EAR ANATOMY HANDOUT The primary auditory receptive area is the anterior temporal transverse gyrus also known as Heschl’s gyri. The main function of the auditory system is to transform sound waves into electrical impulses that can be processed by the nervous system. BAEPs are good for showing posterior fossa demyelination, but MRIs are good for supratentorial plaques (in white matter or internal capsule). Retrocochlear lesions that have absent BAEP waves are located in the distal portion of the acoustic nerve, near the cochlea. Normal BAEPs are rarely seen when the pons is involved. The auditory system can be separated into the peripheral which is contains the ear, and the central which is made up of the brainstem and cortex (brain). PERIPHERAL NERVOUS SYSTEM The ear is divided into three parts, 1) external ear, 2) middle ear and 3) the inner ear. (Picture on page 6 of this handout.) EXTERNAL EAR External ear- also called the pinna and the auricle is the part of the ear that we can see from the outside. (1) This is where sound impulses start. It’s distinctive contour assists in funneling sound inward, it thins to form the external auditory meatus (the opening of the ear) stopping at the eardrum or tympanic membrane (which is in the middle ear). External auditory meatus- (2) is the opening of the ear, which leads inward toward the middle ear to the tympanic membrane. MIDDLE EAR Middle ear- is a chamber filled with air made up of the tympanic membrane and the three ossicles. (3) Sound waves hit the huge surface of the tympanic membrane (also called ear drum), causing a vibration that sets off the ossicles. Ossicles- three small bones in the middle ear called 1) malleus or hammer, 2) incus or anvil and 3) stapes or stirrup. Movement of the ossicles is restricted to avoid harm from loud noises. The middle ear has two tiny muscles that are attached to the ossicles that relax and stiffen to control the ossicles. (4) The ossicles transfer the sound waves onto the footplate of the stapes. 1 Footplate of the stapes- (5) forces fluid of the cochlea backwards and forwards with every new sound wave. INNER EAR Inner ear- includes the cochlea, scali, basilar membrane and the organ of corti Oval window- is the entrance to the scala vestibuli, which contains the footplate of the stapes (1 of the ossicles). (6) The movement of the stapes on the oval window creates a wave in the fluid of the upper 2 scalae. As the wave gets stronger, the basilar membrane starts to vibrate, shifting the wave to the scala tympani. Semicircular canals- contain hair cells that generate impulses. The canals are oriented at 90 degrees to each other, providing a mechanism that is sensitive to rotation along any axis. They are involved with the vestibular portion of the acoustic nerve, and are not a part of the BAEP recording. They orient the head to the environment. The semicircular canals and the cochlea make up the labyrinth. The utricle and the saccule are both apart of the inner ear and are also involved with the vestibular portion of the acoustic nerve. They orient the body to the environment. They too are not a part of the BAEP recording. Cochlea- (7) a hollow, coiled tube filled with fluid in the inner ear, if uncoiled it is comprised of 3 smaller fluid filled tubes known as 1) scala vestibuli, 2) scala media and 3) scala tympani. The basilar membrane divides the upper scalae (vestibuli & media) from the lower tympani. (media is in middle) The Reissner’s membrane separates the scala vestibule and the scala media. The entrance to the scala vestibuli is the oval window. The round window is at the end of the scala tympani. The two windows separate the scala vestibule and the scala tympani the two windows also separate the middle ear from the inner ear. The cochlear portion of the VIIIth cranial nerve takes the impulse into the central portion of the auditory system. Middle Ear Oval Window Scala Vestibuli Reissner’s Membrane Scala Media Basilar Membrane Scala Tympani Round Window Inner Ear 2 Basilar membrane- (8) is very thin and firm at the bottom of the cochlea, it gets broader and more flexible toward the cochlea’s apex. The thin firm end, recognizes high frequencies and the broader more flexible end, recognizes low frequencies. This is how the basilar membrane helps the ear recognize different sounds that are created. Organ of Corti- (9) is above the basilar membrane in the scala media, it is in charge of changing the electrical impulses created by the vibrations of the hair cells