O na hoku no na kiu o ka lani.

The stars are the spies of heaven.

The stars look down on everyone and everything.

‘Ōlelo No‘eau, compiled by Mary Kawena Pukui, #2513

Introduction

Fig. 15.1: Hōkūleʻa

Imagine that you are at the Hawaiian Lūʻau where you are smelling the plumeria, tasting the food, listening to the waves, and watching the sunset (figure 15.2). You are using your special senses to enjoy the moments with your family and friends. Unlike general senses, special senses are localized to specific organs and tissues in your body. You can feel general senses such as the temperature of the ocean water in multiple locations throughout your body, but special senses are much more specific in where they are located. For example, your eyes detect light for your sense of vision, and a specialized layer of cells in your nasal cavity detects chemicals and transmits your sense of smell. No other organs or parts in your body can detect those signals and cause the same sensations, which is why olfaction, gustation, vision, vestibular, and auditory senses are special senses.

Fig. 15.2: ʻOno Hawaiian food

15.1 Olfaction and Gustation

Our special senses of smell and taste work together to help us in our daily lives. They are chemical senses in which molecules work as stimuli to activate our sensations and perceptions.

Olfaction

The sense of smell, or olfaction, is caused by the detection of chemicals inhaled through a small region of your nasal cavity. This region is the olfactory epithelium and it contains the olfactory sensory neurons. Olfactory sensory neurons are specialized neurons that detect odorants in their dendrites (figure 15.3). You may see olfactory sensory neurons being named as olfactory receptor cells in other textbooks. Odorants are chemicals that cause sensations of smells and odors when they bind to olfactory receptor proteins located on the dendrites of olfactory sensory neurons. Olfactory sensory neurons are embedded throughout the olfactory epithelium. The olfactory epithelium also contains regenerative basal cells that replace damaged or dead olfactory sensory neurons and supporting cells that maintain the overall structure of the tissue.

Figure 15.3 The Olfactory System (a) The olfactory system begins in the peripheral structures of the nasal cavity. (b) The olfactory receptor neurons are within the olfactory epithelium. (c) Axons of the olfactory receptor neurons project through the cribriform plate of the ethmoid bone and synapse with the neurons of the olfactory bulb (tissue source: simian). LM × 812. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) (OpenStax) Note that the olfactory receptor neurons in the figure are the same as the olfactory sensory neurons.

Olfactory sensory neurons experience constant turnover, in that damaged or dead cells are replaced by new olfactory neurons. Basal cells are the stem cells of these neurons, and basal cells divide and differentiate to replace olfactory sensory neurons. However, the rate at which basal cells replace olfactory neurons decreases with age, which is why the sense of olfaction gradually becomes less sensitive as we age.

The axons of the olfactory sensory neurons form the right and left olfactory (I) nerve, the first cranial nerve. The axons extend from the basal surface of the epithelium, through olfactory foramina in the cribriform plate of the ethmoid bone, and into the brain. The olfactory tract is a group of axons that connect to the olfactory bulb on the ventral surface of the frontal lobe. From there, the axons split to travel to several brain regions. Some travel to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe of the cerebral cortex. Others project to structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Olfaction is unique among the special senses in that it is the only sense with no thalamic filtering as it does not synapse with the thalamus. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion.

Remember the olfactory pathway next time you smell a lei with tuberose and pikake!

Olfactory transduction is a series of steps that happens inside the olfactory sensory neuron when it is stimulated by the scent of your morning coffee. If enough coffee molecules bind to the olfactory receptor proteins, they activate a type of membrane protein called a G protein. The G protein activates the enzyme adenylyl cyclase to produce cyclic adenosine triphosphate (cAMP), a type of the second messenger. The cAMP opens up the cation channels that allow Na+ and Ca2+ to rush into the cell and cause depolarization. This results in a generator potential that can trigger action potentials in the olfactory neuron that can be relayed along olfactory nerve fibers and tracts.

Olfactory discrimination is the ability to distinguish between different odorants. The number of unique smells and odorants that can be detected by the human nose is still being researched, but scientists have identified multiple genes that code for different olfactory receptor proteins which may be involved in this process.

Once generator and action potentials occur in an olfactory neuron, this triggers negative feedback mechanisms inside the cell that eventually block the ion channels that contribute to the olfactory neuron’s action potentials. An example of this is when you may, at first, notice a strong odor such as perfume or cologne, but you become less aware of it as time passes, even though the odorant is still there.

Gustation

As you are sampling the ʻono food at the Hawaiian lūʻau, your sense of taste will help you tell the different tastes. Have you ever wondered how your body does this? The sense of taste, or gustation, involves the detection of chemicals in the oral cavity.

Fig 15.4 Structures Associated with Taste. The tongue is covered with papillae (a), which contain taste buds (b and c). Within the taste buds are specialized taste cells (d) that respond to chemical stimuli dissolved in the saliva and, in turn, activate sensory nerve fibers in the facial and glossopharyngeal nerves. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/). (https://courses.lumenlearning.com/austincc-ap1/chapter/special-senses-taste-gustation/)

The tongue contains bumpy structures called papillae, which have smaller structures called taste buds (figure 15.4). Taste buds have small openings facing toward the inside of the oral cavity, which exposes specialized cells called gustatory receptor cells, to chemicals in the oral cavity. These gustatory receptor cells have taste hairs (gustatory hairs), which are small microvilli that project into the oral cavity and increase the surface area of the cell to detect more chemicals. Be mindful that the taste hairs are very different from the hair on the skin surface.

Next to the gustatory receptor cells in the taste buds, basal cells are stem cells that help to regenerate and replace lost or damaged gustatory cells. The gustatory receptor cells have a lifespan of 10 days and that’s the reason you will regain your sense of taste after being burned by a hot cup of coffee.

