Projection Pathways for Pain

Pain signals from the face travel mainly by way of the trigeminal nerve to the pons, while signals from the neck down travel by way of spinal nerves to the dorsal horn of the spinal cord. They synapse in the dorsal horn with second-order neurons that decussate and ascend the contralateral spinothalamic tract. The gracile fasciculus carries signals for visceral pain. By any of these pathways, pain signals arrive at the thalamus, where they are relayed to neurons that carry them to their final destination in the primary somesthetic cortex (postcentral gyrus) of the cerebrum (fig. 16.2). Pain signals also travel up the spinoretic-ular tract to the reticular formation and ultimately to the hypothalamus and limbic system. Pain signals arriving here activate visceral, emotional, and behavioral reactions to pain.

Pain in the viscera is often mistakenly thought to come from the skin or other superficial sites—for example when the pain of a heart attack is felt "radiating" along the left shoulder and medial side of the arm. This phenomenon is called referred pain. It results from the convergence of neuronal pathways in the CNS. In the case of cardiac pain, for example, spinal cord segments T1 to T5 receive input from the heart as well as the chest and arm. Pain fibers from the heart and skin in this region converge on the same spinal interneurons, then follow the same pathway from there to the thalamus and cerebral cortex. The brain cannot distinguish which source the arriving signals are coming from. It acts as if it assumes that signals arriving by this path are most likely coming from the skin, since skin has more pain receptors than the heart and suffers injury more often. Knowledge of the origins of referred pain is essential to the skillful diagnosis of organ dysfunctions (fig. 16.3).

Limbic System Pain

Primary somesthetic cortex Somesthetic association area

Third-order neuron Thalamus

Hypothalamus and limbic system

Spinothalamic tract

Reticular formation

Spinoreticular tract

Spinal cord

First-order (afferent) neuron Nociceptor-

Figure 16.2 Projection Pathways for Pain. A first-order neuron conducts a pain signal to the dorsal horn of the spinal cord, a second-order neuron conducts it to the thalamus, and a third-order neuron conducts it to the cerebral cortex. Signals from the spinothalamic tract pass through the thalamus. Signals from the spinoreticular tract bypass the thalamus on the way to the sensory cortex.

Primary somesthetic cortex Somesthetic association area

Third-order neuron Thalamus

Hypothalamus and limbic system

Spinothalamic tract

Reticular formation

Spinoreticular tract

Spinal cord

First-order (afferent) neuron Nociceptor-

Area Sensory Cortex
Second-order (projection) neurons Anterolateral system

Figure 16.2 Projection Pathways for Pain. A first-order neuron conducts a pain signal to the dorsal horn of the spinal cord, a second-order neuron conducts it to the thalamus, and a third-order neuron conducts it to the cerebral cortex. Signals from the spinothalamic tract pass through the thalamus. Signals from the spinoreticular tract bypass the thalamus on the way to the sensory cortex.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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