structural:scholar_edition
Central

The main olfactory bulb transmits pulses to both mitral and tufted cells, which help determine odor concentration based on the time certain neuron clusters fire (called 'timing code'). These cells also note differences between highly similar odors and use that data to aid in later recognition. The cells are different with mitral having low firing-rates and being easily inhibited by neighboring cells, while tufted have high rates of firing and are more difficult to inhibit. How the bulbar neural circuit transforms odor inputs to the bulb to the bulbar responses that are sent to the olfactory cortex can be partly understood by a mathematical model.

The uncus houses the olfactory cortex which includes the piriform cortex (posterior orbitofrontal cortex), amygdala, olfactory tubercle, and parahippocampal gyrus.

The olfactory tubercle connects to numerous areas of the amygdala, thalamus, hypothalamus, hippocampus, brain stem, retina, auditory cortex, and olfactory system. In total it has 27 inputs and 20 outputs. An oversimplification of its role is to state that it: checks to ensure odor signals arose from actual odors rather than villi irritation, regulates motor behavior (primarily social and stereotypical) brought on by odors, integrates auditory and olfactory sensory info to complete the aforementioned tasks, and plays a role in transmitting positive signals to reward sensors (and is thus involved in addiction).

The amygdala (in olfaction) processes pheromone, allomone, and kairomone (same-species, cross-species, and cross-species where the emitter is harmed and the sensor is benefited, respectively) signals. Due to cerebrum evolution this processing is secondary and therefore is largely unnoticed in human interactions. Allomones include flower scents, natural herbicides, and natural toxic plant chemicals. The info for these processes comes from the vomeronasal organ indirectly via the olfactory bulb. The main olfactory bulb's pulses in the amygdala are used to pair odors to names and recognize odor to odor differences.

Stria terminalis, specifically bed nuclei (BNST), act as the information pathway between the amygdala and hypothalamus, as well as the hypothalamus and pituitary gland. BNST abnormalities often lead to sexual confusion and immaturity. BNST also connects to the septal area, rewarding sexual behavior.

Mitral pulses to the hypothalamus promote/discourage feeding, whereas accessory olfactory bulb pulses regulate reproductive and odor-related-reflex processes.

The hippocampus (although minimally connected to the main olfactory bulb) receives almost all of its olfactory information via the amygdala (either directly or via the BNST). The hippocampus forms new and reinforces existing memories.

Similarly, the parahippocampus encodes, recognizes and contextualizes scenes. The parahippocampal gyrus houses the topographical map for olfaction.

The orbitofrontal cortex (OFC) is heavily correlated with the cingulate gyrus and septal area to act out positive/negative reinforcement. The OFC is the expectation of reward/punishment in response to stimuli. The OFC represents the emotion and reward in decision making.

The anterior olfactory nucleus distributes reciprocal signals between the olfactory bulb and piriform cortex. The anterior olfactory nucleus is the memory hub for smell.

When different odor objects or components are mixed, humans and other mammals sniffing the mixture (presented by, e.g., a sniff bottle) are often unable to identify the components in the mixture even though they can recognize each individual component presented alone. This is largely because each odor sensory neuron can be excited by multiple odor components. It has been proposed that, in an olfactory environment typically composed of multiple odor components (e.g., odor of a dog entering a kitchen that contains a background coffee odor), feedback from the olfactory cortex to the olfactory bulb suppresses the pre-existing odor background (e.g., coffee) via olfactory adaptation, so that the newly arrived foreground odor (e.g., dog) can be singled out from the mixture for recognition.