Chapter 17: The Special Senses

Question 1

describe the structure of the olfactory receptors and other cells involved in olfaction.

Answer 1

olfaction – sense of smell

olfactory epithelium – region that occupies the superior part of the nasal cavity, covering the inferior surface of the cribriform plate and extending along the superior nasal concha. Area of 5cm^2

Consists of three kinds of cells: olfactory receptor cells, supporting cells, and basal cells.

1. olfactory receptor cells
   
   • Each olfactory receptor cell is a bipolar neuron with an exposed knob-shaped dendrite and an axon projecting through the cribriform plate that ends in the olfactory bulb. Found only in the superior portion of the nasal cavity.

– the first-order neurons of the olfactory pathway.

• olfactory cilia (haires) – extend from the dendrite of an olfactory receptor cell
  
  • non-motile
  • The sites of olfactory transduction (the conversion of stimulus energy into a graded potential in a sensory receptor) Actually bind odarants and begin signal transduction
  • Within the plasma membranes of the olfactory cilia are olfactory receptors that detect inhaled chemicals.
• Odorants – chemicals that have an odor that bind to and stimulate the olfactory receptors in the olfactory cilia
  
  • Olfactory receptor cells respond to the chemical stimulation of an odorant molecule by producing a generator potential, thus initiating the olfactory response.

1. supporting cells

– columnar epithelial cells of the mucous membrane lining the nose.

• Provide physical support, nourishment, and electrical insulation for the olfactory receptor cells and help detoxify chemicals that come in contact with the olfactory epithelium.
  1. basal cells

– stem cells located between the bases of supporting cells.

• Continually undergo cell division to produce new olfactory receptor cells, which live for only a month or so before being replaced.
• Remarkable, considering olfactory receptor cells are neurons and mature neurons are not generally replaced.

olfactory glands or Bowman’s glands – mucus producing glands within the connective tissue that supports the olfactory epithelium.

• Mucus is carried to the surface of the epithelium by ducts
• The secretions moisten the surface of the olfactory epithelium and dissolves odorants so that transduction can occur.

Innervation – both supporting cells of the nasal epithelium and olfactory glands are innervated by parasympathetic neurons within branches of the facial (VII) nerve, which can be stimulated by certain chemicals.

• Impulses in these nerves in turn stimulate the lacrimal glands in the eyes and nasal mucous glands, resulting in tears and runny nose after inhaling pepper or vapors such as household ammonia.

physiology of olfaction

– genetic evidence suggests the existence of hundreds of primary odors.

• Our ability to recognize about 10,000 odors probably depends on patterns of activity in the brain that arise from activation of many different combos of olfactory receptor cells.
• In response to odorant, olfactory receptor cells create a generator potential (depolarization) which triggers one or more nerve impulses.

This process, called olfactory transduction occurs as follows:

1. Binding of odorant to olfactory receptor protein in an olfactory cilium stimulates a membrane protein called a G protein.
2. The G protein in turn activates the enzyme adenylate cyclase to produce a substance called cyclic adenosine monophosphate (cAMP)
3. The cAMP opens a sodium ion channel that allows Na+ to enter the cytosol, which causes a depolarizing generator potential to form in the membrane of the olfactory receptor cell.
4. If the depolarization reaches threshold, an action potential is generated along the axon of the olfactory receptor cell.

odor threshold and adaptation

– like all special senses, olfaction has a low threshold. Only a few molecules of certain substances need to be present in the air to be perceived as an odor. Ex. Methyl mercaptan is a chemical that smells like rotten cabbage/egg and can be detected in concentrations as low as 1/25 billionth of a mg per mL of air. Added to natural cooking gas to enable people to detect gas leaks.

olfactory pathway

• olfactory (I) nerves – Left and Right. Collectively formed by about 40 bundles of slender, unmyelinated axons of olfactory receptor cells that extend through about 20 olfactory foramina in the cribriform plate of the ethmoid bone.
• olfactory bulbs – paired masses of gray matter where the olfactory nerves terminate in the brain. Located below the frontal lobes of the cerebrum and lateral to the crista galli of the ethmoid bone. Within the olfactory bulbs, the axon terminals of olfactory receptor cells form synapses with the dendrites and cell bodies of olfactory bulb neurons in the olfactory pathway.
• olfactory tracts – bundle of axons that extends from the olfactory bulb posteriorly to olfactory regions of the cerebral cortex. projects directly to the primary olfactory cortex and to the limbic system and hypothalamus
• primary olfactory area – in the cerebral cortex, located at the inferior and medial surface of the temporal lobe.
• Where conscious awareness of smell begins.
• Some axons of the olfactory tract project here.
• Other axons of the olfactory tract project to the limbic system and hypothalamus (these connections account for memory/emotional response to smell)
• From the primary olfactory area, pathways extend to the frontal lobe.

1. The orbitofrontal area is an important region for odor identification and discrimination.
2. People who suffer damage to this area have difficulty identifying different odors.
3. The orbitofrontal area of the RIGHT hemisphere exhibits greater activity during olfactory processing.

Question 2

identify the five primary tastes.

Answer 2

gustation or taste – the sense of taste. Is a chemical sense. Much simpler than olfaction in that only 5 primary tastes can be distinguished: sour, sweet, bitter, salty, and umami “meaty or savory.” Most flavors are a combination of the 5 primary tastes, plus accompanying olfactory and tactile sensations. Odors from food can pass upward from mouth to nasal cavity and stimulate olfaction. Olfaction is much more sensitive than taste and a given concentration of a food substance may stimulate the olfactory system 1000’s times more strongly than it stimulates the gustatory system.

Cold = no smell, not no taste.

taste buds – oval body consisting of 3 kinds of epithelial cells: supporting cells, gustatory receptor cells, and basal cells.

