Once nociceptive activity in the spinal cord is above threshold levels, action potentials from the first order neuron are transmitted to the second order neuron of the spinothalamic tract. From there they are transmitted to higher pain-regulating centers In the substantia gelatinosa of the posterior column of the spinal cord, pain afferent stimuli directly, via interneuronal connections, interact with the lateral column of the spinal cord and transfer to the motor tract of the anterior column. It is here, where in the same segment or via collateral fibers to several neighboring segments, nociceptive impulses are switched to vegetative as well as motor neurons (= conversion afferents; Figure I-37b). This explains the development of muscle spasms of the abdominal wall during pain, the expansion of visceral-sensitive pain (Figure I-37c), accompanying vegetative circulatory effects, activation of sweat glands, and the projection of pain from deep visceral structures to specific areas of the skin ("head zones"; Figure I-37a). The latter are transmitted from the same spinal cord segment as the affected internal organ. Nociceptive visceral activity can be transmitted on to motor neurons with activation of smooth muscle activity and contraction, resulting in a vicious circle. Such a development is elucidated by free nerve endings within the smooth muscle, which are sensitive to contraction, resulting in a reinforcement of the nociceptive barrage and an increase in pain sensation (Figure I-38).
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