Question

Briefly explain the differences in how information within the olfactory system and gustatory system are organized.

Answer 1

GUSTATORY SYSTEM.

Morphology of Taste Buds and Cell Types

Taste buds are located on papillae and distributed on the surface of the tongue. Taste buds are also found on the oral mucosa of the palate and epiglottis. These pear-shaped structures contain about 80 cells arranged around a central taste pore.

Taste receptor cells are spindle shaped, modified neuro-epithelial cells that extend from the base to the apex of the taste buds. Voltage-gated channel proteins for Na+, K+ & Ca2+ are present in the plasma membrane with the K+-gated channel proteins located in larger numbers on the apical membrane of the taste cells. Synaptic vesicles are present near the apex and the basal region in many taste cells. Microvilli from each taste cell project into the taste pore which communicate with the dissolved solutes on the surface of the tongue. These receptor cells are innervated by afferent nerve fibers penetrating the basal lamina. The nerve fibers branch extensively and receive synaptic input from the taste receptor cells. A group of non-receptor columnar cells and basal cells are present within taste buds. The basal cells migrate from adjacent lingual epithelium into the buds and differentiate into taste receptor cells which are replaced about every 9-10 days.

Transport of Solutes

Taste solutes are transported to the taste pore and diffuse through the fluid layer to make contact with membrane receptor proteins on the microvilli and apical membrane. Taste sensitivity is dependent upon the concentration of the taste molecules as well as their solubility in saliva. Many bitter tasting hydrophobic solutes interact with an odorant binding protein produced by von Ebner’s glands in the posterior region of the tongue.

Sensory Transduction

Taste sensation can be evoked by many diverse taste solutes. The pattern of membrane potential change include depolarization, depolarization followed by hyperpolarization, or only hyperpolarization. Action potentials in the taste receptor cells lead to an increase Ca2+ influx through voltage-gated membrane channels with the release of Ca2+ from intracellular stores. In response to this cation, neurotransmitter is released, which produces synaptic potentials in the dendrites of the sensory nerves and action potentials in afferent nerve fibers.

Salts

The taste of salts is mediated by Na+ ions which do not interact with a membrane receptor but diffuse through a Na+ channel located in the microvilli and apical membrane. Anions such as Cl- contribute to the salty taste, but anions are transported into these cells by a paracellular route. The influx of these ions of salt evokes a depolarization in the apical membrane.

Acids and Sour Tastes

The hydrogen proton of acids and sour foods can influx through the Na+ channels, or through a proton transport membrane protein. Some acids block the efflux of K+ at the microvilli. The resulting influx of protons or a reduction in K+ conductance will initiate receptor potentials in response to the quality of sour tastes.

Sweet

Sweet tasting solutes, sugars and related substances, bind to membrane receptor proteins which are coupled to a G-s protein (gustducin), which activates adenylyl cyclase (AC). Cyclic AMP (cAMP) dependent protein kinase (PKA) reduces K+ efflux in the apical membrane and produces membrane depolarization. Some sweet solutes and non-sugar sweeteners interact with a receptor membrane protein through a G protein, which activates phospholipase C. A second messenger, inositol triphosphate (IP3), is synthesized which releases Ca2+ from intracellular stores. Accumulation of Ca2+ depolarizes the cell, releasing neurotransmitter at the synapse.

Bitter

Bitter tasting solutes include many non-toxic and toxic alkaloids, hydrophilic quinine and some divalent ions. The transduction of bitter tastes involves several mechanisms: 1) blockage of the efflux of K+ by a number of hydrophilic bitter substances generates a depolarizing potential; 2) interaction with a receptor membrane receptor coupled to the G protein, gustducin, and activation of cAMP dependent protein kinase with blockage of K+ channels; and 3) involves a receptor protein linked to G-protein and activation of phospholipase C, which results in substrate hydrolysis to IP3, releasing Ca2+ from intracellular stores.

These mechanisms for taste transduction were identified in laboratory animals and are probably present in the microvilli and apical membrane of taste receptor cells in humans. A fifth taste quality, umami, is predicted to interact with a ligand-gated inotropic glutamate receptor coupled to gustducin and to Ca2+ channel membrane proteins.

Taste stimuli produce depolarizing and hyperpolarizing potentials in individual taste cells. Excitation of voltage-gated Na+, K+, and Ca2+ channels can generate action potentials which are propagated toward the basal region of the taste cell. These currents open the voltage-gated Ca2+ channels near the base of the taste cells, which leads to the subsequent release of neurotransmitter. These transmitters diffuse across the synaptic cleft and lead to the initiation of action potentials in the afferent nerve fibers.

