| acoustic impedance | The resistance that a material offers to the passage of a sound wave (colloquial); a property of a medium computed as the product of density and sound propagation speed (characteristic acoustic impedance). Discontinuities in acoustic impedance are responsible for the echoes on which ultrasound imaging is based. Unit: the rayl. (05 Mar 2000) |
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| acoustic impedance tests | Objective tests of middle ear function based on the difficulty (impedance) or ease (admittance) of sound flow through the middle ear. These include static impedance and dynamic impedance (i.e., tympanometry and impedance tests in conjunction with intra-aural muscle reflex elicitation). This term is used also for various components of impedance and admittance (e.g., compliance, conductance, reactance, resistance, susceptance). (12 Dec 1998) |
| acoustic lemniscus | A bundle of ascending fibres that originate from the cochlear and auditory relay nuclei of the rhombencephalon, enter the trapezoid body, a transverse fibre stratum in which about half their number decussate, and from here turn rostrally along the lateral side of the spinothalamic tract; in the midbrain, it arches dorsally and enters the inferior colliculus in which all of its fibres terminate; the auditory pathway is transsynaptically extended from here by the brachium of the inferior colliculus to the medial geniculate body of the thalamus, from which in turn the auditory radiation leads to the auditory cortex; intercalated in the trapezoid body and along the ascending trajectory of the lemniscus are several cell groups in which part of the fibres synapse. Synonym: lemniscus lateralis, acoustic lemniscus, auditory lemniscus, auditory tract, lateral fillet. (05 Mar 2000) |
| acoustic lens | In ultrasonography, a lens used to focus or diverge a sound beam; may be simulated by electronic manipulation of signals. (05 Mar 2000) |
| acoustic maculae | Thickened areas of the saccule and utricle where the termination of the vestibular nerve occurs. (12 Dec 1998) |
| acoustic meatus | The passage leading inward through the tympanic portion of the temporal bone, from the auricle to the tympanic membrane; it consists of a bony (inner) portion and a fibrocartilaginous (outer) portion, the cartilaginous external acoustic meatus. Synonym: meatus acusticus externus, acoustic meatus, antrum auris, auditory canal, external auditory meatus. (05 Mar 2000) |
| acoustic nerve | <anatomy, nerve> The 8th cranial nerve. The acoustic (or vestibulocochlear) nerve has a cochlear part which is concerned with hearing and a vestibular part which mediates the sense of balance and head position. The fibres of the cochlear nerve originate from neurons of the spiral ganglion and project to the cochlear nuclei. The fibres of the vestibular nerve arise from neurons of scarpa's ganglion and project to the vestibular nuclei. (12 Dec 1998) |
| acoustic nerve diseases | Diseases of the eighth cranial nerve. These include vestibular neuronitis, cochlear neuritis and acoustic neuroma. (12 Dec 1998) |
| acoustic neurilemoma | <tumour> Schwannoma arising from cranial nerve eight. (05 Mar 2000) |
| acoustic neuroma | <oncology, tumour> This is a benign tumour of the 8th cranial nerve. This tumour arises from the myelin forming Schwann cells that coat the 8th cranial nerve (acoustic nerve). Clinical presentation usually includes hearing deficit. Diagnosis can be made with auditory evoked potentials and/or MRI scanning of the brain. Vertigo and tinnitus may be associated symptoms. Surgical removal of this tumour involves dissection, cautery and obliteration with laser. Incidence: 1 in 100,000. (13 Nov 1997) |
| acoustic papilla | A prominent ridge of highly specialised epithelium in the floor of the cochlear duct overlying the basilar membrane of cochlea, containing one inner row and three outer rows of hair cells, or cells of Corti (the auditory receptor cells innervated by the cochlear nerve) supported by various columnar cells: the pillars of Corti, cells of Hensen, and cells of Claudius; the spiral organ is partly overhung by an awning-like shelf, the tectorial membrane, the free marginal zone of which is covered by a gelatinous substance in which the stereocilia of the outer hair cells are embedded. Synonym: organum spirale, acoustic papilla, Corti's organ. (05 Mar 2000) |
| acoustic pressure | In ultrasound, the instantaneous value of the total pressure minus the ambient pressure; unit is pascal (Pa). (05 Mar 2000) |
| acoustic radiation | The fibres that pass from the medial geniculate body to the transverse temporal gyri of the cerebral cortex by way of the sublentiform part of the internal capsule. Synonym: radiatio acustica. (05 Mar 2000) |
| acoustic reference level | The biological reference level for sound measurements. When the term decibel is used to indicate the noise level, a reference quantity is implied; this reference value is usually expressed as a sound pressure of 20 micronewtons per square meter. The reference level is referred to as 0 decibels, the baseline of the scale of noise level's; this baseline is considered the weakest sound that can be heard by a person with very good hearing in an extremely quiet location. Other equivalent reference level's still being used include 0.0002 microbar and 0.0002 dyne per square centimeter. (05 Mar 2000) |
| acoustic schwannoma | <tumour> A benign neoplasm of the intracranial segment of the eighth cranial nerve, producing cerebellar, lower cranial nerve, and brainstem signs and symptoms. Synonym: acoustic neuroma, cerebellopontine angle tumour, eighth nerve tumour. (05 Mar 2000) |