The Nervous System
4.1a The physiology of the nervous system
This section focuses on the physiology of the nervous system. Together with the endocrine system, the nervous system is one of the topics explored in this text that arguably has great relevance to the understanding of the physical basis of holistic approaches to healing. Together with the endocrine system, the nervous system is involved in the process of communication of one body part with another. Together, these systems form, from a medical perspective, part of the physical foundation of what makes the body an integrated whole.
It is important to point out here that a prevailing and convincing holistic understanding on interconnectedness (exemplified by Myers (2001)1 and Keown (2014)2) is that the myofascial system (comprising the muscles and the body-wide interconnected web of fascial connective tissue that links them with the periosteum of the bones) is another integrated body system that provides the medium for the communication of energy/Qi between disparate body parts. Whilst there is good embryological and anatomical evidence that this fascial basis for energy flow is consistent with acupuncture meridian theory, the medical establishment still doesn’t generally recognize the potentially profound role that fascia and its unique electric properties might have to play in linking the function of diverse body parts. From a medical perspective this role is all down to nervous and endocrine systems.
The complexity of the nervous system is so great that it is very likely that it can never be fully understood by human minds. The number of brain cells alone approximates 10 billion, while the total number of connections made between these cells runs into the trillions. These connections are unique to each individual, not only as a consequence of a particular genetic makeup, but also because these connections continually form and reform in response to the personal experiences of that individual.
In the light of this complexity, even with increasingly complex computer modeling, it makes sense that only the most basic features in this mysterious neural landscape have been understood and described by scientists. The subtleties that form the foundations for many aspects of human experience, such as the diverse emotions and spiritual experiences, have simply not yet been mapped. It is very possible that these experiences, and also some aspects of the subtle phenomena that holistic therapists attribute to the movement of energy described as Qi or Prana, could be explained scientifically if this unmapped part of the nervous system were explored using reductionist experimental methods. However, it may be that such knowledge will always be out of reach of the scientific experiment, being protected within the unique complexity of the structure of each individual’s brain.
For this reason it may well be that the mechanism of the more subtle energetic approaches to healing will never be fully explained by science. For example, there is strong scientific evidence to support the role of acupuncture in the modulation of pain, but little or no evidence to explain the more subtle effects on the body’s function, emotions, and even the destiny of an individual, as described in Chinese medicine theory. However, this does not mean that a physical mechanism for these subtle effects does not exist, but possibly that it is far too delicate, complex and unique to an individual healing encounter for current scientific experiment (and possibly for the human mind) to explain in physical terms.
This section explores only the most basic of the principles of the physiology of the nervous system. Nevertheless, it might be helpful to consider that the structures described here are those parts of the body that regulate those aspects that many complementary therapists might recognize as essential foundations to health, such as the control of homeostasis, thoughts, emotional feelings and even the spirit.
As there is a great deal of material to study, the section is divided into two parts. If you are following this text as part of a study course, you are recommended to work through this section in two separate study sessions. Part I describes the nervous tissue, which is the basic building material for the nervous system. The three parts of the nervous system, the central nervous system, the peripheral nervous system and the autonomic nervous system, are then introduced. Finally, the central nervous system is explored in more detail. In Part II the peripheral nervous system and the autonomic nervous system are described in more detail.
Part I: The physiology of the nervous system3
THE NERVOUS TISSUE
The nerve cells (neurons)
The foundation of the nervous system is the nerve cell, which is scientifically termed the neuron. Nerve cells have the ability to relay messages between each other, and are specialized to perform this role.
The way in which this communication occurs can be compared to an old-fashioned telephone exchange that allows communication to take place between any two households that are connected to that exchange. In a telephone exchange the physical basis of the communication is an electrical current that is conducted from households through telephone wires to the exchange. The telephone acts as the receiver of information, as well as relaying the message from the household at the other end of the line. At the exchange the telephone message from one household can be specifically processed and directed to another household by means of a telephone operating system. The presence of the exchange dramatically reduces the need to have wires linking every home with all the others in the system. Each home needs to be connected only to the exchange, and similarly, each body part needs to be connected only to the central nervous system in order to have a nerve-mediated influence on another body part.
In the nervous system some of the specialized nerve cells are able to receive information from the environment and translate it into an electrical message (akin to the telephone receiver). Nerve cells that receive external information and relay it to the nervous system are known as sensory nerve cells (or effector nerve cells). This term reflects the fact that these nerve cells sense the environment.
Other cells are adapted to act like the telephone wires to the exchange, and relay the message via the spinal cord to other nerve cells or to the brain. The spinal cord and the brain can be seen as the telephone exchange, from which messages originating in the body are redirected to other parts of the nervous system. The nerve cells that link up different parts of the nervous system in this way are sometimes called connector nerve cells.
