special senses

Question 1

Identify the general function of the following accessory structures of the eye: eyelids, eyebrows, and eyelashes; conjunctiva; lacrimal apparatus (lacrimal glands, lacrimal canals & ducts); extrinsic eye muscles.

Answer 1

Eyebrows
- Help shade the eyes from sunlight and prevent perspiration trickling down the forehead from reaching the eyes

Eyelids
- Protect the eye anteriorly from foreign objects by blinking

Eyelashes
- Blinking reflex triggered by anything that touches the eyelashes (root hair plexuses)

Conjunctiva
- Produce a lubricating mucus that prevents the eye from drying out

Lacrimal apparatus

• Lacrimal glands - continually releases a dilute saline solution (lacrimal solution aka tears)
• Lacrimal canals - tears enter here after traveling from lacrimal gland down across the eyeball
• Lacrimal duct (nasolacrimal duct?) - empties tears into the nasal cavity

Extrinsic eye muscles
- Control movement of each eyeball - allow eyes to follow a moving object, help maintain shape of eyeball, and hold it in orbit

Question 2

What type of tissue comprises the conjunctiva? Distinguish between the palpebral conjunctiva and bulbar conjunctiva. Is the cornea covered by conjunctiva?

Answer 2

Transparent mucous membrane
Palpebral conjunctiva - lines the eyelids
Bulbar conjunctiva - folds back over the anterior surface of the eyeball - DOES NOT cover the cornea, only the white of the eye

Question 3

Name the extrinsic eye muscles, describe their innervation, and tell what movement each produces.

Answer 3

Superior rectus muscle

• Elevates eye and turns it medially
• Innervated by Oculomotor nerve (CN III)

Inferior rectus muscle

• Depresses eye and turns it medially
• Innervated by Oculomotor nerve (CN III)

Medial rectus muscle

• Moves eye medially
• Innervated by Oculomotor nerve (CN III)

Lateral rectus muscle

• Moves eye laterally
• Innervated by Abducens nerve (CN VI)

Superior oblique muscle

• Elevates eye and turns it laterally
• Innervated by Trochlear nerve (CN IV)

Inferior oblique muscle

• Depresses eye and turns it laterally
• Innervated by oculomotor nerve (CN III)

Question 4

Name the three layers of the wall of the eye from outermost to innermost.

Answer 4

Fibrous layer, vascular layer, inner layer (sensory layer)

Question 5

Describe the general structure and function of each of the layers named above (#4) for the wall of the eye.

Answer 5

Fibrous layer - dense avascular CT, 2 parts, protects and shapes eyeball and provides anchoring sites for extrinsic eye muscles (sclera), lets light enter eye (cornea)

Vascular layer - pigmented layer, 3 regions, nourishes eye and helps absorb light (choroid), helps control lens shape (ciliary body), contains pupil and changes size of pupil (iris)

Inner layer/sensory layer/retina - 2 layers, transduces light energy (photoreceptors), involved in processing responses to light

Question 6

Identify and describe the 2 parts of the fibrous layer.

Answer 6

Sclera

• White and opaque, seen anteriorly as “white of the eye”
• Protects and shapes the eyeball
• Provides anchoring site for extrinsic eye muscles
• Pierced posteriorly by optic nerve, continuous with dura mater of brain

Cornea

• Bulges anteriorly from its junction with sclera
• Clear, lets light enter, major part of light bending apparatus
• External stratified squamous ET protects cornea from abrasion
• Deep simple squamous ET lines inner surface of cornea and has active Na pumps that maintain clarity of cornea by keeping its water content low
• Well supplied with nerve endings (most are pain receptors), extraordinary repair and regeneration capacity, avascular so beyond immune system

Question 7

Identify and describe the three parts of the vascular layer of the eye.

Answer 7

Choroid

• Blood vessel rich, blood vessels nourish all eye layers
• Brown pigment helps absorb light - prevents light from scattering and reflecting within the eye (would cause visual confusion)
• Has a posterior opening where optic nerve leaves eye

Ciliary body

• Thickened ring of tissue that encircles lens
• Consists of interlacing smooth muscle bundles (ciliary muscles) that control lens shape
• Ciliary processes - secrete fluid that fills the cavity of the anterior segment
• Ciliary zonule/suspensory ligament - extends from ciliary processes to lens and holds lens in upright position

Iris

• Has round central opening (pupil) that allows light to enter the eye
• Has 2 smooth muscle layers, muscle fibers allow it to vary pupil size - constriction and dilation

Question 8

What are the primary functions of the choroid?

Answer 8

Has many blood vessels that nourish all eye layers

Pigmented brown, which helps absorb light and prevent it from scattering and reflecting within the eye, which would cause visual confusion

Question 9

Describe the structural relationship between the ciliary body, ciliary processes, ciliary zonule (aka suspensory ligament), and lens of the eye. How do these structures work together in accommodation for near vision?

