Did you know that in your brain there is somewhere in the region of 10 million cells with literally trillions of connections between them? The patterns made by these connections are completely unique to you, firstly because of your genetics but also because those connections are continually forming and reforming in response to your unique life experiences. In other words when we think we’re changing our minds, we really are…
The Nervous System is in control of many of the aspects of being human that seem to separate us from the rest of life including thoughts, memory and emotions.
Along with the endocrine system, the nervous system is the main form of communication, both within the body and with the outside world. The difference is that the messages received via the nervous system are much more immediate than the prolonged action of hormones release via the endocrine system.
These two forms of communication are heavily involved in the complicated mechanisms of homeostasis. This is the term that describes the body’s ability to cope with fluctuations in our basic bodily functions, returning them back to health and stability. Broadly speaking homeostasis is the function that acupuncture aims to stimulate in individuals through treatment.
When we talk about the nervous system we are referring to the brain, the spinal cord and the cranial and peripheral nerves that originate from them.
Nerve cells are called neurones or nerves. However, the term nerve is more commonly used to describe a whole bundle of neurones all working together, as in ‘The sciatic nerve’, therefore I will use the term neurones when I am talking about individual nerve cells.
Although there are several different types of neurons that have different roles, they all share the same basic structure. Like other cells they contain a nucleus within a cell body but the membrane that surrounds this is very different to other cells. Neurones have projections that stick out from them, called dendrites. This is a term that means little tree in Greek. This provides an excellent description of the elongated branching nature of the dendrites as you can see in the diagram below. Some dendrites reach out to communicate with other neurones, others terminate in sensory receptors, like those in the skin or the eye. Either way, all dendrites convey messages from the outside into towards the body of the cell.
Diagram of a Nerve Cell
The diagram, also shows a longer projection extending out of the cell body. This is the cells axon. Axons convey messages away from the cell and can be as long as 100cm. They are also sometimes called nerve fibres. At the end of the axon there is another arrangement of branching structures know as synaptic knobs or Bouton.
Some nerve fibres or axons are coated with a material known as myelin. Myelin sheaths allow electrical impulses to be transmitted more quickly.
In both the brain and the spinal cord, we tend to see collections of cell bodies together forming grey matter, with their myelinated nerve fibres linking up to form white matter.
As we have said dendrites carry information into the cell body and axioms carry information away from the cell body. They do this through nerve impulse. These impulses rely on two important and specialised properties of neurones: irritability and conductivity.
Irritability is the ability to initiate a nerve impulse in response to an external stimulus. This might come from outside the body, such as sound, or the sensation of heat or pressure; or it may be from inside the body, as when food in the stomach activates gland function, or stretch receptor in the bladder register that we need to pass water.
Conductivity is the ability to transmit impulses on to other nerve cells, including those of the brain, muscles, organs, sense organs and glands.
The nerve impulses is also called an action potential. In its resting state the membrane of the neurone is positively charged on the outside and negatively charged on the inside. This is because it is selectively permeable and does not allow ions to completely balance out across it.
The cell membrane contains a sodium-potassium pump that expels three sodium ions for each two potassium ions it lets in. This means that outside the cells there is a higher concentration of Sodium (Na+) and inside there is a higher concentration of Potassium (K+). The positive electrical charge this creates along the cell membrane is called the resting membrane potential.
An action potential or nerve impulse happens when a stimulus excites one part of the membrane of the neurone. When this happens that part of the membrane, suddenly becomes more permeable to Sodium. Because the concentration of Sodium is higher outside the cell than inside, the Sodium ions rush into the cell and the cell membrane changes from being positive to negative. This excites the next part of the cell membrane and the impulse is carried on like a wave.
Diagram from Ross and Wilson Page 143 (Figure 7.5)
In order to return the cell to its original state the cell membrane next becomes more permeable to Potassium, which moves down its concentration gradient and out of the cell. Eventually the membrane potential returns to normal. The fastest nerve fibres can conduct impulses at a rate of 130m per second.
There are three main types of neurones:
- Sensory neurones
- Motor neurones
- Connector neurones
Sensory neurones have specialise endings that allow them to pick up stimulus from inside and outside the body. Sensory nerves in the skin may be sensitive to hot, cold, pain or touch. Sensory nerves are also found in the eyes, ears, nose and mouth as well as the muscles, joints and organs.
