Olfaction And Taste (sensory System) Part 1

Olfaction (smell) and gustation (taste) constitute chemical senses. Like other sensory systems, olfactory and taste systems provide information regarding the external environment. The two sensory systems are anatomically and morphologically distinct. They are discussed together in this topic because their specialized sensory receptors are stimulated by chemical molecules, and the functions of the two systems often complement each other as special visceral afferents. For example, wine tasters typically depend on the sensation of taste (flavor) and olfaction (smell) to distinguish between different wines.

Olfactory System

Stimulus

Chemicals that generate odors stimulate specialized receptors of the olfactory system. Human beings can detect these odors at very low concentrations (a few parts per trillion); thousands of such chemicals can be distinguished.

Receptors

Unlike in other sensory systems, the bipolar olfactory sensory (receptor) neurons are not located in a ganglion. Instead, these neurons, along with their processes, are present in the specialized olfactory mucosa of the nasal cavity just below a thin sheet of bone called the cribriform plate of the ethmoid bone of the skull (Fig. 18-1, A and B). The olfactory sensory neurons have single dendrites on one end that terminate in the surface of the olfactory mucosa as expanded olfactory knobs (Fig. 18-1B). A single unmyelinated axon arises on the opposite end of the sensory neuron. Collectively, these axons are called the olfactory nerve (cranial nerve [CN] I). The axons of olfactory sensory neurons do not form a single nerve as in other cranial nerves. Instead, small clusters of these axons penetrate the cribriform plate and synapse in the ipsilateral olfactory bulb. Sustentacular (supporting) cells present in the olfactory epithelium help in detoxifying chemicals that come in contact with the olfactory epithelium (Fig 18-1B).


 Organization of the human olfactory system. (A)The bipolar olfactory sensory neurons are present in the olfactory mucosa just below the cribiform plate. (B) Note the location of olfactory receptor cells, including their expanded ends (olfactory knobs), cilia arising from the olfactory knobs, the olfactory nerve, and supporting (sustentacular) cells.

FIGURE 18-1 Organization of the human olfactory system. (A)The bipolar olfactory sensory neurons are present in the olfactory mucosa just below the cribiform plate. (B) Note the location of olfactory receptor cells, including their expanded ends (olfactory knobs), cilia arising from the olfactory knobs, the olfactory nerve, and supporting (sustentacular) cells.

Sensory Transduction

A protein, called olfactory binding protein, is secreted by the Bowman’s glands, which are located in the olfactory mucosa, and is more abundant around the cilia of the olfactory sensory neurons. Although the exact function of the olfactory binding protein is not known, it is believed that it carries and/or concentrates the odorant (a substance that stimulates olfactory receptors) around the cilia. The steps involved in the sensation of olfaction are shown in Figure 18-2. At least two second-messenger systems—cyclic adenosine monophosphate (cAMP) and inositol triphosphate (IP3)—are involved in the transduc-tion of olfactory signals. When an odorant molecule binds to the receptor protein on the cilia, a receptor-odorant complex is formed, which activates a G protein. The activated G protein (Golf) combines with guanosine triphosphate (GTP), displacing guanosine diphosphate (GDP).

Sensory transduction in olfaction. The olfactory binding protein carries odorant molecules to the cilia of the olfactory sensory neurons. A receptor-odorant complex is formed, which activates a G protein. Second-messenger systems are activated, Na+ (sodium) and Ca2+ (calcium) or Ca2+ channels are opened, and the cilia are depolarized. This depolarization is conducted to the axon hillock of the olfactory sensory neuron where action potentials are generated, which are conducted along the axons of the olfactory sensory neurons. Cl- = chloride; ATP = adenosine triphosphate; cAMP = cyclic adenosine triphosphate.

FIGURE 18-2 Sensory transduction in olfaction. The olfactory binding protein carries odorant molecules to the cilia of the olfactory sensory neurons. A receptor-odorant complex is formed, which activates a G protein. Second-messenger systems are activated, Na+ (sodium) and Ca2+ (calcium) or Ca2+ channels are opened, and the cilia are depolarized. This depolarization is conducted to the axon hillock of the olfactory sensory neuron where action potentials are generated, which are conducted along the axons of the olfactory sensory neurons. Cl- = chloride; ATP = adenosine triphosphate; cAMP = cyclic adenosine triphosphate.

The GTP-Golf complex activates adenylate cyclase, leading to the generation of cAMP, which, in turn, opens Na+ (sodium) and Ca2+ (calcium) channels. The influx of Na+ and Ca2+ results in depolarizing generator potential in the cilia. In another pathway, the GTP-Golf complex activates phospholipase C, which generates IP3. IP3 activates and opens Ca2+ channels, causing depolarizing generator potentials. In both second-messenger pathways, an increase in intracellular Ca2+ concentration results in the opening of Ca2+-gated Cl- (chloride) channels, efflux of Cl- ions, and further depolarization of the cilia. This depolarization is conducted passively from the cilia to the axon hillock of the olfactory sensory neuron. When the axon hillock reaches a threshold, action potentials are generated, which are conducted along the axons of the olfactory sensory neurons. These signals are processed in the central olfactory pathways for the sense of smell.

