Section Summary

This section summarizes the neural structures in the peripheral sensory system as the starting point to receive, transduce, and propagate various modalities of somatosensory information to the central nervous system. Understanding various types of sensory receptors, their axon types, cell body location, and major neurotransmitters used will provide a solid foundation for understanding at the next level of neural transmission, the central nervous system.

This section covers the anatomical organization of the spinal cord, pathways to the brainstem and thalamus, and the primary somatosensory cortex, in addition to the gate control system, descending inhibitory system, and limbic system. With this knowledge, not only do we understand the somatosensory information flow to the brain to enable perception, but also understand how the peripheral stimulation to the body (touch, burning, pain, and other inputs) can generate emotional responses. Returning to our example from the beginning of this section, we can appreciate how getting stepped on once activated our physical and emotional response to pain, but we could use our posterior parietal cortex to integrate multiple sources of information (the apology) and plan a calm response (using our prefrontal cortex). But when we get stepped on again, the same stimulus integrates with our memories to yield a stronger emotional response the second time. This example shows how sensation is more than just the activation of peripheral receptors; it interacts with multiple brain systems to help guide our behaviors.

The emotional experience of pain is contributed to significantly by the limbic system structures, which also play a critical role in the regulation of our emotions, such as depression. Clinically, increased pain will make depression worse, and vice versa; proper treatment of one will also relieve the other. Anatomical and physiological evidence suggests that they share the same neural circuitry.

Although pain and itch share similarities in activities in both C- and Aδ-fibers and both are subjective in nature, their functional mechanisms are quite different. While pain can be elicited by various stimulation (mechanical, thermal, chemical, and electrical), itch is mainly induced by chemical stimulation.

Treatment of pain can be delivered through various strategies, from the over-the-counter medication to prescription drugs, behavioral cognitive treatment to physical therapy, and other instrumental measures including invasive or non-invasive stimulation of the nervous system (peripheral and central), and finally, surgical options.