Within the body’s “telephone exchange” the message is processed and leads to one or more response messages, which are relayed back out to the body. The nerve cells that relay messages back out to the body to bring about a change in the body’s function are called effector nerve cells or motor nerve cells. In this text the term motor nerve cell is used to describe these nerve cells. It may help to remember this term by reflecting that a motor in mechanical terms is a structure that converts energy into action. It is the same for motor nerve cells, because they allow the transformation of the energy of the electrical message that they are carrying into the action of a contraction of a single muscle cell fiber.
A simple example to illustrate this system is the bodily response to an unexpected pin-prick. The message of pain is sensed and relayed by sensory nerve cells to the spinal cord. Messages returning via motor nerve cells from the spinal cord in response to this pain lead to action in the form of a rapid withdrawal of the body from the offending pin. This example describes the most basic form of reflex, which will be explained in more detail presently.
THE PHYSIOLOGY OF THE NERVE CELL
The membrane that surrounds each nerve cell is very sensitive to changes in its environment. External changes to which a nerve cell is sensitive are termed stimuli (singular stimulus). Sensory nerves, depending on their specific role, can respond to a diverse range of external stimuli, such as pressure, light, heat, cold and irritants. All other nerve cells respond to the stimulus of chemicals called neurotransmitters, which are released from the nerve cells that are near to them.
A stimulus will cause the membrane of a nerve cell to change so that an electrical impulse is transmitted to all parts of the cell membrane. The ability to convert an external stimulus to an electrical charge is called irritability. The transmission of the electrical impulse to all parts of the cell is called conductivity. It is these properties of irritability and conductivity that allow a stimulus at one part of the cell to be transmitted, in an electrical form, to all other parts of the cell.
All nerve cells are closely linked to other nerve cells by means of fine-branching extensions of the cell membrane called dendrites (from the Greek meaning little trees).
Figure 4.1a-I illustrates a generalized nerve cell, and shows the long extension of the axon and the smaller tree-like extensions called dendrites. The branching dendrite-like structures at the bottom of the axons have endings called boutons, which are adapted to release tiny quantities of neurotransmitter chemicals from secretory vesicles inside the cell. The axon is an extension that can be very long, and for this reason it can also be termed a nerve fiber. For example, the nerve cells that relay information from the feet have axons or nerve fibers that run from the spinal cord right down to the toes.
When a nerve cell receives a stimulus, the electrical impulse is conducted to the tips of all the dendrites and also down the length of the axon. Some axons are coated in a white substance called myelin. Although this myelin sheath is simply represented in Figure 4.1a-I, the structure of myelin is actually very complex as it consists of a fine layer of fatty substance secreted between layers of a glial cell membrane wrapped many times around the nerve fiber. The glial cell that forms myelin in this way is called the Schwann cell. Myelin enables these cells to transmit an impulse more quickly than those cells that do not have this coating.
Figure 4.1a-I The structure of a typical neuron
The tips of the dendrites make contact with other nerve cells. In the case of motor nerves, the tips of some of the axons make contact with muscles. The way in which dendrites or axons make contact with other nerve cells is shown in Figure 4.1a-II. This illustrates that the branching ends of the axons come very close to the dendrites of another nerve cell. However, they do not actually touch, but instead leave a tiny gap called a synapse or synaptic cleft.
Figure 4.1a-II A synapse
When an electrical charge reaches the branching end of the axon it causes an internal change in the cell, which leads to a release of neurotransmitter from the branching tips of the axon at the terminal boutons. This chemical can diffuse across the synapse, and may act as the stimulus for the nerve cell on the other side of the gap. In some cases the neurotransmitter has the opposite effect and reduces the irritability of the nerve cell on the other side of the synapse. In this case, instead of acting as a stimulus, the neurotransmitter inhibits further stimuli for a brief period. This complex process involving both an electrical charge and a release of chemicals is the basis for communication between nerve cells, and also between a nerve cell and a muscle.
As stated earlier, the number of synapses in the body runs into trillions. Whenever a single cell is stimulated, impulses are transmitted via its axons to a number of other cells. Some of these impulses will lead to the stimulation of a series of cells in connection with the first cell. Of course, all these cells have their own connections with many other cells, and this can lead to a chain reaction of impulses, which can spread through the nervous system. However, there will also be inhibition of other cells, which may in turn lead to another chain reaction of inhibition of other cells in the nervous system.
This series of chain reactions from a single stimulus is how an experience such as a single pin-prick not only causes a withdrawal from the source of pain, but also leads to the felt experience of pain, an emotional response to the pain, an expressed cry of indignation, and the laying down of memory about the incident, among many other possible responses.
Nerve cells are highly active because they constantly manufacture neurotransmitter chemicals. The manufacture of substances within the cell requires basic nutrients and energy in the form of energy-charged adenosine triphosphate (ATP), and takes place within the endoplasmic reticulum and the Golgi body (see Section 1.1b). The neurotransmitters are exported to the outside environment by the process of exocytosis of vesicles, which is one of the methods of active transport described in Section 1.1b. This process also requires energy in the form of ATP.
