17 General Senses

General Senses UNIT OUTLINE

General Sensory Receptors Activity 1: Identifying General Sensory Receptors Activity 2: Examining the Microscopic Structure of General Sensory Receptors Activity 3: Performing a Two-Point Discrimination Test

n this unit we investigate sensory receptors, structures that respond to changes, or stimuli, in the external environment and within the body. Most sensory re- Iceptors—those that respond to touch, pressure, cold, heat, pain, stretch, vibra- tion, and changes in body position—are widely distributed throughout the body and are classified as general sensory receptors. Somatic sensory receptors are found in the skin and in musculoskeletal organs, whereas visceral sensory receptors are located in visceral organs. Sensory receptors for the special senses, by contrast, are housed in specialized sense organs in the head. The special senses, which include gustation (taste), olfaction (smell), vision, hearing, and equilibrium, will be our focus in Unit 18.

THINK ABOUT IT Which brain structure serves as a relay station for most sensory impulses? ______Sensory impulses originating from general sensory receptors in the skin are relayed to which functional area of the cerebral cortex? ______What is the difference between sensation and perception and where does each occur? ______▪

355 PRE-LAB ASSIGNMENTS Pre-lab quizzes are also assignable in To maximize learning, BEFORE your lab period carefully read this entire lab unit and complete these pre-lab assignments using your textbook, lecture notes, and prior knowledge.

PRE-LAB Activity 1: Identifying General Sensory Receptors 1. Use the list of terms provided to label the accompanying illustration of cutaneous receptors. Check off each term as you label it.

□ lamellated corpuscle a □ tactile corpuscle b □ free nerve endings □ Ruffini ending c

2. Fill in the blanks with the appropriate term(s): ______a. light touch receptor located in the papillary layer of the dermis ______b. deep pressure receptor ______c. light touch receptor located in the epidermis ______d. receptors that detect pain and temperature

PRE-LAB Activity 2: Examining the Microscopic Structure of General Sensory Receptors 1. Where specifically in the skin are tactile (Meissner) corpuscles located? ______2. Where specifically in the skin are lamellated (Pacinian) corpuscles located? ______

PRE-LAB Activity 3: Performing a Two-Point Discrimination Test 1. Which instrument will be used to determine the ability of a subject to discriminate between two points? ______2. What is the relationship between two-point receptor density and tactile sensitivity? ______

General Sensory Receptors Because action potentials, once generated, are identical, the body distinguishes among different stimuli entirely by the Sensory receptors respond to various stimuli, both external region of the cerebral cortex that receives the sensory input and internal, and they are often classified according to the originating from the receptor. source of the stimulus. These receptors act as transducers— Sensory receptors can be classified in various ways. One that is, they convert stimuli into afferent nerve impulses. classification method groups sensory receptors according to 356 UNIT 17 | General Senses 357 the type of stimulus to which they respond. Based on this musculoskeletal organs and their associated connective tis- criterion, sensory receptors can be divided into five groups: sue coverings. Two types of proprioceptors—muscle spindles mechanoreceptors, which respond to touch or pressure; and Golgi tendon organs—respond to changes in muscle chemoreceptors, which respond to chemicals; photorecep- length and in joint position and movement, respectively. tors, which respond to light; nociceptors, which respond to Receptors in the skin, or cutaneous receptors (Figure 17-1), intense thermal, chemical, or mechanical stimuli that result in are of two general types: free nerve endings and encapsulated pain; and thermoreceptors, which respond to temperature. receptors. Free nerve endings, which are the dendrites of sen- Sensory receptors can also be classified based on the sory neurons, are the least specialized receptors; they respond source of the stimulus. Exteroceptors, which respond to primarily to temperature and pain. Some free nerve endings stimuli from the external environment, are usually located are associated with epidermal cells and are called Merkel cell near the body surface. Examples are the sensory receptors fibers (or tactile discs); others are wrapped around a hair fol- found in the skin and the specialized sensory receptors for vi- licle (found only in thin skin) and are called hair follicle sion and hearing located in the eye and ear, respectively. In- receptors (or root hair plexuses, not shown). Both Merkel cell teroceptors (also called visceroceptors) respond to stimuli fibers and hair follicle receptors respond to light touch. Encap- within the body. Examples include chemoreceptors (such as sulated receptors are nerve endings that are encapsulated by those located in the medulla oblongata that respond to blood connective tissue cells. Tactile corpuscles (also called Meiss- carbon dioxide levels), stretch receptors in the walls of hollow ner corpuscles) are located in the dermal papillae and function organs such as the urinary bladder, and baroreceptors in arte- as light touch receptors. Lamellated corpuscles (also called rial walls that respond to changes in blood pressure. Pacinian corpuscles) are located deeper in the reticular layer A third type of receptor based on stimulus type is pro- of the dermis and respond to deep pressure. Ruffini endings prioceptors. Like interoceptors, proprioceptors respond (also called bulbous corpuscles) lie in the dermis, hypodermis, to internal stimuli, but these receptors are located only in and joint capsule and respond to deep and continuous pressure.

