The ear is a complex organ that permits hearing sound. Aside from hearing, it also has other roles in balance. It is made up of a number of different parts and is a crucial part of the auditory system. The ears are a pair of organs situated on either side of the head.
There are two major systems of the ear. The first is the auditory system and this is the system responsible for hearing sounds. The other system is the vestibular system and this involves balance. The pathway of communication for both these systems is via transmitters and neurons.
Anatomy of the Ear
There are three main parts to the ear which include the external ear, the middle ear and the inner ear, all of which are made up of many different parts themselves. The external ear's main function is to collect the sound waves and it is made up of the pinna and the ear canal. The sound waves collected are passed to the tympanic membrane which is commonly known as the ear drum. The ear drum separates the external ear from the middle ear. The pinna is the external part of the organ seen to the public eye and it is made up of cartilage which is folded into many contortions. This creates the odd concave shape of the ear. These in turn are given different names in order to identify different parts of the pinna. The pinna is also made up of ligaments and muscle. The ear canal is the opening of the ear and it leads down to the ear drum. Its maximum length is 1.5 inches long and it forms a curved shape. The ear canal is formed from both cartilage and bone which separates the canal into two parts. The cartilage part is approximately 8 mm in length and it leans inwards in the ear. It is highly flexible due to a few fissures which are found throughout the vertical part of the cartilage. The bone forms the rest of the ear canal and both are covered with a very thin layer of skin.
The next part of the ear is the middle ear and this is an air filled cavity like the external ear. It is involved in amplifying the sound waves so that they can be transmitted from the air into the fluid in the inner ear. There are three small bones found within the middle ear called the malleus, incus and stapes respectively. This link the ear drum to the oval window which is a very thin membrane. Each of the bones differs in shape and they are all connected. The malleus resembles a hammer and it is from this that it has gleaned its name. The Incus was so named for it looks like an anvil but a closer comparrison is thought to be a tooth. The final piece of bone is the stapes which looks very much like a riding stirrup.
A tube can also be found within the middle ear called the eustachian tube and this connects the middle ear to the throat. This is important in maintaining a constant pressure within the ear. If the pressure changes within the ear, it can have painful consequences and cause the ear drum to burst. Changes to the pressure also cause changes in the ability of the individual to hear as it affects the vibrations. The tube can open and close in order to control the pressure and this can be controlled by swallowing or yawning. Pressure changes may occur due to an ear infection or activities such as flying or scuba diving.
The inner ear is the only fluid filled cavity found within the ear. It contains structures for both hearing and balance. It receives the input from the external and middle ear and distributes it down the auditory nerve. It is also referred to as the labyrinth due to its complex structure. One of the structures in the inner ear is the cochlea which contains the receptors that allows an individual to hear. The vestibular apparatus are also present in this portion of the ear. There is also the vestibulocochlear nerve which seperates the audtiory input from the input for balance.
The cochlea is the place where sound transduction occurs. Its structure resembles a spiral and the end of the spiral is called the helicotrema. Within the spiral are three compartments each filled with fluid and these are separated by two different membranes. The first is known as the vestibular membrane and the second is the basilar membrane. Both of these membranes separate the cochlea into the scala vestibuli, the scala tympani and the scala media. The membranes meet at the helicotrema and there is a small opening at this point between the first two compartments. The fluid found within the first two compartments is called the perilymph and this is a different fluid to that found in the scala media. Perilymph is more like cerebrospinal fluid whereas the endolymph fluid found in the scala media is more like intracellular fluid. The endolymph fluid has a high concentration of potassium and a low concentration of sodium. There is also a potential in this fluid of +80 mv compared to the other compartments.
The oval and the round windows of the middle ear seperate the cochlea from the rest of the ear. The fluid cannot be compressed but needs to move in order to generate the transduction of waves throughout the system. Vibrations occur at the oval window and are generated in the scala vestibuli and these then travel through the scala tympani which in turn causes the round window to move. This prevents a change in volume in the inner ear.
Organ of Corti
Within the cochlea lies the organ of Corti and this is situated on the basilar membrane. It contains a number of hair cells as well as supporting cells, and is covered in a membrane called the tectorial membrane. The hair cells have a number of protrusions called stereocilia and the tips of these protrusions are embedded into the membrane. The hair cells are arranged into different rows and within the organ of Corti there are three rows of outer hair cells and one row of inner hair cells. The sound waves cause the inner hair cells to bend in response which is the start of sound transduction. The outer hair cells are present in order to control the sensitivity of the inner hair cells. The outer hair cells receive efferent innervation.
The hair cells are at a membrane potential of -70mv and their environment is rich in potassium. The hair protrusions are surrounded by the endolymph fluid whereas the majority of the hair cell is surrounded by the perilymph. Potassium channels can be found in the hair protrusions and together with the potential and the concentration of potassium ions, there is an electrochemical force which allows potassium to enter the hair cell and this is important for sound transduction.
The inner hair cells transduce sound and the waves generated in the perilymph cause the two membranes to move in the ear. These in turn cause the tips of the hair cells to bend and depending on which way they bend, the potassium channels either open or close. It is this control that decides whether an action potential is transmitted. When there is no sound, the hair cells are straight and elastic filaments hold the potassium channels open slightly so that partial depolarisation can occur. This causes calcium dependent transmitter release in the cell and low action potentials are produced. When sound waves are present, the tips are bent and the potassium channels are opened wider. This causes a larger depolarisation and eventually a larger action potential is generated. The action potentials frequency can be reduced during transduction of sound waves if the hair cells are bent the other way for the potassium channels are forced to shut. The size of the action potential also depends on the sound itself for larger sounds will cause the hair cells to bend further.
