Hearing aids – Lanka Medical Audio
Basic anatomy and physiology of the auditory system
By Manjula Wanniarachchi, Clinical Audiologist, BSc, (Hons) SHS, Specialized in Audiology.
Hearing aids – Lanka Medical Audio, we help and care of your hearing and this artical will help you to understand the basic anatomy of the auditory system
Anatomical orientation of the human auditory system is concern with hearing and balance. In generally the function of the auditory system is, sounds from the outside of the air converted to the vibration and interpreted as an auditory signal by the brain which termed as hearing. Auditory system function as the one of the functioning system which maintain the body balance and equilibrium.
For the explanatory purposes, human ear divided into three main parts.
- Outer ear
- Middle ear
- Inner ear
Outer ear contains externally visible pinna and external ear canal. External ear canal also termed as external auditory meatus which ending at the ear drum.
Pinna also termed as auricle which principally compose of elastic cartilage. It also contains undifferentiated extrinsic and intrinsic muscles. Funnel like shape of the pinna is important to collect the sound from the air.
The external ear canal is roughly 2.5 cm long and not quite a straight tube. It is slightly “S” shaped tube which extend to ear drum. The outer third of the canal is cartilaginous and inner two third is bony. The cartilaginous portion contains several glands and hairs. It transmits sounds to the eardrum.
Middle ear is an air filled cavity behind the ear drum. Three tiny bones consequently malleus, incus and stapes is located in the middle ear. Middle ear connected with the nasopharynx by Eustachian tube.
The ear drum also termed as tympanic membrane. It is in 55 degrees to the canal. The tympanic membrane composes of three layers and handle of the malleus attached to it. Sound entering the outer ear is picked up by the ear drum and set into vibration. The vibration of the ear drum is transmitted to the ossicular chain of the middle ear. The three tiny ossicles of the middle ear act as a bridge which extend from ear drum to the oval window. Then the vibration goes through the ossicular chain.
Middle ear contains two muscles. The tensor tympani muscle and stapedius muscle. The tensor tympani muscle contact with the malleus at the top of the manubrium near the neck. Contraction of it pull the malleus in the interomedial direction thereby stiffening the ossicular chain.
The stapedius muscle is the smallest skeletal muscle in the body which contact with the neck of the stapes. Contraction of stapedius muscle pull the stapes posteriorly and stiffening the ossicular chain.
The Eustachian tube provides aeration and drainage of the middle ear. Because of that mechanism it is possible to maintain the air pressure same on both sides of the ear drum.
Inner ear contains the sensory organs of hearing and balance. Inner ear consist of vestibule, three semi-circular canals and cochlea. Inner ear is fluid filled area which lies in the temporal bone. Cochlea like a snail –shaped twisted organ that makes 2 and ¾ turns.
The cochlea set as duct inside the duct, outer duct is called bony labyrinth and inner duct called membranous labyrinth. The bony labyrinth filled with perilymphatic fluid and membranous labyrinth filled with endolymphatic fluid.
The cochlea has three portions as shown in above, the scala media between the scala vestibuli above and the scala tympani below. Scala media filled with endolymphatic fluid and scala vestibuli and scala tympani filled with perilymphatic fluid.
The organ of corti is the sensory organ for hearing which located in the scala media. Below the tectorial membrane there is a single row of inner hair cells and three rows of outer hair cells. Middle ear vibration transmitted to the inner ear via the bridge of ossicular chain and it cause to motion of the fluid in the inner ear. This fluid motion cause stereocillia on the inner hair cells bend in a manner which induce the release of neurotransmitters. Then via the afferent nerve fibers signal transmits to the brain. Outer hair cells which supply by efferent nerve fibers boost the low-level sound transmission.
Sensory receptors in the inner ear are in contact with the nerve cells of the eighth cranial nerve, which connect the peripheral ear to the central nervous system.
Three semicircular canals, utricle and saccule of the vestibule concern with balance. Each ear contains three semi-circular canals that respond to angular acceleration. These canals are at right angle to each other, because of that they can respond to angular motions in any direction. Each canal has dilated area called ampulla and within the ampulla sensory organ called crista located.
The utricle and saccule respond to the linear acceleration and gravity. The maculae are located within the utricle and saccule which is the sensory organ of vestibule.
Stimulus from the semi-circular canals and vestibule transmitted to the higher areas through vestibular portion of the eighth cranial nerve.
The outer and middle ear called as the conductive systems because they together bring sound or vibration of the air in to the inner ear. The cochlear of the inner ear and the eighth cranial nerve together termed as sensorineural system because it converts the auditory signal in to neural signal and transmit it into higher central areas to identify and response it.