Because of its intensive energy requirements, the nerve cell is absolutely reliant on a high level of oxygen and glucose to perform this function. A nerve cell that is even temporarily starved of these nutrients cannot function well, and is more likely to die than some other types of cell. Once a nerve cell has died it cannot be replaced, because nerve cells are one of the few types of cells in the body that do not divide. However, although nerve cells cannot be replaced, new connections between remaining cells can be made, as dendrites from nerve cells can grow out and make new contacts with other cells.
Sometimes the loss of nerve cells can be overcome by this mechanism of new growth, although full recovery may take some time. In other cases, the loss of nerve cells leaves a permanent deficit, as occurs in the case of many strokes.
Glial cells (neuroglia)
Glial cells (also known as neuroglia) are the connective tissue cells of the nervous system. They form about one-third of all nervous tissue. These cells act as the scaffolding that holds the nerve cells in place. Unlike nerve cells, glial cells can replicate. For this reason almost all cancers of the nervous system are derived from the glial cells rather than from nerve cells.
One type of glial cell called the astrocyte protects nervous tissue from toxins in the blood by forming a layer around the blood vessels that enter the nervous system. This is called the blood-brain barrier. Some drugs cannot penetrate the blood-brain barrier, a property that has been utilized by drug manufacturers. For example, the newer types of beta blocker drugs are less likely to cause emotional and sleep disturbance because they have been designed not to be able to cross the blood-brain barrier. However, many other drugs, such as alcohol and nicotine, cross this barrier very easily.
Another type of glial cell, called the oligodendrocyte, is responsible for the manufacture of the myelin that enables rapid conduction of electrical impulses down the nerve cell axons.
The last category of glial cell, called the microglia, is derived from the leukocyte. This is similar to the macrophage in ordinary connective tissue, and can phagocytose (see Section 1.1c) unwanted cellular material.
The composition of nervous tissue
The nerve cells and the supporting glial cells are the main components of all parts of the nervous system, from the brain and spinal cord, right down to the thread-like branching nerves that penetrate all the body tissues. The structure and consistency of the different types of nervous tissue depend on the relative proportion of cell bodies and axons (nerve fibers).
The tissue that makes up the brain contains many cell bodies and is very soft in consistency, rather akin to a thick blancmange. This explains why it can be so easily damaged by violent shaking or a blow to the head, and why it requires the rigid casing of the skull for protection. The cell bodies are concentrated in particular areas within different parts of the brain and, when very closely packed, have a grayish color. This is the origin of the term gray matter, which in common usage refers to the part of the brain that does the thinking. The nerve fibers or axons running from these cell bodies to different parts of the brain, or down to the spinal cord, tend to run together rather like all the telephone wires running from one group of houses to the exchange. These pathways of thousands of nerve fibers are called tracts. They have a white appearance because of the white myelin, and are described as the white matter.
In the spinal cord, the tracts of nerve fibers are very prominent. Thousands of nerve fibers run up and down the length of the spinal cord, allowing communication between different parts of the body and the brain. These also appear as white matter when the spinal cord is dissected. However, in the core of the spinal cord are dense clusters of the nerve cell bodies of connector and motor neurons. On dissection these are seen as a central H-shaped area of gray matter. Figure 4.1a-III shows a cross-section of the spinal cord, illustrating the appearance of the white and gray matter. Like the brain, the spinal cord is of a very soft consistency and is also vulnerable to injury.
Figure 4.1a-III also shows the two roots of the nerve fibers that leave this part of the spinal cord. These meet to leave the vertebral column as a single spinal nerve containing thousands of nerve fibers. With the exception of the nerves that supply the head and neck, all the bodily nerves originate from spinal nerves. In this case, the word nerve means thousands of nerve fibers bundled together, rather than a single cell (correctly called nerve cell, nerve fiber or neuron).
Figure 4.1a-III A section of the spinal cord showing the nerve roots on one side
All the nerves that run outside the spinal cord consist entirely of sensory and motor nerve fibers, and have originated from the anterior (containing motor fibers) and posterior (containing sensory fibers) spinal nerve roots (see Figure 4.1a-III). The cell bodies of the motor nerve fibers sit within the gray matter of the spinal cord. However, the cell bodies of the sensory nerve sit outside the spinal cord, mostly within the spinal root ganglion on the sensory nerve root. This is seen as a swelling in Figure 4.1a-III. The spinal root ganglion is, therefore, a little mass of gray matter. (The sensory nerves of the autonomic nervous system have their cell bodies in ganglia situated further away from the spinal cord, as described later.)
THE PARTS OF THE NERVOUS SYSTEM
The nervous system is conventionally described as being composed of three interrelated parts: the central nervous system (CNS), the peripheral nervous system (PNS) and the autonomic nervous system (ANS).
The CNS consists of the brain and spinal cord. It is protected by a thick fibrous coat and sits within the brain and the vertebral column. The brain is further divided on an anatomical basis into the upper brain (cerebral hemispheres), hindbrain (cerebellum) and brainstem (the midbrain, the pons and medulla oblongata).