Most of the motor neurones originate in the brain and spinal cord. They are specially designed to interact with muscle fibres via motor end plates. They transmit messages to the muscles and glands and allow for both voluntary and involuntary movement.
Connector neurons enable communication between sensory or motor neurones and the central nervous system (CNS). Connector neurons found in the spinal cord allow the body to respond quickly to stimulus without referring to the brain. This is called reflex and can be demonstrated by our normal response to pain. If we touch something hot a message is sent from the heat receptors in the skin to the spinal cord. Here connector neurons relay the message to the motor neurones to move the hand away. By-passing conscious involvement makes the process quicker.
Synaptic Clefts and Neurotransmitters
It is important to remember that neurons do not act in isolation, they communicate with one another. However, they have to employ a different method here as in most places they are not directly connected. In fact, as we saw earlier, the free end of each axon breaks up into branches know as synaptic knobs or Boutons. These are situated close to, but not touching the dendrites and cell bodes of the next cell. The tiny gap between these structures is know as the synaptic cleft. In order to bridge this gap neurons produce chemical, called neurotransmitter, that they store in vesicles at the end of the synaptic knobs.
When a neuron is sufficiently stimulated, an electrical impulses is sent to the synaptic knob. The synaptic knob responds by releasing a neurotransmitter into the fluid that circulates within the synaptic cleft – these chemicals bind to specific receptors in adjacent cells. The way in which different neurotransmitter effect the cells they interact with varies. By and large neurotransmitters either stimulate or inhibit adjacent nerve cells.
When their job is done neurotransmitters either diffuse away from the synaptic cleft, are inactivated by enzymes, or are taken back into the synaptic knob.
Once the neurotransmitter is released the previously electrical message becomes chemical. These chemicals can cross the synapse cleft and effect the membrane of the neighboring neurons. At this point the reaction within the new cell is electrical once more. Therefore, we refer to the nervous relay system as electrochemical.
Neurotransmitters are peptides – small groups of amino acids. At least a hundred peptides are known to act as neurotransmitters including, serotonin, adrenaline, dopamine, acetylcholine, histamine, glutamate and aspartate.
However, reality isn’t as clear-cut as this description makes it seem. This is because in the body nothing happens in isolation. The many dendrites of just one neurons can have numerous neurotransmitters attached at the same time. Some of these may be stimulating and some may be inhibiting. What actually happens depends upon the balance. As neurons also communicate with one another impulses can spread very quickly in lots of different directions. Some cells will be stimulated and others will be inhibited. From a human point of view, one single experience can lead to several different ‘automatic’ responses. Physical, emotional, intellectual and vocal.
It also seems that receptors for neurotransmitters are not only found in nerve cells, as once thought. Research has shown that they are found in and effect other tissue of the body as well, such as glial cells or neuralgia which are the structural cells that hold the neurons of the CNS in place.1
Neurones are constantly manufacturing neurotransmitters and to do this they require large amounts of both nutrients and energy. Energy is also required to emit the neurotransmitters from the axon. All this means that neurones are particularly vulnerable to deficiencies in either nutrients or energy. If the supply dries up even for a relatively short time the cell will die. Sometimes the body can repair damage that occurs in this way, as the dendrites of other neurons can grow out to re-establish connections that have been lost, but sometimes this is impossible. Unlike most cells of the bodies, neurones can not divide. Therefore, until recently it was believed that they could not be replaced at all. However it is now know that we do continue to manufacture neurons in the brain, even as adults.2
For more information on how to stimulate this process I recommend this fascinating talk:
Neurons also make new dendrites and create more connections with each other. “Deepak Chopra suggests this is a physiological expression of the increasing wisdom people may show as they age, when different parts of life and knowledge connect more and more.”3
“An action potential is a travelling wave of electrochemical excitation. In order for one to happen there must be sufficient excitement. There is a threshold below which nothing will happen, but above which will trigger an action potential that is all-or-nothing; in other words it is always of the same magnitude.”
THE CENTRAL NERVOUS SYSTEM (CNS)
When we talk about the CNS we are referring to:
- The Brain, including the Upper brain (Cerebral Hemisphere), Brain Stem (the Midbrain, the Pons and the Medulla oblongata), Hind-Brain (Cerebellum)
- The Spinal Cord
The Meningeal Membranes
For protection, the brain and spinal cord are covered by several layers of protective membrane. The layer in contact with the brain and spinal cord is called the pia mater, the middle layer is the arachnoid mater and the outer layer is called the dura mater. In fact, there are two layers of dura mater surrounding the brain, but in most places there is only a potential space between them. The dura mater lines the inside of the skull.
