Review of Basic Neuroanatomy
In the neuroanatomy of the brain, a nucleus is a neural brain structure consisting of a relatively compact cluster of neurons. It is one of the two most common forms of nerve cell organization, the other being layered structures such as the cerebral cortex (or cerebellar cortex) and cingulate gyrus. The vertebrate brain contains hundreds of distinguishable nuclei, varying widely in shape and size. A nucleus may itself have a complex internal structure, with multiple types of neurons arranged in clumps (subnuclei) or layers, such as in the pituitary (Filshie & White, 1998).
The term “nucleus” is in some cases used rather loosely, simply to mean an identifiably distinct group of neurons, even if they are spread over an extended area. The reticular nucleus of the thalamus, for example, is a thin layer of inhibitory neurons that surrounds the thalamus (Marieb & Hoehn, 2009). It is these layers that have terminal ends that extend from the focal area of some acupuncture points (Filshie & White, 1998; White et al., 2008).
Some of the major anatomical components of the brain are organized as clusters of interconnected nuclei. Notable among these are the thalamus and hypothalamus, each of which contains several dozen distinguishable substructures. These substructures explain the profound responsibilities and vast areas of influence these components have. The medulla and pons also contain numerous small nuclei with a wide variety of sensory, motor, and regulatory functions (e.g. breathing and cardiac). This is all contained in the central nervous system (CNS) and, respectively, the brain, but have neural tracts that feed information back to them from the peripheral (Marieb & Hoehn, 2009). Furthermore, the recent discoveries in neuroscience involving revolutionary concepts in neuroplasticity open new worlds for exploration and medicine.
Since acupuncture is performed on the peripheral of the CNS, let’s examine the neuroanatomy relevant to that as well. The peripheral nervous system (PNS) has specialized structures that respond to changes in our environment. They are grouped as sensory receptors and include:
•exteroceptors, which sense stimuli arising outside the body
•interoceptors or visceroceptors, which respond to stimuli inside the body
•proprioceptors, which tell the brain where we are relative to other body parts’ nociceptors, which respond to potentially damaging threats to tissue.
(Marieb & Hoehn, 2009)
These specialized groups of structures include nerve tracts, ganglia (clusters of neurons in the PNS), and specific receptors. It has been concluded from the imaging studies of acupuncture points that ganglia occur near or directly subdermal to many traditional acupuncture points (Marieb & Hoehn, 2009). It is at these Neuropuncture acupoints that ganglia are accessed, and the stimulation can be sent up tracts to specific regions of brain tissue, as well as to target specific spinal segments and plexuses.
Below is a short list of terms that need to be understood and conceptualized in order to grasp this system. I would encourage practitioners of acupuncture to undertake further studies in neuroscience, neurophysiology, neurophysics, or molecular biology and pharmacology as, in my opinion, to be a physician of acupuncture, you must first understand neuroscience. It is the micro and macro of acupuncture today! The following terms are explained and defined as they apply to this topic, but these are and by no means conclusive definitions.
•Central nervous system (CNS): The CNS comprises the brain and spinal cord. The brain is the central command center, and the spinal cord is a highway of neurotracts. The spinal cord is segmented by different levels of visceral and somatic neuro-innervations. Those innervations converge and travel up the spinal cord in tracts that terminate in specific regions of the brain, relaying information to the central command center.
•Brain: This is the central command center, the largest and most sophisticated pharmaceutical outfit in the world today. Even now it is not fully understood and is referred to generally as the “last frontier for medicine.” Composed of 60 percent fatty acids and cholesterol, and 40 percent proteins, functional magnetic resonance imaging (fMRI) studies have revealed corrugations between very specific regions and areas of the brain with very specific functions and responsibilities in relation to acupuncture’s mechanisms.
•Cerebral cortex: This is the folded, outermost, layer of tissue covering the brain. It is responsible for higher functioning in humans. Peripheral nerve stimulation (i.e. Electro-Acupuncture) sends nerve impulses along larger nerve tracts to reach the brainstem; they then fire up and stimulate regions of the cerebral cortex before terminating in other regions of the inner brain.