• Receptors for taste sensation are located in the taste buds.
• Most taste buds are on the tongue but some are on the soft palate, pharynx, and epiglottis.
• 10,000 taste buds in young adult, number declines with age.

supporting cells – contain microvilli and surround about 50 gustatory receptor cells in each taste bud.

gustatory receptor cells – about 50 in each taste bud, surrounded by supporting cells

• Each has a life span of about 10 days.
• At their base, the cells synapse with dendrites of the first-order neurons that form the first part of the gustatory pathway.
• The dendrites of each first-order neuron branch profusely and contact many gustatory receptor cells in several taste buds.

gustatory microvilli or gustatory hairs – project from each gustatory receptor cell to the external surface through the taste pore.

taste pore – an opening in the taste bud.

basal cells – stem cells found at the periphery of the taste bud near the connective tissue layer.

• Basal cells produce supporting cells which then develop into gustatory receptor cells.

papilla (plural is papillae) – elevations on the tongue in which taste buds are found. They increase the surface area and provide a rough texture to the upper surface of tongue. 3 types of papillae contain taste buds: vallate, fungiform, and foliate.

1. vallate or circumvallate papillae – about 12 very large, circular papillae that form an inverted V-shaped row at the back of the tongue Each papilla houses 100-300 taste buds.
2. fungiform papillae – mushroom-shaped elevations scattered over the entire surface of the tongue Contain about 5 taste buds each
3. foliate papillae – located in small trenches on the lateral margins of the tongue.
4. Most of their taste buds degenerate in early childhood.
5. filiform papillae – on the entire tongue surface no taste buds Pointed, threadlike structures that contain tactile receptors but no taste buds. Increase friction between the tongue and food, making it easier for the tongue to move food in the oral cavity.

Question 3

explain the process of taste transduction.

Answer 3

physiology of gustation

plasma membranes of the gustatory microvilli are sites of taste transduction. Depolarizing receptor potential that stimulates exocytosis from synaptic vesicles in gustatory receptor cell/. Liberated neurotransmitter triggers graded potential that produce nerve impulse in 1st order sensory neurons.

Tastants – chemicals that stimulate gustatory receptor cells. Once dissolved in saliva, can make contact with the plasma membranes of the gustatory microvilli, which are the sites of taste transduction.

• The result is a receptor potential that stimulates exocytosis of synaptic vesicles from the gustatory receptor cell.
• In turn, the liberated neurotransmitter molecules trigger nerve impulses in the first-order sensory neurons that synapse with gustatory cells.

The receptor potential arises differently for different tastants.

1. The sodium ions in salty food enter gustatory receptor cells via the Na+ channels in the plasma membrane.
2. The accumulation of Na+ inside the cell causes depolarization, which leads to release of neurotransmitter.
3. The hydrogen ions in sour tastants may flow into gustatory receptor cells via H+ channels. They also influence opening and closing of other ion channels. Again, the result is depolarization and liberation of neurotransmitter.
4. Other tastants responsible for stimulating sweet, bitter, and umami tastes do not themselves enter gustatory receptor cells. Rather, they bind to receptors on the plasma membrane that are linked to G proteins. The G proteins activate several different chemicals knows as second messengers inside the gustatory receptor cell. Different second messengers cause depolarization in different ways but the result is the same, neurotransmitter release.

Different food tastes – all tastants cause release of neurotransmitter, so how do foods taste different?

• Answer: probably the patterns of nerve impulses in groups of first-order taste neurons that synapse with the gustatory receptor cells. Different tastes arise from activation of different groups of taste neurons. Individual gustatory receptor cells may react more strongly to one type of tastants than others.

taste thresholds and adaptation – threshold for taste varies for each of the primary tastes.

• Bitter threshold is lowest.
• Poisonous substances are often bitter, ? protective function?
• Sour threshold is somewhat higher.
• Salty and sweet threshold are similar and higher than bitter or sour.
• Adaptation to a specific taste can occur in 1-5 minutes of continuous stimulation. occurs rapidly

Taste adaptation is due to changes that occur in the taste receptors, in olfactory receptors, and in neurons of the gustatory pathway in the CNS.

gustatory pathway – 3 cranial nerves carry axons of the first-order gustatory neurons that innervate for taste buds.

a. The facial (VII) nerve – serves taste buds in the anterior two thirds of the tongue
b. The glossopharyngeal (IX) nerve – serves taste buds in posterior one third of tongue
c. Vagus (X) nerve – serves taste buds in the throat and epiglottis.

Gustatory pathway:

1. from the taste buds, nerve impulses propagate along these cranial nerves to the gustatory nucleus in the medulla oblongata.
2. From the medulla, some axons carrying taste signals project to the limbic system and the hypothalamus, others project to the thalamus.
3. Taste signals that project from the thalamus to the primary gustatory area in the parietal lobe of the cerebral cortex give rise to the conscious perception of taste.

Question 4

discuss why vision is important.

Answer 4

Vision – the act of seeing. Extremely important to human survival.

• More than half the sensory receptors in the human body are located in the eyes, and a large part of the cerebral cortex is devoted to processing visual information.

Question 5

define visible light.

Answer 5

The eyes are responsible for the detection of visible light, the part of the electromagnetic spectrum with wavelengths ranging from about 400 to 700 nm.

Visible light exhibits colors: The color of visible light depends on its wavelength. For example, light that has a wavelength of 400 nm is violet, and light that has a wavelength of 700 nm is red.

An object can absorb certain wavelengths of visible light and reflect others; the object will appear the color of the wavelength that is reflected. For example, a green apple appears green because it reflects mostly green light and absorbs most other wavelengths of visible light.

An object appears white because it reflects all wavelengths of visible light. An object appears black because it absorbs all wavelengths of visible light.

Question 6

identify the accessory structures of the eye.