Between the pia and the arachnoid mater we find the subarachnoid space, which is filled with cerebrospinal fluid. Between the arachnoid mater and the dura mater, there is another potential space called the subdural space.
All three layer also extend down to protect the spinal cord. The dura and pia mater go all the way down to the coccyx, but the arachnoid mater ends at the top of the sacrum.
The Cerebrospinal Fluid (CSF) that fills the subarachnoid space also flows through holes in the brain, know as ventricles. There are four irregular shaped ventricles in the brain that are all linked together by canals. There is also a canal that runs down the centre of the spinal cord. This is called the central canal and it is also continuous with the ventricular spaces in the brain. These structural spaces allows CSF to circulate through the CNS.
CSF moves in and out of the brain through the blood and its volume is controlled by the pressure in the brain. This means that a constant pressure is maintained within the skull. CSF also keeps the brain and spinal cord moist, supports and protects them and acts as an effective shock absorber. Modern research also suggests that the circulation of CSF around and through the brain, while we sleep, removes toxins and wastes, much like the lymph system does for the rest of the body.4
[DIAGRAM Ross and Wilson – Page 150 Combine Figure 7.15 The parts of the central nervous system and Fig 7.16 The lobes and sulci of the cerebrum]
The cerebrum is the largest part a of the brain, and occupies the area under the front of our skulls. It is the part of the brain that is specifically associated with humans. The cerebrum has a convoluted structure with lots of ridges and fissures which increases its surface area. It is split down the middle into two hemispheres, that are connected by the corpus callosum – a wide tract to nerve fibres. For descriptive purposes the cerebrum is dived into areas: frontal, parietal, temporal and occipital.
The cerebrum is involved in the most complex functioning of the brain, and much of the activity that we associate with the mind:
- Sensory perception of sight, taste, smell and touch, including pain
- Control of voluntary movement
- Thinking, reasoning, intelligence, learning
- Memory and emotions
- Sense of responsibility and mortality
It seems that different areas of the cerebrum are in control of certain aspects of the mind, such as movement, speech or memory. However, this reductionist view of the brain is not the whole story. As we have seen, neurons do not act in isolation, and more and more evidence is showing that individual parts of the brain rarely act in isolation either.
Nevertheless, for a broad view, let us look more closely at the specific areas of the cerebrum in turn.
Cerebrum – The Frontal Lobe
The motor cortex lies in the frontal lobe – motor areas of the right hemisphere of the cerebrum control voluntary muscles on the left side of the body and visa versa. This is because the nerve cells involved – Betz’s cells – are pyramid-shaped. At the medulla oblongata at the base of the brain the nerve impulse cross the spinal cord and descend on the opposite side. The neuron with its body in the cerebrum is the upper motor neuron and the one with its body in the spinal cord is the lower motor neurone.
In the motor area of the brain, the body is represented upside down. This means that the neurones nearest the top of the head control the feet and lowest ones control the head and neck. The area of the brain devoted to them is also not directly proportional to their size but to the complexity of the movement of the area. Therefore, the area that controls the hands and the face is much larger than that involved in movement of the wrist or trunk.
The premotor area is also found in the frontal lobe. It is positioned just in front of the motor area and appears to be involved in managing the smooth order of complex motions, such as those involved in driving a car, or painting. Tasks like these that involve complex muscular actions, carried out in a specific order are referred to as manual dexterity.
Another important area found in the frontal lode is the motor speech area. This area seems to have dominant control over the manual dexterity required for speech. It is more prominent in the left hemisphere for right-handed people and visa versa.
The rest of the frontal lobe is situated just behind the forehead. This is the area of the brain that is thought to be the seat of our character, our behaviour and our emotional lives.
Cerebrum – The Parietal lobe
The sensory cortex is found in the parietal lobe. It is the part of the brain that perceives pain, heat, pressure and touch. It is also aware of the position of the joints and has knowledge of muscular movements. The sensory areas of the right hemisphere receives impulses from the left side of the body and visa versa, much like the motor area.
