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 generalised 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 fibre’. For example, the nerve cells that relay information from the feet have axons or nerve fibres that run from the spinal cord right down to the toes.

Figure 4.1a I • The structure of a typical neuron.

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’. This myelin sheath is simply represented in Figure 4.1a-I, but the structure of myelin is complex as it is actually a fine layer of fatty substance secreted between layers of a glial cell membrane wrapped many times around the nerve fibre. 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.

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. In can be seen from the figures that the branching ends of the axons come very close to the dendrites of another nerve cell. However, they do not actually touch, but 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 (see Q.4.1a-1 and Q4.1a-2).

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 Chapter 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 Chapter 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.

The meninges

The meninges (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 Chapter 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 anaesthetic is one that is injected carefully in the lumbar region of the spinal cord so that the anaesthetic drug collects outside the dura and so 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 the spinal cord.

Beneath the layers of the dura is the subdural space. This contains fluid which 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 haemorrhage (see Chapter 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 which 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 haemorrhage (see Chapter 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 4.1-V) so that a sample of CSF can be obtained (see Q4.1a-3-Q4.1a-6).

Cranial therapists claim that they can detect the pulsations of the CSF within the meningeal membranes. These pulsations are proposed to be 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

The cerebrum is the upper part of the brain (see Figure 4.1-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 with a lowly vegetable, 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 grey 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.

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 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 Chapter 4.1d) that damages a 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 fibres) 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 fibres called the ‘corpus callosum’. The tracts of fibres 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 grey 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 (see Q4.1a-7 and Q4.1a-8).

Figure 4.1a-VII • The lobes and functional regions of the cerebral cortex.

The brainstem

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 which carry nerve fibres between the cerebrum and the spinal cord.

All three of these areas of the brain contain nuclei of nerve cell bodies which 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 specialised centres of nerve cells in the medulla. Because these functions are all essential for life, these specialised areas of the brainstem are called ‘vital centres’. Other centres in this part of the brain control the reflex actions of coughing, vomiting and sneezing.

A specialised 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 (see Q4.1a-9).

The cerebellum

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 Fengfu Du-16 and Fengchi GB-20) will have the palm overlying the cerebellum.

The cerebellum is another control centre 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 signalling 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 centres 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 most simple 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 ‘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 it 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 which 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 • The spinal cord and spinal nerves.

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.

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.

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 Mingmen Du-4 or Shenshu Bl-23 are situated, is also the level of the emergence of the second lumbar spinal nerve from the vertebral column.

The spinal cord consists of tracts of white matter containing vertically ascending and descending nerve fibres, and a core of grey matter containing cell bodies. This core is approximately H-shaped when seen in horizontal section (see Figure 4.1a-VIII). Some of the nerve fibres 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 grey 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 fibres 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 which is an involuntary response to the stretch of the patellar tendon.

Figure 4.1a-XI • A simple reflex arc.

Spinal tone

The spinal cord is responsible for maintaining a constant level of contraction, or tone, in the body muscles. This is essential as a background upon 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. In this situation 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 (see Q4.1a-10).

The spinal nerves

The structure of the spinal cord was described at the end of Part I of this chapter. The 31 pairs of spinal nerves, which 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 fibres to all the parts of the body that they supply. In the spinal nerves, the motor nerve fibres 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 fibres 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 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 fibres 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 fibres (e.g. the optic nerve, which leads from the retina of the eye to the brain). Five pairs carry only motor nerve fibres (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 which cradles the eyeball).

In these paragraphs 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 fibres 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 in the last part of this chapter.

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 forms 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 fibres in the plexuses is important to understand because the nerve branches that emerge from each plexus contain nerve fibres that originate from more than one spinal nerve. For example, the sciatic nerve, which emerges from the buttock to descend down the leg, carries fibres 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.

Dermatomes

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, 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 (see Q4.1a-11).

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.

Figure 4.1a-XV • Dermatomes of the nerves that supply the legs.

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 fibres from these spinal nerves away from the area, the ulnar and median nerves and their branches direct these motor nerve fibres to this part of the lower arm.

Referred pain

Referred pain is the term used to describe the situation in which pain originating in one part of the body is experienced elsewhere. In general, referred pain is felt either as a cutaneous or muscular sensation, when in fact the pathology is in a deeper part of the body. In sciatica, the problem usually lies either in the structure of the lumbar and sacral bones at the base of the back, where there can be bony or muscular impingement on the L5–S3 nerve roots, or in the proximal region of the sciatic nerve itself as it emerges from the sacral plexus and enters the buttock muscles. However, sciatic pain is experienced at the back of the leg and the foot. The pain is experienced in the leg and foot because the brain misinterprets the origin of the irritation as coming from the distal ends of the spinal nerve branches that terminate in the dermatome, rather than the correct site, which is far more proximally located.

Another cause of referred pain is the pain that originates from the deep organs. Compared to the skin and the skeletal muscles, the deeper body parts do not have a very full sensory nerve supply. For example, there is not a general awareness of how the spleen feels, or of the opening of our heart valves. If a deep organ is in distress, the pain from the organ and its surrounding tissues results in sensory messages that travel to the spinal cord through the spinal nerves which supply nerve fibres to that organ. However, the brain interprets these messages as coming from the nerves that supply its particular dermatome. The organs are connected by nerves often to a number of spinal levels, which means that deep organ pain is often vague and difficult for the patient to localise. In some cases, the dermatome overlies the position of the organ, but in others the pain is felt in a distant site. For example, gallbladder pain may be referred to the shoulder tip, the region of the C4 dermatome, because fibres from the C4 spinal nerve supply the diaphragmatic region as well as the shoulder (see Q4.1a-12).