Unlike olfaction, gustation can be broken down into five known primary tastes: sour, salty, sweet, bitter, and umami (savory). Scientists are researching other possible primary tastes and have proposed potential new tastes such as a sixth “fatty” taste. The classification of each of these tastes is determined by the presence of receptor proteins in the membranes of gustatory receptor cells. Each tastant, chemicals that stimulate gustatory receptor cells, stimulate specific gustatory receptor cells. So each gustatory receptor cell is responsible for sensing one primary taste. In each taste bud, it contains at least five types of gustatory receptor cells to allow us to detect all primary tastes in all parts of the tongue.

The five known primary tastes can be grouped into two broad mechanisms of how they are detected by gustatory receptor cells (figure 15.5). The tastes of salty and sour are detected by gustatory receptor cells that have ion channels that leak cations into cells. Salty tastes are due to the presence of sodium ions (Na+). Table salt (NaCl) tastes salty due to the dissociation of NaCl into Na+ and Cl- ions in solution. The Na+ ions enter into gustatory cells through the Na+ channels. Sour tastes are triggered by the presence of acidic hydrogen ions (H+) which enter into gustatory cells through the H+ channels. The accumulations of Na+ or H+ inside the cells cause depolarization which leads to a salty or sour sensation, respectively.

Fig. 15.5: mechanism of ion channel and GPCR-based tastes

Sweet, bitter, and umami tastes utilize a different mechanism where the tastant molecules do not enter the cells. Instead, the tastants bind to specific protein receptors located on the membranes of gustatory receptor cells. Each of these receptors has a subset of distinct G proteins that carry out the signal transduction pathways in the cytosol. When a sweet, bitter, or umami molecule binds to the appropriate protein receptor, it causes signal transduction and intracellular changes that depolarize the gustatory receptor cell.

Sweet taste is primarily caused by the presence of glucose dissolved in the saliva. Other monosaccharides such as fructose, or artificial sweeteners such as aspartame (NutraSweet™), saccharine, or sucralose (Splenda™) activate the same sweet protein receptors that detect glucose. The affinity for each of these molecules varies, and some will taste sweeter than glucose because they bind to the protein receptors differently. Remember that next time you eat your favorite flavor of shave ice!

Bitter tastes that we sense from eating unsweetened cocoa or citrus peel are caused by the binding of bitter compounds to other sets of protein receptors on the gustatory receptor cells. There is a vast amount of chemicals with diverse chemical structures that can bind to these protein receptors to cause bitter sensations. Some bitter molecules depolarize gustatory cells, whereas others hyperpolarize gustatory cells. The specific response depends on which molecule is binding to the receptor. One major group of bitter-tasting molecules are alkaloids. Alkaloids are nitrogen-containing molecules that are commonly found in bitter-tasting plant products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin. By containing toxic alkaloids, the plant is less susceptible to microbial infection and less attractive to herbivores. Therefore, the function of bitter taste may primarily be related to stimulating the gag reflex to avoid ingesting poisons. Because of this, many bitter foods that are normally ingested are often combined with a sweet component to make them more palatable (for example, cream and sugar in coffee). The highest concentration of bitter protein receptors appear to be in the posterior tongue, where a gag reflex could still spit out poisonous food.

Umami, sometimes referred to as “savory”, is also caused by the binding of chemicals to protein receptors of the gustatory receptor cells. Unlike sweet and bitter sensations, umami receptors are activated by a very specific molecule, the amino acid L-glutamate. Therefore, protein-rich foods cause a strong umami flavor. Accordingly, dishes that contain meat are often described as savory, along with other foods rich in L-glutamate such as mushrooms and tomatoes.

Once gustatory receptor cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of gustatory sensory neurons. Facial (VII) and glossopharyngeal (IX) cranial nerves carry taste sensations to the brain, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two-thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which is more sensitive to noxious stimuli such as bitterness.

The salivary glands constantly produce saliva, and taste sensations gradually decrease over time due to dilution and washing away of taste compounds away from gustatory cells, as well as adaptation of the neurons that receive stimulation from gustatory cells.

15.2 Vision

The sense of sight, or vision, is the special sense based on the receiving and perception of light through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye.

Fig. 15.7 The Eye in the Orbit The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull. (OpenStax)

In addition to the complex of bones that form the orbit around the eye, several accessory structures of the eye helps to protect the eye against the external environment. These include the eyelids, eyebrows, and lacrimal apparatus. The eyelids, with eyelashes at their leading edges, help to protect the eye from abrasions by blocking particles such as dust or sand, which may land on the surface of the eye and damage it. The inner surface of each lid has a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (sclera), connecting the eyelids to the eyeball. Conjunctivitis, also known as pink eye, is the inflammation of the conjunctiva and the inner surface of the eyelids from a viral or bacterial infection.

Tears are produced by the lacrimal gland, located on the superior and lateral edge of the orbital complex. Tears produced by this gland flow through the lacrimal duct and into the lateral corner or lateral canthus of each eye. Tears then flow over the conjunctival surface of the eye, and eventually to the medial corner of each eye. This process washes away any foreign particles from the surface of the eye. Excess tears flow into a small opening called the lacrimal punctum (plural: puncta) located in the lacrimal caruncle, a soft tissue located in the medial canthus of each eye. Each lacrimal punctum leads to a small channel called a lacrimal canaliculus (plural: canaliculi) or lacrimal duct which drains into the lacrimal sac, then the nasolacrimal duct, and eventually into the nasal cavity.

Figure 15.8 Frontal view of the lacrimal apparatus showing the flow of tears from the lacrimal glands to the nasolacrimal duct and nasal cavity. Note that the lacrimal canaliculi are shown as lacrimal ducts in the figure. (By CNX OpenStax — https://cnx.org/contents/5CvTdmJL@4.4, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=53728100)

The eye is also surrounded by extrinsic muscles that move the eye and direct your focus of vision. Four of the muscles, the superior rectus, inferior rectus, medial rectus, and lateral rectus, are arranged at the top, bottom, and both sides of the eye. When each of these muscles contracts, the eye moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up. You can find the figures of these muscles in the Skeletal Muscle chapter.

