Special Senses

The Special Senses

–Olfaction (smell)–Gustatory (taste)–Vision–Hearing and Equilibrium

Olfaction

• Olfactory receptors are in the roof of the nasal cavity– Neurons with long cilia– Chemicals must be dissolved in mucus for detection

• Impulses are transmitted via the olfactory nerve• Interpretation of smells is made in the cortex

Physiology of Olfaction• Many different combinations of receptors produces the

possibility for thousands of different odor sensations.

• Low threshold, only few molecules needed.

• Adaptation - rapid

1. Olfactory receptors

2. Olfactory (I) nerves

3. Olfactory tract

4. Temporal lobe (primary olfactory area)

Olfactory Pathway

Olfactory Pathway

Gustation• Sweet (sugars)

– Saccharine– Some amino

acids• Sour

– Acids• Bitter

– Alkaloids• Salty receptors

– Metal ions• Umami

– Meaty/savory

Anatomy of Taste Buds and Papillae• The tongue is covered with projections called

papillae– Filiform papillae—sharp with no taste buds– Fungiform papillae—rounded with taste buds– Circumvallate papillae—large papillae with taste

buds• Taste buds are found on the sides of papillae

Anatomy of Taste Buds and Papillae

Physiology of Gustation• Gustatory cells are the receptors

– Have gustatory hairs (long microvilli)– Hairs are stimulated by chemicals dissolved in saliva

• Impulses are carried to the gustatory complex by several cranial nerves because taste buds are found in different areas– Facial nerve– Glossopharyngeal nerve– Vagus nerve

• Impulse travels from receptors to medulla→ thalamus→ primary gustatory area of the cerebrum

• Eyelids and eyelashes• Conjunctiva• Lacrimal apparatus • Extrinsic eye muscles

Accessory Structures of Eye

Vision

Accessory Structures of the Eye• Eyelids

– Meet at the medial and lateral commissure (canthus)• Eyelashes

– Tarsal glands produce an oily secretion that lubricates the eye

– Ciliary glands are located between the eyelashes • Conjunctiva

– Membrane that lines the eyelids– Connects to the outer surface of the eye– Secretes mucus to lubricate the eye and keep it moist

Accessory Structures of the Eye

• Lacrimal apparatus = lacrimal gland + ducts– Lacrimal gland—produces lacrimal fluid;

situated on lateral aspect of each eye– Lacrimal canaliculi—drain lacrimal fluid from

eyes medially– Lacrimal sac—provides passage of lacrimal

fluid towards nasal cavity– Nasolacrimal duct—empties lacrimal fluid into

the nasal cavity

Lacrimal Apparatus

Accessory Structures of the Eye

• Function of the lacrimal apparatus – Protects, moistens, and lubricates the eye– Empties into the nasal cavity

• Lacrimal secretions (tears) contain:– Dilute salt solution– Mucus– Antibodies– Lysozyme (enzyme that destroys bacteria)

Accessory Structures of the Eye• Extrinsic eye muscles

– Six muscles attach to the outer surface of the eye– Produce eye movements

Muscles of the Eye

Muscles of the Eye

Structure of the Eye• Three layers:

– Fibrous tunic- outer layer• Sclera “white” of the eye• Cornea-transparent coat

– Vascular tunic or uvea- middle layer• Choroid• Ciliary body consists of ciliary processes and ciliary

muscle• Iris

– Retina- inner layer• Optic disc• Macula lutea- fovea centralis

Structure of the Eye

Structure of the Eye

Structure of the Eye: The Fibrous Layer

• Sclera– White connective tissue layer– Seen anteriorly as the “white of the eye”

• Cornea– Transparent, central anterior portion– Allows for light to pass through– The only human tissue that can be transplanted

without fear of rejection

• Choroid is a blood-rich nutritive layer in the posterior of the eye– Pigment prevents light from scattering

• Modified anteriorly into two structures– Ciliary body—smooth muscle attached to lens– Iris—regulates amount of light entering eye

• Pigmented layer that gives eye color• Pupil—rounded opening in the iris

Structure of the Eye: The Vascular Layer

Muscles of Iris

• Retina contains two layers–Outer pigmented layer– Inner neural layer

• Contains receptor cells (photoreceptors)– Rods– Cones

Structure of the Eye: The Sensory Layer

• Signals pass from photoreceptors via a two-neuron chain– Bipolar neurons– Ganglion cells

• Signals leave the retina toward the brain through the optic nerve

• Optic disc (blind spot) is where the optic nerve leaves the eyeball– Cannot see images focused on the optic disc

Structure of the Eye: The Sensory Layer

Retina

View with Ophthalmoscope

Retina

Interior of the Eyeball• Lens

– lack blood vessels, consists of a capsule with proteins (crystallins) in layers; transparent.

– held in place by a suspensory ligament attached to the ciliary body

– divides the eyeball into two cavities: anterior and posterior.

- Anterior cavity- further divided into anterior and posterior chambers. Both are filled with aqueous humor.

- Posterior cavity (vitreous chamber)-filled with vitreous body.

Chambers of the Eye

Cavities of the Interior of Eyeball• Anterior cavity (anterior to lens)

– filled with aqueous humor• produced by ciliary body• continually drained• replaced every 90 minutes

– 2 chambers• anterior chamber between cornea and iris• posterior chamber between iris and lens

• Posterior cavity (posterior to lens)– filled with vitreous body (jellylike)– formed once during embryonic life– floaters are debris in vitreous

Lens - Cataract

• Refraction: Bending of light as it passes from one substance (air) into a 2nd substance with a different density (cornea)

• In the eye, light is refracted by the anterior & posterior surfaces of the cornea and the lens

Image Formation

• Emmetropia—eye focuses images correctly on the retina• Myopia (nearsighted)

– Distant objects appear blurry – Light from those objects fails to reach the retina and are

focused in front of it– Results from an eyeball that is too long

• Hyperopia (farsighted)– Near objects are blurry while distant objects are clear– Distant objects are focused behind the retina– Results from an eyeball that is too short or from a “lazy

lens”

Vision

Vision Correction

Visual Pathway

Anatomy of Ear

Middle Ear

Inner Ear

Cochlea and

Cranial Nerve VIII

One Turn of

Cochlea and

Organ of Corti

Physiology of Hearing

Auditory Pathway

Balance and Position

Crista and Rotational Movement