Pain is the most frequent cause of suffering and disability and is the most common reason that people seek medical attention. It is a major symptom in many medical conditions, significantly interfering with a person's quality of life and general functioning. To understand the physiology and the mechanism of pain as well as optimal methods of control, one must appreciate the anatomical pathways that transmit nociceptive information to the brain. For a better comprehension of the anatomical pathways we divided it into four parts: the peripheral system, the spinal and medullary dorsal horn system, and the ascending and supraspinal system.

The pain pathway can be envisioned as a three-neuron pathway that transmits noxious stimuli from the periphery of the cerebral cortex.

The primary afferent neurons are located in the dorsal root ganglia, which lie in the vertebral foramina at each spinal cord level. Each primary afferent neuron has a single bifurcating axon, one end going to the peripheral tissue it innervates and the other going to the dorsal horn of the spinal cord, which receives sensory input.

In the dorsal horn of the spinal cord, the primary afferent neuron synapses with a second-order neuron whose axons cross the midline of the cord and ascend in the contralateral spinothalamic tract to reach the thalamus. Once in the dorsal horn, in addition to synapsing with second-order neurons, the axons of first-order neurons may synapse with interneurons, sympathetic neurons, and ventral horn motor neurons.

Second-order neurons synapse in thalamic nuclei with third-order neurons, which in turn send projections through the internal capsule and corona radiata to the postcentral gyrus of the cerebral cortex. At each point along the pathway there are several options for longer routes and for modification, and or integration of the information (Fig. 3.1) (Besson 1999).

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