There are also two oblique extrinsic muscles of the eye. The superior oblique originates at the posterior of the orbital socket, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a sling-like piece of cartilage known as the trochlea. The tendon inserts at an angle into the superior surface of the eye. The oblique angle of the tendon and the pulley-like action at the trochlea causes contraction of the superior oblique to rotate the eye laterally. The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane. When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a 20-degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by the contraction of the inferior oblique.

The levator palpebrae superioris is located in the orbit, but it does not attach to the eye. When this muscle contracts, it is responsible for elevating and retracting the upper eyelid to help you open your eyes. This movement usually occurs in concert with the elevation of the eye by the superior rectus.

The eye is not a completely solid organ but is arranged like a sphere consisting of layers of tissue filled with fluid. The outermost layer of the eye is the fibrous tunic, which includes the white sclera and clear cornea. The sclera accounts for five-sixths of the surface of the eye, most of which is not visible, but you can see the sclera or “white of the eye” surrounding your iris if you look in a mirror. The cornea is a transparent section of the fibrous tunic that covers the anterior tip of the eye and allows light to enter the eye. The corneal limbus is the border that transitions between the white, opaque sclera and the clear cornea of the eye.

Fig 15.9 Structure of the Eye The sphere of the eye can be divided into anterior and posterior chambers. The wall of the eye is composed of three layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two synaptic layers in between. The center of the retina has a small indentation known as the fovea. (OpenStax)

Fig 15.10 Diagram of a human eye (horizontal section section of the right eye)
1. Lens, 2. Zonule of Zinn or Ciliary zonule, 3. Posterior chamber and 4. Anterior chamber with 5. Aqueous humour flow; 6. Pupil, 7. Corneosclera or Fibrous tunic with 8. Cornea, 9. Trabecular meshwork and Schlemm’s canal. 10. Corneal limbus and 11. Sclera; 12. Conjunctiva, 13. Uvea with 14. Iris, 15. Ciliary body (with a: pars plicata and b: pars plana) and 16. Choroid); 17. Ora serrata, 18. Vitreous humor with 19. Hyaloid canal/(old artery), 20. Retina with 21. Macula or macula lutea, 22. Fovea and 23 Optic disc → blind spot. 24. Visual axis (line of sight). 25. Optical axis. 26. Optic nerve with 27. Dural sheath, 28. Tenon’s capsule or bulbar sheath, 29. Tendon. 30. Anterior segment, 31. Posterior segment. 32. Ophthalmic artery, 33. Artery and central retinal vein → 36. Blood vessels of the retina; Ciliary arteries (34. Short posterior ones, 35. Long posterior ones and 37. Anterior ones). 38. Lacrimal artery, 39. Ophthalmic vein, 40. Vorticose vein. 41. Ethmoid bone, 42. Medial rectus muscle, 43. Lateral rectus muscle, 44. Sphenoid bone.

The vascular tunic is the middle layer of the eye and is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that supplies blood to the eyeball. The choroid encompasses the posterior of the eyeball and extends anteriorly to the ciliary body, a circular muscular structure that is continuous with the vascular tunic and choroid. The ciliary body is attached to the lens of the eye by a network of suspensory ligaments or zonule fibers. These suspensory ligaments circle and surround the lens, thus holding it in place and shaping it. The ciliary body and suspensory ligaments can bend and reshape the lens, which allows for adjustment and focusing light on the back of the eye.

The iris is the visible colored part of the eye that gives you your eye color. The iris is continuous with the vascular tunic and is located anterior to the ciliary body and lens. The iris is a network of smooth muscle with a hole at its center called the pupil, which allows light to enter into the eye and reach the posterior of the eye. The iris can contract or relax to open or close the pupil, which adjusts the amount of light entering the eye. This is why the pupil constricts in response to bright light, and why the pupil grows wider and dilates in response to dim light.

The neural tunic, or retina, is the innermost layer of the eye containing the nervous tissue responsible for photoreception. The retina is composed of several layers of specialized cells called photoreceptors (rods and cones) that detect and process light to eventually convert it into visual stimuli. Light carries energy that stimulates photoreceptors, which changes their membrane potential and allows them to transmit action potentials to other cells in the retina. Histologically, the retina consists of 3 layers of nuclei. Did you know that one possible consequence of diabetes, particularly, diabetes type 2, is blindness? This condition is known as diabetic retinopathy and it affects the blood vessels in the retina. A comprehensive eye exam must be done at least once a year with people with diabetes.

Fig 15.11:  Figure retina histology

The outermost layer consists of the photoreceptor cells called rods and cones. Next is a layer consisting of bipolar cells (first-order sensory neurons), horizontal cells, and amacrine cells. Finally, the innermost layer consists of ganglion cells. The axons of all of the ganglion cells travel towards the optic disc where they converge to form the optic nerve.

The interior of the eye contains two major cavities, the anterior cavity and the posterior cavity, which are filled with special fluids. The lens forms the border between the anterior and posterior cavities. The anterior cavity is located between the cornea and the lens and is further divided into an anterior chamber located between the cornea and the iris, and a posterior chamber located between the iris and the lens. The anterior cavity has a clear, watery fluid called aqueous humor that helps maintain the shape of the eye. The aqueous humor also provides a small amount of transport and circulation within the anterior cavity of the eye. The pressure of the aqueous humor in the eye must be controlled, as high pressures in the eye (intraocular pressure) can lead to glaucoma, a group of conditions that can harm the optic nerve and cause a loss in vision.

The posterior cavity consists of the entire inner portion of the eye posterior to the lens and is filled with a thick, jelly-like substance called vitreous humor (vitreous body). Vitreous humor contains more proteins and polysaccharides than aqueous humor, which makes it relatively thicker than aqueous humor. Vitreous humor helps to maintain the overall shape of the eye and prevent it from being distorted or crushed by the extrinsic muscles that surround and move the eye in the orbit. The vitreous humor also keeps the retinal layer of the eye pressed against the walls and other layers of the eye, thus keeping it smooth and holding its shape. Changes in the vitreous humor of the eye can result in the retina falling away from the walls of the eye, leading to a condition called retinal detachment, which can lead to loss of vision in detached areas.

