9.16 .Pain Explained Neurologically

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16. The Neurology of Pain (Update 10 Septr 2021) The below headings in descending order as met with in text give an idea of the chapter and may be used by search & find or scroll-down to locate the subject for immediate reading.

Let us follow a pain stimulus and its impulse

Sensory pain synapses use neurotransmitters

 In sensory stimuli, there is a basic separation

As sensory fibers enter the spinal cord at each higher level

Pain, its subjective experience and other aspects

More Thoughts on Dealing with Pain

Let us follow a pain stimulus and its impulse from its start in sticking your right fingertip with a pin. The sharp pin contacts free nerve-ending pain receptors just beneath the skin and the physical disturbance sensed by the free nerve-endings generates an electrical depolarization membrane potential change that, once it rises above a threshold level, makes an all or none digital action-potential (seen as an electric blip on a cathode screen from electrodes over the nerve). The action potential impulse runs up the right forearm in a nerve fiber (afferent fiber because it runs toward the neuron from which the fiber grew) in a sensory nerve branch and continues from the upper arm and shoulder to the dorsal spinal root in the Cervical-7 spinal segment (Assuming it is 3rd to 5th finger, which is in C7 spinal zone). The arriving signal runs through the dorsal spinal root ganglion (without going through a synapse because it is passing through a pseudo-unipolar ganglionic neuron) into the spinal cord gray matter at the C7 level and there inside the spinal cord gray matter it synapses with a relay neuron whose output fiber (axon) and runs up the spinal cord in a white matter sensory fiber bundle along with other such fibers in the spinal cord in its very front and lateral part (anterior spinothalamic tract on both sides of the cord white matter) on the right side. And, near the highest level in the spinal cord, the right-side fibers cross over to left side and synapse again to a next-relay neuron and continue up through the brainstem and then the signal has a 3rd synapse relay in the left thalamus and from there is dispersed into several signals that give meanings (danger, self administered as test, risk of transmitting HIV, etc). One set of neuron fibers from the thalamus send the signal to left sensory cortex in forward part of parietal lobe where it is processed and appears in your consciousness. It should be mentioned that interneurons in the spinal cord can gate (block) the pain signal if there are good reasons (high emotion, a soldier in combat).

   Sum up the big picture: a skin sharp stick on one side of the body passes to consciousness in the brain's cerebral cortex on the other side via a 3- or 4-neuron relay through thalamus and dispersal of signal to many parts of the brain's cortex for appropriate behavioral responses. Note the crossing of signaling, a general feature of motor and sensory signals to cerebral cortex and also note the thalamus as central switchboard.

Also the pain feeling has two expressions - immediate sharp locating pain (epicritic) and slightly delayed, duller less localized diffuse pain (protopathic) usually accompanied by reddening and slight swelling of the surrounding skin area.  Each of these 2 feelings of pain is transmitted by separate nerve fibers from the source of the needle stick.

The neuron just before the cerebral cortex, in the thalamus, modifies the signal either by inhibiting (damp it down) or exciting (ramp up the pain). It may, in the thalamus, attach an emotion (fear, depression or happiness). This explains why a local feeling of pain may be blocked in certain psychological situation (e.g., a bullet wound in combat may not be felt at all; while, if it happens as an accident at home, it is felt as severe pain).

Sensory pain synapses use the neurotransmitters glutamate (glutamic acid) and gamma amino butyric acid (GABA) respectively as excitatory (increase feeling of the stimulus) or inhibitory (reduce). Each of these amino acid neurotransmitters, attaching to receptor sites, assist in the release of either morphine-like endorphin pain modifier or pain-increasing P substance.

