Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous System
Sensory Systems
Auditory and Vestibular Sensory Systems

General Information. The auditory and vestibular receptor apparatuses are located in the Inner ear. The vestibular apparatus is associated with statokinetic stimulation and is activated by any change in THE POSITION OF the HEAD, and consequently, the entire body in space. It is involved in maintaining balance and a specific body posture.

Both receptor apparatuses share a common phylogenetic origin. In their simplest form, they are represented by a vesicle whose walls are lined with ciliated epithelium. Such a vesicle is found in some Coelenterates (jellyfish). It is filled with fluid and contains a calcareous formation—a statolith. When the body position changes, the statolith rolls and stimulates the endings of the sensory nerves approaching the vesicle wall, giving the Organism a sense of its position in space. In the course of evolution, The Structure of this organ became significantly more complex, dividing into two sections, one of which retains the static function, while the other acquires the auditory function. Both receptor apparatuses are innervated by fibers running within the Vestibulocochlear nerve (VIII). They are stimulated by mechanical vibrations: the vestibular system perceives vibrations associated with changes in body position, while the auditory system perceives airborne vibrations.

Human auditory receptors lie in the Cytology/practical/79.html">Spiral organ of the cochlea, while vestibular receptors are located in the ampullary crests of the semicircular canals and the maculae of the saccule and utricle. While the Organ of Balance consists only of structures containing receptor Cells, the STRUCTURE OF THE Organ of Hearing is significantly more complex due to a system of structures that conduct sound waves to the receptor.

The human auditory organ consists of three parts: 1) the outer ear, which captures airborne vibrations; 2) the Middle ear, which transmits sound waves; and 3) the inner ear, which perceives sound. In addition to auditory receptors, the latter houses the receptors of the vestibular system. The Organs of Hearing and Balance are located mainly within the petrous part of the Temporal bone.

The outer ear (auris externa) developed at the site of the ectodermal outpocketing of the first visceral (mandibular) arch. It consists of the auricle (pinna) and the external acoustic meatus (Atl. Fig. 152).

In animals, the auricle (auricula) has The ability to prick up and serves to capture sounds. In humans, due to the poor Development of the auricular Muscles, it retains only minor mobility.

The framework of the auricle is formed by elastic Cartilage of a complex shape. Below, it is completed by a Skin fold—the lobule (earlobe), which is filled with adipose tissue. The free outer margin of the auricle is curved inward to form the helix, and parallel to it, an elevation called the antihelix rises from its floor. Medial to the antihelix lies the cavity of the concha, at the bottom of which is the opening of the external acoustic meatus. Anteriorly, the meatus is bounded by a prominent tubercle, the tragus, and posteriorly by the antitragus. The shape and size of the auricle vary individually. Sometimes, the posterosuperior part of the helix projects into a tubercle. This is clearly visible in the human fetus and lower primates.

The external acoustic meatus (meatus acusticus externus) is about 24 mm long and ends at the tympanic membrane. Its outer cartilaginous third is a continuation of the auricular cartilage, while the remaining two-thirds are bony and located within the petrous part of the temporal bone. The auditory canal is slightly curved and lined with skin containing fine hairs and modified Sweat Glands (ceruminous glands) that secrete earwax. Both the hairs and the wax protect the tympanic membrane from adverse environmental influences, such as dust.

The tympanic membrane (membrana tympani) separates the outer ear from the middle ear. Its core consists of Collagen fibers, covered on the outside by epidermis and on the inside by a mucous membrane. The membrane is so thin that the malleus of the middle ear is visible through it.

The middle ear (auris media) consists of the tympanic cavity, auditory ossicles, and the auditory (Eustachian) tube. The tympanic cavity (cavitas tympanica) develops from the first visceral pouch. The cavity is filled with air entering from the nasopharynx through the auditory tube, the opening of which is located on the anterior wall of the tympanic cavity. On its posterior wall, the mastoid cells open, which are also filled with air, and on the medial wall are the oval window and the round window, leading to the inner ear. The round window is closed by the secondary tympanic membrane. Within the middle ear cavity are the auditory ossicles (Fig. 3.65; Atl. Fig. 152): the malleus and incus—derivatives of the first visceral (mandibular) arch—and the stapes, which developed from the second visceral (hyoid) arch.