The sympathetic nervous system

It might be said that the SNS acts to promote the more Yang aspects of the maintenance of homeostasis. It is most active when the mind has been alerted, or when the body has been injured, and prepares the body for quick responses, including meeting a challenge head on or the decision to escape (fight or flight).

The nerve cells of the SNS originate in the brainstem, and descend to connect with nerves in the spinal cord between T1 and L3. From these levels, fibres leave the spinal cord through the spinal nerves and intermingle with each other in a chain of nervous tissue that runs on either side of the spinal column at the back of the abdominal and thoracic cavities. There are a number of cell bodies, known as the ‘sympathetic ganglia’, within this chain of nervous tissue, and thus it can be compared to the grey matter of the brain and the spinal cord (the term ‘ganglion’, from the Greek meaning ‘lump’, is used in this case to refer to a cluster of nerve cell bodies). Figure 4.1a-XVI illustrates how nerve fibres leave the sympathetic ganglia to travel to all body parts, and indicates the wide range of tissues and organs that are influenced by the sympathetic nerves.

Figure 4.1a-XVI • The structure and influence of the sympathetic nervous system (SNS).

As mentioned above, the sympathetic part of the ANS is sometimes described as the part of the nervous system that prepares the body for fight or flight. It causes physical changes that allow the body to expend energy in a focused way, such as was described in the example of running for the bus. The various actions of the SNS are listed in Figure 4.1a-XVI, and it might become clear that all these changes, such as dilatation of the pupil, inhibition of mucus secretion, increased heart rate and reduced peristalsis, are appropriate ones for a body that is about to go into action.

The SNS also directly stimulates one endocrine gland, the adrenal medulla (the central part of the adrenal gland), which is situated just above the kidneys. This causes the release of the hormones adrenaline and noradrenaline into the bloodstream. These hormones help the body to deal with impending stress, and complement the function of the sympathetic nerves, as they have effects such as stimulation of the heart rate, increase of blood pressure, widening of the bronchioles and increased mental alertness.

The parasympathetic nervous system

In contrast to the sympathetic system, the parasympathetic nervous system (PSNS) might be described as promoting the maintenance of homeostasis. It is most active when the mind and body are at rest.

The parasympathetic nerves originate in the brainstem and the sacral part of the spinal cord. In particular, the vital centres in the brainstem, which deal with essential functions such as respiration and the heart rate, contain many parasympathetic nerve cells. These nerve cells have fibres that leave the cord either through the four pairs of mixed cranial nerves (the oculomotor, facial, glossopharyngeal and vagus nerves) or within the S2, S3 and S4 spinal nerves. The mixed cranial nerves have parasympathetic branches that extend to supply the deep organs and glands of the head, neck and abdomen down to the upper part of the large intestine. The parasympathetic fibres in the sacral spinal nerves supply the lower part of the large intestine, the bladder and the genital organs. Figure 4.1a-XVII illustrates the structure of the PSNS.

Figure 4.1a-XVII • The structure and influence of the parasympathetic nervous system (PSNS).

The PSNS promotes those aspects of bodily function that work to nourish and build up the body’s reserves. It enables the body to make use of the resting periods to perform functions such as the digestion of nutrients and the clearance of wastes from the kidneys and the bowel. It is noteworthy that it is when the PSNS is active that sexual function is promoted. This suggests that sex is an activity that ideally takes place when the person is at rest, and has the purpose of nourishing and building up the individual as well as the creation of a new life. It is clear that excessive sympathetic nervous activity, such as would occur in a stressful period or even a situation of excitement, tends to counteract this parasympathetic action, and can lead to impotence.

Figure 4.1a-XVII shows how the various organs and glands are supplied by nerves that originate from the brainstem and the sacral spinal cord. The parasympathetic nerves increase secretions from the eye, the mouth and the digestive tract. Although not indicated in this diagram, the production of mucus of the respiratory tract is also increased. These secretions are all protective and nourishing to the organs that produce them. The heart and respiratory rate slows with parasympathetic stimulation, and digestion, defecation and urination are also promoted.

The balance between the sympathetic and parasympathetic systems

The PSNS can be seen as the aspect of the ANS that always tends to increase in function when the mind and body is at rest. When the mind and body are at rest, the centres of nerve cells in the brainstem and in the spinal cord automatically cause the PSNS to stimulate all the organs and glands of the body. When the mind is alerted or when the body has been injured, messages are relayed to inhibit the action of the parasympathetic centres in the brainstem, and the sympathetic nerve centres in the brainstem come into action instead.

It would make sense that the periods of stress that lead to sympathetic stimulation should be short-lived, so that a time of parasympathetic stimulation can allow the body to recover from the stress. It is part of human life to have to deal with stress, not least because the creative and competitive aspects of human nature often lead us to instigate situations of stress. This is not necessarily a bad thing, as long as there are adequate periods of recuperation between these stressful episodes.

It is not easy to assess what the ideal balance of parasympathetic and sympathetic stimulation should be, and this ideal may vary from individual to individual, depending on personality amongst other things. However, what seems to be clear is that many people in modern society do not have sufficient periods of parasympathetic activity to make up for the times during which their SNS is overactive.

Persistent stress, be it social, spiritual or physical, leads to excessive stimulation of the fight-or-flight response of the sympathetic nerves. Blood pressure and heart rate become persistently high, digestion worsens, the blood sugar levels rise, muscle tension is increased and the mind is unable to relax. This, of course, becomes a vicious circle, as a tense body and alerted mind further stimulate the sympathetic nerves. In such a situation, if the body and mind could be allowed to rest, the activation of the PSNS would help the organs to begin the self-healing process of the body. This is why rest is so important, as it allows the heart and blood vessels to relax, nutrients to be well absorbed, wastes to be fully cleared, and muscles and joints to be free of tension.

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