into nerve impulses that move through the cochlear nerve. The cochlear microphonic potential is formed from these impulses. Corti is the doctor who discovered it. Wave I- (10) is generated in the distal or extracranial (outside the brain) portion of the auditory nerve (VIIIth cranial nerve) (peripheral). CENTRAL NERVOUS SYSTEM (11) Wave II is generated as the auditory nerve (VIIIth cranial nerve) enters the cranium (skull), at the medullary pontine junction and may be a proximal or intracranial portion of the auditory nerve in the cochlear nucleus. (12) The auditory nerve (VIIIth cranial nerve) enters the brainstem between the medulla and pons and ends in the ipsilateral dorsal and ventral cochlear nuclei. This is where wave III is generated. It may correspond to action of the superior olivary nucleus/complex and trapezoid body. (13) Almost all of the neurons decussate (cross) to the opposite side of the brainstem, going through the trapezoid body to the superior nucleus in the upper pons and then climbs up to the lateral lemniscus. This is where wave IV is generated. Wave IV is the first to get contributions from both ears. (14) The next part of the trail generates wave V as the signal climbs the lateral lemniscus in the mid to upper pons and ends in the inferior colliculus in the midbrain. (15) From the midbrain the signal spreads to the medial geniculate bodies (metathalamus) of the thalamus. The geniculocortical (goes from the geniculate fibers to the cortex) fibers, also called the acoustic radiations, climb to the primary auditory receptive areas in the anterior transverse temporal gyrus also know as Heschl’s gyrus. This is where wave VI is generated. A lesion in this area could cause an auditory hallucination which may or may not be epileptic in nature. Acoustic or auditory radiations- (also called the geniculocortical fibers) are higher in the brain, between the thalamus and the auditory cortex. The BAEPs we record are short latency. We would have to record middle or long latency BAEPs to record from the acoustic radiations. If we had enough time on our screen, we could record wave VII from the acoustic radiations. 3 GENERATOR SITES FOR THE BAEP These are the best guestimates for the generator sites of the BAEP. There is ongoing controversy regarding the actual generators for these waves. Waves III on, may have several sources that when combined, generate them. The distance between the auditory nerve’s entry point into the brainstem and the inferior colliculus is 3 to 4 centimeters. A Confused Surfer Landed In My Auditorium. I It comes from the distal or extracranial (outside the brain) portion of the auditory nerve. (spiral ganglion) It is a nearfield negative wave recorded from Ai (ipsi). In our montage, the ear is negative, so the pen goes up. II It comes from the entrance of the auditory nerve at the medullary pontine junction and may be a proximal or intracranial portion of the auditory nerve in the cochlear nucleus. It is a farfield positive wave recorded from Cz. In our montage, Cz is positive, so the pen goes up. III It comes from the lower mid pontine level where the auditory nerve enters the brainstem between the medulla and pons. It may correspond to action of the superior olivary nucleus/complex and the trapezoid body. It is a farfield positive wave recorded from Cz (positive), so the pen goes up. IV It is mostly from the mid to upper pontine level, contributions from the cochlear nucleus and nucleus of the lateral lemniscus are probable as well. The majority of neurons decussate (cross) to the opposite side of the brainstem, traveling through the trapezoid body to the superior olivary nucleus in the upper pons. It is a farfield positive wave recorded from Cz. V It comes from fibers of the lateral lemniscus ending in the inferior colliculus. It is generated in the upper pontine level or by the inferior colliculus at the lower mid brain level. It is a farfield positive wave recorded from Cz (positive), so the pen goes up. (On the ABRET exam, it may just have the “brainstem” as the generator for wave I.) VI It is generated in the medial geniculate body. It is possibly due to continuous firing of the inferior colliculus. It is a farfield positive wave. VII It is generated in the auditory radiations. It is also possibly due to continuous firing of the inferior colliculus. It is farfield and positive. From here the signal goes to the primary auditory receptive area also known as the anterior transverse temporal gyrus or Heschl’s gyri. 4