Anatomy and Physiology Marieb’s Human Anatomy and Physiology Marieb w Hoehn Chapter 13 – General Sensory Receptors Chapter 15 - Special Sensory Receptors Lecture 22 55 slides, 155 min. (1 -> 26, 80 min; 28 -> end, 75 min.)

Lecture Overview Introduction to the senses and sensation Types of sensors Classification of sensory receptors Anatomy of the ear Physiology of hearing/equilibrium Anatomy of the eye Physiology of vision Video 1 Video 2 Video 3

Special Senses sensory receptors are within large, complex sensory organs in the head hearing and equilibrium in ears sight in eyes smell in olfactory organs taste (gustation) in taste buds (Video 2) (Video 3)* Not covered in video – see master slide set

External Anatomy of the Orbital Region Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

The Eye and Deep Orbital Region Visual Accessory Organs eyebrows eyelids (palpebrae) conjunctiva lacrimal apparatus extrinsic eye muscles Lacrimal caruncle - sebaceous plus sweat gland produces whitish oily secrn (‘sand’ in eyes) Limbus

Eyelids palpebrae = eyelids composed of four layers skin muscle connective tissue conjunctiva orbicularis oculi – closes eye (CN VII) levator palpebrae superioris – raises eyelid (CN III) tarsal (Meibomian) glands – secrete oil onto eyelashes; keep lids from sticking together conjunctiva – mucous membrane; lines eyelid and covers portion of eyeball; keeps eye from drying out Fornix Tarsal plates are CT and serve as insertion points for orbicularis oculi and lev. palpebrae sup. muscles. Blink reflex occurs every 3-7 sec. to spread oil, mucus, and saline across surface of eye. Eyelash roots are richly innervated and will cause blinking if disturbed Tarsal (Meibomian) glands embedded in tarsal plates – modified sebaceous gl produce oily secrn to help keeps lids from sticking together. Ciliary gland are located between eyelashes (sebaceous and modified sweat gl). Infection of a tarsal gl = chalazion; infection of smaller glands = sty. Conjunctiva is a transparent mucus membrane containing blood vessels that produces a lubricating mucus that helps keep the eye from drying out. Sagittal section of right eye Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

Some External Disorders of Eye Sty (Infection of smaller glands (eyelashes) Chalazion (Infection of tarsal glands) Conjunctivitis (Inflammation of conjunctiva)

Lacrimal (Tear) Apparatus lacrimal gland lateral to eye secretes tears canaliculi collect tears lacrimal sac collects from canaliculi nasolacrimal duct collects from lacrimal sac empties tears into nasal cavity Lacrimal gland is visible through the conjunctiva when the upper lid is everted. Lacrimal puncta are visible as tiny red dots on the medial margin of each eyelid. Tears: - supply oxygen and nutrients to cornea (avascular) - are antibacterial (contain antibodies and lysozyme) - lubricate and bathe the conjunctiva

Extraocular Eye Muscles Superior rectus rotates eye up and slightly medially Inferior rectus rotates eye down and slightly medially Medial rectus rotates eye medially Four rectus muscles originate from a common tendinous ring, the annuar ring. The trochlea is a fibrocartilagenous ring through which the superior oblique muscle runs. Superior and inferior oblique muscles are needed since the superior and inferior rectus muscles approach the eye from a posteromedial direction and their action turns the eye medially. The obliques balance this medial pull and allow the eye to be directly elevated or depressed. Extrinsic muscles of the eye have a high axon:muscle fiber ratio (8-12 muscle cells per motor unit; some as few as two or three).

