The human nervous system will be dissected into its major parts, discussed in detail and then wrapped up with a look at what can go wrong and how to provide the care and feeding that it requires.
The quick and dirty organization of our nervous system is as follows:
I. Central Nervous System - Brain and spinal cord
II. Peripheral Nervous System - Cranial nerves and spinal nerves
Starting with the central nervous system (CNS) and its major component, the brain, we learn from an old French proverb that a brain is worth little without a tongue.
We can put that to rest by spending a little time listening to talk radio or watching the evening news.
We will quickly conclude that there is no physical connection between the brain and the vocal organs.
To say that the brain is the center of the human nervous system just doesn't do it justice. It is the life force of the body. Consider that it is one of the largest organs of the body, Weighs about 3 pounds and has more than 100 billion cells.
It is just 2% of the body's weight but consumes 20% of the body's blood supply. All of the activities of the body are devoted to the care and protection of the brain.
Is it complex? How about an organ estimated to contain 100 billion neurons which would be about 30,000 miles if laid end-to-end. Each neuron has about 10,000 synapses. What a challenge it is to unravel the workings of such a complex system.
The Great Courses has tackled the issue of Understanding the Brain head on. Their description of this course says that:
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Click on the Great Courses button link below, navigate to the course by putting Understanding the Brain in the search box and decide if you want to make this incredible learning experience part of your DVD library.
Ask any neurologist and they will quickly affirm that we have much to learn about the human nervous system. For example, David Eagleman, writing in Discover magazine says that "even partial answers to these 10 questions could restructure our understanding of the three-pound mass" inside our skull.
Rest assured, there is a ton of research currently underway to answer those questions.
We will close this introduction with the question. "When are we dead"? We are dead when the brain dies. Brain dead is as dead as it gets. OK, we're talking about the body here, not the soul or consciousness, which may go into that great unknown when the brain dies.
Take a look at the brain in the human nervous system and what do we see? On the surface it resembles a gray blob, separated into two halves or hemispheres with convoluted ridges and valleys apparently meandering over its surface.
However, if we were able to examine a large number of brains from the human nervous system, we would see that the patterns of those meandering ridges and valleys are not random at all. They are consistent and so well defined that they can be divided into identifiable sections and mapped.
People who study the brain (neurologists) have given these areas descriptive names and numbered them according to function. In the book shown, "The Brain Atlas", you will find a professional text on the subject. Click on the link below for more information or to order.
Time for a new word...Encephalon. Medical words aren't just made up, they have meaning and encephalon is no exception. The word comes from the Greek word "enkephalos" meaning brain.
So it follows that neurologists would have named the front part of the brain the proencephalon; the middle part, the mesencephalon; and the back part the rhombencephalon, consisting of the pons, cerebellum and medulla oblongata.
In the human nervous system there is a telencephalon that is the largest part of the brain and comprises all the other encepalons. The lobes of the brain are all contained in the telencephalon. The diagram below shows the four lobes of the brain.
The cerebrum is divided into two cerebral hemispheres, each of which is subdivided into four lobes.
In the human nervous system, the frontal lobe controls conscious thought and damage to it can cause personality and mood changes.
The Parietal lobe integrates sensory input and allows the manipulation of objects.
The occipital lobe deals with sight and damage can result in hallucinations and the temporal lobe deals with smell and sound and processes complex inputs such as facial recognition and scenes.
In addition to the four lobes, we need to know something about the Cerebellum, the Pons and the Medulla Oblongata.
Starting with the Cerebellum, in the big picture it links sensory input with motion and controls balance. Like the brain, it consists of two deeply-convoluted hemispheres which comprise about 10% of the weight of the brain.
Nevertheless, it contains as many neurons as all the rest of the brain combined.
Since its main function is to coordinate body movements, damage to the cerebellum produces muscle motions that are jerky and uncoordinated. In the human nervous system, the cerebellum is a center for learning motor skills.
The pons serves as a relay station carrying signals from various parts of the cerebral cortex to the cerebellum. Nerve impulses coming from the eyes, ears, and touch receptors are sent on the cerebellum. It also participates in the reflexes that regulate breathing.