The malleus is fused to the tympanic membrane by its manubrium (handle), which pulls its center inward; the head of the malleus articulates with the body of the incus, whose long limb, in turn, articulates with the head of the stapes. The base (footplate) of the stapes rests against the membrane closing the oval window. Thus, all three ossicles form a mobile chain connecting the tympanic membrane to the inner ear. Consequently, air vibrations in the form of sound waves acting on the tympanic membrane are transmitted to the receptors of the inner ear. At the same time, the tympanic membrane, which perceives the vibrations, has a significantly larger surface area than the oval window, into which the footplate of the stapes fits. Thus, the ossicular System of the middle ear can be considered to concentrate the stimulus.

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Fig. 3.65. Auditory ossicles of the middle ear:

1 — malleus; 2 — incus; 3 — stapes; 4 — footplate of stapes

The auditory tube (tuba auditiva) is a long (3.5 cm) and narrow (2 mm) canal, bony on the ear side, where it passes through the petrous part of the temporal bone, and cartilaginous along the rest, larger part of its length. On the inferoventral side, the cartilage is completed by a fibrous membrane to which the Muscles of the soft palate attach. The tube is crucial for equalizing air pressure on the tympanic membrane. The pharyngeal opening of the tube is normally collapsed, and air enters the tympanic cavity during swallowing and yawning, when the contracting muscles of the soft palate pull back the fibrous part of the tube wall, opening its lumen. The mucosa of the tube is continuous with the walls of the tympanic cavity and the mastoid cells.

The inner ear (auris interna), or labyrinth, has the most complex structure (Atl. Fig. 152). A complex system of membranous tubes filled with endolymph forms the membranous labyrinth. It is, as it were, inserted into the bony labyrinth, which mirrors the shape of the membranous one. In some places, the membranous labyrinth is attached to the periosteum of the bony labyrinth.

The bony labyrinth is embedded within the petrous part of the temporal bone, between the tympanic cavity and the internal acoustic meatus. It consists of three parts: the vestibule occupies the central position, with the cochlea located anteriorly and the semicircular canals posteriorly (Fig. 3.66).

The vestibule (vestibulum) of the bony labyrinth communicates with the tympanic cavity via the oval window and the round window. The vestibule is an oval-shaped cavity separated from the tympanic cavity by a septum. There are two openings in the septum: the upper one is the oval window (fenestra vestibuli), which receives the footplate of the stapes, and the lower one is the round window (fenestra cochleae), closed by an elastic membrane. On the inner surface of the vestibule, there are two recesses—spherical and elliptical—separated by a crest. The walls of these recesses, as well as an area at the Base of the cochlea, are perforated by numerous small openings and are called cribriform plates (maculae cribrosae).

Fig. 3.66. Bony labyrinth:

1 — ampullae of semicircular canals; 2 — cochlea; 3 — vestibule; 4 — round window; 5 — oval window; 6 — posterior, 7 — lateral, 8 — anterior semicircular canals

The spiral canal of the cochlea, the openings of the three semicircular canals, and the narrow vestibular aqueduct, which opens with its outer end on the posterior surface of the petrous part of the temporal bone, all open into the vestibule.

All PARTS OF THE membranous labyrinth are smaller than the corresponding sections of the bony labyrinth. Between their walls lies a cavity filled with perilymph, called the perilymphatic space. The cavity of the membranous labyrinth is filled with endolymph. Its wall consists of three layers: an outer Connective Tissue layer, a middle membranous layer (a thin plate of Dense connective tissue), and an inner epithelial layer.

The central part of the membranous labyrinth of the vestibule consists of two chambers. One of them is rounded—the saccule (spherical sac); the other is oval—the utricle (elliptical sac) (Fig. 3.67; see Atl.). They are connected to each other by the bifurcated end of the endolymphatic duct, which passes through the petrous part of the temporal bone in a bony cleft—the vestibular aqueduct. On its posterior surface, within the dura mater, the duct ends in a dilation—the endolymphatic sac. Blood Vessels in its wall come into contact with the Vessels of the dura mater. When the endolymph pressure inside the membranous labyrinth increases, it drains through the endolymphatic duct into the subdural space. The cochlear duct opens into the saccule via the ductus reuniens (connecting duct), while the membranous semicircular ducts open into the utricle.

