Anatomy, embryology and neurophysiology

Chapter 2 Anatomy, embryology and neurophysiology



Pierre Rabischong



CHAPTER CONTENTS


General principles 15
Men did not build men 15

Embryological development 16

The nervous system 16

Transmission and processing of neural signals 16

Morphogenesis of the head 18

The neurophysiological basis of auriculotherapy 21
The identification of the acupuncture point 22

Neurophysiology of pain 27

Conclusions 34


GENERAL PRINCIPLES



MEN DID NOT BUILD MEN


We have the solution and not the problem.


Humans are the most recent species to appear on Earth and the only one able to understand the great complexity of all biological systems. The comprehensive approach that we advanced many years ago presupposes that the technical specifications of organs and functions correspond to a logic and intelligent program of construction. Therefore it is necessary to begin observation of a function by identifying the technical requirements necessary to achieve it, like an engineer investigating a machine that he did not design. After that, moving to the anatomical solution allows us to validate it and to appreciate the choice of biocomponents and the organization of command and control systems. In other words, this approach, going from function to morphology, can be compared to the inverse dynamics used in mechanics. In fact being obliged, after collecting scientific data from the observation of a biological problem, to formulate an explanatory theory, it is very important for us to keep in mind that a logical plan of construction exists and that this is no place for fantasy or personal feelings. At the present time we have measurement technology available for evaluating any kind of biological, physical or chemical phenomenon. In the clinical field, however, the situation can be a little bit different: sometimes a good clinical result indicated by a patient and partially validated by a physician may generate an interpretation lacking in precise scientific proof. The empirical practice of medicine is full of such interpretations which do not change the results but which can form the basis for a more or less convincing education program. Nevertheless, a hypothesis remains a hypothesis until it has been scientifically validated. This rigorous approach is mandatory when we are examining a clinical therapy such as auriculotherapy. We will try later to integrate the clinical data obtained for this technique within an acceptable functional scientific framework, leaving some doors open for future investigation.



EMBRYOLOGICAL DEVELOPMENT


The development of the embryo is guided by very precise instructions contained in the genetic code, with a specific reproducible timetable for every species. Starting from a small group of polyvalent cells, each organ and function is built and interconnected. The crucial role of the nervous system as control integrator explains its early identification in the ectoderm of the embryo as neural plate, neural groove and then neural tube induced by the notochord which also organizes the construction with the metameric segmented mesoblastic components of the spine and muscles. This initial overdevelopment of the neural tube is responsible, in all species, for the spiral shape of the embryo, rolled up around the vascular system, with the heart pump in the center pulsating very early in order to transport the required building and trophic material to all parts. At the end of the third month, each organ and function is in place. This complex growth program is perfectly regulated in its timing and everything is designed to minimize the influence of external factors, called epigenetic factors, by providing a physical and biochemical protective environment within the uterus, first for the embryo and later for the fetus. Some mistakes do occur, causing, fortunately, a low percentage of malformations.



THE NERVOUS SYSTEM


The nervous system is in general more complex than was thought. The more we know about it, the more we can appreciate its remarkable organization which can explain all biological and physiological phenomena. The incredible miniaturization of the components of neurons and the enormous capacity of their synaptic connections creates a very powerful, flexible and plastic neural network. It is no longer possible to discover new morphological structures within the brain, but the extremely complicated multimedia interconnections that we can increasingly better identify through revolutionary modern methods of imaging are still to be fully understood. A good example is the acupuncture point, which physically and structurally exists without any doubt, but which is difficult to integrate logically in the plan of human construction. The maps we make of neural pathways and centers, in our common tendency to represent them, are in fact very far from being a true representation of nervous organization; in reality it is increasingly difficult to envisage a ‘holistic conception’ of the nervous system. However, some very precise laws of functioning have been defined and accepted. This means that we cannot speculate on the role of a particular nervous structure without precise scientific arguments and evidence. This is, for example, the case with the reticular formation: for many teachers this is developing into a magical key but it lacks irrefutable neuroscientific proof for its use. Two other important points need to be considered: neural signal processing and the relationship between the cerebrospinal system and the vegetative or autonomic system.



