The ear is the organ of hearing and equilibrium. The ear contains receptors that convert sound waves into nerve impulses and receptors that respond to movements of the head. Impulses from both receptor types are transmitted through the vestibulocochlear nerve to the brain for interpretation. The ear has two sensory functions: hearing and equilibrium (balance). The sensory receptors for both of these are located in the inner ear, and each consists of hair cells with stereocilia that are sensitive to mechanical stimulation.The hair cells are mechanoreceptors.
The ear is partitioned into three distinct anatomic regions/sections/parts these are,: externa ear, middle ear, and inner ear as shown in ( figure.1 ).
Figure. 1.The human ear sections.
Outer Ear/External Ear.
The outer (external) ear is essentially a funnel for conducting airborne vibrations to the eardrum. It begins with pinna on the side of the head, shaped and supported by elastic cartilage except for the earlobe. The auricle has a predictable arrangement of whorls and recesses that direct sound into the auditory canal.
The outer ear consists of the auricle and the ear canal. The auricle, or pinna, is made of cartilage covered with skin.
The auricle is funnel shaped, and serves to protect the entry into the ear and to direct sound waves into the bony tube called the external acoustic meatus, which extends medially and slightly superiorly from the lateral surface of the head.The ear canal is lined with skin that contains ceruminous glands. It may also be called the external auditory meatus, and is a tunnel into the temporal bone, curving slightly forward and down.
The middle ear is an air-filled cavity in the temporal bone. The eardrum, or tympanic membrane, is stretched across the end of the ear canal and vibrates when sound waves strike it. These vibrations are transmitted to the three auditory bones: the malleus, incus, and stapes (see Figure 1). The stapes then
transmits vibrations to the fluid-filled inner ear at the oval window.
The inner ear is located within the petrous part of the temporal bone, where there are spaces or cavities called the bony labyrinth. Within the bony labyrinth are membrane-lined, fluid filled tubes and spaces called the membranous labyrinth.Receptors for equilibrium and hearing are housed, along with supporting cells, within a sensory epithelium lining part of the membranous labyrinth. The space between the outer walls of the bony labyrinth and the membranous labyrinth is filled with a fluid called perilymph which is similar in composition to both extracellular fluid and cerebrospinal fluid.
In the inner ear, the perilymph suspends, supports, and protects the membranous labyrinth from the wall of the bony labyrinth. The membranous labyrinth contains a unique fluid called endolymph .Endolymph exhibits a low sodium and high potassium concentration similar to that of intracellular fluid.
Parts of the inner ear.
The cochlea is shaped like a snail shell with two-anda-half structural turns. Internally, the cochlea is partitioned into three fluid-filled canals. The medial canal is the cochlear duct, the floor of which is the basilar membrane that supports the receptors for hearing in the organ of Corti (spiral organ). The receptors are called hair cells which contain endings of the cochlear branch of the 8th cranial nerve. Overhanging the hair cells is the tectorial membrane.
The eustachian tube (auditory tube) extends from the middle ear to the nasopharynx and permits air to enter or leave the middle ear cavity. The air pressure in the middle ear must be the same as the external atmospheric pressure in order for the eardrum to vibrate properly. The eustachian tubes of children are short and nearly horizontal and may permit bacteria to spread from the pharynx to the middle ear. This is why otitis media may be a complication of a strep throat.
Round window .The membrane-covered round window, just below the oval window, is important to relieve pressure. When the stapes pushes in the fluid at the oval window, the round window bulges out, which prevents damage to the hair cells.
The vestibule is the central portion of the bony labyrinth. It contains the vestibular (oval) window, into which the stapes fits, and the cochlear (round) window on the opposite end (figure. 2).
The membranous labyrinth within the vestibule consists of two connected sacs called the utricle and the saccule .The utricle is larger than the saccule and lies in the upper back portion of the vestibule. Both the utricle and saccule contain receptors that are sensitive to gravity and linear movement (acceleration) of the head.
Posterior to the vestibule are the three bony semicircular canals, positioned at nearly right angles to each other. The thinner semicircular ducts form the membranous labyrinth within the semicircular canals (figure. 2). Each of the three semicircular ducts has a membranous ampulla at one end and connects with the upper back part of the utricle. Receptors within the semicircular ducts are sensitive to angular acceleration and deceleration of the head, as in rotational movement.
Figure 2.The labyrinths of the inner ear.
Structures for Hearing
Cochlea.Hearing organs are housed within the cochlea in both inner ears. The cochlea is a snail-shaped spiral chamber in the bone of the inner ear. It has a spongy bone axis, called the modiolus. Protected within the core of the modiolus, the membranous labyrinth houses the spiral organ (formerly called the organ of Corti ), which is responsible for hearing ( figure 19.27 ). The cochlear duct , or scala media , is the membranous labyrinth that runs through the cochlea.
