Community Presentation 6 Basic Ear Anatomy

Hello all thank you for attending this presentation on basic ear anatomy. The human ear is made up of three parts: the outer ear,middle ear and inner ear. The components all work together to transfer sounds from the environment to the brain. We will describe the components, importance and function of each part as it is related to hearing and/or balance. This is a descriptive photo that labels the individual parts of the ear. We will discuss each component separately. We will begin our journey with the outer ear. The outer ear is the only part of the ears that can be visually inspected without the use of special medical equipment or procedures. The only item needed to inspect this section of the ear is your eyes and an otoscope. An otoscope is a medical device used by physicians and audiologists to inspect the auditory canal to determine ear health or disease. The outer ear is made up of two major components: the auricles and the external auditory canal. The auricle is the part of the ears that is visible outside of the skull. An auricle consists of skin over contoured cartilage that is held in place by muscles and ligaments. The shape of the auricle can vary by person. Auricles are located on both sides of the head near the temple and where the jaw meets the skull.The auricle is also referred to as the pinna. The auricles primary function is to collect and amplify incoming sounds. This part of the ear also aids in determining the direction from which the sound came. The external auditory canal is a thin like tube that begins at the auricle and ends at the tympanic membrane. The canal is approximately 1.25 inches long and 0.25 inches in diameter. The inner two-thirds of the canal is embedded in the temporal bone. The outer one-third of the canal is cartilage. This canal is used to funnel sound to the tympanic membrane. Although the shape of each ear canal varies. The typical canal forms an elongated s-shaped curve.The ear can now resonate sound waves and increases the loudness of tones and the 3000 and 4000 Hertz range. The ear canal maintain proper conditions of temperature and humidity necessary to preserve the elasticity of the tympanic membrane. Glands, which produce cerumen or ear wax and tiny hairs in the ear canal provide added protection against insects and foreign particles from damaging the tympanic membrane. The main function of the middle ear is to pass down to the inner ear. This process begins by sound entering the outer ear which then travels through the middle ear and causes the eardrum and obstacles in the middle ear to vibrate. As it travels the vibration becomes louder and changes from an air to a liquid and then travels into the inner ear which my other partner will talk more about later. The anatomical parts of the middle ear include the auditory ossicles, the tympanic cavity and the eustachian tube. Another part of the sound vibration and amplification of the middle ear are the muscles of the middle ear which includes the tensor tympani and the stapedius muscle. The auditory ossicles are some of the smallest bones in the body and include the malleus, incus and stapes. In fact, the term ossicles literally means tiny bones. Although these bones are small they have the important job of amplifying sound and carrying the vibrations from sound waves to the inner ear. The malleus is also known as the hammer because as you’ll see in the diagram on the following slide it slightly resembles one and it’s a long handle that is attached to the eardrum.The incus, which resembles an anvil. is the bone that bridges the malleus and the stapes. And finally the stapes, which looks like the stirrup, is a foot plate and the smallest bone in the body. And now on the next slide you will get to see a diagram that shows a picture of all three of the bones. As you can see from this image of the ear, in which the outer ear and the inner ear are in black and white so as to showcase the middle ear. Here you can see the malleus, the incus and the stapes. In this diagram you can really see how the malleus resemble a small hammer the incus resembles an anvil and the stapes resembles a stirrup, respectively Located in the temporal bone, the tympanic cavity is a major part of the middle ear system .The ossicles, muscles, chorda tympani and the tympanic plexus of nerves are all housed in the tympanic cavity which makes up most of the basic anatomical parts of the middle ear. The tympanic cavity is filled with air and very narrow, it is also lined with six layers of mucous membrane in order to better protect the ossicles and the additional parts of the middle ear from dirt and pathogens. Here we have a dorsal view of the middle ear as you can see from the label which I apologize that they are slightly blurry. Here is the bony edge of the tympanic cavity that I was previously discussing and you can also see the malleus and the cochlear window as well as the opening of the auditory meatus. Both of which are part of the inner ear and will be discussed in more detail later. The eustachian tube connects the middle ear to the nasopharynx. Its main function is to equalize the pressure in the middle ear to make it the same as the pressure that is outside the body. the purpose behind this equalization is so that the tympanic membrane will have the maximum ability to move, which can only happen when the two pressures are equal. The eustachian tube is usually closed, it only opens when someone is yawning, chewing or swallowing in order to allow air through the passage between the middle ear and the nasopharynx. However, the eustachian tube can be opened on its own due to a rapid atmospheric change in pressure. For example, when your lifting off on an airplane. The muscles of the middle ear which include the stapedius muscle and the tensor tympani are located in the tympanic cavity and are actually the smallest striated muscles of the body. Both muscles play a prominent role in middle ear function. The main function in the tensor tympani is to dampen sound like those produced by chewing. This is beneficial because without damping sounds like chewing could potentially damage the inner ear because of louder than average noise or it could distract us from normal activities. The tensor tympani is innervated by the motor fibers of the trigeminal nerve which is also known as cranial nerve five. The way that this muscle works is, when the tensor tympani is tensed the muscles pull the malleus away from the eardrum which causes the tympanic membrane to tense thus damping vibration in