The nervous system can be categorized in many different ways. In understanding nervous system physiology, a simple but accurate way of categorization is to view the nervous system as two separate but integrated systems:
The MOTOR nerve system is the nerves that move our muscles (motor), and also control the function of our visceral organs (like heart, lungs, intestines, pancreas, liver, kidneys, etc.). The nerves that send the electrical signal from our brain and spinal cord to our muscles to control their contraction are actually called motor nerves. The nerves that send the electrical signal from our brain and spinal cord to control the function of our visceral organs are called autonomic nerves. This is because they function automatically, without our thinking and even when we are sleeping. Occasionally these autonomic visceral organ nerves are called visceral motor nerves. The motor nerve systems are output nerves, also called efferent nerves.
The SENSORY nerve system is the nerves that send electrical nerve signals into our spinal cord and brain. Therefore, the sensory nerves travel in the opposite direction of the motor nerves. The sensory nerve systems are input nerves, also called afferent nerves.
The sensory nerves have special endings (receptors) that can take environmental events, convert these events into an electrical signal, and send the electrical signal along sensory nerves to the brain for interpretation. The sensory nerves create our senses. It is our lifelong sensory experiences that “mold” our brain.
There are six primary sensory inputs into our brains:
Touch (#5) requires elaboration. Touch (for our purposes) will also include the sensations of pain and temperature (hot/cold). When there is a perturbation on our skin, we readily can distinguish between touch, pain, hot, and/or cold. All of these sensations are electrical signals that travel to various parts of the brain for interpretation.
The important point is that all perceptions (sight, sound, taste, smell, touch [including pain and temperature] and proprioception) occur in the cortical brain. “All perceptions are cortical.” This means that they occur in our brain. Lay people often believe that their eye sees, their ear hears, their tongue tastes, their nose smells, or that something at their toe or back or neck hurts. But actually these various parts of our body only initiate an electrical signal that is then interpreted in our brain.
The cortical perception of pain is a universal human experience. The electrical signal for the perception of pain in the brain is brought to the brain via special sensory nerves called nociceptive afferents or nociceptors.
All sensory inputs into the brain begin with a special receptor, except for pain (nociception). The receptor that initiates the electrical signal for sight is different than the receptor that initiates the electrical signal for sound or taste or smell. Pain (nociception) is the great receptor exception in that, for the most part, there is no receptor on the end of the nociceptive nerves. Consequently, the nociceptive nerve beginnings are referred to as free or naked receptors.
The pain problem in our country (USA) and in the world is astonishingly huge and it is getting worse. In the United States alone, 116 million Americans suffer from chronic daily pain (1). A recent cover article in the newspaper The Wall Street Journal quantifies the anatomical regions for American’s chronic pain (2):
Hip Pain 07.1%
Finger Pain 07.6%
Shoulder Pain 09.0%
Neck Pain 15.1%
Severe Headache 16.1%
Knee Pain 19.5%
Lower-Back Pain 28.1%
The total cost attributed to America’s pain problem, including treatment, lost productivity, and disability, is approaching $1 trillion per year.
It is useful to discuss pain using the categorizations of C. Chan Gunn, MD (3, 4). Dr. Gunn is a Clinical Professor at the Multidisciplinary Pain Center at the University of Washington Medical School, Seattle, Washington. Dr. Gunn’s pain categories are:
1) Nociception Pain
In this category of pain, there is no tissue damage, and therefore no inflammation. This is the type of pain one would experience if someone stepped on your toe; one would have pain but no tissue damage or inflammation. This type of pain does not require a healthcare provider to diagnose the cause of the pain. The cause of the pain is obvious; someone is standing on your toe.
Likewise, this type of pain does not require healthcare provider treatment. The treatment is obvious; get the person’s foot off your toe. The patient self treats.
With this type of pain, once the person’s foot is off your toe, you experience immediate and lasting relief. The prognosis is excellent.
This is the type of pain that most patients (and insurance companies) hope they are experiencing, hoping for instant relief. Sadly, this type of pain rarely makes it into a doctor’s office because it is self diagnosed and treated.
2) Algogenic Pain
Suppose that instead of someone stepping on your toe, they smacked your toe with a sledgehammer. Even though the hammer is no longer actually on your toe, your toe still hurts. The hammer added something to the equation, trauma, tissue damage, and inflammation. This disruption of the tissues and blood vessels by the trauma produces and releases inflammatory chemicals that are often collectively called algogenic exudates.
The inflammatory algogenic chemicals alter the thresholds of the nociceptive afferent system, increasing the pain electrical signal to the brain. Instant relief for this type of pain is not possible. The pain subsides as inflammation resolves and the nociceptive afferents system becomes sub-threshold.
Individuals suffering from this type of pain often go to healthcare providers for relief. Treatment often involves anti-inflammatory efforts (controlled motion, drugs, omega-3s, ice, electrical modalities, low–level laser therapy, etc.) and efforts to accelerate healing (low-level laser therapy). Depending upon the degree of tissue injury and a myriad of individual unique characteristics, response can last days, week, or months.
Chronic inflammation, caused by scar tissue, autoimmune responses, infection, etc., can cause chronic algogenic pain.
3) Neuropathic Pain
This is pain that persists after all possible tissue healing has occurred. Once again, instant relief for this type of pain is not possible. This is chronic pain that may persist for months, years, or forever.
Lay people often view pain solely as a bad thing, but healthcare professionals recognize pain to be both friend and foe. For example, if one sits on one’s foot for a prolonged period of time, it will eventually begin to hurt. This is an example of nociceptive pain. We simply self diagnose and treat our foot pain by moving it or changing our sitting position, and the pain goes away.
