Auditory translation

1. Acoustic division of CN 8 has its cell bodies in the spiral ganglion of the cochlea; the peripheral processes of the ganglion convey auditory impulses from the specialized neuroepithelium of the inner ear, the spiral organ of Corti
   1. organ of Corti has 15,000 hair cells resting on the basilar membrane along the 2 ½ turns of the cochlea; sound causes the basilar membrane to vibrate; the upward displacement of the basilar membrane bends the hair cells lying against the overlying tectorial membrane; each hair cell corresponds to a different frequency and stimulate the afferent bipolar neurons of the spiral ganglia which project to the cochlear nuclei
   • basilar membrane separates the cochlear duct from the scala tympani and is in contact with both endolymph and perilymph
   1. organ of Corti can respond to frequencies from 20 Hz to 20,000 Hz
2. the cochlear fibers bifurcate and terminate in the dorsal and ventral cochlear nuclei; fibers from one side of the cochlear nucleus are connected to both inferior colliculi (mainly the opposite) via the lateral lemniscus; secondary acoustic fibers project via the trapezoid body and lateral lemniscus to the medial geniculate bodies; from the medial geniculate bodies fibers project to the cortex via the auditory radiations (to Heschle’s gyri, superior temporal gyrus)
   • some cochlear fibers terminate in the trapezoid body and superior olivary complex and subserve such reflex functions as auditory attention, sound localization, auditory startle, and oculo-postural orientation to sound;
     1. • the trapezoid body contains auditory fibers from the cochlear and superior olivary nuclei, medial lemniscus, and corticobulbar fibers; exiting fibers from CN 6 also go through the trapezoid body; the MLF lies posterior to the trapezoid body
3. there are four main language areas situated in the left cerebral hemisphere – two receptive and two executive – all border the Sylvian fissure and are connected by the arcuate fasciculus
   1. receptive
      1. perception of spoken language occupies the posterior-superior temporal area (Area 22) and Heschle’s gyri in the middle, superior portion above Area 22 (Area 41 and 42)
         1. posterior portion of Area 22 in the temporal lobe is called Wernicke’s area
      2. perception of written language is in Area 39 just posterior to Area 22 in the inferior parietal lobe
   2. executive
      1. posterior end of the inferior frontal convolution is referred to as Broca’s area (Area 44 and 45) concerned with the motor aspects of speech (innervated with connections to motor function of the mouth, tongue, and lips)
      2. second executive area is controversial but is supposed to be where visually perceived words are given expression in writing (innervated with connection to motor function of the hand)
   3. Cerebral dominance and its relation to language and handedness
      1. 90-95% of the population is right handed but most individuals are neither completely right-handed or left-handed but favor one hand for more complicated tasks
         1. left handedness may result from disease of the left hemisphere early in life and accounts for the higher incidence of left handedness among the mentally retarded and brain injured
      2. the right hemisphere has an important role in the communication of feelings and emotion (e.g. globally aphasic patients can still shout or curse)
         1. the aspects of language contributed from the right hemisphere are called prosody (the melody, rhythm, intonation, inflection and pauses that transmit emotional overtones in speech)
         2. patients with right hemisphere MCA (inferior branch) strokes have difficulty understanding the emotional content of speech