Painting of Yama, the Indian God of death, made in Tibet around the 17th century. In Tibetan Buddhism Yama protects people against emotional addictions such as lust and hate. While religion, mythology, and medicine have been invoked throughout history to explain our emotions and control our appetites, Buddhism has articulated particularly well the perils of human craving and the benefits of expanding the scope of the conscious mind. This is a detail of the full painting shown above. (Reproduced, with permission, from image copyright: the Metropolitan Museum of Art; image source: Art Resource, NY.)
Many aspects of behavior, especially emotional and homeostatic behaviors, are unconscious and instinctive. They are mediated almost reflexively by systems in subcortical brain regions that are concerned with feeding, drinking, temperature regulation, and sex.
Thus, as Sigmund Freud first pointed out in 1900, we experience emotional states not only consciously but also unconsciously. Many of these emotional states, particularly those involving fear, depend on the amygdala, a subcortical region of the limbic system. The cognitive elements in emotions, which we call feeling states, are thought to be mediated by pathways to the cerebral cortex that originate from the musculature of the body and the internal organs, on the one hand, and from the amygdala, on the other hand. In contrast, unconscious emotional states are thought to depend on autonomic, endocrine, and skeletal motor responses in subcortical parts of the nervous system, especially connections between the nuclei of the amygdala, the hypothalamus, and the brain stem. These unconscious responses can prepare the body for action and communicate internal emotional states to other individuals. An emerging realization in the neurobiology of emotion is that an unconscious representation of our emotional state by the amygdala can lead to a somatic response that often precedes our cognitive awareness—our feeling—of an emotional state. This conscious feeling state presumably involves the cerebral cortex, the outer layer of the brain. When the function of the cerebral cortex is temporarily disrupted, as in an epileptic convulsion, we lose all sense of emotion and feeling, and commonly, we lose consciousness.
We begin our consideration of these systems with the brain stem, a structure critical for wakefulness and conscious attention on the one hand and sleep on the other. Thus, the significance of this small region of the central nervous system—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 the brain stem contains all of the ascending tracts that bring sensory information from the surface of the body to the cerebral cortex and all the descending tracts from the cerebral cortex that deliver motor commands to the spinal cord. Damage to the brain stem also can affect consciousness and sleep because the brain stem contains the locus ceruleus, a center thought to be crucial for attention and therefore for many cognitive functions. In fact, fully half of all noradrenergic neurons of the brain are clustered together in this small nucleus. 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 regulate 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 reinforcement of behavior 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, one of whose functions is 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 volume of the brain. The hypothalamus, with closely linked structures in the brain stem and limbic system, acts directly on the internal environment through its control of the endocrine system and autonomic nervous system to achieve goal-directed behavior. It acts indirectly through its connections to higher brain regions to control emotional and motivational states. In addition to regulating specific motivated behaviors, the hypothalamus, together with the brain stem below and the cerebral cortex above, maintains a general state of arousal, which ranges from excitement and vigilance to drowsiness and stupor.