Chemical senses: Olfaction & smell

OBJECTIVES Discuss : basic smell and taste modalities Olfactory gustatory receptor cells olfactory and taste pathways abnormalities of olfaction taste pathways, abnormalities of gestation.

 Both smell and taste are chemical senses, because the sensations arise from the interaction of molecules with smell or taste receptors. Because impulses for smell and taste propagate to the limbic system (and to higher cortical areas as well), certain odors and tastes can evoke strong emotional responses or a flood of memories. The smell receptors are distance receptors (teleceptors), and the smell pathways have no relay in the thalamus. The taste pathways pass up the brain stem to the thalamus and project to the postcentral gyrus along with those for touch and pressure sensibility from the mouth. The smell receptors are distance receptors (teleceptors), and the smell pathways have no relay in the thalamus. The taste pathways pass up the brain stem to the thalamus and project to the postcentral gyrus along with those for touch and pressure sensibility from the mouth.

Chemical senses (taste & smell) are those conscious sensations that are elicited in response to a chemical stimuli Are mediated by chemoreceptors Are mutually complementary to add to the flavour of food Flavour of food indicates the combined influence of smell, taste, consistency, temperature of food Induce pleasurable feeling for nutritious food Allow animals to avoid harmful or lethal food Olfaction linked to recognition, behavioral functions and emotions

OLFACTION Olfactory receptors, pathway, odoriferous substances, olfactory transduction, adaptation, applied aspects

Olfactory mucous membrane Location: Yellowish pigmented area in the nasal mucosa in the roof of nasal cavity above the superior turbinate Cells present: –Receptor cells : 10 – 20 million –Supporting cells –Basal stem cells : give rise to other types of cells –Bowmann’s glands : located below the epithelium, open to the surface & secrete mucous

Macrosmatic & microsmatic animals Animals have large area olfactory mucous membrane – macrosmatic Humans are called microsmatic- area covered is small (5 cm 2 )

OLFACTORY RECEPTORS Each receptor is a bipolar neuron Have short thick dendrites (olfactory rods) with expanded ends & having cilia projecting to nasal cavity Cilia have receptors for odorant molecules Renewed once in few weeks Nasal cavity cavity mucus Olfactory bulb

Olfactory pathway Receptor cell axon pierces the cribriform plate of ethmoid ↓ enters olfactory bulb ↓ synapse with dendrites of mitral cells to form olfactory glomeruli Olfactory bulb Olfactory glomeruli With mitral cells

Axons of mitral cells pass posteriorly thro’ olfactory stria ↓ Olfactory cortex -Ant. Olfact. nucleus -Olfact. tubercle -Prepyriform cortex -Amygdala -Entorhinal cortex Olfactory bulb Olfactory cortex Olfactory cortex Olfactory glomeruli Olfactory glomeruli Axon of mitral cell Orbitofrontal cortex HypothalamusHippocampus

Olfactory impulses are projected both to NEOCORTEX PERCEPTION & DISCRIMINATION OF ODOURS LIMBIC SYSTEM EMOTIONAL, MOTIVATIONAL, BEHAVIORAL & PHYSIOLOGICAL EFFECTS OF ODOURS

Special features Receptor is a modified neuron Only place where neurons are in direct contact with external environment Though the receptors are neurons, they undergo degeneration periodically & are replaced by new cells Pathway involves only two sets of neurons No definite relay in thalamus (unlike other sensations) Olfactory signals do not reach somatosensory cortex but reach orbitofrontal cortex Impulses reach limbic system

Importance of olfaction Nutritive role In food & sex motivated behaviour, olfaction plays an important role in subhuman species & a minor role in humans –Pheromones are odoriferous hormone like substances secreted by animals to attract its mating partner. –Pheromones are sensed by vomeronasal organ present in animals Also acts as a protective sensation to avoid dangerous & toxic chemicals

Primary smell sensations –Camphoraceous –Floral –Ethereal –Pepperminty –Musky –Pungent –Putrid Humans can distinguish between 2000 – 4000 different odours

Criteria for the substances to be smelt Olfactory receptors respond only to substances that are in contact with olfactory epithelium The substances –Must be volatile –Must be water soluble or lipid soluble

