The chemical senses involve detection of chemical stimuli and transduction of those stimuli into electrical energy that can be transmitted in the nervous system. Olfaction, the sense of smell, is one of the chemical senses. In humans, olfaction is not necessary for survival, yet it improves the quality of life and even protects against hazards.
Anosmia is the absence of the sense of smell, hyposmia is impaired sense of smell, and dysosmia is a distorted sense of smell. Head injury, upper respiratory infections, tumors of the anterior fossa, and exposure to toxic chemicals (which destroy the olfactory epithelium) all can cause olfactory impairment.
Olfactory Epithelium and Receptors
Odorant molecules, which are present in the gas phase, reach the olfactory receptors via the nasal cavity: Air enters the nostril, crosses the nasal cavity, and exits into the nasopharynx. The nasal cavity contains structures called turbinates, some of which are lined with olfactory epithelium containing the olfactory receptor cells. (The remainder of the nasal cavity is lined by respiratory epithelium.) The turbinates act as baffles, causing air flow to become turbulent and, thereby, to reach the upper regions of the nasal cavity.
The olfactory epithelium consists of three cell types: supporting cells, basal cells, and olfactory receptor cells (Fig. 3-25).
Figure 3–25 Olfactory pathways, showing the olfactory epithelium and olfactory bulb.
Supporting cells are columnar epithelial cells lined with microvilli at their mucosal border and filled with secretory granules.
Basal cells are located at the base of the olfactory epithelium and are undifferentiated stem cells that give rise to the olfactory receptor cells. These stem cells undergo mitosis, producing a continuous turnover of receptor cells.
Olfactory receptor cells, which are also primary afferent neurons, are the site of odorant binding, detection, and transduction. Odorant molecules bind to receptors on the cilia, which extend into the nasal mucosa. Axons from olfactory receptor cells leave the olfactory epithelium and travel centrally to the olfactory bulb. These axons must pass through the cribriform plate at the base of the skull to reach the olfactory bulb. Thus, fractures of the cribriform plate can sever olfactory neurons, leading to olfactory disorders (e.g., anosmia). Olfactory nerve axons are unmyelinated and are among the smallest and slowest fibers in the nervous system (recall the relationships between fiber diameter, myelination, and conduction velocity discussed in Chapter 1).
Because the olfactory receptor cells are also primary afferent neurons, the continuous replacement of receptor cells from basal cells means that there is continuous neurogenesis.
Transduction in the olfactory system involves the conversion of a chemical signal into an electrical signal that can be transmitted to the CNS. The steps in olfactory transduction are as follows (Fig. 3-26):
Figure 3–26 Steps in olfactory transduction. Circled numbers correspond to steps described in the text. cAMP, cyclic adenosine monophosphate.
1. Odorant molecules bind to specific olfactory receptor proteins located on the cilia of olfactory receptor cells. There are at least 1000 different olfactory receptor proteins (members of the superfamily of G protein–coupled receptors), each encoded by a different gene and each found on a different olfactory receptor cell.
2. The olfactory receptor proteins are coupled to adenylyl cyclase via a G protein called Golf. When the odorant is bound, Golf is activated, which activates adenylyl cyclase.
3. Adenylyl cyclase catalyzes the conversion of ATP to cAMP. Intracellular levels of cAMP increase, which opens cation channels in the cell membrane of the olfactory receptor that are permeable to Na+, K+, and Ca2+.
4. The receptor cell membrane depolarizes (i.e., the membrane potential is driven toward a value in between the equilibrium potentials for the three cations, which is depolarization). This depolarizing receptor potential brings the membrane potential closer to threshold and depolarizes the initial segment of the olfactory nerve axon.
5. Action potentials are then generated and propagated along the olfactory nerve axons toward the olfactory bulb.
Encoding Olfactory Stimuli
It is not known exactly how olfactory stimuli are encoded; that is, how do we recognize the scent of a rose or a gardenia or a special person, and how do we distinguish a rose from a gardenia?
The following information is known: (1) Olfactory receptor proteins are not dedicated to a single odorant, and each protein can respond to a variety of odorants. (2) Still, olfactory receptor proteins are selective, responding to some odorants more than others, and to some not at all. (3) Different olfactory receptor proteins have different responses to the same odorant. For example, receptor protein “A” has a much stronger response to “apple” than does receptor protein “B.” (4) If the response to a given odorant is examined across many receptors, different patterns emerge for different odorants. This is called an across-fiber pattern code. Each odorant produces a unique pattern of activity across a population of receptors, which is projected onto targeted glomeruli in the olfactory bulb (“odor map”). The CNS then interprets these odor maps (e.g., a rose or a gardenia or a special person).
As noted, olfactory receptor cells are the primary afferent neurons in the olfactory system. Axons from the receptor cells leave the olfactory epithelium, pass through the cribriform plate, and synapse on apical dendrites of mitral cells (the second-order neurons) in the olfactory bulb. These synapses occur in clusters called glomeruli (see Fig. 3-25). In the glomeruli, approximately 1000 olfactory receptor axons converge onto 1 mitral cell. The mitral cells are arranged in a single layer in the olfactory bulb and have lateral dendrites in addition to the apical dendrites. The olfactory bulb also contains granule cells and periglomerular cells (not shown). The granule and periglomerular cells are inhibitory interneurons that make dendrodendritic synapses on neighboring mitral cells. The inhibitory inputs serve a function similar to that of the horizontal cells of the retina and may provide lateral inhibition that “sharpens” the information projected to the CNS.
Mitral cells of the olfactory bulb project to higher centers in the CNS. As the olfactory tract approaches the base of the brain, it divides into two major tracts, a lateral tract and a medial tract. The lateral olfactory tract synapses in the primary olfactory cortex, which includes the prepiriform cortex. The medial olfactory tract projects to the anterior commissure and the contralateral olfactory bulb.