Auditory System
The auditory system forms the basis for communication for the animals, and it is designed to detect, analyse sound in the surrounding environment and react accordingly. For the location of the sound, the auditory system requires both the ears to detect the difference in the time of arrival and sound intensity approaching the two ears, whereas hearing requires at least one ear.
Further, animals’ sense of hearing is also enhanced by their ability to move their ears around to scan the environment for different sounds and locate where the sound is coming from.Sounds are pressure waves in the air with given frequencies and amplitudes. The auditory system perceives the frequency of sounds as pitch and their amplitude as loudness.
12.2.1 Structure of Auditory System
Hearing involves the external ear, middle ear and inner ear where the sensory receptor (i.e. the organ of Corti) is located. The sound waves directed by the external ear enter into the ear canal and induce vibration of the tympanic membrane that separates the external ear from the middle ear.
The external ear is composed of pinna (auricle) and external auditory canal. The pinna is a cartilaginous flap that is used to guide sound waves through ear canal. Pinna is absent in birds, reptiles and amphibians. The external auditory canal is composed of both bone and cartilages. The inner end of external auditory canal is shut by tympanic membrane. The air wax secreted by the sebaceous gland is deposited in the external auditory canal.
The external and the middle ears conduct sound to the auditory receptors (organ of Corti) in the cochlea of the inner ear. The eardrum separates the external and the middle ear. The middle ear consists of air-filled tympanic cavity containing a chain of three auditory ossicles (malleus, incus and stapes) and the Eustachian tube (auditory tube).
As the auditory tube connects the middle ear to the nasopharynx, pressure in the middle ear gets equalised with external atmospheric pressure and also facilitates clearing of fluids from the middle ear. In horse, there is a ventral diverticulum located in the auditory tube called as guttural pouch.The vibrations of the tympanic membrane caused by the sound waves are transmitted through the middle ear by the three ossicles. The malleus (hammer) is attached to the tympanic membrane, while the footplate of the stapes contacts the oval (vestibular) window membrane, in the cochlea of the inner ear, thereby providing a mechanical linkage between the tympanic membrane and the oval window. The unique design and the relative size difference between the bones and the tympanic membrane magnify the vibrating pressure of the tympanic membrane to the stapes which is essential, as the sound waves travel from air to the fluid perilymph in the inner ear. This arrangement also decreases the amplitude of sound waves transferred to the perilymph, thereby providing protection to the sensitive sensory cells of the organ of Corti.
Two small skeletal muscles are located in the middle ear, which alter the transmission of vibrations between the eardrum and the oval window. In birds, only one bone— columella—is present in the middle ear, which connects the eardrum and the oval window, and hence the transmission of sound is less efficient.
The inner ear or labyrinth consists of an acoustic part, the cochlea and a non-acoustic part, the vestibular organ. The inner ear is made of a bony labyrinth; within it is located the membranous labyrinth. The cochlea is formed by the coiling of three fluid-filled tubes known as scala vestibuli (vestibular duct), scala media (cochlear duct) and scala tympani (tympanic duct). The scala vestibuli and scala tympani are connected by a narrow channel, the helicotrema. The receptor cells of the auditory system are located within the cochlea. The scala media is separated from the scala vestibuli by the Reissner’s membrane or vestibular membrane, while the scala media is separated from the scala tympani by the basilar membrane.
Along the floor of the scala media, on the basilar membrane lies the hair cell receptor system, the organ of Corti. The scala tympani and scala vestibuli are filled with Na+ ion-rich fluid called the perilymph. The scala vestibuli at the basal end faces the oval window, whereas the scala tympani faces the round window. Thus, pressure on the oval window due to the movement of the stapes is equalised by fluctuation of the connective tissue sheath covering the round window. The scala media is filled with endolymph, which has a concentration of K+ ions.The cochlea of birds is short, almost straight, and hair cells are not arranged in rows. The hair cells, tectorial membrane and basilar membranes with cochlear nerve terminals form the organ of Corti.
