Anatomy of Internal ear

Anatomy of Internal ear.

The internal ear, also called the labyrinth, is a crucial organ responsible for hearing (via the cochlea) and balance (via the vestibular system). It is one of the most protected structures in the body, safely enclosed within the petrous part of the temporal bone.

Location & Connections

• The internal ear is positioned between the middle ear and the brain, deep within the skull.
• It connects to the middle ear through two small openings:
  • The oval window – where sound vibrations from the stapes enter the cochlea.
  • The round window – which allows pressure release from the cochlear fluid movements.
• It communicates with the brain through two key pathways:
  • The internal acoustic meatus transmits the cochlear nerve (for hearing) and the vestibular nerve (for balance).
  • The cochlear aqueduct is a small canal involved in fluid regulation.

Functions of the Inner ear.

• It is responsible for the transduction of mechanical energy into electrical signals that can then be passed to the brain along the auditory or vestibular nerves. Movements of the stapes footplate are transmitted to the cochlear fluids which moves the basilar membrane and sets up a shearing force between the tectorial membrane and the hair cells. The distortion of hair cells gives rise to cochlear microphonics, which triggers the nerve impulse.
• The internal ear is also responsible for maintaining balance by detecting position and motion.

Parts of the Inner ear. The internal ear is divided into parts:

1. Bony labyrinth – a rigid, protective shell filled with perilymph. It consists of a vestibule, three semi-circular canals and a cochlea.
2. Membranous labyrinth – a delicate structure inside the bony labyrinth, filled with endolymph and housing the sensory receptors. It consists of the cochlear duct, the utricle and saccule, the three semicircular ducts, and the endolymphatic duct and sac.
   
   

 

 

BONY LABYRINTH.

The bony labyrinth develops from the mesoderm that surrounds the membranous labyrinth which first ossifies into cartilage and then ossifies into the bone to form the bony labyrinth. This process is called as enchondral ossification.

The bony labyrinth has three distinct parts:

• Vestibule (central part)
• Semicircular canals (posterior part)
• Cochlea (anterior part)

Bony labyrinth showing the vestibule, the semicircular canals and the cochlea.

 

Overview of Inner ear

 

1. Vestibule. It is a small, oval-shaped central chamber (5 mm) of the bony labyrinth. It is located between the middle ear and the internal acoustic meatus. It is separated from the middle ear by the oval window and communicates anteriorly with the cochlea and posterosuperiorly with the five openings of semi-circular canals. The vestibule communicates with the posterior cranial fossa through the vestibular aqueduct. The endolymphatic duct passes through the vestibular aqueduct.

The vestibule has the following important structures:

• Lateral wall – The oval window (fenestra vestibuli) is present over the lateral wall which is closed by the footplate of stapes.
• Medial wall – Two recesses are present over the medial wall, a spherical recess, which (houses the saccule; and has multiple apertures called the macula cribrosa media for the passage for inferior vestibular nerve filaments, and an elliptical recess, which houses the utricle; and has multiple apertures called the macula cribrosa superior (Mike’s dot) for the passage for superior vestibular nerve filaments. Recesses are part of bony labyrinth while saccule and utricle are part of membranous labyrinth. Opening of the aqueduct of vestibule is also present below the elliptical recess. Through this passes the endolymphatic duct.

2. Semicircular canals. There are three semicircular canals.

• Superior (Anterior) Canal – 15-20 mm long, oriented perpendicular to the petrous bone. It detects the rotation of the head in the sagittal plane.
• Lateral (Horizontal) Canal – 12-15 mm long, forming a bulge in the aditus ad antrum. It detects the rotation of the head in the transverse plane (vertical axis).
• Posterior (Vertical) Canal – 18-22 mm long, running parallel to the posterior part of the temporal bone. It detects the rotation of the head in the coronal plane.

All three lie in planes at right angles to one another. The angle formed by the three semicircular canals is called the solid angle. Each canal has a diameter of 0.8 mm and occupies 2/3rd of a circle. The lateral SCC is related to the middle ear while the superior SCC is related to the base of the skull. The lateral SCC is the shortest SCC and is set at an angle of about 30 degrees to the horizontal plane. Each canal has:

• An ampullated end (containing sensory receptors “cristae” for movement detection) of the three semicircular canals opens separately into the vestibule.
• A non-ampullated end, which is in the superior and posterior canals, joins to form the crus commune (4 mm length), while the non-ampullated end of the posterior canal opens separately into the vestibule. Crus commune opens into the medial part of the vestibule. So, the three SCCs open into the vestibule by five openings and not by six openings.

