Chemical Senses

Myopia

• Nearsightedness

• Can’t see clearly far away objects

• Usually develops in childhood

• Problems with the shape of the eyeball, cornea or lens

Hyperopia

• Farsightedness

• Can’t see objects up close

• lifelong

• problems with the shape of the eyeball cornea or lens

Presbyopia

• Farsightedness

• Can’t see objects up close

• Usually develops after the age of 40

• Reduction in ciliary muscle function, less elasticity in the lens

The chemical sense: smell and taste

• Smell (olfaction) and taste (gustation): complementary sense that let us know whether a substance should be savored or avoided

• Chemoreceptors are used by these systems

• Chemicals must be dissolved in aqueous solution to be picked up by chemoreceptors

  • Smell receptors are excited by chemicals dissolved in nasal fluids

  • Taste receptors respond to chemicals dissolved in saliva

Olfactory Receptors

Olfactory epithelium: organ of smell

• Located in roof of nasal cavity

• Covers superior nasal conchae

• Contains olfactory sensory neurons

  • Bipolar neurons with radiating olfactory cilia

• Supporting cells surround and cushion olfactory receptor cells

• Olfactory stem cells lie at base of epithelium

• Olfactory neurons are unusual bipolar neurons

  • Thin apical dendrites terminate in knob

  • long, largely nonmotile cilia, olfactory cilia, radiate from knob

    • Covered by mucus (solvent for odorants)

• Bundles of nonmyelinated axons of olfactory receptor cells gather in fascicles that make up filaments of olfactory nerve (cranial nerve I)

• Olfactory neurons, unlike other neurons, have stem cells that give rise to new neurons every 30-60 days

Specificity of Olfactory Receptors

• Smells may contain 100s of different odorants

• Humans have ~400 “smell” genes active in nose

• Each encodes a unique receptor protein

  • Protein responds to one or more odors

• Each odour binds to several different receptors

• Each receptor cell has one type of receptor protein

• The genes encoding the receptor molecules were isolated and identified

• Every single olfactory receptor cell expresses one and only one gene of all the genes that code for olfactory receptor molecules

• Neurons with same receptor are confined to one zone, but scattered in that zone

• Nuerons with different receptors are interspersed

• Axons of sensory neurons with the same oforant receptor type converge in 2 glomeruli

• Single odorant activates multiple glomeruli with input from different receptors

• Each glomerulus id dedicated to one type of receptor

The smell test

The relatively small size of the human olfactory bulb - compared to other animals - has long been cited as a reason for human’s “inferior” scent of smell. New research is starting to overturn this notion.

Physiology of Smell

In order to smell substance, it must be volatile

• Must be in gaseous state

• Odorant must also be able to dissolve in olfactory epithelium fluid

Activation of olfactory sensory neurons

• Dissolved odorants bind to receptor proteins in olfactory cilium membranes

• Open cation channels, generating receptor potential

• At the threshold, AP is conducted to first relay station in the olfactory bulb

Smell Transduction

• Odorant binds to receptor, activating a G protein

  • Referred to as G(olf)

• G protein activation causes cAMP (second messenger) synthesis

• cAMP opens Na+ and Ca2+ channels

• Na+ influx cause depolarization and impulse transmission

• Ca2+ influx causes decreased response to a sustained stimulus, referred to as olfactory adaptation

  • People can’t smell a certain odor after being exposed to it for a while

Olfactory Transduction Process

1. Odorant binds to its receptor

2. Receptor activates G protein (Golf)

3. G protein activates adenylate cyclase

4. Adenylate cyclase converts ATP and cAMP

5. cAMP opens a cation channel, allowing Na+ and Ca2+ influx and causing depolarization

The Olfactory Pathway

• Filaments of olfactory nerves synapse with mitral cells located in overlying olfactory bulb

  • Mitral cells are second-order neurons that form olfactory tract

• Synapse occurs in structures called glomeruli

• Axons from neirons with same receptor type converge on given on given type of glomerulus

• Mitral cells amplify, refine, and relay signals

• The olfactory cortex is defined as those brain regions that receive the mitral and tufted cell projections from the olfactory bulb

• Impulses from activated mitral cells travel via olfacotry tracts to piriform lobe of olfactory cortex

• Some information sent to hypothalamus, amygdala, and other regions of limbic system

  • Emotional responses to odour are elicited

Anosmias

Olfactory disorders; most result from

• Head injuries that tear olfactory nerves

• Aftereffects of nasal cavity inflammation

• Neurological disorders, such as Parkinson’s disease

Olfactory Hallucinations (phantosmia)

• Usually caused by temporal lobe epilepsy that involves olfactory cortex

• Some people have olfactory auras prior to epileptic seizures

Where and how is taste sensed?

