The olfactory bulb or bulbus olfactorius processes sensory stimuli from the nose and is part of the olfactory pathway. It is located at the frontal lobe base of the brain and has special types of neurons called mitral, brush, and granule cells. Damage and functional impairment in the olfactory bulb result in various olfactory disorders.
What is the olfactory bulb?
The sense of smell is one of five human senses and enables olfactory perception. With its help, humans recognize edible food and perceive pheromones. In addition, smelling plays an important role in the sense of taste and in detecting invisible dangers such as rot or smoke. The olfactory bulb is also known as the olfactory bulbus olfactorius. The name is derived from the Latin terms for “bulb” (bulbus) and “smell” (olfacere).
Anatomy and structure
Anatomically, the olfactory bulb splits into two structural units: The main olfactory bulb (bulbus olfactorius proper) and the accessory olfactory bulb (bulbus olfactorius accessorius). The olfactory bulb is located at the base of the frontal lobe of the brain, where it forms an elongated structure that protrudes from the surrounding tissue. It lies on the sieve plate (lamina cribrosa) of the ethmoid bone (Os ethmoidale); the ethmoid bone represents a part of the human skull. The bone does not form an impermeable barrier at this point, but has channels for the olfactory nerves (nervi olfactorii). The olfactory nerves connect the olfactory bulb with the sensory cells in the nose. Contrary to a common misconception, the olfactory cells are not distributed over the entire inner wall of the nose, but are confined to the olfactory mucosa (regio olfactoria). The fila olfactoria or olfactory filaments are the axons of these cells and together form the olfactory nerve or nervus olfactorius. Only in the olfactory bulb is there a synapse where the nerve signal from the olfactory nerve passes to the mitral cells of the bulbus olfactorius. The mitral cells are located in the fourth layer from the outside. Above them are the outer plexiform layer, the glomerular layer/ball layer, and the nerve layer. Further inside the olfactory bulb, below the mitral cell layer, lie de inner plexiform layer as well as the granule cell layer.
Function and tasks
The olfactory bulb forms an intermediate site in the processing of olfactory stimuli: information from the olfactory filaments converges in it. For the function of the bulb olfactorius, one of the total of six layers is particularly crucial: the mitral cell layer. Its cells have a pyramid-like shape and collect signals from 1000 individual sensory cells each. At the spherical glomeruli olfactorii, which are located in this layer, the synapses between the mitral cells of the olfactory bulb and the olfactory cords are located. In the opposite direction, toward higher brain areas, the tractus olfactorius exits the olfactory bulb. The tractus olfactorius consists of about 30,000 individual nerve fibers, each originating from a mitral cell, and forms the eye of the needle for further processing of olfactory information. Only via the olfactory bulb and the tractus olfactorius can these sensory stimuli reach the olfactory bulb (tuberculum olfactorium), the nuclei septales and the parahippocampal gyrus. The olfactory brain is closely linked to areas of the brain that process emotions; therefore, the perception of odors often generates automatic emotional responses and can trigger memories. This process can involve positive, neutral, and negative memories, but is particularly well known from the context of post-traumatic stress disorder. In this mental illness, olfactory perceptions and other triggers can cause sufferers to relive severely stressful events. In a positive sense, odors can also promote general well-being in this way.
Diseases
Multiple lesions due to injury, neurodegenerative and inflammatory diseases, malformations, and other pathologic conditions can impair or completely prevent olfactory bulb function. In both cases, medicine refers to central dysosmia; in this type of olfactory disorder, the olfactory nerves as well as the sensory cells may be fully intact, but processing at the cerebral level is impaired. Dysosmia represents an umbrella term and can be divided into quantitative and qualitative disorders.Quantitative olfactory disorders include hyposmia, which is characterized by impaired olfactory perception, and anosmia, in which affected individuals can no longer smell anything in fact or in practice (functional anosmia). Increased olfactory ability or hyperosmia may manifest during pregnancy or be due to diseases involving the central nervous system. Examples include psychoses – mental disorders associated with delusional thoughts, hallucinations, and negative symptomatology such as flattening of affect – and epilepsy. All olfactory disorders constitute diseases only when they are pathologically expressed. For example, people with hyperosmia do not simply possess a good sense of smell, but suffer from the olfactory disorder and its consequences. In addition to the three quantitative olfactory disorders, various qualitative disorders of olfactory perception exist. People with euosmia perceive stimuli as pleasant, the majority of which are considered unpleasant; medicine calls the opposite case cacosmia. Individuals suffering from olfactory dysagnosia are able to perceive scents, but are unable to recognize or associate them. In addition, disorders in the olfactory bulb can lead to phantosmia, which is the perception of odors that are not present. Phantosmia can result from faulty stimulation of the olfactory bulb, in which electrical signals are generated unintentionally in the neurons or reach them via incorrect connections. In contrast, in parosmia, the triggering odor is present, but the subjective perception is altered. When people confuse one odor with another under the influence of certain emotions (but not under other conditions), physicians refer to it as pseudoosmia.
