Olfaction and Gustation

• The special senses include olfaction (smell), gustation (taste), vision, and hearing/equilibrium (balance)

Olfaction and Gustation

• Smell (olfaction) and taste (gustation) are complementary senses, informing whether a substance should be savored or avoided
• Chemoreceptors are utilized by both smell and taste systems
• Chemicals must be dissolved in an aqueous solution to be detected by chemoreceptors
• Smell receptors are stimulated by chemicals dissolved in nasal fluids
• Taste receptors are stimulated by chemicals dissolved in saliva

Olfactory Epithelium and the Sense of Smell

• Olfactory epithelium, the organ of smell, is located in the roof of the nasal cavity
• Olfactory epithelium covers the superior nasal conchae
• Olfactory epithelium contains bipolar olfactory sensory neurons with radiating olfactory cilia
• Supporting cells surround and cushion olfactory receptor cells within the olfactory epithelium
• Olfactory stem cells lie at the base of the epithelium
• Axons of olfactory receptor cells form filaments of the olfactory nerve (cranial nerve I)
• Mucus covers the olfactory cilia, acting as a solvent for odorants
• Olfactory neurons have stem cells, unlike other neurons, allowing for new neuron generation every 30-60 days

Specificity of Olfactory Receptors

• A smell can be made up of hundreds of different odorants
• Humans have approximately ~350 different odorant receptors
• Each receptor encodes a unique protein and responds to one or more odors
• Each odor binds to several different receptors, so each receptor has one type of receptor protein
• Pain and temperature receptors exist in nasal cavities and respond to irritants like ammonia
• Pain and temperature receptors respond to hot or cold sensations, like chili peppers or menthol
• To be smelled, a substance must be volatile (in a gaseous state)
• Odorants must also be able to dissolve in the olfactory
• Filaments of olfactory nerves synapse with mitral cells, which are located in the olfactory bulb
• Mitral cells are second-order neurons that form the olfactory tract
• Synapses occur in structures called glomeruli
• Axons from neurons with the same receptor type converge on a given type of glomerulus
• Mitral cells amplify, refine, and relay signals

Olfactory Pathways

• Olfactory pathways begin with the binding of an odorant to a G protein-coupled receptor
• Binding of an odorant to a G protein-coupled receptor creates a generator potential (depolarization)
• Axons leaving the olfactory bulb travel along the olfactory tract to the olfactory cortex, hypothalamus, and limbic system
• Olfactory information reaches the cerebral cortex directly, without being relayed from the thalamus like other sensations

Olfactory Discrimination

• Humans can distinguish thousands of chemical stimuli
• Dogs possess 72 times more olfactory receptor surface area than humans
• Their sense of smell is more than 10,000 times more sensitive than humans
• Olfactory receptors are replaced frequently
• The total number of olfactory neurons declines with age

Gustation, the Sense of Taste

• Provides information about food solids and liquids consumed
• Gustatory epithelial cells (taste receptors) are located in taste buds
• Taste buds can be found on the superior surface of the tongue, and portions of the pharynx and larynx
• Taste buds are associated with epithelial projections (lingual papillae) on the surface of the tongue
• Taste buds consist of 50-100 flask-shaped epithelial cells of two types: gustatory and basal
• Most of the 10,000 taste buds are on the tongue, in peglike projections of tongue mucosa called papillae

Location and Structure of Taste Buds

• Tops of mushroom-shaped structures (fungiform papillae) house most taste buds and are scattered across the tongue
• Vallate papillae, largest taste buds, contain between 8-12 structures that form a "V" shape at the back of the tongue
• Folate papillae are located on the side walls of the tongue
• A few taste buds exist on the soft palate, cheeks, pharynx, and epiglottis
• Gustatory epithelial cells are taste receptor cells with microvilli, called gustatory hairs
• Gustatory hairs project into taste pores and are bathed in saliva
• Sensory dendrites coiled around gustatory epithelial cells send taste signals to the brain
• Basal epithelial cells are dynamic stem cells that divide every 7-10 days

Five Basic Taste Sensations

• Sweet: sugars, saccharin, alcohol, some amino acids, and some lead salts
• Sour: hydrogen ions in solution
• Salty: metal ions (inorganic salts); sodium chloride tastes saltiest
• Bitter: alkaloids such as quinine and nicotine and nonalkaloids such as aspirin
• Umami: amino acids glutamate and aspartate with examples being beef (meat), cheese taste, and monosodium glutamate
• Possible sixth taste of long-chain fatty acids from lipids, and may explain liking of fatty foods
• Taste likes/dislikes have homeostatic value
• Guide intake of beneficial and potentially harmful substances
• Dislike for sourness and bitterness is a protective way to warn of something spoiled or poisonous

Physiology of Taste

• To be tasted, a chemical must be dissolved in saliva
• The chemical must then diffuse into a taste pore
• The chemical must then contact gustatory hairs to activate taste receptors via two mechanisms
• Food chemical (tastant) binding depolarizes the cell membrane of the gustatory epithelial cell, which causes neurotransmitter release
• Neurotransmitter binds to a sensory neuron dendrite, which initiates a generator potential to create action potentials
• Different gustatory cells have different excitation thresholds, bitter receptors being the most sensitive
• Taste receptors adapt in 3-5 seconds, showing complete adaptation in 1-5 minutes
• Salty taste is created by Na+ influx that directly causes depolarization
• Sour taste happens because of H+ acting intracellularly by opening channels for other cations to enter
• Unique receptors for sweet, bitter, and umami are coupled to G protein gustducin
• This activation causes release of stored Ca2+ that opens cation channels, causing depolarization and release of neurotransmitter ATP

