Many automatic functions are handled by a part of the brain called the hypothalamus. Within this region, there is an oval-shaped group of cells called the preoptic area. Primarily, this area is responsible for thermoregulation, the process by which the body maintains a constant temperature by gaining or losing heat. Certain components of this area serve other functions, such as creating thirst, inducing sleep, and regulating male sexual behavior.
Keeping the body temperature constant is an important unconscious process for warm-blooded animals. Special sensory cells known as thermoreceptors located in the skin and certain membranes detect temperature changes, and relay this information to the preoptic area. After receiving this information, this region sends messages to appropriate parts of the hypothalamus responsible for temperature responses. In turn, these regions generate automatic responses to heat or cold, depending on the preoptic area's output.
This area of the hypothalamus actually contains several smaller nuclei, or groups of neurons, each with their own unique functions. Located in the center of this area, the median preoptic nucleus helps to regulate thirst. Sensory cells that detect a lack of water due to loss of their own volume send signals to the median preoptic nucleus. The nucleus then releases the chemical norepinephrine to higher processing centers that create the conscious feeling of thirst. Norepinephrine production stops after the individual consumes water, stopping the feeling of thirst.
Another important nucleus in the preoptic area is the lateral preoptic nucleus, located toward the edge of this area. Cells involved in thermoregulation are found here, but this nucleus serves another important function. It helps to signal non-rapid eye movement (REM) sleep through the release of molecules like galanin, which inhibits the activity of other neurons.
Between the lateral and median preoptic nuclei lies the medial preoptic nucleus. This region is sometimes called the sexually dimorphic nucleus because it shows a size difference between genders. Males have a larger medial preoptic nucleus, and it is more spherical in shape. Additionally, females tend to experience more cell death in this region due to a lack of activity.
Molecules like dopamine stimulate cells in this portion of the preoptic area, and when stimulated, these neurons regulate male sexual behavior. Animal studies have shown that stimulation results in copulation behavior and the release of compounds like gonadotropin-releasing hormone. Higher dopamine stimulation is seen in response to testosterone levels, as well as by sensory stimulation, such as the presence of a female.