Odour detection Human nose contain 5 million olfactory receptors – 1000 different types Human nose contain 5 million olfactory receptors – 1000 different types An odour is ‘dissected’ into various components Each receptor responds to one component of an odour Various components of an odour are detected by one of the 1000 different receptors A particular receptor responds to a particular component of an odour shared in common by different scents Binding of an appropriate scent signal to an olfactory receptor activates a G protein

Triggers a cascade of cAMP-dependent intracellular reactions Opening of a Na + channels Depolarizing receptor potential Generation of action potential in afferent neurons In the glomeruli (olfactory bulb) the terminal of receptor cells carrying information about a particular scent component synapse with the mitral cells Each glomeruus detect a particular odour component – ‘smell files’

The mitral cells refine smell signals and relay them to the brain Odour discrimination is based on different pattern of glomeruli activated by various scents The human cortex can distinguish over 10,000 different scents (Dogs’ sense of smell is hundreds of times more sensitive than that of humans) Humans have about 5 million receptor cells – Dogs have about 4 billion olfactory receptor cells

Mechanism of stimulation Odouriferous molecules combine with receptors on cilia  Cyclic AMP formation   Na + influx  Development of receptor potential  AP in olfactory nerve fibers  Conducted to olfactory cortex  Interpretation of smell sensation

Adaptation of olfactory sensation Smell sensation shows complete adaptation This is a function of both receptors and CNS

Sniffing The portion of the nasal cavity containing the olfactory receptors is poorly ventilated in humans. Most of the air normally moves smoothly over the turbinates with each respiratory cycle, although eddy currents pass some air over the olfactory mucous membrane. These eddy currents are probably set up by convection as cool air strikes the warm mucosal surfaces. The amount of air reaching this region is greatly increased by sniffing, an action that includes contraction of the lower part of the nares on the septum, deflecting the airstream upward. Sniffing is a semi-reflex response that usually occurs when a new odor attracts attention

Vomeronasal Organ In rodents and various other mammals, the nasal cavity contains another patch of olfactory mucous membrane located along the nasal septum in a well-developed vomeronasal organ. This structure is concerned with the perception of odors that act as pheromones Its receptors project to the accessory olfactory bulb and from there primarily to areas in the amygdala and hypothalamus that are concerned with reproduction and ingestive behavior.

The organ is not well developed in humans, but there is an anatomically separate and biochemically unique area of olfactory mucous membrane in a pit in the anterior third of the nasal septum which appears to be a homologous structure. There is evidence for the existence of pheromones in humans, and there is a close relationship between smell and sexual function. The perfume ads bear witness to this. The sense of smell is said to be more acute in women than in men, and in women it is most acute at the time of ovulation. Smell and, to a lesser extent, taste have a unique ability to trigger long-term memories, a fact noted by novelists and documented by experimental psychologists.

Role of Pain Fibers in the Nose Naked endings of many trigeminal pain fibers are found in the olfactory mucous membrane. They are stimulated by irritating substances, and an irritative, trigeminally mediated component is part of the characteristic "odor" of such substances as peppermint, menthol, and chlorine. They are stimulated by irritating substances, and an irritative, trigeminally mediated component is part of the characteristic "odor" of such substances as peppermint, menthol, and chlorine. These endings are also responsible for initiating sneezing, lacrimation, respiratory inhibition, and other reflex responses to nasal irritants.

Disorders of olfaction Anosmia : loss of smell sensation Hyposmia : diminished sensitivity to smell Dysosmia : disturbed smell sense Parosmia : abnormal smell sense Olfactory hallucinations may precede epileptic seizures Kallman’s syndrome : Hypogonadism with loss of smell sensation

. Hyposmia (hı ￣ -POZ-me ￣ -a; osmi smell, odor), Women often have a keener sense of smell than men do, especially at the time of ovulation. Smoking seriously impairs the sense of smell in the short term and may cause long-term damage to olfactory receptors. With aging the sense of smell deteriorates a reduced ability to smell, affects half of those over age 65 and 75% of those over age 80. Hyposmia also can be caused by neurological changes, such as a head injury, Alzheimer disease, or Parkinson disease; certain drugs, such as antihistamines, analgesics, or steroids; and the damaging effects of smoking

OUTCOME –Describe the gustatory receptors and the neural pathway for gustation. –Describe the olfactory receptors and the neural pathway for olfaction –Describe basic features of the neural elements in the olfactory epithelium and olfactory bulb. –How do olfactory receptors and gustatory receptor cells differ in structure and Function? –Compare the olfactory and gustatory pathways