The uncus is a specific part of the brain located within the temporal lobe. It is a structure involved in various neurological functions, playing a role in sensory processing and memory. This region is a component of the limbic system, which is broadly associated with emotion, motivation, and memory.
Anatomical Location and Structure
The uncus is a hook-shaped limbic structure found on the anteromedial aspect of the parahippocampal gyrus, situated in the mediobasal region of the temporal lobe. Its name originates from the Latin word “uncus,” meaning hook, which describes its characteristic appearance. This brain region lies adjacent to the hippocampus and entorhinal cortex, making it an integral part of the medial temporal lobe.
The uncus is composed of two main segments: an anterior and a posterior part. The anterior segment contains structures like the ambient gyrus and the semilunar gyrus, which overlie the amygdala. The posterior segment houses the head of the hippocampus. An important anatomical landmark on its inferior surface is the band of Giacomini, which represents a continuation of the dentate gyrus. While superficially continuous with the parahippocampal gyrus, it forms a distinct part of the rhinencephalon.
Primary Functions
The uncus plays a role in the processing of olfactory (smell) information. It receives fibers from the olfactory tract, making it a component of the primary olfactory cortex. This connection helps explain how certain smells can evoke strong emotional responses and memories.
Beyond olfaction, the uncus is also involved in memory processing, particularly emotional memory. Its close proximity and connections to the amygdala, central to emotional processing, and the hippocampus, crucial for memory formation, allow for the integration of sensory input with emotional significance. It also influences emotional regulation and the response to stress as part of the broader limbic system.
Clinical Significance
The uncus is clinically significant, particularly when affected by increased intracranial pressure or neurological conditions. One serious medical emergency involving this region is uncal herniation. This occurs when rising pressure inside the skull, often due to a tumor, hemorrhage, or swelling, pushes the uncus over the tentorial notch, a rigid structure within the skull.
Uncal herniation can compress the brainstem and the oculomotor nerve (cranial nerve III). Compression of the oculomotor nerve leads to specific symptoms, such as the pupil on the affected side becoming dilated and unresponsive to light, along with an inability to move the eye medially or superiorly. Continued pressure on the midbrain can result in progressive lethargy, coma, and ultimately death due to disruption of the brain’s arousal system. Early recognition and aggressive treatment are important for potential reversal of this life-threatening condition.
The uncus is also a common origin for temporal lobe seizures, sometimes referred to as uncinate fits. Seizures originating in this area can manifest with characteristic olfactory or gustatory hallucinations, where individuals experience phantom smells or tastes. Damage to the uncus or surrounding medial temporal lobe structures can also lead to memory impairments, especially related to emotional memories. Conditions like Kluver-Bucy syndrome, characterized by behavioral changes such as hypersexuality and hyperorality, can also result from damage to the uncus and its associated structures.