When light enters the eye, it first passes through the cornea. The rounded shape and the tissues of the cornea allow it to bend light, and this helps to focus incoming light on the retina in the posterior of the eye. This bending and change of direction of light are called refraction. It occurs when light passes from one substance to another. In the case of the cornea, light is refracted as it passes from the air and into the cornea. If the cornea is not perfectly rounded, this can distort incoming light and result in a condition called astigmatism, which can cause minor distortions or blurriness in vision.

Fig 15.12: Refraction of light, visual explanation of convex distortion of light: Schematic representation of accommodation, ciliary muscle contracts, the tension on the lens capsule decreases the lens thickens and the pupillary aperture gets smaller.

The aqueous humor in the anterior cavity of the eye causes further refraction of the light before light reaches the lens. The lens is a clear and dense tissue surrounded by a fibrous capsule and contains layers of cells called lens fibers, which are arranged circularly. The lens fibers secrete transparent proteins called crystallins, which are responsible for the clarity of the lens, and the ability of the lens to refract and focus light. As we age, the tissue of the lens becomes discolored and its clarity becomes cloudier — this condition is called cataracts, which can cause blurry and discolored vision.

Fig 15.13. Dense white mature cataract of a 60-year-old male (By Imrankabirhossain — Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=59408737)

The ciliary body and suspensory ligaments attach to the capsule surrounding the lens, and contraction of the ciliary body can change the shape of the lens and alter its focus, which is a process called accommodation. The circular arrangement of the ciliary body causes it to shrink in diameter when it contracts. When the ciliary body contracts, this reduces the tension in the suspensory ligaments and allows the lens to become less stretched and more rounded, which allows for sharper focusing of light on the retina. On the other hand, when the ciliary body relaxes, it increases in diameter, which stretches and tenses the suspensory ligaments, thus pulling and flattening the lens. As we age, the lens becomes stiffer and less responsive to changes in shape caused by the ciliary body. This impaired ability to accommodate and focus the lens is called presbyopia, which is why older individuals often need reading glasses to help make up for the loss in focusing ability.

After light passes through the lens, it passes through the vitreous humor and projects an image onto the retina at the inner posterior surface of the eye. The refraction of light through the cornea, aqueous humor, lens, and vitreous humor results in an upside-down inverted image of what is being viewed. The spherical shape of the eye also affects the focus of the image projected onto the retina. Ideally, the image of what is being viewed should be focused onto the retina for clear vision. Emmetropia is a state that describes normal, clear vision with no defects in focus or refraction. However, differences in the anterior to posterior length of the eye and overall shape can result in the focus being offset from the retina. If an eye is too long from anterior to posterior, the focus of an image will fall short of the retina and end up spreading and blurring when it eventually reaches the retina. This condition is called myopia, commonly known as “near-sightedness”. People who have hyperopia (far-sightedness) have an anterior-posterior eye length that is too short, causing images that are near the eye to not be focused enough by the time it reaches the retina. Myopia, hyperopia, and astigmatism can be corrected with visual aids such as glasses or contact lenses, which add additional layers that refract light to restore focus and clarity to images projected on the retina.

Fig 15.14: Figure Image reversal

Fig. 15.15: Myopia and hyperopia: (a) The nearsighted (myopic) eye converges rays from a distant object in front of the retina, so they have diverged when they strike the retina, producing a blurry image. An eye lens that is too powerful can cause nearsightedness, or the eye may be too long. (b) The farsighted (hyperopic) eye is unable to converge the rays from a close object on the retina, producing blurry near-field vision. An eye lens with insufficient optical power or an eye that is too short can cause farsightedness.

Physiology of Vision

After light is refracted by the cornea and lens, it must travel through the ganglion cells and the bipolar cells before the image reaches the receptors. Sensory transduction of light (phototransduction) occurs in the rods and cones of the retina. Rods and cones are different from other types of receptor cells in that they are depolarized in the absence of a light stimulus. This is called the dark current.

Fig 15.16: Dark Current

Fig 15.17: Photoactivation: Representation of molecular steps in photoactivation (modified from Leskov et al., 2000). Depicted is an outer membrane disk in a rod. Step 1: Incident photon (hv) is absorbed and activates a rhodopsin by conformational change in the disk membrane to R*. Step 2: Next, R* makes repeated contacts with transducin molecules, catalyzing its activation to G* by the release of bound GDP in exchange for cytoplasmic GTP. The α and γ subunits Step 3: G* binds inhibitory γ subunits of the phosphodiesterase (PDE) activating its α and β subunits. Step 4: Activated PDE hydrolyzes cGMP. Step 5: Guanylyl cyclase (GC) synthesizes cGMP, the second messenger in the phototransduction cascade. Reduced levels of cytosolic cGMP cause cyclic nucleotide gated channels to close preventing further influx of Na+ and Ca2+.

In the absence of light, sodium channels remain open, leaving the cells in a state of depolarization with a membrane potential of around -30mV. This causes the receptor cells to release glutamate onto the bipolar cells. Although glutamate is usually an excitatory neurotransmitter, in this case, it is inhibitory and prevents the bipolar cells from stimulating the ganglion cells.

In the presence of light, photoreceptors are stimulated. This results in the closure of the sodium channels which causes the repolarization of the membrane and inhibition of glutamate release. In the absence of inhibition of the bipolar cells, they become stimulated and release neurotransmitters onto the ganglion cells and this triggers action potentials that travel down the optic nerve.

The photoreceptors of the rods and cones are composed of a vitamin A derivative called retinal and an opsin protein. Rods use rhodopsin protein, which is specific for detecting purple. Cones, on the other hand, use photopsin which, depending on the specific amino acids, detect red, blue, or green.

Before interacting with a photon, the retinal’s flexible double-bonded carbons are in the cis conformation. This molecule is referred to as 11-cis-retinal. A photon interacting with the molecule causes the flexible double-bonded carbons to change to the trans-conformation, forming 11-trans-retinal, which has a linear hydrocarbon chain.

