Gross Anaatomy Lf The Ear Flashcards
What is the innervation of the auricle?
Innervation:
Posterior 2/3rd: Great auricular (C2,C3) and lesser occipital nerve (C2) Anterior 1/3rd: Auriculotemporal nerve (V3) with small contributions
from CN VII and X
Describe the development of the auricle
Ear develops at first high up in the neck from 6 auricular hillocks - 3 from 1st arch and 3 from 2nd arch mesenchyme tissue
Malformation of their development can lead to auricular sinuses and cysts
With the development of the mandible it migrates to its definitive position
What causes pre-aucular sinus?
ue to incomplete fusion of the primitive tubercles that form the pinna
Auricular sinuses/pits are usually present anterior to the auricle and considered remnants of the 1st pharyngeal groove
Relatively common
Describe the structure and innervation of the external acoustic meatus
- Canal is 2-3 cm long in adults
- Lateral 1/3rd is cartilaginous
- Medial 2/3rd is bony and part of tympanic portion of temporal bone
- Innervation: Auriculotemporal nerve (V3) except for small area via CN X
Describe the structure function of the middle ear
- Air filled cavity in the petrous part of the temporal bone
- Three ossicles which conduct vibrations of the tympanic membrane:
- Malleus – ‘mallet’
- Incus – ‘anvil’
- Stapes – ‘stirrup’
- Chorda tympani nerve
- Tensor tympani tendon (dampens sound)
- Stapedius muscle (dampens sound)
- Pharyngotympanic tube (to nasopharynx)
Describe structure of the ear
• Translucent membrane separating external meatus from middle ear
• External surface covered by very thin layer of skin, internal surface by thin layer
of mucous membrane
- Membrane is concave externally; central area called umbo, attached to malleus
- Most of membrane is tense but there is a flaccid part superior to the malleus
- Cone of light - a triangular area seen upon otoscopic examination of a normal ear
Describe the innervation of the tympanic membrane
Innervation:
• external surface auriculotemporal nerve (V3) except for small area by CN VII and CN X
• internal surface is by tympanic plexus (CN IX)
Explain how the facial nerve leaves the skull
A.
Exit from skull
1. leaves cranial vault via internal acoustic meatus
2. leaves meatus and passes through facial canal in petrous part of
temporal bone
a. canal takes a sharp posterior turn on medial wall of middle ear
b. geniculate ganglion lies at the turn
c. canal turns inferiorly and ends at stylomastoid foramen
What are the functions of the facial nerve?
- motor root is larger
a. carries motor fibres to stapedius, stylohyoid, posterior belly of
digastric, muscles of facial expression - nervus intermedius is smaller root
a. carries somatic sensory from external auditory meatus, parasympathetic
preganglionics and taste fibres
b. greater petrosal nerve carries parasympathetics to pterygopalatine ganglion
and taste from soft palate
How does the middle ear develop?
Develops from the distal expanded part of the tubotympanic recess arising from
the 1st pharyngeal pouch
What are the auditory ossicles?
Bones:
• Malleus & incus from 1st arch cartilage
• Stapes from 2nd arch cartilage
Muscles:
Muscles:
• Tensor tympani from 1st arch
• Stapedius from 2nd arch
Ossicles develop by endochondral ossification
Ossicles are of adult size at birth
How does the external ear develop?
Surface ectoderm grows & forms a solid meatal plug which undergoes canalization to form the external auditory meatus
The meatus is short and reaches adult size by 9th year of life
How does the tympanic membrane develop?
Tympanic membrane develops from the 1st pharyngeal (branchial) membrane It is derived from all three germ layers:
- ectoderm of the 1st groove
- endoderm of the 1st pouch
- mesoderm invades in between
What are the features of the lateral wall of the middle ear?
Features:
Primarily tympanic membrane with handle of malleus
Chorda tympani nerve between malleus handle and incus
Tensor tympani tendon attached to handle of malleus
What is the branch of the facial nerve of the middle ear?