Central Pathways

The olfactory bulb lies bilaterally on the ventrorostral aspect of the forebrain (Fig. 18-3A). It is the first region of the central nervous system where sensory signals from olfactory sensory neurons are processed. As noted earlier, the axons of the olfactory sensory neurons travel in olfactory nerves and spread over the surface of the ipsilateral olfactory bulb, forming an olfactory nerve layer (Fig. 18-3B). Located near the surface of the olfactory bulb is the glomerular layer. Each glomerulus contains clusters of nerve terminals from olfactory sensory neurons, dendrites of the tufted cells (located in the external plexi-form layer of the olfactory bulb), mitral cells (located in the mitral cell layer), and y-aminobutyric acid (GABA)-ergic interneurons, called the periglomerular cells (located in the glomerular layer of the olfactory bulb). The terminals of first-order olfactory sensory neurons form synapses with the dendrites of the tufted, mitral, and peri-glomerular neurons. The transmitters released at the terminals of olfactory sensory neurons are believed to be peptides. The inner plexiform layer of the olfactory bulb contains GABAergic interneurons called granule cells. The mitral and tufted cells discharge spontaneously. They are excited by the inputs from the olfactory sensory neurons. The inhibitory interneurons (periglomerular and granule cells) modulate the activity of the mitral and tufted cells. The signals from mitral and tufted cells are then conducted to forebrain structures for further processing.

The projections of the axons of the mitral and tufted cells are shown schematically in Figure 18-3C. Olfactory tracts, located on the ventral (inferior) surface of the frontal lobe, arise from their enlarged ends known as the olfactory bulbs. The largest bundle of axons from the mitral and tufted cells exit from the olfactory bulb in the lateral olfactory tract, and their functions are mediated by the excitatory neurotransmitters, glutamate or aspartate. These axons project to the primary olfactory cortex (pir-iform or pyriform cortex), amygdala, and entorhinal cortex. The entorhinal and piriform cortices, hippocampus,and amygdala are located in the temporal lobe; the hippocampus lies in the medial temporal lobe.The neurons in the piriform cortex, amygdala, and entorhinal cortex project to the prefrontal cortex. Note that the olfactory projection system differs from other sensory systems in that the projection pathway can reach the prefrontal cortex without having to make a synapse in the thalamus first, which is typical of other sensory systems.Neurons in the entorhinal cortex project to the hippocampus (a major limbic structure) via a fiber bundle called the perforant fiber pathway. Therefore, olfactory inputs can play an important role in modulating hippocampal functions in a manner similar to that for the amygdala.Although olfactory projections can reach the prefrontal cortex without making a synapse in the thalamus, there are direct tertiary inputs from the piriform cortex to the mediodorsal thalamic nucleus, which projects to wide areas of the frontal lobe, including the prefrontal cortex.

Central olfactory pathways. (A) The axons of the olfactory sensory neurons project to the ipsilateral olfactory bulb via the olfactory nerve.

FIGURE 18-3 Central olfactory pathways. (A) The axons of the olfactory sensory neurons project to the ipsilateral olfactory bulb via the olfactory nerve.

(C) The axons of mitral and tufted cells in the olfactory bulb form the olfactory tracts. The largest bundle of fibers from mitral and tufted cells exit from the olfactory bulb in the lateral olfactory tract and project to the primary olfactory cortex (piriform cortex), amygdala, and entorhinal cortex. The entorhinal and piriform cortices are located in the temporal lobe. The hippocampus lies in the medial temporal lobe. The amygdala lies just rostral to the hippocampus in the temporal lobe. The prefrontal cortex is located in the frontal lobe. Some fibers from the mitral and tufted cells exit the olfactory tract via the medial olfactory tract. For other details, see text.

Some fibers from the mitral and tufted cells exit the olfactory tract via the medial olfactory tract (Fig 18-3C). These axons project ipsilaterally to basal limbic forebrain structures, such as the substantia innominata, medial septal nucleus, and bed nucleus of the stria terminalis. Other fibers in the medial olfactory stria arise from the contralateral anterior olfactory nucleus. This nucleus, located in the posterior part of each olfactory bulb, receives sensory signals from mitral and tufted cells and relays them to the contralateral olfactory bulb via the anterior commissure.

