CH 15- Special Senses- Smell, taste and vision

Olfaction

• Sense of smell

• Occurs in response to odors stimulating sensory receptors in the olfactory region of the nasal cavity

• Olfactory epithelium

Olfactory epithelium

Contains cell bodies and dendrites of ~10 million olfactory neurons

Dendrites extend to the epithelial surface

• Olfactory vesicles

• Olfactory hairs

• Basal cells

Olfactory vesicles

Bulbous enlargements at the ends of dendrites

Olfactory hairs

Cilia on olfactory vesicles that are covered in thin mucous film

Basal cells

Replace olfactory cells every 2 months

Odorants

• Airborne molecules that enter into nasal cavity and dissolve in fluid covering the olfactory epithelium

• Bind to odorant receptors (chemoreceptors)

• 1000 different odorant receptor molecules

• Regulate multiple intracellular pathways involving G proteins, adenylate cyclase, and ion channels allowing for detection of ~4000 smells

• Seven primary classes

Seven primary classes

1. Camphoraceos (mothballs)

2. Musky

3. Floral

4. Pepperminty

5. Ethereal (fresh pears)

6. Pungent 

7. Putrid 

Neuronal Pathways for Olfaction

• Complex pathways involving in multiple areas of the cerebrum

• Olfactory stimuli causes perception of specific odors and emotional and autonomic responses

• Majority of neurons in olfactory cortex areas in the temporal and frontal lobes to perceive odors

  • Piriform cortex

• Some olfactory neurons project to secondary olfactory areas involved in emotional and autonomic responses

  • Include hypothalamus, hippocampus, and structures of the limbic system 

Taste

Gustation

Taste buds

Sensory structures of taste

• Small, oval structures located along the edge of papillae on the tongue, palate, lips and throat

• Taste (gustatory) cells

  • Taste hairs  

• Basal cells

• Supporting cells

Taste gustatory cells

About 50 sensory cells per taste bud

Taste hairs

• Microvilli that extend through the taste pore of the taste bud

• Replaced about every 10 days throughout life

Basal cells 

develop into new taste cells

Supporting cells

Support taste cells

Lingual Papillae

• Filiform

• Vallate

• Folliate

• Fungiform

Filiform

• Filament shaped

• Most numerous 

• No taste buds

• Give rough surface on tongue

Vallate

• Largest and least numerous (8-12)

• Form V-shaped row along the border and anterior and posterior parts of the tongue

Foliate

• Leaf shaped 

• Folds on the sides of the tongue 

• Contain most sensitive taste buds

• Numerous in children and decrease with age 

Fungiform

• Mushroom shaped 

• Scattered on the superior surface of the tonge

Tastants

• Substances that dissolve in saliva and enter taste pores and stimulate taste cells

• Have short connections that release neurotransmitters to secondary sensory neurons

• Five taste classes

Five taste classes

• Salty

• Sour

• Sweet

• Bitter 

• Umami

SALTY

• Na⁺ diffuses through Na⁺ channels on the surface of the taste cells causing depolarization

• Low sensitivity

SWEET

• Tastant (sugar) binds to G protein-couple receptor molecules on taste hairs of taste cells

• Leads to depolarization

• Low sensitivity

SOUR

• H⁺ of acids cause depolarization by three mechanisms

  • Enter the cell directly through H⁺ channels

  • H⁺  bind to ligand-gated K⁺ channels and block K⁺ from exiting the cell

  • H⁺ can open ligand-gated channels for other positive ions allowing them to enter the cell

BITTER

• Alkaloid tastants stimulate via G protein mechanism

• Highly sensitive

• Detects toxins

UMAMI

• Results from amino acids (glutamate)

• Depolarization via G protein mechanism

Influences of taste

• Texture of food

• Temperature of food

• Adaptation of taste can occur within 1-2 seconds after perception complete adaptation within 5 minutes

  • Occurs at the level of the taste bud and in the CNS

• Olfactory sensations

Neural Pathways for taste are carried by three cranial nerves

• Facial nerve (CN VII)

• Glossopharyngeal nerve (CN IX)

• Vagus nerve (CN X)

Neuronal Pathways for taste

• Axons of cranial nerves carry information to the tractus solitarius of the medulla oblongata

• Fibers from the nucleus of the tractus solitarius extend to the thalamus and decussate at the level of the midbrain

• Neurons from the thalamus project bilaterally to the taste areas in the insula of the cerebrum

