Session 2 - The Ear Flashcards
What is the structure of the outer ear?
The most external part of the ear consists of the pinna (or ‘auricle’), the ear canal (or external acoustic meatus) and the tympanic membrane (eardrum). The pinna is shaped to gather sound waves and direct them into the ear canal. The ear canal directs these sound waves towards the tympanic membrane. When sounds waves strike the tympanic membrane, it vibrates like the surface of a drum and transmits them deeper into the ear towards the cochlea.
What is the contained in the middle ear cavity?
Immediately behind the tympanic membrane is the middle ear cavity. Within it, are the ossicles. The ossicles are three of the smallest bones in the body and they carry the sound waves to the oval window which conveys it into the cochlea.
• Malleus – the first ossicle rests against the tympanic membrane. It is shaped like a hammer, from which its name is derived. The handle of the ‘hammer’ rests against the tympanic membrane and can be seen through the membrane during otoscopy (looking in a patient’s ear). The head of the hammer connects to the incus.
• Incus – the second ossicle’s name means ‘anvil’. As the ‘hammer’ (malleus) strikes the ‘anvil’ (incus), sound waves are transmitted, eventually reaching the stapes.
• Stapes – the third and final ossicle’s name means ‘stirrup’, which it clearly resembles. It receives sound wave vibrations from the incus and transmits them onto the oval window which marks the boundary of the middle and inner ear cavities.
What is the Eustachian tube?
The superior opening of the auditory tube (also known as the Eustachian tube) is also located in the middle ear cavity. The inferior opening of the auditory tube is in the posterior and inferior part of the nasal cavity. It therefore provides a connection of airflow between the external environment and the middle ear cavity via the nasal cavity. This connection is vital to maintain equal air pressure on either side of the tympanic membrane. Without it, air pressure would build up on the inside or outside of the tympanic membrane, causing pain and potentially rupture.
How do you counter pressure in ear?
This pressure can be felt when descending in an aeroplane from a significant height, or when diving underwater. As you descend deeper, air (or water) pressure outside the tympanic membrane increases and causes the membrane to bulge inwards. To counter this, air is allowed to pass up through the auditory tube to increase air pressure on the inside of the tympanic membrane. Sometimes, if the pressure correction occurs suddenly, you can feel a ‘pop’ in your ear.
What are the two small muscles in the inner ear?
There are two small muscles within the middle ear cavity that act to dampen sound vibrations and reduce perceived volume.
• The tensor tympani muscle inserts on the malleus and when it contracts, it increases tension in the tympanic membrane, reducing how much it can vibrate. Tensor tympani is supplied by the mandibular branch of the trigeminal nerve (CN V).
• The stapedius muscle inserts on the stapes and acts in similar way. When it contracts, it dampens the vibrations of the stapes. Stapedius is supplied by the facial nerve (CN VII).
What does the inner ear cavity contain?
The inner ear cavity contains the bony labyrinth, a network of small bony passages within the petrous part of the temporal bone of the skull. The bony labyrinth contains two main organs:
• Cochlea – as sound waves and vibrations travel through fluid within the cochlea, they are converted into electrical impulses which are passed via the cochlear nerve to the auditory cortex. This allows us to perceive sound. The sound waves first enter the cochlea via the oval window (which the stapes is in contact with). The round window is located nearby and bulges in and out to allow the fluid within the cochlea to move.
• Vestibular system – consists of the semicircular canals, utricle and saccule. The vestibular system contains fluid, which flows when we move our heads. The movement of this fluid is detected by specialised cells causing them to produce electrical impulses. These impulses are passed along the vestibular nerve towards various parts of the brain including the cerebellum, thalamus and certain cranial nerve nuclei. The three semicircular canals, positioned perpendicular to each other in three dimensions, allow us to perceive movement. The utricle and saccule allow us to perceive linear acceleration.
Where are impulses carried from the cochlea and vestibular system?
Impulses from the cochlea and vestibular system are carried by the vestibulocochlear nerve (CN VIII), through the internal auditory meatus towards the nuclei of the vestibulocochlear nerve in the pons. Here, there are connections to other brainstem nuclei of the oculomotor, trochlear and abducens nerves which control eye movements. Because of these connections, we are able to maintain fixed gaze whilst moving our head. This is called the oculocephalic reflex.
What is vertigo?
Vertigo is the symptom of being able to perceive movement when there is none. This is commonly perceived and recreated if you spin around several times and then stop suddenly. The perception of the world continuing to spin around you, or the floor moving unevenly beneath you, is vertigo. There are various causes of vertigo, but a common presentation to clinical practice is a disorder of the vestibular system. Inflammation, infections, endo/perilymph disorders or cancers of the vestibular system or nerves may cause vertigo.
What is vestibular schwannoma?
Also known as an acoustic neuroma, this is a benign tumour of the Schwann cells surrounding the vestibulocochlear nerve. As it grows, it gradually leads to symptoms of unilateral hearing loss, tinnitus, a feeling of fullness in the ear and vertigo. If it grows large enough, it will start to compress the other cranial nerves that leave the brainstem around the same position. This position is known as the cerebellopontine angle, and other cranial nerves leaving the brainstem here are the trigeminal (CN V) and facial (CN VII) nerves.
What is otitis media?
Otitis media is the term for inflammation within the middle ear cavity. It is most often caused by a simple viral infection and is particularly common in young children. When a patient is suffering from an upper respiratory tract infection, the inflammation can spread throughout the upper respiratory tract from the nose and pharynx to the middle ear cavity via the auditory tube. This can lead to accumulation of inflammatory fluid and pus in the middle ear cavity, impairing conduction of sound waves along the ossicles.
The auditory tube is relatively narrow in young children and therefore less effective at draining the middle ear cavity. Because of this, a build of pus behind the tympanic membrane can increase pressure significantly which can be painful. If the inflammation continues, the tympanic membrane may rupture due to the pressure. This may relieve the pain and allow drainage of the pus. The tympanic membrane will usually later heal on its own. However, more serious cases of otitis media can spread deeper into the ear and cause inflammation of the cochlea or vestibular system, mastoid process of the temporal bone or the meninges.
