Pain, Spinothalamic Pathways and Pain Management

By Emmanuela P François

The International Association for the Study of Pain defined pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” (Lipton, 1991) and Karpurkar et al., defined it as “an unpleasant emotional experience usually initiated by a noxious stimulus, mediated over a specialized neural network to cortical and subcortical centers where it is interpreted as such” (Kapurkar 2015). These noxious stimuli activate nociceptors, the free nerve endings found in muscles, joints, bones, viscera and skin that detect signals from damaged tissue and indirectly respond to chemicals such as bradykinin, ATP and K+ released from the damaged tissue (Snell, 2010).

The nerve fibers transmitting pain permeate almost every part of the body and send electrical signals from the site of the stimulus to the spinal cord when stimulated by an injury. Pain is detected via two main types of nerve fibers: A- fibers and C-fibers. A fibers are thin, myelinated fibers found mostly in subcutaneous tissue that carry sharp, localized pain signals to the brain while C-fibers are classified as slow, unmyelinated fibers. These electrical signals travel to the spinal cord via nerve fibers and most of these fibers enter the dorsal horn and then decussate before travelling to the brain to terminate in the primary somatosensory cortex (Snell, 2010). These spinothalamic tracts include the neospinothalamic tract which forms a “direct connection” with the thalamic nuclei and sensory cortex of the brain to provide the fast and pinprick sensations of pain immediately following an injury, the paleospinothalamic tract which transmits the slow and dull characteristics of pain and the archispinothalamic tract which provides collaterals that innervate the hypothalamus and the limbic system (UTHealth, 2016).

Pain may be classified as nociceptive, inflammatory, neuropathic and referred. Nociceptive pain involves a normal response to noxious stimuli from muscles, joints, bones, viscera and skin. Facial pain, an example of this type of pain, results from trigeminal nerve stimulation and rheumatoid arthritis, herpes zoster, inflammatory bowel disease and appendicitis are examples of inflammatory pain which results from the activation of the nociceptive pain pathway by a variety of chemicals released at a site of tissue inflammation. Post-stroke central pain, diabetic neuropathy, phantom limb and spinal cord injuries are classified as neuropathic pain which is characterized by malfunction in the nervous system itself rather than an injury. A nerve may be severed or it may be stimulated so often that it gets into the habit of sending pain signals to the brain and the cortical neurons may become sensitized so that even when there are no external stimuli, they produce the experience of pain (University of Wisconsin, 2010). Referred pain results when nerve fibers from areas of low sensory input and those from areas of high sensory input enter the spinal cord at the same location. The brain then misinterprets the location of the pain since the brain expects to be receiving the data from areas of high sensory input (Snell, 2010). This is often the case for individuals who are experiencing a heart attack. Nerves from the heart converge with those of the arm as they enter the spinal cord and the brain misinterprets the location of the pain as coming from the arm rather than the heart.

Pain management should take into account the many factors affecting pain. Emotional status and age are important factors. The pain threshold depends greatly on attitude towards the procedure involved. If the individual is anxious or emotionally unstable, the pain threshold may be low but the reaction is high. Pain reaction threshold is usually high in subjects who are relaxed and have a good night sleep compared to those who are tired. Older individuals tend to tolerate pain better and most often understand that unpleasant experiences are part of life (University of Wisconsin, 2010). This wisdom may account for the fact that they have higher pain threshold than young individuals.

Pain cannot be treated on the basis of intensity alone, therefore, the patient must be thoroughly evaluated to determine what would be the most meaningful pain relief for that patient. Many techniques are now available for treatment of pain. Psychological tools involve the use of behavioral methods to help patients manage their pain. Such cognitive therapy involves teaching the patient to alleviate pain by means of relaxation and coping techniques (Ulrich Jr., 2000). In the case of burn victims, pain relief was achieved by using virtual reality as a focal point. Patients were immersed in a virtual cooling environment and this virtual reality was so distracting that less attentional resources were available for the brain to process pain signals (Schuenke et al., 2010). Electronic stimulation and chiropractic manipulation are also thought to help relieve pain by taking the pressure off sensitive neurological tissue. The Gating Theory was suggested to explain the pain relief by such manipulation. The theory suggested that tactile stimulation closes the gate for pain while the analgesia system suggests that stimulation of certain areas of the brainstem can reduce or even block the sensation of pain and may be responsible for suppressing both sharp and pricking pain as well as burning pain (Snell, 2010).

Our thoughts, emotions, and expectations can all have a profound effect on the degree of pain. People can affect pain consciously by directing attention away from it, or even imagining that they are pain-free. It has been found that an intensely imagined experience generates almost identical brain activity to the equivalent real experience, therefore, even when pain fibers are being stimulated, an imagined state of being pain-free may be achieved (Dubuc, 2016). The prefrontal cortex as well as the anterior cingulate cortex play an important role in interrupting pain signals and diverting attention away from pain. This is the reason why the placebo effect may be a successful way to manage pain.

Pain involves practically every part of the brain. The somatosensory cortex, the cingulate cortex, and the insular cortex play a role in registering, assessing and pinpointing the site of pain signals, while the motor cortex and supplementary motor area plan and execute movement to escape pain stimuli (Dubuc, 2016). Part of the parietal cortex is involved in directing attention to the threat and several parts of the frontal cortex are involved in determining the significance of the pain and what actions should be taken against it (Craig, 2003).

Pain and its knowledge of transduction, transmission, perception and modulation are essential in pain management. Understanding how pain signals reach the brain and understanding the many different techniques that may result in pain relief is imperative. Whether treatments involve psychological tools, electrical stimulation, drugs, and/or surgery, healthcare professionals must effectively evaluate the patient and recommend the treatments that will result in maximum physical ease.

Pain affects the whole person and some may believe that the inability to feel pain may be desirable, but in reality such an anomaly may be disastrous to our health. Individuals with congenital analgesia, a rare genetic disorder that results in the lack of pain nerve endings, are in constant danger. They do not have the ability to sense pain and therefore are unable to protect themselves or are unable to remove themselves from harmful threats. Pain, although most often associated with a negative experience, is very important to our survival. 


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