Geruchssystems

Geruchssystems, also known as the olfactory system, refers to the sensory and neural structures that detect and interpret odorant molecules. It includes the peripheral detection apparatus in the nasal cavity and the central neural pathways that process olfactory information, ultimately contributing to perception, memory, and emotion.

In the nose, odorant molecules dissolve in mucus and bind to receptors on the cilia of olfactory receptor neurons housed in the olfactory epithelium. Each neuron expresses one type of odorant receptor from a large gene family (about 350–400 functional genes in humans). Activation of receptors triggers intracellular signaling and electrical impulses that travel along the axons through the cribriform plate to the glomeruli in the olfactory bulb, where signals from many neurons converge.

From the olfactory bulb, mitral and tufted cells project via the olfactory tract to several cortical areas, notably the piriform cortex, the entorhinal cortex, and the amygdala, with additional routes to the orbitofrontal cortex. Unlike other senses, olfactory information reaches the cortex directly, somewhat bypassing the thalamus. The limbic connections account for the strong links between odors, memory, and emotions.

Mechanistically, odor coding is combinatorial: a given odor activates a pattern of receptors, producing a unique spatial-temporal activity across glomeruli. The system also interacts with the vomeronasal organ in many species, which detects pheromones; in humans, the vomeronasal system is largely vestigial.

Clinical aspects: olfactory dysfunction can be congenital or acquired (anosmia, hyposmia). Causes include upper respiratory infections, nasal obstruction, head trauma, and neurodegenerative diseases; post-viral loss is common and COVID-19 has been a notable cause. Diagnosis and prognosis vary with etiology; olfactory testing is used in clinical practice.