Chorda tympani nerve:
- afferent special sensory fibers for taste from ant. 2/3 of tongue
- efferent preganglionic parasympathetic innervation for submandibular and sublingual glands (synapse in submandibular ganglion
What are the clinical significance if the middle ear?
- Disruption of ossicular chain (conductive hearing loss)
- Stapes (otosclerosis)
- Mastoid (mastoidectomy, approach for many procedures)
- Round window (cochlear implant)
- Tympanic membrane (tympanostomy tubes/grommets/ventilating tubes)
- Tympanic plexus (tympanic neurectomy/reduction of salivation)
- Chorda tympani (reduction of salivation)
What are the features of the medial wall of the middle ear ?
Features: Oval window (footplate of stapes) and round window (membrane)
Promontory (with tympanic plexus)
Bony prominences from facial nerve and lateral semicircular canal
What is the tympanic plexus Innervated by?
Tympanic nerve (from CN IX) -carries sensory afferents from entire middle ear region and preganglionic parasympathetics destined for parotid gland (lesser petrosal nerve to synapse at otic ganglion )
Describe the facial nerve throughout the ear
CN VII and VIII leave posterior cranial fossa via internal acoustic meatus
• meatus becomes facial canal after CN VIII portion
• geniculate ganglion located in facial canal, a collection of sensory
neuron cell bodies of CN VII
Greater petrosal nerve - preganglionic parasympathetics to lacrimal and
mucosal glands via pterygopalatine ganglion
Distal to geniculate ganglion, the facial canal runs in the medial wall (horizontal course), then in the posterior wall (vertical course) of the middle ear, where if gives of the stapedial nerve and chorda tympani nerve
What are the boundaries of the middle ear?
Posterior wall
separates cavity from mastoid air cells
Lateral wall - mainly tympanic membrane
Floor - base of skull near jugular foramen
Medial wall
separates cavity from inner ear
Anterior wall
separates cavity from carotid canal
What are the contents of the middle ear?
Contents
a) auditory ossicles
b) chorda tympani nerve and tympanic nerve plexus
c) tendons of tensor tympani and stapedius muscle
What are the contents of the middle ear?
Contents
a) auditory ossicles
b) chorda tympani nerve and tympanic nerve plexus
c) tendons of tensor tympani and stapedius muscle
What’s the innervation 9f the middle ear?
5.
Innervation: glossopharyngeal nerve via the tympanic plexus
What nerve innervates the inner ear?
Intracranial view
CN VIII – has two special sensory divisions: Vestibular nerve -equilibrium and balance
Cochlear nerve -hearing
How does the inner ear develop?
The internal ear is the first part to develop ~ 4th week as an invagination of the surface ectoderm
Otic vesicle will eventually expand into endolymphatic duct
The otic vesicle, derived from the ectoderm, will give rise to the membranous labyrinth
The otic vesicle induces the surrounding mesenchyme to condense and differentiate into a cartilaginous otic capsule
The cartilaginous otic capsule undergoes ossification to form the bony labyrinth (cochlea,
vestibule & semicircular canals)
Describe atresia of the EAM
Absent external acoustic meatus (failure of central cells of meatal plug to canalize)
Normal situation: pharyngeal groove closed by meatal plug up to 7th months when central cells begin to degenerate and form the acoustic meatus
What is the significance if an ear ache?
Earache = otalgia
• Through pathology of the ear itself or via referred pain
• Always think of all the nerves that contribute to the ear:
C3-4, CN V3, VII, IX, X
Temporal bone consists of five parts (squamous, petrous, tympanic, mastoid, styloid)
Mastoid part not yet well developed at birth (facial nerve more exposed, beware with forceps delivery!)
Question:
ICA travels in which bone?
Bell’s palsy is a condition that causes a temporary weakness or paralysis of the muscles in the face.
A facial paralysis caused by a vascular event, severance of the nerve, tumor, is not a Bell’s palsy.
