Oct 9 - Auditory and Vestibular System Flashcards
Name the three parts of the ear
The outer, middle and inner ear
Describe the outer ear
Has an angle; you need to pull your ear upwards to see the tympanic membrane. The angle prevents things from flying into and damaging the tympanic membrane
Describe the middle ear
Separated by the outer ear by the tympanic membrane. There are three bones. It’s embedded deeply in the temporal bone. It’s ventilated (can respond to pressure differences - it feels plugged up when we have a cold)
Name the three bones of the middle ear
The malleus, the incus and the stapes
What divides the middle ear from the inner ear?
The foot of the stapes inserts into a small foramen that connects the middle ear to the inner ear. They are separated by a membrane, called the oval window
Describe the inner ear
It has liquid in it, which is very important as water deflects the majority of sound waves (97%) and only 3% is transmitted. It is encases by bone, which wraps around a membranous inner ear, protecting it from rupture
How is sound transmitted from an air-filled cavity to a liquid-filled cavity?
It requires an amplifier: the tympanic membrane vibrates the bones of the middle ear, the the stapes foot puts pressure in the inner ear, displacing the fluid of the inner ear. The fluid cannot compress, so it has to move somewhere, causing a 22-fold amplification
How big is the tympanic membrane compared to the stapes foot?
The tympanic membrane has an area about 17x that of the stapes foot
Name the two components of the inner ear
The cochlea (for hearing) and the anterior semicircular canal (vestibular)
Name the two labyrinths of the inner ear
The bony labyrinth and the membranous labyrinth
Describe the bony labyrinth
It surrounds and protects the vestibular and auditory sensory structure
Name the two openings of the bony labyrinth
The fenestrum vestibuli (oval) and fenestrum cochleae (round)
Describe the membranous labyrinth
Interconnecting ducts within the bony labyrinth. It contains endolymph, which is similar to CSF (high in potassium)
What is between the membranous and bony labyrinths
Perilymph, which is similar to ECF (high in sodium)
What is the ductus reuniens
It connects the vestibular and cochlear endolymphatic system
Describe the cochlea
Behind our ear. It has many channels/canal-like structures. There are many nerves
Name the two large openings of the cochlea. What is within these openings?
The scala vestibuli (superior) and the scala tympani (inferior). They both are filled with perilymph
What is the third smaller opening of the cochlea. What fills it?
The cochlear duct. It is filled with endolymph
Besides endolymph, what else can be found within the cochlear duct?
There are cilia called hair cells (or outer hair cells) that all project in one direction. On top of the hair cells lies a gelatinous structure called the tectorial membrane; the hair cells project into this membrane and touch it
Describe what happens when the stapes foot vibrates
When there is movement of the stapes foot, it vibrates the oval window, which causes a perilymph wave in the scala vestibuli. The wave migrates along the cochlea until it reaches the apex. When the wave reaches the helicotrema. Then it migrates down the scala tympani. When it reaches round window (membranous structure), the round window bulges out and the energy of the wave disappears.
What is the helicotrema?
It’s where the scala vestibuli and scala tympani meet; it’s the main part of the cochlear apex
What happens to the vestibular and the basilar membrane while the perilymph wave migrates throughout the cochlea?
While the wave migrates down the scala vestibuli, the vestibular membrane is compressed, and while the wave migrages down the scala tympani, the basilar membrane is refracted
What does movement of the basilar membrane do?
Sound induced motion of the basilar membrane and endolymph within the cochlear duct deflects the hair cels, which allow them to touch the tectorial membrane. When deflection takes place, the hair cells start firing and the action potential transmitted to the neuronal cells that go into the brain
How is the frequency of sound determined?
It is based on the location of maximally excited hair cells. The range of frequnencies are tonotopically distributed along the cochlear duct. High tones are at the base and low tones are at the apexThe frequency is measured in hertz (Hz) of sound
What is the organ of corti?
It is the organ that transduces pressure waves into action potentials
How is sound transmitted to the brain?
There are nerve endings in the cochlea that collect the information and they all come together in the cranial nerve VIII aka vestibulocochlear nerve. The information then goes into the spinal cord and it crosses the hemispheres
What is Area 41?
In the brain, it collects hearing information and that information is displayed in the cerebral surface in the same way as it was in the basilar membrane (tonotopically)
What is a decibel?
Measurement of sound intensity. We can hear sound intensity over a wide range (approx 100 decibels)
What is the range of frequencies we can hear?
Between 20 and 20 000 Hz
What is the vestibular system? Describe it
It controls posture, it co-oridnates body, head and eye movements, it maintains balance and spatial orientation. It tells us where we are in relation to our environment. There is a bony case and it is membranous inside. It is filled with endolymph
Name the two labyrinths of the vestibular system
The kinetic labyrinth and the static labyrinth
Describe the kinetic labyrinth
The crista (sensory epithelium/sensory organ) are found within the semi-circular canals/ducts. They detects rotation of the head. It’s a very old system (important for fish)
Describe the static labyrinth
Contains the macula, the sensory epithelium within the otolith orans
Name the two components that comprise the vestibular system
The semicircular canal system, which detects rotational movements and the otoliths, which detect linear acceleration
What make up the otolith organs?
