Transmission starts in the peripheral nerves. In myelinated fibers such as A delta, the action potential leaps from one node of Ranvier to the next between stretches of myelin (saltatory conduction), while unmyelinated C fibers conduct it more slowly as a continuous wave along the membrane. Whatever the fiber, the primary afferent neurons, the first messengers of pain, have their nerve cell bodies neatly tucked away in the dorsal root ganglion.
As the signal zips along, it reaches the spinal cord, entering through the dorsal horn where it makes its first synaptic connection with secondary neurons. These connections are primarily found in specific layers of the spinal cord known as Rexed laminae I, II, or V. Here, the plot thickens—the action potential crosses to the opposite side of the spinal cord, a maneuver known as decussation, and ascends towards the brain via the spinothalamic tract, like an express train heading towards its main station.
The thalamus plays a pivotal role as the relay station, receiving the pain message and forwarding it to the tertiary neurons. These neurons then send the signal to various cerebral cortices including the somatosensory cortex, which helps pinpoint the exact location of the pain and engage the appropriate emotional and physical responses. This complex network also involves other key brain areas like the anterior cingulate cortex, insular cortex, and motor cortex, each adding a layer to the pain experience, from emotional coloring to preparing the body for action.
The neurotransmitters that act as chemical messengers in this journey are diverse; glutamate and substance P take center stage in the spinal cord, while others like GABA, norepinephrine, serotonin, dopamine, and endogenous opioids play supporting roles in the brain, each adding a nuance to how pain is perceived and responded to.
Furthermore, an intriguing aspect of this signaling is referred pain, where signals from the viscera (such as the heart) are misinterpreted by the brain as coming from more commonly stimulated areas of the body. This phenomenon explains why a heart attack might be felt as pain in the arm or shoulder—demonstrating the body’s sometimes flawed interpretation system in the complex map of pain.