The medulla looks like a swollen tip on the spinal cord. Nerve impulses arising here rhythmically stimulate the muscles between the ribs, which form the diaphragm, making breathing possible.
It also regulates the heartbeat in addition to regulating the diameter of arterioles thus adjusting blood flow.
The neurons controlling breathing have receptors to which opiates, like heroin, bind. This accounts for the suppressive effect of opiates on breathing. From the foregoing, it is pretty obvious that destruction of the medulla results in instant death.
The cerebellum, pons, and medulla receive their inputs as nerve impulses from the spinal cord and a dozen pair of cranial nerves, some of which contain both sensory and motor axons.
An axon is a long, slender nerve fiber in the neuron (nerve cell) that conducts electrical impulses away from the neuron's cell body.
Photo: Schematic diagram of a typical neuron in the human nervous system with the structural parts labeled; attribution - Quasar Jarosz at en.wikipedia, 11 Aug. 2009.
The diagram is included as clarification. Neurons are the primary transmission lines of the human nervous system and, as bundles, they help make up nerves.
Individual axons are microscopic in diameter (typically about 1µm across), but may be up to several feet in length.
The midbrain along with the medulla and pons are often referred to as the "brainstem" of the human nervous system and the next few paragraphs will give a short description of its main structures.
First is the reticular formation whose function is to collect input from higher brain centers and send it on to motor neurons.
Second, the substantia nigra helps "smooth" out body movements. Lesions (damage) to the substantia nigra cause Parkinson's disease which is characterized by jerky body movements.
Third, the ventral tegmental area (VTA) is packed with dopamine-releasing neurons that seem to be involved in pleasure. For example, nicotine, amphetamines and cocaine bind to and activate its dopamine-releasing neurons which may account for their addictive qualities.
The forebrain of the human nervous system is made up of two large cerebral hemispheres, called the telencephalon which was briefly mentioned above. Because of crossing over of the spinal tracts, the left hemisphere of the forebrain deals with the right side of the body and vice versa.
The Diencephalon of the human nervous system has four main structures, the thalamus; lateral geniculate nucleus, hypothalamus, and posterior lobe of the pituitary, all located deep in the cerebrum.
The paired structures of the thalamus receive all sensory input, except for smell, on their way up to the regions of the cerebral cortex responsible for processing the senses of touch, pain, temperature and other sensory data.
Signals from the cerebellum pass through the thalamus on the way to the motor areas of the cerebral cortex.
The lateral geniculate nucleus (LGN) takes all signals entering the brain from each optic nerve, processes them, and moves them along to the various visual areas of the cerebral cortex.
The hypothalamus is the seat of the autonomic nervous system. Damage to the hypothalamus is quickly fatal since the normal homeostasis (balance) of body temperature, blood chemistry and other critical regulators go out of control.
It is the source of eight hormones, two of which pass into the posterior lobe of the pituitary gland.
The posterior lobe of the pituitary receives vasopressin and oxytocin from the hypothalamus and releases them into the blood. Vasopressin acts on the kidney ducts to facilitate the reabsorption of water and reduce the volume of urine.
Oxytocin is the love amino acid; it acts to promote bonding, enhance feelings of love and assist in the birth process. Bring on the oxytocin!
Tucked away beneath each of these regions of the cerebral cortex are several vital structures of the human nervous system.
There is an olfactory bulb that makes the sense of smell possible. Smell depends on sensory receptors that respond to airborne chemicals.
In the human nervous system, these receptors sit in a patch of tissue about the size of a postage stamp located high in the nasal cavity.
There is a striatum that receive inputs from the frontal lobes and also from the limbic system, discussed below. Here we find the Nucleus Accumbens, located at the base of each striatum.
The pleasurable (and addictive) effects of amphetamines, cocaine, and perhaps other psychoactive drugs seem to depend on their producing increasing levels of dopamine at the synapses in the nucleus accumbens as well as the ventral tegmental area (VTA).