Gustatory signals → Detected by chemoreceptors in the taste buds

Taste Buds

sensory organs for taste. Most of 10, 000 taste buds are located on tongue in papillae, peglike projections of tongue mucosa

Fungiform Papillae

Tops of these mushroom-shaped structures house most taste buds; scattered across tongue

Foliate papillae

on side walls of tongue

Vallate Papillae

Largest taste buds with 8-12 forming “V” at back of tongue

Gustatory Epithelial Cells

Taste receptor cells have microvilli called gustatory hairs that project into taste pores, bathed in saliva

• Sensory dendrites coiled around gustatory epithelial cells send taste signals to brain

Basal Epithelial Cells

Dynamic stem cells that divide every 7-10 days

Basic Taste Sensations

There are five basic taste sensations

1. Sweet - sugars, saccharin, alcohol, some amino acids, some lead salts

2. Sour - hydrogen ions in solution

3. Salty - metal ions (inorganic salts); sodium chloride tastes saltiest

4. Bitter - alkaloid such as quinine and nicotine, caffeine, and non-alkaloids such as aspirin

5. Umami - amino acids glutamate and aspartate; example: beef (meat) or cheese taste, and monosodium glutamate

• Taste likes/dislikes have homeostatic value

• Guide intake of beneficial and potentially harmful substances

• Dislike for sourness and bitterness is a protective way of warning us if something is spoiled or poisonous

To be able to taste a chemical, it must:

• Be dissolved in saliva

• Diffuse into taste pore

• Contact gustatory hairs

Activation of taste receptors

• Binding of food chemical (tastant) depolarizes cell membrane of gustatory epithelial cell membrane, causing release of neurotransmitter

• Neurotransmitter binds to dendrite of sensory neuron and initiates a generator potential that leads to action potentials

• Different gustatory cells have different thresholds for activation

  • Bitter receptors are most sensitive

• All adapt in 3-5 seconds, with complete adaptation in 1-5 mintues

Taste Transduction

Gustatory epithelial cell depolarization caused by:

• Salty taste is due to Na+ influx that directly causes depolarization

• Sour taste is due to H+ acting intracellularly by opening channels that allow other cations to enter

• Unique receptors for sweet, bitter, and umami, but all are coupled to G protein Gustducin

  • Activation causes release of stored Ca2+ that opens cation channels, causing depolarizationa nd release of neurotransmitter ATP

The Salivary Glands

• 3 Main Pairs

• Saliva acts as a solvent

• faciliates clearance of taste particles

What other sense are involved in taste perception?

• Gustatory - from taste buds (gustatory Pathway VII. IX.)

• Touch - mechanoreceptors in the oral cavity (Somatosensory Pathway) ( V. IX.)

• Temperature - nerve endings in the oral cavity (Somatosensory Pathway) (V. IX.)

• Olfactory - from the olfactory epithelium (olfactory Pathway (I.)

• Taste is 80% smell

• If nose is blocked, foods taste bland

• Mouth also contains thermoreceptors, mechanoreceptors and nociceptors

  • Temperature and texture enhance or detract from taste

  • Spicy hot foods can excite pain receptors in mouth, which some people experience as pleasure

  • Example: hot chili peppers

How are gustatory signals transmitted to the brain

Taste buds sen gustatory signals via:

• Vagus Nerve

• Glossopharyngeal Nerve (Majority of gustatory signals)

• Facial Nerve

Gustatory Pathway

Two main cranial nerve pairs carry taste impulses from tongue to brain:

• Facial Nerve (VII) carries impulses from anterior two-thirds of tongue

• Glossopharyngeal (IX) carries impulses from posterior one-third and pharynx

• Vagus nerve transmits (X) from epiglostis and lower pharynx

• Fibers synapse in the solitary nucleus of the medulla, then travel to thalamus, and then to gustatory cortex in the insula

• Hypothalamus and limbic system are involved; allow us to determine appreciation of taste

Taste perception = Thalamus Cortex

Appetite, Satiety and Emotional Valance = Lateral hypothalamus and Amygdala

Important roles of taste involve:

• Triggering reflexes involved in digestion, such as: increased secretion of saliva into mouth

• Increased secretion of gastric juice into stomach

May initiate protective reactions, such as:

• Gagging

• Reflexive vomiting

Cordination

• Gustatory - from taste buds

• Touch - mechanoreceptors in the oral cavity

• Temperature - nerve endings ini the oral cavity

• Olfactory - from the olfactory epithlium

Primary cortices of sense → Orbitofrontal Cortex → Limbic system, Amygdala

Gustatory Reflex

Taste buds → Sensory input → Solitary nuclei (medulla) → salvatory nuclei (pons) → Salivary Glands

Disgeusia

• Taste disorders are less common than disorders of smell, mostly because taste receptors are served by three different nerves

• Not likely that all three nerves would be damaged at same time

• Distorted or complete loss of taste

Causes of taste disorders include:

• Upper respiratory tract infections

• Head injuries

• Chemicals or medications

• Head and neck radiation for cancer treatment

• Zinc supplements may help some cases of radiation-induced taste disorders