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. Electrophysiologic⁹ 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.

Figure 1-6 Course of sympathetic fibers. The segment shown at the cross-section of the cord is representative of the sympathetic outflow from T1 to L2 or L3. A single preganglionic fiber (solid line) is used to illustrate the possible courses that preganglionic fibers can take. They travel through the ventral root of a spinal nerve and leave the nerve to reach the sympathetic trunk. At the trunk they have one of several courses: they may synapse with the ganglion cells in the first ganglion they reach; they may run up or down the trunk to synapse in ganglia above or below the level at which they enter the trunk; or they may leave the trunk without synapsing, to end in ganglia of the celiac or other ganglia around the aorta. Postganglionic fibers (broken line) arise from cells of the trunk ganglia and return to the spinal nerves to be distributed with them, arise from the ganglia of the celiac and related plexuses to be distributed along the blood vessels to the viscera, or leave as a direct branch of a cervical ganglion to structures in the head and neck. (The association of the cervical ganglia to spinal nerves is the same as that for other ganglia of the trunk, but it is not illustrated.) Note: There are preganglion fibers that go to the medulla of the suprarenal gland to synapse directly on cells there.

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

The primary neurons are named preganglionic neurons. The preganglionic neurons send their axons out to synapse with the secondary neurons. Secondary neurons are located in one of the autonomic ganglions outside the spinal cord. The secondary neurons are named postganglionic neurons. The axons of postganglionic neurons, named postganglionic fibers, travel to the target organs, such as glands in the skin, blood vessels, and organs.

Functionally the ANS is further subdivided into two divisions: sympathetic and parasympathetic. The sympathetic (thoracolumbar) division (Figure 1-7) originates from neurons (preganglionic neurons) in the spinal cord (C8-L2).

Figure 1-7 General plan of the sympathetic system. Preganglionic neurons are located in the lateral gray horn of the spinal cord. Ganglionic neurons are shown in gray. LG, Lumbar ganglia; LSN, lumbar splanchnic nerve; MCG, middle cervical ganglion; SCG, superior cervical ganglion; SG, sacral ganglia; St.G, stellate ganglion; TG, thoracic ganglia; TSN, thoracic splanchnic nerve.

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

The parasympathetic (craniosacral) division (Figure 1-8) comprises preganglionic neurons in the gray matter of the brainstem and of the middle three segments of the sacral cord (S2-4).

Figure 1-8 General plan of the parasympathetic system. Preganglionic neurons are located inside the brainstem and sacral segment of the spinal cord. CG, Ciliary ganglion; HG, heart ganglia; IG, intramural ganglia; MG, myenteric ganglia; OG, otic ganglion; PtG, pterygopalatine ganglion; SG, submandibular ganglion.

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

Sympathetic and parasympathetic nervous systems are functionally opposite to each other in the same way as the yang (active) and the yin (passive) pair in Taoist concept. The functional goal of a two-part nervous system is to balance the visceral activities.

The sympathetic system helps to maintain a constant internal body environment. To perform this function adequately, the sympathetic system is responsible for increasing adrenaline and blood sugar levels, regulating body temperature, and maintaining the contractibility of blood vessels (vasomotor tone). Such activities are needed for surviving in life-threatening situations.

This protective or survival mechanism, which results in hyperactivity of the sympathetic nervous system, is an energy-consuming process. Sometimes the sympathetic nervous system is able to inhibit pain sensation. It is well known that some soldiers do not feel pain for hours after being injured in the battlefield. In our daily lives, if hyperactivity of the sympathetic system persists too long, we become exhausted because of consuming stored energy; our immune system becomes suppressed; we are more likely to get sick; and finally our constant body environment, homeostasis, starts to decline. In this event the body becomes excessively sensitive to pain (hyperalgesia).

When the sympathetic nervous system calms during rest and a period of tranquility, the parasympathetic system becomes active. The decreasing hyperactivity of the sympathetic system ensures, for example, such functions as proper food digestion, which helps to absorb and supply energy flow to the body systems.

Clinical evidence shows that acupuncture stimulation normalizes the activities of the sympathetic and parasympathetic systems to restore optimal homeostasis, which means that acupuncture stimulation calms the sympathetic activities and activates parasympathetic functions.