Answer 9

Ciliary body is a thickened ring of tissue that encircles the lens. Ciliary body is made up of ciliary muscles. Near the lens is the ciliary processes. The ciliary zonule/suspensory ligament extends from the ciliary processes to the lens

Accommodation of near vision:
Ciliary muscles contract → pull ciliary body anteriorly toward the pupil and inward → releases tension in ciliary zonules so the lens is no longer stretched → lens recoils and bulges → provides the shorter focal length needed to foucs the image of a close object on the retina

Question 10

Describe the structure of the iris. How does the iris modify the amount of light that enters the posterior segment of the eye?

Answer 10

• Made up of two smooth muscle layers with bunches of sticky elastic fibers that congeal into a random pattern before birth
• In close vision and bright light, the sphincter pupillae (circular muscles) contracts → pupil constricts → less light enters
• In distant vision and dim light, the dilator pupillae (radial muscles) contracts → pupil dilates → more light enters

Question 11

Which muscle layer of the iris is under parasympathetic control? sympathetic control?

Answer 11

Parasympathetic control - sphincter pupillae muscle

Sympathetic control - dilator pupillae muscle

Question 12

Identify and locate (with respect to each other and to the eyeball as a whole) the two layers of the retina.

Answer 12

Pigmented layer - next to choroid, extends anteriorly to cover the ciliary body and the posterior face of the iris, outer layer of retina

Neural layer - extends anteriorly to the posterior margin of the ciliary body, inner layer of retina

Question 13

What are the primary functions of the pigmented layer with respect to clear vision?

Answer 13

• Composed of pigment cells that absorb light and prevent it from scattering in the eye (like choroid)
• Act as phagocytes participating in photoreceptor cell renewal
• Store vitamin A needed by photoreceptor cells

Question 14

Name the three distinct layers of retinal neurons from outermost to innermost.

Answer 14

Photoreceptors, bipolar cells, ganglion cells

Question 15

Which of the cell layers (#14) transduces visible light energy? which cells generate an action potential?

Answer 15

Transduces visible light energy - photoreceptors

Generate an AP - ganglion cells

Question 16

Name and describe the specific functions of the two types of photoreceptors.

Answer 16

Rods

• Dim light and peripheral vision receptors
• Do not provide sharp images or color vision
• More numerous and far more sensitive to light than cones

Cones

• Vision receptors for bright light
• Provide high-resolution color vision

Question 17

What region of the retina produces the sharpest vision? Explain the two reasons why is acuity so high in this region?

Answer 17

Fovea centralis, inside macula lutea

1. Retinal structure next to the vitreous humor are displaced to sides - allows light to pass almost directly to the photoreceptors rather than through several retinal layers → enhances visual acuity greatly
2. Contains only cones, which provide high-resolution color vision

Question 18

The optic disc is referred to as the “blind spot” because it is a region of the retina that does not contain photoreceptors? What is located in the optic disc?

Answer 18

• The optic disc is where the optic nerve exits the eye, no photoreceptors or sclera
• Central artery and central vein of the retina enter and leave the eye through the center of the optic nerve - vessels radiate outwards from the optic disk

Question 19

Where is aqueous humor located and what is its function?

Answer 19

Location: anterior segment

Functions

• Maintains constant intraocular pressure of about 16 mm Hg to help support eyeball internally - does so by forming and draining continually at the same rate
• Supplies nutrients and oxygen to the lens and cornea and to some cells of the retina
• Carries away metabolic wastes

Question 20

Where is vitreous humor located and what is its function?

Answer 20

Location: in the posterior segment

Functions

• Transmits light
• Supports the posterior surface of the lens and holds the neural layer of the retina firmly against the pigmented layer
• Contributes to intraocular pressure, helping to counteract the pulling force of the extrinsic eye muscles

Question 21

Identify in order the structures or materials through which light passes as it moves from the air to the retina.

Answer 21

Air → cornea → aqueous humor → lens → vitreous humor → retina

Question 22

What two eye structures account for virtually all of the eye’s refractive ability?

Answer 22

Cornea and lens

Question 23

What is the functional significance of the far point of vision?

Answer 23

The distance beyond which no change in lens shape (accommodation) is needed for focusing

Question 24

Describe how the lens shape changes during accommodation for near vision. What is the role of the ciliary muscle and ciliary zonule in the lens shape change during accommodation for near vision? How do the ciliary muscle and ciliary zonule function to flatten the lens for far vision?

