smell (olfactory)

whats the difference between tasting and smelling senses?

♣ Taste - immediate sense ♣ Smell - a more distant sense to detect small concentration of airborne substances

why is the sense of smell transmission different from other sensory modalities?

♣ Unlike other sensory modalities that are first sent to the thalamus and from there relayed to the cerebral cortex, the sense of smell is transmitted DIRECTLY to the cerebral cortex

what is the function of smell and taste?

♣ Warning system to identify potentially toxic food or noxious chemicals ♣ Awareness of many pleasurable sensations- appreciation of foods, beverages, etc.

olfactory receptor cells

♣ binds to chemicals) • Afferent neuron whose receptor portion is in olfactory mucosa in nose and afferent axon traverses into brain • Axons of olfactory receptor cells collectively form olfactory nerve - receptor cells for the smell sensation -♣ are located in the olfactory epithelium ♣ are TRUE neurons that conduct action potentials into the CNS

describe the chemical route to olfactory tract

♣ mucus layer olfactory cell sustentacular (support) cell glomerulus mitral cell olfactory tract

sensory receptors main purpose

to sense the environmental signals that can be detected

mechanoreceptors

(e.g. stretch receptors in muscle and hair cells in auditory and vestibular systems)

sensory receptors

(general receptors on skin) - are cells/neurons that transduce environmental signals into neural signals

What are the steps in transduction of olfactory neurons

1. Odorant molecules bind to receptors on the cilia of the olfactory receptor neurons. 2. When the receptors are activated, they activate G proteins, which in turn activate adenyl cyclase. 3. The increase in intracellular cAMP opens Na+ channels in the olfactory receptor membrane, producing a depolarizing receptor potential. 4. The receptor potential depolarizes the initial segment of the axon to threshold, and action potentials are generated and propagated

what is the mechanism of excitation of the olfactory cells (steps 1-10)

1. The odorant substance, upon contact with the olfactory membrane surface, first diffuses into the mucus that covers the cilia. 2. Olfactory Cilia responds to the chemical stimuli 3. Then it binds with receptor proteins in the membrane of each cilium 4. Receptor proteins are (GPCRs = G Protein Coupled Receptors) with seven transmembrane domains folding inward and outward 5. The odorant binds with portion of the receptor protein that folds to the outside. 6. The inside of the protein is coupled G protein, itself combination with 3 subunits (a, b, g) 7. On excitation of the receptor protein, an alpha subunit breaks away from the G-protein and activates adenyl cyclase. 8. The activated cyclase, in turn converts many molecules of ATP into cAMP 9. This cAMP activates another membrane protein, a gated Na+ ion channel, that opens its gates and allows large amount of Na+ to pour through the membrane into the receptor cell cytoplasm a. Increased Calcium and Sodium influx b. Increased Chloride efflux 10. The Na ions increase the electric potential in the positive direction inside the cell membrane, exciting the Olfactory neurons and transmitting action potentials into the CNS by Olfactory nerve. a. Activated by... i. Chemical stimulus & cAMP o This cascade of events account for the sensitivity of the olfactory neurons to even the slightest amount of odorant

how many different types of olfactory receptors are there

1000

interoceptors

Chemoreceptors that respond to chemicals in internal environment

odorants

Molecules that are/can be smelled

bowman's glands

Spaced among the olfactory membrane are many Bowman's glands that secrete mucus onto the surface of the olfactory membrane ♣ In order to dissolve inhaled chemical

special senses

The receptors that mediate sight, hearing, equilibrium, taste and smell

parosmia

altered perception of smell in the presence of an odor, usually unpleasant

agnosia

inability to classify or contrast odors, although able to detect odors

hyposmia

o - decreased ability to detect odors

about how many olfactory cells are in the olfactory epithelium?

o 100 million of these cells in olfactory epithelium interspersed with sustentacular cells ("support" cells = "sustentacular" cells)

dysosmia

o Distorted identification of smell

describe the organization of the olfactory membrane and olfactory bulb and connection to the olfactory tract.

o Lies in the superior part of each nostril o Medially, folds downward along the surface of the superior septum o Laterally, it folds over the superior turbinate and a small portion of the upper surface of the middle turbinate o In each nostril, the olfactory membrane has a surface area of 2.4 square cm

describe the processing of scents in olfactory bulb.

o Mitral cells - secondary neurons ♣ Each of the glomerulus receives input from only one type of olfactory receptors, regard less of those receptors that are located in olfactory epithelium ♣ Afferent signals are sorted according to scent component by glomeruli within olfactory bulb • Each glomerulus only receives signals from ONE type of chemical ♣ Impulses from mitral cells to TWO destinations for interpretation and identification of odors... • (1) Via thalamus to the piriform lobe of the olfactory cortex • (2) Bypasses the thalamus and flows directly to the regions of the limbic system (smell associated with dangers (smoke, gas leak))

describe the cilia of olfactory cells.