Spatial Organization

A basic question concerns how we discriminate and become aware of different kinds of odors. While little is basically known about this process, it is believed that olfactory discrimination takes place, at least in part, within the olfactory bulb. It has been suggested that different glomer-uli that are located in spatially distinct parts of the olfactory bulb respond to specific odorants. In this manner, olfactory signals become topographically organized within the olfactory bulb much the same as other sensory modalities are topographically arranged (i.e., similar to other modalities of sensation, which are tonotopically, somato-topically, and visuotopically organized for auditory, tactile, and visual sensations, respectively). This topographical arrangement, with respect to olfactory signals, provides a basis by which neuronal pools within the prefrontal cortex can receive and transform such signals into a conscious awareness of a specific odorant.

From a functional perspective, we can, thus, say that affective and emotional aspects of olfactory sensation are mediated by olfactory projections to the limbic system (entorhinal cortex, hippocampal formation, medial septal nuclei, and amygdala). Autonomic responses to olfactory stimuli are mediated via descending projections to the hypothalamus, midbrain periaqueductal gray, and auto-nomic centers of the lower brainstem and spinal cord.

Clinical Conditions in Which the Olfactory Sensation is Altered

In some cases of head trauma, the olfactory bulb moves with respect to the cribriform plate, and the axons projecting from the sensory neurons (located in the olfactory mucosa) to the olfactory bulb may be damaged. This results in a loss (anosmia) or reduction (hyposmia) of olfactory function. These conditions may also result from damage to the olfactory mucosa due to infections. Loss or alteration of olfactory function may occur in Alzheimer’s and Parkinson’s diseases.

Seizure activity involving parts of the temporal lobe produce olfactory hallucinations of unpleasant smells (cacosmia). This condition is referred to as an uncinate fit. The neural structures affected in this condition are the uncus, parahippocampal gyrus, amygdala, and piriform and entorhinal cortices.

Taste

Stimulus

As mentioned in the beginning of the topic, sensory receptors in this system are stimulated by chemical molecules. Basic sensations of taste include sweet, bitter, salty, and sour. The areas of the tongue most sensitive to different taste sensations are: tip of the tongue for sweetness, back of the tongue for bitterness, and sides of the tongue for saltiness and sourness (Fig. 18-4A). The concept that specific areas of the tongue mediate specific taste sensations (so called "taste map" of the tongue) is not universally accepted, and it is currently believed that taste sensation arises from all regions of the oral cavity.

Receptors

The receptor cells that mediate the sensation of taste are located in taste buds, which are the sensory organs for the taste system. Taste buds are located in different types of papillae, which are protrusions on the surface of the tongue. The types of papillae include: filiform, fungiform, foliate, and circumvallate papillae. The filiform and fungi-form papillae are scattered throughout the surface of the anterior two thirds of the tongue, especially along the lateral margins and the tip. The foliate papillae are present on the dorsolateral part of the posterior part of the tongue. The circumvallate papillae are larger than other papillae and are located in a V-shaped line, which divides the tongue into two portions: the anterior two thirds and posterior one third (Fig. 18-4B). The taste buds are located in the lateral margins of the papillae that are surrounded by a deep furrow bathed by fluids in the oral cavity (Fig. 18-4C).

Each taste bud has a pore at its tip through which fluids containing chemical substances enter (Fig. 18-4D). The taste bud contains taste receptor cells in different stages of development. The taste receptor cells live for about 10 days and have to be replaced. Small cells at the base of the taste bud (basal cells) divide to replace the taste receptor cells. Afferent nerve terminals make contact with the base of the taste receptor cells. The cell bodies of these afferent terminals are located in the ganglia of CN VII (facial), IX (glossopharyngeal), and X (vagus).

Components of the taste system. (A) The regions of the tongue that are most sensitive to different taste sensations are: the tip for sweetness, the back for bitterness, and sides for saltiness and sourness (it should be noted that the "taste map" of the tongue is not universally accepted, and it is currently believed that taste sensation arises from all regions of the oral cavity). (B) The filiform and fungiform papillae are scattered throughout the surface of the anterior two thirds of the tongue. The circumvallate papillae are located in a V-shaped line that divides the tongue into the anterior two thirds and the posterior one third. (C) The taste buds are located in the lateral margins of the papillae. (D) Each taste bud has a pore at its tip through which fluids containing chemical substances enter.

FIGURE 18-4 Components of the taste system. (A) The regions of the tongue that are most sensitive to different taste sensations are: the tip for sweetness, the back for bitterness, and sides for saltiness and sourness (it should be noted that the "taste map" of the tongue is not universally accepted, and it is currently believed that taste sensation arises from all regions of the oral cavity). (B) The filiform and fungiform papillae are scattered throughout the surface of the anterior two thirds of the tongue. The circumvallate papillae are located in a V-shaped line that divides the tongue into the anterior two thirds and the posterior one third. (C) The taste buds are located in the lateral margins of the papillae. (D) Each taste bud has a pore at its tip through which fluids containing chemical substances enter.

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