  A key anatomical landmark is Trautmann’s Triangle, a weak area bounded by:

• Anteriorly: Bony labyrinth.
• Posteriorly: Sigmoid sinus.
• Superiorly: Superior sagittal sinus.

This region is important because infections can spread through it to the posterior fossa and also making it a surgical access point.

3. Bony Cochlea. It is present anterior to vestibule. It measures 5 mm in height and 9 mm in base diameter, 30 mm in length. The bony cochlea twists on itself to form a coiled tube (like a snail) making 2.5 to 2.75 turns (30 mm length) around a central pyramid of bone called modiolus. The wide base of modiolus is directed towards the internal acoustic meatus and transmits vessels and nerves to the cochlea. A thin plate of bone called osseous spiral lamina, winds spirally like the thread of a screw around the modiolus. The spiral ganglions are present in Rosenthal’s canal, which runs along the osseous spiral lamina. This osseous spiral lamina gives attachment to the basilar membrane and divides the bony cochlear tube into three compartments:

1. Scala vestibuli or vestibular duct (upper chamber) – The scala vestibuli is in continuity with the vestibule at the oval window and is closed by the footplate of the stapes. 
2. Scala tympani or tympanic duct (lower chamber) – The scala vestibuli and scala tympani communicate with each other through an opening called helicotrema at the apex of the cochlear duct. The scala tympani runs parallel to the scala vestibule. The secondary tympanic membrane also known as the round window membrane closes the scala tympani separating it from the middle ear. 
3. Scala media or cochlear duct (middle chamber) – The cochlear duct (scala media) is separated from the scala vestibule by the Reissner membrane and from the scala tympani by the basilar membrane. It houses the organ of Corti, the primary sensory organ of hearing. It is a part of a membranous labyrinth and not a part of bony labyrinth.

Perilymphatic system. CSF passes into the scala tympani through the aqueduct of the cochlea.

MEMBRANOUS LABYRINTH.

The membranous labyrinth is suspended/ floats inside the bony labyrinth and contains endolymph. The membranous labyrinth has three distinct parts:

• Membranous Vestibular Labyrinth (present inside vestibule)
• Membranous Semicircular Canals (Semicircular ducts)
• Membranous Cochlea (Cochlear duct)

It consists of two sacs (utricle and saccule) and four ducts (three semicircular ducts and one cochlear duct). The utricle, saccule, and three semicircular ducts are related to the balance (equilibrium) while the cochlear duct is related to the sense of hearing.

1. Membranous Vestibular Labyrinth (present inside vestibule). It includes:

• Utricle – The utricle is present in the posterior part of the bony vestibule in the elliptical recess.  The five openings of the three semicircular ducts open in the utricle. The utricle is connected to the saccule through the utriculosaccular duct, which continues as the endolymphatic duct that passes through the vestibular aqueduct. The terminal part of the endolymphatic duct is dilated to form an endolymphatic sac between the two layers of the dura (intradural) on the posterior surface of the petrous bone in the posterior cranial fossa and it is not communicating with the CSF of the brain.  The function of the endolymphatic sac is to absorb endolymph. The endolymphatic sac is exposed for drainage or shunt operation in Meniere’s disease.
• Saccule – The saccule is present in the spherical recess of the bony vestibule, anteroinferiorly to the utricle and opposite the stapes footplate. It detects vertical movements. The utricle is bigger than the saccule and lies superior to the saccule. The saccule is also connected to the cochlear duct (scala media/ membranous cochlea) by a thin duct, ductus reuniens. The cochlear duct empties into the saccule through the ductus reuniens. In Meniere’s disease, the distended saccule lies against the stapes footplate and can be surgically decompressed by perforating the footplate. 
• Maculae – The sensory epithelium (sense organ) of both the utricle and saccule is called the macula. It contains hair cells covered by a statoconial membrane, which has calcium carbonate crystals (otoliths) that are related to linear or gravitational acceleration. The utricle responds to linear acceleration in the horizontal plane and sideways head tilts. The saccule responds to linear acceleration in the vertical plane, such as forward-backward and upward-downward movements.