Merkel cell Free nerve fiber endings

Epidermis

LM (1000×) Tactile corpuscle

Dermis

Axons of Ruffini ending sensory neurons (bulbous corpuscle)

LM (200×) Lamellated corpuscle Figure 17-1 Cutaneous receptors. 358 UNIT 17 | General Senses

Materials Needed ACTIVITY 1 □ Skin model and anatomical charts Identifying General Sensory Receptors □ Muscle model or anatomical charts □ Colored pencils Learning Outcomes 1. Identify cutaneous receptors—free nerve ending, lamellated Instructions corpuscle, hair follicle receptor, Ruffini ending, tactile corpuscle, and Merkel cell fiber—on a skin model or anatomical chart, and 1. CHART Complete the following chart as you identify then describe the location and function of each. each of the types of sensory receptor on an anatomical 2. Identify proprioceptors (Golgi tendon organ and muscle spindle) model (or on an anatomical chart). Information for the on an anatomical model or chart, and then describe the location first structure has been filled in for you. and function of each.

Sensory Receptor Location Stimulus Type Received In What Way(s) Can This Receptor Be Classified? Free nerve Skin (epidermis and dermis) Temperature; Exteroceptor: responds to external stimulus ending pain; Thermoreceptor: responds to temperature light touch Nociceptor: responds to pain Mechanoreceptor: responds to touch

Golgi tendon organ

Lamellated corpuscle

Muscle spindle

Hair follicle receptor

Ruffini ending

Tactile corpuscle

Merkel cell fiber UNIT 17 | General Senses 359

2. Sketch a longitudinal section of the skin. Instructions a. Label and then color the following structures: 1. Observe a longitudinal section (or a photomicrograph) epidermis, blue; dermis, green; free nerve endings, of the skin showing a tactile corpuscle under low power. yellow; tactile corpuscle, brown; lamellated Sketch and label the following structures: epidermis, corpuscle, purple; Ruffini ending, red; hair follicle dermal papilla, dermis, tactile corpuscle. receptor, orange. b. Draw a black box around the labels for the receptors classified as free nerve endings. c. Draw a black circle around the labels for the receptors classified as encapsulated receptors.

Total magnification: _____ ×

Describe the location of the tactile corpuscle.

Switch to the high-power objective (or view a more- magnified photomicrograph), observe the cells of the tactile corpuscle, and describe their appearance.

2. Observe a histological section (or a photomicrograph) of the skin showing a lamellated corpuscle under low power. Sketch and label the following structures: dermis, lamellated corpuscle.