The roof and floor of the cochlear duct are formed by the vestibular and basilar membranes,respectively. These membranes partition the bony labyrinth of the cochlea into two smaller chambers, both filled with perilymph. The superior chamber is the scala vestibuli ( vestibular duct ), and the inferior chamber is the scala tympani ( tympanic duct ). The scala vestibuli and scala tympani merge through a small channel called the helicotrema at the apex of the cochlear spiral apex.
Within the cochlear duct, the spiral organ is a thick sensory epithelium consisting of both hair cells and supporting cells that rests on the basilar membrane. The hair cells extend stereocilia into an overlying gelatinous structure called the tectorial membrane.
Process of Hearing
- Sound waves are collected and funneled by the auricle(pinna) of the external ear. From there, sound waves enter the external acoustic meatus and make the tympanic membrane vibrate.
- The vibration of the tympanic membrane causes movement by the auditory ossicles. Sound waves are amplified, allowing even soft sounds to be heard by the ear. The foot of the stapes moves like a piston in the oval window, thereby transmitting the effect of sound waves into pressure waves in the inner ear.
- Pressure waves originate within the inner ear at the oval window and travel through the perilymph in the scala vestibuli.
- The high-frequency and upper medium-frequency pressure waves in the scala vestibuli cause the vestibular membrane to vibrate, resulting in pressure wave formation in the endolymph of the cochlear duct. Pressure waves in the cochlear duct displace a specific region of the basilar membrane. Hair cells in the spiral organ of this region are distorted, causing a stimulus in the cochlear branch of CN VIII.
- The remaining pressure wave vibrations in the cochlear duct are transmitted to the perilymph of the scala tympani, and they exit the inner ear at the round window. When pressure waves leave the inner ear, the window bulges slightly. The sound waves that give rise to pressure waves in the inner ear are characterized by their frequency and intensity. Frequency is the number of waves that move past a point during a specific amount of time. Frequency is measured in hertz (Hz), and is classified as high, medium, or low.
Figure.3 Hearing process.
In the previous section, I have shared with you on how sound waves travel from pinna to the inner ear .Now important question to ask your self is how are those sound waves transferred to the brain and finally we respond toward a certain information and also, where these sounds waves are interpreted as sound? The following are the processes of auditory pathway for human being.
- When the basilar membrane bounces, the stereocilia on spiral organ hair cells bend against the tectorial membrane, producing a nerve impulse. The nerve impulse travels through the cochlear branch that attaches to the hair cells. The cochlear branch and the vestibular branch merge to form the vestibulocochlear nerve.
- The sensory axons of the cochlear nerve (the “primary neurons” in this sensory pathway) terminate in the paired cochlear nuclei within the brainstem. These sensory neurons synapse with secondary neurons housed within this nucleus.
- After integration and processing of incoming information within the cochlear nucleus, axons from secondary neurons in this nucleus project to both the inferior colliculi within the mesencephalon and the superior olivary nuclei within the myelencephalon.
- Thereafter, inferior colliculi neurons extend axons to the medial geniculate nucleus of the thalamus. 5. Neurons reaching the medial geniculate nucleus of the thalamus synapse with tertiary neurons in the thalamus; axons from the tertiary neurons extend to the primary auditory cortex in the temporal lobe. The nerve impulses in the primary auditory cortex are perceived as sounds.
Figure 4.Auditory pathway of sound waves.
Development of the Ear
Ear development begins during the fourth week . All embryonic germ layers contribute to ear formation, including the surface ectoderm and the layer that is beginning to specialize as the neuroectoderm. Each part of the ear originates separately. External and Middle Ear Development The auricle of the external ear forms from the surface ectoderm. Masses of tissue from the first and second pharyngeal arches come together and fuse, forming the unique shape of the auricle. Each external acoustic meatus forms from an external indentation of the ectoderm called the first pharyngeal cleft (or groove ). The tympanic cavity and the auditory tube of each middle ear are formed from an internal tubelike expansion of the pharynx called the first pharyngeal pouch. (Eventually, four pharyngeal clefts and pouches form, but only the first pharyngeal cleft and pouch develop into external and middle ear structures.) Specifically, as the first pharyngeal pouch grows, it develops into a trumpet-shaped structure called the tubotympanic recess.
Inner Ear Development
Each inner ear forms from an otic placode that is formed from ectoderm and is at the level of the hindbrain. Each otic placode invaginates, forming an otic pit . Eventually, each otic pit closes off and becomes an internal structure called an otocyst or otic vesicle . The otocyst is initially very primitive. During the fifth and sixth weeks of development, the otocyst begins to expand. Its anterior tip elongates and starts to form the future cochlear duct. The utricle and saccule become better differentiated. The hair cells of the spiral organ start to develop by week 7. Not only that,during the seventh week, the three semicircular ducts form along with the ampulla structures within them. The shape of the membranous labyrinth is attained by the end of the eighth week. As the petrous part of the temporal bone (bony labyrinth) develops around the membranous labyrinth, the space between the two labyrinths fills with perilymph.
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