Anatomy and Physiology of the Ear: From Outer to Inner Ear
External Ear
The external ear is composed of the pinna and ear canal.
Pinna
The pinna, or auricle, is an irregularly folded, sheet-like structure that protrudes from the side of the head. It is attached around the external auditory meatus of the temporal bone. The central depressed area is called the concha and is surrounded, especially above and behind, by two prominent ridges: the helix and the antihelix. These ridges are separated by a distinct groove.
In front of the lower concha is the entrance to the ear canal. This entrance is partially protected by two other prominences: the tragus, located anteriorly, and the antitragus, located posteriorly. These prominences are separated by a well-marked notch. All these ridges and prominences of the pinna are produced by folds in the cartilage that forms its skeleton.
The only part of the ear that lacks cartilage is the lobule, which hangs from the bottom. Three small muscles, rudimentary in humans but used in other species to direct the pinna towards sounds, radiate from the ear.
Ear Canal
The ear canal begins at the opening of the ear and extends inward to the tympanic membrane (eardrum), which forms the outer wall of the middle ear. The ear canal measures about 2.5 to 3 cm in length. Its outer third is cartilaginous, while the remainder is bony. The cartilaginous part is a continuation of the ear cartilage, while the bony part is a canal within the temporal bone.
The ear canal is not straight but slightly curved. Overall, it is directed inward, slightly forward, and slightly downward, so that its inner end is deeper and more anterior than the outer opening. The skin lining the ear canal is continuous with that of the pinna. It contains hairs in the outer part and numerous ceruminous glands that secrete earwax.
The anterior wall of the ear canal is closely related to the temporomandibular joint. Movement of the mandible can be perceived by placing a fingertip in the canal and pushing forward on its anterior wall.
Eardrum
At the bottom of the ear canal is the eardrum (tympanic membrane), which separates the ear canal from the middle ear. It is positioned obliquely, with its inferior wall longer than its superior wall. When the bottom of the ear canal is illuminated, the eardrum can be visualized. Because it is translucent, some structures on the other side can be seen. The handle of the malleus (one of the middle ear bones), attached to the inner surface of the eardrum, can be seen descending from the top to the center of the membrane. This point, called the umbo, is pulled inward, and light from the otoscope (the instrument used for examination) reflects off its anteroinferior quadrant, forming a characteristic bright area known as the cone of light.
Middle Ear
The middle ear consists of a central part, the tympanic cavity (middle ear cavity), which is continuous anteriorly with the Eustachian tube and posteriorly with the mastoid antrum and mastoid air cells.
Tympanic Cavity
The tympanic cavity is an air-filled, drum-shaped cavity that is flattened laterally. It is located entirely within the temporal bone. It has two relatively wide walls, the lateral (outer) and medial (inner) walls, and an irregularly cylindrical circumference that is very narrow.
The most notable feature of the lateral wall is the tympanic membrane. However, the membrane does not occupy the entire wall. Above and below the membrane’s attachment are spaces called the epitympanic recess (attic) and the hypotympanum, respectively. The roof of the attic is a thin plate of bone that separates the tympanic cavity from the middle cranial fossa. The fact that the hypotympanum is located below the level of the eardrum facilitates the retention of fluids within the cavity. This is why postural exercises are sometimes recommended to drain fluids when the eardrum is perforated.
The medial wall of the tympanic cavity borders the inner ear. It has a rounded central prominence called the promontory, which is formed by the basal turn of the cochlea. Posterior to the promontory are two openings: the oval window, which is higher and larger, and the round window, which is lower and smaller. Both communicate with the inner ear but are closed by membranes: the stapes footplate covers the oval window, and the secondary tympanic membrane covers the round window.
Anteriorly, the tympanic cavity opens into a canal called the Eustachian tube, which communicates with the nasopharynx at its anterior end. The Eustachian tube’s initial portion is bony, while the rest is fibrocartilaginous and therefore elastic. This elasticity allows the opening of the tube to be modified by the action of two small muscles in the palate. This mechanism regulates the pressure within the tympanic cavity, equalizing it with the pressure in the external auditory canal. This equalization is essential for proper vibration of the tympanic membrane when sound waves, collected by the pinna and conducted through the ear canal, strike it.