The saccule, which is sensitive to vertical acceleration and utricle, which is sensitive horizontal acceleration
What is ampulla?
Dilatation at one end of the semicircular ducts
What is found within the ampulla?
Sensory neuroepithelium called crista, which consists of a thick gelatinous cap called the cupula and embedded within the cupula are many hair cells (approx 60-100 stereocilia and 1 kinocilium)
How are signals sent to the brain from ampullas?
Motion of the head moves the endolymph, pushing and pulling the the cupula, bending the cilia, causing them to fire signals
What is an otolith?
Structures within the saccule or the utricle. They are sensitive gravity and linear acceleration
How many layers does the macula of the vestibular system have?
Three.
Describe the bottom layer of the macula of the vestibular system
It consists of hair cells embedded in a gelatinous layer (middle layer). Each hair cell consists of approx 40-70 stereocilia and in the middle is one kinocilium
Describe the top layer of the macula of the vestibular system
The top layer consists of otolith. The otolith together with the gelatinous layer is referred to at the otolithic membrane
How is the macula of the vestibular system stimulated?
Movement of the otolithic membrane in response to gravity or linear acceleration causes the hair cells to fire, stimulating the afferent nerve
What happens to the information coming from the vestibular system?
The information goes to the nuclei in the brain stem, then it’s distributed to the muscles of the eye. The eye sends information back to the brain and it’s sent to the cerebellum
What is BPPV?
Benign paroxysmal positional vertigo. It’s caused by otoliths becoming dislodged, causing the vestibular system to send signals of movement when there is none, causing the person to become dizzy.
What is Meniere’s disease?
It’s a disease caused by too much endolymphic fluid within the inner ear, causing balance and hearing impairement
Describe, briefly, what happens light hits the eyeball
It hits the cornea, then the lens, then the retina. The retina collects the information and sends it through the optic nerve to the brain
Describe accomodation
It is suspended by zonular fibres that are connected to a circular muscle, called the ciliary muscle. When that muscle contracts, the lens bulges out allowing us to see things close to our face.
What is the role of the iris?
It determines how much light enters our eye
How many layers are in the retina? Describe them
There are three layers. The outermost layer contains the photoreceptors (rods and cones). The middle layer has bipolar cells. The innermost layer contains ganglion cells; the axons of the ganglion cells create the optic nerve
What is the macula of the eye?
It’s the area of retina that contains the majority of the cones (photoreceptors that allow us to see colour). It’s mostly made up of cones. It’s the area where the majority of the photons are focused
Describe the area outside of the macula of the eye
It’s mostly made up of rods. It does not see in colour
What is the fovea
It’s the centre of the macula area in the eye. It’s for sharper vision.
How does light move through the retina
It moves through the three layers of the retina to the photoreceptors on the outermost layer. Activation of the rods and cones is relayed to the bipolar cells, and that information is passed onto the ganglion cells
Name and describe the sources of arterial blood to the eye
There are two main sources: the ophthalmic artery in the internal carotid artery and the central retinal artery from the ophthalmic artery. These two arteries ensure that the outer and inner layers of the retina are supplied with arterial blood
Describe what happens when light hits the lens
The lens reverses the orientation of the light. The left and right optic nerves criss cross at the optic chiasm, immediately below the hypothalamus. Here, the nerve fibres from the medial half of the visual field cross hemispheres, and the fibres from the lateral half do not cross hemispheres, creating the optic tracts. So the left hemisphere sees what is in on the right and vice versa.
What happens if one optic nerve is cut/damaged?
The patient will only see through the opposite eye (i.e., if the left optic nerve is cut, the patient will only see through the right eye)
What happens if the nerve fibres crossing hemispheres at the optic chiasm are cut?
The patient will only see what is in the medial half of the visual field
What happens if one of the optic tracts?
The patient will only see the half of the visual field, the same half as the tract that was cut (i.e., if the left optic tract is cut, the patient will only see the left half of the visual field; the lateral half of the left eye and the medial half of the right eye)
Name the extraocular muscles of the eyeball that control the movement of the eyeball
Superior oblique, inferior oblique, superior rectus, inferior rectus, lateral rectus, medial rectus
Name the extraocular muscle that controls movement of the eyelid
Levator palpebrae superioris
Name the nerves of the eyeball. What muscles does they innervate?
The trochlear nerve (CN IV): superior oblique
The abducent nerve (CN VI): lateral rectus
The oculomotor nerve (CN III): superior rectus, inferior rectus, medial rectus, inferior oblique, levator palpebrae