ACTIVITY 2 Examining the Microscopic Structure of General Sensory Receptors Learning Outcomes 1. Distinguish between a tactile corpuscle and a lamellated corpuscle in a longitudinal section of the skin. 2. Compare and contrast a tactile corpuscle and a lamellated corpuscle with respect to structure and function. Total magnification: _____ × Materials Needed □ Anatomical models Describe the location of the lamellated corpuscle. □ Microscope and skin sections (or photomicrographs) showing a tactile corpuscle and a lamellated corpuscle 360 UNIT 17 | General Senses

Switch to the high-power objective (or view a more- table from 1 to 5 (1 has the greatest receptor density, and magnified photomicrograph), observe the cells of the 5 has the lowest receptor density). Then write a brief ratio- lamellated corpuscle, and describe their appearance. nale for your prediction.

Location Prediction (Ranking) How are the locations of the tactile corpuscle and the lamellated corpuscle related to their functions? Cheek Palm Fingertip Posterior neck Anterior thigh 3. CHART Complete the following chart to note the similarities and differences between these two types of Rationale for prediction: ______cutaneous receptors:

Lamellated Tactile Corpuscle Corpuscle Now perform the following steps: In papillary layer or reticular layer of 1. Designate one student in your lab group as the test subject. the dermis? 2. Obtain a compass and instruct the subject to close both Detects light eyes. Starting with the compass closed all the way, gen- pressure or deep tly touch the subject’s cheek. Ask the subject how many pressure? points he or she can discriminate: one or two. Exteroceptor or 3. If the subject reports feeling only one point, remove the interoceptor? compass and open it one millimeter. Now, touch the subject’s cheek again and ask how many points the subject can discriminate: one or two. Free nerve ending or encapsulated 4. Continue this process until the subject reports feeling ending? two points. Then carefully remove the compass, mea- sure the distance between the two points in millimeters, and record the distance in the following table. 5. Repeat steps 1–4 for each of the four remaining locations. ACTIVITY 3 Location Two-Point Discrimination Distance (mm) Performing a Two-Point Cheek Discrimination Test Palm Learning Outcomes Fingertip 1. Perform a two-point discrimination test on various areas of the Posterior neck body. 2. Relate the results of a two-point discrimination test to receptor Anterior thigh density. 6. Work with your lab group members to answer the Materials Needed following questions based on the data you collected: □ Blunt-tip drawing compass with millimeter scale a. State which body area has the greatest density Instructions of receptors and which body area the least? In this activity, you will use the two-point discrimination test ______to determine the density of tactile receptors in five different b. Draw a conclusion with respect to two-point areas of the body. The greater the touch receptor density in a discrimination distance and receptor density. given area, the greater the tactile sensitivity in that area. ______Before you begin the test, predict the relative density of tactile receptors by ranking the five locations in the following ______

Post-lab quizzes are also assignable POST-LAB ASSIGNMENTS in

Name: ______Date: ______Lab Section: ______

PART I. Check Your Understanding

Activity 1: Identifying General Sensory Receptors

1. Match each of the following descriptions with its receptor. More than one correct answer is possible for each question; include all correct answers. ______a. function as light touch receptors 1. hair follicle receptors ______b. are located in the dermis 2. muscle spindles ______c. respond to stretch 3. free nerve endings _____ d. respond to pain and temperature 4. lamellated corpuscles ______e. function as deep pressure receptors 5. tactile corpuscles _____ f. are also called “naked” receptors 6. Golgi tendon organs _____ g. lie deepest in the dermis 7 Ruffini endings ______h. are located in the epidermis 8. Merkel cell fibers ______i. are classified as proprioceptors ______j. are classified as encapsulated receptors

Activity 2: Examining the Microscopic Structure of General Sensory Receptors

1. Identify the following cutaneous receptor, describe its specific location in the skin, and state its function. ______

LM (400×)

361 362 UNIT 17 | General Senses

2. Identify the following cutaneous receptor, describe its specific location in the skin, and state its function. ______

LM (100×)

Activity 3: Performing a Two-Point Discrimination Test

1. A two-point discrimination test is used to map the distribution of ______receptors in the skin. Which area of the skin has the highest concentration of receptors?