When a transmitter is released from the hair cells, it will bind to receptors that are found on the cochlear nerve which is part of the cranial nerve VIII and this causes depolarisation of the neuron. It can determine the intensity of sound by the extent of depolarisation. The nerve carries the message to the cochlear nuclei which is found in the medulla oblongata in the brain. This then relays the message to the medial geniculate nucleus which is located in the thalamus and it is then taken to the auditory cortex in the temporal lobe. The brain can work out the direction of the sound based on which neurons are firing.
The Ear and Balance
The second role of the ear is to control and maintain balance or equilibrium. There are different anatomical structures needed for this function and they allow the brain to perceive acceleration of the body in relation to the head.
The Vestibular system
The vestibular system is made up of the vestibular apparatus which is located in the cavities of the temporal bones. This space is often referred to as the bony labyrinth. Within the bony labyrinth lies the membranous labyrinth which is filled with endolymph. The surrounding area is filled with perilymph.
There are three main parts that make up the vestibular apparatus and these are the semicircular canals, the utricle and the saccule. There are three canals and these are located perpendicular to each other which allows the ear to detect movement in three different planes. Each canal detects a certain type of movement - the anterior canal detects movement of the head up and down, the lateral canal detects rotations from side to side and the posterior canal can sense when the head has moved up and down to the side. The other two regions of the vestibular apparatus can detect movement also. The utricle can detect acceleration forwards and backwards whilst the saccule can detect up or downward acceleration.
The Semicircular Canals
Transduction in this system also relies on hair cells and there is a different type of hair cell present in this region of the ear. They are found in the ampulla which is the large area at the bottom of each canal and the cristae resides in this area on the bottom of the ampulla and contains both hair and support cells. The cupula overlies this area and is a gelatinous area that is separate from the endolymph due to a single membrane. The hair cells are similar to the other hair cells found in the ear except within the group is a single hair cell which is much larger than the rest and this is referred to as the kinocilium.
In terms of hearing, it was the sound waves that caused the bending of the tips of the hair cells. In this case it is the rotation that causes the bending. The bony labyrinth starts to rotate as the head rotates but the endolymph is much slower in the rotation and this creates a pressure that causes the hair cells to bend. Depending on which way the hair cells bend in relation to the kinocilium, the action potential either increases or decreases. Constant rotation cannot be perceived because eventually the endolymph will catch up with the labyrinth and there will be no more bending.
An individual can feel dizzy and disorientated if they spin for a while and stop. This is because the once the constant rotation stops, the head and the bony labyrinth stop moving but the endolymph keeps moving for a time and this makes the individual feel as if they are moving when they are in fact standing still.
The Utricle and Saccule
The utricle and saccule can be found as bulges between the semicircular canals and the cochlea and they also use hair cells to perceive acceleration. The hair cells extend into a gelatinous area and at the edge of this area are crystals called otoliths which are made from calcium carbonate. The utricle hair cells are in horizontal rows with the hair protrusions extending vertically whereas the hair cells in the saccule are vertical and extend horizontally. This orientation allows the utricle to detect forward and backward and the saccule to detect up and down acceleration.
When an individual walks forward, the otoliths cause the gelatinous mass to fall behind which causes the hair cells to bend towards the kinocilium and a larger action potential is formed. If the individual walks backwards, the hair cells bend away from the kinocilium and a smaller action potential is produced. The same occurs in the saccule for upwards and downwards motion.
The vestibular nerve carries the action potential to the vestibular nuclei. From here they may travel back to the cerebellum in order to provide the brain with immediate feedback for motor coordination. The vestibular nuclei also projects to the cortex to control perception. It also provides feedback for eye movements which is important because of the eyes and head do not move together then the vision will blur and the individual will experience motion sickness.
There are a number of different conditions that can affect a persons hearing. These can range from temporary lapses in hearing to permanent problems and deafness. Hearing loss can be classified into two main types. The first is conductive hearing loss and this is where the sound does not reach the inner ear. This may be due to problems with the ear canal such as malformation or problems with the middle bones. Dysfunction of the ear drum can also cause conductive hearing loss. The second type is sensoineural hearing loss where the cochlea or the nerves are affected. The main cause is usually problems with the hair cells in the cochlea. It is possible that hearing loss can be caused from a combination of the two types.
There are a number of different causes for deafness or impaired hearing loss. Some people are born deaf and this is a genetic condition. This could be a factor caused by other conditions or the individual may simply be just deaf. A number of deaf genes have been identified. Sometimes trauma to the ear can result in loss of hearing. Chemicals and medications can also cause lesions in the cochlea which are often irreversible. Also a number of different illnesses can affect hearing such as mumps, meningitis and HIV. Other factors include age and noise.
Ear infections can cause problems with hearing. These can be reversible or irreversible depending on the situation. Other common problems include Tinnitus which causes a roaring sound in the ear and this can cause problems with sleeping and hearing. Conditions of the ear can also affect balance and the individual can feel dizzy.
Treating Hearing Impairments
There are a number of treatments or devices that can aid an individual with hearing impairment. Hearing aids are commonly used and these amplify the sound that is coming into the ear so that is can be transduced. Cochlea implants are also used where an artificial implant is used to send electrical impulses to the cochlea nerve. However, these are expensive and there are high risks involved such as an increase in the chance of contracting meningitis. Aided devices are often used by the deaf community in order to communicate efficiently. Communication devices are used to send messages over telephone wires using a word processing device. Sign language is also used and is recognised world wide. Hearing dogs are also an advantage and are trained to pick up on key sounds such as the door bell or fire alarm.