Porth's Essentials of Pathophysiology, 4e

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Somatosensory Function, Pain, and Headache

C h a p t e r 3 5

Cortical centers

Somesthetic association cortex

Cognition, anxiety, insomnia

Perception and meaning

Primary somesthetic cortex Discrimination, location, intensity

Thalamus Thalamus

Limbic cortex Emotional experience

Hypothalamus Autonomic and endocrine responses

Midbrain periaqueductal gray Endogenous analgesic center

Pontine noradrenergic neurons

Brain stem RAS Increased alertness

Medullary nucleus raphe magnus

Paleospinothalamic tract (dull, aching pain)

Neospinothalamic tract (sharp, bright pain)

Spinal cord and dorsal horn modulating circuits

Dorsal root ganglia

C-fiber (slow)

A-delta (fast)

Primary touch fibers

Nociceptive stimuli

FIGURE 35-8. Primary pain pathways.The transmission of incoming nociceptive impulses is modulated by dorsal horn circuitry that receives input from primary touch receptors and from descending pathways that involve the limbic cortical systems (orbital frontal cortex, amygdala, and hypothalamus), the periaqueductal endogenous analgesic center in the midbrain, pontine noradrenergic neurons, and the nucleus raphe magnus (NRM) in the medulla. Dashed lines indicate inhibition or modulation of pain transmission by dorsal horn projection neurons. RAS, reticular activating system.

lateral thalamus and the somatosensory cortex are neces- sary to add precision, discrimination, and meaning to the pain sensation. The paleospinothalamic system projects diffusely from the intralaminar nuclei of the thalamus to large areas of the limbic cortex. These connections prob- ably are associated with the hurtfulness and the mood- altering and attention-narrowing effect of pain. Recent research has demonstrated cortical repre- sentation of fast–sharp and slow–chronic types of pain sensation. In healthy adults, nociceptive A δ afferent stim- ulation is related to activation in the contralateral pri- mary somatosensory cortex in the parietal lobe, whereas

C afferent stimulation is related to activation of the sec- ondary somatosensory cortices and the anterior cingu- lated cortex, which is part of the limbic system. With both afferents there is activation of the bilateral secondary somatosensory cortices in the posterior parietal lobes. 11 Central Pathways for Pain Modulation A major advance in understanding pain was the discov- ery of neuroanatomic pathways that arise in the mid- brain and brain stem, descend to the spinal cord, and modulate ascending pain impulses. One such pathway