Olfaction The sense of smell

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The sense of smell

  • Occurs in response to odors

  • Odors stimulate sensory receptors located in the extreme superior region of the nasal cavity

  • This is called the olfactory region

  • Olfactory Epithelium: specialized epithelium

  • The olfactory region of each of the two nasal passages in humans is a small area of about 2.5 square centimeters containing in total approximately 50 million primary sensory receptor cells.

  • About 10 million olfactory neurons are present in the olf. Epi.

  • Axons of these bipolar neurons project through numerous small foramina of the bony cribriform plate to the olfactory bulbs.

  • Olfactory tracts project from the bulbs to the cerebral cortex


Olfactory Region

  • Olfactory vesicles: The ends of dendrites of the olf. neurons modified into bulbous enlargements

  • Olfactory hairs: cilia on olfactory vesicles

  • Odorants: Airborne molecules bind to transmembrane odorant receptors

  • Odorants enter the nasal cavity and are dissolved in the fluid covering the olfactory epithelium


  • A condition in which the sense of smell is reduced or lost entirely.

  • Can be caused by traumatic head injury (e.g. a fall in which the head receives a severe blow) or a virus (a bad cold, or infection of the nasal mucosa).

  • Some people are born without a sense of smell - congenital anosmia, and some develop it as a consequence of another disorder, e.g. Alzheimer's disease.

  • “Money has no smell” Who said it?

  • There's money in smell - around $24 billion is spent on scented products per annum in the US alone - but "Money has no smell"

  • Ninety percent of what is perceived as taste is actually smell

  • Who documented it?


  • Emperor Vespasian AD 9-79

  • Dr Alan Hirsch of the Taste Treatment and Research Foundation in Chicago, quoted in MX, Melbourne, Australia, 28 Jan 2003).

The Human Eye

Structure and Function

Accessory Structures

Eyebrows, Eyelids, and Conjunctiva

  • Eyebrows – Prevent perspiration from getting in your eyes; Shades the eyes from direct sunlight.

  • EyelidsWith associated lashes prevent foreign objects from injuring the eye; Lubricates the eye by spreading tears over surface.

  • ConjunctivaThin, transparent mucous membrane that covers the insides of the eyelids and the anterior surface of the eye.

Lacrimal Apparatus

  • Lacrimal Gland-Produces tears which pass over the surface of the eye

  • Lacrimal Canaliculi-Small ducts located in the medial corner of the eye; collects excess tears

  • Lacrimal Sac-An enlargement of the nasolacriminal duct

  • Nasolacriminal duct-Opens into the nasal cavity

  • Tears-Lubricate the eye and cleanse it. They also contain an enzyme that helps combat eye infections

Lacrimal Apparatus

Extrinsic Eye Muscles

  • Six skeletal muscles which accomplish movement of each eyeball

  • Four muscles run from their origins in the posterior orbit of the eye to attach to the four quadrants of the eyeball: the superior, inferior, medial, and lateral rectus muscles.

  • Two muscles, the superior and inferior oblique muscles, are at an angle to the long axis of the eyeball.

Anatomy of the Eye

Also, Identify all parts of the eye in your textbook!


Anterior Compartment

  • Compartment between the lens and cornea

  • Filled with aqueous humor (watery fluid)

  • Maintains pressure in eye

  • Bends light

  • Provides nutrients to the inner surface of the eye

  • If flow of aqueous humor through the venous ring is blocked, the pressure in the eye increases, resulting in a disease called glaucoma

Posterior Compartment

  • Contains transparent jelly-like substance called vitreous humor

  • Maintains pressure within the eye and holds the lens and retina in place

  • Bends light

Anatomy of human eye

Fibrous Tunic

The Sclera:

-The firm, white, outer connective tissue

-Makes up the posterior 5/6ths of the eye

-Maintains shape of eye

protects internal structure

-attachment for extrinsic eye muscles

The Cornea:

-The transparent anterior 1/6th of the eye

-Permits light to enter the eye

-Refracts entering light

The Retina or Nervous Tunic

Fovea centralis-

  • a small pit in the center of the macula lutea

  • highest concentration of cones

  • light is normally focused here when the eye is looking directly at an object

Macula lutea-

  • a small yellow spot near the center of the posterior retina

Optic disc

-a white spot just medial to the macula lutea

-large blood vessels enter the eye here

-the optic nerve exits the eye

-contains no photoreceptor cells

-doesn’t respond to light

-forms the blind spot

The Blind Spot: see location on the diagram in your text

Functions of the Complete Eye

Light Refraction

  • Concave lens: the light rays diverge as they pass through the lens

  • Convex lens: the light rays converge as they pass through the lens

  • Focal Point: Light rays cross each other at this point


  • What happens when you are driving a car?

  • You look down at the speedometer and then back at the road!

  • What changes have to occur to accommodate the near vision and the distant vision?


Accommodation for near vision:

  • Ciliary muscles are contracted

  • the suspensory ligaments reduce their tension

  • more convergent (spherical) shape of the lens

  • more convex surface of the lens

  • greater refraction

Accomodation for far vision:

  • Ciliary muscles are relaxed

  • tension of the suspensory ligaments increases

  • flatter, more concave lens

  • allows distant vision


The Neurology of Sight

Optic nerve-Nerve that leaves the eye and exits the orbit through the optic foramen to enter the cranial vault

Optic chiasma-Where the optic nerves converge laterally to connect to each other medially

Optic tract-The route of the ganglionic axons

Visual cortex-Part of the brain in the optical lobe which receives projections from ganglionic axons

Visual field-The image seen by each eye (also known as depth perception)

How We See?