There is a limbic system that receives input from various association areas in the cerebral cortex and passes signals on to the nucleus accumbens. Each limbic system is made up of a hippocampus which is essential for the formation of long-term memories.
There is an amygdala which appears to be a center of emotions, fear for example. It sends signals to the hypothalamus and medulla which can activate the flight or fight response of the autonomic nervous system.
In rats, the amygdala is shown to contain receptors for vasopressin whose activation increases aggressiveness and other signs of the flight or fight response and oxytocin whose activation lessens the signs of stress.
The amygdala receives a rich supply of signals from the olfactory system, and this may account for the powerful effect that odor has on emotions and memories.
Both the spinal cord and the brain are made up of White and gray matter where the colors, white and gray, refer to parts of neurons in the human nervous system.
White matter is bundles of axons where each axon is coated with a sheath of myelin. It's the degeneration of myelin that is associated with multiple sclerosis. Remember axons? If not, go back to the neuron diagram above.
Gray matter is the other end of the neuron, masses of the cell bodies and dendrites along with all their synapses. In the human nervous system's spinal cord, the white matter is at the surface, the gray matter inside.
It is postulated that the segregation into gray and white matter in the human nervous system is to provide the highest level of connectivity and the lowest possible delay in the transmission of signals between sensory input and ultimate stimulus.
Both the spinal cord and brain are covered in three continuous sheets of connective tissue, the meninges.
The brain is arguably the best protected organ in the body. From the outside in, the first layer of protection is the skull, the armor that protects the brain from physical trauma.
Next are the meninges, three membranes that surround the brain to keep it from being damaged by contact with the inside of the skull.
It is these membranes that become infected when someone gets meningitis, and it is because the meninges are in direct contact with the brain that meningitis is so dangerous. See the diagram below for their relative positions.
The meninges are the dura mater, the arachnoid and the pia mater.
The region between the arachnoid and pia mater is filled with cerebrospinal fluid (CSF).
The fluid flows uninterrupted throughout the central nervous system through the central cerebrospinal canal of the spinal cord and through an interconnected system of four ventricles in the brain.
Then it returns to the blood through veins draining the brain.
Think of a telephone line or the coaxial cable to your TV or the wires that interconnect computers in a network. All those are simplified versions of the human nervous system's spinal cord.
Yes, it carries information; sensory information from the peripheral nervous system to the brain and carries motor information from the brain to our various skeletal muscles, cardiac muscles, smooth muscles and glands, collectively known as "effectors".
They are called sensory axons or motor axons, depending on the type and direction of information they carry; sensory impulses go to the brain, motor impulses away from the brain.
Thirty-one pairs of spinal nerves, carrying both sensory and motor signals, traverse the spinal column, exiting at various points like the off-ramps of a freeway.
However, within the human nervous system's spinal column, all the sensory axons pass into a tissue mass called the dorsal root ganglion where their cell bodies are located and then into the spinal cord itself.
All the motor axons pass into the ventral roots, the outgoing motor axon, before uniting with the sensory axons to form the mixed nerves.
The spinal cord carries out two main functions:
It connects a large part of the peripheral nervous system to the brain. Information (nerve impulses) reaching the spinal cord through sensory neurons is transmitted up into the brain.
Signals arising in the motor areas of the brain travel back down the cord and leave in the motor neurons.
The spinal cord also acts as a minor coordinating center responsible for some simple reflexes like the withdrawal reflex, such as receiving a sensory signal from touching a hot stove and sending a motor signal to move (withdraw) our hand from the stove.
The peripheral nervous system consists of sensory neurons running from stimulus receptors that inform the CNS of the stimuli and; motor neurons running from the CNS to the muscles and glands that transform the signal into action.
The PNS is subdivided into the somatic (sensory) nervous system and the autonomic nervous system.
The somatic system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves. All our conscious awareness of the external environment and all our motor activity to cope with it operate through the somatic division of the PNS.
The autonomic human nervous system consists of sensory neurons and motor neurons that run between the central nervous system
(especially the hypothalamus and medulla oblongata) and various internal organs such as the heart, lungs, glands (endocrine and exocrine), and viscera.