The largest part of the PNS consists of 31 pairs of spinal nerves, which emerge from the spinal cord to pass through the spaces between each vertebra and through the holes in the sacrum bone known as the sacral foramina. These nerves contain sensory and motor nerve fibers, and divide to penetrate most of the body tissues. The motor nerve fibers of this part of the PNS are primarily concerned with the voluntary control of muscle action.
The PNS also embraces 12 pairs of cranial nerves, which originate not from the spinal cord but from the lower portions of the brain itself. These also contain sensory and motor nerve fibers. Although some of the motor nerve fibers in the cranial nerves are concerned with the voluntary control of muscular action (e.g. the nerve that supplies the masseter muscle, the muscle of chewing), many carry nerve impulses to control involuntary muscle action (e.g. the vagus nerve, which supplies the cardiac muscle).
Those nerves in the PNS that are involved in the involuntary control of body activity are conventionally considered as a separate system, the ANS. Strictly speaking, the ANS is a subdivision of the PNS.
The central nervous system (CNS)
The CNS consists of the brain and the spinal cord, and sits protected within spaces formed within the bony architecture of the brain and the vertebral column. Figure 4.1a-IV illustrates the structure of the main parts of the CNS. Note that this diagram is not drawn to scale; the spinal cord is, in reality, much longer than indicated in this drawing.
Figure 4.1a-IV The parts of the central nervous system
This delicate nervous tissue of brain and spinal cord is protected within the bony housing of the skull and vertebral canal by three layers of tissue, the dura mater, arachnoid mater and the pia mater, which collectively are called the meninges.
The meninges (see Figure 4.1a-V) consist largely of fibrous tissue. The outermost layer, known as the dura mater (dura), is composed of dense white fibrous tissue, the outer layer of which is adherent to the skull, where it forms the periosteum (see Section 4.2a) and the bony margins of the vertebral canal (central canal) right down to the coccyx. In certain regions in the skull the dura splits into two layers, and venous blood courses in the spaces between them in canals called sinuses. An epidural anesthetic is one that is injected carefully in the lumbar region of the spinal cord so that the anesthetic drug collects outside the dura and acts to numb the lower thoracic and lumbar spinal nerve roots as they course down to supply the abdomen and legs.
Figure 4.1a-V The meninges covering the brain and spinal cord
Beneath the layers of the dura is the subdural space. This contains fluid that separates the dura from the next layer, the arachnoid mater, which is also composed of fibrous tissue. The arachnoid mater loosely follows the internal contours of the dura mater. Veins also pass through the subdural space. These can become vulnerable to damage in old age as the substance of the brain shrinks, and there is more possibility of traction on these vessels if the skull is suddenly moved, as can occur in a fall. This is the cause of subdural hemorrhage (see Section 4.1d).
Beneath the arachnoid mater is the subarachnoid space, which separates the arachnoid mater from the deepest meningeal layer, the pia mater. The delicate pia courses along the surface of the brain and the spinal cord, and in some areas there is quite a pronounced space between it and the arachnoid mater. This space is filled with a fluid known as cerebrospinal fluid (CSF). This space is in connection with the CSF, which bathes internal portions of the brain within the chambers known as ventricles. It is in the subarachnoid space that the major arteries that course along the surface of the brain are located, and it is into this space that blood can leak should one of these arteries become damaged. This is the situation of subarachnoid hemorrhage (see Section 4.1d). A lumbar puncture is an investigation that involves inserting a needle into the region of the subarachnoid space as it widens out in the lower lumbar area below the termination of the spinal cord (see Figure 4.1a-V) so that a sample of CSF can be obtained.
Craniosacral therapists work with what they perceive to be the pulsations of the CSF within the meningeal membranes, transmitted throughout the whole body via the myofascial system. Cranial therapy works to influence deep structures by allowing the release of the myofascial system, through very gentle manipulation techniques, and through such manipulation effect healing at physical, emotional and spiritual levels.
The cerebrum is the upper part of the brain (see Figure 4.1a-IV), and is divided into two cerebral hemispheres. These have a convoluted surface, giving this part of the brain a cauliflower-like appearance. Despite this visual comparison, the cerebrum is the most complex part of the brain, and is the part that is most enlarged in human beings compared to other animals.
The cerebrum deals with the most complicated aspects of nervous activity. To use once again the example of the pin-prick, although the reflex withdrawal from the pain need not involve the brain at all, the emotional experience of pain, the cry of indignation and the laying down of memory about the event all involve the nerve cells in the cerebral hemispheres. In keeping with this complexity, the cerebrum contains a lot of gray matter (nerve cell bodies) concentrated on its convoluted surface. This part of the cerebrum is called the cerebral cortex. The functions of particular parts of the cerebral cortex have now been extensively mapped. For instance, the voluntary control of movement of all the body parts can be located onto precise segments of a single ridge of cortex, called the motor area, situated on the side of each cerebral hemisphere. The complexity of such representation is illustrated in Figure 4.1a-VI. The mapping of the motor area and other surface regions of the brain has influenced the location of points used in the modern Chinese development of scalp acupuncture.4
Figure 4.1a-VI The mapping of the motor cortex to bodily function
This diagram illustrates how different strips of the motor cortex have been mapped to regions of muscle function in the body. The distorted humanoid figure is called a homunculus and is drawn to illustrate that some parts of the body (the hand, face and tongue) have disproportionate representation on the motor cortex, in keeping with the complexity of their muscle action.
