Chapter 15 Special Senses

Question 1

Special Senses

Answer 1

Vision, taste, smell, hearing and equilibrium
have distinct receptor cells localized in the head
housed in complex sensory organs (eye, ear) or in distinct epithelial structures (taste buds and olfactory epithelium)

Question 2

Eye

Answer 2

Eye – houses structures associated with vision
• vision = dominant sense in humans and contains 70% of all sensory receptors in body
• most of eye enclosed and protected by a cushion of fat and walls of the bony orbit

Question 3

Eyes

Accessory structures protect anterior visible portion of eye

Answer 3

Accessory structures protect anterior visible portion of eye

Eyebrows
Eyelids (palpebrae)
Conjunctiva
Lacrimal apparatus

Question 4

Accessory structures protect anterior visible portion of eye

Answer 4

Eyebrows – shades eyes from sunlight and prevent perspiration from reaching eye

Question 5

Accessory structures protect anterior visible portion of eye

Eyelids

Answer 5

Eyelids (palpebrae) – skin covering tarsal plates (connective tissue) anchored to orbicularis oculi and levator palpebrae superioris

protect eye from foreign objects
activated reflexively to cause blinking every 3-7
spreads secretions across eyeball surface keeping eyes moist
**lacrimal caruncle (in medial corner of palpebral fissure) – contains sebaceous and sweat glands
**eyelashes (with nerve endings) and **tarsal glands (modified sebaceous glands – oily secretions that prevent lids from sticking together)

Question 6

Accessory structures protect anterior visible portion of eye

Conjunctiva

Answer 6

Conjunctiva = very thin transparent **mucous membrane – blood vessels easily seen
lines eyelids  (palpebral conjunctiva) and covers anterior surface of eyeball (bulbar conjunctiva) except for the cornea
produces lubricating mucous that prevents eyes from drying out
conjunctivitis = inflammation – bacterial or viral infection that is highly contagious

Question 7

Accessory structures protect anterior visible portion of eye

Lacrimal Apparatus

Answer 7

Lacrimal apparatus = lacrimal gland and ducts that drain secretions into nasal cavity

lacrimal gland – continually releases a dilute saline solution (tears)
also contains mucus, antibodies and **lysozyme – enzyme that destroys bacteria
cleanses, protects, moistens and lubricates

Question 8

Extrinsic eye muscles

Answer 8

Six extrinsic eye muscles – control movement of each eyeball

**Diplopia = double vision – condition that occurs when movements of two eyes are not perfectly coordinated
Paralysis, weakness of certain extrinsic muscles, or neurological diseases

**Strabismus = congenital weakness of external eye muscles (affected eye rotates medially or laterally)

Question 9

Structure of Eyeball

Answer 9

• sphere-shaped wall composed of 3 layers: outer fibrous, middle vascular, inner neural layer
• anterior and posterior inner cavities filled with fluid (humors) and separated from each other by the lens of the eye

Question 10

Structure of Eyeball

Fibrous layer

Answer 10

• fibrous layer – outer layer = avascular dense regular connective tissue

Sclera = “white of eye” – bulk of fibrous layer
tough, opaque, protects and shapes eyeball and provides anchoring site for extrinsic eye muscles

Cornea = anterior transparent ”window-like” portion of fibrous layer
major part of light bending apparatus of eye
has no blood vessels and is beyond reach of immune system – only tissue that can be transplanted from one person to another with little risk of rejection
stratified squamous epithelium covers superficial surface of cornea
• protects from abrasion
• rich with nerve endings – most = pain receptors that activate blink reflex
deep simple squamous epithelium with sodium pumps help with clarity by keeping water content of cornea low

Question 11

Structure of Eyeball

Vascular layer

Answer 11

Vascular layer (uvea) – middle layer that contains choroid, ciliary body and iris

Choroid = blood vessel rich, and melanocytes result in dark brown membrane – posterior 5/6 of layer
nourish all eye layers and dark colour helps absorb light preventing it from scattering and reflecting within eyeo

Ciliary body = thickened anterior ring of choroid that encircles the lens
mainly interlacing smooth muscle bundles (ciliary muscles) that control the shape of the lens
folded cilliary processes secrete fluid that fills the cavity of the anterior segment
ciliary zonule = suspensory ligaments that extend from the ciliary body to the lens

Iris = pigmented and most anterior portion of vascular layer
two smooth muscle layers that act reflexively to control amount of light entering the eye through the **pupil = round central opening

Question 12

Structure of Eyeball

Inner neural layer = retina

Answer 12

Inner neural layer = retina

Outer pigmented layer = single layer of cells next to choroid – absorbs light, store Vitamin A

Neural portion composed of three main cells types (outer to inner): **photoreceptors, bipolar cells, and ganglion cells
**horizontal and amacrine cells – participate in visual processing

Question 13

Retina
Neural portion
photoreceptors

Answer 13

**Photoreceptors (quarter billion) – transduce light (photon) energy into change in membrane potential

**Rods = most numerous mainly in periphery of retina – very sensitive to light
• dim light and peripheral vision receptors
• do not provide sharp images or colour vision

**Macula lutea - small area where light is focused onto back of eye – mostly contains cones

**Cones – function in bright light
• **fovea centralis = center focus point and only contains cones
• provide high-resolution (detailed) and colour vision

Question 14

Retina
Neural portion
Bipolar Cells

Answer 14

Bipolar cells – stimulated by glutamate (excitatory neurotransmitter) released from rod and cones

Question 15

Retina
Neural portion
Ganglion cells

Answer 15

Ganglion cells – stimulated by bipolar cells – **where true action potentials are generated
axons come together at the **optic disc = the blind spot (contains no photo receptors); and leave the eye as the optic nerve