It is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them. The contraction of both smooth muscle and cardiac muscle is controlled by motor neurons of the autonomic system.
The actions of the autonomic nervous system are largely involuntary (in contrast to those of the somatic system). It also differs from the somatic system in using two groups of motor neurons to stimulate the effectors instead of one.
The autonomic nervous system has two subdivisions, the sympathetic nervous system and the parasympathetic nervous system.
In the human nervous system, the sympathetic nervous system is totally concerned with survival; fight, fright and flight. Its job is "up regulation".
Physiologically, the neurotransmitter of the transmitting (preganglionic) sympathetic neurons is acetylcholine (ACh). It stimulates an action response in the receiving (postganglionic) neurons.
The neurotransmitter released by the postganglionic neurons is noradrenalin (also called norepinephrin). The action of noradrenalin on a particular gland or muscle is excitatory is some cases, inhibitory in others.
The release of noradrenalin:
In short, stimulation of the sympathetic branch of the autonomic human nervous system prepares the body for emergencies: for "fight or flight" (and, perhaps, enhances the memory of the event that triggered the response).
When activation of the sympathetic system puts us on alert, we are alert all over.
It is quite general because a single preganglionic transmitting neuron usually synapses with many postganglionic neurons; the release of adrenaline from the adrenal medulla into the blood ensures that all the cells of the body will be exposed to sympathetic stimulation.
The parasympathetic human nervous system is the exact opposite of the sympathetic and is concerned with rest, recovery and relaxation. Its job is "down regulation".
The main nerves of the parasympathetic system are the tenth cranial nerves, known as the vagus nerves. They originate in the medulla oblongata.
Other preganglionic parasympathetic neurons also extend from the brain as well as from the lower tip of the spinal cord.
Each preganglionic parasympathetic neuron synapses with just a few postganglionic neurons, which are located near or in a muscle or gland.
Acetylcholine (ACh) is the neurotransmitter at all the pre, and many of the post, ganglionic neurons of the parasympathetic system.
However, some of the postganglionic neurons release nitric oxide (NO) as their neurotransmitter.
Parasympathetic stimulation causes:
In short, the parasympathetic system returns the body functions to normal after they have been altered by sympathetic stimulation. In times of danger, the sympathetic system prepares the body for violent activity. The parasympathetic system reverses these changes when the danger is over.
The vagus nerves also help keep inflammation under control. Inflammation stimulates nearby sensory neurons of the vagus. When these nerve impulses reach the medulla oblongata, they are relayed back along motor fibers to the inflamed area.
The acetylcholine from the motor neurons suppresses the release of inflammatory cytokines, e.g., tumor necrosis factor (TNF), from macrophages in the inflamed tissue.
Although the autonomic nervous system is considered to be involuntary, this is not entirely true. A certain amount of conscious control can be exerted over it as has long been demonstrated by practitioners of Yoga and Zen Buddhism.
During their periods of meditation, these people are clearly able to alter a number of autonomic functions of the human nervous system including heart rate and the rate of oxygen consumption. These changes are not simply a reflection of decreased physical activity because they exceed the amount of change occurring during sleep or hypnosis.
From all the foregoing, it is should be pretty clear that the human nervous system is a very complex system. In general, the more complex the system or organ, the more can go wrong.
Click on the book cover to the left and find out just how complex it is and how to keep it humming.
There is a lot that can go wrong with the human nervous system. The focus here will be on the brain due to its complexity although from the foregoing paragraphs, it should be clear that damage or lesions in the spinal column or nerves of the human nervous system will interrupt the sensory and/or motor pathways with accompanying loss of function.
Earlier we mentioned that those big names given to medical conditions mean something; but again, sometimes they just disguise the fact that the doctors don't have a clue what to do about it. For example, take PML, progressive multifocal leukoencephalopathy. I mention that because a friend has just been diagnosed with it.
From the NIH's National Institute of Neurological Disorders and Stroke, we learn that "the symptoms of PML are the result of an infection that causes the loss of white matter (which is made up of myelin, a substance the surrounds and protects nerve fibers) in multiple areas of the brain.
