Know your brain: Olfactory bulb

Where is the olfactory bulb?

The olfactory bulb is a structure found on the inferior (bottom) side of the cerebral hemispheres, located near the front of the brain. There is an olfactory bulb at this location in both cerebral hemispheres. The olfactory bulb is attached to the cerebral hemisphere by a long stalk often referred to as either the olfactory stalk or olfactory peduncle.

What is the olfactory bulb and what does it do?

The olfactory bulb is an essential structure in the olfactory system (the system devoted to the sense of smell). Olfaction begins when odorant molecules enter the nasal cavity through inhalation or by rising from the mouth (e.g. during the chewing of food). Those molecules interact with olfactory receptors, which are part of a family of G-protein coupled receptors. Stimulation of these receptors causes the production of second messengers like cyclic AMP (cAMP), which leads to the opening of ion channels and the generation of action potentials in olfactory receptor cells.

The axons of these olfactory receptor cells terminate in the olfactory bulb, where they converge on the dendrites of olfactory bulb neurons in small clusters called glomeruli (plural for glomerulus, which is a term sometimes used in anatomy to refer to a small cluster of structures). Each glomerulus consists of the axons of several thousand olfactory receptor neurons converging on the dendrites of a small collection (~40 to 50) of olfactory bulb neurons, and each glomerulus only receives input from one type of odorant receptor. This does not mean that each glomerulus is only capable of detecting one odor, as each type of odorant receptor is capable of detecting multiple odorants. The olfactory bulb is patterned in such a way, however, that similar odorants often stimulate glomeruli found close to one another in the olfactory bulb. This creates an organization in the olfactory bulb that seems to be related to odorant structure.

There are several types of neurons in the olfactory bulb. These include mitral cells, tufted relay neurons, granule cells, and periglomerular neurons. The mitral cells and tufted relay neurons form connections with olfactory receptor neurons in the glomeruli. They receive olfactory information and then carry it from the olfactory bulb to the olfactory cortex, the main site for the processing of olfactory information. The olfactory cortex consists of several cortical areas that receive information from the olfactory bulb, including the piriform cortex, entorhinal cortex, an area of cortex covering the amygdala known as the periamygdaloid cortex, and two regions known as the olfactory tubercle and anterior olfactory nucleus, respectively. Granule cells and periglomerular neurons are both interneurons that are thought to be involved with fine-tuning the processing of olfactory information by doing things like helping to sharpen the contrast between different odorants.

The olfactory bulb tends to be much smaller in humans and other primates than in animals that rely more heavily on a sense of smell to provide them with information about their environment (e.g. rodents, dogs, etc.). Assertions that the human sense of smell is "underdeveloped" due to lack of importance may be overblown, however. Studies suggest humans may be able to detect up to a trillion different odors and that we are capable of using olfaction much more extensively when asked to complete a task that relies heavily on olfaction. Also, those who intentionally test their olfactory system regularly (e.g. wine tasters) are able to demonstrate vastly refined olfactory perception. Humans have even been found to be able to utilize the same type of scent tracking used by animals like bloodhounds. Thus, it may be that we have the capacity for greater olfactory discrimination but not a pressing need to refine these skills except in certain circumstances.

The olfactory bulb is also a brain region of interest because it is one of the few places in the brain where new neurons appear over the course of the lifespan. This phenomenon has mostly been observed in rodents, however, and there is some debate about its prevalence and/or importance in humans. In rodents, the new neurons that are added to the olfactory bulb are primarily produced in an area known as the ventricular zone, which lines the walls of the lateral ventricles. The new neurons then migrate to the olfactory bulb, where they differentiate into specific functional cell types. Estimates are that thousands of new olfactory bulb neurons are produced every day in the rodent brain. The reasons for this prolific neurogenesis in the olfactory bulb are not clear, although it has been proposed that it is an important component of synaptic plasticity in the structure and that it might help the olfactory bulb to adapt to the frequently changing composition of olfactory receptor neurons, which only have about a 60 day lifespan in rodents.

References (in addition to linked text above):

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York.