Again, like the motor area, in the sensory area of the brain, the body is represented upside down and the areas devoted to different parts are sized in proportion to the complexity of the sensory information they provide. This means that the areas devoted to the hands, the face and the lips are much larger than those of the elbow or arm.
The parietal area is also involved in our ability to obtain and retain accurate information about objects. It is thought that it is this area that allows us to identify things with our eyes shut.
The sensory speech area is the part of the brain, located in the parietal lobe, that allows us to understand speech. It predominates in the left hemisphere for right-handed people and visa versa.
Deep inside the parietal lode we find the taste area of the brain. This area receives nerve impulses from the mouth and tongue, that it translates into what we know as taste.
Cerebrum – The Temporal Lobe
The temporal lobe house the auditory area of the brain, that translated impulses from the ear into sound. Deep inside the temporal lobe we find the olfactory area, which converts messages from the nose into smells.
Again deep inside the temporal lobe we find the hippocampus. This is now known to be where new neurones are created. This process is called neurogenesis. We are thought to make around 70 new neurones a day.5 The hippocampus also has an important role in learning and memory – particularly in the conversion of short-term to long-term memory.
The temporal lobe also houses the Amygdala. The amygdala plays a role in emotion, memory and fear.
Cerebrum – The Occipital Lobe
The occipital lobe is found under the skull at the back of the head. It houses the area of the brain that appears to be heavily involved in vision. The optic nerves travel from the eyes to this area of the brain, where the information is translated into visual impressions.
The cerebral hemisphere also contain several vitally important groups of cell bodies called nuclei that act as relay stations, where impulses can be passes from one neurone to the next in a chain. These are, the Basal nuclei, the Thalamus and the Hypothalamus.
The basal nuclei appears to control the smooth, coordinated movement of skeletal muscle.
The thalamus receives sensory input from the skin, the special sense organs and the viscera before it is passed onto the appropriate area of the cerebrum. It is also central to emotional experience.
The hypothalamus is situated just above the pituitary gland. Nerve fibres connect the hypothalamus to the pituitary and this connection allows it to control the output of hormones from this important gland. The hypothalamus is also involved in many of the functions related to homeostasis, including control of the autonomic nervous system, huger and thirst, body temperature, emotional reactions, such as anger, fear and joy, sexual behavior and the cycles of our body clocks and circadian rhythms.
The Brain Stem
The Brain Stem is made up of several sections. These are known as: The Midbrain, Pons varolii, Medulla oblongata, Reticular formation.
Brain Stem – The Midbrain
The Mid brain is situated above the Pons varolii and below the Cerebrum.
It is made up of lots of nerve cells and fibres which connect the cerebrum to the lower parts of the brain and the spinal cord. These nerve cells act as relay stations for the ascending and descending nerve fibres.
Brain Stem – The Pons Varolii
The Pons varolii is situated in front of the cerebellum, below the midbrain and above the medulla oblongata. It is largely made up of nerve fibres that form a bridge between two hemisphere of the cerebellum. It also connects the higher brain to the spinal cord. This part of the brain is ‘inside out’ in that here the grey matter is on the inside and the white matter in on the outside.
Brain Stem – The Medulla Oblongata
The Medulla oblongata is situated below the Pons varolii and is continuous with the spinal cord below. It is about 2.5cm long and is shaped like a pyramid with its base upwards. Like the Pons it has white matter on the outside and grey matter on the inside.
Deep inside the Medulla oblongata there are vital centres that have autonomic control of breathing, blood pressure, heart rate and reflexes associated with coughing, sneezing, vomiting and swallowing.
The pyramidal shape of the medulla oblongata is responsible for the phenomena known as the decussation of the pyramids. This has been discussed before and refers to the fact that information from one side of the body crosses over here and effects the other side of the brain.
Brain Stem – The Reticular Formation
The Reticular formation is found at the base of the brain stem, just above the spinal cord.
It has numerous functions including the co-ordination of skeletal muscle activity required for balance and voluntary movement, the co-ordination of the activity of the autonomic nervous system and control over selective awareness. Selective awareness determines how much sensory info makes it through to the cerebral cortex, through the Reticular Activating System (RAS). This is how we can ‘drown out’ regular, but unimportant information such as traffic noise or a ticking clock.
The Cerebellum is found behind the Pons varolii and just below the back portion of the Cerebrum. It is ovoid in shape and has two hemispheres.
