Hearing (zoology)

Sensory perception provides the only means of communication between the external world and the nervous system. The process of sensory perception begins in the sense organs, where specially designed receptor cells are stimulated by various types of energy. These receptor cells are highly selective for specific forms of stimulus energy. For example, the photoreceptors in the eye are specific for light energy and largely ignore other forms of stimuli. Each receptor cell transduces (changes) the stimulus into an electrical charge (nerve impulse) which travels through nerve fibers to the brain, where the electrical impulse is translated into a particular sensation. The major types of sensory receptor cells are the chemoreceptors (sense chemical energy), mechanoreceptors (sense mechanical energy), photoreceptors (sense light energy), thermoreceptors (sense thermal energy), and electroreceptors (sense electrical energy). The sensory receptors for the organs of hearing are called hair cells, which are extraordinarily sensitive mechanoreceptors. Tiny filaments (like hair follicles, only much smaller) called cilia project from the ends of the receptor cell. This filament bends in response to mechanical pressure, and the bending generates the nerve impulse.

Throughout evolution, the sensory systems developed from single, independent receptor units into complex sense organs, such as the vertebrate ear, in which receptor cells are organized into a tissue associated with accessory structures. The organization of the receptor cells and the architecture of the accessory structures allow far more intricate and accurate sampling of the environment than is possible by independent, isolated receptor cells. Some invertebrates, such as anthropods, have developed a sense of hearing in several systematic groups at differing degrees of anatomical and functional complexity. Others, like those in the class Insecta, are the only invertebrates known to produce and receive airborne sounds. However, while invertebrates possess receptor cells that sense vibrational (mechanical) energy, the true sense of hearing originated with vertebrates.

The Vertebrate Ear

All vertebrates possess a pair of membranous labyrinths (cavities lined by a membrane) embedded in the cranium, lateral to the hindbrain. This region is often referred to as the inner ear. Each labyrinth consists of three semicircular canals, a utriculus, and a sacculus, which, during development, become filled with a fluid called endolymph. The semicircular canals and utriculus are primarily associated with equilibrium, but the sacculus has evolved into an organ of hearing. In fishes, the lagena, a depression in the floor of the sacculus, has its own maculae (patches of hair cells), which respond to vibratory stimuli of relatively high frequency (sound waves). In tetrapods, the lagena has evolved into an additional fluid-filled duct of the labyrinth called the spiral duct or cochlea. With the evolution of the cochlea, the sensory region enlarged into the organ of Corti. The hair cells are located in the organ of Corti. Most of the structures of the ear assist in transforming of sound waves (airborne vibrations) into movements of the organ of Corti, which stimulate the hair cells. These hair cells then excite the sensory neurons (nerve cells) of the auditory nerve.

Mammals are the only vertebrates to possess a true cochlea, but birds and crocodilians have a nearly straight cochlear duct that contains some of the same features, including an organ of Corti. Detection of sound in the lower vertebrates that have no cochlear ducts is carried out by hair cells associated with the utriculus and lagena. The cochlea is coiled somewhat like the shell of a snail and is divided into three longitudinal compartments. The two outer compartments, the scala tympani and the scala vestibuli, are filled with a fluid called perilymph and are connected to one another by a structure called the helicotrema. The scala media, filled with endolymph, is located between the two outer compartments and is bound by the basilar membrane and Reissner’s membrane. The organ of Corti lies within the scala media and sits upon the basilar membrane. Four rows of hair cells are present in adult mammals—one inner and three outer rows. The cilia of the inner row are thought to be sensitive primarily to the velocity (speed) at which they are displaced by sound waves. The cilia of the outer rows are more sensitive and can detect the degree of deflection and the speed.

