Taste and Olfactory Systems Quiz

Olfaction: Sense of Smell

• Smell and taste are chemical senses
• The human nose has 10 million to 100 million receptors for smell (olfaction) in the olfactory epithelium of the superior nasal cavity
• The olfactory epithelium covers the inferior surface of the cribriform plate (of the ethmoid bone) and extends along the superior nasal concha
• There are three types of cells: olfactory receptor cells, supporting cells, and basal cells

Olfactory Pathway

• Receptors in the nasal mucosa send impulses along branches of olfactory (I) nerve
• Impulses travel through the cribriform plate to the olfactory bulb
• Synapse with the olfactory bulb
• Impulses travel along the olfactory tract
• Interpretation occurs in the primary olfactory area in the cerebral cortex (temporal lobe)

Olfactory Transduction

• Olfactory transduction is the binding of an odorant molecule to an olfactory receptor protein
• Chemical reactions involving cyclic AMP (cAMP) cause depolarization
• Action potential travels to the primary olfactory area
• Impulse travels to the frontal lobe (orbitofrontal area) for odor identification

Gustation: Sense of Taste

• Taste is a chemical sense, simpler than olfaction
• Five primary tastes: sour, sweet, bitter, salt, and umami (meaty, savory)
• Flavors other than umami are combinations of the four primary tastes
• Taste buds contain receptors for taste sensation
• Approximately 10,000 taste buds are found on the tongue of a young adult and on the soft palate, pharynx and epiglottis
• Taste buds contain three kinds of epithelial cells: supporting cells, gustatory receptor cells, and basal stem cells
• Taste buds are located in elevations on the tongue called papillae.
• Three types of papillae that contain taste buds: vallate papillae (about 12 that contain 100-300 taste buds), fungiform papillae (scattered over the tongue with about 5 taste buds each), and foliate papillae (located in lateral trenches of the tongue—most of their taste buds degenerate in early childhood)
• Filiform papillae cover the entire surface of the tongue and contain tactile receptors but no taste buds

Cranial Nerves Involved in Taste

• Three cranial nerves are involved in the sense of taste:
  • Facial (VII) nerve carries taste information from the anterior 2/3 of the tongue.
  • Glossopharyngeal (IX) nerve carries taste information from the posterior 1/3 of the tongue.
  • Vagus (X) nerve carries taste information from taste buds on the epiglottis and in the throat

Gustatory Pathway

• Taste information is carried to the brain by three cranial nerves (VII, IX, X).
• Information goes to the thalamus then to the primary gustatory area in the cerebral cortex.

Vision

• Vision uses visible light from the electromagnetic spectrum with wavelengths from about 400 to 700 nm

• Wavelength is the distance between two consecutive peaks of an electromagnetic wave

• Accessory structures of the eyes include the eyelids, eyelashes, eyebrows, lacrimal (tear-producing) apparatus, and extrinsic eye muscles

• Palpebral muscles control eyelid movement and extrinsic eye muscles move the eyeball in different directions.

• The conjunctiva is a thin mucous membrane lining the eyelids and covering the sclera.

• The tarsal plate is a connective tissue fold that gives form to the eyelids and contains sebaceous glands (tarsal glands/Meibomian glands) to prevent eyelids from sticking together.

• The lacrimal apparatus produces and drains tears. The pathway includes: lacrimal glands, lacrimal ducts, lacrimal puncta, lacrimal canaliculi, lacrimal sac, and nasolacrimal ducts.