The Cerebellum is involved in the coordination of voluntary muscle movement, of posture and balance. Much of what it does is not under voluntary control. It co-ordinates balance and equilibrium using information from the ears, eyes, muscles and joints. The Cerebellum is also involved in the co-ordination of various muscle groups that allows for smooth, fluid and precise movement.
It is also thought to be involved in learning actions that can eventually be performed without conscious effort, such as driving a car or riding a bike.
The spinal cord is suspended in the vertebral canal. Like the brain it is surrounded by meninges and cerebral spinal fluid. It leaves the medulla oblongata through the large hole in the base of the skull, called the foreman magnum and travels down approximately 45cm, to the level of the 1st lumbar vertebra. It is about the width of the little finger.
Solid horse shoe shaped vertebra sit in front of the spinal cord and bony spinous processes project out to the back and sides, to protect the delicate structure.
The spinal cord links the rest of the body to the brain. 31 pairs of spinal nerves branch out from the cord and its protective meningeal membranes to the various organs and tissue of the body, transmitting information from the brain to the body and back.
Nerve fibres in the spinal cord are bundled together.
Like the brain, the spinal cord contains both grey and white matter. The grey matter is in the centre of the spinal cord and the white matter surrounds it.
The neurones in the spinal cord are either: sensory cells, that receive impulses from the periphery of the body; lower motor neurones that communicate with the skeletal muscle or connector neurones which connect the other two to form reflex arcs.
Sensory nerve fibres ascend to the brain and motor nerve fibres descend from the brain.
The Peripheral Nervous System
The peripheral nervous system is made up of 31 pairs of spinal nerves and 12 pairs of cranial nerves. It includes the autonomic part of the nervous system.
31 pairs of spinal nerves
The spinal nerves leave the vertebral canal and emerge from either side of the spinal cord.
You can see in the diagram above, that each nerve has both an anterior and posterior root. Anterior nerve roots are made up of motor nerve fibres and posterior nerve roots consist of sensory nerve fibres. From this point on, the two halves unite to form mixed nerves.
From their initial trunk they branch out many times to feed the whole body. The nerves are made up of bundles of nerve fibres, each protected by several layers of connective tissue. These nerves are mostly concerned with body sensation and the voluntary control of muscles. However, some motor nerves close to the spine actually supply internal, smooth and cardiac muscles of the deep organs, blood vessels and glands. This action is generally involuntary, which makes them part of the Autonomic Nervous System.
Each nerves also has an extension known as a preganglionic fibre, which is under sympathetic control.
For descriptive purposes the 31 pairs are divided into:
8 cervical pairs
12 thoracic pairs
5 lumbar pairs
5 sacral pairs
1 coccygeal pair
As you can see they are named and grouped according to the vertebra from which they emerge.
After leaving the spinal cord all of the nerves except for the thoracic nerves intermingle and weave together to form networks known as plexus.
From these plexus nerve branches can emerge that have originated from more than one spinal nerve.
This is made of the first four cervical nerves. It lies under the sternocleidomastoid muscle at the level of the 1st, 2nd, 3rd and 4th cervical vertebra in the neck.
It has three branches that affect various depths within the body. It the most superficial it supplies the structures at the back and side of the head and the skin of the front of the neck to the level of the sternum. Beneath that it also supplies the muscles of the neck (sternocleidomastoid and trapezius). Deeper still the phrenic nerve that branches out from the 3rd, 4th and 5th vertebra travels down in front of the lungs to supply the muscles of the diaphragm.
This is made up of the lower four cervical nerves and a large part of the first thoracic nerve. This plexus travels down behind the collar-bone into the arm pit. Branches from this plexus supply the skin and muscles of the upper arm and some of the chest muscles. Five large nerves form out of this plexus and go on to serve the arm and hand. These are: the axillary nerve, the radial nerve, the musculocutaneous nerve, the median nerve, the ulnar nerve and the medial cutaneous nerve. Collectively these supply the muscles and skin of the shoulder, arm, wrist and hand.
The lumbar plexus is made up of the first three and part of the forth lumbar nerves.
It is situated in the lower back, just out from the spine. Again various nerve emerge from this plexus to supply the muscles and skin of the lower abdomen, groin, thigh and the inside of the leg ankle and foot.