TRANSMISSION AND PROCESSING OF NEURAL SIGNALS


Transmission and processing of neural signals occur in two conditions: first there is a direct nervous conduction along the fibers by a depolarization wave traveling from one node of Ranvier to another at a velocity that depends on the diameter of the myelin sheath. The maximum speed is in the order of 120 m/s corresponding to the largest sensory fibers of 25 μ diameter. Second, this neural conduction, neuron to neuron, is also regulated by the synaptic doors using neurotransmitters as keys for opening them and passing through. This technical originality, introducing a chemical code to facilitate or inhibit a signal, explains the clinical importance of neurochemistry as crucial to the understanding of nervous function. In addition there are some specific sites within the central nervous system responsible for the secretion of specific neurotransmitters, such as the monoaminergic centers (noradrenergic, serotoninergic and dopaminergic) in the brainstem and the cholinergic centers. The neuromodulation produced by all these neurotransmitters uses, like the endocrine glands, the circulation of the blood and also the cerebrospinal fluid. The action can be slower than direct nervous conduction and in most cases is more prolonged in time.


The cerebrospinal system has two parts: a central part located in the encephalon (brain, brainstem and cerebellum) and in the spinal cord, where in the gray matter the sensory inputs are separated from the motor outputs, and a peripheral part grouping all the sensory and motor nerves.


Regarding the general organization of the sensory inputs, it is important to remember that the different sensory fibers coming from different types of receptors with their different calibers are the dendritic expansions of the first neuron of the sensory pathway located in the spinal or cranial ganglion outside the central nervous system. There are more skin receptors than sensory fibers conveying the signal, which is the expression of a peripheral sensory convergence still not fully understood: is it the same type of receptor on the same fiber or different types? In addition, in their distribution, spinal and cranial nerves have a cutaneous and muscular territory and also, by sympathetic or parasympathetic fibers, a visceral territory, the main conscious expression of which is logically manifested on the skin by projected pain, according to its poor representation within the conscious somatosensory cortex. This central neural interference mechanism between different kinds of input has to be explained by the organization of the spinal and thalamic relays described later. This is therefore the neurophysiological justification for the dermatological metameric reflexes with an important sympathetic component provoked by pinching the skin in relation to visceral dysfunction, described after Head1 by Jarricot,2,3 which also exists at the level of the auricle. In addition, reflex activity needs to be defined from the physiological point of view. It is the result of the conjunction between a stimulus and a reaction.


A reflex can be monosynaptic, like the myotatic reflex due to the link of a Ia sensory fiber from a muscular spindle with an alpha motoneuron of the anterior horn of the spinal cord generating a muscular contraction, or polysynaptic, with a delayed reaction like the spinal withdrawal reflex of lower limbs after painful pinching of the skin, or the contraction of the iris after light stimulation. It can also concern neural multicentric activity such as general vegetative reactions (tachycardia, skin pallor, etc.) after mental induction (fright, anxiety, stress, etc.) or, particularly, conditioning situations as demonstrated by Pavlov. In this context, it is possible to talk of reflexotherapy if certain well-defined stimuli result in pain relief or restore normal functioning. Neural architecture is composed of different interconnected levels creating a complex heterarchic control system which finally operates at the cortical level in conscious and unconscious mode. The commonly poor knowledge of the pilot of the human machine in biology or bioengineering was provided in the program, allowing him to decide and command difficult tasks executed by unconscious processes within the very complex and rich neural network. This can explain why, in order to understand any physiological problem, it is not enough to refer to the conscious part of nerve function, which represents only a small part of it. In fact, in order to have a clear view of a function it is mandatory to try to perceive all the parameters and interconnections acting in a completely unconscious manner.


The vegetative or autonomic system has two polarities controlled by the sympathetic and parasympathetic nervous systems. Their architecture is the same: first, specific centers within the central nervous system (brainstem and sacral spinal cord for the parasympathetic and C8/L2 spinal cord level in the intermediate gray matter for the sympathetic); second, ganglions connected by preganglionic myelinated fibers (white communicating ramus) with the centers and supplying vegetative organs by postganglionic unmyelinated fibers (gray communicating ramus). Those fibers are small in diameter (in the order of 3–6 μ) and therefore have a low conduction velocity. The differences between sympathetic and parasympathetic systems lie in the organization of the ganglions.


In the sympathetic system, the ganglions are located in a long interconnected chain on both sides of the spine with some plexus grouping of fibers along arterial vessels before distribution: cardiac, pulmonary, esophageal, coeliac, superior and inferior mesenteric, hypogastric plexuses. The superior cervical ganglion supplies the encephalic territory by fibers traveling along the vertebrobasilar artery and the internal and external carotid arteries.


