Ear

Development of ear

The ear consists of three parts which function together but have different origins. The membranous part of the internal ear originates from the otic vesicle(surface ectoderm origin) during the fourth week. The otic vesicle divides into an anterior part which forms the saccule and the cochlear duct, and a dorsal part forming the utricle, semicircular canals and endolymphatic duct. The surrounding bony labyrinth develops from adjacent mesenchyme. Except for the cochlear duct, from which develops the organ of Corti(spiral organ), the membranous labyrinth is concerned with maintaining balance.

The epithelial lining of the middle ear(tymphanic cavity, mastoid antrum and auditory) tube is derived from the endoderm of the tubotympanic recess of the first branchial pouch. The auditory ossicles develop from the dorsal ends of the cartilages of the first(malleus and incus) and second(stapes) branchial arches.

The external auditory(acoustic) meatus develops from the first branchial cleft, and is separated from the tympanic cavity by the tympanic membrane, which is derived from three sources: the ectoderm of the first branchial cleft, an intermediate mesodermal layer, and the endoderm of the first branchial pouch. The external ear(auricle) develops from six mesenchymal swellings around the margin of the first branchial pouch.

Components of ear

The three parts of the ear(external, middle and internal) are all, except for the auricle, found within the temporal bone; the auricle is attached to the tympanic part of the temporal bone. The external ear collects the sounds and conveys these to the tympanic membrane causing it to vibrate. The tympanic membrane forms the boundary between the external and middle parts of each ear. Vibration of this membrane is transmitted across the middle ear by the three auditory ossicles(incus, malleus and stapes) to the internal ear. The middle ear communicates with the nasopharynx via the Eustachian(auditory) tube. The internal ear consists of two functionally distinct parts, that concerned with hearing(the cochlear part) and that with balance and position(the vestibular part). The sensory endings of both parts are supplied by the vestibulocochlear nerve, the eighth cranial nerve.

External ear

The external ear consists of the auricle and the external acoustic(auditory) meatus(see figure) which collect and convey sound respectively towards the tympanic membrane. The auricle projects backwards and laterally from the side of the head, being connected to the fascia by three small, insignificant muscles. It is a single piece of elastic cartilage, except for the fibrofatty lobule, covered with skin; the named parts are given in figure a. In adults its shape is extremely variable, increasing threefold in length from birth to adulthood: it also tends to increase in size and thickness in old age.

The external acoustic meatus is 25mm long. It is cartilaginous in its outer third, being continuous with the cartilage of the auricle, and bony in its medial two thirds, being formed by the tympanic part of the temporal bone. The meatus curves upwards and backwards as it passes medially, its inferior wall being 5mm longer than the superior wall because of the obliquity of the tympanic membrane. The skin lining the meatus is firmly attached to the underlying bone and in the outer third of the canal contains numerous ceruminous(wax secreting) cells and hairs. The meatus lies behind the temporomandibular joint, with the mastoid air cells being immediately posterior.

Middle ear

This is a narrow, irregular cavity containing the auditory ossicles immediately medial to the tympanic membrane(figure a). The middle ear can be conveniently viewed as a six-sided space, with that part above the tympanic membrane being known as the epitympanic recess. The cavity communicates with the nasopharynx via the Eustachian tube which opens into the anterior wall, and with the mastoid air cells via the aditus in the posterior wall(figure b). The auditory tube enables the pressure on both sides of the tympanic membrane to be equalized; it is opened during swallowing.

The tympanic membrane is circular and concave laterally, and consists of three layers: modified skin externally, mucous membrane internally with an intermediate fibrous layer. The majority of the membrane is tense, however a small flaccid area exists anterosuperiorly. Between the internal and external layers runs the chorda tympani branch of the facial nerve conveying taste sensations from the anterior two thirds of the tongue.

The auditory ossicles articulate by synovial joints and transmit the vibrations of the tympanic membrane to the inner ear. The malleus attaches to the inner surface of the membrane and articulates with the incus, which in turn articulates with the stapes, the oval base of which lies in the oval window. Movements of the malleus and stapes are contolled and reflexly dampened down by contraction of the tensor tympani and stapedius muscles respectively, both of which are found within the middle ear. Tensor tympani is innervated by the mandibular division of the trigeminal nerve and stapedius by the facial nerve.

Internal ear

This is situated within the petrous part of the temporal bone and consists of a complex series of fluid – filled spaces, known as the membranous labyrinth, occupying a similarly shaped cavity, the bony labyrinth. Displacement of the fluid in these spaces stimulates the sensory endings of the lining epithelium.

The bony labyrinth consists of three parts, the vestibule(containing the utricle and saccule of the membranous labyrinth), the semicircular canals(anterior, posterior and lateral) and the cochlea(figure below). The anterior and posterior semicircular canals are at right angles to each other and lie 45 degrees to the sagittal plane, with the anterior being anterior and lateral, and posterior, posterior and lateral. The membranous semicircular ductus are dilated at one end(the ampulla) in which there is a thickening(the ampullary crest) where endings of the vestibulocochlear nerve terminate. The three ducts open into the utricle, which communicates with the saccule, which in turn communicates with the cochlea. Thickenings in both the utricle and saccule are known as the maculae and contain terminations of the vestibulocochlear nerve. The ampullary crests of the semicircular canals convey information about rotatory and angular movements of the head, while the maculae convey information regarding linear and tilting movements. Disease of the semicircular ducts, utricle and saccule gives rise to giddiness of varying degrees.

The bony cochlea consists of two and three-quarter turns of a spiral, and resembles a shell lying on its side. It has a central supporting column of bone(the modiolus) to which is attached a thin lamina of bone partially dividing the spiral into two parts, the scala vestibule above and scala tympani below(picture b up).  The membranous cochlear duct lines the bony cochlea and is triangular in cross section. The outer wall of the triangle is thickened to form the spiral ligament, the lower part is the basilar membrane while the upper part is the vestibular membrane. The thickened and highly specialized spiral organ(organ of Corti) lies on the basilar membrane. Pulsations transmitted to the perilymph within the membranous cochlea by movement of the stapes in the oval window pass through the scala tympani, and are transmitted to the fluid in the scala vestibule, being adjusted by compensatory movements of the round window, thus causing movement of the basilar membrane, thereby stimulating the hair cells of the spiral organ(picture c); the end result being auditory perception. Low frequency sounds cause maximum activity in the basilar membrane; high frequency sounds are limited to the basal portion of the cochlea.