In order to comprehend the complicated effects of cerebral diseases it helps to understand that, if parts of the cerebrum are lost through injury or disease, the precise function of those parts is also lost. For example, a person who has a small stroke (see Section 4.1d) that damages part of the motor area of the cortex only will suffer an inability to move the corresponding part of their body. In this situation, other functions such as sensation and speech will be unaffected. In contrast, a person who has a stroke that affects the visual cortex only will suffer from visual problems, while movement will be unimpaired.
Tracts of white matter (nerve fibers) descend through the brain from the cerebral cortices (plural of cortex). These connect with other parts of the brain, and also with the spinal cord. The two halves of the upper brain are intimately connected with each other by a wide tract of nerve fibers called the corpus callosum. The tracts of fibers that link the cortices with the rest of the body tend to cross over (decussate) within the brainstem, so that the left side of the brain controls the right side of the body, and vice versa. For this reason the left side of the brain is the dominant hemisphere in a right-handed person.
Deep within the cerebral hemispheres are islands of gray matter called nuclei. These have essential control functions. The largest of these nuclei are the basal ganglia (important for the control of movement), the thalamus (important for the control of sensation) and the hypothalamus (important for the control of the hormones of the pituitary gland and for other aspects of homeostasis such as temperature control).
The main areas and functions of the cerebral cortex are illustrated in Figure 4.1a-VII. The cortices are each anatomically divided into four lobes named after the skull bone that overlies them. Functions can therefore be associated anatomically with lobes; for instance, vision is a function of the occipital lobe. The areas associated with speech production (frontal lobe) and processing of language (temporal lobe) are concentrated on the so-called dominant hemisphere only, which is usually on the left-hand side in right-handed people and the right-hand side in left-handed people. In contrast, the areas responsible for spatial awareness (parietal lobe) tend to be more developed on the non-dominant side of the cerebrum.
Figure 4.1a-VII The lobes and functional regions of the cerebral cortex
The brainstem consists of the midbrain, the pons variolii (the pons) and the medulla oblongata (the medulla) (see Figure 4.1a-IV). These three sections of the brain contain large tracts of white matter that carry nerve fibers between the cerebrum and the spinal cord.
All three of these areas of the brain contain nuclei of nerve cell bodies that have the function of the control of the most basic aspects of maintaining life in a complicated organism. For this reason, the structure of these areas is not too dissimilar to the equivalent parts of the brain in other animals. For example, the control of breathing, heart rate and blood pressure is coordinated by specialized centers of nerve cells in the medulla. Because these functions are all essential for life, these specialized areas of the brainstem are called vital centers. Other centers in this part of the brain control the reflex actions of coughing, vomiting and sneezing.
A specialized area in the brainstem called the reticular activating system (RAS) is responsible for the control of how much sensory information gets through to the cerebral cortex. It is the RAS that controls our level of awareness of the environment, and which allows us to focus on one thing to the exclusion of all others in certain circumstances. It is believed that sleep, a time in which our conscious awareness is very much inhibited, is controlled in part by the RAS.
The cerebellum, or hindbrain, sits at the back of the brainstem. A hand placed over the upper part of the back of the neck and supporting the base of the skull (while covering the area that includes acupoints Feng Fu DU-16 and Feng Chi GB-20) will have the palm overlying the cerebellum.
The cerebellum is another control center of the brain. It is responsible for the smooth coordination of movement and balance. To perform this function it receives information from the muscles, ears and eyes about the position of the body.
It is believed that the cerebellum also holds memory about repeated actions, so that with practice these can be performed smoothly and without conscious effort. For example, it is the cerebellum that allows a person who is driving to perform a complex task such as changing gear and signaling when decelerating towards a junction, even while focusing consciously on a conversation on the radio.
In a disease that damages the cerebellum only, the conscious mind and the vital centers are left intact. A person with such a condition will have no problem with thinking and the physical body will be in generally good health. However, balance will be poor, speech will be clumsy and the simplest tasks will have to be performed slowly and deliberately, with many errors of movement occurring as the task is undertaken.
THE SPINAL CORD
The spinal cord is the long extension of nervous tissue that leaves the brain at the base of the brainstem. It projects through the large hole at the base of the skull called the foramen magnum, and extends down as far as the first or second lumbar vertebrae. The spinal cord is protected as it runs in the vertebral canal (also known as the spinal canal), formed by the ring-shaped vertebrae that sit one on top of the other. The solid vertebral bodies lie in front of the spinal cord, and the spinous and transverse processes project out to the back and the side of the vertebral rings that embrace the cord along its length. A section of the thoracic spinal cord is illustrated in Figure 4.1a-VIII.