In the parasympathetic system there is no chain but isolated ganglions appended commonly to the nerve, resulting in the innervation of a territory, as for the three branches of the trigeminal nerve with the ciliary, sphenopalatine and otic parasympathetic ganglions having their centers close to the IIIrd, VIIth and IXth cranial nerve (CN) nuclei.


The domain includes all the visceral organs located within the trunk, all the digestive glands and all the arterial vessels equipped with a contractile system allowing regulation of the blood flow. The best indicator of the equilibrium between the two opposite components of the vegetative nervous system is the iris in the anterior chamber of the eye, which has a smooth sphincter innervated by the parasympathetic system (myosis) and a dilatator radial muscle innervated by the sympathetic system (mydriasis). The intercommunication between the two big systems is made at the highest level in the brain, mainly by the hypothalamus. This center, which represents only 4 g of neural substance, plays a powerful role in the control of specific functions such as hunger, thirst, temperature, aggression, sexual behavior and the whole of the endocrine system through its rich vascular and nervous connections with the hypophysis (pituitary gland). Its influence on the cardiovascular and digestive systems and its connection with the central nucleus of the amygdala make it an important component of the limbic system, particularly the part concerning emotional expression. Finally, it will be necessary to associate these two cerebrospinal and vegetative systems when looking for the particular physiology of the ear pavilion. The vascular reaction perceived in the pulsations of the radial artery after its manipulation corresponds to a non-specific vegetative vascular reflex existing in all cutaneous territories.



MORPHOGENESIS OF THE HEAD


At the initial embryonic stage, three tubes are aligned longitudinally: the neural tube, the notochord and the primitive digestive tube closed at both its caudal and cranial extremities. Two major developments will occur, drastically changing the morphology. First of all, the rapid overgrowth of the neural tube, particularly in its cranial part, will generate the five encephalic vesicles with the cephalic and pontine flexures after closing the neural tube like a ‘zipper model’ during primary neurulation (2–4 weeks) and secondary neurulation (4–6 weeks). Isolated on its lateral border is the neural crest, responsible for the spinal and cranial sensory ganglions as well as postganglionic fibers, glial cells and the adrenal medulla. The construction of the primitive skull base has two parts: first, the posterior part built around the notochord, which plays a role in the genesis of the vertebral bodies, ending in the clivus joining the occipital bone process and a part of the sphenoid at the level of the sella turcica; second, the anterior prechordal part built around the olfactory ethmoid bone and the orbital neurocranium with a typical human skull base angle (kyphotic skull base as opposed to the lordotic skull base of carnivores). The calvarium, the second part of the neurocranium, becomes osseous by a different osteogenesis than the cartilaginous matrix of the skull base, made by direct apposition of bone on the continuous dural membrane. Therefore the growth of the telencephalon is the ‘motor’ of the expansion of the skull vault which is possible because the cranial sutures are growth lines. The telencephalon initially has a smooth aspect and progressively makes folds called gyri to increase the surface of the cortex. When it finishes its growth, the sutures close, starting from the endocranial side. The anencephaly which is the malformative lack of telencephalon means the absence of a skull vault. The hydrocephalic deformation in young children with anomaly of the circulation of cerebrospinal fluid demonstrates the importance of intracranial pressure for the expansion of the skull. The suture drawing is linear on the endocranial side and sinuous on the exocranial side, due to the alternating tractions of the muscles during growth. However, after a certain age it is absolutely impossible to move the bones, and the explanation for the clinical success of cranial osteopathy has to be found in manipulation not of cranial bones but of the skin covering the skull.


The second major growth process takes place around the primitive intestinal digestive tube which will first be actively open, creating the primitive mouth at the cranial extremity, later dividing into nasal cavity and oral cavity, and the anal canal and its opening at the caudal extremity. The ectoblast and mesoblast of the area will generate some folds around the digestive tube that we must call ‘visceral arches’ and not branchial arches. In fact, this denomination originated in Ernst Haeckel’s theory of recapitulation, which said the human embryo follows different stages as a phylogenetic recapitulation, which is nonsense. The four visceral arches, contributing by their growth to the setting up of the embryonic face, correspond to the segmentation of the mesoblast with ectodermal and endodermal fissures.