Fig. 3.67. Bony and membranous labyrinths:

1 — endolymphatic sac; 2 — semicircular canals; 3 — semicircular ducts; 4 — dura mater; 5 — ampulla of the semicircular duct; 6 — vestibule; 7 — stapes; 8 — connecting duct; 9 — saccule; 10 — scala tympani and 11 — scala vestibuli; 12 — perilymphatic space; 13 — utricle; 14 — endolymphatic duct

The bony cochlea has a conical shape and a complex structure (see Atlas). It is a spiral canal that makes two and a half turns around a conical central pillar, the modiolus. The axis of the modiolus lies almost horizontally. A bony spiral lamina projects from the modiolus but does not reach the outer wall of the canal. The modiolus is composed of spongy Bone tissue permeated by longitudinal canals. These canals extend into the spiral lamina.

Two membranes—the basilar and vestibular membranes—stretch from the free and upper edges of the spiral lamina to the opposite wall of the cochlea. They enclose the cochlear duct, which is part of the membranous cochlea. This spirally coiled duct follows the course of the cochlear canal. It begins as a blind end near the saccule, close to where a thin connecting duct from the utricle enters it. The cochlear duct also ends blindly at the apex of the bony cochlea. The cross-section of the cochlear duct is triangular, allowing three walls to be distinguished (Atlas Fig. 153).

The lower wall, or basilar (spiral) membrane, lies as an extension of the bony spiral lamina and is fused with its free edge. It is formed by a dense network of collagen fibers. At the opposite end, the basilar membrane attaches to the thickened periosteum covering the bony wall of the cochlea. This thickened area also ascends spirally to the apex of the cochlea and is called the spiral ligament. Located on this wall is the spiral organ, or organ of Corti, which is the peripheral part of the auditory sensory system.

The outer wall is fused with the spiral ligament, which separates it from the bony cochlea. The inner surface of the cochlear duct is lined here by a simple cuboidal epithelium. Beneath it lie numerous blood vessels that form the stria vascularis.

The upper wall, or vestibular membrane, is stretched between the outer wall and the upper edge of the bony spiral lamina. It is a thin membrane formed by two layers of epithelial cells.

The lumen of the cochlear duct is filled with endolymph, which is produced with the involvement of the stria vascularis in the outer wall of the duct.

The cochlear duct divides the cavity of the bony cochlear canal into two parts, or scalae. The upper part, or scala vestibuli, begins at the oval window of the vestibule and reaches the apex of the cochlea, where it communicates with the lower part of the canal cavity, the scala tympani, through a small opening. The latter extends from the apex of the cochlea to its base, where it opens into the Vestibule of the bony labyrinth via the cochlear window (round window). This window is closed by an elastic membrane. The scala vestibuli and scala tympani are filled with perilymph.

The spiral organ lies on the basilar membrane and is a rather complex structure. A row of cells, including supporting and Hair cells, rests on the basilar membrane. The supporting (phalangeal) cells are cylindrical and provide support for the receptor hair cells. In the Cytoplasm of the supporting cells, a prominent bundle of microtubules and fibrillar structures runs from the base of The Cell to its apical portion. One of the projections of this bundle approaches the basal part of the hair cells, forming a plate-like structure. Another part of the fibrous bundle, surrounded by a layer of cytoplasm, extends to the apical surface of the cell, where it flattens. It forms contacts with the apical parts of the receptor cells. Sensory nerve fibers are also in contact with the membrane of the supporting cells, forming endings on the receptor cells.

The receptor cells occupy the upper part of the cell layer. On their apical surface, there are projections that are large microvilli (stereocilia). A distinction is made between outer and inner hair cells.

The outer hair cells lie closer to the outer wall of the duct in three rows. The inner hair cells form only a single row. The cilia of both types of receptor cells are in contact with the tectorial membrane. This membrane is a thin, homogeneous, gelatinous mass attached at one end to the epithelial cells covering the thickened periosteum of the spiral lamina.