Fig 15.18: Retinal/opsin and the photobleaching cycle

This process is called photobleaching because conversion to the trans-retinal conformation causes the retinal molecule to detach from the opsin pigment leaving a colorless product. When photobleaching occurs, it triggers a reaction cascade causing the breakdown of cyclic guanosine monophosphate (cGMP). It is cGMP that keeps the ligand-gated sodium channels in the open position so photobleaching results in the cessation of the dark current.

After photobleaching occurs, the system must be reset so that light can be detected again. This is the job of the enzyme retinal isomerase, which converts the trans-retinal back into cis-retinal which can then associate with its opsin protein again. cGMP is then reformed causing the sodium channel to open, thus restoring the dark current. The system is now ready to detect light again.

Because rods are much more sensitive to light than cones, they are continuously photobleached during the day, leaving the rods inoperative. For this reason, we have developed a dual system of light detection. One for the day, the photopic system, and another for the night, the scotopic system. Although the cones are much less sensitive to light, they unbleach relatively quickly and thus can be used over and over in high light intensity during the day. However, due to that low sensitivity, they are relatively useless at night. The rods, on the other hand, are much more sensitive and unbleach slowly. Therefore they are ideally adapted for low light conditions at night. This dual system is well suited for both low light intensity at night and high light intensity during the day. We experience the high sensitivity of the rods when we wake up in the morning and open our eyes, causing the simultaneous photobleaching of every rod receptor in our retina. This can be somewhat uncomfortable. On the other hand, when we move from a well-lit room into darkness it takes some time for the rods to unbleach. It takes 30 to 40 minutes to reach full light sensitivity in darkness.

The rods and cones in our retinas are separated physically. Cones are packed densely into a small region called the fovea centralis which is in the center of our visual axis. The fovea centralis is localized within the macula lutea region. When we look at something, we are aiming our region of high cone density, or foveating, on that object. Although we think that our eyes are detecting a visual field incredibly rich in detail, in actuality, we detect very little of what we think we see. In absence of lines or contrast, our visual cortex can fill in the spaces without those features. In the next section, you will learn about the visual pathway and binocular vision.

Fig 15.19 Nasal/temporal retina and projection pathways

Our eyes allow us to have a binocular vision where the visual fields of the two eyes overlap. The visual field of each eye is divided into two regions, the right and the left visual fields. Because of the curvature of the retina, the lateral or temporal half of each retina (temporal retina) points medially and receives lights from the left visual field. The medial or nasal halves of each retina (nasal retina) are directed laterally and receive light from the right visual field. Overall, this means that the right and left visual fields are both detected by each retina. The axons of the retinal ganglion cells form the optic (II) nerve and it passes through the optic chiasm (crossover). After the optic chiasm, each optic tract is only carrying information from the opposite visual field. After these second-order sensory neurons synapse in the lateral geniculate nucleus (LGN) of the thalamus, some processing of visual information occurs before third-order sensory neurons project in the optic radiations to the primary visual cortex in the occipital lobe. It is at this level that conscious perception of vision occurs. However, we do not put all of the information together until it is processed at successively higher levels of the association cortex. Ultimately it is all consolidated at the highest level of the visual association cortex (primary visual area) in the posterior parietal and inferior temporal lobes.

However, in the brain, as with general sensation, information is lateralized so that the left half of the visual cortex receives information from the right visual field while the right half receives information from the left visual field. Therefore, the information from the left and right visual fields traveling in each optic nerve needs to be separated.

Although the visual pathway projects primarily to the visual cortex, the optic nerve extends collateral fibers to several other important areas. These include the suprachiasmatic nucleus for control of circadian rhythms, the superior colliculus for subconscious control of object tracking, and the oculomotor nucleus which functions as the reflex center for the pupillary light reflex.

15.3 Hearing and Equilibrium

Outer/Middle/Inner Ear Anatomy

The ear contains all of the structures necessary for the detection of sound, static equilibrium, and dynamic equilibrium. Static equilibrium is the detection of the position of the head relative to gravity whereas dynamic equilibrium is the detection of linear and rotational movement. These 3 sensory modalities are detected deep in the temporal bone in a region called the inner ear. However, for the detection of sound, we also need a pathway for the movement of soundwaves that travel through the outer ear and the middle ear before being transmitted to the inner ear.

Fig 15.20: outer/middle/inner ear anatomy

Sound is produced by vibration of molecules and can pass through air, liquid, or solid. The vibration of an object, such as a tuning fork or our vocal cords, causes molecules in the air to vibrate and collide with molecules in front of them, creating a traveling wave. The frequency of the vibrations of the air molecules generates the pitch of the sound with higher frequencies (shorter wavelengths) generating higher pitch and lower frequencies (longer wavelengths) generating lower pitch. The frequency of these waves are measured in cycles per second or hertz (Hz). Loudness, on the other hand, is generated by the intensity, or amplitude of the vibration waves, and is measured in decibels (dB).

The amplitude or height of a wave is measured from the peak to the trough. The wavelength is measured from peak to peak.	This figure illustrates waves of differing wavelengths/frequencies. At the top of the figure, the red wave has a long wavelength/short frequency. Moving from top to bottom, the wavelengths decrease and frequencies increase.

Fig 15.21: Sound waves and graph showing frequency/amplitude

The outer ear consists of the externally visible portion of the ear called the pinna (auricle) which funnels sound waves into the external auditory canal. These structures function to transmit the traveling sound waves to the tympanic membrane or eardrum. When soundwaves reach the tympanic membrane, they cause it to vibrate at the same frequency as the sound vibrations.

Fig 15.22:  Middle ear

The tympanic membrane separates the outer ear and the middle ear. It is connected to a set of tiny bones called the auditory ossicles which are the smallest bones in the human body. These are the malleus, which is connected to the incus which is in turn connected to the stapes. The stapes is connected to a structure called the oval window which separates the middle ear from the inner ear. The auditory ossicles function as a lever system and will move with the vibrations of the tympanic membrane, causing those vibrations to be transmitted to the oval window. On the other side of the oval window is a fluid called perilymph. When the tympanic membrane, auditory ossicles, and oval window vibrate it causes the sound waves traveling through air to be conducted into sound waves traveling through the perilymph fluid. This conduction of sound from air to fluid is critical in our ability to detect sound. Anything that prevents this process results in a loss of hearing called conductive hearing loss.

