Islands of Calleja - Structure and Neuronal Pathways

Structure and Neuronal Pathways

In rodents, the islands of Calleja are composed of seven distinct clusters within the olfactory tubercle, with the major island creating a border between the septum, the nucleus accumbens, and the diagonal band. Some of the islands contain a "core" of neuropil, or unmyelinated axons and dendrites, filled with a large cell in some cases. Projections to and from the islands connect the structures to the piriform cortex, which is responsible for processing smell, as well as areas of the basal forebrain, a region responsible for determining an animal's level of wakefulness. The projections with the piriform cortex align with the rest of the olfactory system, the path beginning in the sensory cells of the nose and then proceeding through the olfactory bulb to regions such as the piriform cortex, olfactory tubercle, and amygdala. However, the projections to the basal forebrain structures actually originate from the smaller outer cells of the island clusters as opposed to the large cells within the "core."

The islands of Calleja receive inputs from the back of the amygdala, which processes emotional memory, as well as the septum, nucleus accumbens, and piriform cortex. The islands also receive information in the form of dopamine from the substantia nigra and ventral tegmental area, located in the midbrain of the brainstem. Information traveling from these midbrain structures is triggered in response to rewarding activities or feelings. The islands project to the back of the thalamus, a region with many responsibilities such as sensory information processing, regulation of wakefulness, and relaying information to the cerebral cortex.