Chemical Senses Taste and Smell PREPARED BY:- SHEIKH ATEEQ MOHD YASEEN

Chemoreceptors in the Human Body Several types of chemoreceptors are present: Taste buds - receptors of taste (gustatation) Olfactory receptors- receptors of smell (olfaction) Cutaneous nociceptors -Irritating chemicals on skin Muscle sensors - burning during heavy exercise, acidity receptors Circulatory sensors - oxygen & CO2 receptors Digestive tract sensors - receptors for various ingested substances

Taste & Smell We will consider only those dealing with taste and smell These two senses are mediated quite differently, but are perceived together Other receptors in the mouth also participate: Texture Temperature

Gustation Gustation = taste Basic tastes: Sweetness - outer tip of tongue Saltiness - lateral edge of tongue behind tip Sourness - lateral edge of tongue behind saltiness region Bitterness - back edge of tongue toward throat Umami - taste of glutamate - not localized Tastes triggers are not unique many chemicals can trigger each receptor type Loss of taste is ageusia

Organs of Taste The tongue is the major organ of gustation There are also some tastebuds on the palate, pharynx, and epiglottis Papillae - contain taste buds Fungiform papillae (mushrooms) - top surface of tongue Foliate papillae (ridges) - rear, lateral margin of tongue Vallate papillae (pimples) - rear, medial margin of tongue

Mapping Taste

Taste Buds Only 1% of the epithelial cells on tongue’s surface are taste buds Contain 50 to 75 sensitive cells each Taste-sensitive cells have a limited lifetime, and are constantly being replaced. Taste pore permits substances to enter Three cell types in each taste bud: Taste receptors – chemoreceptors Basal cells - source of new taste receptors Gustatory afferent fibers - conduct action potentials to brain

Taste Cells - Taste Receptors Come from the division and differentiation of the basal cells Not neurons in the strict sense, but have synapses with gustatory afferent fibers Arrival of chemical molecules on surface leads to change in membrane potential - receptor potential Most taste receptors respond to at least two of the basic tastes Sometimes taste receptors generate action potentials Transduction differs for each type of taste receptor

Taste Transduction - Salty Sodium in food goes through a sodium channel (amiloride sensitive sodium channel) which always sits open Sodium influx causes membrane depolarization (receptor potential) Voltage-gated calcium channel opens Inward calcium current causes synaptic release onto gustatory afferent at taste cell synapse

Taste Transduction - Sour Hydrogen ions in sour food goes through the amiloride sensitive sodium channel and closes an open potassium channel Membrane depolarization (receptor potential) Voltage-gated calcium channel opens Inward calcium current causes synaptic release

Taste Transduction - Sweet The sweet molecule in food binds to G-protein coupled receptor Effector protein is adenylyl cyclase cAMP activated Protein Kinase A activated PKA closes a potassium channel Membrane depolarization (receptor potential) Voltage-gated calcium channel opens Inward calcium current causes synaptic release

Taste Transduction - Bitter Bitter molecule in food blocks potassium channel Membrane depolarization (receptor potential) Voltage-gated calcium channel opens Inward calcium current causes synaptic release

Taste Transduction – Bitter II Bitter molecule in food binds G-protein coupled receptor Effector protein is phospholipase C IP3 produced Calcium released from internal stores Calcium causes synaptic release No change in membrane potential (no receptor potential)

Taste Transduction – Umami Glutamate in food binds to transmitter-gated sodium channel Membrane depolarization (receptor potential) Voltage-gated calcium channel opens Inward calcium current triggers synaptic release

Neural Pathways for Taste Afferents Anterior 2/3 of tongue - VII (facial nerve) Posterior 1/3 of tongue - IX (glossopharyngeal nerve) glottis, epiglottis, pharynx, palate - X (vagus nerve) All afferent fibers end in gustatory nucleus in the medulla

Pathways to the Brain Taste fibers proceed along several pathways to the medulla oblongata or brain stem, then to the thalamus, and finally to the taste area on the anterior cortex. For taste sensation, gustatory nucleus neurons send fibers to the ventral posterior medial nucleus (VPM) of thalamus VPM sends fibers to the primary gustatory cortex (Brodmann's area 43 in ventral parietal lobe For autonomic functions, gustatory nucleus sends fibers to the many brainstem regions involved in swallowing, salivation, gagging, vomiting, digestion, respiration and to the hypothalamus and amygdala appetite and food preferences

Neural Coding for Taste Labeled line hypothesis - if each receptor only responds to a specific flavor, then each axon would represent a particular taste Population coding - if each receptor responds to many flavors, but differentially, then the population of activity across all the fibers would represent a particular taste Gustation uses a population coding scheme Other inputs participate in the overall perception of taste as mentioned above

Common Ideas from Other Senses Taste perception is a result of differences in neural stimulation Different perceptions can arise from the same cells. There are synaptic connections between neighboring cells, as in the case of vision and hearing. The taste sense exhibits adaptation and masking, like the other senses.