Nerves and Nerve Fibers

Input signals from a peripheral nerve, carried by the dendrites of neurons, reach the cell bodies located in the ganglions, in the spinal cord, or in the brain. The processed output signals come out along the axons from the cell bodies and reach the peripheral organs, such as skin, muscles, glands, and viscera.

A nerve, also called a nerve fiber, may contain many dendrites and axons. The dendrites and axons existing in the same nerve fiber may convey different input or output in the form of electrical or chemical signals. A nerve or nerve fiber may carry thousands of axons and dendrites or carry just one axon or dendrite.

Different signals can travel in different directions in the same nerve. To ensure that the delivery of the functional signaling reaches the exact address, something must function like an insulating tube to cover a dendrite or axon to protect the signals. Schwann cells provide this insulating layer, called a myelin sheath (Figure 1-9).

Figure 1-9 Schwann cell and myelination in peripheral nervous system.

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

As noted, all peripheral nerves are functionally categorized as either efferent nerves or afferent nerves. An efferent nerve (motor fiber) contains only axons and carries the signals from the CNS to the peripheral target organ. An afferent nerve (sensory fiber) is formed by the dendrites and carries signals to the CNS. The afferent nerve is also called a sensory nerve. A nerve is called a mixed nerve when it contains both afferent (sensory) and efferent (motor) fibers.

The Nerve Bundles

Acupuncture needles stimulate nerve fiber receptors, generating sensations such as tenderness, soreness, heaviness, and sometimes pain. All these receptors are associated with nerve bundles or nerve trunks. A nerve bundle or trunk is a congregation of nerve fibers. Some nerve trunks are so fine that they are invisible to the naked eye, whereas others are thicker and distinctly identifiable, such as the sciatic nerve.

Acupuncture needling concerns two types of nerve trunks: muscular nerves and cutaneous (skin) nerves. A muscular nerve innervates skeletal muscles and consists of three types of fibers:

A cutaneous nerve is distributed to the skin and carries afferent fibers for the general senses and postganglionic fibers for the glands (such as sweat glands) or the blood vessels in the skin.

The skin contains no skeletal muscles. Note that acupuncture needles always stimulate either cutaneous or muscular nerves. To understand the degree of therapeutic effects of needling during acupuncture treatments, it is important to pay attention to the nerve fiber components stimulated by needling.

Some nerve fibers are thicker because they are wrapped with a thick myelin sheath. Some nerve fibers are thinner because they are wrapped with a thin myelin sheath. The finest nerve fibers are not covered by any myelin sheath and are called unmyelinated nerve fibers.

Note that the electrical signals travel faster along thicker nerve fibers. The skin is innervated with three types of sensory fibers: A beta, A delta, and C fibers. A beta fibers are sensitive to gentle pressure and vibration, the thinner A delta fibers are sensitive to heavy pressure and temperature, and the very thin and unmyelinated C fibers are responsive to pressure, chemicals, and temperature. When needles are inserted into the skin or muscle tissue, they stimulate the two finest fibers: A delta fibers and C fibers. A small electrical stimulus, such as that used in transcutaneous electrical nerve stimulation (TENS), preferentially excites the large A beta fibers. The physiology of these fibers is summarized in Table 1-1.

Our clinical experience shows that when needle stimulation is applied, different tissues may generate different sensations. For example, the sensation of numbness is generated mostly by the nerve trunks; the sensation of soreness is generated by the bone membrane (periosteum); soreness and the feeling of distention are generated by muscles; and the sensation of sharp pain is generated by pierced small blood vessels. Clinically it is advisable to relocate the needle slightly when a patient feels sharp, burning, or stinging pain.

C fibers have free endings that are not covered by the myelin sheath. These free endings are specifically sensitive to tissue-threatening (noxious) stimuli such as heavy pressure, intense heat, and strong chemicals. C fibers are called polymodal nociceptors in pain management. Acupuncture needling mostly stimulates C fibers by producing in the tissues a tiny lesion, which may last 2 to 5 days.

Anatomic Configuration of Acupoints

At present the exact anatomic identity of acupoints has not been clearly established. After careful clinical observation and meticulous laboratory research, Dr. H.C. Dung suggested 10 anatomic features associated with acupoints.¹⁰ Each acupoint may contain one or more of these features.