Between the outer and inner hair cells lies the tunnel of Corti. Along its edges are the outer and inner pillar cells, whose structure resembles that of supporting cells.

Sound waves cause vibrations of the tympanic membrane, which are transmitted through the chain of auditory ossicles and the oval window to the perilymph of the vestibule. The perilymph waves travel sequentially through the scala vestibuli of the cochlea, then the scala tympani, causing the membranous walls of the cochlear duct to vibrate. The vibrations of the perilymph are possible because, at the end of their path, the waves meet the compliant membrane (secondary tympanic membrane) of the round window of the cochlea. As a result of the vibrations of the basilar membrane, the receptor cells bring their stereocilia into contact with the tectorial membrane, perceiving the sound stimulus.

From the receptor cells, excitation is transmitted to the nerve fibers contacting their basal parts. These fibers pass through the basilar membrane underlying the supporting cells and then enter the canal (or cleft) of the spiral lamina. They run to the Neurons of the spiral ganglion, which lies closer to the bony modiolus of the cochlea.

Physiological experiments have demonstrated that sound waves of different wavelengths excite receptor cells in different Regions of the cochlea. Thus, nerve fibers coming from adjacent turns of the cochlear spiral carry information about sounds of different frequencies (pitches)—a tonotopic Organization.

The utricle and saccule of the vestibule are interconnected by a duct (Fig. 3.67). This duct continues as the endolymphatic duct. At the entry points of the nerves, the wall of the membranous labyrinth is rigidly fixed to the bony wall. The saccule communicates with the cochlear duct.

Development of The Organ of Hearing and Equilibrium during the Prenatal period of ontogeny. In the human embryo at the beginning of the third week of development, thickenings of the cranial ectoderm appear on both sides of the still open neural plate at the level of the Hindbrain, initiating The formation of the otic pit. During the fourth week, the ectoderm of the pit invaginates and pinches off from the cranial ectoderm, transforming into the otic vesicle—the primordium of the inner ear. The endolymphatic duct grows out from it (Fig. 3.71). Around this primordium, a cartilaginous capsule forms from the mesoderm, which later ossifies. Subsequently, the otic vesicle divides into the primordia of the vestibular part of the membranous labyrinth and the cochlea. At the end of the 6th week, ridge-like outgrowths—the future semicircular canals—are formed in the vestibular part. Each of the three canals lies in a plane perpendicular to the other two. At the beginning of the 3rd month, the utricle and saccule separate within the vestibule. Neuroepithelial hair cells appear in them, as well as at the junctions of the semicircular canals with the utricle, and the fibers of the vestibulocochlear nerve grow toward them.

The sound-perceiving region of the inner ear develops from the cochlear part. During the first 6 weeks of embryonic development, the cochlear region rapidly elongates, and its end coils. As a result, by the 7th–8th week, a spiral with 2.5 turns is formed. The fibers of cranial nerve VIII are distributed along the entire length of the cochlear duct.

duct, and the utricle with the semicircular ducts.

The semicircular ducts (ductus semicirculares), lying in three mutually perpendicular planes, very closely mimic the shape of the bony semicircular canals in which they are located (Fig. 3.67). The anterior, vertical canal lies in the frontal plane, causing the arcuate eminence on the anterior surface of the pyramid. The posterior canal, also vertical, is located in the sagittal plane, and the lateral one is in the horizontal plane. Each semicircular canal, and consequently each duct, has two limbs—a simple limb and an expanded, ampullary limb. The simple limbs of both vertical semicircular canals fuse together into a single common limb. Therefore, five openings, rather than six, open into the utricle of the vestibule.

The utricle, saccule, and semicircular ducts are functionally connected to the vestibular sensory system. All of these structures are filled with endolymph. Their membranous wall consists of fibrous tissue lined internally by a simple squamous epithelium.

In the areas of flattened elevations on the inner surface of both sacs—the maculae—and in each ampulla of the semicircular ducts—the cristae—are located receptors for body position. The epithelium of the maculae is formed by receptor and supporting cells. The receptor cells are hair cells; on their apical surface, there are elongated microvilli—stereocilia, and a single cilium—the kinocilium. The surface of the cells is covered by a gelatinous membrane containing numerous microscopic crystals of calcium carbonate—otoliths. During movement that occurs in the endolymph bathing the epithelium, with changes in head position, linear acceleration, or changes in gravity, the gelatinous membrane shifts and stimulates the hairs of the sensory cells.