•Limbic area: This is a set of evolutionary primitive brain structures located on top of the brainstem and buried under the cortex. Limbic system structures are involved in many of our emotions and motivations, particularly those that are related to survival. Such feelings include fear, anger, and emotions related to sexual behavior. The limbic system is also involved in feelings of pleasure that are related to our survival, such as those experienced from eating and having sex. It also plays a major role in chronic pain.
•Hypothalamus: This portion of the brain contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland (hypothalamo-pituitary-adrenal axis). The hypothalamus is located below the thalamus, just above the brainstem. In the terminology of neuroanatomy, it forms the ventral part of the diencephalon. All vertebrate brains contain a hypothalamus. In humans, it is roughly the size of an almond. The hypothalamus is responsible for the release of beta-endorphins. These endorphins in turn stimulate the PAG for systemic pain relief. The hypothalamus also controls body temperature, hunger, thirst, fatigue, sleep, and circadian cycles.
•Thalamus: This is a limbic system structure connecting areas of the cerebral cortex involved in sensory perception and movement with other parts of the brain and spinal cord that also have a role in sensation and movement. As a regulator of sensory information, the thalamus controls sleep and states of consciousness when we are awake, and also plays a role in pain management. The auricular acupuncture point exerts an influence and stimulates the thalamus for pain management.
•Cingulate gyrus: Cingulum is the Latin word for “belt.” The name was probably chosen because this cortex, in great part, surrounds the corpus callosum. It receives input from the thalamus and the neocortex. It is an integral part of the limbic system and is involved with emotion formation and processing, learning, and memory. It is also important for executive function and respiratory control. It consists of the “folded layers of tissue” that along with the cerebral cortex help to make up 40 percent of the brain.
•Amygdala: The amygdala is an almond-shaped mass of nuclei located deep within the temporal lobe of the brain. It is a limbic system structure that is involved in many of our emotions and motivations, particularly those related to survival. The amygdala is involved in the processing of emotions such as fear, anger, and pleasure. It is also responsible for determining what memories are retained and where they are stored in the brain. It is thought that this determination is based on the size of an emotional response to an event. The amygdala is also a major area for sufferers of chronic pain and post-traumatic stress disorder.
•Pituitary: The pituitary gland is a small endocrine organ that controls a multitude of important functions in the body. It is divided into an anterior lobe, intermediate lobe, and posterior lobe, all of which are involved in hormone production. The posterior pituitary is composed of axons from the neurons of the hypothalamus. Blood vessel connections between the hypothalamus and pituitary allow hypothalamic hormones to control pituitary hormone secretion. The pituitary gland is termed the “master gland” because it directs other organs and endocrine glands, such as the adrenal glands, to suppress or induce hormone production.
•Periaqueductal gray (PAG): This is the part of the brainstem that is involved in pain suppression and is found in mammals. A large integral component for the endogenous descending pain system, it is stimulated by beta-endorphins, which are produced and secreted by the hypothalamus. There are certain Neuropuncture points with nerve endings that terminate in the hypothalamus and stimulate the secretion of beta-endorphins for pain management.
•Somatosensory regions: These areas of the cerebral cortex are responsible for the production of the sensory modalities such as touch, temperature, proprioception (body position), and nociceptive pain (the sensory nervous system’s response to the experience of pain). We see direct relations with the classical scalp acupuncture motor and sensory lines.
•Primary afferent nociceptive system (PANS): These are specialized sensory nerve endings that include the A-delta and C-fibers. They are generally the first nerves to be involved in nociception. The system includes the network of A-delta fibers, retracing to the dorsal horn, up the spinothalamic tract, and terminating in the brain.
•Cervical plexus: C1–C4 is the nerve root plexus that exits the cervical spine at the levels of C1–C4, to become the lesser occipital nerve, greater occipital nerve, the transverse cervical nerve, and the supraclavicular nerve.
•Brachial plexus: C5–T1 is the nerve root plexus that exits the spinal column at the levels of C5–T1, which supply the arm.
•Lumbar plexus: L1–L4 is the nerve root plexus that exits the spinal column at the levels of L1–L4, which penetrate the lower limbs.
