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Chapter 17

Anatomy and Physiology HAP101 Chapter Notes - Chapter 17: Special Senses, Olfactory Receptor, Olfactory Bulb

Anatomy and Physiology
Course Code
Anatomy and Physiology HAP101
Tania Killian

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HAP101 Week 12/Chapter 17: The Special Sense
LO 17.1: Describe the structure of the olfactory receptors and other cells involved in olfaction
Olfaction: the sense of smell, which the nose is responsible for. The olfactory receptors are contained within a region called the
olfactory epithelium occupies the superior part of the nasal cavity, covering, the inferior surface of the cribriform plate and
extending along the superior nasal concha
The smell receptors are sensitive to seven types of sensations that can be characterized as camphor, musk, flower, mint, ether,
acrid, or putrid
The olfactory epithelium consists of three kinds of cells
o Olfactory receptor cells: these are first-order neurons of this pathway. Each cell is a bipolar neuron with an exposed
knob-shaped dendrite and an axon projecting through the cribriform plate that ends in the olfactory bulb. Extending from
those dendrites are several olfactory cilia the sites for olfactory transduction (conversion of stimulus energy into a
graded potential in a sensory receptor). Within the plasma membrane of the olfactory cilia are olfactory receptors
these detect inhaled chemicals. Odorous chemicals bind to stimulate the olfactory receptors in the cilia these are called
odorants; responsive by producing a generator potential.
o Supporting cells: these cells line the mucous membrane of the nose. They provide physical support, nourishment and
electrical insulation for the olfactory receptor cells; also help detoxify chemicals in epithelium. Basal cells are stem cells
located between the bases of the supporting cells.
o Olfactory glands: also known as Bowman’s glands, these are within the tissue that support the epithelium. They produce
mucous and this secretion moistens the surface to dissolve odorants for transduction to occur. The VII nerve innervates
the nasal epithelium and olfactory glands. Sometimes, the impulses in these nerves stimulate lacrimal glands in the eyes
and nasal mucous glands to produce tears and a runny nose after inhaling things like pepper.
Physiology of Olfaction
o Human abilily to recognize numerous odors depends on patterns of activity in the brain that arise from activation of
many different combinations of olfactory receptor cells
o Olfactory receptors react to odorant molecules similar to how sensory receptors react to their specific stimuli, i.e.
olfactory transduction:
Odor binding to an olfactory receptor protein in the cilium stimulates a membrane protein called G protein
The G protein activates adenylate cyclase to produce a substance called cyclic adenosine monophosphate
cAMP opens a sodium ion channel that allows sodium to enter the cytosol, causing a depolarizing generator
potential to form in the membrane of the olfactory receptor cell
If the depolarization reaches threshold, action potential generated along the axon of the olfactory receptor cell
Odor Thresholds and Adaption
o Like all special senses, olfaction has a low threshold; only a few molecules of a substance need to be present to be
perceived as an order
o Adaption (decreasing sensitivity) to odors occurs rapidly. Complete sensitivity occurs about a minute after exposure.
Reduced sensitivity involves an adaption process in the central nervous system as well
LO 17.2: Outline the neural pathway for olfaction
The Olfactory Pathway
o Odor/odor binding protein binds to olfactory cilia; triggers olfactory (I) nerve; G protein activates adenylate cyclase;
ATP activates cAMP; sodium channels open, causing depolarization; action potential travels to olfactory bulbs; from the
bulbs, it goes to the olfactory cortex or the hypothalamus & limbic system
LO 17.3: Describe the structure of the gustatory receptors and the neural pathway for gustation
Gustation: sense of taste; only five primary tastes can be distinguished: sour, sweet, bitter, salty, and umami (meaty or savory).
All other flavors are a combination of these five senses, accompanying olfactory and tactile sensation
food odors can pass upward from the mouth into the nasal cavity, where they stimulate olfactory receptors
When suffering from a cold and you are unable to taste your food, the olfaction is blocked, not taste
Anatomy of Taste Buds and Papillae
o Receptors for sensations of taste are located in the taste buds, most of which are located on the tongue, but also found on
the soft palate, pharynx, and epiglottis
o Taste bud: an oval body consisting of three kinds of epithelial tissue
Supporting cells: contain microvilli and surround around 50 gustatory receptor cells
Gustatory receptor cells: gustatory microvilli project from each cell to the external surface through the taste
pore, an opening in the taste bud
Basal cells: are found at the periphery of taste bud near the connective tissue layer and produce supporting cells
that develop into gustatory receptor cells.
Taste buds are found on the papillae, which are elevations on the tongue, and increase surface area and provide a
rough texture to the upper surface of tongue. There are three types that contain taste buds