Three cranial nerves that carry taste

• Facial nerve (CN VII)

  • Chorda tympani – branch of the facial nerve that transmits taste sensation from anterior 2/3 of the tongue

• Glossopharyngeal nerve (CN IX)

  • Carries taste sensation from posterior 1/3 of the tongue, vallate papillae, and superior pharynx

• Vagus nerve (CN X)

  • Carries taste sensation from the root of the tongue and epiglottis

Visual system 

• Includes eyes, accessory structures, optic nerves (CN II), and pathways 

• Eye

• Optic nerve and tracts

EYE

• Includes eyeball and lens

• Respond to light and initiate afferent action potentials

Optic nerve tracts

• Transmit action potentials from the eye to the brain

Accessory Structures of the Eye

• Important for maintenance and protection of the eyes

• Protect, lubricate, move, and aid in the function of the eye

Accessory Structures of the Eye includes:

• Eyebrows

• Eyelids

• Eyelashes conjunctiva

• Lacrimal apparatus

• Extrinsic eye muscles

EYEBROWS

• Hairs superior to the orbits

• Prevents perspiration from running into the eye

• Helps shade eye from direct sunlight

EYELIDS

• With eyelashes protect eyes from foreign objects by blinking

• Blinking helps lubricate the eye by spreading tears

• Regulates amount of light entering the eye

• Palpebral fissure – space between eyelids

• Canthi – angles where the superior and inferior eyelids meet (medial and lateral)

• Caruncle – small reddish/pink mound in the medial canthus, houses modified sebaceous and sweat glands

• Eyelashes – 2-3 rows of hairs at the free edges of the eyelids

• Ciliary glands – modified sweat glands that lubricate the eyelashes

  • Sty – inflammation of the ciliary glands

• Meibomian (tarsal) glands – sebaceous glands near the inner margins of the eyelid that secrete sebum to lubricate the lids

  • Chalazion – infection or blockage of the meibomian gland

Sty

Inflammation of the ciliary glands

Meibomian (tarsal) glands

Sebaceous glands near the inner margins of the eyelid that secrete sebum to lubricate the lids

Chalazion

Infection or blockage of the meibomian gland

Layer of the eyelid superficial to deep

Thin layer of skin

Thin layer of areolar connective tissue

Layer of skeletal muscle (orbicularis oculi and levator palpebrae superioris muscles)

•Tarsal plate – crescent shaped layer of dense connective tissue helping to maintain shape of the eye

•Palpebral conjunctiva

Accessory Structures of the Eye: Tarsal Plate

Crescent shaped layer of the dense connective tissue helping to maintain shape of the eye

What is the conjunctiva

Thin, transparent mucous membrane associated with the eyelids and exposed areas of the eye

What does the conjunctiva produces

Secretions help lubricate the surface of the eye

Parts of the conjunctiva

• Palpabreal conjunctiva

• Bulbar conjunctiva

• Superior and inferior conjunctival fornices

Conjunctiva: Palpebral conjunctiva

Covers the inner surface of the eye

Conjunctiva: Bulbar conjuncitva

Covers the anterior white surface of the eye

Conjunctiva: Superior and inferior conjunctival fornices

Points where the palpebral and bulbar conjunctive meet

Conjunctivitis

Inflammation of the conjunctiva caused by infection or other irritation

Acute contagious conjunctivitis (pink eye) is caused by____

bacterium

The lacrimal apparatus is formed by ____

• Lacrimal gland

• Tears

• Nasolacrimal duct

Lacrimal gland is located in

Superolateral corner of the orbit

Lacrimal glands produce

tears that exit through several lacrimal ducts

The lacrimal glands are innervated by____

parasympathetic fibers from facial nerve (CN VII)

What do tears do?

Moisten surface of the eye, lubricate eyelid, wash away foreign objects

How many tears are produce in a day?

~1 mL/day

Tears contain_____

salt, mucus, and lyzozomes (enzymes that kills certain bacteria)

Nasolacrimal duct

Beginning in the inferomedial corner of the orbit

How many extrinsic eye muscles are there?

