Ear
Question 1. What is an ear? List its subdivisions.
Answer.
The ear is the organ of hearing and balance. It is subdivided into the following three parts (from lateral to medial):
- External (outer) ear
- Middle ear
- Internal (inner) ear
Question 2. Discuss the sensory nerve supply of the auricle/pinna.
Answer.
Lateral (facial) surface
- Lower one-third by great auricular nerve (C2, C3)
- Upper two-thirds by the auriculotemporal nerve
Medial (cranial) surface
- Lower one-third by great auricular nerve (C2, C3)
- Upper two-thirds by lesser occipital nerve (C2)
Question 3. Write a short note on external auditory (acoustic) meatus.
Answer.
- The external acoustic meatus is an ‘S’-shaped osseocartilaginous tube, that extends from the bottom of the concha to the tympanic membrane.
- It is about 24 mm long, of which the medial two-thirds (16 mm) is bony and the lateral one-third (8 mm) is cartilaginous.
- It conducts sound waves from the concha to the tympanic membrane.
- It develops from the first pharyngeal cleft.
Question 4. Write a short note on the tympanic membrane.
Answer.
Tympanic Membrane Introduction
- The tympanic membrane or eardrum is a semitransparent oval membrane, which separates the external acoustic meatus from the middle ear cavity.
- Its diameter measures about 9 × 10 mm and is placed obliquely at an angle of 55° with the floor of the external acoustic meatus. It faces downwards, forwards, and laterally. The circumference of the membrane is made up of a fibrocartilaginous ring. The sulcus is absent between anterior and posterior malleolar folds. The part of the membrane-enclosed between malleolar folds is called pars flaccida.
Tympanic Membrane Structure It consists of three layers:
- An outer cuticular layer (ectodermal in origin), continuous with the skin of the external auditory meatus.
- A middle fibrous layer (mesodermal in origin), consisting of superficial radiating fibers and deep circular fibers.
- An inner mucous layer (endodermal in origin), lined by ciliated columnar epithelium, continuous with the mucosa of the middle ear.
Tympanic Membrane Features
- Most of the tympanic membrane is tightly stretched and called pars tensa. A small upper part between two malleolar folds is loose and called pars flaccida (vide supra). The pars flaccida is crossed internally by the chorda tympani nerve.
- The tympanic membrane has outer and inner surfaces. The outer surface is concave. The inner surface is convex and provides attachment to the handle of the malleus, which extends up to its center. The point of maximum convexity on the inner surface is called the umbo. The cone of light is the reflection of light from the otoscope.
- The handle of the malleus is embedded in the middle fibrous layer.
Tympanic Membrane Applied anatomy
- The otoscopic examination may reveal the bulging, perforation, or retraction of the tympanic membrane.
- The membrane is incised (myringotomy) to drain the pus present in the middle ear.
Read And Learn More: Selective Anatomy Notes And Question And Answers
- The incision should be given in the posteroinferior quadrant of the membrane to avoid injury to the chorda tympani nerve.
- The rupture of the tympanic membrane usually occurs in pars flaccida.
Question 5. Describe the middle ear under the following headings:
- Location, shape, and dimension,
- Contents,
- Boundaries,
- Nerve supply and
- Applied anatomy.
Answer.
Middle Ear Location
- The middle ear is a narrow, slit-like, air-filled cavity in the petrous part of the temporal bone.
- It communicates anteriorly with the nasopharynx through the auditory tube and posteriorly with the mastoid antrum through the aditus ad antrum.
Middle Ear Shape Biconcave hollow disc, resembling the red blood cell (RBC).
Middle Ear Dimension
Vertical: 15 mm
Transverse:
- 6 mm: At roof
- 2 mm: In the center
- 4 mm: At floor
Middle Ear Contents
- Air
- Two muscles: Tensor tympani and stapedius
- Three ear ossicles: Malleus, incus and stapes
- Two nerves: Chorda tympani nerve and tympanic plexus.
Middle Ear Boundaries
Middle Ear Roof or tegmental wall:
Formed by tegmen tympani, a thin plate of bone that separates the middle ear from the middle cranial fossa.
Middle Ear Floor or jugular wall
- Formed by the jugular fossa of the temporal bone.
- Separates the middle ear from the superior bulb of the internal jugular vein.
Middle Ear Anterior wall or carotid wall
- The upper part presents Canals for tensor tympani and auditory tube.
- Lower part forms: Posterior wall of carotid canal.
Middle Ear Medial or labyrinthine wall: It separates the middle ear from the internal ear and presents the following features:
- Promontory: A rounded elevation produced by the first turn of the cochlea.
- Oval window (fenestra vestibuli): An oval opening posterosuperior to the promontory that leads to the vestibule of the internal ear. It is occupied by the base of stapes.
- Round window (fenestra cochleae): A round opening posteroinferior to the promontory that leads to the scala tympani of the cochlea and is closed by a secondary tympanic membrane.
- Sinus tympani: A depression behind promontory.
- The prominence of the facial canal is just above the oval window.
- The prominence of the lateral semicircular canal is above the prominence of the facial canal.
Middle Ear Lateral or membranous wall
- Separates the middle ear from the external ear.
- It is formed:
- Mainly by the tympanic membrane.
- Partly by the squamous part of the temporal bone in the region of the epitympanic recess.
Middle Ear Posterior wall or mastoid wall:
It presents the following features. From above downwards, these are:
- Aditus to mastoid antrum (aditus ad antrum)
- Fossa incudis: Depression for incus.
- Pyramid: A conical bony projection. The apex of the pyramid presents an opening for the tendon of the stapedius.
- Posterior canaliculus for chorda tympani, lateral to pyramid.