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Vallate papillae (circumvallate papillae): form an inverted V-shaped row at the back of the tongue. Each
of these house 100-300 taste buds
Fungiform papillae: mushroom-shaped elevations scattered over the entire surface of tongue; contain five
taste buds each
Foliate papillae: located in small trenches on lateral margins of tongue, but most buds degenerate in early
o Filiform papillae: structures located on the entire surface of the tongue; contain no taste buds but have tactile receptors;
increase friction between tongue and food to make it easier for the tongue to move food in the oral cavity
Physiology of Gustation
o Tastsants: chemicals that stimulate gustatory receptor cells.
Once dissolved, it will contact the plasma membrane of the gustatory microvilli (sites of taste transduction)
A receptor potential will result that stimulates exocytosis of synaptic vesicles from the gustatory receptor cell
Liberated NT trigger nerve impulses in the first-order sensory neurons that synapse with gustatory receptor cells
Salty and sour receptors are chemically gated
Sweet, bitter umami: G protein to second-messenger
Taste Thresholds and Adaption
o Threshold varies for each of the primary tastes: low thresholds are bitter and sour substances; high thresholds are sweet
and salty substances
The Gustatory Pathway
o The three cranial nerves involved are facial (VII) nerve, glossopharyngeal (IX) nerve, and the vagus (X) nerve.
o From the taste buds, a nerve impulse will travel along these nerves to the gustatory nucleus in the medulla oblongata
o From the medulla, axons carrying taste signals project to the limbic system and the hypothalamus; others go to
thalamus (from thalamus they go to the primary gustatory area)
LO 17.4: Identify the components of the eyeball and the accessory structures of the eye
Vision: these sense of seeing
Electromagnetic Radiation: the energy in the form of waves that radiates from the sun; include gamma rays, x-rays, UV rays,
visible lights, infrared radiation, microwaves, and radio waves (electromagnetic spectrum)
o The eye is responsible for the detection of visible light light the exhibits color, which depends on its wavelength
Accessory Structures of the Eye
o Eyelids (palpebrae): the upper and lower shade the eyes during sleep, protect the eyes from excessive light and foreign
objects and spread lubricating secretions over the eyeballs.
o Eyelashes and Eyebrows: the eyelashes project from the border of each lid, and the eyebrows, which arch above the
upper eyelid, help protect eyeballs from foreign objects, perspiration, and direct sun rays. Oil glands at follicle of eyelash
release a lubricating fluid into follicles, however, these can be infected by bacteria and cause a sty
o Lacrimal Apparatus: a group of structures that produces and drains lacrimal fluid (tears) through lacrimation. The
lacrimal glands secrete lacrimal fluid which drains into lacrimal ducts that empty tears onto surface of conjunctiva of
the upper lid (rd. more on pg. 581). Lacrimation is a protective mechanism, as tears dilute and wash away the irritating
o Extrinsic Eye Muscles: the orbits help protect eyes, stabilize and anchor them to the muscles that produce their essential
movements. These muscles extend from the walls of the orbit to the sclera and are surround by periorbital fat in the
orbit. These muscles are capable of moving the eye in any direction; there are 6 muscles: superior rectus, inferior
rectus, lateral rectus, medial rectus, superior oblique, and inferior oblique. They are supplied by the oculomotor
(III), trochlear (IV), or abducens (VI) nerves
Anatomy of the Eyeball
o The wall of the eyeball consists of three layers:
Fibrous tunic: the superficial layer of the eye, consisting of the anterior cornea and posterior sclera. The cornea
helps focus light onto the retina. The sclera (white of eye) is made from dense connective tissue, collagen fibers
and fibroblasts; covers most of eye and gives shape to eyeball, making it more rigid and protecting its inner parts.
Vascular tunic: the middle layer of the eyeball; composed of the choroid, ciliary body and iris. The choroid lines
most of the internal surface of the sclera and provides nutrients to the posterior surface of the retina; also contains
melanin which causes layer to be dark brown in color. The ciliary body contains muscles which alter the shape of
the lends, adapting it for near or far vision. The iris is the colored portion of the eye that contains melanin; helps
regulate the amount of light entering the eye through the pupil (black because it is heavily pigmented); bright
light causes pupil constriction (parasympathetic), and dim light causes pupil dilation (sympathetic)
Retina: lines the posterior three-quarters of the inner layer of the eyeball. It may be viewed using an
ophthalmoscope; optic (II) nerve is also visible; point at which the optic nerve exits the eye is the optic disc (blind
spot); exact center of the retina is the macula lutea (center is the fovea centralis - area of highest visual acuity). It
also contains rods (allow to see dim light) and cones (produce color vision). From these sensors, information
flows through the outer synaptic layer to bipolar cells through the inner synaptic layer to ganglion cells.
Axons of these exit as the optic (II) nerve.
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