Central olfactory connections

M/T cell axons project via the olfactory tract (OT) to the olfactory cortex. This cortex is not common in two respects. At first, it is palaeocortex (old cortex) containing only three layers. Secondly, the only cortex is to receive sensory input directly instead of through the thalamus.

There are five areas of the olfactory cortex with different connections and functions though all receive input from the olfactory tract. The anterior olfactory nucleus project axons across the midline in the anterior commissure to the contralateral olfactory bulb as shown in figure below. The anterior perforated substance (i.e., olfactory tubercle) sends output to the posterior hypothalamus. This pathway, altogether with that to the corticomedial amygdala that then projects to the medial hypothalamus is concerned with the motivational aspects and affective of odors. A pathway from the olfactory tract to the entorhinal cortex, that sends its whole output to the hippocampus, presumably encodes olfactory components of episodic memories.

The pyriform cortex is concerned with olfactory discrimination. It projects, through the medial dorsal thalamus, to the orbitofrontal cortex that mediates the conscious perception of smell. The olfactory processing is subject to considerable modulation. The olfactory bulb receives inputs from noradrenergic and serotonergic neurons in the brainstem and cholinergic neurons in the forebrain. Additionally the anterior perforated substance receives a projection from the brainstem dopaminergic system. These different inputs are implicated in olfactory learning and altering olfaction on the basis of appetite and arousal. Therefore, mitral cell responses to food odors are modulated depending on whether an animal is hungry or sated.

                                      Figure: Connections of the left olfactory cortex. Here, M/T=mitral and tufted cells.