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 (X): carries impulses from posterior one-third of tongue and pharynx
• The Vagus nerve transmits from the epiglottis and lower pharynx
• Fibers synapse in the solitary nucleus of the medulla, then travel to the thalamus, and then to the gustatory cortex in the insula
• Hypothalamus and limbic systems involved for enabling appreciation of taste

Hearing

• Inner ear mechanoreceptors enable hearing and balance
• Fluids must be stirred by hearing and balance receptors of the inner ear
• Receptors for hearing and balance respond to activate separately but independently to different stimuli

Structure of the Ear:

• The ear has three major areas: external, middle and internal
• The External and Middle ears are for hearing only
• The Internal (inner) ear is for hearing and equilibrium

Anatomy of the Ear

• The external part of the ear includes the auricle (pinna), helix, lobule, external acoustic meatus
• The Middle ear contains the tympanic membrane and the pharyngotympanic (auditory) tube
• The internal ear (labyrinth) includes the Oval window, Entrance to mastoid, Malleus (hammer), Incus (anvil), Stapes (stirrup) Tympanic membrane, Round window, Semicircular canals, Vestibule, Vestibular nerve, Cochlear nerve, Cochlea, Pharyngotympanic (auditory) tube
• The inner ear consists of rotational and static equilibrium
• Utricle and saccule are located in the vestibule
• Hearing happens from fluid to bend hair cells

Anatomy of the Inner Ear

• Helicotrema, Modiolus, Cochlear nerve, Spiral ganglion, Osseous spiral lamina, Vestibular membrane, Cochlear duct (scala media), Stria vascularis are located inside the inner ear
• Spiral organ, Basilar membrane, Scala tympani are also features of the inner ear
• Inner and Outer hair cells along with supporting cells are also located in the inner ear
• Equilibrium: rotational (angular) acceleration happens at the Semicircular ducts and the Crista ampullaris
• Equilibrium: head position relative to gravity, linear acceleration happens at the Utricle and saccule and Macula
• Hearing happens at cochlea and spiral organ

Physiology of Hearing

• Sound: is a pressure disturbance (alternating areas of high and low pressure) produced by a vibrating object and propagated by molecules
• Sound waves travel more slowly than light
• Two physical properties of sound are: frequency and amplitude

Frequency

• Number of waves that pass a given point in a given time
• Pure tone has crests and troughs that repeat at specific intervals and have Wavelength
• Wavelength is the Distance between two consecutive crests or troughs and is constant for a particular tone
• Shorter wavelength = higher frequency

Amplitude

• Frequency range of human hearing is 20-20,000 hertz (Hz = waves per second), but most sensitive between 1500 and 4000 Hz
• Pitch: perception of different frequencies where higher the frequency, higher the pitch
• Height of sine wave crests are used for assessing sound Amplitude
• Loudness: human interpretation of sound intensities, which can be measured in decibels (dB)
• Normal range of hearing function is 0-120 decibels (dB)
• Normal conversation is around 50 dB and threshold of pain is 120 dB
• Severe hearing loss can occur with prolonged exposure above 90 dB (amplified concert music is often 120 dB or more)
• Frequency causes pitch, while amplitude causes noise

Transmission of Sound to the Internal Ear

• Pathway of Sound: sound passes through air, membranes, bones, and fluid to be mechanically detected
• Sound waves enter the external acoustic meatus and strike the tympanic membrane, to cause the eardrum to vibrate.
• Higher intensity causes greater eardrum vibration
• Auditory Ossicles then transfer the eardrum vibration to Oval Window.

Hair Cells

• Sounds with frequencies below hearing traveling through the helicotrema do not excite hair cells
• Sounds in hearing range go through cochlear duct, vibrating basilar membrane, which deflects hairs on the inner hair cells
• Hair Cell Bending then opens and closes mechanically gated ion channels in hair cells
• Steady stream of action potentials in the vestibular nerve provide the basis of hearing and spatial awareness

Equilibrium

• Response to various head movements: relies on inputs from inner ear, eyes, and stretch receptors of muscles and tendons
• Static equilibrium is the position your head needs to be in at a stationary point in time
• Equilibrium receptors in the semicircular canals and vestibule
• Vestibular receptors respond to static equilibrium: (linear acceleration and head position with respect to gravity)
• Semicircular canals receptors monitor dynamic equilibrium: (changes in head rotation)

The Maculae

• Maculae function as sensory receptor organs
• Maculae are located in each saccule wall and one in each utricle wall
• Maculae monitor the position of the head through the control of posture. They only respond to linear acceleration however.

Anatomy of a Macula

• Hair cells release neurotransmitter continuously
• Head movement causes the amount of neurotransmitter released to the brain to change

The Cristae Ampullares

• Receptors exist for rotational acceleration and are are crista ampullaris (crista)
• Small elevation in ampulla of each semicircular canal
• Cristae are excited by head movement (acceleration and deceleration)
• Major stimuli are rotational (angular) movements, such as twirling when dancing
• Semicircular canals are located in all three planes of space, so cristae can pick up on all rotational movements