•Sacral plexus: L4–S4. The sciatic branch, L4–S3, is the largest nerve in the plexus and the entire body. S3 and S4 refer to the levels of S3 and S4 of the sacral fused vertebral bodies.
•Neurotransmitters: These are chemicals that are released by the axons and communicate and transmit information along a specific neuro-tract. There are many different types with vast responsibilities.
•Axon: Nerve extension that conducts impulses away from the soma (cell body).
•Dendrite: Nerve extension that conducts electrical impulses received from other neural cells.
•Resting membrane potential: Na++ (sodium ions) outside of the cell with K+ (potassium ions) on the inside of the cell. Negative inside versus positive on the outside (-70 mV).
•Membrane potential: Depolarization, a change on the resting membrane potential.
•All or none theory: Utilizing Electro-Acupuncture to bring about the action potential. This theory states that a nerve tract will change its membrane potential at a specific point of stimulus (-50 mV) and then anything after that point, or will not change at all.
•Descending pain system: The endogenous circuit that enables us to tolerate pain.
•Membrane threshold: -70 mV is the electrical charge required for a nerve cell membrane to hold before it changes its membrane potential, resulting in depolarization or repolarization of nerves.
•Myelin sheath: This is the lining around the nerve and consists of docosahexaenoic acid (DHA) molecules.
•Opioid receptors: G-protein-shaped neurons. Their stimulation releases endorphins.
•Endorphins: These are natural endogenous opioid polypeptide compounds—simply our body’s natural pain killers.
•Mu: An opioid receptor with a high affinity for beta-endorphins.
•Kappa: An opioid receptor with a high affinity for dynorphins.
•Delta: An opioid receptor with a high affinity for enkephalins.
•Beta-endorphin: An endogenous poly-opioid peptide that regulates the perception of pain. It is produced and released by the hypothalamus.
•Dynorphin: An endogenous poly-opioid peptide that regulates the perception of pain. It has a high affinity to the kappa receptor. It is released from the hindbrain.
•Enkephalin: An endogenous poly-opioid peptide that regulates the perception of pain. It has a high affinity to the delta receptor and mu. It is released at the dorsal horn of the spinal cord.
•Serotonin: Biochemically derived from tryptophan, serotonin aids in down-regulating pain signaling from many locations of the body. Approximately 80 percent of the human body’s total serotonin is located in the enterochromaffin cells in the gut, where it is used to regulate intestinal movements. This is another reason why we say “the stomach is our second mind.” With regard to acupuncture and pain relief, we look at the specific release of serotonin at the dorsal horn.
•Afferent nerve fibers: Also referred to as sensory neurons, the afferent nerve fibers carry nerve impulses from receptors or sense organs toward the CNS.
•Efferent nerve fibers: Also referred to as motor neurons, the efferent nerve fibers carry nerve impulses away from the CNS to effectors such as muscles or glands.
•A-delta fibers: In acupuncture studies, this term refers to the broad section of A-gamma and A-beta fibers that create the neural network at the Neuropuncture acupoints.
•II and III muscle fibers: These are the fibers in the muscle that are responsible for some of the De Qi sensation in acupuncture stimulation.
•C-fibers: These are afferent nerve fibers found in the somatosensory system. They are unmyelinated and have a slow conduction velocity. They are mostly associated with “sharp” pain.
•Greater auricular nerve: Access to this nerve is via the Neuropuncture acupoint affecting C1–C5 of the cervical plexus.
•Radial nerve: The largest branch of the brachial plexus.
•Median nerve: A branch of the brachial plexus and the nerve injured in carpal tunnel syndrome.
•Ulnar nerve: A distal branch of the brachial plexus and the largest unprotected nerve in the body. It is felt when the “funny bone” is struck.
•Sciatic nerve: This innervates the spinal segment L4–S3. It bifurcates into the tibial, common, and deep peroneal nerves.
•Femoral nerve: This innervates the spinal segment L2–L4. It bifurcates into the saphenous and sural nerves.
•Peroneal nerve: This forms from the bifurcation of the sciatica nerve around the head of the fibular.
