PHA 538 Lab 11: Special Senses Flashcards

Lab Overview and Preparation

Lab Preparation Steps:
- Read the lab study guide in its entirety before beginning.
- Watch supplemental videos listed on the Supplemental Videos Document posted in Moodle.
In-Lab Procedure:
- Collaborate with lab groups to review materials at each of the $6$ lab stations.
- Utilize an anatomy atlas to locate specific anatomical structures in models, cadavers, and specimens.
- Consult with the instructor and TA for clarification on any questions before exiting the lab.
Post-Lab Follow-up:
- Read the corresponding textbook sections related to special senses.
- Rewatch Moodle supplemental videos.
- Attend office hours and open labs with specific prepared questions.

Learning Objectives

Identify and understand the functions of accessory eye structures in models and specimens.
Identify the function and structures of the three tunics (layers) of the eye.
Trace the visual pathway from the external environment to the primary visual cortex, identifying all intermediary structures.
Identify macroscopic anatomy of the external, middle, and internal ear on models and explain their functions.
Trace the sound path from the external ear to the primary auditory cortex.
Identify and describe functions of structures for olfaction (smell) and taste (gustation).
Trace the sensory paths of smell and taste to their primary cortical areas in the brain.
Review Lab $9$ material regarding the brain and cranial nerves.
Apply anatomical knowledge to understand the pathology of various medical conditions.

Anatomy of the Eye: Accessory Structures

Eyebrows:
- Description: Short hairs located on the supraorbital margins.
- Function: Prevent sweat from entering the eyes.
Palpebrae (Eyelids):
- Description: Skin-covered upper and lower lids featuring eyelashes at their free margins.
- Function: Protect the eyes and spread lacrimal fluid (tears) through blinking.
Conjunctiva:
- Description: A clear mucous membrane lining the eyelids and the anterior white of the eye (note: it does not cover the cornea).
- Function: Secretes mucus to provide lubrication for the eye.
Lacrimal Caruncle:
- Description: A fleshy, reddish elevation in the medial aspect of the eye containing sebaceous and sweat glands.
- Function: Secretes an oily secretion for eye lubrication.
Lacrimal Gland:
- Description: Located on the superior lateral aspect of the orbit.
- Function: Secretes lacrimal fluid (tears) containing mucus, antibodies, and lysozyme.
Lacrimal Puncta:
- Description: Two tiny openings located in the medial aspect of each eyelid.
- Function: Allows lacrimal fluid to drain into the nasolacrimal duct.
Nasolacrimal Duct:
- Description: A tube that empties into the nasal cavity.
- Function: Facilitates the drainage of lacrimal fluid into the nasal cavity.

Extrinsic Eye Muscles and Innervation

Inferior Oblique:
- Origin: Floor of the bony orbit.
- Insertion: Sclera of the eye.
- Function: Elevates and laterally rotates the eye.
- Innervation: Oculomotor nerve (CN III).
- Exit Foramen: Superior orbital fissure.
Medial Rectus:
- Origin: Common tendinous ring of the eye.
- Function: Adducts the eye.
- Innervation: Oculomotor nerve (CN III).
- Exit Foramen: Superior orbital fissure.
Superior Rectus:
- Function: Elevates and medially rotates the eye.
- Innervation: Oculomotor nerve (CN III).
- Exit Foramen: Superior orbital fissure.
Inferior Rectus:
- Function: Depresses and laterally rotates the eye.
- Innervation: Oculomotor nerve (CN III).
- Exit Foramen: Superior orbital fissure.
Superior Oblique:
- Function: Abducts, depresses, and medially rotates the eye.
- Innervation: Trochlear nerve (CN IV).
- Exit Foramen: Superior orbital fissure.
Lateral Rectus:
- Function: Abducts the eye.
- Innervation: Abducens nerve (CN VI).
- Exit Foramen: Superior orbital fissure.