People are constantly doing things that begin to generate pain, and the pain afferents send a sensory signal to our brain reminding us to stop doing that activity. In this regard, pain keeps us safe, reminding us not to do certain things or to stop doing certain things. Without pain, we would not survive childhood and make it into adulthood.
Chronic pain is another story.
Of the many structures that make up the spine, most of them are capable of generating pain. All of the spinal structures that can initiate the pain signal to the brain have a common factor: they are innervated by sensory afferent nociceptive neurons that carry the pain electrical signal to the brain. As noted by exceptional spine care pioneer Alf Nachemson, MD, whatever causes spine pain must have a nerve (5). In 1991, Stephen Kuslich, MD, and colleagues clarified and quantified the spinal tissues that were capable of initiating the pain electrical signal to the brain as (6):
Any of these tissues are capable of initiating acute spinal pain. Chronic spinal pain perception was primarily attributed to the intervertebral disc and facet capsules, in that order (6). Other studies have primarily attributed chronic neck pain perception to the facet capsules and the intervertebral disc, in that reverse order (7).
For a new incidence of non-traumatic low back or neck pain, it is important for both clinicians and patients to make an initial quick assessment of the severity of the problem. Absent other historical indicators, it is common to assume the pain is algogenic in nature. This means there is an accumulation of algogenic inflammatory exudates that are increasing the sensitivity of the pain sensory nerves. The first step in doing this is to categorize the symptoms into one of three groups:
Group 1: Spinal pain alone
Either neck pain or back pain without pain radiation into the arms or legs. In general, algogenic spine pain that does not radiate is not serious. It is usually the consequence of a local inflammatory condition. It can be chronic and even disabling, but it is not dangerous.
Group 2: Sclerogenic pain; also known as sclerotomic pain or sclerotogenous pain
Sclerogenic pain radiates from the neck into the arm(s) or from the low back into the leg(s). Classically the pain radiation will not extend below the elbow (from the neck) or below the knee (from the low back).
As a rule, sclerogenic pain is difficult for the patient to localize. The pain presentation is often described as being deep and dull in character, similar to a toothache.
In general, sclerogenic pain is not dangerous. It is a form of referred pain that occurs as a consequence of a shared neuromere during embryonic development. In other words, the neurology of the back and the leg, or of the neck and the arm, are shared embryological, which can cause some confusion as to the exact location of the irritation when the electrical signal is sent to the brain. Originally based on the research of JH Kellgren and colleagues in 1938 and 1939, irritations of deep spinal tissues can cause sclerogenic pain referral to the arm or to the leg (8, 9).
In the sclerogenic pain patient, successful management of the deep spinal tissue irritations will resolve the sclerogenic pain referral. Deep spinal tissue irritations include irritations to the intervertebral disc, the facet joint capsules, and the core stabilization segmental mover muscles. These tissues respond excellently to spinal adjusting.
Group 3: Radicular pain, radiculitis, radiculopathy
The technical definition of radicular pain is that the spinal nerve root is inflamed, and the classic symptomatology is radiating arm or leg pain. In contrast to sclerogenic pain (a deep dull ache), the pain is often sharp and easily localized by the patient. Also, the pain often travels below the elbow (into the hands and fingers) and/or below the knee (into the foot and toes).
Radicular pain is more serious than sclerogenic pain. It is therefore a good idea to determine (to the best of one’s ability) if the pain is radicular or sclerogenic. History and physical examination can be quite accurate in establishing a differential diagnosis. However, conformation will require advanced diagnostic imaging. The current gold standard in advanced diagnostic imaging is magnetic resonance imaging, or MRI.
Radicular pain is often caused by compression of the nerve root, the compression causing nerve root irritation and/or inflammation. This pathology is commonly referred to as compressive radiculopathy. Interestingly, the compression itself is not necessarily painful. Rather, the pain arises when the compression initiates irritation and/or inflammation of the nerve root. The degree of nerve root compression and its seriousness is estimated with MRI scans.
The most classic cause of radicular compression is herniation of the intervertebral disc. Other causes include arthritic changes (degenerative joint disease, degenerative disc disease, spondylosis) causing osseous (bone spurs, hypertrophic changes, osteophytes) narrowing of the intervertebral foramen.
Each nerve root supplies a specific patch of skin (a dermatome) and a specific muscle (a myotome). Consequently, radicular compression is often associated with specific myotomal muscle weakness and altered sensation in the dermatomal patch of skin (paresthesias).
The deep tendon reflex is a common component of establishing if the extremity pain is sclerogenic or radicular. With radicular compression, the deep tendon reflex is diminished or possibly even absent. There are three common deep tendon reflexes in the arms (assessing the nerve roots of the neck), and two in the legs (assessing the nerve roots in the low back).
Radicular compressive pathology can result in permanent death of some of the neurons in the nerve root, resulting in permanent loss of various functions. Consequently, when compressive radiculopathyis suspected, “red flags” of such pathology should be watched for and assessed. These “red flags” include:
There are two (one for the neck and one for the low back) very simple tests that are commonly done by healthcare providers to help determine if radiating pain is sclerogenic referral or as a consequence of radicular compression. These tests can also be easily performed by patients to help determine the seriousness of compression and its progress while under treatment.
Both tests are stretch tests. If the nerve root is compressed, irritated, and inflamed as it exits the spinal column, stretching it will aggravate the discomfort and the radiation.
Low Back Pain With Leg Radiation Test