The uncus, also called the uncinate gyrus, is a region of the brain made of white matter that is located on the tip end of the medial surface of the parahippocampal gyrus. It is part of the temporal lobe, which is situated at the bottom of the brain underneath the frontal lobe directly near the base of the brain stem. As part of the olfactory cortex, the uncus processes information from the sense of smell.
Most of the olfactory system of the brain is located in the nasal cavity and the frontal lobe of the brain, not as part of the temporal lobe. Frontal lobe and temporal lobe olfactory regions are connected by the olfactory tract and the anterior commissure. These nerves bring information stored on neurons to the uncus for processing.
Nasal cavities have a mucus membrane that collects sensory information in the form of axons that travels along the first cranial nerve to their first stop at the olfactory bulb. This region is part of the frontal lobe olfactory system. Once moved out of the bulb area, axons head to the uncus, which is part of the olfactory cortex. The two parts of the olfactory cortex are the hook-shaped uncus and another separate portion on the frontal lobe.
After transitioning through the uncus, further processing of olfactory information happens at the insular cortex. Smell and taste information is combined here to send signals to the brain for additional levels of processing, such as the perception of taste and flavor. When there is olfactory nerve damage from head injuries or tumors resulting in the loss of the sense of smell, taste is also usually impacted. Should seizures originate in this area, many individuals report first experiencing strange phantom smells. When the sense of smell is damaged, only one nostril may be affected because the nerve endings are separate for each.
The uncus is located directly on top of the amygdala, almost surrounding it. This area of the brain processes emotion and emotional memory. Many different parts of the brain communicate with the amygdala, including the olfactory bulb.
Increased size of the temporal lobe can cause an uncal herniation. The olfactory system often shows the first indication that there are any major issues with brain function. When this happens, the uncus presses down on a cranial nerve that is adjacent to the brain stem. Untreated, this can lead to central herniation and the possibility of coma and diabetes insipidus.