Figure 4.1a-VIII Horizontal section through the vertebral column and spinal cord at the level of the thorax
As shown in Figure 4.1a-IX, the 31 pairs of spinal nerves leave the cord at regular intervals. These spinal nerves project out of the meningeal membranes, which protect the cord and leave the vertebral column through spaces between the vertebrae. Figure 4.1a-X illustrates the base of the spinal cord as it looks on a vertical section of the vertebral column. This diagram shows how the cord only goes down as far as the first lumbar vertebra. The labels T1, T7, T12, L1, S1 and S3 point to the solid bodies of three thoracic vertebrae, one lumbar vertebra and two of the sacral vertebrae that are fused to form the sacrum bone. This diagram also shows the spinous processes projecting back from the vertebral canal. These processes are linked by a tough fibrous ligament, not shown in this diagram, which runs from the base of the skull to the sacrum. It is this ligament that is penetrated when a lumbar puncture is performed.
Figure 4.1a-IX shows the paired spinal nerves. In this diagram the nerves are cut off just at the point where they exit the vertebral canal between the vertebrae. This shows how the lower thoracic, lumbar and sacral nerves descend by increasing lengths through the vertebral canal before they actually leave the space between the vertebrae.
Figure 4.1a-IX The spinal cord and spinal nerves
The first to seventh cervical nerves leave above the corresponding cervical vertebra. However, the eighth cervical nerve leaves below the seventh cervical vertebra. The 12 thoracic and five lumbar spinal nerves all exit below the corresponding thoracic and lumbar vertebrae. The sacral and coccygeal nerves all leave through gaps in the fused sacrum and bones of the coccyx.
With this information it is possible to work out which spinal nerve emerges at a particular spinal level. For example, the level L2/L3, the level at which the acupoints Ming Men DU-4 or Shen Shu BL-23 are situated, is also the level of the emergence of the second lumbar spinal nerve from the vertebral column.
Figure 4.1a-X Section of the distal end of the vertebral canal
This diagram also shows horizontal sections of the spinal column at four levels. These illustrate how the spinal cord terminates at the level of L1/L2 and distal to this only nerve roots travel in the spinal canal.
The spinal cord consists of tracts of white matter containing vertically ascending and descending nerve fibers, and a core of gray matter containing cell bodies. This core is approximately H-shaped when seen in horizontal section (see Figure 4.1a-VIII). Some of the nerve fibers in the white matter of the spinal cord travel to and from the brain, whereas others simply link different levels in the spinal cord.
The spinal cord receives sensory information from most parts of the body below the head and the neck through sensory nerve cells that enter the cord through the sensory root of the spinal nerves. The muscle action within the body is controlled by information carried by motor nerve cells that leave the spinal cord at the motor root of the spinal nerves (see Figures 4.1a-III and 4.1a-VIII).
The gray matter in the spinal cord can be considered as having a similar function to the nuclei of the brain. Some basic aspects of muscle movement are controlled by the clusters of nerve cells in the spinal cord alone. The protective reflex arc and spinal tone are two such aspects that function adequately without any input from the nerve fibers of the brain.
THE PROTECTIVE REFLEX ARC
The protective reflex of withdrawal from pain is one example of a movement that is controlled by the spinal cord alone. Once again the example of the response to a pin-prick of the finger is a useful illustration. In this case, the pain of a pin-prick leads to impulses in the sensory nerves that supply the finger. These travel up the arm to where the nerves enter the spinal cord at the thoracic level of T6/T7. Here the sensory nerve cells make connections with a number of motor nerve cells in the spinal cord by means of connector nerve cells. These motor nerve cells send impulses down to various muscles in the arm and shoulder to cause a withdrawal of the arm from the pain. In such a reflex, the brain is not involved. A diagrammatic representation of a simple reflex arc is given in Figure 4.1a-XI. In this case the diagram illustrates the “knee jerk” reflex that is an involuntary response to the stretch of the patellar tendon.
Figure 4.1a-XI A simple reflex arc
The spinal cord is responsible for maintaining a constant level of contraction, or tone, in the body muscles. This is essential as a background on which upright posture and smooth body movements can occur. The existence of this tone is clear in the situation when all connection between the limbs and the brain is lost, as can occur in an injury of the spinal cord. In this situation, the muscles do not just go limp and useless. Instead, they manifest a sometimes intense level of this sustained contraction, a result of impulses from motor nerves originating in the spinal cord. This tone tends to cause the lower limbs to extend out straight, and the upper limbs to flex so that a bend is held at the elbow. This limb position is characteristic of patients who have suffered a spinal injury at a high level such as at the neck. In such patients the involuntary muscular contraction is often over-forceful, and over time the muscles become tight and contracted.