To facilitate the conduction of sound, the tympanic membrane must be able to move freely so it is important to keep the pressure on both sides of the membrane equal so that pressure from one side does not restrict movement or, in the worst case, cause the membrane to rupture. Equalization of pressure is the function of the eustachian tube (auditory tube) which connects the middle ear to the nasopharynx space in the throat so that the air pressure in the middle ear is the same as the air outside of the body. Although the eustachian tube is beneficial for hearing, it allows a pathway for the bacteria of the pharynx (especially streptococcus) to invade the middle ear. When this happens, it results in a middle ear infection or otitis media. Otitis media must be treated with antibiotics to prevent a strong inflammatory reaction that can damage or even destroy the tympanic membrane. When babies get frequent otitis media infections, doctors often place small tubes through the membrane allowing inflammatory cells and bacteria to drain to prevent serious damage. A different type of ear infection, that of the external auditory canal, is called otitis externa and is common in swimmers and surfers. This type of infection can often be treated with topical antibiotic ear drops.

Although the movement of the tympanic membrane is important, if the tympanic membrane and ossicles move too freely, the powerful vibrations that they create in the perilymph of the inner ear can damage the delicate hair cells that detect sound. Any damage to these receptor cells (or to the vestibulocochlear nerve) results in another type of hearing loss called permanent sensorineural hearing loss, except in some specific types of cases where cochlear implants can be used in infants or very young children.

Fig 15.23 Cochlear implants (From Wikipedia)

To prevent too much vibration, two muscles called the tensor tympani and the stapedius muscles insert on the malleus and stapes bones, respectively, so that when loud sounds are detected, muscular contraction can partially immobilize the ossicles preventing the damage that can result from prolonged loud sounds. This protective response to loud sounds is called the tympanic reflex.

On the other side of the oval window, the hollow space in the tympanic bone filled with perilymph fluid is called the bony labyrinth and contains all of the structures of the inner ear.

Fig 15.24 Inner ear

The contours of the bony labyrinth are mirrored by a hollow membranous structure called the membranous labyrinth which is filled with a fluid called endolymph. It is within this membranous labyrinth that we find the receptor cells that are responsible for static equilibrium, dynamic equilibrium, and hearing. These receptor cells are separated into three regions called the semicircular canals, the vestibule, and the cochlea. The cochlea is where sound is detected.

Fig 15.25 Cochlea including cross-section with vestibular, tympanic, and cochlear ducts

When the perilymph is vibrated by the oval window, the movement of the fluid travels through the cochlea in a space called the vestibular duct which travels around the cochlea until it reaches the innermost ring to end at a structure called the helicotrema where the vestibular duct joins another duct called the tympanic duct. The fluid movement continues through the rings of the tympanic duct until it reaches the end, causing bulging of another membrane called the round window. The vibrating perilymph in the vestibular and tympanic ducts are separated from the endolymph in the membranous labyrinth by two membranes called the vestibular membrane and the basilar membranes, respectively. On the basilar membrane sits the spiral organ (organ of Corti) where sound is detected. The spiral organ includes thousands of receptor cells, called the inner hair cells. Depending on the frequency of the vibrations in the perilymph, different regions of the basilar membrane will vibrate and stimulate the receptor cells of the spiral organ. In this way, the hair cells of the basilar membrane are similar to a piano keyboard, though instead of generating pitch, they detect it.

Fig 15.26 Unfolded basilar membrane

Fig 15.27: Schematic Uncoiled Cochlea

When vibrated, the inner hair cells of the basilar membrane are pushed upwards into a gelatinous structure called the tectorial membrane causing them to bend. This triggers the opening of a mechanically gated ion channel.

Fig 15.28: Hair Cell

Fig 15.29: Mechanoelectrical transduction in hair cells: (1) Mechanically gated ion channel (orange) is attached to a tip link, which consists of homodimers of cadherin 23 and protocadherin 15. (2) Inside a stereocilium, harmonin (green) links the cytoplasmic terminus of cadherin 23 (blue) to myosin 7A (black), a motor protein that tightly binds filamentous actin of the cytoskeleton (pink). (3) The vibrational energy in sound waves physically displaces the bundle toward the tallest stereocilium, increasing tension in the tip link that forces the ion channel to open. (4) Influx of the cations calcium (Ca2+; red) and potassium (K+; yellow) depolarizes the hair cell to trigger neurotransmitter release. Created with BioRender.com.

Because the endolymph has an especially high potassium concentration, the result is an influx of potassium ions into the hair cell, causing it to depolarize. The depolarization causes voltage-gated calcium channels to open and it triggers the release of neurotransmitters onto the first-order sensory neurons of the vestibulocochlear nerve. The cell bodies of these sensory neurons are grouped in the spiral ganglion which sits in a shallow depression behind the cochlea called the modiolus. Because each neuron is associated with only one region of the basilar membrane, each pitch will only stimulate one set of sensory neurons. This tonotopic arrangement is maintained throughout the projection pathway all the way to the auditory cortex. Similarly, the loudness of the sound is coded by the frequency of firing because stronger vibrations of the perilymph cause greater bending of the hair cell and more neurotransmitter release.

Fig 15.30: Auditory projection pathway

The first-order sensory neurons of the cochlea project to the cochlear nucleus in the pons. From there, projections of second-order neurons from both right and left cochlear nuclei travel to the superior olivary nucleus where the timing of their arrival is compared to allow localization of sound in space. Without being able to compare the sound from both ears (binaural hearing), we cannot detect the direction from which a sound originates. The second-order sensory neurons continue to the inferior colliculus in the midbrain where auditory reflexes are controlled. For example, when you hear a door slam you will reflexively turn your head/eyes towards the sound. From the inferior colliculus, third-order sensory neurons project to the medial geniculate nucleus (MGN) of the thalamus from which fourth-order sensory neurons project to the primary auditory cortex in the temporal lobe of the cerebral cortex. It is here in the primary auditory cortex where we become aware of sounds. However, it is not until integration occurs in the auditory association cortex that we become fully aware of the details of the sound and can identify it. Due to extensive decussation along the auditory projection pathway, damage to only the left or the right auditory cortex does not result in unilateral hearing loss.