2. Membranous Semicircular ducts. It includes:

• Semicircular ducts – Each membranous semicircular duct runs inside the corresponding bony semicircular canal. They are three in number and open in the utricle. Each semicircular duct is the same in shape as its complementary bony semicircular canal including a dilated end forming the ampulla. They correspond exactly to the three bony canals.
• Cristae – The sensory epithelium (sense organ) of the three semicircular ducts is the cristae ampullaris. It is present at the ampullated end of each duct and it is related to rotational (head rotation) or angular acceleration.

3. Cochlear duct (membranous cochlea or scala media). It occupies the middle portion of the cochlear canal and it is connected to the saccule by ductus reuniens. It is a blind coiled tube.

It appears triangular on cross-section and its three walls are formed by:

• Floor – It is formed by the Basilar membrane, which supports the organ of Corti and separates the scala media endolymph from the perilymph in the scala tympani. The length of basilar membrane increases as we proceed from the basal coil (base of modiolus) to the apical coil (apex of modiolus). It is for this reason that higher frequencies of sound are heard at the basal coil while lower ones are heard at the apical coil. 
• Roof – It is formed by the Reissner’s membrane, which separates it from the scala vestibule.
• Lateral wall – It is formed by the Stria vascularis, which contains vascular epithelium and is concerned with secretion of endolymph.

 

A section through the cochlea to show scala media (cochlear duct), scala vestibuli and scala tympani.

Organ of Corti (the main sensory organ for hearing).

The Organ of Corti is the primary sensory organ of hearing, located on the basilar membrane within the scala media of the cochlea. It is responsible for converting sound vibrations into electrical signals, which are then transmitted to the brain.

Key Components of the Organ of Corti

1. Tunnel of Corti. It is formed by two rows of pillar (rod) cells, which separate the inner hair cells and outer hair cells. It creates a triangular structure with the basement membrane and contains cortilymph, though its exact function remains unclear.
   
   • Inner Hair Cells (IHCs): They are arranged in a single row. They are richly supplied by afferent cochlear nerve fibres, making them crucial for transmitting auditory signals.
   • Outer Hair Cells (OHCs): They are arranged in three to four rows. The tips of the outer hair cells are attached to the undersurface of the tectorial membrane. They receive efferent innervation from the olivary complex, playing a role in modulating Inner hair cells function.
   • Both the IHC and OHC are supported by pillar cells, Deiters’ cells, and Hensen’s cells, which provide structural stability.
2. Hair Cells – The Sensory Receptors. They are important receptor cells of hearing and transduce sound energy into electrical energy.
3. Tectorial Membrane. It consists of a gelatinous material with delicate fibres that overlies the Organ of Corti. When sound waves cause vibrations of the basilar membrane, the shearing force between the hair cells and the tectorial membrane produces a stimulus to the hair cells.
4. Stria Vascularis. It is located on the lateral wall of the scala media (cochlear duct), which contains vascular epithelium and is concerned with secretion of endolymph. It plays a crucial role in maintaining the ionic composition and electric potential of the endolymph, which is essential for proper hair cell function.

 

Clinical Significance:

• Cochlear Implants. The cochlear implant electrodes are inserted through the round window (i.e. secondary tympanic membrane) to reach the internal ear. The electrodes are placed in the scala tympani to bypass the damaged organ of Corti and directly stimulate the auditory nerve.
• Drugs. Drugs like gentamicin or steroids also pass through the round window to reach the internal ear from the middle ear.
• Infections. Infections from the brain can easily enter the internal ear through the internal acoustic meatus and the cochlear aqueduct leading to labyrinthitis. Conversely, infections from the internal ear can spread to the brain. Therefore, hearing tests, such as BERA, are recommended, particularly in pediatric patients with meningitis, to rule out hearing loss.

INNER EAR FLUIDS AND THEIR CIRCULATION. There are two fluids in the inner ear: perilymph and endolymph. Perilymph fills the space between bony and membranous labyrinth while endolymph fills the entire membranous labyrinth.

1. Perilymph. It resembles extracellular fluid and is rich in Na ions. It communicates with CSF through the aqueduct of cochlea which opens into the scala tympani near the round window. In fact this duct is not a direct communication but contains connective tissue resembling arachnoid through which perilymph percolates. There are two views regarding the formation of perilymph: (i) It is a filtrate of blood serum and is formed by capillaries of the spiral ligament and (ii) it is a direct continuation of CSF and reaches the labyrinth via the aqueduct of cochlea.  

