Comprehensive Study Guide: Anatomy and Physiology of the Ears

Introduction to the Anatomy and Physiology of the Ear

The ear functions as the primary organ for hearing and balance, divided structurally and functionally into three main regions: the external ear, the middle ear, and the internal ear. Thong Mengyou, MD, a radiologist at Preah Kossamak Hospital who graduated from France, defines the external ear as the portion attached to the lateral aspect of the head and the canal leading inward. The middle ear is characterized as a cavity within the petrous part of the temporal bone, bounded laterally by a membrane that separates it from the external canal and connected internally to the pharynx via a narrow tube. The internal ear consists of a series of complex cavities located within the petrous part of the temporal bone, situated between the middle ear laterally and the internal acoustic meatus medially. The classification of the ear includes components such as the auricle, external acoustic meatus, cartilage, and tympanic membrane for the external ear; the ossicles and pharyngotympanic tube for the middle ear; and the internal acoustic meatus, cochlea, and semicircular canals for the internal ear.

Anatomy and Embryology of the External Ear

The external ear consists of three primary parts: the pinna (auricle), the external auditory canal (EAC), and the tympanic membrane. The anatomical features of the pinna include the helix, anti-helix, tragus, anti-tragus, lobule, concha, and the external acoustic meatus. The external acoustic meatus itself extends from the deepest portion of the concha to the tympanic membrane, covering a distance of approximately $1\,inch$ or $2.5\,cm$. Embryologically, the external ear develops from the ectodermal and mesodermal layers of the first and second branchial arches. The first branchial arch gives rise to the tragus and most of the helix, while the second branchial arch forms the antihelix, antitragus, lobule, and the inferior portion of the helix. Additionally, the dorsal portion of the first branchial cleft extends toward and eventually makes contact with the endoderm of the expanding tubotympanic recess.

Vascularization, Lymphatic Drainage, and Innervation of the External Ear

The arterial supply to the auricle is derived from numerous sources, primarily the posterior auricular artery, the anterior auricular artery, and the occipital artery. More specifically, the auricle receives blood from the posterior auricular and small auricular rami of the superficial temporal vessels, which originate from the external carotid artery. The external auditory canal is supplied by the posterior auricular artery, the auricular rami of the superficial temporal vessels, and the deep auricular artery, which is a branch of the first part of the maxillary artery. Venous drainage follows the path of these arteries. Lymphatic drainage of the auricle passes anteriorly into the parotid nodes and posteriorly into the mastoid (retroauricular) nodes, potentially reaching the upper deep cervical nodes. Other nodes involved in the region include the superficial cervical, occipital, buccal, submandibular, and submental nodes. Sensory innervation is complex, involving the auriculotemporal branch of the trigeminal nerve (V3), the great auricular nerve (C2, C3), and the lesser occipital nerve (C2, C3) from the cutaneous branch of the cervical plexus. It also incorporates the auricular branch of the vagus nerve (known as the nerve of Arnold) as well as cranial nerves VII (facial) and IX (glossopharyngeal).

Structure and Clinical Significance of the Tympanic Membrane

The tympanic membrane, or eardrum, serves as the boundary between the external acoustic meatus and the middle ear. It measures approximately $8-9\,mm$ by $9-10\,mm$. Structurally, it consists of three layers: an outer epithelial layer, a middle layer of yellow elastic fibrous tissue containing radial and circular layers, and an inner layer of mucus. The membrane is divided into the pars tensa and the pars flaccida, also referred to as Shrapnell's membrane. In the pars flaccida, the middle fibrous layer is absent; this area is located in the small uppermost part of the eardrum, often called the attic part. Chronic perforations in the pars flaccida are noted as potentially dangerous. Anatomical landmarks on the membrane include the umbilicus, the handle of the malleus, the short process of the malleus, the anterior and posterior mallear folds, and the light reflex. The membrane maintains an air cushion that prevents the insufflation of foreign material from the nasopharynx and protects the middle ear space. Its blood supply includes a small peripheral vascular ring formed by the deep auricular branch of the maxillary artery, the anterior tympanic branch of the maxillary artery, and the stylomastoid branch of the posterior auricular artery. Innervation of the outer surface is provided by the auriculotemporal branch of the trigeminal nerve and the auricular branch of the vagus nerve, while the inner surface is supplied by the tympanic plexus.

Anatomy of the Middle Ear and Ossicles

The middle ear is an air-filled, mucous membrane-lined space within the temporal bone, comprising the tympanum, the Eustachian tube, and the mastoid antrum and cells. The tympanum is vertically divided into three portions: the attic (uppermost), the mesotympanum (middle), and the hypotympanum (lowest). Within this space are the three smallest bones in the human body, known as the ossicles: the malleus (hammer), the incus (anvil), and the stapes (stirrup). The handle of the malleus is firmly embedded in the middle layer of the tympanic membrane. The incus consists of a body, a short limb, and a long limb. The stapes consists of a head, neck, anterior crus, posterior crus, and a footplate which occupies the oval window of the inner ear. Two striated muscles protect the ear: the tensor tympani, which attaches to the malleus and is innervated by the trigeminal nerve, and the stapedius, which attaches to the stapes and is innervated by the stapedial branch of the facial nerve.