Internal Eye Structures and Tunics

Fibrous Tunic (External Layer)

Sclera:
- Description: Dense connective tissue forming the "white" of the eye.
- Function: Maintains eyeball shape and provides an attachment site for extrinsic muscles.
Cornea:
- Description: Modified transparent sclera that bulges anteriorly; avascular (contains no blood vessels).
- Function: Acts as a clear window and the primary light-refracting (bending) medium.

Vascular Tunic (Middle Layer / Uvea)

Choroid:
- Description: Thin, pigmented, highly vascular loose connective tissue membrane; extends from the ora serrata to the optic nerve.
- Function: Supplies oxygen and nutrients to the outer retina; absorbs excess light to prevent scattering.
Ciliary Body:
- Description: Ring-shaped structure at the anterior choroid end encircling the lens; contains smooth ciliary muscles and ciliary processes.
- Function: Controls lens shape (contraction increases convexity for near vision accommodation) and produces aqueous humor.
Iris:
- Description: Pigmented anterior portion containing two smooth muscle layers (sphincter pupillae and dilator pupillae).
- Function: Regulates the amount of light entering the eye by adjusting pupil diameter.
Pupil:
- Description: Round central opening of the iris.
- Function: Passageway for light.

Nervous Tunic (Inner Layer)

Retina:
- Structure: Consists of two layers: Pigmented (absorbs light) and Neural (contains photoreceptors).
- Function: Photoreceptors respond to light and convert it into action potentials sent to the primary visual cortex.
Ora Serrata:
- Description: The jagged anterior edge of the neural retina.
- Function: Transition zone between ciliary body and retina.
Macula Lutea:
- Description: Small area on the posterior retina lateral to the optic disc; high cone density.
- Function: Focal point for incoming light rays.
Fovea Centralis:
- Description: Depression in the center of the macula lutea with the highest cone percentage and near-zero rods.
- Function: Site of greatest visual acuity (resolution).
Optic Disc (Blind Spot):
- Description: Oval-shaped elevation on the medial posterior retina.
- Function: Exit for optic nerve axons and blood vessels; lacks photoreceptors.
- Clinical Note: Enlargement (papilloedema) is an early sign of increased intracranial pressure.
Optic Nerve (CN II):
- Description: Purely sensory nerve exiting the posterior eye through the optic canal.
- Function: Transmits visual information to the brain.

The Lens

Structure: Transparent disc with simple cuboidal epithelium on the anterior surface and simple columnar epithelium on the posterior surface.
Location: Posterior to the iris, anterior to the vitreous humor.
Function: Helps focus light specifically onto the fovea centralis.

Chambers and Humors of the Eye

Anterior Cavity:
- Location: Space between the cornea and the lens.
- Subdivisions:
  - Anterior Chamber: Between cornea and iris.
  - Posterior Chamber: Between iris, ciliary body, and lens.
- Fluid: Contains Aqueous humor, secreted by ciliary bodies; nourishes the avascular lens and cornea.
Posterior Cavity:
- Location: Space between the lens and the posterior inner surface of the eye.
- Substance: Contains Vitreous humor, a gelatinous/liquid substance.
- Function: Contributes to light refraction, maintains eyeball shape, and holds the retina against the choroid.

The Visual Pathway

The sequence of light and signal transmission from the environment to the brain is as follows:

Cornea
Aqueous humor
Pupil
Lens
Vitreous humor
Retina
Blind spot
Optic nerve
Optic chiasm
Left optic tract
Left lateral geniculate nucleus of the thalamus
Left optic radiations
Left primary visual cortex

Anatomy of the Ear

External Ear

Auricle (Pinna): Elastic cartilage covered with skin; collects and directs sound waves into the external acoustic meatus.
External Auditory Meatus: Short narrow canal in the temporal bone lined with skin containing ceruminous glands. Transmits sound and uses cerumen (earwax) to trap foreign objects and inhibit microbial growth.
Tympanic Membrane (Eardrum): Thin, semitransparent membrane separating external and middle ear; vibrates in response to sound waves.