Answer 6

accessory structures of the eye:

eyelids or palpebrae – upper and lower. Shade eyes during sleep, protect eyes from excessive light and foreign objects, spread lubricating secretions over the eyeballs. The upper eyelid is more moveable than the lower and contains the levator palpebrae superioris muscle in its superior region.

palpebral fissure – the space between the upper and lower eyelid that exposes the eyeball. The angles of the palpebral fissure are known as:

• Lateral commissure – narrower and closer to temporal bone
• Medial commissure – broader, nearer the nasal bone

Lacrimal caruncle – small, reddish elevation within the medial commissure which contains sebaceous glands and sudoriferous glands. Eye ”sleep” comes from these glands.

tarsal plate – a thick fold of connective tissue that gives form and support to the eyelids.

tarsal glands or Meibomian glands – embedded in each tarsal plate; a row of modified sebaceous glands that secrete a fluid that helps keep the eyelids from adhering to each other.

• Chalazion – a small bump on the eyelid caused infection of the tarsal glands that produces a tumor or cyst.

Conjunctiva – a thin, protective mucous membrane composed of nonkeratinized stratified squamous epithelium with numerous goblet cells that is supported by areolar connective tissue.

Palpabral conjunctiva – lines the inner aspect of the eyelids

Bulbar conjunctiva – passes from the eyelids onto the surface of the eyeball, where it covers the sclera but not the cornea

• Vascular over the sclera. Dilation and congestion of blood vessels of the bulbar conjunctiva due to local irritation or infection is the cause of blood-shot eyes.

Eyelashes – project from the border of each eye lid

Sebaceous ciliary glands – sebaceous glands at the base of the hair follicles of the eyelashes. Infection of these glands causes a painful, pus-filled swelling called a sty.

Eyebrows – transversely above the upper eyelids

• Both eyelashes and eyebrows help protect eyeballs from foreign objects, perspiration, and sun rays.

lacrimal apparatus – a group of structures that produces and drains lacrimal fluid in a process called lacrimation.

• lacrimal fluid or tears – tears

1. The fluid protects, cleans, lubricates, and moistens the eyeball.
2. After being secreted from the lacrimal gland, lacrimal fluid is spread medially over the surface of the eyeball, by the blinking of the eyelids.

lacrimation – production of tears

1. Only humans show emotion by crying.
2. In response to parasympathetic stimulation, lacrimal glands produce excessive lacrimal fluid, which may spill over the edges of the eyelids and fill the nasal cavity with fluid (this is why crying causes a runny nose)

Lacrimal gland, lacrimal duct, superior or inferior lacrimal canal, lacrimal sac, nasolacrimal duct, nasal cavity (order of tears)

lacrimal glands – secretory cells, located at the superior anterolateral portion of each orbit

1. Secrete tears into 6-12 excretory ducts that open onto the surface of the conjunctiva.
2. Each about almond sized.
3. Supplied by parasympathetic fibers of the facial (VII) nerves.
4. Each gland produces about mL of lacrimal fluid per day.

lacrimal ducts – 6-12 ducts that empty tears onto the surface of the conjunctiva of the upper eyelid.

lacrimal puncta – two small openings at medial aspect of eye

lacrimal canaliculi – a duct, one on each eyelid, beginning at the lacrimal punctum at the medial margin of an eyelid and conveying tears medially into the nasolacrimal sac.

lacrimal sac – the superior expanded portion of the nasolacrimal duct that receives tears from a lacrimal canaliculus

nasolacrimal duct – a canal that transports the lacrimal secretion from the nasolacrimal sac into the nose.

Lysozyme – a protective bactericidal enzyme found in tears, saliva, and perspiration.

extrinsic eye muscles (six muscles) – extend from the walls of the bony orbit to the sclera of the eye and are surrounded in the orbit by periorbital fat.

• periorbital fat – surrounds the muscles of the eyes in the orbits

The muscles can move the eye in almost any direction:

1. Superior rectus
2. Inferior rectus
3. Lateral rectus
4. Medial rectus
   
   • One each on top, both sides, and bottom. Common origin: tendinous ring (attached to orbit around optic foramen). Insertions are top, sides, and bottom of eyeball.
5. Superior oblique
6. Inferior oblique -Stabilize the eyeballs. Preserve rotational stability of the eyeball.
7. These eye muscles are supplied by the oculomotor (III), trochlear (IV), or abducens (VI) nerves.
8. In general, the motor units in these muscles are small.
9. Some motor neurons serve only 2 or 3 muscle fibers
10. Provides smooth, precise, and rapid movement of the eyes.
11. Neural circuits in the brain stem and cerebellum coordinate and synchronize the movements of the eyes.

Question 7

identify the components of the eye.

Answer 7

anatomy of the eyeball

wall of the eyeball (three major layers): fibrous tunic, vascular tunic, retina

1) fibrous tunic – the superficial layer (coat) of the eyeball, consists of the posterior sclera and anterior cornea.

• Cornea – a transparent fibrous coat that covers the colored iris.

1. Curved, helps focus light onto the retina
2. Outer surface consists of nonkeratinized stratified squamous epithelium.
3. Middle coat consists of collagen fibers and fibroblasts
4. Inner surface is simple squamous epithelium.

Sclera - the white coat of dense connective fibrous tissue that forms the superficial protective covering over the eyeball except over the cornea.

1. Made up mostly of collagen fibers and fibroblasts
2. Covers the entire eyeball except the cornea
3. Gives shape to the eyeball, makes it more rigid
4. Protects its inner parts
5. Serves as a site of attachment (insertion) for the extrinsic eye muscles.

scleral venous sinus or canal of Schlemm – at the junction of the sclera and cornea. An opening through which aqueous humor drains into.