Vascular and Neural Network of the Middle Ear

Arterial supply to the middle ear is extensive, involving the anterior tympanic artery (from the maxillary artery), the inferior tympanic artery (from the ascending pharyngeal artery), the stylomastoid artery (from the posterior auricular artery), the posterior tympanic artery, the middle meningeal artery (from the maxillary artery), and the caroticotympanic artery (from the internal carotid artery). Venous drainage occurs through the pterygoid plexus and the superior petrosal sinus. Lymphatic drainage involves the retropharyngeal and parotid lymph nodes. Innervation is primarily provided by the tympanic plexus, which includes the tympanic nerve from the glossopharyngeal nerve (IX), branches from the internal carotid plexus (caroticotympanic nerve), and the lesser petrosal nerve. The chorda tympani and the mandibular nerve also play roles in the middle ear's neural landscape.

The Eustachian Tube and Neonatal Variations

The Eustachian tube serves as a conduit for air exchange between the middle ear and the upper aerodigestive tract, specifically connecting the tympanum to the nasopharynx. In adults, it is approximately $35\,mm$ long ($11\,mm$ proximal, $24\,mm$ distal) and is situated at a $45\text{-degree}$ angle. It functions to maintain barometric pressure equality, drain secretions, and protect against nasopharyngeal infection via its respiratory epithelium. In children, the tube is significantly different; it is shorter ($17-18\,mm$ at birth), straighter, and wider, remaining open all the time. Furthermore, in newborns, there are remainders of embryonic mixoid tissue in the tympanum until age 3, and the walls are not fully ossified, allowing contact between the middle ear mucous membrane and the dura mater. The eardrum in newborns is thicker and set at a sharper angle than in adults. Regarding the mastoid bone, newborns possess only one air cell, the antrum or cave, meaning they cannot suffer from mastoiditis, though antritis is a frequent pathology.

Anatomy of the Inner Ear and the Organ of Corti

The inner ear is located in the petrous part of the temporal bone and consists of the bony labyrinth (containing perilymph) and the membranous labyrinth (containing endolymph). The bony labyrinth includes the vestibule, three semicircular canals (anterior, posterior, and lateral), and the snail-shaped cochlea, which is $35\,mm$ long and has $2.5$ turns. The membranous labyrinth includes the semicircular ducts, utricle, saccule, utriculosaccular duct, endolymphatic duct/sac, and the cochlear duct. The Organ of Corti is the essential organ of hearing located within the cochlear duct on the basilar membrane. It contains inner and outer hair cells that transduce mechanical energy into electrical neural energy. Supporting structures include the tectorial membrane, Reissner's membrane (vestibular membrane), stria vascularis, spinal ligament, and various cells such as Deiters', Hensen's, and Claudius' cells. The arterial supply comes from the internal auditory artery, which originates from the anterior inferior cerebellar artery ($83\%$) or the basilar artery ($17\%$).

Physiology of Hearing and Sound Transmission

Sound waves enter the ear and travel to the tympanic membrane, striking it and moving it medially. This medial movement is transferred to the handle of the malleus, which then moves the head of the malleus laterally. This articulates with the incus, moving its head laterally and its long process medially. Consequently, the stapes moves medially, pushing its base into the oval window. This process performs impedance matching, converting large-amplitude, low-force airborne waves into small-amplitude, high-force vibrations in the fluid-filled cochlea, providing a gain of $25-30\,dB$. The external ear itself provides a $15\,dB$ gain at $3\,kHz$ and a $10\,dB$ gain between $2-5\,kHz$. In the cochlea, the wave moves through the perilymph of the scala vestibuli, causing the secondary tympanic membrane of the round window to bulge outward. This vibrates the basilar membrane, stimulating hair cells in the Organ of Corti. These hair cells send impulses through the cochlear part of the vestibulocochlear nerve (VIII) to the brain. For protection, the middle ear muscles dampen vibrations for sounds exceeding $80\,dB$ at frequencies below $2\,kHz$.

Balance, Neural Pathways, and the Auditory Cortex

The inner ear serves dual functions: the cochlear duct is for hearing, while the semicircular ducts, utricle, and saccule are the organs of balance. In the Organ of Corti, transduction occurs as the displacement of the basilar membrane responds to the stapes. This involves the movement of potassium ($K^+$) ions. The endolymph in the scala media has a potential of $+80\,mV$. The displacement of stereocilia via tip links opens potassium channels, leading to depolarization. Neural signals travel via the vestibulocochlear nerve (CN VIII) through the spiral ganglion to the cochlear nuclei. From there, signals progress through the superior olivary complex, lateral lemniscus, and medial geniculate body to the auditory cortex. The primary auditory cortex (area A1) corresponds to Brodmann's area 41, while the associated auditory cortex (area A2) corresponds to Brodmann's areas 22 and 42.