Extrinsic Eye Muscles Lateral rectus rotates eye laterally Superior oblique rolls eye, rotates eye down and laterally Inferior oblique rolls eye, rotates eye up and laterally Diplopia = double vision. Inability to coordinate both eyes to look at same object. Stabismus = crossed-eyes. Which cranial nerves innervate each of the muscles in the diagram above? LR6SO4AO3

Extraocular Eye Muscles & their CN Which cranial nerves innervate each of the muscles in the diagram above? LR6SO4AO3

Structure of the Eye - Overview Figure from: Martini, Fundamentals of Anatomy & Physiology, Pearson Education, 2004 Three layers (tunics) of the eye: - Outer fibrous tunic - Sclera and cornea - Middle vascular tunic (uvea) – Iris, ciliary body, and choroid - Inner neural tunic - Retina

Outer (Fibrous) Tunic Cornea anterior portion transparent light transmission light refraction well innervated avascular Sclera posterior portion opaque protection support attachment site for extrinsic eye muscles Cornea is covered by epithelial sheets on both surfaces; outside = str. Squamous epithelium that protects from abrasion and merges with the ocular conjunctiva (cells for renewal come from this layer), inside is a simple squamous epithelium that contains sodium pumps that maintain the clarity of the cornea by keeping water content low. Cornea is well supplied with nerve endings (mostly pain fibers). Touching of cornea causes reflexive blinking (corneal reflex; afferent V, efferent VII). Cornea has great capacity for repair. Transverse section, superior view

Aqueous Humor fluid in anterior cavity of eye secreted by epithelium on inner surface of the ciliary processes provides nutrients maintains shape of anterior portion of eye leaves cavity through canal of Schlemm (scleral venous sinus) Nomral intraocular pressure is about 16 mm Hg, maintained by constant production and drainage of aqueous humor. Aq humor supplies nutrients and oxygen to the lens and cornea and to some cells of the retina, and carries away metabolic wastes. Pressure build up in the eye compresses the retina and optic nerve (glaucoma). Late signs include halos around light and blurred vision. Normally about 1-2 ul / min of aqueous humor is secreted, the same amount being drained via the scleral venous sinus

Lens Loss of lens transparency = cataracts transparent, avascular biconvex lies behind iris largely composed of lens fibers enclosed by thin elastic capsule held in place by suspensory ligaments of ciliary body focuses visual image on retina (Crystallins) Loss of lens transparency = cataracts

SEM of Lens Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

Cataracts

Accommodation changing of lens shape to view objects nearby Far vision (emmetropia) (20 ft. or greater) The far point of vision is the distance beyond which no change in lens shape is needed for focusing. This is about 20 ft. for the normal (emmetropic) eye. Closer than 20 ft, the lens must round up (accommodate) in order to focus light acutely on the retina (and the pupils constrict and the eyeballs converge). This close vision reflex appears to be triggered by blurring of the retinal image. Pupils constrict to prevent the most divergent light rays from entering the eye since they would pass through the edges of the lens and not be focused properly. Convergence occurs in order to keep the object’s light rays falling on the foveae. Myopia = nearsightedness, i.e., NEAR vision is UN impaired. Hyperopia = farsightedness, i.e., far vision is unimpaired. Presbyopia is the loss of the ability to accommodate with age Near vision

Middle (Vascular) Tunic = Uvea 1. Iris anterior portion pigmented CT controls light intensity 2. Ciliary body anterior portion pigmented holds lens muscles reshape lens for focusing aqueous humor Blood vessels of uvea provide nutrition to all eye tunics. Iris is most anterior portion of the uvea. Made up of two smooth muscle layers, one circular and one radial. Most babies irises are slate gray or blue because their iris pigment is not yet developed. 3. Choroid coat provides blood supply pigments absorb extra light This layer contains the intrinsic muscles of the eye - Regulate the amount of light entering the eye - Regulate the shape of the lens

Iris How would viewing near objects affect pupil size? composed of connective tissue and smooth muscle pupil is hole in iris dim light stimulates (sympathetic) radial muscles and pupil dilates bright light stimulates (parasympathetic, CN III) circular muscles and pupil constricts mydriasis Cycloplegia = paralysis of the ciliary smooth muscles miosis How would viewing near objects affect pupil size?