This is formed from one branches of the lumbar plexus along with the 1st, 2nd and 3rd sacral nerves. It is found behind the wall of the pelvic cavity. It has several branches that serve the muscles and skin of the pelvic floor, muscles of the hip-joint, the pelvic organs and the muscles and skin of the anus and urethra. It is also the origin of the largest nerve in the body – the sciatic nerve. In the middle of the thigh the sciatic nerve divides into the tibial and common peroneal nerve. The tibial nerve travels down the back of the calf and the sole of the foot and the common peroneal nerve supplies the front and side of the calf and foot.
This is made up of part of the 4th and the 5th sacral nerve, along with the coccygeal nerves. This supplies the small area of skin around the coccyx along with the muscles of the pelvic floor the coccygeus muscles and the anal sphincter.
The thoracic nerves do not form plexus. The first 11 are intercostal nerves meaning they pass between the ribs that supply the muscles and skin around them. The 12th pair are known as the subcostal nerve.
The area of the body that each nerve supplies is very similar for everyone and has been mapped. The term used to describe these areas is Dermatomes. As you can see from the diagram below, the various plexus allow the nerves to serve areas of the body that are further away.
This explains why we can have referred pain away from the sight of the problem. For example in sciatica the irritation of the nerve at the level of the spine is misinterpreted by the brain as coming from the terminal branches of the nerve in the leg.
We can also get referred pain from the organs. They do not have a very rich supply of sensory nerves but if they are in distress the message is sent up to the spinal cord. This can then sometimes be interpreted by the brain as coming from the dermatome that is supplied by the level of the spinal cord. Organs can be connected to nerves at several levels which can make organ pain vague and difficult to identify.
There are 12 pairs of cranial nerves. They originate from the underside of the brain. They are:
These are the sensory nerves that give us our sense of smell. The nerve endings are found in the upper part of the mucous membrane of the nose. Massages from here are transmitted to the area for the perception of smell in the temporal lobe of the cerebrum.
These are the sensory nerves of the sight. The fibres originate in the retina of the eyes. The fibres of the two eyes cross in an area known as the optic chiasma, which is just above the pituitary gland. From here optic tracts travel onto the centres for sight in the occipital lobes of the cerebrum and also to the cerebellum, where visual stimulus is used to maintain balance.
Oculomotor, Trochlear and Abducent Nerves
Together these three nerves are the motor nerves of sight. They allow us to move our eyes around, to change the shape of the lens of the eye, to alter the size of the pupil and to open and close the eyelids.
This is a mixed nerve containing both sensory and motor nerves. It is the main sensory nerves of the face, monitoring temperature, touch and pain. It also contain the motor nerves that allow us to chew. This nerve has three branches:
The Ophthalmic branch, which is sensory only and supplies the front part of the scalp and fore head, the eyes, tear duct, eyelids and the inside of the nose.
The Maxillary branch, which are again sensory and supply the cheeks, upper gums, upper teeth and lower eyelids.
The Mandibular branch, which contains both sensory and motor nerves. They supply the teeth and gums of the lower jaw, the front part of the ear, the lower lips and tongue and the muscles associated with chewing.
These are mixed nerves. The motor nerves allow us to show facial expression. The sensory nerves send messages from the taste buds in the front part of the mouth.
Vestibulocochlear (auditory) Nerves
This is actually two sets of nerves combined
The Vestibular nerve originates in the semicircular canals of the inner ear. It sends messages to the cerebellum to help us maintain posture and balance.
The cochlear nerve is the sensory nerve of hearing and it sends messages to the hearing area of the cerebral cortex.
These are mixed nerves. The motor nerves stimulate the muscles of the tongue, the pharynx and the salivary glands. The sensory nerves send messages from the taste buds at the back of the mouth.
This is the cranial nerve with the widest scope. It travels down from the head through the neck into the thorax and the abdomen. The motor nerves supply the smooth muscle and secretary glands of the pharynx, larynx, trachea, heart, oesophagus, stomach, intestines, pancreas, gall bladder, bile ducts, spleen, kidneys, ureter and blood vessels in the thoracic and abdominal cavities. The sensory nerves send messages from the lining membranes of the same structures to the brain.
These motor nerves supply the sternocleidomastoid and trapezius muscles. This allows us to move the head, shoulders and neck. Branches join the Vagus nerve and also supply the pharyngeal and laryngeal muscles.