On the posterior wall of the tympanic cavity is an opening called the aditus to mastoid antrum. This opening leads to a cavity called the mastoid antrum, from which numerous small cavities and air cells, known as mastoid air cells, radiate in all directions. All these air spaces are located within the mastoid portion of the temporal bone. Posteriorly, they are separated from the posterior cranial fossa by a thin layer of bone. The fact that these cavities communicate directly with the tympanic cavity, which in turn communicates with the pharynx through the Eustachian tube, means that an infection localized in the pharynx can spread to the middle ear and even to the mastoid air cells. Similarly, the thinness of the bony walls separating the middle ear from the middle and posterior cranial fossae can facilitate the spread of infection to the cranial cavity.
Ossicles and Associated Muscles
Within the tympanic cavity is a chain of three tiny bones called the ossicles. These bones, named for their shapes, are the malleus (hammer), incus (anvil), and stapes (stirrup).
- The malleus has an elongated shape with a rounded head and a handle. Its handle is attached to the inner surface of the tympanic membrane. The head of the malleus articulates with the body of the incus.
- The incus has a central body from which two limbs, or processes, extend. The short process rests on the posterior wall of the tympanic cavity, while the long process articulates with the head of the stapes.
- The stapes resembles a stirrup. It consists of a head, two limbs (crura), and a footplate. The footplate is attached to the edge of the oval window by a ring-shaped ligament.
The ossicles form a chain that transmits mechanical vibrations from the tympanic membrane to the oval window. When sound waves strike the eardrum, it vibrates. These vibrations are transmitted through the ossicles to the stapes footplate, which in turn vibrates against the oval window. This vibration sets the fluids of the inner ear in motion, ultimately leading to the sensation of hearing.
Two small muscles, the tensor tympani and the stapedius, regulate the movement of the ossicles.
- The tensor tympani muscle is innervated by the trigeminal nerve (cranial nerve V). Its contraction increases the tension on the tympanic membrane, which dampens the amplitude of vibrations and protects the inner ear from loud sounds.
- The stapedius muscle is innervated by the facial nerve (cranial nerve VII). Its contraction pulls the stapes footplate away from the oval window, also reducing the amplitude of vibrations.
Inner Ear
The inner ear, located medial to the middle ear, is a complex system of fluid-filled chambers and canals embedded within the temporal bone. It is often referred to as the labyrinth due to its intricate shape. The inner ear has two main parts: the vestibular apparatus, responsible for balance, and the cochlea, responsible for hearing.
Bony Labyrinth
The bony labyrinth is a series of cavities within the temporal bone that houses the membranous labyrinth. It consists of three main parts: the vestibule, the semicircular canals, and the cochlea.
Vestibule
The vestibule is the central part of the bony labyrinth. It contains two membranous sacs, the utricle and the saccule, which are important for sensing linear acceleration and head position. The oval window and round window are located on the lateral wall of the vestibule.
Semicircular Canals
Three semicircular canals, oriented at right angles to each other, arise from the vestibule. They are named the superior, posterior, and lateral semicircular canals. Each canal contains a membranous duct, the semicircular duct, which is filled with endolymph. The semicircular canals are responsible for sensing angular acceleration, or rotational movements of the head.
Cochlea
The cochlea is a spiral-shaped structure that resembles a snail shell. It contains the cochlear duct, a fluid-filled tube that runs along its entire length. The cochlea is responsible for converting sound vibrations into electrical signals that are transmitted to the brain.
Membranous Labyrinth
The membranous labyrinth is a series of interconnected sacs and ducts located within the bony labyrinth. It is filled with a fluid called endolymph and is surrounded by another fluid called perilymph, which separates it from the bony labyrinth.
Vestibular Membranous Labyrinth
The vestibular membranous labyrinth consists of the utricle, saccule, and semicircular ducts. The utricle and saccule contain sensory receptors called maculae, which detect linear acceleration and head position. The semicircular ducts contain sensory receptors called cristae ampullaris, which detect angular acceleration.
Cochlear Duct
The cochlear duct is a spiral-shaped tube located within the cochlea. It is filled with endolymph and contains the organ of Corti, the sensory organ for hearing. The organ of Corti sits on the basilar membrane, which vibrates in response to sound waves. These vibrations are converted into electrical signals by hair cells within the organ of Corti.