Answer 24

Near vision

• Ciliary muscles contract, pulling ciliary body anteriorly toward the pupil and inward → releases tension in ciliary zonule
• Therefore, the lens is no longer stretched
• Lens will then recoil and bulge

Far vision

• Sphincterlike ciliary muscles are completely relaxed
• Tension in the ciliary zonule stretches the lens flat

Question 25

What is the functional significance of the near point of vision? Describe the typical age- related changes in the near point of vision.

Answer 25

• Closest point on which we can clearly focus, represents the maximum bulge the lens can achieve
• The near point of vision = 10 cm/4 in from the eye ← in young adults. Near point is even closer in children and gradually recedes in age because the lens decreases in elasticity as we age until the lens may become nonaccommodating in the elderly (presbyopia)

Question 26

Describe the abnormality in refraction which characterizes each of the following conditions: myopia; hyperopia; astigmatism.

Answer 26

Myopia

• Nearsightedness - can see close objects but distant objects are blurred
• Occurs when distant objects focus in front of the retina rather than on it, happens typically bc eyeball is too long
• Can correct with concave lenses

Hyperopia

• Farsightedness - can see distant objects but not close objects as well
• Occurs when parallel light rays from distant objects focus behind the retina
• Ciliary muscles contract almost continuously to increase the light bending power of the lens, which moves the focal point forward onto the retina. However, diverging light rays from nearby objects focus so far behind the retina that the lens cannot bring the focal point onto the retina even at its full refractory power. Happens usually because eyeball is too short.
• Can correct with convex lenses

Astigmatism

• Unequal curvatures in different parts of the cornea or lens
• Leads to blurry vision
• Specially cylindrically ground lenses or laser procedures are used to correct

Question 27

Describe how the accommodation pupillary reflex and eyeball convergence contribute to clear and focused near vision.

Answer 27

Accommodation pupillary reflex (constriction of the pupil done by sphincter pupillae muscles of the iris)
- Prevents the most divergent light rays from entering the eye - helps prevent blurry vision that would be caused by those most divergent light rays

Eyeball convergence (medial rotation of the eyeballs by the medial rectus muscles)
- Direct the eyeball toward the object being viewed

Question 28

Reading for long periods of time causes eye strain (or eye fatigue). There are 3 mechanisms that contribute to the fatigue. What are they?

Answer 28

Accommodation of the lenses, pupillary constriction, and eyeball convergence

Question 29

Identify how you would assess each of the following and why:
visual acuity; observation of the retina, arteries and veins; accommodation; color blindness;
convergence

Answer 29

Visual acuity
- Snellen chart, E chart, near vision chart

Observation of the retina, arteries, and veins

• Use ophthalmoscope
• Retina - finding a vessel and tracing it to the optic disk
• Arteries - light red, smaller
• Veins - dark red, larger

Accommodation
- Pupillary constriction bilaterally and convergence

Color blindness
- Snellen chart

Convergence
- Use the pen and bring it close to nose, ask patient to look at it

Question 30

Describe the general structure of photoreceptor cells.

Answer 30

• Modified neurons, resemble tall epithelial cells turned upside down
• “Tips” immersed in the pigmented layer of the retina
• “Tips” are actually receptive regions of rods and cones, called outer segments
• Connecting cilium joins the outer segment to the inner segment which connects to the cell body which is continuous with an inner fiber bearing synaptic terminals
• Rods outer segment is slender and rod shaped
• Cones outer segment is short and conical

Question 31

In which part of the photoreceptor cell are the photopigments located?

Answer 31

Outer segment, embedded in areas of the plasma membrane that form discs

Question 32

Compare and contrast (structure and function) rods and cones.

Answer 32

Structure

Rods

• Inner segment connects to the cell body by the outer fiber
• Discs are discontinuous - stacked inside a cylinder of plasma membrane like pennies in a coin wrapper
• Participate in converging pathways - as many as 100 rods may ultimately feed into each ganglion cell, resulting in rod effects being summated and considered collectively → fuzzy indistinct vision
• Mostly in peripheral retina

Cones

• Inner segment joins the cell body directly
• Disc membranes are continuous with the plasma membrane, so interiors of cone discs are continuous with the extracellular space
• Each cone in the fovea has a straight-through pathway via its own bipolar cell to a ganglion cell → detailed high-acuity views of very small areas of the visual field
• Mostly in central retina

Function

Rods

• Very sensitive, function in dim light
• Best suited for night vision and peripheral vision
• Contain a single kind of visual pigment → inputs perceived only in gray tones

Cones

• Need bright light for activation
• React more rapidly
• Have one of three different pigments → inputs perceived in color

Question 33

Name and describe the two parts of all photopigments.