______

What is the physiological significance of these data? ______

PART II. Putting It All Together

A. Review Questions Answer the following questions using your lecture notes, your textbook, and your lab notes.

1. Explain what it means to say that sensory receptors act as transducers. ______

2. Why is the thalamus called a “gateway” for sensory impulses? ______UNIT 17 | General Senses 363

3. How does the size of the somatosensory cortex area that receives sensory input from a particular area of the body relate to cutaneous receptor density? ______

4. Why is it important for tactile receptors to exhibit adaptation? ______

5. Why are you not aware of sensory signals originating in proprioceptors? ______

B. Concept Mapping

1. Fill in the blanks to complete this concept map outlining the classification of sensory receptors.

exteroceptor mechanoreceptor muscle spindle photoreceptor proprioceptor

SENSORY RECEPTORS Respond to internal stimuli Found only in Respond to musculoskeletal external stimuli organs

Interoceptor

Respond Respond to touch Example to light or pressure

2. Construct a unit concept map to show the relationships among the following set of terms. Include all of the terms in your diagram. Your instructor may choose to assign additional terms.

cerebellum cerebrum encapsulated ending epidermis exteroceptor

free nerve ending lamellated corpuscle mechanoreceptor muscle spindle

photoreceptor primary somatosensory cortex proprioceptor

tactile corpuscle thalamus thermoreceptor This page intentionally left blank 18

Special Senses UNIT OUTLINE

Olfaction and Gustation Activity 1: Exploring the Gross Anatomy of Olfactory and Gustatory Structures and Demonstrating the Effect of Olfaction on Gustation Vision Activity 2: Examining the Gross Anatomy of the Eye Activity 3: Dissecting a Mammalian Eye Activity 4: Performing Visual Tests Hearing and Equilibrium Activity 5: Examining the Gross his unit entails a detailed study of the structure and function of the body’s Anatomy of the Ear complex special senses: olfaction, gustation, vision, hearing, and equilib- Activity 6: Performing Hearing Trium. Whereas most general sensory receptors are modified nerve endings and Equilibrium Tests of sensory neurons, special sense receptors are highly specialized receptor cells. Sensory information from these specialized receptor cells is transmitted via complex neural pathways to specific areas of the cerebral cortex for integration and interpretation.

Think AbouT iT Which cerebral lobe receives and interprets incoming sensory impulses for each of the following? smell ______taste ______hearing ______equilibrium ______vision ______▪

365 PRE-LAB ASSIGNMENTS Pre-lab quizzes are also assignable in To maximize learning, BEFORE your lab period carefully read this entire lab unit and complete these pre-lab assignments using your textbook, lecture notes, and prior knowledge.

PRE-LAB Activity 1: Exploring the Gross Anatomy of Olfactory and Gustatory Structures and Demonstrating the Effect of Olfaction on Gustation 1. Olfactory and gustatory receptors are classified as which receptor type based on stimulus type received? ______2. The nerve fibers of the olfactory nerves pass through the ______foramina of the ______plate of the ______bone en route to the cerebral cortex. 3. When gustatory cells in taste buds are stimulated by dissolved chemicals, sensory messages are relayed to the medulla oblongata via which cranial nerves? ______

PRE-LAB Activity 2: Examining the Gross Anatomy of the Eye 1. Use the list of terms provided to label the accompanying illustration of the eye. Check off each term as you label it.

□ aqueous humor □ iris □ retina □ ciliary body □ lens □ sclera □ choroid □ optic nerve □ vitreous humor □ cornea □ pupil a b h c (opening) i d (watery fluid) j e

k f (gel-like substance) g

366 UNIT 18 | Special Senses 367

2. Match each eye structure with its correct description: _____ a. structure filled with aqueous humor 1. outer fibrous layer _____ b. structure continuous with the optic nerve 2. middle vascular layer _____ c. attachment site for extrinsic eye muscles 3. inner neural layer _____ d. retina 4. anterior cavity _____ e. iris, ciliary body, and choroid 5. posterior cavity _____ f. structure that houses the photoreceptors _____ g. structure filled with vitreous humor _____ h. sclera and cornea

PRE-LAB Activity 3: Dissecting a Mammalian Eye 1. How does the cow eye differ from the human eye? ______2. Which tissue type surrounding the eyeball must be cut away before dissecting it? ______

PRE-LAB Activity 4: Performing Visual Tests 1. Color-blind individuals are missing one or more of which type of photoreceptor? ______2. ______is an eye disorder caused by an irregular curvature of the cornea or lens. 3. As people age, why does their near point of accommodation increase? ______

PRE-LAB Activity 5: Examining the Gross Anatomy of the Ear 1. Use the list of terms provided to label the accompanying illustration of the ear. Check off each term as you label it.

□ auricle □ tympanic membrane □ semicircular canals □ cochlea □ incus □ stapes □ external auditory canal □ malleus □ vestibule d a e

f b

h i 368 UNIT 18 | Special Senses

2. Match each of the following ear structures with its description: _____ a. malleus, incus, and stapes 1. tympanic membrane _____ b. houses receptor organ for hearing 2. auditory ossicles ______c. function in equilibrium 3. cochlea _____ d. connects middle ear to nasopharynx 4. semicircular canal ______e. is found in middle ear 5. vestibule _____ f. eardrum 6. auditory tube ______g. is found in the inner ear _____ h. snail-like structure 3. Which of the following structures vibrates in response to sound waves? a. vestibulocochlear nerve d. semicircular canal b. tympanic membrane e. auditory tube c. vestibule

PRE-LAB Activity 6: Performing Hearing and Equilibrium Tests 1. The ______and ______tests are tests health care providers use to determine if hearing loss is conductive or sensorineural. 2. The ______test evaluates the ability of the vestibular apparatus to maintain equilibrium in the absence of visual stimuli.

Olfaction and Gustation cribriform plate of the ethmoid bone and synapse in the olfactory bulbs (see Figure 18-1b). There olfactory nerve The olfactory epithelium, a specialized area located in the fibers synapse with mitral cells in complex structures called superior portion of the nasal cavity (Figure 18-1a), con- glomeruli (“little balls”). When stimulated, the mitral cells sists of three types of cells: olfactory neurons, basal cells, trigger impulses that travel via the olfactory tracts to the and supporting cells (see Figure 18-1b). Olfactory neurons, primary olfactory cortex in the temporal lobe, where aware- which are bipolar neurons, possess cilia that project into the ness and identification of odors occur. mucous layer overlying the surface of the olfactory epithe- The receptors for gustation, called gustatory cells lium. The cilia (see Figure 18-1c) contain olfactory receptors (Figure 18-2c), are in taste buds located primarily on the su- to which chemical odorants bind as they enter the nasal cav- perior surface of the tongue (see Figure 18-2a), but a few are ity. Basal cells are stem cells that divide to produce new ol- also located on the soft palate, pharynx, epiglottis, and inner factory neurons, and supporting cells support and nourish surface of the cheeks. A taste bud contains up to 100 gus- the olfactory neurons (see Figure 18-1b). The olfactory epi- tatory cells, as well as basal cells and supporting cells. Each thelium contains olfactory glands that secrete mucus. Chem- gustatory cell has a lifespan of about 10 days; basal cells ical odorants entering the nasal cavity must diffuse through divide regularly to form supporting cells, which then differ- this watery mucous layer before they can stimulate receptors entiate into new gustatory cells. on the olfactory cilia. Taste buds on the tongue are located within surface The axons of olfactory neurons form the olfactory nerves projections called papillae, which are of four types: val- (CN I), which pass through the olfactory foramina of the late, foliate, fungiform, and filiform. The largest, called UNIT 18 | Special Senses 369

Olfactory bulb

Olfactory tract

Cribriform plate Axon of olfactory neuron

Olfactory gland Olfactory bulb Olfactory neuron Olfactory Olfactory Supporting cell epithelium tract Basal cell Cribriform plate of ethmoid bone Middle and inferior nasal Dendrite conchae Olfactory epithelium (b) Structure of olfactory epithelium Olfactory cilia

Inhaled air containing odorants (a) Location of olfactory epithelium in the nasal cavity Dendrite of olfactory epithelium

Figure 18-1 The olfactory epithelium.

vallate papillae, are arranged in an inverted V shape on the instead of taste buds; these papillae give the tongue a rough posterior surface of the tongue (see Figure 18-2a). Foliate texture that aids in manipulating food. When gustatory cells papillae, located on the lateral surfaces of the tongue, are pres- in taste buds are stimulated by dissolved chemicals, sensory ent mostly in children. Fungiform papillae (see Figure 18-2a) messages are relayed to the medulla oblongata via the facial and filiform papillae (not shown) are scattered across the an- (CN VII), glossopharyngeal (CN IX), and vagus (CN X) terior two-thirds of the tongue’s surface. Although filiform pa- nerves. Impulses are then transmitted to the thalamus and ul- pillae are the most abundant, they contain tactile receptors timately to the gustatory cortex located in the insula. 370 UNIT 18 | Special Senses

Taste bud Microvilli

Vallate papilla

Foliate Sensory papillae neurons

Fungiform papillae Basal cells Supporting Gustatory Taste cells cells pore (b) Longitudinal section of a vallate papilla

(a) View of the tongue

Figure 18-2 The location and structure of gustatory cells in taste buds.

ACTIVITY 1 Instructions Exploring the Gross Anatomy A. Olfactory and Gustatory Structures of Olfactory and Gustatory Structures 1. Look into a mirror and identify vallate and fungiform papillae on your tongue. Sketch your tongue and and Demonstrating the Effect indicate the locations of these two types of papillae. of Olfaction on Gustation Learning Outcomes 1. Identify the olfactory structures associated with the nasal cavity, skull, and brain. 2. Identify the locations of the four types of papillae on the tongue. 3. Identify the cranial nerves that transmit sensory information from the taste buds to the cerebral cortex. 4. Demonstrate the effect of olfaction on taste by identifying unknown food samples with nostrils closed and with nostrils open.

Materials Needed □ Mirror □ Midsagittal head model □ Skull □ Brain model 2. On a brain model, identify the three cranial nerves that □ Paper towels carry sensory information from the taste buds to the □ Plastic container containing cubes of four different foods brain. List the three nerves here: ______(provided by instructor and unknown to test subject) □ Toothpicks ______UNIT 18 | Special Senses 371

3. Identify each of the following olfactory structures on 5. Repeat steps 3 and 4 for the remaining three food an appropriate anatomical model or chart, and then samples. indicate the function of each: Food (1) No (2) Chewing, (3) Chewing, a. Olfactory epithelium ______Sample Chewing, Eyes Eyes and Eyes Closed ______and Nostrils Nostrils Closed and Nostrils Closed Open b. Fibers of olfactory nerve ______#1 ______#2 c. Olfactory foramina ______#3

d. Olfactory bulb ______#4 ______e. Olfactory tract ______6. Answer the following questions based on your results: ______a. Did your experimental results match your prediction? ______4. Optional Activity Explain: ______Practice labeling gustatory and olfactory structures on

™ human cadavers at > > Study Area > ______Practice Anatomy Lab > Human Cadaver: Nervous System > ______Special Senses > Images 1–5 b. Compare and explain the effects of no chewing and chewing on the ability to taste an unknown food. B. Demonstrating the Effect of Olfaction on Gustation ______Demonstrate the relationship between olfaction and taste by completing the following activity: ______1. Before performing the experiment, make the following ______predictions: a. What will be the effect of an inability to smell on your ability to taste and identify unknown food Vision samples? ______Accessory Structures of the Eye ______The accessory structures of the eye (Figure 18-3) include the b. What is the basis of this prediction?______eyelids, conjunctiva, lacrimal apparatus, and the extrinsic eye muscles. The eyes are covered anteriorly by the superior and ______inferior eyelids (see Figure 18-3a), which meet medially and 2. Work in pairs and choose one student to be the test laterally at the medial commissure and lateral commissure, subject. respectively. The medial commissure contains a small fleshy mass of tissue called the lacrimal caruncle, which contains 3. Have the subject dry her or his tongue with a clean modified sweat glands that release a lubricating secretion. Each paper towel, close both eyes, and pinch both nostrils eyelid contains tarsal glands (see Figure 18-3b), modified se- shut. baceous glands that produce an oily secretion that keeps the 4. Using a clean toothpick, place a bit of the first food eye’s surfaces lubricated. The conjunctiva, a thin mucous mem- sample (sample #1) on the subject’s tongue. Then ask brane, lines the internal surface of the eyelids (the palpebral the subject to identify the food (1) immediately—no conjunctiva) and then folds to cover much of the anterior sur- chewing allowed!—with eyes and nostrils closed; face of the eyeball (the bulbar conjunctiva). The conjunctiva (2) after chewing, with eyes and nostrils closed; and secretes mucus that lubricates the surface of the eyeball. (3) after chewing with eyes closed and nostrils open. In each case where the subject is able to correctly identify the food, write “Yes” in the following table; if the subject could not correctly identify the food, write “No.” 372 UNIT 18 | Special Senses

Lacrimal caruncle Eyebrow

Levator palpebrae superioris muscle

Orbicularis oculi muscle Tarsal plate Tarsal gland Superior eyelid Eyelashes

Inferior eyelid Tarsal gland

Tarsal plate

Medial Lateral Orbicularis oculi muscle commissure commissure Palpebral conjunctiva Bulbar (ocular) conjunctiva (a) Anterior view (b) Lateral view

Lacrimal gland Superior rectus muscle Optic nerve (II) Trochlea Lacrimal punctum

Lacrimal sac Superior oblique muscle

Medial rectus muscle

Lacrimal canaliculi Lateral rectus muscle

Nasolacrimal duct Inferior rectus muscle Inferior concha Inferior nasal Inferior oblique muscle meatus Common tendinous ring

The lacrimal apparatus (see Figure 18-3c) consists of Table 18-1 Extrinsic Eye Muscles the lacrimal gland and its accessory structures. The lacrimal gland secretes watery, alkaline tears containing an Extrinsic Eye Muscle Action Innervation antibacterial enzyme called lysozyme. The tears clean, lu- Inferior oblique Elevates the eye and CN III bricate, and protect the eye. Tears travel across the surface rotates it laterally. of the eye and drain through small pores called lacrimal Inferior rectus Depresses the eye. CN III puncta (singular = punctum) into the lacrimal canaliculi (singular = canaliculus) and then into the lacrimal sac. The Medial rectus Moves the eye medially. CN III nasolacrimal duct receives tears from the lacrimal sac and Lateral rectus Moves the eye laterally. CN VI transports them into the nasal cavity. The actions and innervation of each of the six extrinsic Superior oblique Depresses the eye and CN IV eye muscles (see Figure 18-3d)—skeletal muscles that move rotates it laterally. Table 18-1 the eyeball—are summarized in . Superior rectus Elevates the eye. CN III UNIT 18 | Special Senses 373

Suspensory Ora serrata ligaments

Fibrous layer: Sclera Cornea Macula Fovea lutea centralis

Pupil

Optic disc Vascular Layers layer: Lens of the Iris eyeball Ciliary body Choroid

Neural layer: Retina Central artery and Optic nerve (II) vein of the retina

Figure 18-4 The internal structure of the eye (sagittal section).

Structure of the Eyeball The neural layer consists of the retina, which begins at the ora serrata, the serrated boundary between the retina The eye (Figure 18-4) is a complex sensory organ composed and the ciliary body. The retina is composed of two lay- of three layers: the outer fibrous layer, the middle vascular ers: an outer pigmented epithelium that absorbs light and layer, and the inner neural layer. prevents it from scattering, and an inner neural layer that The fibrous layer consists of the cornea and the sclera. contains the photoreceptors (rods and cones). When stimu- The transparent cornea, the anterior one-sixth of the fibrous lated by light, the photoreceptors trigger stimulation of the layer, bends light as it enters the eye. The sclera—the white optic nerve and the transmission of impulses to the thala- of the eyeball—makes up the posterior five-sixths of the fi- mus and eventually to the occipital lobe of the cerebrum. brous layer; it protects the eye and serves as an attachment Photoreceptors are located throughout the retina, except at site for the extrinsic eye muscles. the optic disc (also called the blind spot), the point at which The vascular layer consists of the choroid, ciliary body, the optic nerve leaves the eyeball. Just lateral to the optic disc and iris. The choroid is a highly vascular structure that sup- is the macula lutea; in the center of it is the fovea centralis, plies oxygen and nutrients to the cells of the eye. It contains a small area where the concentration of cones is greatest. an abundance of melanocytes, cells that produce the pigment The retina (Figure 18-5) contains three major populations melanin, which absorbs excess light. Anteriorly, the choroid of cells: photoreceptors (rods and cones), bipolar cells, and coat thickens to form the ciliary body, a structure that reg- retinal ganglion cells. Rods enable vision in low-light condi- ulates the shape of the lens to enable light to be focused on tions and are important to peripheral vision; they are highly the retina. The ciliary body also secretes aqueous humor, concentrated along the periphery of the retina. By contrast, the clear watery fluid that fills the anterior cavity of the eye. cones are important for both visual acuity (sharpness) and The most anterior portion of the vascular layer is the iris, color vision; their concentration increases as you move from which contains pigmented cells that are responsible for eye the periphery of the retina to its center, where the macula color and smooth muscle fibers that regulate the size of the lutea is located. The fovea centralis contains cones only. Rods pupil—and thus the amount of light that enters the eye. and cones transmit electrical signals to bipolar cells, which 374 UNIT 18 | Special Senses

Choroid

Light Pigmented layer of retina

Rods

Cones

Horizontal cell

Bipolar cells

NOTE: File has been checkedAmacrine and this cellis the most current Amerman art for this figure.

Retinal ganglion cells

LM (450×) Light (a) Cellular structure of retina (b) Photomicrograph of retina Figure 18-5 The retina. Ciliary body synapse with the retinal ganglion cells. The axons of the Suspensory ligaments ganglion cells fuse to form the optic nerve. Figure 18-6 shows the structures within the internal cavi- Scleral venous ties and chambers of the eye. The lens, a biconcave structure sinus that bends and focuses incoming light on the retina, is com- Cornea posed of highly specialized lens fiber cells packed with trans- Pupil parent proteins called crystallins. The lens is attached to the Lens ciliary body via the suspensory ligament. Iris The lens separates the eye into anterior and posterior cavities. The anterior cavity is subdivided by the iris into anterior and posterior chambers. The ciliary body continuously secretes aqueous humor into the posterior chamber. This watery fluid, which is formed as a filtrate of the blood from cap- illaries in the ciliary body, flows through the pupil into the Scleral anterior chamber and then drains into the scleral venous venous Vitreous sinus humor in sinus, an enlarged vessel that returns the aqueous humor to posterior the blood. Aqueous humor contributes to intraocular pres- Anterior Posterior cavity sure and provides oxygen and nutrients to the avascular cor- chamber chamber nea and lens. The posterior cavity is filled with vitreous humor, a gel-like substance formed prior to birth. The vitre- Anterior cavity: Anterior and posterior chambers form the anterior ous humor provides internal support for the posterior part cavity and are filled with aqueous humor. The arrows show the flow of aqueous humor before it drains into the scleral venous sinus. of the eyeball and helps keep the retina pushed firmly against the vascular choroid. Figure 18-6 Cavities and chambers of the eye.