Vertebrate hearing has been the subject of much study. The anatomy of the vertebrate ear is complex, but can be subdivided into three general regions, the outer ear, the middle ear, and the inner ear. The outer ear consists of the pinna and a duct that leads to the middle ear. The pinna’s primary function is amplification, allowing for sounds dispersed over the large surface area of the ear to be channeled into a small air duct towards the middle ear. The middle ear converts this sound energy into mechanical energy via the tympanic membrane. The distance this membrane is stretched is a function of the sound’s amplitude, the number of times per second the membrane is stretched from crest to trough is a function of the sound’s frequency. The shifts in the tympanic membrane are translated into three ear ossicles that function like a lever, amplifying the sound as it travels to the entrance of the inner ear, the oval window. The oval window is a membrane on the cochlea that vibrates in response to the last ossicle’s movement and translates this mechanical energy into hydraulic energy. The frequency of the sound is translated by how quickly the liquid revolves through the cochlea and sound amplitude is translated by the amount of compression applied to the fluid. Translation of this movement into sound perception is done through the organ of Corti, a series of membranes and special nerve cells called hair cells. The organ of Corti is composed of two membranes, the lower basilar membrane which is flexible, and the upper tectorial membrane which is more rigid. Lodged into the basilar membrane are the bottom ends of the hair cells, whose namesake hair-like protrusions stick out at the top and into the tectorial membrane. The basilar membrane will resonate based on the movement of the fluid, which is passed on to the hair cells through their axonal projections. Sound amplitude is translated by the amount these hair cells move, and hence the amount of neurotransmitter they release into the tectorial membrane, the more neurotransmitter, the louder we perceive that sound. Sound frequency is more complicated. The stiffness of the basilar membrane differs at its base and at its apex. The basilar fibres are shorter and stiffer at the base, while longer and more flexible at the apex. Higher frequency sounds cause the shorter fibres to vibrate and lower frequency sounds cause the longer fibres to vibrate. An animal’s hearing range increases as the difference between basal and apical fibre length increases. The shorter the fibre, the higher the frequency that can be heard, the longer the fibre, the lower the frequency that can be heard.
Recent comments