•Tibial nerve: This is the other bifurcation of the sciatica nerve.
•Femoral nerve: The largest branch of the lumbar plexus.
•Saphenous nerve: The largest branch of the femoral nerve.
•Sural nerve: A short branch of the saphenous nerve.
•Trigeminal nerve: This is the fifth cranial nerve (CNV). It is primarily a sensory nerve but it also has certain motor functions (biting, chewing, and swallowing).
•Vagus nerve: This is the tenth cranial nerve (CNX). The vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech and keeping the larynx open for breathing. It also has some afferent fibers that innervate the inner (canal) portion of the outer ear, via the auricular branch and part of the meninges. This explains why a person may cough when tickled on the ear (such as when trying to remove ear wax with a cotton swab).
•Calcitonin gene-related peptide (CGRP): This is the most potent peptide vasodilator and can function in the transmission of pain. Locally, for acupuncture purposes, it helps to vasodilate the surrounding capillaries to further release other powerful bio-chemicals.
•Nitric oxide synthase (NOS): An enzyme that produces nitric oxide from L-arginine. The neural type activates K+ channels, resulting in the hyperpolarization and relaxation of smooth muscles and nerves.
•Myotomes: A group of muscles whose supplied nerve is innervated at specific spinal segments and motor regions (see Table 2.2).
|
Table 2.2 Myotomes |
|
|
Spinal exiting nerve |
Muscle innervation |
|
C1 |
None |
|
C2 |
Longus colli, sternocleidomastoid, rectus capitis |
|
C3 |
Trapezius, splenius capitis |
|
C4 |
Trapezius, levator scapulae |
|
C5 |
Supraspinatus, infraspinatus, deltoid, biceps |
|
C6 |
Biceps, supinator, wrist extensors |
|
C7 |
Triceps, wrist flexors |
|
C8 |
Ulnar deviators, thumb extensors, thumb adductors |
|
T1–T2 |
Minor innervations of intrinsic muscles of the hand, elbow, forearm, shoulder, scapulae, upper back, and neck |
|
T3–T12 |
Innervations of the upper torso, as well as posterior and anterior aspects |
|
L1 |
None |
|
L2 |
Psoas, hip adductors |
|
L3 |
Psoas, quadriceps, thigh atrophy |
|
L4 |
Tibialis anterior, extensor hallucis |
|
L5 |
Extensor hallucis, peroneals, gluteus medius, dorsiflexors, hamstrings and calf atrophy |
|
S1 |
Calf and hamstring, wasting of gluteals, peroneals, plantar flexors |
|
S2 |
Calf and hamstring, wasting of gluteals, plantar flexors |
|
S3 |
None |
|
S4 |
Bladder, rectum |
•Dermatomes: Skin regions whose nerves are innervated at specific spinal segments and sensory regions (see Figure 2.3).
Figure 2.3 Dermatomes
•Viscerotomes: Organs and their spinal segment innervations (see Table 2.3).
|
Table 2.3 Viscerotomes |
|
|
Spinal segment |
Organ |
|
T2–T4 (C3/C4) |
Lung |
|
T1–T5 |
Heart |
|
T6–T10 |
Diaphragm |
|
T6–T10 |
Stomach |
|
T7–T10 |
Spleen |
|
T7–T10 |
Pancreas |
|
T7–T9 |
Liver and gall bladder |
|
T9–T10 |
Small intestines |
|
T11–L1 (to splenic flexure), L1–L2 (splenic flexure to rectum) |
Large intestines |
|
T10–L2 |
Kidneys |
|
T11–L2 |
Bladder |
|
T10–S3 |
Reproductive organs |
|
S2–S4 |
Parasympathetic pathways of genital sex organs |
|
T11–L2 |
Sympathetic pathways of genital sex organs |
|
T10–T12 |
Ovaries |
|
T11–L1 |
Fallopian tubes |
|
T11–T12 |
Uterus |
|
S2–S4 |
Vagina |
|
L2–L1 |
Testicles |
|
T12–L2 |
Prostate |
|
L1–L2 |
Penis |