Middle Ear (Tympanic Cavity)

Malleus: Hammer-shaped bony ossicle attached to the eardrum; transmits/amplifies vibrations to the incus.
Incus: Anvil-shaped ossicle; transmits/amplifies vibrations to the stapes.
Stapes: Stirrup-shaped ossicle; base fits into the oval window; transmits/amplifies vibrations to the internal ear.
Oval Window: Membrane deep to the stapes; transmits vibrations to the perilymph of the scala vestibuli.
Pharyngotympanic (Auditory) Tube: Connects middle ear to the nasopharynx; equalizes pressure between the middle ear and external atmosphere.

Internal Ear

Cochlear Component (Hearing):
- Cochlea: Spiral bony cavity containing the Organ of Corti for sound processing.
- Cochlear Nerve (CN VIII): Transmits electrical impulses to the brainstem.
Vestibular Component (Equilibrium):
- Vestibule: Bony region containing the utricle and saccule; detects gravity and linear acceleration.
- Semicircular Canals/Ducts: Detect rotational equilibrium.
- Vestibular Nerve (CN VIII): Transmits equilibrium impulses to the brainstem.

The Auditory Pathway

The sequence of sound transmission is as follows:

Auricle
External auditory meatus
Tympanic membrane
Malleus
Incus
Stapes
Oval window
Cochlea
Cochlear component of vestibulocochlear nerve (CN VIII)
Medulla oblongata (second-order neurons)
Pons
Inferior colliculi
Medial geniculate nucleus (MGN) of the thalamus
Primary auditory cortex (third-order neurons)

Olfection (Smell)

Anatomical Structures:
- Olfactory Epithelium: Patch in the roof of the nasal cavity covering the superior nasal concha; contains olfactory sensory neurons.
- Olfactory Bulb: Located on the cribriform plate of the ethmoid bone; contains second-order neurons.
- Olfactory Tract: Axons on the anterior inferior surface of the frontal lobe.
- Primary Olfactory Cortex: Located on the anterior, inferior, medial surface of the temporal lobes.
Path of Smell:
- Odor particles are sensed by bipolar receptor cells in the olfactory epithelium.
- Axons combine to form the olfactory nerves (CN I).
- CN I perforates the cribriform plate to synapse with second-order neurons in the olfactory bulb.
- Axons extend through olfactory tracts to areas including the amygdala, hypothalamus, brainstem, and prefrontal cortex.

Gustation (Taste)

Taste Receptors: Taste buds are located on the dorsal tongue (within papillae), pharynx, soft palate, epiglottis, and inner cheeks. Saliva dissolves food particles to contact gustatory epithelial cells.
Path of Taste (Innervation):
- Facial Nerve (CN VII): Serves the anterior $2/3$ of the tongue.
- Glossopharyngeal Nerve (CN IX): Serves the posterior $1/3$ of the tongue.
- Vagus Nerve (CN X): Carries fibers from the pharyngeal region.
Cortical Pathway: Cranial nerves convey signals to the medulla oblongata, then to the thalamus, and finally to the primary gustatory cortex in the insula.

Applied Anatomy and Clinical Conditions

Surface Anatomy: Refer to p. $16$ (Fig $2.19$ , $2.20$ ) and p. $17$ (Fig $2.21$ ) of the Surface Anatomy Textbook for eye identification.
Conjunctivitis (Pinkeye): Inflammation of the conjunctiva causing redness and irritation. Infectious pinkeye is highly contagious and caused by bacteria or viruses.
Cataracts: Cloudiness of the lens resulting from the breakdown and clumping of lens proteins and fibers. This prevents light from reaching the retina, causing blurry vision; requires surgical replacement.
Retinal Detachment: A medical emergency where the retina pulls away from the choroid. Lack of blood supply can lead to permanent vision loss if not immediately treated.
Glaucoma: Excessive build-up of aqueous humor due to clogged drainage pathways. Increased intraocular pressure damages the retina and optic nerve fibers, leading to potential blindness.
Ear Infection: Infection of the middle ear caused by fluid and mucus build-up. This occurs when the pharyngotympanic tube is inflamed/irritated and fails to drain excess fluid into the throat.
Anosmia: Loss of smell. Causative factors include injury, neurodegenerative disorders, cold viruses, or congenital conditions.