The vestibular system is closely related to the auditory system both in location and physiology. As with hearing, static and dynamic equilibrium are maintained by detecting forces via hair cells associated with gelatinous membranes.

In the vestibule region, there are two regions, the utricle, and the saccule which contain sensory structures called maculae. These receptors are oriented at right angles and are specialized to detect the position of the head relative to gravitational force along the Z-axis. They can detect linear acceleration and deceleration. The hair cells of the maculae are specialized to detect gravity because they are embedded within a gelatinous membrane called the otolithic membrane. This membrane contains deposits of protein and calcium carbonate called otoliths (ear stones). When the head bends in any direction it causes the otolithic membranes, which are heavily weighted due to the otoliths, to slide through the endolymph fluid causing the hair cells to bend.

Fig 15.31 Maculae bending with head tilt

As in the spiral organ, the bending of hair cells causes a potassium channel to open resulting in a depolarization and neurotransmitter release onto first-order sensory neurons.

Rotational movement is detected in the semicircular canals. There are three canals, oriented at nearly right angles, for detection of rotation in the three planes X, Y, and Z. Each of the three canals has an expanded region at its base called the ampulla. Within the ampullae are the structures called cristae that detect rotational motion. These cristae contain hair cells embedded in a tall gelatinous membrane called the cupula. When the head rotates, the cupulae also rotate. However, the endolymph within the canals does not. This causes the cupulae to bend along with their associated hair cells. As in the spiral organ and maculae, the bending of hair cells causes a potassium channel to open resulting in depolarization and neurotransmitter release onto first-order sensory neurons.

Fig 15.32 Rotational stimulation of crista ampullaris

(a) The cupula of the human semicircular canal. Top: The cupula spans the lumen of the ampulla from the crista to the membranous labyrinth. Bottom: Since head acceleration exceeds endolymph acceleration, the relative flow of endolymph in the canal is opposite to the direction of head acceleration. This flow produces a pressure across the elastic cupula, which deflects in response.	(b) Angular acceleration in semicircular canals

(c) Vestibular system’s semicircular canal – a cross-section

Fig 15.34 Semicircular ducts and endolymph movement

First-order sensory neurons of the vestibular system project to the vestibular nuclei in the pons and medulla which project to several regions. These include the cerebellum for coordination of movement and the nuclei of the cranial nerves that innervate the extraocular muscles for control of the vestibulo-ocular reflex (VOR). The VOR allows us to maintain the gaze on an object even when the head is moving. The application of this reflex can be seen after spinning around and becoming dizzy. Due to sensory adaptation, the sensory neurons begin firing and the brain interprets this as continued spinning. This is the feeling when you get off a roller coaster. The eyes are therefore directed, erroneously, to move to maintain gaze on an object. This can be seen as rapid back and forth eye movements called nystagmus. The vestibular nuclei also project to the spinal cord via the vestibulospinal tract for reflexive maintenance of posture and balance when the body changes position. Finally, the vestibular nuclei project to the primary and then associate sensory cortices where we become aware of head position, acceleration/deceleration, and rotation. This information, along with proprioceptive information from muscles and joints and visual information, combines to give us a sense of body position.

Now that you have read this chapter, the next time you’re at a lūʻau, you will know exactly how your body is processing all of the amazing smells, tastes, sounds, and visuals.

Chapter Summary

Key Terms

alkaloid

substance, usually from a plant source, that is chemically basic with respect to pH and will stimulate bitter receptors

amacrine cell

type of cell in the retina that connects to the bipolar cells near the outer synaptic layer and provides the basis for early image processing within the retina

ampulla

in the ear, the structure at the base of a semicircular canal that contains the hair cells and cupula for transduction of rotational movement of the head

auricle

fleshy external structure of the ear

basilar membrane

in the ear, the floor of the cochlear duct on which the organ of Corti sits

bipolar cell

cell type in the retina that connects the photoreceptors to the RGCs

choroid

highly vascular tissue in the wall of the eye that supplies the outer retina with blood

ciliary body

smooth muscle structure on the interior surface of the iris that controls the shape of the lens through the zonule fibers

circadian rhythm

internal perception of the daily cycle of light and dark based on retinal activity related to sunlight

cochlea

auditory portion of the inner ear containing structures to transduce sound stimuli

cochlear duct

space within the auditory portion of the inner ear that contains the organ of Corti and is adjacent to the scala tympani and scala vestibuli on either side

contralateral

word meaning “on the opposite side,” as in axons that cross the midline in a fiber tract

cornea

fibrous covering of the anterior region of the eye that is transparent so that light can pass through it

cupula

specialized structure within the base of a semicircular canal that bends the stereocilia of hair cells when the head rotates by way of the relative movement of the enclosed fluid

equilibrium

sense of balance that includes sensations of position and movement of the head

extraocular muscle

one of six muscles originating out of the bones of the orbit and inserting into the surface of the eye which are responsible for moving the eye

fibrous tunic

outer layer of the eye primarily composed of connective tissue known as the sclera and cornea

fovea

exact center of the retina at which visual stimuli are focused for maximal acuity, where the retina is thinnest, at which there is nothing but photoreceptors

general sense

any sensory system that is distributed throughout the body and incorporated into organs of multiple other systems, such as the walls of the digestive organs or the skin

gustation

sense of taste

gustatory receptor cells

sensory cells in the taste bud that transduce the chemical stimuli of gustation

hair cells

mechanoreceptor cells found in the inner ear that transduce stimuli for the senses of hearing and balance

incus

(also, anvil) ossicle of the middle ear that connects the malleus to the stapes

inferior colliculus

last structure in the auditory brainstem pathway that projects to the thalamus and superior colliculus

inferior oblique

extraocular muscle responsible for lateral rotation of the eye

inferior rectus

extraocular muscle responsible for looking down

inner ear

structure within the temporal bone that contains the sensory apparati of hearing and balance