Olfaction - Smell Inside each side of the nose is an air chamber, the nasal cavity. Air comes in through the nostril and flows down, around the rear of the roof of the mouth, into the throat. When you sniff, air swirls up into the top of the cavity. A small patch of about 10 million specialized olfactory (smelling) cells are in the cavity Loss of smell is anosmia

Organs of Smell Olfactory epithelium in roof of nasal cavity Olfactory receptors – chemoreceptors Supporting cells - secrete mucous Basal cells - generate new olfactory receptors Size of the olfactory epithelium is a measure of keenness of smell humans have about 10 sq. cm dogs have 170 sq. cm and dogs may also have 100 times the olfactory receptor density

Visualizing Smell

Olfactory Receptor Cells The olfactory sensors are located on each side of the inner nose. True neurons: Dendrite ends in knob with multiple cilia (containing receptors) at surface of epithelium Odor particles in the air stick to the cilia Unmyelinated axon leaves base of epithelium thru cribiform plate and ends in olfactory bulb Born, live, and die with a 4-8 week cycle - only neurons regularly replaced throughout life The olfactory cells produce nerve signals, which travel to the olfactory bulb

Mechanisms of Olfactory Transduction Odorant binds to G-protein coupled receptor protein There are 500 - 1000 different olfactory receptor proteins coded genetically Effector protein is adenylyl cyclase cAMP produced and binds to cation channel (sodium + calcium) Calcium influx opens calcium-sensitive chloride channel Calcium and chloride cause membrane depolarization (receptor potential) Action potentials in olfactory receptor fibers

The Olfactory Bulb A pre-processing center that sorts the signals before they travel along the olfactory tract to the brain Axons from receptor cells project to the olfactory bulbs Here the glomeruli, receive signals from 26,000 receptors The olfactory bulbs on either side are cross-connected.

Pathways to the Brain Nerve fibers project from the olfactory bulb to the olfactory tubercle Olfactory tubercle neurons project to medial dorsal nucleus of thalamus Thalamic neurons project to orbitofrontal cortex reach the olfactory areas in the neocortex for the sensation of smell

Neural Coding in the Olfactory System Olfactory receptors respond to a variety of odorants Population code is used Both spatial distribution and timing of action potentials is important

Other Pathways Olfactory bulb neurons also end in other places Olfactory cortex that is not part of neocortex old system Effects many brain systems Odor discrimination Odor perception Motivation Emotions Reproduction, feeding imprinting, memory

Olfactory Senses in Other Animals In insects, the olfactory sense is located on the antennae. Snakes and lizards possess a Jacobson's organ in the front of the mouth that is directly connected to the olfactory center in the brain. The flicking tongue transfers scents to this organ for analysis. Scents seem to have a strong influence on the social interactions of many animals Birds have a well-developed olfactory sense, which was not appreciated until recently.

Some Facts About Smell The olfactory sensations are given as fruity, flowery, resinous, spicy, foul, and burned. Smell is 10,000 times as sensitive as taste Smell is primarily responsible food flavors. The two nostrils receive slightly different chemical signatures allows finer discrimination of odors. There is strong adaptation; one soon becomes accustomed to an odor and unaware of it One odor can be masked by another the theory of perfume. Need a 20% increase in concentration to cause a perceptible increase in the strength of perception.

Odor & Memory Odors call up memory. Smell is the only sense with direct access to the amygdala, the 'emotional center' of the brain.

Interaction of Taste & Smell Information on taste is organized in the brain separately from that of smell When the brain processes this information, nerve signals from the two senses unite and create a third, different representation. The latter represents flavor the combination of taste and smell. Flavor is handled in a distinct region of the brain, separate from those where smell and taste are processed.

Taste & Smell

Taste, Smell, & Memory The chemical senses are sometimes associated with vivid mental images and recollections an unexpected connection to higher mental processes. Scientists investigated the way in which both taste and smell sensations are stored in memory.