Middle Ear Nerve supply
- Tympanic branch of glossopharyngeal nerve.
- Superior and inferior caroticotympanic nerves from sympathetic plexus around the internal carotid artery.
Middle Ear Applied anatomy
Otitis media:
The throat infections commonly spread to the middle ear through the auditory tube. It is more common in children because in children the tube is shorter, wider, and horizontal.
The longstanding otitis media often leads to a collection of pus in the middle ear – a condition called chronic suppurative otitis media (CSOM). The pus from the middle ear:
- May be discharged in the external ear following the rupture of the tympanic membrane.
- May erode the roof leading to meningitis and temporal lobe abscess.
- May erode the floor causing thrombosis of the internal jugular vein.
- May spread backward into the mastoid antrum leading to a mastoid abscess.
Bleeding from ear: Fracture of the middle cranial fossa can cause bleeding through the ear.
Question 6. Write a short note on mastoid antrum.
Answer.
- The mastoid antrum is an air space in the mastoid portion of the temporal bone.
- It communicates anteriorly with the tympanic cavity through aditus ad antrum (entrance to the mastoid antrum).
Mastoid Antrum Boundaries
Roof is formed by tegmen tympani.
The lateral wall is formed by a plate of bone about 1.5 cm thick just deep into a supramental triangle.
The posterior wall is formed by a thin plate of bone that separates it from the sigmoid sinus.
Mastoid Antrum Functions
- Provides resonance to the voice.
- Acts as an acoustic insulator and provides protection to the middle ear from physical damage.
- Acts as sound receptor.
Mastoid Antrum Applied anatomy
- Mastoid air cells are a major contributor to the middle ear inflammatory disease.
- The lateral wall of the antrum is approached for surgery, through the supramental triangle.
Question 7. Write a short note on ear ossicles.
Answer.
- There are three tiny bones present in the middle ear cavity.
- From medial to lateral, these are malleus (hammer), incus (anvil), and stapes (stirrup).
Malleus is the lateral ossicle. It has a head, neck, and three processes, i.e. handle, lateral process, and anterior process.
- The head articulates with the incus
- The handle passes downwards and is attached to the tympanic membrane. The medial aspect of the handle receives insertion of tensor tympani.
- The anterior process is attached to the spine of the sphenoid by a ligament.
- The lateral process is attached to the tympanic sulcus (bony) by anterior and posterior malleolar folds.
Incus is the middle bone. It has a body, a short process, and a long process.
- The body articulates with the head of the malleus.
- A short crus is attached to the floor of aditus by a ligament.
- Long crus articulates with the head of stapes.
Stapes is the most medial (innermost) ossicle. It has a head, neck, anterior and posterior limbs, and footplate.
- The head articulates with the long process of the incus.
- Footplate is held in fenestra vestibule (oval window) by annular ligament.
- The neck of the stapes receives insertion of the stapedius.
Ear Ossicles Applied anatomy
The vibrations of sound waves are transmitted from the tympanic membrane to the fluid (perilymph) of the inner ear by the ossicular chain.
- Paralysis of stapedius leads to hyperacusis.
- Otosclerosis (abnormal ossification of the annular ligament that anchors the stapes footplate leads to conduction deafness).
Question 8. Briefly describe the internal ear.
Answer.
- The internal ear is involved in both hearing and balance.
- It consists of two components: a membranous labyrinth and a bony (osseous) labyrinth.
Internal Ear Membranous labyrinth
It consists of four membranous parts/structures:
- Cochlear duct
- Saccule
- Utricle
- Three semicircular ducts
All these parts are interconnected to each other to form a labyrinth.
Internal Ear Functions
- The sensory receptor within the cochlear duct is a spiral organ of the Corti. It is concerned with hearing.
- The sensory receptors present within the saccule and utricle are maculae. They are concerned with static balance.
- The sensory receptors within the semicircular ducts are cristae ampullaris. They are concerned with kinetic balance.
Internal Ear Bony labyrinth It consists of intercommunicating bony spaces in the petrous part of the temporal bone.
The bony labyrinth consists of three parts:
- Cochlea
- Vestibule
- Three semicircular canals
Question 9. Write a short note on the spiral organ of Corti.
Answer.
It is an end organ of hearing, located on the basilar membrane of the cochlear duct.
Spiral Organ of Corti Components Microscopically, the organ of Corti consists of five components:
- Basilar membrane: A fibrous membrane that extends from the osseous spiral lamina to the outer wall of the cochlear duct.
- Tunnel of Corti: Formed by inner and outer rod cells and contains corticolymph.
- Hair cells are receptor cells of hearing located on the basilar membrane. These cells bear stereocilia and form the most important component of the spiral organ of Corti. They are divided into inner and outer hair cells.
The hair cells perform the following functions:- Detect movements of endolymph
- Detect vibrations of the basilar membrane
- Transfer vibrations into nerve impulses to the cochlear nerve
- Supporting cells: The inner hair cells are supported by phalangeal cells, while the outer supporting cells are called Henson’s cells.
- Membrana tectoria: It is a gelatinous membrane that overlies the hair cells. The shearing force between the hair cells and tectorial membrane stimulates the hair cells.
Innervation of the organ of Corti The hair cells are innervated by the peripheral processes of bipolar neurons of the spiral ganglion located within the modiolus near the base of the spiral lamina. There are two types of neurons:
Type 1 neurons: They are myelinated and afferent. They innervate inner hair cells and are responsible for auditory sensation.
Type 2 neurons: They are unmyelinated and efferent from the contralateral superior olivary nucleus. They innervate outer hair cells and are responsible for auditory discrimination.
Leave a Reply