2) vascular tunic – the middle layer of the eyeball, composed of the choroid, ciliary body, and iris. AKA uvea

• choroid – highly vascularized posterior portion of the vascular tunic. darkly pigmented structure reduces light reflection within the eyeballs

1. Lines most of the internal surface of the sclera.
2. Numerous blood vessels provide nutrients to the posterior surface of the retina.
3. Also contains melanocytes that produce melanin Causes this layer to appear dark brown in color
4. Melanin absorbs stray light rays, which prevents reflection and scattering of light within the eyeball.

• Result is image cast on retina by the cornea and lens remains clear and sharp.
• Albinos lack melanin even in eyeballs. Often have to wear sunglasses even inside because moderately bright light is perceived as bright glare due to light scattering.
• ciliary body – in the anterior portion of the vascular tunic, the choroid becomes the ciliary body.

1. Extends from the ora serrata to a point just posterior to the junction of the sclera and cornea.
2. Also contains melanocytes and appears dark brown.
3. Ciliary body consists of ciliary processes and ciliary muscle.

ora serrata – the irregular margin of the retina lying internal and slightly posterior to the junction of the choroid and ciliary body.

ciliary processes – protrusions or folds on the internal surface of the ciliary body.

• Contain blood capillaries that secrete aqueous humor.

zonular fibers (or suspensory ligaments) – extend from the ciliary processes; attach to the lens

• Consist of thin, hollow fibrils that resemble elastic connective tissue fibers.

ciliary muscle – a circular band of smooth muscle.

• Contraction or relaxation of the ciliary muscle changes the tightness of the zonular fibers, which alters the shape of the lens, adapting it for near or far vision.

Iris – the colored portion of the eyeball.

1. Shaped like a flattened donut. I love donuts!
2. Suspended between the cornea and the lens and is attached at its outer margin to the ciliary body.
3. Consists of melanocytes and circular and radial smooth muscle fibers.
4. Amount of melanin in the iris determines eye colour

• High melanin = brown to black
• Moderate melanin = green
• Low melanin = blue

1. One principal function of the iris is to regulate the amount of light entering the eyeball through the pupil

• Pupil – the hole in the center of the iris; the area through which light enters the posterior cavity of the eyeball. Appears black because as you look through the lens, you see the heavily pigmented back of the eye (choroid and retina). Red eye in photos: when light is directed into the pupil, the reflected light is red because of the blood vessels on the surface of the retina.
• Autonomic reflexes regulate pupil diameter in response to light levels.
• When stimulated by bright light, parasympathetic fibers of the oculomotor (III) nerve stimulate the circular muscles of the iris to contract, causing decrease in size of pupil.
• circular muscles (constrictor) or sphincter pupillae – iris muscles. Contract in bright light in response to stimulation by parasympathetic fibers of the oculomotor (III) nerve.
• radial muscles (dilator) or dilator pupillae – other iris muscles. In dim light, sympathetic neurons stimulate these muscles to contract, causing an increase in the pupil’s size.

3. Retina – the inner and third layer of the eyeball.

1. Lines the posterior three quarters of the eyeball
2. Is the beginning of the visual pathway
3. Consists of nervous tissue and a pigmented layer of epithelial cells that contact the choroid. (See iii and iv below)
4. This layer can be viewed with an ophthalmoscope that shines light into the eye and provides a magnified image of the retina and its blood vessels as well as the optic (II) nerve.
5. The retina’s surface is the only place in the body where blood vessels can be viewed directly and examined for pathological changes (ex. DM, Htn, cataracts, macular disease)

optic disc – the site where the optic (II) nerve exits the eyeball.

• AKA the blind spot; a small area of the retina containing openings through which the axons of the ganglion cells emerge as the optic (II) nerve.
• Bundled together with the optic nerve are the central retinal artery and the central retinal vein.

pigmented layer – of the retina, a sheet of melanin-containing epithelial cells located between the choroid and the neural part of the retina.

1. The melanin in the pigmented layer of the retina also helps to absorb stray light rays.
2. This layer is between the outer protective covering of the eyeball and the neural part of the eyeball that processes vision.

neural layer (3 major sublayers): multilayered outgrowth of the brain that processes visual data extensively before sending nerve impulses into axons that form the optic nerve.

The three layers are separated by 2 zones: the outer and inner synaptic layers, where synaptic contacts are made.

• II. Light passes through the ganglion and bipolar cell layers and both synaptic layers before it reaches the photoreceptor layer.

photoreceptor layer – contains photoreceptors (specialized cells) that begin the process by which light rays are converted to nerve impulses.

• Two types of photoreceptors: rods and cones.
• Rods – allow sight in dim light. Do not provide color vision so in dim light we see only black, white, and all shades of gray between.
• Cones – stimulated by brighter light.

1. Produce color vision.
2. 3 types of cones:
   
   • Blue cones – sensitive to blue light
   • Green cones – sensitive to green light
   • Red cones – sensitive to red light
3. Color vision results from the stimulation of various combinations of these 3 types of cones.
4. Most of our visual experiences are mediated by the cone system. Loss of cone system produces legal blindness. Loss of rod vision causes difficulty seeing in dim light, should not drive at night.
   
   1. Layers of bipolar and ganglion cells (which scatter light) do not cover the cones here.

From photoreceptors, info flows through the outer synaptic layer to bipolar cells and then from bipolar cells through the inner synaptic layer to ganglion cells.

Blind spot – where the optic (II) nerve exits theretina. There are no photoreceptors here. AKA

optic disc.

Macula lutea – the yellow spot in the exact center of the posterior portion of the retina, at the visual axis of the eye

Central fovea – a depression in the center of the macula lutea of the retina, containing cones only and lacking blood vessels

1. Is the area of highest visual acuity.
2. Rods are more plentiful toward the periphery of the retina. Rod vision is more sensitive than cone vision, so a faint object is seen better slightly to one side rather than directly at it.

bipolar cell layer – middle sub-layer of the neural layer

I. horizontal cells – present in the bipolar cell layer of the retina.

II. Amacrine cells – present in the bipolar cell layer of the retina.

• These two types of cells form laterally directed neural circuits that modify the signals being transmitted along the pathway from photoreceptors to bipolar cells to ganglion cells.
  