Without the protection of myelin, nerve signals can’t travel successfully from the brain to the rest of the body. Typical symptoms associated with PML are diverse, since they are related to the location and amount of damage in the brain, and evolve over the course of several days to several weeks.
The most prominent symptoms are clumsiness; progressive weakness; and visual, speech, and sometimes, personality changes. The progression of deficits leads to life-threatening disability and death over weeks to months."
So having a description for PML, it is logical that they named it "progressive" (gets worse), "multifocal" (hits different areas", "leuko" (white), "encephalo" (brain) and "pathy" (disease). In the language of real people, we have a disease that gets worse and attacks various points of white matter in the brain. Just because they can name something doesn't mean they know how to cure it.
Sounds a lot like multiple sclerosis, doesn't it? The medics haven't quite figured that one out either. Both are conditions of the human nervous system that still need a lot of work.
Trauma or lesions (damage) to the brain from physical causes like accidents or sports injuries, especially full-contact sports like boxing or football, can cause the brain to bounce around, resulting in bruising or more permanent damage. Strange things can happen as a result of physical brain damage.
One of the most often cited cases of how brain damage can change the personality is that of Phineas Gage. Gage was a railroad foreman who had a steel rod driven through the frontal cortex area of his brain during a dynamite accident. He survived but reportedly had such negative personality changes that he was "no longer Gage" according to those who knew him.
Photo: Depiction of the trauma suffered by Phineas Gage.
Agnosia is a neurological term for "loss of Knowledge" and numerous types of agnosia have been documented as a result of damage to the human immune system's brain.
For example if damage occurs to the visual cortex area, an individual may lose the ability to identify common objects. Damage to other higher order areas of the brain may result in the loss of ability to perceive motion, or color, or the sense of touch or to recognize faces.
One of the most curious conditions occurs when the right parietal cortex is damaged. Cases have been documented where patients no longer recognize the left side of their body as being part of them and the left side of the visual world no longer exists. Neurologists call the condition "contralateral neglect".
For example, a person with this disorder will take care of and groom the right side of their body and totally ignore the left side.
The message is that every area of the brain has a well defined purpose and if any one of them is damaged, we are likely to experience loss of some ability or function.
The brain thrives on oxygen and nutrients, both of which it gets from its blood supply. If anything happens to interrupt or reduce the brain's blood supply, death of brain tissue and all kinds of problems can manifest.
Typical interruptions to the blood supply are strokes which is the third largest killer in the U.S.
Strokes are either ischemic, where a blood vessel gets blocked, or hemorrhagic, where a vessel ruptures. In either case, it is a medical emergency and the affected parts of the brain will quickly die unless intervention is immediate.
Seizures are typified by epilepsy, the cause of which is still largely unknown, "ideopathic" in the words of the medical profession. One common theory is that scarring in the brain causes a firing of the neurons. The scarring could be a result of inflammation or trauma.
Other theories are that tumors or aneurisms exerting pressure could set off the uncontrolled firing of neurons. Usually the neuron firing starts in one area and progresses across the brain.
It may start with just a thumb twitch that moves to the whole hand, then the arm, soon the whole body is twitching. The person will likely fall down, unconscious for a time. After the seizure, a 5 to 30 minute period of nausea, headache, confusion, or depression may be experienced.
Medications can depress brain activity and Phenobarbital was an early drug that was widely used to dampen such activity.
Many seizures are preceded by an aura, sounds, colors, or some abnormal presence that signal something is about to start and epileptic sufferers know that they had better sit down or pull over if driving.
Memory and cognitive disorders of the brain are typically, but erroneously, ascribed to old brains. It is a myth that memory loss in old age is always caused by Alzheimer’s or dementia. Some memory loss in aging is normal but it can be forestalled for a long time.
The vascular problems discussed above can cause some memory loss through diminished blood flow. Alzheimer's disease is a whole different matter for the human nervous system and is characterized by neuron tangles and plaques in the brain, but is that the problem or a symptom?