On the outer wall of the semicircular ducts, in the region of their ampullae, lie transverse ridges (cristae). They are covered by an epithelium similar to that of the maculae, containing supporting and receptor cells. The surface of the crista is covered by a gelatinous, acellular membrane—the cupula. It closes the lumen of the ampulla like a valve. This membrane does not contain crystals and begins to move when angular acceleration occurs during rotation. The movement of the membrane is caused by the flow of endolymph in the semicircular duct. Afferent nerve fibers, which originate from the neurons of the vestibular ganglion, approach the basal parts of the receptor cells through the base of the crista.

The conducting and central Divisions of the auditory sensory system. The auditory and vestibular Sensory systems are linked together into a single whole at the beginning of the conducting pathway—in the vestibulocochlear nerve.

The pathway of the Auditory Analyzer begins with the sensory neurons of the spiral ganglion (Atl. Fig. 154). The latter is located within the modiolus of the bony cochlea, where the osseous spiral lamina originates. Dendrites of the spiral ganglion cells pass through the canaliculi of the osseous spiral lamina to the receptors of the spiral organ, while axons run through the longitudinal canals of the modiolus to enter the internal acoustic meatus, where they merge with the fibers of the vestibular nerve to form the common ROOT of the VIII cranial nerve. The latter enters the Brainstem between the inferior cerebellar peduncles and the Pons, and its auditory fibers project into the pontine tegmentum to the dorsal and ventral cochlear nuclei. Fibers from different regions of the cochlea project in an orderly tonotopic manner onto different neurons of the cochlear nuclei. Some fibers project to the olivary nuclei of the ipsilateral and contralateral sides, which serve as centers for binaural sound localization. From the olive, individual fibers travel back to the cochlea via the olivocochlear bundle, exerting centrifugal control over the hair cells.

Fig. 3.68. Diagram of the structure of the macula and crista: A — diagram of the macula structure: 1 — supporting epithelial cells; 2 — receptor cells (a — cilia); 3 — nerve endings; 4 — Myelinated nerve fibers; 5 — gelatinous membrane; 6 — otoliths (after Kolmer); B — diagram of the crista structure: I — crista; II — cupula; 1 — supporting cells; 2 — sensory cells (a — hairs); 3 — nerve endings; 4 — myelinated nerve fibers; 5 — gelatinous adhesive substance of the cupula; 6 — epithelium lining the wall of the membranous canal (after Kolmer, modified)

Most of the fibers from the cells of these nuclei decussate to the contralateral side: from the dorsal Nucleus along the floor of the Fourth ventricle as part of the striae medullares, and from the ventral nucleus as part of the trapezoid body. On the contralateral side, the fibers form the lateral lemniscus. Some of its fibers terminate on the Cells of the inferior colliculus, from which impulses travel along the tectospinal tract to trigger motor responses to auditory stimuli. Other fibers of the lateral lemniscus, within the brachium of the inferior colliculus, project to the medial geniculate body. The processes of the cells of the latter form the auditory radiation, which terminates in the cortex of the superior temporal gyrus, deep within the lateral sulcus (Brodmann areas 41 and 42).

In the inferior colliculi, the medial geniculate body, and the auditory cortex, There is a clear tonotopic projection of different parts of the cochlea onto specific groups of neurons. This enables the precise discrimination of sounds of different frequencies.

Fig. 3.69. Connections of the vestibular nuclei:

1 — vestibulocerebellar system; 2 — to the Cerebellum; 3 — superior, 4 — lateral, 5 — medial, and 6 — inferior vestibular nuclei; 7 — from vestibular receptors; 8 — lateral vestibulospinal tract; 9 — to motor neurons of the limbs; 10 — to motor neurons of the neck and trunk; 11 — medial vestibulospinal tract; 12 — medial longitudinal fasciculus; 13 — to motor neurons of the extraocular muscles (III, IV, VI); 14 — vestibulo-ocular system

