Key Points

The autonomic nervous system influences the general emotional status of the individual. The network involved originates from small groups of neurons within the brain stem, with extensive projections to widespread areas throughout the neural axis.

Pleasurable feelings are associated with activity in the parasympathetic nervous system; feelings of anxiety are associated with activity in the sympathetic nervous system.

The Papez circuit links the site of emotional awareness, the cingulate cortex, with appropriate portions of the autonomic nervous system and thereby triggers the physical expression of the emotion.

The hypothalamus generates aggressive behavior patterns based on input from the Papez circuit and from the amygdala. Neuronal circuits that result in pleasurable sensations constitute a reward or reinforcement system for certain survival behaviors. Neuronal groups within the brain stem that utilize dopamine, norepinephrine, and serotonin provide diffuse modulatory systems that influence pleasure centers and establish the general state of awareness of the brain. The brain stem reticular-activating system helps establish different functional states of the brain resulting in coordinated patterns of neuronal activity ("brain waves'') that can be monitored by means of an electroencephalogram.

In contrast to the rapid, point-to-point transfer of information within sensory and motor pathways, other types of neuronal circuits are present in the nervous system that operate on a much slower time scale and have much broader, less-defined targets. These are described as diffuse modulatory systems. One of the best examples is the autonomic nervous system (ANS) (described in

Chapter 9). As implied by its name, the ANS is primarily a reflex, nonvolitional system that is only indirectly influenced by the motor cortex. Preganglionic motor neurons that function in the sympathetic division of the ANS are located in the thoracic and upper lumbar portions of the spinal cord gray matter with fibers exiting in the spinal nerves. In the parasympathetic division, preganglionic motor neurons are located in brain stem nuclei, with exiting fibers in the cranial nerves, or in the sacral portion of the spinal cord, with fibers exiting accordingly. These preganglionic fibers project to postganglionic motor neurons within sympathetic and para-sympathetic ganglia that, in turn, provide dual and, in most cases, antagonistic innervation of glands and smooth muscles. Input to preganglionic neurons comes from auto-nomic nuclei in the brain stem that act as centers for coordinating autonomic responses of spinal and cranial nerves to regulate breathing rate, heart rate, bladder function, and so on. These autonomic centers receive direct input from the amygdala and the hypothalamus but no direct innervation from the somatic motor cortex. Thus, there is no conscious control of autonomic function (Fig. 1).

The diffuse, modulatory nature of the ANS is best exemplified by the parasympathetic portion of the vagus nerve (cranial nerve X), which originates from a small group of neurons in the vagal motor nucleus of the medulla. These neurons provide preganglionic fibers that synapse in a large number of diverse parasympathetic ganglia controlling muscles of the esophagus, heart, peritoneal cavity, and intestine. Compared to the somatic motor and sensory system, the vagal parasympathetic pathway evokes relatively slow, sustained responses that are coordinated throughout the extensive and broadly distributed group of target tissues. The vagal motor nucleus functions as a master control point, receiving and summating a wide range of sensory inputs and sending signals that in essence elicit a whole-body response. In some cases, all components of the parasympathetic division may react together. For example, vagal stimulation slows the heart and, along with other parasympa-thetic centers, facilitates food ingestion and digestion.

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