This is also a motor nerve. It supplies the muscles of the tongue and those surrounding the hyoid bone. They help with swallowing and speech.
The Autonomic Nervous System (ANS)
As the name suggests the autonomic nervous system is the part of the nervous system that makes things happen automatically, outside of conscious control.
It manages things like heart rate, breathing, digestion, vision, blood pressure and temperature control. The ANS actually uses the nerves of the PNS. Therefore, the distinction between the PNS and the ANS is functional not anatomical.
The ANS is made up of two parts
The Sympathetic nervous system (SNS)
The Parasympathetic nervous systems (PSNS)
These two aspects of automatic response are normally in opposition to one another. Their interaction allows the body to rapidly respond to change and to maintain the dynamic balance of homeostasis.
The Sympathestic Nervous System (SNS)
The SNS is active when we have been excited, injured or alerted to a threat.
The nerve cells of the SNS originates in the hypothalamus, the reticular formation and the medulla oblongata and connect with nerves in the spinal cord between T1 and L3. From here the nerves then move out through the spinal nerves and combine in a band of nervous tissue which runs down either side of the spinal column behind the thoracic and abdominal cavities. Contained within this band are numerous sympathetic ganglia not dissimilar to the grey mater of the spinal cord.
The SNS is also involved in the fight or flight response. Its impact prepares the body for action. For example, sympathetic stimulation, speeds up the beating of the heart and increases the force with which it pumps, increases blood flow to the heart, brain and muscles, constricts blood vessels on the surface of the body, dilates pupils, dilates skeletal blood vessels, dilates the bronchi of the lungs so that oxygen intake and carbon dioxide output are increased and inhibits digestion and kidney function. It also increases the rate at which the liver converts glycogen to glucose to provide fuel.
Also the SNS effects the adrenal medulla (an endocrine gland). This leads to the secretion of adrenaline and noradrenaline into the blood. This has a similar effect on the body – increasing mental alertness, blood pressure, heart rate and the widening of the bronchioles.
The Parasympathetic Nervous System (PSNS)
The nerve cells of the PSNS originate in the brain stem and the sacral region of the spinal cord. Vital centres of the brain stem involved in breathing and heart rate contain many parasympathetic nerve cells. From the brain they also move out into the body through the 4 mixed cranial nerves mentioned earlier or at S2,S3 and S4
The cranial nerves connect with the glands and the deep organs of the head, neck and abdomen as well as the upper part of the large intestine. The sacral aspect supplies the lower part of the large intestine, the bladder and the genitalia.
The PSNS is largely involved in the slowing down of bodily process and in increase in the digestion of nutrients, removal of wastes and self-healing. It is also involved in sexual function. It can be seen as the peacemaker and healer involved in preparation and restoration.
For example, parasympathetic stimulation leads to increased production of mucous in the respiratory tract, pupil constriction, increased saliva and tear secretion, constriction of the coronary arteries, trachea and bronchi, increased gastric juices, pancreatic juices, intestinal juices, bile and insulin leading to increased digestion and nutrient absorption, increased urine secretion, increased motility in the stomach and intestines and reduced heart and respiratory rate.
Normally these two systems function simultaneously to maintain an appropriate response to the external and internal requirements. However, if we are put under prolonged stress the body spends too much time exposed to sympathetic nerve activity, with not enough time in parasympathetic activity to allow the body to recover. This can create a negative feedback loop, as what begun as an external stress becomes a physical stress because the body is not given the opportunity to regain homeostasis.
As we said at the beginning, this is where acupuncture seems to be particularly helpful. It encourages the body to deeply relax, moving us away from prolonged expose to the sympathetic response that can come from. physical or emotional stress.
This quality of deep relaxation seems to encourage the body it to recognise and reinstate its own mechanisms for self-healing.
Ross and Wilson – Anatomy and Physiology
The complimentary therapist guide to conventional medicine – Dr Clare Stephenson
Wholistic Anatomy – an integrative guide to the human body – Pip Waller
TED Talk: Sandrine Thuret – You can grow new brain cells, here’s how…
TED Talk: Elliot Krane – The mystery of chronic pain
TED Talk: Jeff Iliff – One more reason to get a good nights sleep
3Holistic Anatomy: An Integrative Guide to the Human Body – Pip Waller