The important point is to remember that every visceral arch will have a skin cover from ectoblast, skeleton and muscles from mesoblast building the splanchnocranium, and a specific cranial nerve from brainstem having motor, sensory and vegetative fibers innervating the territory (Fig. 2.1). Inside, the separation between the respiratory and digestive visceral tubes organizes the mandatory aero-digestive crossroads within the pharynx, allowing first the larynx to inject sounds into the mouth to be ‘masticated’ as phonemes and second, food to be injected into the esophagus without entering the respiratory tract, explaining the crucial role of the soft palate. The appropriate nomenclature is as follows:


• First visceral arch: ‘mandibular arch’, later developing the superior maxillary bone, the malar bone, the jaw with the temporomandibular joint (TMJ) and all the masticatory muscles (temporal, masseter, lateral and medial pterygoid, tensor tympani and tensor veli, the anterior part of the digastric, mylohyoid). The cranial nerve here is the trigeminal nerve (CN V) with its motor root and sensory root originating from the trigeminal ganglion with its three branches (ophthalmic, maxillary, mandibular) serving the three areas of the face (Plates IA, IB). A parasympathetic ganglion is appended to each trigeminal branch. The orbits contain the eyeballs which are diencephalic nervous expansions and have a roof of neural origin and a floor of visceral origin.

• Second visceral arch: ‘hyoid arch’. This refers to the upper half of the hyoid bone with the muscles coming from the mandible (geniohyoid, mylohyoid) and the styloid process (stylohyoid, digastric). The cranial nerve which serves this area is the facial nerve (CN VII) with its motor root innervating all the muscles of facial expression, the stylohyoid, the posterior part of the digastric muscle and the stapedian muscle. The facial nerve during its trajectory on the face is a pure motor nerve because the facial muscles responsible for expression and language have no proprioceptive receptors and the goniometric control needed for movement coordination is given by CN V innervating the skin in which the muscles are included. The sensitive root of CN VII has a ganglion known as the geniculate ganglion from its location on the flexure of the facial nerve within the ear. It serves the anterior two-thirds of the tongue and a part of the auricle and external auditory meatus. The intermediate nerve (CN VII bis) or Wrisberg nerve should not be further mentioned as such, because it is really the sensory root of CN VII and not a correction of a mistake in counting the cranial nerves. The vegetative centers related to salivary glands (submaxillary and sublingual) are in the part of the brainstem related to the facial nucleus, but the distribution of the postganglionic fibers is made by CN V.

• Third visceral arch: ‘hyothyroid arch’. This concerns the lower half of the hyoid bone and the upper part of the thyroid cartilage with the stylopharyngeal muscle, the upper and middle pharynx constrictor muscles and the levator veli. The cranial nerve in this area is the glossopharyngeal nerve (CN IX) with motor and sensory root for the posterior third of the lingual mucosa, the upper part of the pharynx and middle ear mucosa and vegetative salivary center for the parotid gland. However, the extrinsic laryngeal muscles (thyrohyoid, sternothyroid, sternocleidohyoid, omohyoid) are innervated by branches of the cervical plexus (C2, C3) which use the hypoglossal nerve (CN XII) as a tutor to make their distribution by forming a loop.

• Fourth visceral arch: ‘thyrocricoid arch’. This concerns the lower part of the thyroid cartilage and cricoid cartilage with all the intrinsic laryngeal muscles (thyroarytenoid as vocal cord, posterior and lateral cricoarytenoid, interarytenoid). The cranial nerve here is the vagus nerve (CN X) which has three unequal territories. First, it has a sensory cranial distribution corresponding to its two sensory ganglions (upper jugular and lower plexiform) for mucosa of the pharynx, larynx by the superior laryngeal nerve, tuba auditiva and part of the external auditory meatus. Second, it supplies all the intrinsic laryngeal muscles by the recurrent inferior laryngeal nerve corresponding to the motor root of CN X visible on the lower part of the lateral aspect of the myelencephalon. This nerve has a recurrent trajectory to reach the lower part of the larynx, which represents the anatomical limit of the visceral portion of the head and neck. Third, CN X is the most important parasympathetic nerve, conveying vegetative fibers to all viscera of the trunk as far as the pelvis.
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Jul 18, 2016 | Posted by in MANUAL THERAPIST | Comments Off on Anatomy, embryology and neurophysiology

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