Chemical Senses

Question 1

Chemoreceptive Senses

Answer 1

– olfaction (smell) and gustation (taste)
o Taste – plays vital role in food selection
 Sweet and umami (mono-sodium glutamate) are associated with nutritious food
 Bitter associated with presence of toxins and are avoided
o Taste and smell are closely linked but involve different receptors and receptive processes
 May suggest an overlap in central processing

Question 2

Gustation Characteristics

Answer 2

o Taste buds are segregated into 4 groups on specific regions of the tongue
 Anterior  posterior: sweet, salty, sour, bitter
 Individual taste cells are relatively selective to one particular taste but can respond to more than one modality of taste
o 3 types of taste cells (light, dark, intermediate) – specialized epithelial cells (NOT NEURONS)
 Regeneration can occur from basal cells (renewed every ~25 days)

Question 3

Gustation Transduction

Answer 3

– taste buds contain microvilli which contain taste receptors
o Bitter & Sweet ligands – use G-protein coupled membrane receptors
 Bitter ligand (transducin) – releases Ca+ from intracellular stores via IP3 pathway
 Sweet ligand (gustducin) – activates cAMP second messenger that closes K+ channels and depolarizes the cell  voltage gated Ca+ open  Ca+ influx
o Sour & Salt – ionic ligands (Na+ and H+) alter ion channels  depolarizes the cell  voltage gated Ca+ open  Ca+ influx  neurotransmitter release
 Sour – H+ closes K+ channels
 Salt – enters via leaky channels and directly cause cell depolarization
o Umami (deliciousness) – associated with glutamate and other nucleotides
 Receptors are located at the back of the pharynx

Question 4

Theories of Gustation Coding

Answer 4

– how we organize multiple taste cell input
o Labeled-line Coding – each nerve fiber is activated/perceived by a specific stimuli (ex: sucrose = sweet tasting); sweet fiber/nerve gets activated then it recognizes the taste as sweet even though other fibers can get activated by other tastes; they are most sensitive to a particular one
 Still get a mixed signal because the nerve fiber itself branches and innervates a few different taste cells
o Across-fiber Coding – in order to detect a taste you have to look at and sum all the fibers of an individual nerve and the taste is determined accordingly

Question 5

Gustation Projections

Answer 5

o Tongue, soft palate and glottis  solitary nucleus of brainstem  VPM of thalamus OR hypothalamus OR amygdala  VPM of thalamus projects to Insula and Frontal cortex
 Taste afferents from midbrain are involved in viscera reflexes (secretion of GI juices)
 Hypothalamus – center for hunger/satiety
 Amygdala – regulatory center for feelings/emotions
o Primary sensory input to cortex travels via: carry information to nucleus of the solitary tract
 Facial nerve (VII) – anterior 2/3 of tongue  anterior region of nucleus of solitary tract
 Glossopharyngeal nerve (IX) –back of tongue middle region of nucleus of solitary tract
 Vagus nerve (X) – soft palate and mouth  posterior region of nucleus of solitary tract
• Gagging reflex – detects water & closes epiglottis to prevent going into lungs
o Trigeminal Nerve – innervates the oral, nasal, and ocular cavities; gets stimulated by spice
 Mechanism: via TPRV1 Receptor – activated by capsaicin, heat, or protons

Question 6

Olfaction Characteristics

Answer 6

o Anosmic – inability to smell
o More to do with motivation  goes straight to cortex
o Receptors are confined to 5cm2 of the olfactory mucosa and lie deep within the nasal cavity
 Receptors organized based on 3 family of genes
o Greater range of primary olfactory sensitivity (1,000s) than that for taste (5)
 Subdivided into 7 primary qualities of smell: peppermint, musck, floral, ethereal, pungent, putrid, and camphoreaceous

Question 7

Olfaction: Form, Function, Transduction Mechanism

Answer 7

o Sniffing directs air onto olfactory mucosa  odorants absorbed into mucous layer overlying the receptors  diffusion through mucosa brings odorants into contact w/ olfactory cilia/receptors
o Activation of G protein second messenger systems that increase cAMP levels and cause Ca+ influx and depolarization; initiating action potential, which is propagated to the olfactory bulb
 Decrease in chloride channels (Cl- outward flow)
 Olfactory bulb sits superior to the cribiform plate with afferent fibers connecting the bulb to the olfactory cilia and cell on inferior side of cribiform plate
o Olfactory epithelium is highly vascularized  why drugs are better when snorted
o Spacial coding – olfactory bulb transduces information to the cortex through synchronous firing
 Different odorants that arrive will activate different neurons and odorants A+B arriving synchronously activate a specific neuron indicating that small

Question 8

Laminar Organization of the olfactory epithelium and topographic projections to the olfactory bulb

Answer 8

o Functional mapping of specific odors to specific regions within the olfactory tract
o Epithelium organized into 4 zones; send projections to their respective zones in olfactory bulb

Question 9

Multi-Layered Olfactory Bulb

Answer 9

o Neural axons that reach olfactory bulb are separated specific to its glomerulus
 Complex smell results in multiple odorants being distinguished by their own glomerulus
 Co-activation of particular glomeruli encode information for a unique smell
o Glomeruli – present at the glomerular layer – segregation of signals in olfactory bulb occurs here
o Periglomerular cell – innervates neighboring glomeruli; activation of one glomeruli can suppress activity of other glomeruli
o Mitral cell (mitral cell layer) and tufted cell (external plexiform layer) have large neurons that project information to brain via lateral olfactory tract

Question 10

Olfactory Projection

Answer 10

o Receptors  olfactory bulb  lateral olfactory tracts (olfactory nerve CN 1)  olfactory cortex
 Olfactory bulb also sends projections to amygdala  assigns feelings/emotions
• Very fast - doesn’t go to primary sensory nuclei
o Information from one side of olfactory bulb projects to other side thus allowing for modifying/suppressing output from the adjacent side
 Localization of smell by comparing sensory reception of bi-directional projections
o Orbitalfrontal cortex – where we have convergence of taste (from thalamus) and smell info

Question 11

Vermonasal System

Answer 11

–Pheromone Receptors – Sex and Social Behavior
o Receptors located near olfactory neurons in nasal cavity
o Signals to accessory olfactory bulb; tract mainly goes to amygdala

Question 12

Papillae

Answer 12

– small projections all the tongue that detect food textures and contain taste buds
o Circumvallate papillae – posterior – taste buds on side of deep invaginations
 Glossopharyngeal nerve
o Folliate papillae – side and middle – taste buds on side of deep invaginations
 More prominent than fungiform papillae
o Fungiform papillae – interior of tongue – taste buds on bottom of shallow invaginations