Anatomy of Internal ear

Anatomy of Internal ear. The internal ear, also known as the labyrinth is an important organ of hearing via the cochlea and balance via the vestibular system. The internal ear is located between the middle ear and the brain. The internal ear is connected to the middle ear via the oval and round windows. In contrast, the internal ear is connected to the brain via the internal acoustic meatus and the cochlear aqueduct.

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.

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 outer bony labyrinth and inner membranous labyrinth. Perilymph is filled in the space present between membranous and bony labyrinths. The membranous labyrinth is suspended/ floating in the perilymph and is filled with a clear fluid called endolymph.

1. The membranous labyrinth consists of the cochlear duct, the utricle and saccule, the three semicircular ducts, and the endolymphatic duct and sac.
2. Bony labyrinth consists of a vestibule, three semi-circular canals and a cochlea.

MEMBRANOUS LABYRINTH. 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.

 

The general structure of the membranous labyrinth is:

• The utricle and saccule are present in the vestibule of the bony labyrinth. Its sensory epithelium is called the macula.
• The three semicircular ducts are present in the three semicircular canals of the bony labyrinth. Its sensory epithelium is called the cristae.
• The cochlear duct is present in the cochlea of the bony labyrinth. The sensory epithelium of the cochlea is the organ of Corti.
• All of the sensory epithelia (macula, crista and organ of Corti) are composed of two main cell types: sensory hair cells and non-sensory supporting cells.

1. Utricle and saccule.

• 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. 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.
• The utricle is bigger than the saccule and lies superior to the saccule. The sensory epithelium (sense organ) of both utricle and saccule is called the macula and the sensory epithelium of the three semicircular ducts is the cristae. 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. The macula is related to linear or gravitational acceleration and cristae is related to rotational or angular acceleration.

2. Semicircular ducts. They are three in number and open in the utricle. Each semicircular duct is same in shape to its complementary bony semicircular canal including a dilated end forming the ampulla. They correspond exactly to the three bony canals. The ampullated end of each duct contains a thickened ridge of neuroepithelium called the crista ampullaris. Its sensory epithelium is called cristae which is related to linear or gravitational acceleration and cristae is related to rotational or angular acceleration.

3. Cochlear duct. Also called membranous cochlea or the scala media. It is a blind coiled tube.

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

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

• the basilar membrane (floor), which supports the organ of Corti and separates the scala media endolymph from the perilymph in the scala tympani. Its length increases as it proceeds from the basal coil to the apical coil. So, the higher frequencies of sound are heard at the basal coil while lower tones at the apical coil.
• the Reissner’s membrane (roof), which separates it from the scala vestibule. It separates scala endolymph from the perilymph in the scala vestibuli.
• the stria vascularis, which contains vascular epithelium and is concerned with secretion of endolymph. It contains vascular epithelium and secrets endolymph.

Cochlear duct is connected to the saccule by ductus reuniens. 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. The utricle is the larger of the two sacs. It is oval, elongated and irregular in shape and is in the posterosuperior part of the vestibule of the bony labyrinth.

Overview of Inner ear

 

BONY LABYRINTH. It consists of three parts: a vestibule, three semi-circular canals and a cochlea. 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.

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

1. Vestibule. It is the central chamber (5 mm) of the labyrinth. It is separated from the middle ear by the oval window and communicates anteriorly with the cochlea and posteriosuperiorly with the five openings of semi-circular canals. The vestibule communicates with the posterior cranial fossa through the vestibular aqueduct. The endolymphatic duct that passes through the vestibular aqueduct.

Vestibule has following important structures:

• On its lateral wall – Oval window (Fenestra vestibuli) is present which is closed by footplate of stapes.
• On its medial wall – Two recesses are present, a spherical recess, which lodges the saccule, and an elliptical recess, which lodges the utricle. Recesses are part of bony labyrinth while saccule and utricle are part of membranous labyrinth. Opening of aqueduct of vestibule is also present below the elliptical recess. Through this passes the endolymphatic duct.

2. Semicircular canals. There are three semicircular canals (SCC), the lateral (horizontal), posterior and superior (anterior). Each canal has a diameter of 0.8 mm and occupies 2/3rd of a circle. The length of lateral SCC is 12–15 mm, posterior SCC is 18–22 mm and superior SCC is 15–20 mm. 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. All three lie in planes at right angles to one another.

• Lateral SCC detects rotation of the head in the transverse plane (vertical axis).
• Posterior SCC detects rotation of the head in the coronal plane.
• Superior SCC detects rotation of the head in the sagittal plane.

Each canal has an ampullated end which opens independently into the vestibule. The non-ampullated ends of posterior and superior canals unite to form a common opening/ channel called crus commune (4 mm length). Crus commune opens into the medial part of vestibule. So, the three SCCs open into the vestibule by five openings and not by six openings.

3. Bony Cochlea. It is present anterior to vestibule. 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 which contains perilymph.
2. Scala tympani which contains perilymph.
3. Scala media or the membranous cochlea which contains endolymph. It is a part of a membranous labyrinth and not a part of bony labyrinth.

The scala vestibuli and scala tympani communicate with each other through an opening called helicotrema at the apex of the cochlear duct. The footplate of stapes closes the scala vestibuli separating it from the middle ear. Secondary tympanic membrane also known as the round window membrane closes the scala tympani separating it from the middle ear. The scala tympani runs parallel to the scala vestibuli and terminates at the round window. It is also connected with the subarachnoid space through the aqueduct of cochlea. The bony bulge in the medial wall of middle ear, the promontory, is due to the basal coil of the cochlea.

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

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 ——–

Reference Textbooks.

• Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
• Glasscock-Shambaugh, Textbook of Surgery of the Ear.
• Susan Standring, Gray’s Anatomy.
• Frank H. Netter, Atlas of Human Anatomy.
• B.D. Chaurasiya, Human Anatomy.
• 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.
• Ganong’s Review of Medical Physiology.
• Guyton & Hall Textbook of Medical Physiology

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