  • Focusing- Light rays that enter the eye must come to the retina. The cornea and lens bend the rays toward one another and travel to the vitreous humor and strike the retina.

  • Depth Perception- The lens system is like a camera and reverses images. Optic nerves from the two eyes meet at the base of the brain at a point called the optic chiasma. The brain then processes the image.

Eye Disorders


Test for colorblindness online: http://www.toledo-bend.com/colorblind/Ishihara.html

Disorders of the eye’s outer parts

  • Sty- An eye lash follicle becomes inflamed. A sty looks like a pimple on the edge of the eyelid. Sties may be treated by applying a moist, warm cloth to the area and by using antibiotics prescribed by a physician. In some cases, pus from a sty may have to be drained by minor surgery.

  • Conjuctivitis- Conjuctivitis is an inflammation of the conjunctiva. This is commonly called pinkeye. This is often contagious and treated with eye drops.

Eye Disorders

Nearsightedness and Farsightedness

  • Nearsightedness occurs if light rays from distant objects meet before they reach the retina.

  • Farsightedness occurs if light rays from distant objects reach the retina before they meet.

  • If you're nearsighted, your prescription is a negative number

  • The higher the numeral, the stronger your lenses will be

  • If your glasses or contact lens prescription begins with plus numbers, like +2.50, you are farsighted


  • Astigmatism results if all the light rays from an object do not come together at one point in the eye.

  • Always get your eyes checked so you have a clear view to life!

  • What is Macular Degeneration?

  • Macular degeneration is a condition that affects the macula, the central part of the retina

  • The macula is responsible for central vision and the ability to see detail

Cow Eye Dissection: DO NOT MISS THIS ONE!!

The Ear

The External Ear

  • The fleshy part of the external ear is the auricle

  • The auricle opens into the external auditory meatus, which is a passageway that leads to the eardrum

  • The meatus is lined with hairs and ceruminous glands

  • The tympanic membrane, or eardrum, is a thin membrane that separates the external ear from the middle ear

Hearing and Balance

  • Ear: the external, middle, and inner ear

  • The external and middle ears: hearing only

  • The inner ear functions in both hearing and balance

  • The external ear: from the outside of the head to the eardrum

  • The middle ear: an air filled chamber medial to the eardrum

  • The inner ear: a set of fluid filled chambers medial to the middle ear

The Middle Ear

  • There are two openings, the oval opening and the round window, that connect the middle ear to the inner ear

  • The middle ear has three auditory ossicles: malleus, incus, and stapes

  • There are two unblocked openings in the middle ear, the auditory, or eustachian tube, and one that opens into the temporal bone of the ear

The Inner Ear

  • Consists of interconnecting tunnels and chambers within the temporal bone called the bone labyrinth

  • In the bone labyrinth there is a smaller set of tunnels called the membranous labyrinth

  • Two fluids in the inner ear: endolymph and perilymph

  • The endolymph is the fluid inside the membranous labyrinth of the inner ear

Can you identify all the parts of an ear?

See diagram in your textbook!

The Inner Ear


  • Cochlea: shaped like a snail shell and contains a bony core shaped like a screw

  • Cochlea involved in hearing and balance both

  • Y-shaped membranous complex divides the cochlea into 3 portions

  • Spiral lamina: the threads of this screw; base of the Y

  • Vestibular membrane: one branch of Y

  • Basilar membrane: other branch of Y

  • Spiral Organ: specialized structure inside the cochlear duct

  • Contains specialized sensory cells called hair cells

  • Hair cells have microvilli on their surfaces

  • Microvilli are embedded in the gelatinous shelf called tectorial membrane


  • Static Equilibrium: associated with the vestibule

  • Involved in evaluating the position of the head relative to gravity

  • Kinetic Equilibrium: associated with the semicircular canals

  • Involved in evaluating the change in rate of head movements

  • Vestibule: the utricle and the saccule

  • Each chamber contains specialized patches of epithelium called maculae (surrounded by endolymph)

  • Maculae, like the spiral organ, contain hair cells

  • Hair cells have microvilli on their tips

  • Microvilli of the hair are embedded in a gelatinous mass with otoliths

  • Otoliths: particles composed of protein and calcium carbonate

  • The mass moves in response to gravity, bending the microvilli and initiating the action potentials

  • These action potentials are relayed to the brain by vestibulochochlear nerve (cranial nerve VIII)

  • Semicircular canals: involved in kinetic equilibrium and placed at nearly right angles to each other

  • Pacement of semicircular canals detects movements in different directions

  • Ampulla: the base of each SCC is expanded into an ampulla

  • Crista ampullaris: within each ampulla, epithelium is specialized

  • Tectorial Membrane: Hair tips are embedded into an acellular gelatinous shelf called the tectorial membrane

Hearing and Balance Mechanism

  • Maculae, like the spiral organ have hair cells. The tips of microvilli

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