Mechanisms of esophageal pain

Esophageal pain is transmitted via the sympathetic nervous system to the spinal cord, in which pain from visceral and somatic sources ascends to higher centers in the brain. Primary afferent neurons are bipolar, with the peripheral end specialized to be a sensory receptor. Nociceptors of somatosensory afferents are free nerve endings that can be activated by mechanical, thermal, or chemical stimuli. Esophageal nociceptive neurons have not been specifically identified but probably are also free nerve endings. Most esophageal spinal mechanoreceptors have been shown to be nociceptive. Some esophageal mechanonociceptors have a wide dynamic range and respond to physiologic and painful stimuli, while others have a high threshold of stimulation and are solely nociceptive. Esophageal spinal afferents have their cell bodies in the dorsal root ganglia and contain substance P and calcitonin gene-related peptide. These putative neurotransmitters are transported in both the peripheral and central directions of bipolar afferent neurons. Primary afferent neurons are likely to also contain an excitatory amino acid neurotransmitter such as glutamate. Centrally, nociceptive primary afferents terminate on neurons in specific layers of the dorsal horn of the spinal cord. Convergence of multiple visceral afferents with somatic afferents onto the same dorsal horn neurons may explain referred pain. A patient's inability to distinguish esophageal from cardiac pain may be due to convergence of pain pathways. Second-order neurons in the dorsal horn project in the anterolateral system to the brain. Within the anterolateral system, nociception ascends in the spinothalamic, spinoreticular, and spinomesencephalic tracts. The thalamus relays fast pain to the postcentral areas of the parietal lobe of the cortex. Pathways to the reticular formation are slow and may mediate the increased arousal that occurs in response to pain. The spinomesencephalic tract projects to midbrain sites including the periaqueductal gray. Organ-specific pathways in the brain have yet to be defined, but neuroanatomic tracing techniques employing neurotropic viruses are being developed. The perception of pain can be influenced at multiple levels, such as the receptor in the esophagus, the synapses in the dorsal horn of the spinal cord or thalamus, or the cortex. A fundamental mechanism of modulating nociception is descending inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)