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Embracing couple mourning someone’s death, perhaps buried in a nearby funerary urn. (Mali, Djenné style. Inland Delta of the Niger River, 13th–15th centuries AD. University of Iowa Stanley Museum of Art, The Stanley Collection of African Art. X1986.451.)


EMOTIONAL AND HOMEOSTATIC BEHAVIORS ALL INVOLVE the coordination of one or more somatic, autonomic, hormonal, or cognitive processes. Subcortical brain regions concerned with a range of functions—including feeding, drinking, heart rate, breathing, temperature regulation, sleep, sex, and facial expressions—play a critical role in this coordination. Subcortical brain regions are bidirectionally connected with cortical brain areas, providing a means for reperesentations of internal state variables (eg, visceral information) to influence cognitive operations, such as subjective feelings, decision-making, and attention, and for cognitive functions to regulate or extinguish neural representations in subcortical brain areas that help coordinate behavior reflecting emotional states.

Our consideration of these systems begins with the brain stem, a structure critical for wakefulness and conscious attention on the one hand and sleep on the other. The significance of this small region of the brain—located between the spinal cord and the diencephalon—is disproportionate to its size. Damage to the brain stem can profoundly affect motor and sensory processes because it contains all of the ascending tracts that bring sensory information from the surface of the body to the cerebral cortex and all of the descending tracts from the cerebral cortex that deliver motor commands to the spinal cord. Finally, the brain stem contains neurons that control respiration and heartbeat as well as nuclei that give rise to most of the cranial nerves that innervate the head and neck.

Six neurochemical modulatory systems in the brain stem modulate sensory, motor, and arousal systems. The dopaminergic pathways that connect the midbrain to the limbic system and cortex are particularly important, because they are involved in processing stimuli and events in relation to reinforcement expectation, and therefore contribute to motivational state and learning. Addictive drugs such as nicotine, alcohol, opiates, and cocaine are thought to produce their actions by co-opting the same neural pathways that positively reinforce behaviors essential for survival. Other modulatory transmitters regulate sleep and wakefulness, in part by controlling information flow between the thalamus and cortex. Disorders of electrical excitation in corticothalamic circuits can result in seizures and epilepsy.

Rostral to the brain stem lies the hypothalamus, which functions to maintain the stability of the internal environment by keeping physiological variables within the limits favorable to vital bodily processes. Homeostatic processes in the nervous system have profound consequences for behavior that have intrigued many of the founders of modern physiology, including Claude Bernard, Walter B. Cannon, and Walter Hess. Neurons controlling the internal environment are concentrated in the hypothalamus, a small area of the diencephalon that comprises less than 1% of the total brain volume. The hypothalamus, with closely linked structures in the ...

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