Inside the utricle and saccule are maculae containing hair cells with a membranous covering of microscopic otoconia that detect motion of the endolymph. With head motion or pressure impulses of sound, the endolymph vibrates and creates a stimulus for the receptors of the vestibular system - the utricle and saccule. Discrimination of sound is via the location of the original nerve impulses from different areas of the cochlea. This depolarization allows for neurotransmitter release at the auditory nerve in the postsynapse, generating nerve impulses to be propagated from stereocilia of hair cells to the central nervous system via glutamate transmission. When the stereocilia on the hair cells bend towards the longest cilia, potassium and voltage-gated calcium channels open, and ion influx increases resulting in depolarization. Inner hair cells have an attachment with a tectorial membrane to which they bend against with movement of the cochlear duct membranes and fluids. Inner cells transmit information to the auditory nerve, and outer cells mechanically amplify low-level sound entering the cochlea. There are two varieties of hair cells: inner and outer. The organ of Corti is on the basilar membrane surface, and it contains hair cells which are the primary receptors in sound signal creation. The cochlea has three layers called scala vestibuli (the ascending portion), scala media, and scala tympani (the descending portion). The stapes are in close proximity to the oval window, and it amplifies the mechanical energy to the cochlea, a fluid-filled structure with a fluid called perilymph, by directly pushing on it. This energy transforms into mechanical energy to the malleus, incus, and stapes. Sound waves travel to the ear creating a vibration in the tympanic membrane. To discuss how sound receptors work, first, we must mention the order of events.
This hyperpolarization results in a decreased amount of glutamate released to the postsynaptic membrane, signaling a change to the brain. Interestingly, in this scenario, it is hyperpolarization that occurs with light signaling. This signals the closure of sodium channels that are otherwise open when it is dark. Signal transduction then involves transducin, a multisubunit protein, by binding it to rhodopsin and causing conversion of GDP to GTP this leads to the release of the alpha subunit allowing it to bind to cGMP phosphodiesterase - which lowers levels of cGMP. With this conformational change, rhodopsin transforms into an activated form called meta-rhodopsin. The absorption of energy transforms cis-retinal into trans-retinal.
Light is the stimulus and retinal is the receptor. Its isomer (Cis-retinal) is present in rhodopsin, which is a photosensitive transmembrane G-protein that exists in rods and cones it contains both cis-retinal and opsin. It can absorb different frequencies of light. The retinal is the principal molecule of vision in the retina. The following is a detailed discussion of major sensory receptor types.
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