Bio Chapter 17 - Olfaction & Gustation

The human tongue has four types of lingual papillae

(1) filiform (filum, thread) papillae (2) fungiform (fungus, mushroom) papillae, (3) vallate (vallum, wall) papillae (4) foliate papillae. The distribution of these lingual papillae varies by region. Filiform papillae provide friction that helps the tongue move objects around in the mouth, but do not contain taste buds. Each small fungiform papilla contains about five taste buds. There are up to 12 vallate papillae that form a V near the poste- rior margin of the tongue. Each large vallate papilla contains as many as 100 taste buds. The foliate papillae are folds found on the lateral margins of the posterior tongue.

Olfactory stimulation is the only type of sensory informa- tion that reaches the cerebral cortex directly.

All other sen- sations are relayed from processing centers in the thalamus. Certain smells can trigger profound emotional and behavioral responses, as well as the memories, due to the parallel dis- tribution of olfactory information to the limbic system and hypothalamus. The perfume industry understands the practical implications of these connections and works to develop odors that trigger sexual responses.

Olfactory Discrimination

One example is mercaptan, an odorant commonly added to natural gas, propane, and butane, which are otherwise odorless. Because we can smell mercaptan in extremely low concentrations (a few parts per billion), its addition enables us to detect a gas leak almost at once and take steps to prevent an explosion.

Taste Receptors

Taste buds are recessed into the surrounding epithelium, iso- lated from the unprocessed contents of the mouth. Each taste bud contains about 40-100 receptor cells and many small stem cells, called basal cells

Olfaction

The sense of smell is made possible by paired olfactory organs. These organs are located in the nasal cavity on either side of the nasal septum. The olfactory organs are made up of two layers: the olfactory epithelium and the lamina propria.

Taste receptors, or gustatory receptors, are distributed over the superior surface of the tongue and adjacent portions of the pharynx and larynx.

The most important taste receptors are on the tongue. By the time we reach adulthood, the taste receptors on the pharynx, larynx, and epiglottis have decreased in number. Taste receptors and specialized epithelial cells form sensory structures called taste buds. An adult has about 10,000 taste buds.

olfactory epithelium

The olfactory epithelium contains the olfactory recep- tor cells, supporting cells, and regenerative basal cells (stem cells). This epithelium covers the inferior surface of the cribriform plate, the superior portion of the perpen- dicular plate, and the superior nasal conchae of the ethmoid. The second layer, the underlying lamina propria, consists of areolar tissue, numerous blood vessels, and nerves. This layer also contains olfactory glands. Their secretions absorb water and form thick, pigmented mucus.

Life of smell

The olfactory receptor population undergoes considerable turnover. Basal cells in the epithelium divide and differentiate to produce new receptor cells. This turnover is one of the few examples of neuronal replacement in adult humans. Despite this process, the total number of receptors declines with age, and those that remain become less sensitive. As a result, elderly people have difficulty detecting odors in low concentrations. This decline in the number of receptors accounts for Grandma's tendency to use too much perfume and explains why Grandpa's aftershave seems so strong: They must apply more to be able to smell it.

smelling

What happens when you inhale through your nose? The air swirls within your nasal cavity. This turbulence brings airborne compounds to your olfactory organs. A normal, re- laxed inhalation carries a small sample (about 2 percent) of the inhaled air to the olfactory organs. If you sniff repeatedly, you increase the flow of air across the olfactory epithelium, increasing the stimulation of the olfactory receptors. only the molecules of water-soluble and lipid-soluble materials that can diffuse into the overlying mucus can stimulate those receptors. It appears likely that the CNS interprets each smell on the basis of the overall pattern of receptor activity.

Gustation

taste, provides information about the foods and liquids we eat and drink

Olfactory Pathways

the activation of an afferent fiber does not guarantee an awareness of the stimulus. Considerable convergence takes place along the olfactory pathway, and inhibition at the intervening synapses can prevent the sensations from reaching the olfactory cortex of the cerebral hemispheres. The olfactory receptors them- selves adapt very little to an ongoing stimulus. Rather, central adaptation ensures that you quickly lose awareness of a new smell but remain sensitive to others. Axons leaving the olfactory epithelium collect into 20 or more bundles that penetrate the cribriform plate of the ethmoid bone to reach the olfactory bulbs of the cerebrum, where the first synapse occurs (Figure 17-1). Efferent fibers from nuclei else- where in the brain also innervate neurons of the olfactory bulbs. This arrangement provides a mechanism for central adaptation or facilitation of olfactory sensitivity. Axons leaving the olfac- tory bulb travel along the olfactory tract to the olfactory cortex, the hypothalamus, and portions of the limbic system. Olfactory stimulation is the only type of sensory informa- tion that reaches the cerebral cortex directly. All other sen- sations are relayed from processing centers in the thalamus. Certain smells can trigger profound emotional and behavioral responses, as well as the memories, due to the parallel dis- tribution of olfactory information to the limbic system and hypothalamus. The perfume industry understands the practical implications of these connections and works to develop odors that trigger sexual responses.

Olfactory reception begins with.........

with the binding of an odorant to a G protein-coupled receptor in the plasma membrane of an olfactory receptor cell dendrite