Answer 33

• Retinal - light absorbing molecule, made from vitamin A, can assume many 3D forms (isomers) (11-cis-retinal when bound to opsin and all-trans-retinal when absorb a photon of light)
• Opsins - proteins, different kinds
• Retinal and opsin combine to form four types of visual pigments. Depending on the type of opsin to which the retinal is bound, the retinal absorbs different wavelengths of the visible spectrum → makes blue cones, red cones, and green cones that can be activated one at a time (seeing just red, green, or blue) or can have more than one stimulated at a time to different degrees (how we see intermediate colors like orange, yellow, purple, etc)

Question 34

Which of the parts described above (#33) is the light-absorbing part of the photopigment? which part differs from one type of photopigment to another?

Answer 34

• Light absorbing part of the photopigment = retinal

- Differs from one type of photopigment to another = opsin

Question 35

Describe the cyclical pathway of photopigment bleaching and regeneration.

Answer 35

Rhodopsin = visual pigment of rods, deep purple pigment, arranged in a single layer in the membranes of the disks in rods’ outer segments

1. Pigment synthesis, cis- trans- conversion of retinal
   - In the dark, vitamin A is oxidized (and isomerized) to the 11-cis-retinal form
   - So, in the dark retinal is in the cis-form
   - Cis-retinal can complex with opsin and become rhodopsin
   - Rhodopsin forms and accumulates in the dark
   - When rhodopsin absorbs light, the cis-retinal straightens into trans-retinal form
2. Pigment bleaching
   - In the trans- form, trans-retinal cannot complex with opsin, so they separate
   - The complete separation of trans-retinal and opsin takes about 1 minute → becomes colorless - “bleaching of the pigment”
3. Pigment regeneration
   - Once the trans-retinal separates from the opsin, retinal isomerase (← enzyme) within pigmented epithelium reconvert trans-retinal to cis-retinal
   - Cis-retinal heads to the photoreceptor cells’ outer segments
   - Rhodopsin is regenerated when 11-cis-retinal is rejoined to opsin
   - Continual flow of trans-retinal into pigment epithelium (to retinal isomerase where it can be converted into cis-retinal) and cis-retinal back to outer segment (to form rhodopsin)
   - This step takes some number of minutes but it’s okay because there will be other cis-retinals in outer segment

Question 36

How does the release of activated opsin during this cycle (#35) alter the rate of neurotransmitter release by photoreceptor cells?

Answer 36

• Light-activated rhodopsin activates a G protein called transducin → transducin activates PDE (phosphodiesterase) (enzyme that breaks down cGMP) → cGMP-gated channels close → cation influx from the cGMP-gated channels stops → photoreceptor hyperpolarizes → voltage-gated Ca2+ channels close in synaptic terminals → no neurotransmitter is released
• In the dark, neurotransmitter is continuously being released
• In the light, no neurotransmitter is released

Question 37

What is the “dark current”? In darkness, what is the membrane potential of photoreceptor cells? What happens to its membrane potential when a photoreceptor cell is exposed to light?

Answer 37

• The “dark current” is the continuous influx of Na+ and Ca2+ to the photoreceptor cell in the dark, therefore depolarizing it
• Membrane potential in the dark = -40 mV
• Membrane potential when photoreceptor cell is exposed to light = -70 mV (hyperpolarization)

Question 38

Describe the path of light from the point where it enters the eye until the photoreceptors are stimulated, noting each structure through which the light passes.

Answer 38

Air → cornea → aqueous humor → lens → vitreous humor → through the entire neural layer of the retina (ganglion cells → bipolar cells → photoreceptors)

Question 39

Explain the basic processing of visual input that occurs in the retina.

Answer 39

Info has to cross two synapses before it heads to the CNS

• Photoreceptor cell to bipolar cell
• Bipolar cell to ganglion cell

Can either inhibit or excite cells at these synapses

Question 40

Continue to follow the path of the visual stimulus as it is transduced to an electrical impulse by the photoreceptors of the retina, and carried from the eye to the brain

Answer 40

Optic nerves (axons of retinal ganglion cells)

• At optic chiasma, fibers from medial aspect of each eye cross over to opposite side then continue via optic tracts
• Most fibers in paired optic tracts pass to lateral geniculate nuclei of thalamus
• Axons of thalamic neurons project through internal capsule (form optic radiation of fibers in cerebral white matter) to primary visual cortex of occipital lobes
• Some fibers in optic tract send collaterals to superior colliculi of midbrain (visual reflex centers) and to suprachiasmatic nucleus of hypothalamus (uses input to help set daily biorhythms)
• After processing in primary visual cortex, visual info flows anteriorly to association areas

Question 41

What occurs in the optic chiasm (aka optic chiasma)?

Answer 41

Fibers from the medial aspect of each eye cross over to the opposite side and then continue on via the optic tracts

• Contains fibers from the lateral (temporal) aspect of the eye on the same side and fibers from the medial (nasal) aspect of the opposite eye.
• Carries all the information from the same half of the visual field.