  1. Posterior to the pupil and iris

ganglion cell layer – innermost sublayer of the neural layer. Towards the inside of the eyeball.

Lens – a transparent organ constructed of proteins (crystallins)

1. Anterior to the vitreous body
2. Crystallins – arranged like layers of an onion; make up the refractive media of the lens
3. The lens is normally transparent and lacks blood vessels.
4. Enclosed by a clear connective tissue capsule and held in position by encircling zonular fibers which attach to the ciliary processes.
5. The lens helps focus images on the retina to facilitate clear vision.

interior of the eyeball – the lens divides the interior of the eyeball into two cavities: anterior cavity and vitreous chamber

1. anterior cavity – the space anterior to the lens, consists of two chambers separated by the iris:
2. anterior chamber lies between the cornea and the iris
3. posterior chamber – lies behind the iris and in front of the zonular fibers and lens
4. aqueous humor – a transparent watery fluid that nourishes the lens and cornea.

• Continuously filters out of blood capillaries in the ciliary
• processes of the ciliary body and enters the posterior chamber.
• It flows forward between the iris and the lens, through the pupil, and into the anterior chamber.
• From the anterior chamber, aqueous humor drains into the scleral venous sinus (canal of Schlemm) and then into the blood.
• Normally, aqueous humor is replaced completely about every 90 minutes.

1. vitreous chamber – the larger posterior cavity of the eyeball. A. Lies between the lens and the retina.
2. vitreous body – within the vitreous chamber.
   
   1. A transparent jelly like substance that holds the retina against the choroid, giving the retina an even surface for the reception of clear images.
3. Occupies about 80% of the eyeball.
4. Does not undergo continuous replacement: Formed during embryonic life.
5. Consists mostly of water plus collagen fibers and hyaluronic acid.
6. Also contains phagocytic cells that remove debris, keeping this part of the eye clear for unobstructed vision
7. Vitreal floaters – harmless collections of debris that cast a shadow on the retina and create an appearance of specks that dart in and out of vision. More common as you age.
8. Hyaloid canal – narrow channel that is inconspicuous in adults
   
   1. Runs through the vitreous body from the optic disc to the posterior aspect of the lens.
   2. In the fetus, it is occupied by the hyaloid artery.
9. intraocular pressure – the pressure in the eyeball
   
   1. Produced mainly by aqueous humor, partly by the vitreous body.
   2. Maintains the shape of the eyeball, prevents it from collapsing
   3. Puncture wounds to the eyeball may lead to loss of pressure which in turn can cause a detached retina and blindness.

Question 8

discuss how an image is formed by the eye.

Answer 8

image formation

refraction of light rays - bending of light rays when passing from one transparent substance to another of different density.

1. As light rays enter the eye they are refracted at the anterior and posterior surfaces of the cornea.
2. Both surfaces of the lens of the eye further refract the light rays so they come into exact focus on the retina.
3. Images focused on the retina are inverted and left-to-right reversed.
4. The brain learns to coordinate visual images with the orientations of objects.
5. 75% of refraction of light occurs at cornea
6. 25% remaining is done by the lens.
7. Lens also changes the focus to view near or distant objects
8. Objects more than 6m away have nearly parallel light rays.
9. Objects closer than 6m have divergent rays, must be refracted more to focus on retina: accommodation

Accommodation - increase in curvature of the lens for near vision

1. Parasympathetic fibers of the oculomotor (III) nerve innervate the ciliary muscle of the ciliary body and mediate the process of accommodation
2. Distant viewing = relaxed ciliary muscle of ciliary body and flatter lens stretched in all directions by taut zonular fibers.
3. Close viewing = ciliary muscle contracts, pulls ciliary process and choroid forward toward the lens, releases tension on the lens and zonular fibers, lens becomes more spherical (more convex) which increases focusing power and causes greater convergence of light rays.

near point of vision - the minimum distance from the eye that an object can be clearly focused with maximum accommodation (about 4 inches in a young adult)

Presbyopia - loss of elasticity of the eye lens due to aging with resulting inability to focus clearly on near objects.

refraction abnormalities

1. emmetropic eye - the normal eye; can sufficiently refract light rays from an object 6 m away so a clear image is focused on the eye.
2. Myopia - defect in vision in which objects can be seen distinctly only when very close to the eyes; nearsightedness
3. hypermetropia or hyperopia - farsightedness, eyeball length is short relative to the focusing power of the cornea and lens or the lens is thinner than normal, so an image converges behind the retina. Can see distant objects clearly but not close ones.

Astigmatism - an irregularity of the lens or cornea of the eye causing the image to be out of focus and producing faulty vision.

constriction of the pupil- narrowing of the diameter of the hole through which light enters the eye due to constriction of the circular muscles of the iris.

1. Automatic reflex occurs simultaneously with accommodation
2. Prevents light rays from entering the eye through the periphery of the lens.
3. Light rays entering at the periphery would not be brought into focus on the retina and would result in blurred vision.

Convergence - the medial movement of the two eyeballs so that both are directed toward a near object being viewed in order to produce a single image Occurs by the coordinated action of the extrinsic eye muscles

binocular vision - both eyes focus on only one set of objects Allows for depth perception and 3-D nature of objects

Question 9

Photoreceptors and pigments

Answer 9

Photoreceptors - rods and cones

Photopigment - a coloured protein that can absorb light and undergo structural changes that can lead to the development of a receptor potential. AKA visual pigment.

First step in visual transduction: absorption of light by photopigment

Rhodopsin - single photopigment in rods

cone photopigments - 3 different ones, one in each of the three types of cones.