The reason why this excessive level of tone is not experienced in a healthy person is that nerve impulses originating from the brain are transmitted down the spinal cord to partially inhibit the spinal motor nerves, and to reduce the muscle tone to a level that enables upright posture and smooth movements.
Motor nerve cells that originate in the spinal cord are called lower motor neurons. Motor nerve cells that originate in the brain and descend to control the function of lower motor nerves are called upper motor neurons. This distinction is important to make, as it will help in understanding the underlying problems in the severe diseases that cause impaired muscular movement, such as multiple sclerosis, motor neuron disease and stroke.
In a simple reflex arc and the production of tone after a spinal injury it is the lower motor nerves only that are responsible for the muscle contraction. However, in most other movements the action of the lower motor nerve cells has been stimulated or inhibited by the impulses from upper motor nerves originating from the brain.
Part II: The physiology of the nervous system continued
The peripheral nervous system (PNS)
The peripheral nervous system (PNS) consists of all the parts of the nervous system that exist outside the protective skeletal casing of the skull and the vertebral column. It comprises the 31 pairs of spinal nerves and the 12 pairs of cranial nerves. The autonomic nervous system (ANS) is the part of the PNS that is concerned with the involuntary control of the deep organs, blood vessels and glands, and thus it is essential for the maintenance of homeostasis.
THE SPINAL NERVES
The structure of the spinal cord was described at the end of Part I. The 31 pairs of spinal nerves, which each leave the cord at regular intervals (see Figure 4.1a-IX), divide to form the largest part of the PNS.
All the spinal nerves are mixed nerves in that they carry both motor and sensory nerve fibers to all the parts of the body that they supply. In the spinal nerves, the motor nerve fibers are largely responsible for the voluntary movement of the skeletal muscles (this includes reflex actions such as the withdrawal of parts of the body from painful stimuli). However, some of the motor fibers divide away from the spinal nerves close to their exit from the vertebral column to control the contraction of the smooth and cardiac muscle of the blood vessels, deep organs and glands. These functions are mainly not under the control of the will. These involuntary motor nerves are considered to be part of the ANS.
The first pair of spinal nerves (the first cervical nerves) passes out of the vertebral column between the skull and the first cervical vertebra. All the other cervical nerves, the thoracic nerves and the first four lumbar nerves pass out between the vertebrae that make up the vertebral column. The fifth lumbar nerve exits between the fifth lumbar vertebra and the sacrum. The sacral nerves leave via holes (foramina) within the sacrum, and the coccygeal nerves leave at the level of the fusion of the bones of the sacrum and the coccyx. The names of the spinal nerves are usually abbreviated according to the associated spinal level. For example, the third cervical nerve is called C3, and the fifth lumbar nerve is called L5.
After its exit from the vertebral column, each spinal nerve divides many times to form all the nerves that together carry motor and sensory nerve fibers to most parts of the body below the head and the neck.
THE CRANIAL NERVES
In addition to the spinal nerves, 12 pairs of nerves (the cranial nerves) leave the base of the brain to supply the tissues of the head and the neck. In addition, the tenth cranial nerves (the vagus nerves) each divide to form nerve branches, which extend deep into the thorax and abdomen to supply the deep organs within. The optic nerves, which supply the eye, and the auditory nerves, which supply the ear, are two other examples of cranial nerves.
The cranial nerves are named either by a name derived from a Latin root or by a number (abbreviated as the Roman numeral). For example, the optic nerve is also known as the second cranial nerve (II), and the vagus nerve is also known as the tenth cranial nerve (X).
Unlike the spinal nerves, not all the cranial nerves are mixed nerves. Three of the pairs of cranial nerves carry only sensory fibers (e.g. the optic nerve, which leads from the retina of the eye to the brain). Five pairs carry only motor nerve fibers (e.g. the oculomotor nerves, which supply some of the muscles that move the eye). The remaining four pairs of cranial nerves are mixed nerves. Like the spinal nerves, these four pairs of mixed cranial nerves also carry motor nerves, which are responsible for involuntary action of the muscles that control the functions of the deep organs and glands.
The cranial nerves pass out from the bony skull through holes in or between the cranial bones (e.g. through the hole at the back of the orbit, the cavity that cradles the eyeball).
In this section on the PNS, only those aspects of the cranial nerves that are responsible for sensation and the voluntary control of the skeletal muscles are considered. Those nerve fibers that are responsible for the involuntary control of smooth muscle and cardiac muscle in the blood vessels, deep organs and glands are generally considered as forming a distinct system, the ANS. This system is discussed at the end of this section.
THE DIVISION OF THE SPINAL AND CRANIAL NERVES
Each spinal and cranial nerve branches in a tree-like fashion to supply a particular portion of the body. In the cervical, lumbar and sacral regions, the adjacent spinal nerves intermingle with each other close to the spinal cord before they separate out again into a number of nerve branches that then supply the tissues of the neck, arm, leg and genital areas. This intermingling produces a net-like web of nerves called a plexus. Figure 4.1a-XII illustrates the location of the major nerve plexuses.
