Drives and Emotions: The Hypothalamus and Limbic System

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23 Drives and Emotions: The Hypothalamus and Limbic System

There is a whole sphere of mental activity that goes beyond simple perception of stimuli and logical formulation of responses. We have drives and urges, and most of our experiences are emotionally colored. This emotional coloring and its relationship with basic drives is the province of the limbic system. The hypothalamus regulates autonomic function and drive-related behavior, and limbic structures serve as bridges between the hypothalamus and neocortex.

The Hypothalamus Coordinates Drive-Related Behaviors

The hypothalamus is a nodal point in the neural circuits underlying drive-related behaviors (Fig. 23-1). It’s got interconnections with visceral parts of the nervous system, through which it is informed of and controls things like blood glucose, blood pressure, and body temperature. It’s also got interconnections with limbic structures, through which you become aware of homeostatic needs (“I’m hungry”). Finally, the hypothalamus has not just neural outputs but also ways to control the pituitary gland.

Hypothalamic Inputs Arise in Widespread Neural Sites

The hypothalamus receives lots of inputs (Fig. 23-3), but most of them are from two general categories: those from nuclei in the brainstem and spinal cord conveying information about the state of your body, and those from limbic structures like the amygdala, hippocampus, and septal nuclei. Inputs about the state of the body (“It’s getting warm in here,” or “Blood glucose is getting low”) arrive from places like the nucleus of the solitary tract by way of the dorsal longitudinal fasciculus, which travels through the periaqueductal gray into the periventricular zone; through the medial forebrain bundle, which travels through the reticular formation into the lateral hypothalamus; and as branches from tracts like the spinothalamic tract. Limbic inputs arrive from the amygdala, from the hippocampus (through the fornix), and from the septal nuclei and other sites (through the medial forebrain bundle); collectively they keep the hypothalamus updated on other aspects of the environment (“Not a good place to take off my shirt”).

Inputs also reach the hypothalamus from the retina and in the form of direct physical stimuli. Axons of some retinal ganglion cells terminate in the small suprachiasmatic nucleus on each side of the anterior hypothalamus. The suprachiasmatic nucleus is the “master clock” for most circadian rhythms, and information from the retina helps get these rhythms synchronized with the 24-hour day (THB6 Figure 23-5, p. 585). Finally, some hypothalamic neurons are sensory receptors themselves, directly responsive to temperature, blood osmolality, or the concentration of some chemicals in blood passing through the hypothalamus.

Hypothalamic Outputs Largely Reciprocate Inputs

Hypothalamic connections with visceral nuclei and limbic structures are largely reciprocal (Fig. 23-4). Projections through the dorsal longitudinal fasciculus and the medial forebrain bundle reach sites like the nucleus of the solitary tract, the dorsal motor nucleus of the vagus, and the intermediolateral cell column

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