Vestibular and Auditory System Flashcards
What are the two types of hearing loss?
- Conductive: blockage of sound conducting path from source to cochlea–treatable and reversible
- Sensorineural: damage to inner ear or central auditory pathway (hair cell loss, auditory nerve loss, vascular damage)–incurable, permanent hearing loss
What causes pain in the ear?
Pain from noise is from activation of nerve endings which occurs over about 150 dB depending on the frequency of the sound
Describe the flow of sound in the outer and middle ear.
Sound is air pressure/movement of molecules (condensation and rarefaction of air molecules). Sound flows from the environment to the outer ear. The pinna funnels sound into the ear and sound hits the tympanic membrane. The middle ear consists of the malleus, incus, and stapes which converts and amplifies the air pressure waves into fluid pressure waves in the inner ear.
Inward movement of the tympanic membrane occurs during the compression phase of a sound wave; outward movement of the tympanic membrane occurs during the rarefaction phase of the sound wave.
Where does vibrational input to the inner ear get transmitted?
The vibrational input from the stapes footplate is into the scala vestibuli and the pressure release from these vibrations is via the round window and the end of the scala tympani. For sound to get from the upper to lower chamber, the vibrations must pass through the scala media where the hair cell of the Organ of Corti are located. This causes vibration of the basilar membrane.
What are the endolymph and perilymph? What is the relevance of their ionic concentrations?
Endolymph is located in the scala media while perilymph is located in the scala tympani and vestibuli. The ionic gradients between the endolymph and perilymph generate a positive voltage of +80 mV in the scala media called the endolymphatic potential, which is fundamental to cochlear function.
What do the outer and inner hair cells do? How do they work?
Inner hair cells are in contact with the tectorial membrane, and movement of the organ of Corti causes deflection of hair cells. When the hair cells move toward the largest hair, filaments stretch and the channels open, depolarize, and release neurotransmitter. This causes action potentials in the nerve. When the channels close, the hair cells hyperpolarize.
Basilar membrane movement causes stereocilia to bend which is translated into opening and closing of channels which becomes synaptic release and causes an action potential.
What do high frequency versus low frequency sounds cause? How are these signals transmitted?
High frequency sounds elicit vibrations at the base (thin) of the basilar membrane while low frequency sounds elicit vibrations at the apex (thick) of the basilar membrane.
Changes in hair cell potential follow low frequency sounds because there is rhythmic opening and closing of channels (and therefore vesicle release). Temporal information is maintained by spacing between action potentials.
Hair cell potential does not return to normal during high frequency sounds because membrane depolarization is constant.
What is range of hearing determined by?
The frequencies at which the basilar membrane vibrates–first third 20-200 Hz, second third 200-2000 Hz, last third 2000-20,000 Hz.
Speech is made of multiple frequencies so different parts of the membrane vibrate at the same time which stimulates different axons which carry this information to the brain.
What types of fibers does the cochlear nerve contain?
- Type I afferents: 8-12 per inner hair cell (myelinated)
- Type II afferents: 1 afferent per 14 outer hair cells (unmyelinated)
- Both types of cells receive efferent feeback
Describe the properties of the cochlear nerve–i.e. what is it about them that permits the brain to process sound?
Different fibers in the nerve will be activated depending on what region of the basilar membrane vibrates. When a particular neuron is firing, it represents the frequency (so the temporal pattern of activity is not the only thing that dictates frequency).
Describe the function of the outer hair cells.
Outer hair cells exhibit mechanical contractility–they are motor effectors. Outer hair cells feed back energy into the basilar membrane which boots the stimulus to the inner hair cell. If outer hair cells die, the sharpness of type I afferent tuning curves from the inner hair cells are blunted (poorer frequency selectivity)
What are the six major synaptic steps?
- Hair cells
- Cochlear nuclei
- Superior olivery nuclei
- Inferior colliculus
- Medial geniculate nucleus
- Auditory cortex
What is the interaural time difference? How is it computed?
The difference in the time of arrival of a sound at two ears. The eighth nerve synapses in the cochlear nucleus which projects bilaterally to many cells in the medial superior olive which act as coincidence dectectors. When a cell is stimulated by signals from both cochlear nuclei simultaneously it fires stronly and projects up through the lateral lemniscus to the inferior colliculus. The inferior colliculus relays this information to the medial geniculate complex of the thalamus which sends it to the primary auditory cortex.
What is the interaural level difference?
Sounds that are not directly ahead reach the ears with different intensities. This information can also be used to localize sound. Exons from the cochlear nucleus projects bilaterally to the lateral superior olive, however, ipsilateral input is excitatory and contralateral input is inhibitory. If the exicitory input reaching the LSO exceeds the contralateral inhibitory input, there is a net excitation which is transmitted to higher cortical centers.
What are monoaural cues used to determine?
Sound bounces around the pinna so it helps you determine if it is coming from the front/back or above/below.
Describe how vestibular receptors respond to linear acceleration.
In the utricle and saccule, the cilia protrude through a gelatinous layer that contains calcium carbonate crystals (otoconia) which cause the cilia to bend and depolarize when accelerated. Signals are tonic in the case of head tilt and transient in the case of linear acceleration.
How do kinocilia in the semicircular canals respond to angular acceleration?
Cilia are embedded in a gelatinous mass called the cupula which spans the width of the ampulla, blocking the cirulation of endolymphatic fluid. When the head rotates, the inertia distorts the cupula which bends the cilia, causing them to fire. Linear acceleration produces equal forces on the two sides of the canal and results in no net signal. The cupulas in analogous semicircular canals on opposite sides of the head causes one side to increase firing rate and the other to decrease firing rate–this allows rotation to be sensed. Since canals are orthogonal, rotation in any direction can be sensed.
What is the vestibulo-ocular reflex?
The VOR provides stability of gaze in spite of head movements. The vestibular apparatus senses head movement and drives the oculomotor neurons so as to produce counterrotation of the eyes in the head.
The vestibular nerve synapses in the vestibular nuclei which sends two projections–first, it crosses the midline and travels in the contralateral MLF to the abducents nucleus to stimulate contraction of the contralateral lateral rectus, second, it travels in the ipsilateral MLF to inhibit the abducens nucleus on the same side. Each abducens nucleus projects through the MLF to the opposite medial rectus to mediate conjugate gaze in saccadic and smooth pursuit movements.