1. Colour vision results from different colours of light selectively activating the different cone photopigments.
2. Photopigments have two parts: opsin, retinal

Opsin - a glycoprotein

1. In the human retina, there are 4 different opsins: 3 in the cones and one in the rods (rhodopsin)
2. Small variations in the amino acid sequences of the different opsins permit the rods and cones to absorb different colours of incoming light.

Retinal - a derivative of vitamin A

1. Vitamin A derivatives formed from carotene – good vision depends on adequate dietary intake of carotene-rich foods: carrots, spinach, broccoli, yellow squash, or vitamin A rich foods: liver.
2. Retinal is the light-absorbing part of all visual photopigments.

bleaching and regeneration of photopigment - 4 steps of photopigments responding to light:

1. In darkness, retinal has a bent shape, called Cis-retinal which fits snugly into the opsin portion of the photopigment. When a photon of light is absorbed by sicretinal, it straightens into trans-retinal, process called isomerization. Also, several unstable chemical intermediates form and disappear, chemical changes that can lead to production of a receptor potential.
2. Next, in about a minute, trans-retinal completely separates from opsin and the final products look colourless. “bleaching of photopigment”
3. An enzyme called retinal isomerase converts trans-retinal back to cis-retinal
4. Cis-retinal then binds to opsin, reforming a functional photopigment “regeneration”

light adaptation - visual system adjusts in seconds to the brighter environment by decreasing its sensitivity.

dark adaptation - visual system’s sensitivity increases slowly over several minutes.

release of neurotransmitter by photoreceptor - the neurotransmitter in rods and perhaps cones is the amino acid glutamate (glutamic acid)

1. Glutamate is an inhibitory neurotransmitter.
2. In darkness, ligand-gated Na channels are open. The ligand is cGMP – cyclic guanosine
3. The partial depolarization during darkness triggers continual release of glutamate.
4. Glutamate triggers IPSPs that hyperpolarize the bipolar cells and prevent them from transmitting signals to the ganglion cells.
5. When light strikes the retina, enzymes that break down cGMP are activated. Some cGMP gated Na+ channels close, Na+ inflow decreases, membrane potential becomes more negative, approaching 70mV.hyperpolarization
6. This decreases the release of glutamate. Therefore light excites the bipolar cells that synapse with rods by turning off the release of the inhibitory neurotransmitter glutamate.

colour blindness - an inherited inability to distinguish between certain colours

1. Results from the absence or deficiency of one of the three types of cones.
2. Most common is red-green colour blindness in which red cones or green cones are missing and a person cannot distinguish between red and green.

night blindness or nyctalopia - inability to see well at low light levels

• prolonged vitamin A deficiency and resulting below-normal amount of rhodopsin

visual pathway - visual signals in retina undergo considerable processing at synapses among the various types of neurons in the retina (horizontal cells, bipolar cells, amacrine cells) and then the axons of retinal ganglion cells provide output from the retina to the brain, exiting the eyeball as the optic (II) nerve.

processing of visual input in the retina - within the neural layer of the retina, certain features of visual input are enhanced while other features may be discarded. Input from many cells may either converge on a smaller number of postsynaptic neurons (convergence) or diverge to a larger number (divergence). Overall, convergence predominates: there are 1 million ganglion cells, but 126 million photoreceptors in the human eye

Question 10

describe the processing of visual signals in the retina.

Answer 10

brain pathway and visual fields

1. optic chiasm - a crossing point of the two branches of the optic (II) nerve, anterior to the pituitary gland.
2. optic tract - a bundle of axons that carry nerve impulses from the retina of the eye between the optic chiasm and the thalamus. Most terminate in the lateral geniculate nucleus of the thalamus. Others terminate in the superior colliculi which control the extrinsic eye muscles and others in the pretectal nuclei which control pupillary and accommodation reflexes.
3. optic radiations - axons of neurons in the lateral geniculate nucleus of the thalamus which project to the primary visual areas in the occipital lobes of the cerebral cortex.
4. visual field - everything that can be seen by one eye
5. binocular visual field - large region where the visual fields of the two eyes overlap.
6. nasal and temporal half - two regions of the visual field of each eye.
7. Nasal – central half – light rays fall on the temporal half of the retina
8. Temporal – peripheral half. - light rays fall on nasal half of the retina

Question 11

describe the anatomy of the structures in the three main regions of the ear.

Answer 11

hearing and equilibrium

1. anatomy of the ear - 3 main regions: external, middle, internal

outer ear, consisting of:

1. auricle or pinna - the projecting part of the external ear, composed of elastic cartilage and covered by skin.
2. external auditory canal – a curved tube in the temporal bone that leads to the middle ear. AKA meatus
3. ceruminous glands - specialized sudoriferous (sweat) gland in the external auditory canal/meatus that secretes cerumen.
4. Cerumen - earwax.
5. eardrum or tympanic membrane - thin, semitransparent partition of fibrous connective tissue between the external auditory meatus and the middle ear.

• Covered by epidermis and lined by simple cuboidal epithelium.
• Between the epithelial layers is connective tissue composed of collagen, elastic fibers, and fibroblasts.

middle ear - small, air-filled cavity in the petrous portion of the temporal bone lined by epithelium. Separated from the external ear by the tympanic membrane and internal ear by a thin bony partition with two small openings.

1. auditory ossicles - the three smallest bones in the body, connected by synovial joints, attached to middle ear by ligaments
2. Malleus – “hammer” - attaches to the internal surface of the tympanic membrane. Head of malleus articulates with the body of the incus
3. Incus – “anvil” - middle bone in the series. Articulates with the head of the stapes.
4. Stapes – “stirrup” - the base or footplate of the stapes fits into the oval window.
5. oval window - a small, membrane covered opening between the middle ear and inner ear into which the footplate of the stapes fits
6. round window - a small opening between the middle and internal ear, directly inferior to the oval window, covered by the secondary tympanic membrane.
7. auditory tube or eustachian tube - the tube that connects the middle ear with the nose and nasopharynx region of the throat. AKA pharyngotympanic tube.