the obstacles and reducing the perceived sounds. For relatively quick sounds in the ear that we need protection from, you can also rely on the tympanic or acoustic reflex. It is important to note though that this reflex takes about 40 milliseconds to kick in so if the sound is very sudden and immediate like an explosion for example you can still risk damage to the inner ear. Research on the purpose of the tympanic reflex speculates that it may have been designed to prevent humans from ear damage due to thunder claps. The main function of the stapedius muscle is to stabilize the stapes, try saying that three times fast (laughs). It does this reflexively in response to loud sounds by pulling the head of the stapes backwards outside of the oval window which effectively dampens the noise. Just like the tensor tympani, the purpose of this damping is to prevent excessive vibration and damage to the inner ear. The stapedius muscle is innervated by cranial nerve seven, also known as the facial nerve. One interesting fact about the stapedius muscle is that because it’s integrated by cranial nerve seven, facial nerve paralysis could cause uninhibited movement of the stapes because that muscle can no longer activate to dampen sound and this could cause a lot of damage to the inner ear. Now let’s talk about the inner ear. On the following slides we will discuss the purpose, the components and the function of the inner ear. The inner ear contains the sensory organs of hearing and equilibrium. In this diagram you see the bony labyrinth colored in blue, which is divided into three sections, the vestibule, the semicircular canals and the cochlea. Within the bony labyrinth is a membranous labyrinth colored in orange which is also divided into three parts. You’ll learn more about these on the following slide. One part of the membranous labyrinth is the cochlear duct, the part of the inner ear involved in hearing. The vestibule and the semicircular canals are involved in the sense of equilibrium the three semicircular canals, located on the left slide side of this diagram, are arranged at right angles to each other so that they measure motion in all three planes. It is through these canals and motion of endolymph fluid that send signals to the brain via the vestibular nerve about changes in position. Let’s go over the components of the inner ear. I will explain those that are involved in hearing first and then equilibrium. The cochlea is the major part involved in hearing. It is a snail like structure and is approximately 35 millimeters in length and five millimeters in height. The coils make up about two and a half full turns. This diagram shows the coronal view into the cochlea. It contains the organ of corti,two fluids hair cells and membrane. The organ of corti is the organ associated with hearing. Both the scala vestibuli and the scala tympani, the bluish components of the diagram, contain a sodium filled liquid called perilymph which is necessary for the transferring of impulses involved in hearing. They also protect the membranous layer of the inner ear. The cochlear duct,shown in orange, contains endolymph fluid this fluid is mostly made up of potassium and assists in stimulating receptor cells that send nerve impulses to the brain and that is what signals hearing.There are two types of hair cells inner and outer. There are about 3,500 inner hair cells and 12,000 outer hair cells in a single cochlea. These hair cells transfer sound vibration into neural signals for the brain to interpret. The cochlea also contains two important membranes: reissner’s membrane and basilar membrane. The primary function of the reissner’s membrane is a diffusion barrier allowing nutrients to travel from the perilymph fluid to the endolymph fluid and the basilar membranes primary function is to keep the two fluid separate. But it is also the base at which hair cells are located. The semicircular canals are three interconnected tubes in a bony labyrinth. They are arranged horizontal, superior and posterior respectively. Each canal contains endolymph fluid which senses motion. As your head moves the endolymph fluid lags due to inertia the hair cells dectect this and send the message to the brain that indicates acceleration. Within about 10 seconds of constant motion the endolymph is caught up and the sensation of acceleration decreases. The vestibular nerve is part of the auditory nerve which carries sensory information related to equilibrium to the brain. The saccule is a bed of sensory cells and it translates head movements into neural impulses which the brain can interpret. The saccule detects linear accelerations and head tilt in the vertical plane. When the head moves vertically, the sensory cells of the saccule are disturbed and the neurons connected to them begin transmitting impulses to the brain.These impulses travel along the vestibular portion of the eighth cranial nerve to the vestibular nuclei in the brainstem.The utricle is larger than the saccule but it has a similar function. Any orientation of the head causes a combination of stimulation of the utricles and saccules of the two ears. The brain interprets head orientation by comparing these two inputs to each other and to other input from the eyes and the stretch receptors in the neck, there by detecting whether only the head is being tilted or the entire body is tipping. So how did the outer, middle and inner ear work together to produce hearing? Well we hear by funneling sound from the environment by the outer ear.And it causes the tympanic membrane to vibrate. Those sound wave vibrations are transferred into mechanical vibrations of the ossicles, which you learned are in the middle ear. Those mechanical vibrations caused the oval window to move back and forth.This activates the inner ear and causes perilymph fluid to begin wave-like motions. The perilymph fluid motion is transferred to the endolymph and wave motion is transformed into electrical impulses picked up by the hair cells of the cordi and sent to the brain via the vestibular nerve. The round window is responsible for absorbing the fluid wave vibrations and releasing any increased pressure in the ear caused by the wave motion. The brain interprets the neural impulses from the inner ear as sounds and that’s how hearing occurs. The connection between the outer, middle and inner ear functions are synergistic and systemic, think about all that takes place just to hear one sound. Now imagine all the work your ear has done to listen to our presentation.

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