Part 1: History / Introduction
Part 3: Pain Signal Transmission and Pain Pathways (please scroll down)
Part 6: Pain in the Avian Species
Part 8: Quality-of-Life Issues
Part 3: Pain Signal Transmission and Pain Pathways
Once pain signals are picked up by the nociceptors, they travel to the spinal cord’s dorsal root, located on the top of the vertebral area. There they connect (synapse) with other neurons which send the signal up the spinal column along the spinothalamic tract. This area is like a highway on which all sensory nerves travel to get to the brain. The messages are sent on afferent nerve fibers, meaning they travel to the spinal cord from the site of injury and then ascend to the brain. Efferent nerve fibers travel from the brain to the spinal cord and to the site of injury.
The messages first enter the brain stem (medulla) and synapse with neurons in the thalamus, the brain’s relay center. Some neurons there control physical behavior, such as pulling away from a painful stimulus. Nerves from the thalamus then relay the signal to the somatosensory center located in the cerebral cortex. From there, the neurons send signals to the motor cortex and down the spinal cord to the motor nerves. These tell you to move your hand away from the source of the pain. All this happens at one hundred meters (0.06 mile) per second; this seems instantaneous to the person injured.
The Brain’s Involvement
Scientists believe that the brain can influence pain perception. Researchers have observed that:
Pain reduces in intensity over time.
Patients who distract themselves consistently find that the pain doesn’t bother them as much.
People who receive placebos for pain control often report that the pain ceases or diminishes.
This indicates that the pain-influencing neural pathways must exist in the efferent (descending) pathways. These efferent pathways originate in the somatosensory cortex, travel through the thalamus, then descend to the midbrain. There, they connect (synapse) with the ascending pathways in the medulla and spinal cord and actually inhibit the ascending nerve signals. The descending pathways prevent the painful nerve signals in the ascending pathways from continuing on to the brain! This produces pain relief (analgesia). In the process of this connection, natural, pain-relieving opiate neurotransmitters are stimulated. These are called endorphins, dynorphins and enkephalins. In a nutshell, the body has a way of diminishing the pain it experiences naturally, offering some relief. In more severe cases, it is insufficient, but in minor pain situations, this works very well.
So what happens in the body when it is in sudden, severe pain? The heart rate and blood pressure increase, the body begins to sweat, and rapid breathing occurs. The more intense the pain, the more extensive are these reactions. They may be depressed somewhat by the brain centers in the cortex as the signals travel through descending pathways, but the ascending pain pathways, travelling through the spinal cord and medulla, can be set off by neuropathic pain—pain caused by damage to peripheral nerves, the spinal cord or the brain itself. This damage can limit the ability of the brain’s descending pathways to produce pain relief.
This may also cause psychogenic pain; pain which has no obvious physical cause.
Thoughts, emotions and nerve circuitry can affect both afferent and efferent pain pathways, resulting in physiological and psychological influences on pain perception. These are:
Age: Brain circuitry usually degenerates with age, so older people have lower pain thresholds and have more difficulty dealing with pain.
Gender: Women have a higher sensitivity to pain than men do. This could be due to sex-linked genetic traits and hormonal changes that alter pain perception. Psychosocial factors might influence the response to pain; men are expected not to show or report their pain, to be more stoic.
Fatigue: lack of sleep and stress both lead to a higher level of pain response.
Memory: Past experiences with pain can influence the neural responses; memory comes from the limbic system, which is related to the somosensory system.
Because animals have a somosensory and limbic system, they do feel emotions and pain. This is not a system such as the digestive system, but a collection of areas of the brain that control emotions. Many of these areas house the two systems collectively.
Muir W III. The Ethics of Pain Management. In: Handbook of Veterinary Pain Management. Ed: James Gaynor, Wm Muir III. Mosby Inc, 2009.
Freudenrich C. How Pain Works.