• Six extrinsic eye muscles: superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique and inferior oblique

• The eyeball has two tunics (coats): the fibrous tunic (cornea and sclera) and the vascular tunic (choroid, ciliary body and iris)

Light Refraction in the Eye

• Light refracts when it passes through a transparent substance with one density into a second transparent substance with a different density, which occurs at the junction of the substances
• The cornea and lens focus the image on the retina

Accommodation of the Eye

• The lens must accommodate to properly focus the object
• The image is projected onto the central fovea, the site where vision is the sharpest

Myopia (Nearsightedness)

• The eyeball is longer than it should be and the image converges in front of the retina, causing blurry distant objects.
• A concave lens is used to correct this

Hyperopia (Farsightedness)

• The eyeball is shorter than it should be and the image converges behind the retina, causing blurry close objects.
• A convex lens is used to correct this

Astigmatism

• Astigmatism is a condition where the cornea or lens (or both) has an irregular curve, causing blurred or distorted vision

Rods and Cones

• The retina contains photoreceptors known as rods and cones
• Rods are used to see in dim light
• Cones produce color vision (red, green, blue)
• Photopigments respond to light in a cyclical process

Light and Dark Adaptation

• Light adaptation: occurs when moving from dark surroundings to light. This happens in seconds.
• Dark adaptation takes place when moving from a lighted area to a dark one, and it takes minutes

Neural Pathway for Vision

• The neural pathway for vision begins when rods and cones convert light energy into neural signals directed to optic (II) nerves
• The pathway includes the optic chiasm, optic tract, lateral geniculate nucleus of the thalamus, optic radiations, and the primary visual areas of the occipital lobes for perception.

Visual Fields and Binocular Vision

• The anterior location of our eyes leads to visual field overlap, which gives us binocular vision
• Visual information from the right half of each visual field travels to the left side of the brain
• Visual information from the left half of each visual field travels to the right side of the brain

Hearing and Equilibrium

• The ear is divided into three regions: external, middle and internal ear.

• External ear: includes the auricle (pinna), external auditory canal, and tympanic membrane (eardrum)

• Ceruminous glands secrete cerumen (earwax) to protect auditory canal.

• Middle ear: contains the auditory ossicles (malleus, incus, stapes)

• Auditory tube equalizes air pressure

• Internal ear: contains cochlea which translates vibrations into neural impulses and semicircular canals for balance

• Pressure waves from the scala vestibuli move to the vestibular membrane, endolymph of cochlear duct, causing the basilar membrane to vibrate.

• Hair cells in the spiral organ (organ of Corti) against the tectorial membrane generate nerve impulses.

• Cochlear nerve fibers form the cochlear branch of the vestibulocochlear (VIII) nerve. The axons synapse with neurons in the cochlear nuclei which send impulses.

• Impulse travel to the medial geniculate nucleus of the thalamus and ends in the primary auditory area of the cerebral cortex in the temporal lobe

• Equilibrium (balance) exists in two forms: static and dynamic equilibrium

• • Vestibular apparatus that maintains equilibrium and includes saccule, utricle (otolithic organs), and semicircular canals

• Otolithic membrane sits on top of the macula. Movement of the head causes gravity to move this membrane over the hair cells generating impulses in the vestibular branch of the vestibulocochlear (VIII) nerve

• Three semicircular canals are responsible for dynamic equilibrium. Ampulla in each canal contains the crista with hair cells. Movement of the head affects endolymph, generating a nerve impulse down the vestibular branch of vestibulocochlear (VIII) nerve

Development of the Eyes and Ears

• Eyes begin develop about 22 days after fertilization.

• Ectoderm of the forebrain forms the optic grooves, which become optic vesicles

• Optic vesicle reaches the surface ectoderm, which thickens to form the lens placodes

• Distal portion of the optic vesicles form the optic cups, remaining attached to the prosencephalon by optic stalks

• Internal ears develop first (about 22 days after fertilization), surface ectoderm thickens to form otic placodes which invaginate to form otic pits then otic vesicles.

Aging and the Special Senses

• Smell and taste are less affected by aging until around age 50 with gradual loss of receptors.
• The lens loses elasticity around age 40, difficulty focusing on close objects.
• Muscles of iris weaken leading difficulty adjusting to changes in light conditions.
• Retinal diseases, macular disease, detached retina and glaucoma, occur frequently in elderly.
• Age related hearing loss (presbycusis) in 25% individuals by age 60.
• Tinnitus, ringing in the ears, and vestibular imbalance occur more frequently among the elderly.