminal nerve (V, three branches: ophthalmic, maxillary, and mandibular), facial nerve (VII), and accessory nerve (XI). Also note that, activated by acupuncture stimulation, the vagus nerve plays a very important role in restoring homeostasis and antiinflammatory immune reaction (see Chapter 4 for further explanation).

(From Jenkins D: Hollinshead’s functional anatomy of the limbs and back, ed 8, Philadelphia, 2002, WB Saunders.)

According to their location in relation to the spinal cord, the 33 pairs of spinal nerves are categorized into five groups: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 3 coccygeal. The coccygeal nerves are less important in acupuncture because of their relatively small size, although severe pain in the coccyx or tailbone may occur and will need to be attended (Figure 1-5).

Figure 1-5 The spinal nerves that make up most of the peripheral nerves of the body.

(From Jenkins D: Hollinshead’s functional anatomy of the limbs and back, ed 8, Philadelphia, 2002, WB Saunders.)

The PNS also contains ganglions, which are groups of nerve cells outside the spinal cord. The nerve fibers of the PNS are distributed to all areas of our body except the nails or hair, which is why nails and hair can be cut without pain.

Functionally the PNS comprises sensory neurons, motor neurons, and sympathetic ganglion neurons. Whenever we insert needles into the body, the needles will stimulate sensory nerve endings, which cover the body; sympathetic nerves, which control the blood vessels and glands; and motoneurons if the needled location is innervated by the motoneuron fibers.

Somatic Nervous System

We classify the nervous system into the CNS and the PNS according to anatomic structure. Functionally we divide the nervous system into two groups: the SNS and the ANS. The SNS is the part of the nervous system that innervates the structures of the body wall: muscles, skin, and mucous membranes. The part of the nervous system that controls visceral organs is referred to as the ANS. From the viewpoint of acupuncture therapy, the SNS is similar to parts of the PNS.

Autonomic (Visceral) Nervous System

The ANS consists of portions of the CNS and PNS. It controls the physiologic activities of cardiac muscles, the smooth muscles in viscera (such as in the blood vessels), and the glands. The ANS also transmits sensory information from the previously mentioned target organs to the brain. The ANS includes efferent pathways (motor), afferent pathways (sensory), and groups of neurons (nuclei) in the brain and spinal cord that regulate the target organs.

Anatomically the efferent components, the outflow pathway of the ANS, are characterized by a two-neuron chain (Figure 1-6). The first neurons, or the primary neurons, are located in the CNS in the brainstem nuclei (groups of neurons in the brain are called nuclei) or in the lateral gray column of the spinal cord.