The pathway and central divisions of the vestibular sensory system. In the vestibular system (Atl. Fig. 155; Fig. 3.69), the pathway begins with sensory neurons of the vestibular ganglion, located at the Fundus of the internal acoustic meatus. The dendrites of these neurons extend to the vestibular receptors in the maculae and cristae, while their axons form the vestibular root, which joins the cochlear root to form the vestibulocochlear nerve (cranial nerve VIII). This nerve travels through the internal acoustic meatus into the cranial cavity, entering the Medulla Oblongata. Here, most of the fibers terminate on the neurons of the vestibular nuclei in the Rhomboid fossa. The fibers of these nuclear cells transmit impulses along three pathways. The first pathway projects to the motor neurons of the Spinal Cord within the vestibulospinal tract. These fibers form two branches: medial and lateral. Fibers originating from the cristae of the semicircular ducts project onto the neurons of the vestibular nuclei that give rise to the medial branch. The medial branch enters the medial longitudinal fasciculus and, as part of it, reaches the motor neurons controlling Movements of the trunk and neck. This pathway is involved in organizing Reflexes that maintain the Head and Neck in a normal position during trunk rotations. Along the lateral branch, fibers travel to motor neurons controlling limb Muscle movements to maintain balance. The neurons whose axons form the lateral tract receive afferent input primarily from the maculae.

The second pathway of impulse transmission from the vestibular nuclei is associated with coordinated Eye Movements (Fig. 3.70). This is essential for maintaining a stable image on the retina during body movements. Fibers from the vestibular nuclei project to the motor nuclei of the extraocular muscles (oculomotor, trochlear, and abducens nerves). This pathway is supplemented by connections with the reticular Formation of the brainstem, which underlies the autonomic reactions occurring during intense stimulation of vestibular receptors (nausea, sweating, etc.).

Fig. 3.70. Pathways mediating voluntary and reflex conjugate eye movements:

1 — abducens and 2 — oculomotor nerves; 3 — superior and 4 — inferior colliculi; 5 — lateral geniculate body; 6 — medial longitudinal fasciculus; 7 — superior, 8 — lateral, 9 — medial, and 10 — inferior vestibular nuclei; 11 — lateral vestibulospinal tract; 12 — from the cervical spinal cord; 13 — pontine field mediating horizontal eye movements; 14 — autonomic nuclei of the Oculomotor nerve; 15 — tectal field mediating vertical eye movements; 16 — pathways of voluntary and 17 — reflex eye movements. Roman numerals indicate the nuclei of Cranial Nerves

The third pathway, along which fibers from the vestibular nuclei travel, runs through the inferior cerebellar peduncles to the neurons of the fastigial nucleus and the globose nucleus, as well as to the flocculus of the cerebellum. The processes of neurons from the cerebellar nuclei and the cortex of the vermis project back to the vestibular nuclei. These pathways are involved in maintaining balance.

A small number of fibers project from the vestibular nuclei to the ventral posterior Nucleus of the thalamus, and from there to the part of the somatosensory cortex that receives impulses from the face and upper limbs, as well as to area 21 of the temporal region. This cortical zone is likely involved in the conscious perception of balance and motion determined by vestibular inputs. In addition, some fibers project to the frontal lobe, where neurons controlling voluntary eye movements are located.

Fig. 3.71. Diagram of The Development of the organ of hearing (after Patten):

A — 4 weeks; B — 6 weeks; C — 3 months; 1 — endolymphatic duct; 2 — primordium of the inner ear; 3 — primordium of the auditory ossicles; 4 — I pharyngeal groove; 5 — I pharyngeal pouch; 6 — wall of the neural tube; 7 — primordium of the temporal bone; 8 — mesenchyme; 9 — primordium of the incus; 10 — primordium of the malleus; 11 — stapes; 12 — primordium of the external acoustic meatus; 13 — primordium of the middle ear cavity; 14 — Eustachian tube; 15 — mesenchyme in the tympanic cavity; 16 — outer ear; 17 — malleus; 18 — external acoustic meatus; 19 — tympanic membrane; 20 — tympanic cavity; 21 — round window; 22 — cochlea; 23 — stapes; 24 — oval window; 25 — saccule; 26 — incus; 27 — Facial Nerve; 28 — utricle; 29 — ampullae of the semicircular canals