The Ear - Human Anatomy and Physiology B. Pharma 1st Semester

The Ear

Objectives

At the end of this lecture, student will be able to

• Describe the anatomy of ear

• Identify the receptor organs for equilibrium

• Describe the function of receptor organs for equilibrium

• Describe the auditory pathway

• Explain the major events in the physiology of hearing

Content

• Auditory Pathway

• Physiology of hearing

The Ear

• Organ of hearing

• Supplied by the 8th cranial nerve, stimulated by vibrations caused by sound waves

• Entire structure encased within the petrous portion of temporal lobe, except pinna

Anatomy of the ear

Ear is divided into three main regions

1. External (Outer) Ear

      - Auricle (Pinna)

      - Eardrum

      - External auditory canal

2. Middle Ear

      - Auditory ossicles

      - Auditory (eustachian) tube

3. Internal (inner) ear

      - Cochlea

      - Vestibular apparatus

                                - Semicircular ducts

                               -  Utricle

                                - Saccule

External (Outer) Ear

Auricle (Pinna)

• Flap of elastic cartilage

• Rim of the auricle is the helix

• The inferior portion is the lobule

• Ligaments and muscles attach the auricle to the head

External auditory canal

• A curved tube, 2.5 cm long

• Lies in the temporal bone and leads to the eardrum

Eardrum/ Tympanic membrane

• A thin, semitransparent partition between the external auditory canal and middle ear

• Covered by epidermis; lined by simple cuboidal epithelium

• Tearing of the tympanic membrane - perforated eardrum

Ceruminous glands - specialized sweat glands in the external auditory canal; secrete wax called cerumen

• Hairs + cerumen  à  prevent entry of dust and foreign objects

• Prevents damage

Middle Ear

• Small, air-filled cavity in the petrous portion of the temporal bone, lined by epithelium

• Separated from the external ear by the tympanic membrane

• From the internal ear by a thin bony partition

• Contains two small membrane-covered openings:   

- The oval window

- The round window

Auditory ossicles

• Three smallest bones in the body

• Extending across the middle ear and attached to it by ligaments

• Connected by synovial joints

The bones, named for their shapes

• The malleus (Hammer)

• Incus (Anvil)

• Stapes (Stirrup)

• Incus, the middle bone in the series, articulates with the head of the stapes

• Base or footplate of the stapes fits into the oval window

• Round window

– Opening below the oval window

– Enclosed by a membrane, the secondary tympanic membrane

Two tiny skeletal muscles attach to the ossicles

Tensor tympani muscle	Stapedius muscle
• Supplied by the trigeminal (V) nerve	• Supplied by the facial (VII) nerve
• Limits movement	• Smallest skeletal muscle in the human body
• Increases tension on the eardrum	• Protects the oval window
• Prevent damage to the inner ear from loud noises	• decreases sensitivity of hearing

The Auditory ossicles

Auditory (eustachian) tube

• Opening in the anterior wall of the middle ear

• Consists of both bone and elastic cartilage

• Connects the middle ear with the nasopharynx

• Normally closed at its medial (pharyngeal) end

• During swallowing and yawning, it opens

• Allows air to enter or leave the middle ear until the pressure in the middle ear equals the atmospheric pressure

Internal (inner) ear

• Also called the labyrinth

• Because of its complicated series of canals

Consists of two main divisions:

• An outer bony labyrinth encloses an inner membranous labyrinth

Bony labryinth - A series of cavities in the petrous portion of the temporal bone

Divided into three areas:

(1) The semicircular canals, contain receptors for equilibrium

(2) The vestibule, contain receptors for equilibrium

(3) The cochlea, contains receptors for hearing

• Bony labyrinth, lined with periosteum and contains perilymph

• Epithelial membranous labyrinth contains endolymph

• Potassium ions in endolymph helps in generation of auditory signals

• Vestibule, the oval central portion of the bony labyrinth

• Membranous labyrinth in the vestibule consists of two sacs, the utricle and the saccule

• Semicircular canals – 3 superior and posterior bony projections from the vestibule

• Ampulla - swollen enlargement at one end of each canal

• Semicircular ducts - Portions of the membranous labyrinth inside the bony semicircular canals

Cochlea

• A bony spiral canal, anterior to the vestibule

• Resembles a snail’s shell

• Makes almost three turns around a central bony core called the modiolus

Divided into three channels

• Cochlear duct

• Scala vestibuli

• Scala tympani

Cochlear duct (Scala media)

– A continuation of the membranous labyrinth into the cochlea

– Filled with endolymph

• Channel above the cochlear duct, Scala vestibuli; ends at the oval window

• Channel below, Scala tympani; end at the round window

• Vestibular membrane separates the cochlear duct and Scala vestibuli

• Basilar membrane separates the cochlear duct from the Scala tympani

Section through one turn of the cochlea

• Resting on the basilar membrane is the spiral organ or organ of Corti

• Spiral organ, coiled sheet of epithelial cells

• Consists of supporting cells and about 16,000 hair cells

• Hair cells - receptors for hearing

• Tectorial membrane, a flexible gelatinous membrane, covers the hair cells of the spiral organ

• Sensory neurons + motor neurons à cochlear branch of the vestibulocochlear (VIII) nerve

• Cell bodies of the sensory neurons are located in the spiral ganglion

Enlargement of spiral organ (organ of Corti)

Physiology of hearing: Events are involved in hearing

Hair cells of the spiral organ convert a mechanical vibration (stimulus) into an electrical signal (receptor potential)

The auditory pathway

Physiology of equilibrium

Two types of equilibrium (balance)

Static equilibrium- maintenance of the position of the body (mainly the head) relative to the force of gravity

• The maculae of the utricle and saccule are the sense organs

• Body movements like tilting the head and linear acceleration or deceleration stimulate the receptors

Dynamic equilibrium - maintenance of body position (mainly the head) in response to rotational acceleration/ deceleration

Position of a cupula with the head in the still position (left) and when the head rotates

• Cristae in the semicircular ducts, main sense organs

• Any change of position of the head causes movement in the perilymph and endolymph, bends the hair cells, stimulates sensory receptor in utricles, saccule and ampullae

• The nerve impulse passed to cerebellum through vestibulo cochlear nerve

Summary

• Ear is the organ for hearing

• External (outer) ear - auricle, external auditory canal, and tympanic membrane (eardrum)

• Middle ear - auditory tube, ossicles, oval window, and round window

• The internal (inner) ear consists of the bony labyrinth and membranous labyrinth

• The internal ear contains the spiral organ (organ of Corti), the organ of hearing

• Hair cells of the spiral organ convert a mechanical vibration (stimulus) into an electrical signal (receptor potential)

• Static equilibrium is the orientation of the body relative to the pull of gravity; utricle and saccule are the sense organs of static equilibrium

• Dynamic equilibrium is the maintenance of body position in response to rotational acceleration or deceleration; ristae in the semicircular ducts are the main sense organs of dynamic equilibrium