6

The Extrinsic eye muscles 6 muscles_____

4 run straight anteroposteriorly

2 run at an angle to the globe of the eye

4 extrinsic eye muscles that run anteroposteriorly

• Superior rectus m.- innervated by the oculomotor nerve (CN III)

• Inferior rectus m.- innervated by the oculomotor nerve (CN III)

• Medial rectus m.- innervated by the oculomotor nerve (CN III)

• Lateral rectus m.- innervated by the abducens nerve (CN VI)

2 extrinsic eye muscles that run at an angle to the globe of the eye

• Superior oblique m.- innervated by the trochlear nerve (CN IV)

• Inferior oblique m.- innervated by the oculomotor nerve (CN III)

Eye

Hollow, fluid filled sphere

The eye is composed of ____

Three layers (tunics)

• Fibrous tunic- outer layer

  • sclera and cornea

• Vascular tunic- middle layer

  • choroid, ciliary body, iris

• Nervous tunic- inner layer

  • retina

The fibrous tunic is composed of____

Sclera and Cornea

Fibrous tunic:Sclera

• White outer layer

• Posterior 5/6 of the eyeball

• Loosely attached to the bulbar conjunctiva

Fibrous tunic:Sclera is composed of ____

Dense collagenous connective tissue with elastic fibers

Fibrous tunic:Sclera maintains and protects _____

Maintains the shape of the eyeball, protects its internal structures

Fibrous tunic:Sclera provides____

Attachment point for the muscles that move it

Fibrous tunic:Cornea

• Continuos with the sclera

• Avascular, transparent structure that allows light to enter

• Refracts (bends) light

Fibrous tunic:Cornea consists of____

Connective tissue matrix containing collagen, elastic fibers and proteoglycans with simple squamous epithelial cells on either side

Parts of the Vascular Tunic

• Choroid

• Ciliary body

• Iris

The vascular tunic contains_____

• Most of the blood vessels of the eye

• Short ciliary arteries

• A large number of melanin containing pigment cells and appears black in color

What are short ciliary arteries in vascular tunic

Arteries that pierce the sclera and provide arteries to the vascular tunic

• branches of the ophthalmic artery that branches off the internal carotid artery

Vascular Tunic: Choroid

• Thin layer

• Portion associated with the sclera

Vascular Tunic: Ciliary body parts

Continuous with the choroid

• Ciliary ring- outer portion

• Ciliary processes- inner processes that attach to suspensory ligaments

  • produce aqueous humor

• Ciliary muscles- smooth muscle organized into outer radial fibers and inner circular fibers that regulate the thickness of the lens

Vascular Tunic: Iris

• Portion visible through the cornea

• Color depends on the amount of melanin present

• Attached to the ciliary body

Vascular Tunic: Iris is composed of

Mainly smooth muscle that regulates the size of the pupil (allows light to enter)

• Sphincter pupillae- circular group, innervated by parasympathetic fibers from the oculomotor nerve (CN III)

• Dilator pupillae- radial group, innervated by sympathetic fibers

Nervous Tunic: Retina consists of

• Pigment layer- composed of pigmented simple cuboidal epithelium

• Neural layer- responds to light, contains numerous photoreceptors

  • 120 million rods

  • 6-7 million cones

  • numerous relay neurons

Nervous Tunic: Retina

• Macula

• Fovea centralis

• Optic disc

Nervous Tunic: Retina Macula

Small, yellow spot near the center of the posterior retina

Nervous Tunic: Retina Fovea centralis

Center of the macula where light is most focused when looking directly at an object that contains only cones

• Highest point of visual acuity

Nervous Tunic: Retina Optic disc

White spot medial to macula that the central retinal artery enters and the central retina artery enters and the central retinal vein exits and where axons exit to from the optic nerve

• Lacks photoreceptors (blind spot)

Chambers of the eye

• Anterior chamber

• Posterior chamber

• Vitreous chamber

Anterior chamber

• Between the cornea and iris

• Filled with aqueous humor

Posterior chamber

• Between iris and lens

• Filled with aqueous humor

Vitreous Chamber

• Larger chamber posterior to the lens

• Filled with vitreous humor

The aqueous humor helps maintain______

Intraocular pressure that keeps the eyeball inflated and helps maintain eyeballs shape

The aqueous humor _____ light

Refracts

The aqueous humor provides_____

Nutrition for the structures of anterior chamber

The aqueous humor is produced by____

Ciliary processes

The Aquou

Lens-Capsule

Highly elastic, transparent cover of the lens

Electromagnetic Spectrum

Entire range of wavelengths (frequencies) of electromagnetic radiation

Electromagnetic spectrum- Visible light

Portion that can be detected by the human eye

• Visible spectrum- 380-750nm