o The mucosal end contains cilia (0.3um in diameter and 200 um in length) project into mucus o It is these cilia that react to odors in the air and stimulate the olfactory cells

anosmia

o inability to detect odors

phantosmia

o perception of smell without an odor present ♣ Uncinated fits (psychiatric disorder) - olfactory hallucinations

describe the olfactory mucosa

o ~3cm2 of mucosa in ceiling of nasal cavity o Contains three cell types: olfactory receptor cell, supporting cells, basal cells

♣ Odorants act through ______________________ to trigger action potentials

second-messenger systems

chemoreceptors

sense chemical stimuli (molecules must be dissolved in a liquid to be sensed) in the environment or blood two types: interoceptors and exteroceptors

what is the least understood of our senses and why is it not understood?

smell (olfaction) o The major problems are that... ♣ (1) Is a subjective phenomena • Everyone has different sensations to a stimulus ♣ (2) Is poorly developed in humans as compared to many lower animals, such as dogs ♣ (3) Air entering the nasal cavity must make a hairpin turn to stimulate olfactory receptors, so the human olfactory epithelium is in a poor position for doing its job effectively • Small number of chemicals reach the human receptors

olfactory receptors

specialized endings of renewable afferent neurons (not a cell!)

mitral cells

take signals to the olfactory tract o in the olfactory bulb ♣ are the second-order neurons ♣ output of the mitral cells forms the olfactory tract, which projects to the prepiriform cortex

what two senses are closely related and complement each other

taste and smell

photoreceptors

that detects light (rods and cones of the retina)

nocireceptors/pain receptors

that responds to tissue damage (extreme temperature and pain)

vomeronasal organ

• (VNO) - "sixth sense" o Common in mammals, but until recently was thought to nonexistent in humans o Located about half an inch inside human nose next to vomer bone o Detects pheromones... ♣ Nonvolatile chemical signals passed subconsciously from one individual to another ♣ Helps to attract opposite genders o Role in human behavior has not been validated

exteroceptors

• Chemoreceptors that respond to chemical changes in external environment o Olfactory receptors - sense smell in nose o Taste receptors - sense tastes on tongue

basal cells

• Precursors of new olfactory receptor cells (replaced about every two months) o of the olfactory epithelium are undifferentiated stem cells that continuously turn over and replace the olfactory receptor cells (neurons). ♣ These are the ONLY examples in the human adult where neurons replace themselves (these neurons divide and replace old ones)

smell and taste are what type of senses?

• chemical senses o Give us information about the chemicals in our surroundings (NOT detailed information, just general information) o Both detect dissolved chemicals (cannot sense without dissolving chemicals)

smell disorders

• classified as "-osmias" = inability to detect odors o Disorders of smell are very subjective ♣ Loss is linked to... • Inadequate nutritional intake • Reduced social pleasure • Decreased psychological wellbeing ♣ Examples... smoke detection ♣ Can be total (all odors) or partial (affecting several) or specific (only one or select few odors)

describe the olfactory pathway

♣ (1) Axons of the olfactory receptor cells form the olfactory nerve, which synapse in the olfactory bulbs, the distal ends of olfactory tracts ♣ (2) There, the filaments of the olfactory nerve synapse with mitral cells (which are second order neurons) in complex structures called glomeruli. ♣ (3) Axons from neurons bearing the same kind of receptor converge on glomerulus. That is, each glomerulus represents a single odor

supporting cells

♣ (Bowman Gland) • Secrete mucus (necessary for chemicals to be dissolved) • Are epithelial cells rich in enzymes that oxidize hydrophobic, volatile odorants, make less lipid soluble, less penetrate the membrane and enter the brain (reduce hydrophobicity of chemicals so they will NOT pass through the membrane and enter the brain)

to be smelled, what must a substance be?

♣ ) Sufficiently volatile that some of its molecules can enter nose in inspired air ♣ (2) Sufficiently water soluble that it can dissolve in mucus coating the olfactory mucosa to reach the olfactory cilia (without be dissolved, chemicals would be unrecognizable) ♣ (3) Substances be slightly lipid soluble, because lipid constituents of the cilium itself are a weak barrier to non-lipid soluble odorants • If the chemical is completely lipid soluble, the chemical would not be dissolvable and it would not reach the receptor of the sensory cells

CN 1 (olfactory cells)

♣ Carries information from olfactory receptor cells to the olfactory bulb ♣ Olfactory epithelium is also innervated by CN V (trigeminal), which detects noxious or painful stimuli such as ammonia ♣ The olfactory nerves pass through the cribriform plate on their way to the olfactory bulb. Fractures of the cribriform plate sever input to the olfactory bulb and reduce (hyposmia) or eliminate (anosmia) the sense of smell. ♣ However, responses to ammonia will be intact following fracture of the cribriform plate since these responses are carried on by CN V

richard axel and linda buick

♣ Nobel Prize in 2004 ♣ Land mark paper in 1991 on olfactory receptors ♣ Cloned olfactory receptors-G protein coupled receptors ♣ 1,000 different genes for olfactory receptors in the genome ♣ Each olfactory receptor neuron expresses only ONE KIND of olfactory receptor protein • Each olfactory receptor can only recognize one chemical ♣ Input from all neurons expressing the same receptor is collected by a single dedicated acceptor called glomerulus of the olfactory bulb