(if med school is a Minnesota forest with millions of trees,
these are the red pines)
- The mechanoreceptors that transduce fluid movements induced by sound and head movement are found in the inner ear, deep within the petrous portion of the temporal bone. The complex accessory structures that make up the labyrinths are necessary for both hearing and balance, while the external and middle ears modulate sounds before they are transduced by the cochlea.
- Large modified apocrine glands deep in the dermis of the external auditory meatus produce cerumen (ear wax), which normally protects the tympanic membrane. If the wax becomes impacted, and/or is pushed deep into the ear canal, it can produce a conductive hearing loss.
- Vibration of the tympanic membrane causes the malleus and incus to pivot, resulting in the stapes footplate vibration at the oval window. The gain in pressure due to the actions of the ossicles prevents some of the energy loss inherent in an air/fluid transition and results in air conduction being better than bone conduction (the basis of the Rinne test).
- A conductive hearing loss is an increase in hearing thresholds due to a decrease in sound transmission through the external and/or middle ears. Air conduction is reduced but bone transmission is unaffected. Common causes of CHL include a ruptured eardrum, intra-tympanic fluid (usually caused by otitis media), and otosclerosis.
- The inner ear consists of 2 structural regions that contain different fluids: the bony (osseous) labyrinth, which contains Na+-rich perilymph, and the membranous labyrinth, which contains K+-rich endolymph. The histology in each of the 6 receptive areas of the inner ear is similar.
- Inner ear receptors are divided into two types; both types convert mechanical energy into receptor potentials. TYPE I (INNER HAIR CELLS) are the true sensory receptors that convey information to the brainstem. TYPE II (OUTER HAIR CELLS) function as biological amplifiers, essentially acting as motor units.
Inner ear transduction is DIRECTIONAL: displacement toward the tallest stereocilia (positive deflection) results in DEPOLARIZATION. In the cochlea, this occurs when the basilar membrane moves toward scala vestibuli. Negative deflection (toward scala tympani) results in HYPERPOLARIZATION.
The SEMICIRCULAR CANALS detect head rotation (angular acceleration). The OTOLITH ORGANS (UTRICLE and SACCULE) detect gravity (linear acceleration). The vestibular system is involved in balance and posture, co-ordination of head and body movements and in fixating the visual image on the fovea.
SEMICIRCULAR CANALS WORK IN PAIRS, with depolarizaation occuring in the SAME direction as the head rotation (HORIZONTAL CANALS: head left →depolarization left, hyperpolarization right). The natural pairing is of LEFT ANTERIOR with RIGHT POSTERIOR CANAL (and vice versa).
The VESTIBULO-OCULAR REFLEX is a 3 neuron arc (hair cell/vestibular nerve, vestibular nuclei, cranial nerve motor nuclei) that is used to adjust eye position to compensate for changes in head position (i.e., it keeps the visual image centred on the fovea). Remembering the pairings listed in fact #4, there is depolarization/excitation/contraction in one of the pathways of the pair, and hyperpolarization/inhibition/relaxation in the other. Rotation of the head in one direction results in rotation of the eyes in the opposite direction.
NYSTAGMUS consists of a slow drift of the eyes in one direction (PURSUIT) followed by a rapid recovery movement in the opposite direction (SACCADE). The direction is named for the fast component i.e., a RIGHTWARD NYSTAGMUS consists of slow movement of eyes to the left, followed by fast recovery to the right. The PURSUIT is controlled by vestibulo-ocular reflex; the SACCADE by higher centers (e.g., cortex). Nystagmus can be observed in normal people following stimulation of the vestibular system; in the absence of stimulation, it is a sign of underlying pathology.
The caloric test is used to assess brain function. In a person with a normally functioning cortex, injection of cool water into the right ear, will produce a LEFTWARD NYSTAGMUS (COLD=OPPOSITE, WARM=SAME → COWS). If the patient is COMATOSE, the SACCADE WILL BE ABSENT (the VOR, which operates in the brainstem is still functional and the pursuit will be intact). If the patient is BRAIN DEAD, both the PURSUIT and SACCADE WILL BE ABSENT.
Vestibulospinal reflexes coordinate the position of the head with the trunk and body, with the goal of maintaining the head in an upright position during movement. There are two systems. The LATERAL VESTIBULOSPINAL SYSTEM is responsible for postural changes to compensate for tilts and movements of the body. The MEDIAL VESTIBULOSPINAL SYSTEM stabilizes head position during walking. The two systems differ in anatomical connections, function and the control mechanisms that they use to affect alpha motor neuron function. The function of the VST systems is evident during decerebrate rigidity.
The middle ear transfer function determines the absolute threshold of hearing at each frequency in normal individuals – the cochlea is so sensitive, it can transduce any signal that reaches it. This implies that anything that alters middle ear function (like an infection) will significantly impact hearing thresholds.
Sound waves pass through the cochlea INSTANTANEOUSLY. The traveling wave pattern on the basilar membrane is established more gradually and is INDEPENDENT of how the motion is initiated i.e., don't need to deliver sound via the oval window --- can use bone! The traveling wave establishes a frequency vs. place relationship along the length of the cochlea, with high frequencies being transduced in the base, and low frequencies in the apex.
Outer hair cells use their receptor potential to exert force on the basilar membrane ---thereby generating a POSITIVE FEEDBACK MECHANISM which amplifies the vibration of the membrane in a nonlinear, highly frequency specific manner. This force produces its own fluid wave, which is conducted back through the perilymph, vibrating the middle ear apparatus and generating sounds that are emitted from the ear (OTOACOUSTIC EMISSIONS).
The STRIA VASCULARIS produces the endolymph (high K+) and the endocochlear potential (+80 mV). Many of the ion transporters of the stria are the same as those in the kidney, so drugs that affect renal function are often ototoxic – esp. loop diuretics (which affect the Na+/K+/2Cl- transporter).
Sounds are localized by the differences in timing and intensity between the two ears. Lateral superior olive (LSO) neurons localize high frequency stimuli by comparing interaural intensity differences (IIDs); medial superior olive (MSO) neurons use interaural timing differences (ITDs) to localize low frequency stimuli.
- Because of the extensive bilateral connections of the auditory system,
- the only way to have an ipsilateral hearing loss from a single lesion is to have a peripheral defect i.e., at the cochlea, auditory nerve or cochlear nucleus
- bilateral hearing loss from a single lesion is invariably due to a lesion located centrally
N.B. Noise exposure, ototoxic drugs and congenital malformations can cause simultaneous damage bilaterally but these are considered to be multiple lesions.
- Distinguishing among conductive and sensorineural hearing loss and central auditory processing disorders is important clinically because the causes, treatments and outcomes are radically different. The Rinne and Weber tests and audiograms (obtained by behavioural, ABR or otoacoustic emission techniques) are used to distinguish between conductive and sensorineural hearing loss.