Ciliary Body forms internal ring around front of eye ciliary processes – radiating folds ciliary muscles – contract and relax to move lens

Inner (Neural) Tunic retina contains visual receptors continuous with optic nerve ends just behind margin of the ciliary body composed of several layers macula lutea – yellowish spot in retina surrounds fovea fovea centralis – center of macula lutea; produces sharpest vision; only cones optic disc – blind spot; contains no visual receptors vitreous humor – thick gel that holds retina flat against choroid coat Visual axis Retina has an inner neural layer and an outer pigmented layer (single layer thick, covers ciliary body and posterior face of the iris). The pigmented cells also function as phagocytes and store vitamin A needed for photoreceptors. Anterior neural layer extends to posterior margin of the ciliary body (junction is called the ora serrata rentinae). The two layers are closely approximated but are not fused. Neural retina receives blood from: 1) Outer third from vessels in the choroid, 2) Inner two-thirds by central artery and central vein of retina that enter and leave via through the center of the optic nerve. Retinal detachment allows vitreous humor to seep in between the retinal layers and deprives the outer neural retinal layer of blood supply from the choroid. Transverse section, superior view

Optic Disc (Blind Spot) Figure from: Martini, Fundamentals of Anatomy & Physiology, Benjamin Cummings, 2004 Fovea centralis is about 0.4 mm in diameter (about the size of the head of a pin).

Layers of Retina receptor cells, bipolar cells, and ganglion cells - provide pathway for impulses triggered by photoreceptors to reach the optic nerve horizontal cells and amacrine cells – modify impulses Some ganglion cells absorb light directly for circadian rhythms and control of pupillary diameter.

Visual Receptors Rods long, thin projections contain light sensitive pigment called rhodopsin hundred times more sensitive to light than cones provide vision in dim light produce colorless vision produce outlines of object view off-center at night Cones short, blunt projections contain light sensitive pigments called erythrolabe, chlorolabe, and cyanolabe (photopsins) provide vision in bright light produce sharp images produce color vision About 130 million rods and about 6.5 million cones – and only about 1.2 million nerve fibers in the optic nerve. Dark adaptation by the rods takes approximately 30 minutes. This adaptation can be destroyed by white light in just milliseconds

Rods and Cones Storage site of vitamin A Figure from: Martini, Fundamentals of Anatomy & Physiology, Benjamin Cummings, 2004 Discs are continually replaced; old used discs are shed and phagocytized by pigmented epithelium. Rods participate in converging pathways (as many as 100 rods may converge on a single ganglion cell); cones have a one-to-one relationship with bipolar and ganglion cells. Rhodopsin maximally absorbs in green range in rods. Cones also use retinal and opsin, but the cone opsins differ both from the opsin of the rods and from one another. Blue cones absorp maximally at around 420 nm, green cones at about 530 nm, and red cones at or close to 560 nm. Nyctalopia = night blindness. Retinal is chemically related to vitamin A and is made from it.

Mechanism of Light Transduction Ganglion cells generate about 20-30 action potentials per second even in dark. Phosphodiesterase (PDE) in the retina is a form of the enzyme that Viagra inhibits. Could this cause visual problems? Figure from: Marieb, Human Anatomy & Physiology, Pearson Education, 2004

Rods and Cones – Neural Connections Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007 Rods have a high degree of spatial summation since about 600 rods impinge upon 1 biplolar cell, and many bipolar cells impinge upon a single ganglion cell. This allows an additive effect of rod stimulation so a weak intensity stimulus can be detected. Tradeoff is that rods cannot resolve finely detailed images. (in fovea centralis) Many rods synapse with a single bipolar cell giving poor resolution (acuity). In fovea, 1 cone synapses with one bipolar cell. Therefore, the resolution (acuity) is better using cones and they produce sharp vision.

Image Information Figure from: Martini, Fundamentals of Anatomy & Physiology, Benjamin Cummings, 2004

Stereoscopic Vision Because the pupils and fovea are 6-7 cm apart, each eye receives a slightly different image. This allows the slightly different pictures to be integrated by the brain resulting in stereoscopic vision and depth perception.

Visual Pathway The right side of the brain receives input from the left half of the visual field The left side of the brain receives input from the right half of the visual field Figure from: Martini, Fundamentals of Anatomy & Physiology, Benjamin Cummings, 2004 Hemidecussation – half the fibers of the optic nerve cross over to the other side of the brain.

Please Take the Short Quiz Remember to take the brief quiz now to see if you’ve gotten the major concepts from the video. You can always go back and review the video as many times as you like – and can retake the quiz, as well.