 In this video, I will describe the sensory transduction mechanisms and sensory pathways for the auditory modality. Audition is commonly known as the sense of hearing. Vibration of air causes movement of the tympanic membrane, commonly referred to as the eardrum. As the tympanic membrane vibrates, auditory ossicles, which are three small bones of the middle ear, relay vibrations to the inner ear, producing pressure waves that travel through the fluid of the inner ear. These pressure waves in the fluid of the inner ear will cause vibration in the basilar membrane of the inner ear. The mechanical properties of the basilar membrane change along its length so that different regions of the basilar membrane vibrate according to the frequency of the sound. A high frequency sound is a high pitch sound, and a low frequency sound is a low pitch sound. Adjacent to the basilar membrane is the more rigid tectorial membrane that connects to the stereocilia of auditory hair cells. Vibration of the basilar membrane causes opening and closing of mechanically gated ion channels on the stereocilia of auditory hair cells. Cations enter the auditory hair cells causing depolarization that stimulates the release of neurotransmitters onto the dendrites of afferent neurons. An interesting detail about this transduction mechanism is the cations that enter the auditory hair cells through the mechanically gated ion channels are potassium ions. This is unusual that potassium can enter a cell by facilitated diffusion and only occurs in this situation because the auditory hair cells are surrounded by endolymph fluid that has an extremely high potassium concentration. So there's a higher concentration of potassium outside of the auditory hair cells compared to the cytosol enabling potassium to enter the auditory hair cells producing depolarization. The sensory pathway for the auditory modality begins with the auditory hair cells releasing neurotransmitter to stimulate the afferent neurons. The axons of the afferent neurons enter the cranium through the vestibular cochlear nerve, cranial nerve number 8, and then these afferent neurons will form synapses in the medulla oblongata. In two regions of the medulla oblongata, the superior olivary nucleus and the cochlear nucleus. Then the neurons of the medulla oblongata relay this sensory information to a region in the midbrain known as the inferior colliculus. Then the inferior colliculus neurons relay information to the thalamus, to a region of the thalamus known as the medial geniculate nucleus, and then the neurons from the thalamus relay sensory information to the primary auditory cortex in the lateral temporal lobe.