So far no Alzheimer's bacteria or virus has been discovered. Alzheimer's is just a name given to a scarred and damaged brain but the question is, "where did it originate?” Fortunately, researchers are starting to unravel many of the secrets of Alzheimer's disease and the implications are revolutionary.
Inflammation is suspect number one and a high sensitivity C-reactive protein test should always be given before jumping on an Alzheimer’s diagnosis.
Inflammation has been mentioned repeatedly as a source of brain scarring, plaque build-up and eventual blockage of blood vessels.
Chronic stress is a major cause of inflammation and is one that can be controlled naturally with stress management training, numerous dietary supplements and some basic lifestyle changes.
The culprit in stress-caused inflammation is cortisol, one of the "fight or flight" chemicals released into the blood stream in response to a threat. It can raise havoc throughout the whole human nervous system.
Under normal conditions the cortisol is cleared from the bloodstream once the threat is gone but under conditions of chronic (on-going) stress, it never gets cleared out and is left to do its damage.
Risk factors for the human nervous system and cardiovascular system are the same; high blood pressure, diabetes, arterial inflammation, low HDL, high LDL, sedentary lifestyle, smoking, obesity, EFA imbalance (too little omega 3, too much omega 6), and consuming trans fats and saturated fats.
Attention to ones diet is crucial and if we are eating right for cardiovascular health, we are automatically eating right for brain health.
Tomatoes, tomato juice, spaghetti sauce and real chocolate (high cocoa content) all help to keep the arteries young.
The tomato products contain folate, lycopene and several phytochemicals and the chocolate has flavonoids that increase
dopamine, all of which slow the aging of arteries.
The aspirin therapy is still in wide use and frequently recommended by doctors. Two baby aspirin a day will keep arterial inflammation at bay, will keep clots from forming in the arteries and keep the blood and thus oxygen flowing to the brain.
People on the aspirin routine have fewer strokes and it may help the body build new blood vessels to form alternate routes if clots do form
A warning is not to take aspirin and a non-steroidal anti-inflammatory such as ibuprofen (Advil) on the same day. They tend to cancel out the beneficial effects of each other.
Low fat and no-fat diets can cause cognitive problems. Cells need cholesterol and brain cells are no exception. The cellular membrane is largely cholesterol.
Furthermore the myelin sheath surrounding the neurons axon is fat; actually it is 80% lipid (fat) and 20% protein. The lipid part is glycolipid and galactocerebroside.
So if you go on Lipitor, Zocor, Crestor, Plavix or any other cholesterol lowering statin, be especially alert for changes in your cognitive ability. Fat is good for the brain as long as it is the right kind of fat and getting the right balance and amount of essential fatty acids is critical.
Swing the diet toward more healthy fats derived from olive oil (virgin, cold pressed), fish oils, nuts, fish, soybeans, avocados, whole grains, flax, etc.
Engaging in challenging mental tasks, learning new skills, taking a course, doing puzzles, all work to build new brain connections and keep the existing ones healthy.
My Brain Trainer, accessed via the link below, is an excellent way to start your brain on the road to fitness.
It can't be said any louder or longer...Get the stress out of your life! Stress is a matter of how we react to events out of our control, not the event itself.
At the risk of being repetitive, stress management is a matter of lifestyle changes including diet, exercise, sleep, and keeping a balance between work and recreation. Turn off MSNBC or Fox News, which ever is your poison, and take a vacation or go for a walk in the park.
Don't discount the value of dietary supplementation for the human nervous system. Folate, B6 and B12, Coenzyme Q10, EFAs
Alpha Lipoic acid and L-carnitine, reservatrol and the amino acid S-adenosylmethionine all contribute to a healthy human nervous system.
Our commercial food supply isn't what it used to be, nutritionally, that is.
Green harvesting, processing, preservatives and farm fertilizers and pesticides all tend to degrade the nutrient value of food.
The result is that supplementation is not just nice, it's mandatory. Don't "Ask Your Doctor", just do it. Tune out the media knuckleheads, politicians and "healthcare professionals" that say supplements are useless, or worse, dangerous.
And finally, absolutely, positively make glyconutrients a part of your daily supplementation.
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