The numeric sequence of these anatomic features is important because it is not random and is based on the anatomic features of the acupoints. To understand this subject better, the neuroanatomically defined acupoints are compared with classic meridian acupoints whenever possible.

1. Size of the Nerve Trunk

Acupoints are always associated with either cutaneous nerves or muscular nerves. The acupoints associated with a larger nerve trunk are more likely to become tender than those associated with a smaller nerve trunk. As stated previously the electrical signals travel faster along thicker nerve fibers. For example, in headache patients, the infraorbital nerve acupoint (H19, trigeminal V2) invariably becomes tender first and the supraorbital nerve point (H23, trigeminal V1) becomes tender later. The fact that the infraorbital nerve is larger than the supraorbital nerve explains this sensitization order of the two acupoints.

Clinically when we palpate these two acupoints, if only one acupoint is tender on a headache patient, it is usually the infraorbital nerve acupoint (H19) and it may indicate that in this case the headache is not difficult to treat. If the headache persists, the second acupoint, the supraorbital nerve acupoint (H23), becomes tender. The presence of two tender points often suggests a longer history of headache and the need for more treatments compared with a patient with only one tender point. If the third trigeminal nerve, which passes through the mental foramen on the lower jaw (trigeminal V3), also appears to be tender, the headache is a difficult case because of the extensively sensitized trigeminal nerve in all three branches. When other anatomic determinants are involved, however, nerve size may not be the only factor that dictates the pathophysiologic dynamics of the acupoints.

2. Depth of the Nerve

More acupoints are formed along superficial nerve trunks than along deeper ones. The superficial nerve receptors become sensitized more easily than do the nerves located deep in tissue. For example, the sciatic nerve is the largest nerve trunk in the human body, but only a few acupoints are attributable to this nerve in the gluteal and thigh region because in this region the nerve lies deeply beneath the thick gluteal and hamstring muscles. As the sciatic nerve enters the posterior compartment of the thigh and popliteal fossa and reaches the leg, it emerges superficially and gives out branches. In this region more acupoints are found along the nerve’s branches in the leg. The same principle is applicable to other nerve trunks.

The upper extremity has the same pattern of acupuncture formation as the lower extremity. Nerve trunks in the upper limbs are located either deep beneath the muscles or inside the neurovascular compartment. As a result only a few acupoints are formed in the upper arm. On their way to the forearm, nerve trunks emerge closer to the surface, and therefore more acupoints appear in this region. This is why more acupoints form below the elbows and knees.

The following is an example of how acupoint formation is affected by the depth of a nerve. The deep radial nerve is derived from the brachial plexus and courses through the upper arm without forming any important acupoints. When the deep radial nerve emerges from the deep fascia to the superficial fascia in the forearm, it forms an important acupoint in the body (H1, deep radial nerve).

The superficial acupoints become tender more often than do the deeply located acupoints because of the abundant assemblage of sensory receptors around the location where the acupoints are formed. An interesting neurologic fact is that the limbs below the elbows and knees occupy larger areas in the sensory gyrus in the brain. Therefore the acupoints below the elbows and knees also occupy a larger area in the cortical representation in the postcentral sensory gyrus in the brain. This may explain why the acupoints below the elbows and knees contain more sensory receptors and why needling stimulation to these points may induce a greater reaction and activity in the brain. This principle clearly supports the concept of using certain acupoints below the elbows and knees (the so-called five-Shu points in the classic meridian system) as diagnostic and treatment points during acupuncture treatment.

3. Penetration of Deep Fascia

The term fascia is rather loosely applied in anatomy. Most fascias are connective tissue layers, and they are arranged in sheets or tubes between or around anatomic structures. Superficial fascia is a padding connected with dermis located above the fascia. Deep fascia lies deep under the superficial fascia and often forms the outer connective layer or covering of the structures underneath, such as blood vessels, nerves, or muscles. Acupuncture points are formed at the locations where a nerve trunk penetrates the deep fascia and emerges close to the surface.

