ear

General Functions and Structure of the Ear

• The ear, known in Latin as Auris and in Greek as Otis, serves as the primary organ for hearing and balance (equilibrium).
• The main functions of the ear are:
  • Collecting auditory stimuli from the environment.
  • Transducing auditory stimuli first into mechanical stimuli and then into nervous stimuli.
  • Transmitting nerve impulses to the Central Nervous System (CNS) via the vestibulocochlear nerve (CN VIII).
• The ear is divided into three primary anatomical regions:
  • External Ear: Functions as a sound-collecting apparatus. It consists of the auricle or pinna and the external acoustic meatus (ear canal).
  • Middle Ear: Functions as a sound-conducting apparatus. It consists of the tympanic cavity, the tympanic membrane (ear drum), and three auditory ossicles with their associated ligaments and muscles. It is connected to the pharynx via the auditory (Eustachian) tube.
  • Inner Ear: The organ for both hearing and equilibrium, which is part of the vestibular system. It is composed of a membranous labyrinth enclosed within the petrous part of the temporal bone.

Microanatomy of the External Ear

• Pinna (Auricle):
  • The pinna consists of elastic cartilage covered by skin.
  • Histological components of the pinna include:
  • Chondrocytes: Residing within lacunae, which sometimes appear empty if the section does not pass through the cell.
  • Dermis: The layer of skin covering the cartilage.
  • Elastic cartilage: Contains a matrix with distinct, darkly stained elastic fibers.
  • Perichondrium: Divided into a fibrous layer and a chondrogenic layer.
  • Matrix: Divided into territorial matrix (surrounding lacunae) and interterritorial matrix.
  • Isogenous groups: Clusters of chondrocytes.
  • Glands and hair follicles are also present in the dermis of the pinna.
• External Acoustic Meatus:
  • This structure includes both cartilaginous and osseous (bony) portions.
  • It is divided into a vertical canal and a horizontal canal.
  • The structure of the cartilaginous portion is formed by the auricular cartilage and the annular cartilage.
  • Dense, white, fibrous connective tissue attaches the annular cartilage to the osseous rim of the external acoustic meatus.
  • Ceruminous Glands:
  • These are a specialized type of apocrine gland found in the external acoustic meatus.
  • They secrete cerumen, commonly known as ear wax.

The Middle Ear and Auditory Ossicles

• Tympanic Membrane (Ear Drum):
  • The membrane separates the external acoustic meatus from the tympanic cavity.
  • In dogs, the tympanum is oval to comma-shaped, and the manubrium of the malleus is C-shaped.
  • In horses, the tympanum is more circular, and the manubrium of the malleus forms a very shallow arc and is centrally located.
  • The membrane extends medially toward the tympanic cavity at an approximate $45$-degree angle from dorsal to ventral relative to the central axis of the horizontal ear canal.
  • The connective tissue of the membrane is thicker where the manubrium of the malleus is embedded, and blood vessels are abundant in this area.
• Auditory Ossicles:
  • The three ossicles are the malleus (hammer), incus (anvil), and stapes (stirrup).
  • The stapes is the smallest bone in the body. It consists of a head, crus, and a footplate.
  • Sound Transmission Process:
  • Sound waves enter the ear canal and vibrate the malleus.
  • The malleus vibrates the incus.
  • The incus vibrates the stapes.
  • The stapes acts as a piston, "stomping" on the oval window (vestibular window) to send signals to the inner ear.
• Auditory (Eustachian) Tube:
  • Connects the middle ear to the nasopharynx to equalize pressure.
  • Guttural Pouch (Equine): A ventral expansion of the auditory tube in horses. Clinical issues include dysphagia caused by focal guttural pouch mycosis, which can lead to mononeuropathy of the pharyngeal ramus of the vagal nerve.

The Inner Ear and Vestibular Apparatus

• The inner ear is housed within the petrous temporal bone and consists of the vestibular apparatus (for motion and balance) and the cochlea (for hearing).
• Vestibular Apparatus Components:
  • Semicircular Ducts: Three ducts (anterior, posterior, and lateral) that detect angular acceleration, deceleration, and rotational movement.
  • Utricle and Saccule: Located within the vestibule, these detect static equilibrium and linear acceleration.
• Specialized Sensory Structures:
  • Maculae: Located in the utricle (macula utriculi) and saccule (macula sacculi).
  • The macula utriculi lies horizontally, and the macula sacculi lies vertically.
  • Structure: Consists of neuroepithelial hair cells with stereocilia embedded in a gelatinous otolithic membrane. Otoliths (calcium carbonate crystals) sit atop this membrane.
  • Function: Head tilting or linear acceleration causes the heavy otoliths to shift the membrane, bending the stereocilia and sending signals to the CNS.
  • Crista Ampullaris:
  • Located in the ampulla (widened base) of each semicircular duct.
  • Structure: A ridge of sensory epithelium covered by a gelatinous mass called the cupula.
  • Function: When the head rotates, endolymph fluid lags due to inertia, bending the cupula and the embedded stereocilia, which initiates nerve impulses.
• Fluid Systems of the Inner Ear:
  • Endolymph: Fluid found within the membranous labyrinth. It is high in potassium ($K^+$). It is essential for the normal function of inner ear cells.
  • Perilymph: Fluid within the bony labyrinth (specifically the scala vestibuli and scala tympani).

The Cochlea and Hearing

• The cochlea is a spiral structure divided into three parallel chambers:
  • Scala Vestibuli: Part of the bony labyrinth, filled with perilymph.
  • Scala Tympani: Part of the bony labyrinth, filled with perilymph.
  • Cochlear Duct (Scala Media): Part of the membranous labyrinth, filled with endolymph.
• Spiral Organ (Organ of Corti):
  • Located within the cochlear duct on the basilar membrane.
  • Contains hair cells that sense vibrations perceived as sound.
  • Cell Types and Structures:
  • Inner Hair Cells: A single row of sensory cells.
  • Outer Hair Cells: Three rows of sensory cells.
  • Phalangeal Cells: Support the inner and outer hair cells and are anchored to the basilar membrane.
  • Pillar Cells: Form a triangular inner tunnel and serve as a fulcrum for the organ.
  • Mechanics of Hearing:
  • Fluid waves in the scala vestibuli cause deviation of the basilar membrane.
  • This leads to the deflection of the apical hair cell stereocilia.
  • Associated nerve endings at the basolateral edge of the hair cells transduce this mechanical deflection into electrical signals for sound detection.

Comparative Anatomy and Other Special Senses

• Species Adaptations in the Endolymphatic System:
  • In amphibians and reptiles, the endolymphatic system can form large sac-like diverticula behind the eyes.
  • These structures may function in controlling pressure, fluid homeostasis, and ion (specifically calcium) homeostasis.
• Lateral Line System:
  • Found in fish and both larval and adult aquatic amphibians.
  • Functions as a tactile sense organ to detect movements and pressure changes in the surrounding water.
  • Histology of the lateral line organ includes sensory hairs, hair cells, sustentacular (supporting) cells, basal cells, and mantle cells.
• Chemoreception and the Vomeronasal Organ