The receptors responsible for olfaction, the sense of smell, are located in the olfactory epithelium. The olfactory apparatus consists of receptor cells (which are bipolar neurons), supporting (susten-tacular) cells, and basal (stem) cells. The basal cells generate new receptor cells every 1 to 2 months to replace the neurons damaged by exposure to the environment. The supporting cells are epithe lial cells rich in enzymes that oxidize hydrophobic, volatile odorants, thereby making these molecules less lipid-soluble and thus less able to penetrate membranes and enter the brain.
Each bipolar sensory neuron has one dendrite that projects into the nasal cavity, where it terminates in a knob containing cilia (figs. 10.9 and 10.10). The bipolar sensory neuron also has a single unmyelinated axon that projects through holes in the cribriform plate of the ethmoid bone into the olfactory bulb of the cerebrum, where it synapses with second-order neurons. Therefore, unlike other sensory modalities that are relayed to the cerebrum from the thalamus, the sense of smell is transmitted directly to the cerebral cortex. The processing of olfactory information begins in the olfactory bulb, where the bipolar sensory neurons synapse with neurons located in spherically shaped arrangements called glomeruli (fig. 10.9). Evidence suggests that each glomerulus receives input from one type of olfactory receptor. The smell of a flower, which releases many different molecular odorants, may be identified by the pattern of excitation it produces in the glomeruli of the olfactory bulb. Identification of an odor is improved by lateral inhibition in the olfactory bulb, which appears to involve dendroden-dritic synapses between neurons of adjacent glomeruli.
Neurons in the olfactory bulb project to the olfactory cortex in the medial temporal lobes, and to the associated hippocampus and amygdaloid nuclei. These structures are part of the limbic system, which was described in chapter 8 as having important roles in both emotion and memory. The human amygdala, in particular, has been implicated in the emotional responses to olfactory stimulation. Perhaps this explains why the smell of a particular odor can so powerfully evoke emotionally charged memories.
The molecular basis of olfaction is complex. At least in some cases, odorant molecules bind to receptors and act through
Olfactory bulb Olfactory tract
Tufted cell (secondary neuron)
Cribriform plate of ethmoid bone
Olfactory receptor neurons
■ Figure 10.9 The neural pathway for olfaction. The olfactory epithelium contains receptor neurons that synapse with neurons in the olfactory bulb of the cerebral cortex. The synapses occur in rounded structures called glomeruli. Secondary neurons, known as tufted cells and mitral cells, transmit impulses from the olfactory bulb to the olfactory cortex in the medial temporal lobes. Notice that each glomerulus receives input from only one type of olfactory receptor, regardless of where those receptors are located in the olfactory epithelium.
■ Figure 10.10 A scanning electron micrograph of an olfactory neuron. The tassel of cilia is clearly visible.
G-proteins to increase the cyclic AMP within the cell. This, in turn, opens membrane channels and causes the depolarization of the generator potential, which then stimulates the production of action potentials. Up to fifty G-proteins may be associated with a single receptor protein. Dissociation of these G-proteins re leases many G-protein subunits, thereby amplifying the effect many times. This amplification could account for the extreme sensitivity of the sense of smell: the human nose can detect a billionth of an ounce of perfume in air. Even at that, our sense of smell is not nearly as keen as that of many other mammals.
A family of genes that codes for the olfactory receptor proteins has been discovered. This is a large family that may include as many as a thousand genes. The large number may reflect the importance of the sense of smell to mammals in general. Even a thousand different genes coding for a thousand different receptor proteins, however, cannot account for the fact that humans can distinguish up to 10,000 different odors. Clearly, the brain must integrate the signals from several sensory neurons that have different olfactory receptor proteins and then interpret the pattern as a characteristic "fingerprint" for a particular odor.
1. Explain how the mechanisms for sour and salty tastes are similar to each other, and how these differ from the mechanisms responsible for sweet and bitter tastes.
2. Explain how odorant molecules stimulate the olfactory receptors. Why is it that our sense of smell is so keen?
■ Figure 10.11 The cochlea and vestibular apparatus of the inner ear. The vestibular apparatus consists of the utricle and saccule (together called the otolith organs) and the three semicircular canals. The base of each semicircular canal is expanded into an ampulla that contains sensory hair cells.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.