4. Passage Through Bone Foramina

Acupoints are found at the bone foramina, where nerve trunks emerge to distribute cutaneously. The cutaneous branches of the trigeminal nerves (V) give rise to such acupoints at the foramina as the infraorbital (H19, V2), the supraorbital (H23, V1), and the mental (V3) nerves (see Figure 5-2, p. 58).

5. Neuromuscular Attachments

Acupoints are formed at the loci where the nerve trunks enter the muscle. Muscular nerve trunks contain afferent (sensory) fibers, efferent (motor) fibers, and sympathetic fibers. Two acupoints formed this way can be found at the centers of supraspinous and infraspinous fossae, where the suprascapular nerve enters supraspinatus and infraspinatus muscles.

The preceding example one nerve forms two attachments with two muscles, but the infraspinatus attachment is more superficial because the muscle is thinner; so this locus (H8, infraspinatus) usually becomes tender before the supraspinatus point as the latter lies deeper below the round, thick supraspinatus muscle.

Theoretically each muscle has at least one neuromuscular attachment; however, most similar attachments do not form acupoints because the attachments are located deep in the muscle masses and do not often become sensitive.

6. Concomitant Blood Vessels

Concomitant arteries and veins course along the nerve trunks to form neurovascular bundles to reach the muscle attachments. Acupoints associated with neurovascular bundles tend to become tender more readily than do acupoints associated with only cutaneous nerves because the cutaneous nerves are not accompanied by concomitant blood vessels.

The deep radial nerve is a good example of how an acupoint is formed by concomitant blood vessels and nerves. At the lateral surface of the proximal portion of the forearm, the deep radial nerve is buried inside the intermuscular septum between the muscles of the brachioradialis and the extensor carpi radialis longus. Both nerves, the deep radial and the lateral antebrachial cutaneous nerves, are located in similar depth and have similar size except the deep radial nerves are associated with rich concomitant blood vessels. The acupoint H1 deep radial appearing in this location always becomes tender before the acupoint H9 lateral antebrachial cutaneous associated with lateral antebrachial cutaneous nerve. (Refer to the figure in the inside cover.)

The physiologic role of blood vessels in forming acupoints is unknown. Presumably more sensory receptors are formed where the nerve innervates the smooth muscles of the blood vessel.

7. Nerve Fiber Composition

As noted a cutaneous nerve contains only afferent (sensory) and postganglionic sympathetic fibers, whereas a muscular nerve contains these two types of fibers as well as efferent (motor) fibers to the skeletal muscles. Thus the cutaneous and muscular nerves differ in their nerve fiber composition. When all other anatomic features are similar, acupoints associated with nerve trunks containing more nerve fibers are more likely to become tender.

The afferent (sensory) fibers provide sensory receptors attached to the blood vessels, muscle fibers, muscle spindles, muscle tendons, and skin. The sensory fibers collect sensory information from all these structures.

The efferent (motor) fibers innervate skeletal muscles. The postganglionic sympathetic fibers innervate glands in the skin and internal organs. The motor fibers activate the muscle contraction, whereas the sympathetic fibers control the activities of the glands and internal organs.

8. Bifurcation Points

Acupoints are found at locations where a large nerve trunk branches into two or more smaller branches. For example, acupoints are found in the distal portion of four limbs, particularly in the dorsal surfaces of the hand and foot (Figure 1-10).

Figure 1-10 Acupoints are formed at the locations where a nerve trunk divides into smaller branches.

(Modified From Gosling J, et al: Human anatomy: color atlas and text, ed 4, St Louis, 2002, Mosby.)

9. Sensitive Ligamentous Structures

Sensitive loci can become acupoints. Sensitive ligamentous structures include ligaments, muscle tendons, bone retinacula, thick fascial sheets, joint capsules, and collateral ligaments. They are all formed by dense fibrous connective tissue and are sensitive to pressure, palpation, or stretching because of rich afferent nerve receptors in the tissues. For example, a number of tender or even painful points can be found on the collateral ligaments when the knee hurts.

10. Suture Lines of the Skull

Acupoints are formed along the suture lines of the skull. The acupoints can be palpated along the coronal suture, sagittal suture, and lambdoidal suture (see Figure 5-2, p. 58). Such acupoints appear at the nasion, fontanelle, bregma, pterion, and so on. When chronic headache is not adequately treated, eventually tender points will appear at these locations.