Question 16

Retinal detachment

Answer 16

Blood vessels enter and exit with optic nerve = only place where small blood vessels visible in living person
vascularization of retina makes it susceptible to **retinal detachment – pigmented and neural layers separate
can cause permanent blindness as neural tissues deprived of nutrients

Question 17

Structure of Eyeball

Anterior segment

Answer 17

Anterior segment – contains aqueous humor – fluid similar to plasma
forms and drains continuously
**glaucoma – occurs when drainage of aqueous humor is blocked
pressure builds - can compress retina and optic nerve eventually resulting in blindness

Question 18

Structure of Eyeball

Posterior segment

Answer 18

Posterior segment – contains vitreous humor – clear gel
Supports posterior surface of lens and holds neural layer of retina firmly against pigmented layer
helps counteract pulling force of extrinsic eye muscles

Question 19

Structure of Eyeball

Lens

Answer 19

Lens = biconvex, transparent, flexible structure
can change shape to focus light onto retina
lens cuboidal epithelium found on anterior
differentiate into lens fibers that form bulk of lens
Contains precisely folded proteins called crystallins
Continuously added – lens enlarges throughout life – become denser, more convex and less elastic

**Cataract = clouding of lens – most result from age-related hardening or thickening

Question 20

Light and Optics

Answer 20

light = electromagnetic radiation that travels in waves
• visible light = 400-700 nm and travels in straight lines and with constant speed in a given medium
•**photon = packet of energy associated with visible light
• blocked by non-transparent object

•**refraction = bending of light when meets a different medium at an oblique angle:
**visual spectrum = separation of visible light into component waves due to differential bending when passing through a prism produces colour bands
**focal point = convergence of light to a single point when light rays are bent by passing through a lens
the more convex that lens – the more the rays are bent
image formed by convex lens is inverted, and reversed from left to right

Question 21

Light and Optics

Focusing light onto retina

Answer 21

Focusing light onto retina
Light passes from air in eye moves through convex cornea (bends light) aqueous humor, lens, vitreous humor, and through entire neural layer of retina to reach photoreceptors

**Far point of vision = distance beyond which no change in lens shape needed for focusing
eyes “at rest” – best adapted for distant vision
eyes directed straight ahead (or to one side) to the same degree
ciliary muscles relaxed and lens flat
**emmetropic (normal eye) – far point = 6 m (20 feet)

**accommodation = process that increases refractory power of lens – for focusing objects closer than 6 m
ciliary muscles contract (increased parasympathetic output)
lens bulges
**near point of vision = closest point which can be clearly focused upon
• young emmetropic eye = 10 cm (4 inches)
• loss of lens elasticity with age – **presbyopia = gradual loss of accommodation with age (~50 yrs)

**accommodation pupillary reflex – constrict s the pupil (contracts sphincter muscle of the iris through parasympathetic activity)
enhances accommodation by preventing divergent rays from entering eye
**convergence of eyeballs – medial rotation of eyeballs by contraction of medial rectus muscle
• controlled by somatic motor fibers of oculomotor nerve (III)

Question 22

Light and Optics
Focusing light onto retina
Eye ball shape influences focusing ability

Answer 22

Eye ball shape influences focusing ability

**myopia (nearsightedness) – eyeball too long so focal point occurs in front of retina
• distant objects blurry but can accommodate for near objects
• corrected by concave lens

**hyperopia (farsightedness) – eyeball too short so focal point occurs behind retina
• can see distant objects through continuous accommodation but nearby objects require too much accommodation to bring into proper focus
• corrected by concave lens

**Astigmatism = unequal curvature in different parts of cornea or lens leading to blurry, warped images
• corrected by specially ground lens

Question 23

Phototransduction

Answer 23

Phototransduction = process by which light energy is converted to change in membrane potential
• begins when visual pigment within photoreceptors capture photon of light
• photoreceptors = modified neurons

Question 24

Phototransduction
photoreceptors
Outer segment

Answer 24

Outer segment (tip) = receptive ends

Rods = slender and rod-shaped – contain only one kind of photopigment = **rhodopsin
• perception of light from rods results in grey tones

Cones = conically-shaped – contains one of three different photopigments that are excited by different waves lengths of light in visual spectrum
• allows colour vision
• **colour blindness = X-linked congenital lack of one or more cone pigments – most commonly resulting in red-green colour blindness
easily damaged and constantly renewed
folded plasma membrane increases surface area and contains discs embedded with photopigments

**photopigments – protein (opsin) containing light-absorbing chemical (retinal) that changes shape as they absorb light
• retinal made from Vitamin A (retinol) and has different isomers (3-D shapes)
• 11-cis-retinal = bent form that is bound to opsin
• when absorbs light energy – becomes “activated” and undergoes series conformational changes ending as **all-trans-retinal that **detaches from protein = bleaching of pigment
• enzymes slowly re-convert all-trans-retinal to 11-cis-retinal that recombines with protein

**cGMP gated Na+ channels – cGMP when internally bound to Na+ channel – open Na+ channel and Na+ enters cell

**transducin = membrane **G-protein that is activated by ** “light-activated” photopigment (Fig. 15.17)
• triggers cascade reaction
activated G protein activates the enzyme **cyclic GMP phosphodiesterase
breaks down cGMP
cGMP-gated Na+ channels close

Question 25

Phototransduction
photoreceptors
Inner segment

Answer 25

Inner segment – packed with mitochondria, lots of Na+/K+ ATPase pumps
connects to cell body and synaptic terminals
synaptic terminals contain:
• **voltage-gated Ca++ channels
• vesicles containing **glutamate = excitatory neurotransmitter