Hearing, Ear Anatomy & Auditory Transduction
This video follows the path of the sound waves traveling through each part of the ear (outer ear, middle ear, and inner ear), interacting with the tympanic membrane, auditory ossicles, and the bony labyrinth of the cochlea, until it arrives to the hair cells (auditory receptors), located further within the cochlea, that generate nerve impulses in response. -Sound waves enter the ear, passing along the external auditory canal (meatus) to the tympanic membrane (eardrum). The membrane vibrates in response to sound. Low pitch sounds produce low vibration frequencies. Low volume sounds produce low vibration amplitudes. High frequency sounds produce faster vibrations of higher pitch. -The tympanic membrane articulates with the auditory ossicles (three smallest bones in the body, the malleus, incus and stapes). They pass vibrations that initially hit the tympanic membrane to the oval window of the bony labyrinth in the cochlea displacing a fluid called perilymph. The round window at the end of the bony labyrinth facilitates this displacement by allowing the perilymph movement. -The auditory vibrations move in the cochlea (snail-shaped) ascending via the scala vestibuli (scala means stairs) and descending via the scala tympani. Between these two is the cochlear duct, which is filled with endolymph, it is separated from the scala vestibuli by the Reissner’s (vestibular) membrane and from the scala tympani by the basilar membrane. The vibrations ascending the scala vestibuli are transferred to the Reissner’s membrane and the cochlear duct. Here, the hair cells in the organ of Corti, between the basilar membrane and tectorial membrane, receive the vibrations and generate nerve impluses. These impulses are sent to the brain via the cochlear nerve.
Video Produced by: Brandon Pletsch. “Auditory Transduction (2002).” YouTube. Brandon Pletsch, Aug 26, 2009.
Music by: “Beethoven: Symphony No. 9 in D Minor, Op. 125 – “Choral”: II. Molto Vivace” by Orchestre Révolutionnaire et Romantique & John Eliot Gardiner
Video Edited by: V. Kolchenko & Tristan Charran, New York City College of Technology, 2017.
Ear and the mechanism of hearing
We can get an idea about the mechanism of hearing from this video.
Journey of Sound to the Brain
Learn how sounds make their way from the source to your brain.
How the ear works
Video by javitzproductions: http://www.javitz3d.com
Many of us take for granted a very extraordinary organ… our ears. To understand the ear, we need to understand what sound is. The speakers you are listening to right now are vibrating…flexing in and out causing a wave of pressure through the air The frequency of these waves, or the speed at which the sound creating surface moves back and forth affects the pitch of the sound. The level of air pressure in each wave is directly related to how loud the sound is. The outer part of our ear catches these waves. It faces forward and has a specially designed structure of curves helping us to determine the direction of sound, and emphasize frequencies used in human speech Now that the sound waves are caught, they travel through the ear canal and strike against our eardrum…a thin membrane about 10 mm wide. Now that we received the sound, the middle ear transfers this energy. The smallest bones in your body, the Malleus, Incus, and Stapes start in motion. The Malleus is attached to the eardrum, and as the sound travels along the force is amplified by leverage until it arrives at the Stapes which acts like a reverse piston creating waves in the fluid of the inner ear. The most significant increase in pressure is caused by pneumatic amplification. The face of the stapes has a surface area of 3.2 square mm, while the eardrum has a surface area of 55 square mm. Using this, along with leverage through the Malleus and Incus, the final pressure is 22 times greater than when the sound first arrived. Now we come to the most complicated part of hearing… the Cochlea. In reality, it is coiled up, but it is easier to understand straightened out. There are actually three chambers inside, but lets take a look at the central part. The stapes is causing pressure waves to travel through the structure. Along the inside wall is about 20-30k reed like fibers. As the waves move along they encounter fibers with the correct resonant frequency and energy is released. These fibers aren’t actually what give us the signal that we heard something. There is a special structure next to these fibers containing hair cells. When the hair fibers resonate, they cause the hair cells to move, which then sends an electrical impulse to the cochlear nerve, and on to the brain. Certain pitches of sound will resonate in specific locations, and louder sounds will cause more hair cells to move. Our brain interprets all this raw data, making it possible to enjoy things like music, or an engaging conversation. Just to think that all of this is happening in your head right now at full speed. And not just one, but two of these sophisticated instruments are giving you the amazing sense of hearing. This is just one of the amazing systems found in the human body that go far beyond our humble human understanding.
by Khan Academy
ASCENDING AUDITORY PATHWAY
ASCENDING AUDITORY PATHWAY
How do neural signals travel from hair cells in the Organ of Corti to the primary auditory cortex? Well, let’s examine the ascending auditory pathway. Why ascending you ask? Well, ascending means info goes TO the brain, while descending means info goes from the brain elsewhere. Inner hair cells in the Organ of Corti are connected to type I spiral ganglion neurons (whose axons represent 95% of the cochlear nerve). So firstly, signals travel along the cochlear nerve to the cochlear nuclei in the brainstem on the ipsilateral side. Ipsilateral means same side, contralateral means opposite side. Kay, so here is where is starts to get a little complicated. Most auditory information crosses over, however, each cerebral hemisphere processes stimuli from both the ipsi and contralateral sides. This is advantageous for two reasons. A) If you get brain damage in one hemisphere, your sense of hearing goes “meh, whatever!”. B) we as humans process some pretty complicated sounds – like those involved in speech, and getting input from both ears to both hemispheres allows for more processing potential. Anyway, back to the ascending auditory pathway. Our information is at its first stop – the cochlear nucleus. From here, most of the neurons cross over to the contralateral side. This is the primary pathway that the information takes. However, there is also a secondary pathway, in which some neurons stay on the ipsilateral side. In both cases, the neurons synapse in the superior olivary complex, which is also in the brainstem.p The signal continues to be relayed along the lateral lemniscus to the inferior colliculus in the midbrain. From the inferior colliculus, the information is relayed to the medial geniculate nucleus of the thalamus. Most of the neurons taking this trip stay on the ipsilateral side. However, some cross over. Finally, the information continues into the auditory cortex. The auditory cortex is tucked into the lateral sulcus. The auditory core region contains the primary auditory cortex, or A1, which is organized tonotopically – in other words, it’s arranged by frequency. There are ACTUALLY tuned neurons that respond only to specific frequencies, and they are arranged in a tonotopic map – how cool is that?