TABLE 1.2 COMPOSITION OF INNER EAR FLUIDS	Endolymph	Perilymph	CSF
Na+ (mEq/L)	5	140	152
K+ (mEq/L)	144	10	4
Protein (mg/dL)	126	200–400	20–50
Glucose (mg/dL)	10–40	85	70

Values are average and may differ slightly according to the site of collection of endolymph (cochlea, utricle, sac) and perilymph (scala tympani or scala vestibuli).

2. Endolymph. It fills the entire membranous labyrinth and resembles intracellular fluid, being rich in K ions. It is secreted by the secretory cells of the stria vascularis of the cochlea and by the dark cells (present in the utricle and also near the ampullated ends of semicircular ducts). There are two views regarding its flow: (i) longitudinal, i.e. endolymph from the cochlea reaches saccule, utricle and endolymphatic duct and gets absorbed through the endolymphatic sac, which lies in the subdural space and (ii) radial, i.e. endolymph is secreted by stria vascularis and also gets absorbed by the stria vascularis. This view presumes that the endolymphatic sac is a vestigial structure in man and plays no part in endolymph absorption.

BLOOD SUPPLY OF LABYRINTH

The entire labyrinth receives its arterial supply through labyrinthine artery, which is a branch of anterior-inferior cerebellar artery but sometimes from the basilar.

Divisions of the labyrinthine artery which supply blood to various parts of the labyrinth.

Venous drainage is through three veins, namely internal auditory vein, vein of cochlear aqueduct and vein of vestibular aqueduct, which ultimately drain into inferior petrosal sinus and lateral venous sinus.

It is to be noted that:

1. Blood supply to the inner ear is independent of blood supply to middle ear and bony otic capsule, and there is no cross circulation between the two.
2. Blood supply to cochlea and vestibular labyrinth is segmental, therefore, independent ischaemic damage can occur to these organs causing either cochlear or vestibular symptoms.

———— End of the chapter ————

Download full PDF Link:
Anatomy of Internal Ear Best Lecture Notes Dr Rahul Bagla ENT Textbook

Reference Textbooks.

• Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
• Glasscock-Shambaugh, Textbook of Surgery of the Ear.
• P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
• Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
• Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck Surgery
• Hans Behrbohm, Textbook of Ear, Nose, and Throat Diseases With Head and Neck Surgery.
• Salah Mansour, Middle Ear Diseases – Advances in Diagnosis and Management.
• Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
• Rob and smith, Textbook of Operative surgery.
• Anirban Biswas, Textbook of Clinical Audio-vestibulometry.
• Arnold, U. Ganzer, Textbook of  Otorhinolaryngology, Head and Neck Surgery.

Author:

Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Renowned Teaching Faculty
Mail: msrahulbagla@gmail.com
India

———– Follow us on social media ————

• Follow our Facebook page: https://www.facebook.com/Dr.Rahul.Bagla.UCMS
• Follow our Instagram page: https://www.instagram.com/dr.rahulbagla/
• Subscribe to our Youtube channel: https://www.youtube.com/@Drrahulbagla
• Please read. Anatomy of External Ear. https://www.entlecture.com/anatomy-of-ear/
• Please read. Anatomy of Temporal Bone. https://www.entlecture.com/anatomy-of-temporal-bone/
• Please read. Stenger’s, Chimani Moos, Teal test. https://www.entlecture.com/special-tuning-fork-tests/

Keywords: Helicotrema, Endolymphatic sac, Endolymph, Perilymph, Ductus Reunions, Macula, Saccule, Cristae, Cochlear aqueduct, Anatomy of Internal ear, Internal ear anatomy, Structure of the inner ear, Parts of the internal ear, Function of the inner ear, Cochlea and vestibular system, Detailed anatomy of the internal ear, How does the inner ear work? Bony labyrinth and membranous labyrinth explained, Role of the cochlea in the hearing process, Vestibular system and balance control, What are the parts of the inner ear? How does the cochlea help in hearing? What is the function of semicircular canals? How does the vestibular system maintain balance? What is the difference between the bony and membranous labyrinth? Anatomy of the Internal Ear: Structure, Function, and Clinical Significance, How the Inner Ear Works: Understanding Cochlea & Vestibular System, Complete Guide to Inner Ear Anatomy – Bony & Membranous Labyrinth Explained, Hearing and balance system, Auditory pathway of the inner ear, Sensory organs of the ear, Inner ear fluid dynamics, Ear equilibrium function/p>