Figure 4.1a-XII Diagrammatic representation of the level of the five major nerve plexuses
The major nerve branches that emerge from the plexuses are given descriptive names derived from Latin. For example, the radial nerve is the nerve in the arm that runs close to the radius bone, and the peroneal nerve describes the nerve in the leg that runs close to the peroneal muscles.
The intermingling of fibers in the plexuses is important to understand because the nerve branches that emerge from each plexus contain nerve fibers that originate from more than one spinal nerve. For example, the sciatic nerve, which emerges from the buttock to descend down the leg, carries fibers that originate from five spinal nerves (L4, L5, S1, S2 and S3). This may seem to be a rather esoteric point, but is very relevant to an understanding of the physical effect of acupuncture and other manipulative therapies.
The parts of the body that are supplied by each spinal and cranial nerve are clearly understood. The areas of skin and tissue supplied by the sensory nerves of each spinal nerve have been mapped, and are much the same for each person. These areas are called dermatomes, and are illustrated in Figure 4.1a-XIII. Dermatomes represent those strips of skin and areas of the body that are supplied by the individual spinal and cranial nerves. Although the diagram suggests that there is a clear demarcation between dermatomes, in reality this is not the case, with there being considerable overlap of the regions supplied by adjacent spinal nerves. Nevertheless, these diagrams can be very useful for health practitioners because they indicate which spinal nerve supplies the nerves to whatever body part is a source of symptoms or a focus for treatment. Some dermatomal diagrams also indicate which nerve branch (e.g. the radial nerve or the sciatic nerve) supplies a particular area. Figures 4.1a-XIV and 4.1a-XV show examples of such diagrams for the arm and the leg, respectively.
Figure 4.1a-XIII Dermatomes of the spinal roots and the three divisions of the fifth (V) cranial nerve on the face (the trigeminal nerve)
Figure 4.1a-XIV Dermatomes of the nerves that supply the arms
Although the aim of dermatomal diagrams is to illustrate the distribution of the sensory nerves, they can also give approximate information about the motor nerves that supply the muscles in a particular region. For example, the muscles that control plantar flexion of the wrist and gripping of the fingers are largely supplied by motor nerves that originate from the C6, C7, C8, T1 spinal nerves. Just as they direct the sensory nerve fibers from these spinal nerves away from the area, the ulnar and median nerves and their branches direct these motor nerve fibers to this part of the lower arm.
Figure 4.1a-XV Dermatomes of the nerves that supply the legs
The dermatomes: comments from a Chinese medicine perspective
The discussion about referred pain illustrates recognition in conventional medicine terms that distant parts of the body are linked because they are associated with the same spinal levels. For example, there is a link in the nervous system between the heart and the front of the arm and the neck. In this example, the spinal levels of C3 through to T6 are linked, probably by connector nerve cells in the spinal cord.
In the examples of referred pain, the site of the pain from these deep organs can relate to the position of channels and points that have a close connection with the treatment of these conditions from the Chinese medicine viewpoint.
The referred pain in a heart attack is felt not only in the center of the chest, but down the arm in the area of the Pericardium and Heart Channels, and up the neck, through which the deep pathway of the Heart Channel runs. The referred pain of cholecystitis is felt at the shoulder, over which the Gallbladder Channel passes. The referred pain of the ovary is felt on the front and inner aspect of the thigh, over which pass the Stomach, Spleen and Liver Channels. In all three conditions, points can be chosen from these Channels to relieve the symptoms.
It is now scientifically accepted that acupuncture has a measurable analgesic effect mediated by impulses traveling up to the spinal cord by sensory nerves. Acupuncture stimulation appears to modulate nervous activity in the spinal cord at the level of the stimulated spinal nerve, which then results in inhibition of pain-carrying nerve fibers associated with the same spinal level. Impulses also travel up from the affected spinal cord level to the regions of the midbrain that deal in the general modulation of pain. This supports the observation that acupuncture can have both a local and general effect on the experience of pain. The neurotransmitters B endorphin and metenkephalin are recognized as two chemical mediators that are instrumental in these acupuncture effects.
The non-analgesic effects of acupuncture are less well explained scientifically. These include improvements in wound healing, nausea reduction, treatment of addiction, improved organ function, stimulation of the immune system and enhancement of mood. However, given the complex interconnections of the nervous and endocrine systems, it is feasible that acupuncture-mediated stimulation of nerves at a particular spinal level may have diverse ramifications both at a neural and the endocrine level.
Keown (2014),5 however, persuasively argues that acupuncture meridians seem to follow anatomical pathways formed between planes between the connective tissue that divides the various anatomically and embryologically distinct body parts. Fascia is made up from collagen that is a protein with unique piezoelectric qualities. This means that electrical charge is created when its structure is deformed. It is possible that the acupuncture needle stimulates not only the nervous system but also this anatomically hidden system of fascial pathways, and so leads to energetic change in diverse ways.