Sound waves are transported to the inner ear via the outer ear and middle ear. The outer ear consists of the tympanic membrane (ear drum), which is situated on the surface of the head in frogs and toads. In reptiles, birds, and mammals, the tympanic membrane is located deeper in the head at the dead end of an air-filled passageway called the outer ear canal or external auditory meatus. In mammals, there is also an outer appendage, the pinna, which collects sound waves and directs them into the outer ear canal. The tympanic membrane makes contact with the bones (ossicles) of the air-filled inner ear. In amphibians, birds, and reptiles, there is a single bone called the columella (or stapes). In mammals, there is a series of three bones. The malleus (hammer) is in contact with the tympanic membrane at one end and articulates with a second bone, the incus (anvil). The incus then articulates with a third bone, the stapes (stirrup), which connects to a structure called the oval window of the cochlea.

Detection of Sound

Sound waves striking the tympanic membrane cause it to vibrate. These vibrations are transmitted through the auditory ossicles of the middle ear and through the oval window to the perilymph. The bones of the middle ear amplify the pressure of the vibrations set up in the eardrum by airborne vibrations. Vibrations reaching the oval window pass through the cochlear fluids and the Reisner’s and basilar membranes separating the cochlear compartments before dissipating their energy through the membrane-covered round window of the cochlea. The distribution of perturbations (disturbances) within the cochlea depends on the frequencies of the vibrations entering the oval window. Very long, low frequencies travel through the perilymph of the scala vestibuli, across the helicotrema to the scala tympani, and finally toward the round window. Short wave frequencies take a shortcut from the scala vestibuli through the Reissner’s membrane and the basilar membrane to the perilymph of the scala tympani. Movement of perilymph from the scala vestibuli to the scala tympani produces a displacement of both Reissner’s membrane and the basilar membrane. Movement of Reissner’s membrane does not directly contribute to hearing, but the displacement of the basilar membrane is required for pitch discrimination. Displacement of the basilar membrane into the scala tympani produces vibrations of the basilar membrane. Each region of the basilar membrane vibrates with maximum amplitude to a different sound frequency. Sounds of higher frequency (pitch) cause maximum vibrations of the basilar membrane at the apical region (closest to the stapes), while sounds of low frequency produce maximum vibrations at the distal region of the basilar membrane.

The sensory hair cells are situated on the basilar membrane, with the cilia projecting into the endolymph of the cochlear duct. The cilia of the outer hair cells are embedded with the tectorial membrane located above the hair cells within the cochlear duct. Displacement of the cochlear duct by pressure waves of perilymph produces a shearing force between the basilar membrane and the tectorial membrane. This causes the cilia to bend, and the bending of the cilia produces a nerve impulse in the sensory nerve endings that synapse with the hair cells. The higher the intensity of the sound, the greater the displacement of the basilar membrane, which results in greater bending of the cilia of the hair cells. Increased bending of the cilia produces a higher frequency of nerve impulses in the fibers of the cochlear nerve that synapse with hair cells. Since a specific region of the basilar membrane is maximally displaced by a sound of a particular frequency, those nerve cells that originate in this region will be stimulated more than nerve cells that originate in other regions of the basilar membrane. This mechanism results in a neural code for pitch discrimination. Within the brain, sensory neurons of the eighth cranial (auditory) nerve synapse with neurons in the medulla, which project to the inferior colliculus of the brain. Neurons from this region of the brain project into the thalamus, which in turn sends nerve fibers to the auditory cortex of the temporal lobe of the brain. Through this pathway, neurons in different regions of the basilar membrane stimulate neurons in corresponding areas of the auditory cortex. Hence, each area of this cortex represents a different part of the basilar membrane and a different pitch.

Principal Terms

Auditory Nerve: the cranial nerve that conducts sensory impulses from the inner ear to the brain

Pitch: the frequency of sound—the higher the frequency, the greater its pitch

Sound Frequency: the distances between crests of sound waves measured in hertz

Sound Intensity: the loudness of a sound directly related to the amplitude of the sound waves measured in decibels

Synapse: the functional connection between nerve cells or an effector cell, such as a sensory receptor and a nerve cell

Tetrapods: vertebrates with four limbs

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