Special Senses:The Ear chapter #13By: Precious Blanton
Auricula- The external ear consists of the expanded portion named the auricula or pinna, and the external acoustic meatus. The former projects from the side of the head and serves to collect the vibrations of the air by which sound is produced; the latter leads inward from the bottom of the auricula and conducts the vibrations to the tympanic cavity. Structure of the External ear: Anatomy and physiology of the ear.
Anatomy and physiology of the ear, con’tExternal (outer)Ear, middle ear, and Inner ear:
The ear canal (external auditory meatus, external acoustic meatus), is a tube running from the outer ear to the middle ear. The human ear canal extends from the pinna to the eardrum and is about 26 mm in length and 7 mm in diameter. Auditory Canal:
The tympanic membrane is also called the eardrum. It separates the outer ear from the middle ear. When sound waves reach the tympanic membrane they cause it to vibrate. The vibrations are then transferred to the tiny bones in the middle ear. The middle ear bones then transfer the vibrating signals to the inner ear. The tympanic membrane is made up of a thin connective tissue membrane covered by skin on the outside and mucosa on the internal surface. Tympanic Membrane:
The middle ear, or tympanic cavity, is a narrow, air-filled chamber lined with mucous membrane and is situated between the external acoustic meatus and the labyrinth. It communicates with the mastoid air cells and with the nasal pharynx via the Eustachian (auditory) tube. The auditory ossicles, forming a chain of three small bones, connect the tympanic membrane with the inner ear . The manubrium (handle) of the malleus is attached to the tympanic membrane. The tensor tympani muscle, acting on the malleus, regulates the tension on the tympanic membrane, resulting in two identifiable regions of the tympanic membrane: pars tensor and pars flaccida. The incus is attached to the malleusand to the third ossiclein the chain, the stapes, which in turn is attached via its footplate to the oval window of the cochlea. The stapediusmuscle regulates the range of motion of the stapes; the two muscles (tensor tympani and stapedius) thus regulate to some extent the amplitude sensitivity of the ear. Middle ear:
Ossicles: The ossicles (also called auditory ossicles) are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss. The term "ossicles" literally means "tiny bones" and commonly refers to the auditory ossicles, though the term may refer to any small bone throughout the body.
Structure of The Malleus The malleus is the most lateral of the three middle ear bones. The malleus lies adjacent to the tympanic membrane, and transfers the vibrations of the typmanic membrane to the incus, which then vibrates the stapes and transmits the vibrational signal to the oval window. The manubrium is often referred to as the handle of the malleus. The anterior process fits into the petrotympanic fissure, and can be of variable size, and is often found to be fractured or partially resorbed without apparent hearing loss (Anatomy of the temporal bone with surgical implications.
The Eustachian tube (or auditory tube or pharyngotympanic tube) is a tube that links the pharynx to the middle ear. In adults the Eustachian tube is approximately 35 mm long. It is named after the sixteenth century anatomist Eustachius. Some modern medical books call this the pharyngotympanic tube Eustachian Tube
The inner ear is called the labyrinth because of the complexity of its shape . It contains six mechanoreceptive structures: three semicircular canals, utricle, and saccule, which serve the sense of equilibrium, and the cochlea, which is specialized for detection of sound waves. The inner ear consists of two parts: the osseous (or bony) labyrinth, a series of cavities within the petrous portion of the temporal bone, and a membranous labyrinth, which is a series of communicating sacs and ducts within the bony labyrinth. Also,theinner ear is easily damaged by intense sound, head injury, and ototoxic drugs. It can be affected by microorganisms and is susceptible to degenerative and metabolic disease. It may also suffer abnormal development. Structure of the Inner Ear:
The temporal bone shell of the inner ear is one of the hardest bones in the body. It is lined with periosteum and is filled with perilymph, a fluid closely resembling cerebral spinal fluid in its chemical composition. Midway between the semicircular canals and the cochlea is the vestibule. It is just medial to the tympanic cavity. The oval window, into which fits the footplate of the stapes, is the lateral wall of the vestibule. Note that motion of the stapes that results from sound waves striking the drum meets considerable resistance at this air (middle ear) - fluid (inner ear) boundary. Mechanisms by which this impedance (resistance) mismatch is overcome are covered in the later section on middle ear function.
cochlear partition is a complex structure of the membranous labyrinth that is described in a later section. At the basal end of the cochlea (that end nearest the vestibule) there are two openings to the tympanic cavity, one on each side of the cochlear partition, that are covered by membranes. One is called the oval window and, as we already mentioned, is in contact with the stapes foot plate. The other, called the round window, is just below the oval window and in contact with no structure. As we shall see in a later section this membrane yields under pressure developed at the oval window by stapes motion. The two channels formed by the cochlear partition are called the scalavestibuli and scala tympani, respectively. Again, they are filled with perilymph. A tiny canal, called the cochlear aqueduct (or perilymphatic duct), leads from the lowest turn of the cochlea through the temporal bone to the CSF-containing subarachnoid space at the base of the brain. The Cochlea:
The Semicircular canals The semicircular canals are three half-circular, interconnected tubes located inside each ear. The three canals are the horizontal semicircular canal (also known as the lateral semicircular canal), superior semicircular canal (also known as the anterior semicircular canal), and the posterior semicircular canal. The canals are aligned approximately orthogonally to one another. The horizontal canal is aligned roughly horizontally in the head. The superior and anterior canals are aligned roughly at a 45 degree angle to a vertical plane drawn from the nose to the back of the skull.  Thus, the horizontal canal detects horizontal head movements (such as when doing a pirouette), while the superior and posterior canals detect vertical head movements. Each canal is filled with a fluid called endolymph and contains a motion sensor with little hairs (cilia) whose ends are embedded in a gelatinous structure called the cupula. As the skull twists in any direction, the endolymph is thrown into different sections of the canals. The cilia detect when the endolymph rushes past, and a signal is then sent to the brain. The semicircular canals are a component of the Labyrinth. Among species of mammals, the size of the semicircular canals is correlated with their type of locomotion. Specifically, species that are agile and have fast, jerky locomotion have larger canals relative to their body size than those that move more cautiously
The process of hearing begins with the outer ear, which collects sound (acoustic) energy and directs it through the ear canal to the eardrum. The incoming waves of sound energy cause the eardrum to vibrate, setting into motion the malleus, incus and stapes (also known as the hammer, anvil and stirrup) bones that make up the ossicular chain of the middle ear chamber, which is connected to the Eustachian tube*. The middle ear’s anatomical structure and conductive motion combine to amplify the sound by appx. 2.3 decibels (dB) and transform it effectively into a fluid (hydraulic) vibration inside our inner ear.Also, the ear is a complex organ of hearing and balance. It is capable of perceiving an incredible range from the tiniest audible sound, which would be 1 trillion times weaker than that of a sound thatwould cause pain. This range is about 130 decibels. Besides loudness, the human ear can detect pitch. This relates to the sound vibrations per second or the frequency, which is measure in Hertz. The higher the frequency, the higher the pitch. This range is incredible, from about 20 to 20,000 Hertz. Finally, the human ear can detect tone. How We Ear: