Anatomy and physiology Human olfactory system. 1: Olfactory bulb 2: Mitral cells 3: Bone 4: Nasal epithelium 5: Glomerulus (olfaction) 6: Olfactory receptor cells Olfactory receptors The olfactory neuroepithelium, located in the roof of the nasal chambers, comprises bipolar receptor cells, supporting cells, basal cells, and brush cells. There are approximately 6 million bipolar sensory receptor neurons whose cell bodies and dendrites are in the epithelium. The axons of these cells aggregate into 30-40 fascicles, called the olfactory fila, which project through the cribriform plate and pia matter. These axons collectively make up the olfactory nerve (CN I) and serve the purpose of mediating the sense of smell. Characteristics of the bipolar receptor neurons include cilia on the dendritic ends which project into the mucus, regeneration from basal cells after damage, and each receptor neuron is also a first order neuron. The first order neurons project axons directly from the nasal chamber to the brain. Its first neuron characteristic allows direct exposure to the environment, which makes the brain vulnerable to infection and invasion of xenobiotic agents. Supporting cells, called sustentacular cells, provide metabolic and physical support to the receptors by insulating the cell and regulating the composition of the mucus. Basal stem cells give rise to both neurons and non-neuronal cells and allow for constant regeneration of receptor cells and its surrounding cell. Olfactory transduction Olfactory transduction begins with the movement of odorants from the air phase to the aqueous phase in the olfactory mucus. Odorants are transported by odorant binding proteins or diffuse through the mucus and reach the cilia on the dendritic ends of bipolar receptor neurons. Stimulation causes action potentials to be initiated and the signals are sent to the brain via the olfactory fila. Olfactory bulb Olfactory receptor neuron axons project through the cribriform plate to the olfactory bulb. The olfactory bulb is a structure at the base of the frontal lobe. It comprises neurons, nerve fibers, interneurons, microglia, astrocytes, and blood vessels. It is made up of 6 layers: olfactory nerve layer, glomerular layer, external plexiform layer, mitral cell layer, internal plexiform layer, and granule layer. The terminals of the receptor axons synapse with the dendrites of mitral and tufts cells within the glomeruli of the olfactory bulb. The axons of the mitral and tufts cells send signals to the olfactory cortex. Olfactory cortex Signals from odor sensation are sent from the olfactory bulb through mitral and tufts cell axons via the lateral olfactory tract and synapse at the primary olfactory cortex. The primary olfactory cortex includes the anterior olfactory nucleus, the piriform cortex, the anterior cortical nucleus of the amygdala, the periamygdaloid complex, and the rostral entorhinal cortex. A unique characteristic of olfaction is its independence from the thalamus. The odor signals are sent directly from the sensory receptor neuron to the primary cortex. However, communication between the primary and secondary olfactory cortex requires connections with the thalamus. Odor perception Odor identity, quality, and familiarity are mainly deciphered by the piriform cortex. Consciousness of smell is achieved by projections from the piriform cortex to the medial dorsal nucleus of the thalamus and to the orbitofrontal cortex, which the secondary olfactory cortex is part of. There are approximately 1,000 olfactory receptors coded for in the human genome. Less than 500 receptors are functional in the nasal epithelium. Each receptor neuron is a single type of olfactory receptor and is not specific to any one odorant. An odorant is recognized by more than one type of receptor and thus odorants are recognized by a combination of receptors. The olfactory system relies on different excitation patterns to obtain different codes for different odorants. Nobel prize recipient Linda B. Buck compared this system to combining different letters of the alphabet to produce different words. In this case, each word represents an odor. This coding explains why we can detect more odors than there are receptors in the nasal epithelium. Olfaction and flavor Flavor is perceived by the combination of the sense of taste, sense of smell, and the trigeminal nerve (CN V). The gustatory system is responsible for differentiation between sweet, sour, salty, bitter, and umami. The olfactory system recognizes the odorants as they pass to the olfactory epithelium via a retronasal pathway. This explains why we can identify a variety of flavors in spite of only having five types of taste receptors. The trigeminal nerve senses texture, pain, and temperature of food, and related qualities such as the cooling effect of menthol or the burning sensation of spicy food.
Dysosmia
Anosmia
Aspermia
Dysmelia
Phantosmia
Hyperhidrosis
Dysacusis
Cynanthropy