Propagation of a Neural Code to the Gustatory Center

Branches of the facial cranial nerve, the chorda tympani, innervate taste buds in the anterior 2/3 of the tongue and part of the soft palate. The glossopharyngeal innervate the posterior 1/3 of the tongue. Both the vagus and glossopharyngeal nerves innervate the pharynx and epiglottis. Axons of these three cranial nerves terminate on 2nd order sensory neurons in the nucleus of the solitary tract. From this site in the rostral medulla, axons project into the parabrachial nucleus in lower animals but not in humans. In humans, fibers of the 2nd order neurons travel through the ipsilateral central tegmental tract to the 3rd order sensory neurons in the ventroposterior medial nucleus (VPM) of the thalamus. The VPM projects to the ipsilateral gustatory cortex located near the post-central gyrus representing the tongue or to the insular cortex.

OLFACTORY SYSTEM.

Morphology of Olfactory Mucosa

The olfactory mucosa consists of a layer of columnar epithelium, surrounding millions of olfactory neurons, which are the only neurons to communicate with the external environment and undergo constant replacement. Basal cells near the lamina propria undergo differentiation and develop into these neurons about every 5-8 weeks. The glial-like columnar cells surround and support the bipolar neurons. These columnar cells have microvilli at their apex and secrete mucus which is layered on the surface of the olfactory mucosa.

The bipolar olfactory neurons have a single dendrite which projects towards the apical mucosa. The terminal ending of the dendrites are flattened and have 5-25 cilia that are embedded in the mucosa on the surface. Each cilia may have as many as 40 specific receptor membrane proteins for interaction with different odorant molecules.

Dissolution of Odorant Molecules and Interaction with Sensory Receptors

Unbound hydrophilic odor molecules diffuse across the layer of mucus, whereas hydrophobic odors must become bound to a specific odorant binding protein to be transported to each cilium for interaction with specific receptors. All of these receptors have the same general structure, seven hydrophobic transmembrane regions, but the amino acid sequence within the cylinders spanning the membrane are extremely diverse which permits the discrimination of a large number of odors.

Transduction of Olfactory Stimuli

Odorant molecules bind reversibly to the diverse receptor membrane proteins which are coupled to a G-s group of proteins called Golf. Activation of adenylyl cyclase leads to the formation of cAMP with the activation of Ca2+/ Na+ cation channels. The primary effect of influx of these ions is depolarization and the generation of a generator potential (Figure 9.9). Generated ionic currents are graded in response to the flow rate of the odorant molecules and to their concentration. Sites of summated generator potentials occur across the olfactory mucosa to produce specific spatial pattern of activity for each stimulating odorant molecules, which may contribute to neural coding of odors.

Propagation of Action Potentials and Convergence upon the Olfactory Bulb

The resulting influx of Na+ and Ca2+ produces a depolarizing generator potential that spreads to the axon hillock. There, action potentials are generated, which are propagated to the synaptic endings in the olfactory bulb.

The action potential frequency is proportional to the concentration of specific odorant molecules. However, action potential frequency will be attenuated by adaptation or desensitization of the receptor and reduction in the production of cAMP.

Rapid adaptation and removal of the odorants permit continued recognition and discrimination of new aromas that are inhaled in the next respiratory cycle. Action potentials generated in the axon terminals of activated neurons are propagated into the glomeruli within the olfactory bulb. The olfactory bulbs have many different types of neurons and these have a laminar distribution. On the ventral side of the olfactory bulbs is a layer of glomeruli. This is a site at which axon terminals of several thousand olfactory neurons synapse with numerous dendrites from large mitral cells and tufted cells. Interneurons such as the inhibitory periglomerular cells synapse with the nerve endings within adjacent glomeruli.

Neural pathway into the olfactory cortex.

Axons from mitral and tuft cells project caudally into the olfactory tract. Fibers diverge and synapse with neurons of the anterior olfactory nucleus (AON). Axons from the AON cross to the opposite side of the hemisphere through the anterior commissure. The majority of the axons from the olfactory bulb diverge laterally and form the lateral olfactory tract which synapse with nuclei of the olfactory cortex. These are the piriform cortex (pc), the periamygdaloid cortex, part of the amygdala, and hippocampus. There are no direct relays from the olfactory bulb into the thalamus, but a few fibers synapse with 3rd order sensory neurons in the thalamic dorsomedial nucleus which are projected to the ipsilateral cerebral hemisphere.