iris

colored portion of the anterior eye that surrounds the pupil

lacrimal duct

duct in the medial corner of the orbit that drains tears into the nasal cavity

lacrimal gland

gland lateral to the orbit that produces tears to wash across the surface of the eye

lateral geniculate nucleus

thalamic target of the RGCs that projects to the visual cortex

lateral rectus

extraocular muscle responsible for abduction of the eye

lens

component of the eye that focuses light on the retina

levator palpebrae superioris

muscle that causes elevation of the upper eyelid, controlled by fibers in the oculomotor nerve

macula

enlargement at the base of a semicircular canal at which transduction of equilibrium stimuli takes place within the ampulla

malleus

(also, hammer) ossicle that is directly attached to the tympanic membrane

medial geniculate nucleus

thalamic target of the auditory brain stem that projects to the auditory cortex

medial rectus

extraocular muscle responsible for adduction of the eye

middle ear

space within the temporal bone between the ear canal and bony labyrinth where the ossicles amplify sound waves from the tympanic membrane to the oval window

neural tunic

layer of the eye that contains nervous tissue, namely the retina

olfaction

sense of smell

olfactory bulb

central target of the first cranial nerve; located on the ventral surface of the frontal lobe in the cerebrum

olfactory epithelium

region of the nasal epithelium where olfactory neurons are located

olfactory sensory neuron

receptor cell of the olfactory system, sensitive to the chemical stimuli of smell, the axons of which compose the first cranial nerve

opsin

protein that contains the photosensitive cofactor retinal for phototransduction

optic chiasm

decussation point in the visual system at which medial retina fibers cross to the other side of the brain

optic disc

spot on the retina at which RGC axons leave the eye and blood vessels of the inner retina pass

optic nerve

second cranial nerve, which is responsible visual sensation

optic tract

name for the fiber structure containing axons from the retina posterior to the optic chiasm representing their CNS location

organ of Corti

structure in the cochlea in which hair cells transduce movements from sound waves into electrochemical signals

ossicles

three small bones in the middle ear

otolith

layer of calcium carbonate crystals located on top of the otolithic membrane

otolithic membrane

gelatinous substance in the utricle and saccule of the inner ear that contains calcium carbonate crystals and into which the stereocilia of hair cells are embedded

oval window

membrane at the base of the cochlea where the stapes attaches, marking the beginning of the scala vestibuli

palpebral conjunctiva

membrane attached to the inner surface of the eyelids that covers the anterior surface of the cornea

papilla

for gustation, a bump-like projection on the surface of the tongue that contains taste buds

photon

individual “packet” of light

photoreceptor

receptor cell specialized to respond to light stimuli

pupil

open hole at the center of the iris that light passes through into the eye

receptor cell

cell that transduces environmental stimuli into neural signals

retina

nervous tissue of the eye at which phototransduction takes place

retinal

cofactor in an opsin molecule that undergoes a biochemical change when struck by a photon (pronounced with a stress on the last syllable)

retinal ganglion cell (RGC)

neuron of the retina that projects along the second cranial nerve

rhodopsin

photopigment molecule found in the rod photoreceptors

round window

membrane that marks the end of the scala tympani

saccule

structure of the inner ear responsible for transducing linear acceleration in the vertical plane

sclera

white of the eye

semicircular canals

structures within the inner ear responsible for transducing rotational movement information

special sense

any sensory system associated with a specific organ structure, namely smell, taste, sight, hearing, and balance

spiral ganglion

location of neuronal cell bodies that transmit auditory information along the eighth cranial nerve

stapes

(also, stirrup) ossicle of the middle ear that is attached to the inner ear

superior colliculus

structure in the midbrain that combines visual, auditory, and somatosensory input to coordinate spatial and topographic representations of the three sensory systems

superior oblique

extraocular muscle responsible for medial rotation of the eye

superior rectus

extraocular muscle responsible for looking up

suprachiasmatic nucleus

hypothalamic target of the retina that helps to establish the circadian rhythm of the body on the basis of the presence or absence of daylight

taste buds

structures within a papilla on the tongue that contain gustatory receptor cells

tectorial membrane

component of the organ of Corti that lays over the hair cells, into which the stereocilia are embedded

transduction

process of changing an environmental stimulus into the electrochemical signals of the nervous system

trochlea

cartilaginous structure that acts like a pulley for the superior oblique muscle

tympanic membrane

ear drum

umami

taste submodality for sensitivity to the concentration of amino acids; also called the savory sense

utricle

structure of the inner ear responsible for transducing linear acceleration in the horizontal plane

vascular tunic

middle layer of the eye primarily composed of connective tissue with a rich blood supply

vestibular nuclei

targets of the vestibular component of the eighth cranial nerve

vestibule

in the ear, the portion of the inner ear responsible for the sense of equilibrium

vestibulo-ocular reflex (VOR)

reflex based on connections between the vestibular system and the cranial nerves of eye movements that ensures images are stabilized on the retina as the head and body move

vestibulospinal tract

extrapyramidal connections between the vestibular nuclei in the brain stem and spinal cord that modulate movement and contribute to balance on the basis of the sense of equilibrium

vision

special sense of sight based on transduction of light stimuli

vitreous humor

viscous fluid that fills the posterior chamber of the eye

zonule fibers

fibrous connections between the ciliary body and the lens

Sources

1. OpenStax A&P textbook (Ch 14): https://openstax.org/books/anatomy-and-physiology/pages/14-1-sensory-perception?query=sensation&target=%7B%22index%22%3A0%2C%22type%22%3A%22search%22%7D#fs-id2124989
2. https://manoa.hawaii.edu/exploringourfluidearth/physical/navigation-and-transportation/wayfinding-and-navigation
3. www.hokulea.com
4. Fundamentals of Anatomy & Physiology, 11th edition, Frederic Martini; Judi Nath; Edwin Bartholomew
5. https://courses.lumenlearning.com/austincc-ap1/chapter/special-senses-taste-gustation/