Structural and Functional Biology: The Ear

Organs of Special Senses

• Vet Prep Structural and Functional Biology will cover these organs
• Dr. Cristian Martonos is the instructor
• Dr. Melissa Kehl was the previous lecturer.

The Ear

• Main Learning Objectives:
  • List the main segments of the ear
  • Describe the main features of each segment and their function
  • Differentiate between the organs of hearing and sense of balance
  • Determine the cranial nerve serving each sense

Segments of the Ear

• The ear has 3 non-communicating segments:
  • External Ear
  • Middle Ear
  • Inner Ear

External Ear

• Captures air vibrations and directs them toward the tympanic membrane
• Parts of the external ear include:
  • Auricle (pinna) which is funnel shaped
  • Ear canal with a vertical and horizontal part

Otitis Externa (FYI)

• Causes:
  • Long, floppy ears (warm, dark, moist)
  • Aberrant ear fur
  • Yeast or bacterial overgrowth
  • Mites
  • Environmental or Food Allergies
  • Tumor/polyp
  • Cut/scratch
  • Foreign body
• Diagnostics:
  • Oil impression slide
  • Cytology
  • Otoscopy

Middle Ear

• Functions to modulate sound
• Located in the tympanic part of the temporal bone of skull
• The tympanic membrane (ear drum) is connected to oval window by 3 auditory ossicles
• The middle ear communicates with the pharynx via the auditory tube
  • It ensures air pressure inside the middle ear is equal to outside air pressure

Inner Ear

• Housed deep inside the temporal bone (24)
• Vestibulocochlear nerve (cranial nerve VIII) (21) comprises:
  • Cochlear nerve for hearing
  • Vestibular nerve for balance
• Comprises of:
  • Organ for hearing (cochlea – 16)
  • Organs for balance (vestibular apparatus)
    • Semicircular canals – 13
    • Utricle - 14
    • Saccule - 15

The Eye

• List the three tunics of the eye and describe their sub-components and functions
• Describe how the optic nerve emerges from the retina
• Describe the main compartments of the eye
• Differentiate the aqueous humor from the vitreous body
• List the main protective adnexa of the eye
• List the intrinsic and extrinsic muscles of the eye and differentiate between their type, their specific functions and innervation

Eye Bulb Tunics

• Eye bulb has 3 layers (tunics):
  • Fibrous tunic which provides structural integrity and allows light to enter the eye
  • Vascular tunic which modulates the amount of light, vision focus and provides blood supply and innervation
  • Nervous tunic which reacts to light stimuli and outputs nervous impulses for the brain to be interpreted as images

Fibrous Tunic

• Fibrous tunic has two parts:
  • Cornea (transparent anterior ⅓)
  • Sclera (opaque posterior ⅔) – “white of the eye"

Vascular Tunic

• Consists of the: -Choroid (14) - Dense network of blood vessels in heavily pigmented connective tissue - Tapetum lucidum – dorsal part, light reflecting area, avascular, when light hits reflects in iridescence - (nocturnal adaptation)
  • Ciliary body (4)
    • Thickening of choroid
    • Has ciliary muscles attached to lens (3) to focus
  • Iris (7)
    • Suspended between cornea and lens
    • Opening in the center is the pupil
    • Smooth muscle constrict and dilate pupil to regulate amount of light reaching the retina.

Nervous Tunic (Retina)

• Anterior, thinner pigmented layer has no photoreceptors
• Posterior, thicker layer of the retina contains photoreceptors and connects to the optic nerve (cranial nerve II)
• The optic disk is the region where axons emerge from the eye bulb creating a "blind spot"

Intrinsic Muscles of the Eye

• Under involuntary control
  • Ciliary Muscles
    • Changes the shape of the lens for focusing
    • Poorly developed in domestic animals
  • Pupillary muscles (in the Iris)
    • Constrictor pupillae (circular fibers)
    • Dilator pupillae (linear, radial fibers)
    • Regulates the amount of light

Extrinsic Muscles

• Under voluntary control
  • 4 Rectus (straight) muscles
    • Dorsal rectus (1)
    • Ventral rectus (3)
    • Lateral rectus (2)
    • Medial rectus (4)
  • 2 Oblique (curved) muscles
    • Dorsal oblique (6)
    • Ventral oblique (5)
  • Retractor bulbi mm. (7)

Compartments and Chambers of the Eye

• The space between the cornea and the lens is divided by the iris in 2 CHAMBERS:
  • Anterior chamber (from cornea to iris (3))
  • Posterior chamber (from iris to lens (2))
    • They communicate through the pupil
    • They contain aqueous humor
  • Vitreous chamber (4) is from lens to fundus
    • Does not communicate with anterior compartment
    • Contains vitreous humor

Adnexa of the Eye

• The orbit is incomplete in the dog
  • It connects via the orbital ligament
• Eyelids (palpebrae)
  • Lateral commissure
  • Medial commissure
  • Superior palpebra
  • Inferior palpebra
  • Palpebral fissure

Third Eyelid (Plica Semilunaris)

• Concave fold of palpebral conjunctiva that protrudes from the medial angle of the eye.
• Supported by a T shaped cartilage
• Contains the superficial gland of the third eyelid

Lacrimal Gland

• Function:
  • To make tears which keep the surface of the eye moist and clean and provide nourishment
• Lacrimal gland (9) located dorso-lateral to eye secretes serous fluid
  • Secretion flows over cornea
  • Passes through lacrimal puncta and into lacrimal canaliculi (1, 5)
  • Drains into nasolacrimal duct (3)

The Nose

• Location of the olfactory mucosa in relation to the nasal cavity
• Path of the olfactory nerve in relation to the canine skull

Olfactory Mucosa

• The olfactory mucosa covers thin scrolls of bone belonging to the ethmoidal bone called ethmoidal conchae (red arrow)
  • Located in the caudal region of the nasal cavity
  • Contains over 250 million olfactory receptors in their noses

Olfactory Sense

• The olfactory nerve (cranial nerve I) is made up of numerous axons that arise in the olfactory mucosa
  • Passes through the cribriform foramina to the olfactory bulbs of the brain

FYI Nose

• Olfactory Epithelium:
  • Catacomb at the back of the nasal passage that houses sensory receptors
• Humans:
  • 1 in² surface area with ~6 million receptors
• Dogs:
• 30 in² surface area with ~250 million receptors
• Olfactory Bulb
  • A brain region that processes signals from the olfactory epithelium.
• Canine olfactory bulbs are 3 times larger than humans, even though their brains are 10 times smaller.
• Vomeronasal Organ
  • A sensory organ that detects pheromones picked up by a dog's wet nose
• Nostrils:
  • Air is exhaled through the side slits, so it doesn't dilute the scent of incoming air.

The Tongue

• Location of the taste buds in relation to the tongue
• Sensory nerves servicing the taste sense and their distribution on the tongue

Main Functions of the Tongue:

• Prehension for picking up food
• Mastication for chewing the food
• Deglutition for swallowing the food
• Grooming
• Heat loss (in panting) in dogs
• Gustatory sensation for taste

Taste Buds and Lingual Papillae

• Taste buds have receptor cells that get stimulated by soluble substances.
• There are taste buds on vallate, foliate, and fungiform papillae.

Taste Vs Touch Sensations – Innervation

• The lingual branch of the Glossopharyngeal n. (IX) is sensory (taste + touch) from the caudal one third of the tongue
• The lingual nerve is sensory from the rostral two thirds of the tongue
  • Touch sensation is sent to the brain from the trigeminal n. (V)
  • Taste sensation is sent to the brain from a part of the facial n. (VII)