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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
Odorant binds to its receptor
Receptor activates G protein (Golf)
G protein activates adenylate cyclase
Adenylate cyclase converts ATP and cAMP
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
Sweet - sugars, saccharin, alcohol, some amino acids, some lead salts
Sour - hydrogen ions in solution
Salty - metal ions (inorganic salts); sodium chloride tastes saltiest
Bitter - alkaloid such as quinine and nicotine, caffeine, and non-alkaloids such as aspirin
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