 In sensory stimuli, there is a basic separation of the type of sensory signal. Each of the main stimuli (pain, touch, brushing, heat, cold) has a separate, specific receptor body in the skin at the distal end of the fiber and all the types of sensory feeling remain separate because they travel in separate fibers up to the thalamus. One bundle of neuron fibers in the spinal cord passes pain and thermal stimuli separated from the other bundle whose individual fibers pass touch, pressure and position sense. The pain and thermal fibers run up the spinal cord in a white matter fiber tract that is distinct and is located in lateral frontward outer edge of the cord with destination the thalamus (spino-thalamic tract).  If you could dissect the nerve fiber that is transmitting pain and attach it to an electric stimulus, it would cause the same conscious experience that you feel for example if a pin pricks your finger.  

This separation of different types of sensory feeling in separate nerve fiber bundles is important to the neurosurgeon operating on the spinal cord. The separation of these types of pain starts at their source in the skin or other origin of the pain stimulus, with specific heat, pin-prick, touch, pressure, vibration, etc. pain receptors. This system of neural signals originating separately at the periphery and running in parallel fiber bundles to the thalamus is called labeled lines. The other mode of neural signaling is distributive, where a set of receptors at the periphery is not stimulated and what you experience is related to frequency and amplitude of the electrical signal built into the receptors and interpreted by the thalamus. Pain and other sensory skin feelings use labeled lines.

The next, as sensory fibers enter the spinal cord at each higher level, the last entering level fibers insert close to center, pushing the lower fibers laterally, resulting, at a point in the uppermost cervical spinal cord, in lamina (layers), with the lowest part of body - the buttocks and feet - having its fibers at the very lateral and front of the spinal cord (on both sides, right and left) and the highest part -  the neck and arms - being I nearer the cord's center. This is important to the surgeon doing cancer pain-modification surgery on the cord and also in diseases of the spinal cord white matter and explains location of symptoms. Also important in explaining symptoms of central spinal cord disease - like syringomyelia which causes pressure from center of spinal cord outward - as localizing to neck and arms and sparing lower extremities.

Pain, its subjective experience and other aspects:  All readers know the expected pains - headache, stomach and intestinal cramps, etc. These pains may have various specific qualities - sharp, dull, cramp-like, burning, and may vary in severity but one thing all of them share is that they are expected as part of living and generally the causes are known and not death-dealing.

More Thoughts on Dealing with Pain 

This, the latest addition, is some off-the-top-of-my-head comment on one's approach to pain based on neurological knowledge. It is not remedies although it may be usefully applied. It should be considered experimental in that one tries applications of the knowledge and sees the effect in relieving or preventing a pain. First, as above, realize that one's response to pain has a large emotional, knowledge basis. It means. that mysterious pains (undiagnosed) are the worst type. The first approach to an unusual, pain of uncertain cause is to discover its cause and if that cause is not life-threatening or life-shortening, the pain will fade. Also a life psychoanalysis should have as one aim a generalized reduction in response to pain: a well psychoanalyzed person may still feel the same pain he felt before; but it no longer bothers him as it did before because its mystery and paranoid implications are now understood, and this neutralizes the pain's severity. Keep in mind also, that pain from the skin and underlying tissue is based on the fixed property of the pain receptors: temperature is a key one. A temperature above 42 degrees C will start to be felt as uncomfortably hot.

And knowledge that morphine and other opioids work because they structurally fit keys in the main pain/pleasure mechanisms should help in the use of these medications to treat and to prevent pain and also to produce a psychological anti-paranoid, anti-depression state. Opioids are best for pain that has a heavy psychological overlay (Cancer pains) but for well known pains like in the joints and muscles the NSAIDs starting with acetaminophen (Not a standard NSAID but practically alike.) are best.    

Here I write from direct personal, professional experience of now 25-year use of opioids that has lightened my life and caused no harm. (Because I am a physician and have easy legal, inexpensive, knowledgeable access.) But it is still not for the non-physician especially in areas where opioid-use is criminally treated.  

OK. These are thoughts based on a close knowledge of the neurology of pain that may be added to or modified in further updates.

      END CHAPTER. To read next click 9.17 The Neurology of Emotion