Auditory Pathways
Sound waves are collected by the pinna and travel through the ear canal to the tympanic membrane. The tympanic membrane vibrates, and these vibrations are transmitted through the ossicles to the oval window. The vibration of the oval window creates pressure waves in the perilymph of the scala vestibuli, one of the fluid-filled chambers of the cochlea. These pressure waves travel along the scala vestibuli to the apex of the cochlea and then back down along the scala tympani, another fluid-filled chamber. The pressure waves cause the basilar membrane, on which the organ of Corti sits, to vibrate. The location of maximum vibration along the basilar membrane depends on the frequency of the sound wave. High-frequency sounds cause maximum vibration near the base of the cochlea, while low-frequency sounds cause maximum vibration near the apex.
The vibration of the basilar membrane causes the hair cells within the organ of Corti to bend. This bending generates electrical signals that are transmitted to the brain via the auditory nerve (cochlear nerve). The auditory nerve fibers synapse in the cochlear nuclei in the brainstem. From there, auditory information is relayed to higher brain centers, including the thalamus and the auditory cortex, where it is processed and interpreted as sound.
Vestibular Pathways
, these neurons are gathered in the vestibular ganglion, located at the bottom of internal auditory canal. The dendrites of ganglion cells end around existing hair cells in the maculae of utricle and saccule and the cristae of the semicircular canals. From then until the ganglion, these dendrites pass through the holes that connect the lobby with the internal auditory canal. The axons of ganglion cells come together and form the vestibular portion (static) of the eighth nerve, or nerve estatoacústico, which is associated with the portion to form a thick cochlear nerve trunk. When leaving the internal auditory canal the nerve lies in the posterior fossa, heading angle pontobulbocerebeloso and, once inside the brain stem, distributes its axons to the vestibular nuclei, located in medulla and pons and into the cerebellum. These centers are interconnected by nerve fibers in both directions. Arise from the vestibular nuclei fibers, commuting up and down, running through the brain stem and upper spinal cord, both on the same side as the opposite side. Van for the motor nuclei of cranial nerves III, IV, VI and XI, which supply the muscles responsible for eye movements, head and neck. The fibers that descend from the bone forming the vestibulospinal tracts and control the axial musculature of the trunk. The coordination of these movements is necessary for maintaining balance. Some fibers terminate in the brainstem reticular formation and through it connect with the parasympathetic centers, responsible for the production of nausea and vomiting in relation to inadequate vestibular stimuli. Further, upward, reaching the thalamus and following the appropriate relief, vestibular information reaches a small portion of the cerebral cortex of the parietal lobe, calledvestibular area, which makes consciente.La sensitivity of balance, captured by the vestibular receptors of the inner ear, is received in the primary vestibular area, located in the parietal lobe, just behind the somesthetic area. ROUTES CLUBS ACOUSTICS. The thickness of the columella is a long duct that houses the spiral ganglion cells associated with the acoustic tracks. This set of neurons known as the spiral ganglion or ganglion of Corti. The ends of their dendrites originate around the hair cells of the organ of Corti and through the ducts of the columella reaching the ganglion. The axons of these neurons descended toward the base of the columella following other ducts that empty into the bottom of the internal auditory canal. There, associate to form the cochlear part (noise) of the eighth nerve. After reaching the brain stem, these fibers terminate in the cochlear nuclei located in the pons. Neurons in these nuclei send fibers, homo and contralateral, amounting to less quadrigeminal nucleus of the midbrain, although some previous relay do in small groups of the pons. The quadrigeminal axons ascend to the thalamus, which takes place the last over before reaching the auditory area of the cerebral cortex. These projections are bilateral, so that injury to the cortical area of one hemisphere does not result in total deafness of the ear opposite. From the small relay nuclei of the pons arise connections to the reticular formation and motor nuclei of cranial nerves V and VII Musculito innervating the middle ear. Thus, in relation to the acoustic stimuli, motor responses occur that regulate the transmission of impulses along the ossicles perilinfa.Por and below the lateral fissure in the temporal operculum, is the primary auditory area , where axons arrive, from the thalamus, leading information captured in the recipients of the snail. These axons terminate neatly under the magnitude of the frequency of the sounds perceived by the ear. Information from one ear is projected primarily in the contralateral hemisphere, but a portion goes to the same side hemisphere. For this reason, if this area does not appear changes in hearing, but not a complete deafness ear contralateral.Las association auditory areas located in the temporal lobe, around the primary auditory area; also involved in auditory functions. Your back is included in Wernicke’s area, important for understanding spoken language.