• Consists of both bone and elastic cartilage
• Connects the middle ear with the nasopharynx
• Normally closed at the medial end. During swallowing and yawning, it opens allowing air to enter or leave the middle ear until the pressure in the middle ear equals the atmospheric pressure.

internal ear or inner ear or labyrinth - lying inside the temporal bone, containing the organs of hearing and balance.

1. bony labyrinth - a series of cavities within the petrous portion of the temporal bone forming the vestibule, cochlear, and semicircular canals of the inner ear
2. Perilymph - the fluid contained between the bony and membranous labyrinths of the inner ear
3. membranous labyrinth - the part of the labyrinth of the internal ear that is located inside the bony labyrinth and separated from it by the perilymph; made up of the semicircular ducts, the saccule and utricle, and the cochlear duct.
4. Endolymph - the fluid within the membranous labyrinth of the internal ear.
5. Vestibule - the oval central portion of the bony labyrinth.
6. Utricle - the larger of the two divisions of the membranous labyrinth located inside the vestibule of the inner ear, containing a receptor organ for static equilibrium.
7. Saccule - the inferior and smaller of the two chambers in the membranous labyrinth inside the vestibule of the internal ear containing a receptor organ for static equilibrium.
8. semicircular canals - three bony channels (anterior, posterior, lateral), filled with perilymph, in which lie the membranous semicircular canals filled with endolymph. Contain receptors for equilibrium.
9. Ampulla - a saclike dilation at one end of each semicircular canal.
10. semicircular ducts - the portions of the membranous labyrinth that lie inside the bony semicircular canals. Connect with the utricle of the vestibule.
11. Cochlea - anterior to the vestibule, a winding, cone-shaped tube forming a portion of the inner ear and containing the spiral organ (organ of Corti)
12. cochlear duct or scala media - continuation of the membranous labyrinth into the cochlea, filled with endolymph.
13. scala vestibuli - the channel above the cochlear duct in the bony cochlea, filled with perilymph, ends at the oval window
14. scala tympani - inferior spiral shaped channel of the bony cochlea, filled with perilymph, ends at the round window.
15. Helicotrema - an opening at the apex of the cochlea where the scala vestibuli and the scala tympani are not completely separated.
16. vestibular membrane - the membrane that separates the cochlear duct from the scala vestibuli.
17. basilar membrane - separates the cochlear duct from the scala tympani and on which the spiral organ rests.
18. spiral organ or organ of Corti - the organ of hearing, consisting of supporting cells and hair cells that rest on the basilar membrane and extend into the endolymph of the cochlear duct.
19. hair cells - about 16,000, which are the receptors for hearing. Two groups of hair cells: inner hair and outer hair cells

• Inner hair cells – arranged in a single row
• Outer hair cells – arranged in three rows

1. Stereocilia - at the apical tip of each hair cell; 40-80 that extend into the endolymph of the cochlear duct Long, hair-like microvilli arranged in several rows of graded height.
2. tectorial membrane - a flexible gelatinous membrane covering the hair cells of the spiral organ

Question 12

list the major events in the physiology of hearing.

Answer 12

nature of sound waves - alternating high and low pressure regions travelling in the same direction through some medium (such as air).

• Originate from a vibrating object in much the same way that ripples arise and travel on the surface of water.

Frequency – the frequency of a sound vibration is its pitch.

Pitch – the higher the frequency, the higher the pitch. Audible range extends from 20- 20,000 Hz. 500-5000 Hz heard most acutely by human ear. Sounds of speech primarily involve 100-3000 Hz.

Intensity – the larger the size or amplitude of the vibration, the louder the sound.

decibels (dB) - measurement of sound intensity. Increase of one decibel represents a 10- fold increase in sound intensity.

• The hearing threshold – point at which an average young adult can just distinguish sound from silence is defined as 0 dB at 1000 Hz.
• Sound becomes uncomfortable to a normal ear at about 120 dB and painful at 140 dB.

loud sounds and hair cell damage - exposure to loud sounds damages hair cells of the cochlea. Prolonged noise exposure causes hearing loss.

physiology of hearing - the following events are involved in hearing:

1. Auricle directs sound waves into the external auditory canal
2. Sound waves strike the tympanic membrane, the alternating high and low pressure waves in the air cause the tympanic membrane to vibrate; slowly in response to low frequency sounds and more rapidly in response to high frequency sounds.
3. The malleus is connected to the tympanic membrane, so it vibrates along with it. This vibration is transmitted to the incus and the stapes
4. As the stapes moves back and forth, its oval shaped foot plate vibrates in the oval window. These vibrations are 20x more vigorous than the tympanic membrane because the ear is efficient.
5. The movement of the stapes at the oval window sets up fluid pressure waves in the perilymph of the cochlea. As the oval window bulges inward, it pushes on the perilymph of the scala vestibuli
6. Pressure waves are transmitted from the scala vestibuli to the scala tympani and eventually to the round window, causing it to bulge outward into the middle ear.
7. The pressure waves travel through the perilymph of the scala vestibuli, then the vestibular membrane, and then move into the endolymph inside the cochlear duct.
8. The pressure waves in the endolymph cause the basilar membrane to vibrate, which moves the hair cells of the spiral organ against the tectorial membrane. This leads to bending of the stereocilia and ultimately to the generation of nerve impulses in first order neurons in cochlear nerve fibers
9. Sound waves of various frequencies cause certain regions of the basilar membrane to vibrate more intensely than other regions. Each segment of the basilar membrane is “tuned” for a particular pitch.

• Membrane is narrower and stiffer at the base of the cochlea, so high frequency sounds induce maximal vibrations in this region
• Membrane is wider and more flexible toward the apex of the cochlea, so low frequency sounds cause maximal vibration of the basilar membrane there.

