Essentials of Human AnatomyEssentials of Human Anatomy

Special Senses

Special Senses

• sensory receptors are within large, complex sensory organs in the head

• smell in olfactory organs

• taste in taste buds

• hearing and equilibrium in ears

• sight in eyes

Sense of Smell

Olfactory Receptors• chemoreceptors• respond to chemicals dissolved in liquids

Olfactory Organs• contain olfactory receptors and supporting epithelial cells

• cover parts of nasal cavity, superior nasal conchae, and a portion of the nasal septum

Olfactory Receptors

Olfactory Nerve Pathways

Once olfactory receptors are stimulated, nerve impulses travel through• olfactory nerves olfactory bulbs olfactory tracts limbic system (for emotions) and olfactory cortex (for interpretation)

Olfactory Stimulation

• olfactory organs located high in the nasal cavity above the usual pathway of inhaled air

• olfactory receptors undergo sensory adaptation rapidly

• sense of smell drops by 50% within a second after stimulation

Sense of Taste

Taste Buds• organs of taste• located on papillae of tongue, roof of mouth, linings of cheeks and walls of pharynx

Taste Receptors• chemoreceptors• taste cells – modified epithelial cells that function as receptors• taste hairs –microvilli that protrude from taste cells; sensitive parts of taste cells

Taste Receptors

Taste Sensations

Four Primary Taste Sensations• sweet – stimulated by carbohydrates• salty – stimulated by salts • sour – stimulated by acids• bitter – stimulated by many organic compounds

Spicy foods activate pain receptors

Taste Nerve Pathways

Sensory impulses from taste receptors travel along• cranial nerves to • medulla oblongata to• thalamus to • gustatory cortex (for interpretation)

Hearing

Ear – organ of hearing

Three Sections• External• Middle• Inner

External Ear

• auricle• collects sounds waves

• external auditory meatus• lined with ceruminous glands• carries sound to tympanic membrane• terminates with tympanic membrane

• tympanic membrane • vibrates in response to sound waves

Middle Ear

• tympanic cavity• air-filled space in temporal bone• auditory ossicles

• vibrate in response to tympanic membrane• malleus, incus, and stapes

• oval window • opening in wall of tympanic cavity• stapes vibrates against it to move fluids in inner ear

Auditory Tube

• eustachian tube • connects middle ear to throat• helps maintain equal pressure on both sides of tympanic membrane• usually closed by valve-like flaps in throat

Inner Ear

• complex system of labyrinths• osseous labyrinth • bony canal in temporal bone• filled with perilymph

• membranous labyrinth• tube within osseous labyrinth• filled with endolymph

Inner Ear

Three Parts of Labyrinths• cochlea

• functions in hearing• semicircular canals

• functions in equilibrium

• vestibule• functions in equilibrium

Cochlea

Scala vestibuli• upper compartment• leads from oval window to apex of spiral part of bony labyrinth

Scala tympani• lower compartment• extends from apex of the cochlea to round window• part of bony labyrinth

Cochlea

Cochlear duct• portion of membranous labyrinth in cochlea

Vestibular membrane• separates cochlear duct from scala vestibuli

Basilar membrane• separates cochlear duct from scala tympani

Organ of Corti

• group of hearing receptor cells (hair cells)• on upper surface of basilar membrane• different frequencies of vibration move different parts of basilar membrane• particular sound frequencies cause hairs of receptor cells to bend• nerve impulse generated

Auditory Pathway

To inferior colliculus of opposite side of midbrain

To thalamus

To auditory cortex

Cochlear branch of CN VIII

To cochlear nucleus of medulla

The Sense of Vision

• Visual receptors (photoreceptors) in the eyes to detect light, color, and movement.

• Accessory structures of the eye. – provide a superficial covering over its anterior exposed surface

(conjunctiva) – prevent foreign objects from coming into contact with the eye

(eyebrows, eyelashes, and eyelids) – keep the exposed surface moist, clean, and lubricated (lacrimal

glands)

Lacrimal Apparatus

• lacrimal gland• lateral to eye• secretes tears

• canaliculi• collect tears

• lacrimal sac• collects from canaliculi

• nasolacrimal duct• collects from lacrimal sac• empties tears into nasal cavity

Structure of the Eye

• hollow• spherical• wall has 3 layers• outer fibrous tunic• middle vascular tunic• inner nervous tunic

Cavities and Chambers of the Eye

• The internal space of the eye is subdivided by the lens into two separate cavities. – anterior cavity – posterior cavity

Cavities and Chambers of the Eye

• The anterior cavity is – the space anterior to the

lens and posterior to the cornea

• The iris of the eye subdivides the anterior cavity further into two chambers. – anterior chamber is between

the iris and cornea

– posterior chamber is between the lens and the iris

Vitreous Humor

• Posterior cavity is posterior to the lens and anterior to the retina.

• Transparent, gelatinous vitreous body which completely fills the space between the lens and the retina.

Optic Disc

• Optic disc lacks photoreceptors.• Called the blind spot because no image forms there.• Just lateral to the optic disc is a rounded, yellowish

region of the retina called the macula lutea containing a pit called the fovea centralis (the area of sharpest vision). – contains the highest proportion of cones and almost no rods

Visual Pathways• Each optic nerve conducts

visual stimulus information. • At the optic chiasm, some

axons from the optic nerve decussate.

• The optic tract on each side then contains axons from both eyes.

• Visual stimulus information is processed by the thalamus and then interpreted by visual association areas in the cerebrum.

Extrinsic Eye Muscles

Superior rectus• rotates eye up and medially

Inferior rectus• rotates eye down and medially

Medial rectus• rotates eye medially

Lateral rectus• rotates eye laterally

Superior oblique• rotates eye down and laterally

Inferior oblique• rotates eye up and laterally

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