The Sense Of Smell (Olfaction)
Introduction
Smell and taste are generally classified as visceral senses because of their close association with gastrointestinal function. Physiologically, they are related to each other. The flavors of foods are in large part a combination of their taste and smell. Food may taste different if one has a cold that depresses the sense of smell. Both taste and smell receptors are chemoreceptors. They are stimulated by molecules in solution in mucus in the nose and saliva in the mouth.
Olfactory Epithelium
The nose contains 10-100 million olfactory receptors contained within an area called the olfactory epithelium. Olfactory epithelium lies in the superior part of each nostril. The olfactory membrane has a surface area of about 2.4 square centimeters.
The olfactory epithelium consists of 3 kinds of cells:
- Olfactory receptors: These are receptor cells for the smell sensation (the olfactory cells). They are bipolar nerve cells derived from the CNS. There are about 100 million of these cells in the olfactory epithelium. The mucosal end of the olfactory cell forms a knob. From each knob, 4 to 25 olfactory cilia project into the mucus that coasts the inner surface of the nasal cavity. These projecting olfactory cilia form a dense mat in the mucus. They react to odors in the air and stimulate the olfactory cells.
- Supporting cells/sustentacular cells: The receptor cells in the olfactory epithelium are interspersed among sustentacular cells or supporting cells. The supporting cells are columnar epithelial cells. They provide physical support, nourishment and electrical insulation for the olfactory. They also help to detoxify chemicals that come in contact with the epithelium.
- Basal cells: Basal cells are stem cells located between the bases of the supporting cells. They continually undergo cell division to produce new olfactory receptors, which live for only a month or so before being replaced. This process is remarkable because olfactory receptors are neurons, and mature neurons are generally not replaced. The olfactory renewal process is carefully regulated. A bone morphogenic protein (BMP) exerts an inhibitory effect. BMPs are a large family of growth factors originally described as promoters of bone growth.
Spaced among the olfactory cells in the olfactory membrane are many small Browman’s glands that secrete mucus onto the surface of the olfactory membrane. Mucus is carried to the surface of the epithelium by ducts. The secretion moistens the surface of the olfactory epithelium and dissolves odorants so that transduction can occur.
Olfactory Pathway
Short axons from the olfactory cells terminate in multiple globular structures within the olfactory bulb called glomeruli. Each glomerulus is the terminus for dendrites from about 25 large mitral cells and about 60 smaller tufted cells, the cell bodies of which lie in the olfactory bulb superior to the glomeruli. Other neurons in the olfactory bulbs include:
- Granule cells
- Periglomerular cells
Mitral and tufted cells send axons through the olfactory tract to transmit olfactory signals to higher levels in the CNS.
The olfactory pathway can be summarized as follows:
- Mucus
- Cilia
- Axons of olfactory cells
- Glomeruli in the olfactory bulb
- Dendrites of mitral and tufted cells in the bulb
- Axons of mitral and tufted cells in the olfactory tract
- CNS
Axons of bipolar neurons pierce the cribiform plate of the ethmoid bone and reach the olfactory bulb. Here, axons synapse with dendrites of mitral cells. Different groups of these synapses form globular structures called olfactory glomeruli. Axons of the mitral cells leave the olfactory bulb and form the olfactory tract. The olfactory tract runs backward and ends in the olfactory cortex, through the intermediate and lateral olfactory stria. The olfactory cortex includes structures that form a part of the limbic system. These structures are:
- Anterior olfactory nucleus
- Prepyriform cortex
- Olfactory tubercle
- Amygdala
Olfactory Cortex
Axons of mitral and tufted cells terminate on apical dendrites of pyramidal cells in several areas:
- Anterior olfactory nucleus
- Olfactory tubercle
- Piriform cortex
- Amygdala
- Entorhinal cortex
Information travels from these areas to:
- Frontal cortex or orbitofrontal cortex: Conscious discrimination of odors
- Amygdala: Emotional response
- Entorhinal cortex: Olfactory memories
Olfactory Signal Transduction
The cilium is the portion of the olfactory receptor that responds to chemical stimuli. The odorant substance diffuses into the mucus that covers the cilia and then binds with a receptor protein (Odorant Binding Protein) that protrudes through the ciliary membrane. The odorant binds with a portion of the receptor and is coupled to a G-protein.
The G-protein is a combination of 3 subunits. An alpha subunit breaks away from the G-protein and activates adenyl cyclase. Activated cyclase converts many molecules of intracellular adenosine triphosphate into cyclic adenosine monophosphate (cAMP).
cAMP activates another nearby membrane protein, a gated sodium ion channel, allowing a large number of sodium ions to pour into the receptor cell cytoplasm. Sodium ions help in exciting the olfactory neuron and transmitting action potential into the CNS through an olfactory nerve.
Olfactory transduction is the process by which olfactory receptors convert chemical energy into action potentials in the olfactory nerve fiber. The odoriferous substance stimulates the olfactory receptors only if it dissolves in the mucus covering the olfactory mucus membrane. Molecules of the dissolved substance bind with receptor proteins in the cilia and form a substance-receptor complex. The substance-receptor complex activates adenyl cyclase, which causes the formation of cyclic AMP.
Cyclic AMP, in turn, causes the opening of sodium channels, leading to an influx of sodium and the generation of receptor potential. Receptor potential causes the generation of action potential in the axon of the bipolar neuron.
Adaptation
Olfactory receptors are phasic receptors and adapt very rapidly. They adapt about 50 percent in the first second or so after stimulation.
Threshold For Smell
A minute quantity of stimulating agent in the air can elicit a smell sensation. The substance methylmercaptan can be smelled when only one 25 trillionth of a gram is present in each milliliter of air. Because of this very low threshold, this substance is mixed with natural gas to give the gas an odor that can be detected when even small amounts of gas leak from a cylinder.
Vomeronasal Organ
In rodents and various other mammals, the nasal cavity contains another patch of olfactory mucous membrane located along the nasal septum in a well-developed vomeronasal organ. This structure is concerned with the perception of odors that act as pheromones. Its receptors project to the accessory olfactory bulb and from there primarily to areas in the amygdala and hypothalamus that are concerned with reproduction. The organ is not well developed in humans.
Applied Physiology
- Pain fibers in the nose: Naked endings of many trigeminal pain fibers are found in the olfactory mucous membrane. They are stimulated by irritating substances. Peppermint, menthol, and chlorine cause sneezing, lacrimation, and respiratory inhibition.
- Anosmia: Absence of the sense of smell
- Hyposmia: Diminished olfactory sensitivity
- Dysosmia: Distorted sense of smell
Olfactory thresholds increase with advancing age. More than 75% of humans over 80 have an impaired ability to identify smells.
- Anosmia + Hypogonadism: Kallmann’s syndrome