Transduction channel – a mechanically gated ion channel in the stereocilium of the ear. A tip link protein connects the tip of each stereocilium to the transduction channel in its taller stereocilium neighbour. As the stereocilia bend, the channels open and allow K+ to enter the hair cell cytosol, producing a depolarizing receptor potential. This opens voltage gated Ca2+ channels in the base of the hair cell. The inflow of Ca2+ ions triggers exocytosis of synaptic vesicles containing a neurotransmitter (probably glutamate). As more neurotransmitter is released, the frequency of nerve impulses in the first-order sensory neurons that synapse with the base of the hair cell increases. Bending of the stereocilia in the opposite direction closes the transduction channels, allows hyperpolarization to occur, and reduces neurotransmitter release from hair cells, decreasing the frequency of nerve impulses in the sensory neurons.

auditory pathway

I. Bending of the stereocilia of the hair cells of the spiral organ causes neurotransmitter release which generates nerve impulses in the sensory neurons that innervate the hair cells.

II. The cell bodies of the sensory neurons are located in the spiral ganglia.

III. Nerve impulses pass along the axons of these neurons, which form the cochlear branch of the vestibulocochlear(VIII) nerve. structures carries action potentials generated by sound transduction

IV. These axons synapse with neurons in the cochlear nuclei in the medulla oblongata on the same side.

V. Some of the axons of the cochlear nuclei decussate in the medulla, ascend in a tract called the lateral lemniscus on the opposite side, and terminate in the inferior colliculus in the mid brain.

VI. Other axons from the cochlear nuclei end in the superior olivary nucleus in the pons on each side. Slight difference in the timing of nerve impulses arriving from the two ears at the superior olivary nuclei allow us to locate the source of a sound.

VII. Axons from the superior olivary nuclei also ascend in the lateral lemniscus tracts on both sides and end in the interior colliculi. From each inferior colliculus, nerve impulses are conveyed to the medial geniculate nucleus in the thalamus and finally to the primary auditory area of the cerebral cortex in the temporal lobe of the cerebrum.

VIII. Because many auditory axons decussate in the medulla while others remain on the same side, the right and left primary auditory areas receive nerve impulses from both ears.

Question 13

explain the function of each of the receptor organs for equilibrium.

Answer 13

physiology of equilibrium - two types of equilibrium: static and dynamic.

static equilibrium

• the maintenance of posture in response to changes in orientation of the body, mainly the head, relative to the ground.
• Body movements that stimulate the receptors for static equilibrium include tilting the head and linear acceleration or deceleration, such as when the body is being moved in an elevator or a car that is speeding up or slowing down.

dynamic equilibrium - the maintenance of body position, mainly the head, in response to sudden movements such as rotation.

vestibular apparatus - collective term for the organs of equilibrium, which includes the saccule, utricle, and semicircular ducts.

otolithic organs - the saccule and utricle.

saccule and utricle - both contain a macula.

Macula - two small, thickened regions on the walls of the utricle and saccule (macula of the saccule containe otoliths (ear stones) that contains receptors for static equilibrium. Perpendicular to each other.

1. Provide sensory info on the position of the head in space, essential for maintaining appropriate posture and balance
2. Also detect linear acceleration and deceleration.
3. Consist of two kinds of cells: hair cells and supporting cells

hair cells - the sensory receptors

1. Have 40-80 stereocilia (actually microvilli) on their surface of graduated height plus one kinocilium, a conventional cilium anchored firmly to its basal body and extending beyond the longest stereocilium.
2. As in the cochlea, the stereocilia are connected by tip links.
3. Collectively, the stereocilia and kinocilium are called a hair bundle.

• supporting cells - scattered among the hair cells
• Columnar
• Probably secrete otolithic membrane:

otolithic membrane - thick gelatinous glycoprotein layer located directly over hair cells of the macula in the saccule and utricle of the internal ear.

1. Sits on top of the macula.
2. If you tilt head forward, the otolithic membrane (along with otoliths) is pulled by gravity. Slides “downhill” over the hair cells in the direction of the tilt, bending the hair bundles. Alternately, sitting in a car that suddenly jerks = otolithic membrane lags behind the head movement, pulling on hair bundles and bending them in the opposite direction.
3. Bending of hair bundles in one direction or another causes transduction channels to open and close, causing depolarization and hyperpolarization, releasing neurotransmitter at a faster or slower rate.
4. The hair cells synapse with first order sensory neurons in the vestibular branch of the vestibulocochlear (VIII) nerve which fire impulses at a slow or rapid pace depending on the amount of neurotransmitter present.
5. Motor neurons also synapse with the hair cells and sensory neurons and regulate the sensitivity of hair cells and sensory neurons.

Otoliths - layer of dense calcium carbonate crystals that extends over the entire surface of the otolithic membrane.

semicircular ducts - three ducts, function in dynamic equilibrium.

1. Lie at right angles to one another in three planes: the two vertical ducts are the anterior and posterior semicircular ducts. The horizontal one is the lateral semicircular ducts.
2. This positioning permits detection of rotational acceleration or deceleration.

Ampulla - the dilated portion of each duct. Contains a small elevation called the crista.

Crista - a small elevation in the ampulla of each semicircular duct that contains receptors for dynamic equilibrium Each crista contains a group of hair cells and supporting cells.

Cupula - a mass of gelatinous material covering the hair cells of a crista

• Is a sensory receptor in the ampulla of a semicircular canal, stimulated when the head moves. When you move your head, the attached semicircular ducts and hair cells move with it. The endolymph within the ampulla is not attached and lags behind. As the moving hair cells drag along the stationary endolymph, the hair bundles bend. Bending of the hair bundles produces receptor potentials. In turn, the receptor potentials lead to nerve impulses that pass along the vestibular branch of the vestibulocochlear (VIII) nerve.