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