The auditory receptors are the hair cells embedded in the basilar membrane and their apical surface contains hair-like cilia—stereocilia that project into the endolymph-filled scala media. The apical surface of the hair cells has 50-100 stereocilia, which are connected by filamentous material called the tip link. The tip link is attached to a K+ channel and thus, when the bending of the stereocilia pulls the tip links, the K+ channels are opened. The tectorial membrane, the specialised gelatinous flap of the basilar membrane composed primarily of glycoprotein, overhangs the cilia of hair cells. Due to this structural arrangement, vibrations of the basilar membrane cause bending of the hair cell stereocilia, which is translated into voltage changes of the hair cell membrane. The terminals of the cochlear nerve fibres synapse with the basal ends of each hair cell. The auditory impulses are transmitted through vestibulocochlear nerve to higher brain centres.
12.2.2 MechanismofHearing
Sound waves from the external environment cause vibrations of the tympanic membrane. These vibrations are transmitted through the middle ear by the auditory bones and are presented to the oval window.
This sets up travelling waves in the perilymph of scala vestibuli, which in turn cause vibrations in the basilar membrane. Since the stereocilia of the hair cells are embedded in the tectorial membrane, the up- and-down movement of basilar membrane causes the stereocilia of hair cells to bend. This bending pulls the tip links, which in turn sets up an action potential in hair cells by inducing opening of K+ ion channels at the tip of the stereocilia, opening of voltage-gated Ca2+ channels at the base of the cells and subsequent release of neurotransmitter into the synaptic cleft between sensory hair cells and cochlear nerve terminals. As the basilar membrane has graded stiffness and width being narrow near the base, and wider near the apex of the cochlea, the location of maximum displacement of the basilar membrane is related to the frequency of the tone. Low-frequency tones tend to distort the entire basilar membrane with maximum displacement of the membrane occurring near the apex of the cochlear duct, intermediate tones distort the basilar membrane from the base to an intermediate region and high-frequency tones selectively distort the basilar membrane close to the base of the cochlea. Hair cells at different locations along the basilar membrane respond to different frequencies of sound waves. Stimulation of nerve fibres from different regions of the basilar membrane provides the central nervous system about the frequency of the sound. Humans can detect sounds in the range of 20-20,000 Hz. In dogs, the upper limit is about that of humans. Frequencies of about 98,000 Hz produce potential changes in the cochlea of bats.12.2.3 Auditory Pathway
The vestibulocochlear nerve terminates in the cochlear nucleus of the medulla, and then the impulses are transmitted through superior olivary nuclei, inferior colliculus and median geniculate body to the temporal lobe of the primary auditory cortex. Each cochlea is mapped bilaterally in the auditory cortex, wherein the decoding and feature extraction of complex auditory information occur.
The primary auditory cortex is surrounded by association cortical areas that integrate various sensory stimuli and are thus critical in understanding the surrounding environment.On exposure to loud sounds, the stereocilia may be forced to move excessively leading to their destruction and resulting in hearing loss. This type of hearing loss is prevented by reflex contraction of the tensor tympani and stapedius muscles in response to loud sound, thereby protecting the sensory cells. This middle-ear reflex (acoustic stapedius reflex) protects the sensory cells by reflexively contracting the tensor tympani and stapedius muscles in response to loud sound. The movement of the tympanic membrane and stapes is limited by the contraction of these muscles, thereby reducing the force and amplitude of sound applied to the organ of Corti. The motor nuclei of the trigeminal and facial nerves are involved in mediating this reflex. As it takes time for these muscles to contract fully, the protection offered by this reflex against loud sound is only partial. The reflex mediated by bilateral loud sound to one ear also triggers the reflex in the opposite ear. This is done by sending signals from the cochlear nuclei to the contralateral dorsal nucleus of the trapezoid body. Hearing is a function of the cerebral cortex, whereas the auditory reflex, such as turning the head in response to sound, is mediated by the brainstem.
12.2.4 Equilibrium
The vestibular apparatus of the ear helps to maintain the posture and equilibrium of the body. It is mainly controlled by a gravity sensor (vestibular apparatus) in response to visual and proprioceptive information. The body movements cause the stimulation of hair cells, and afferent nerve impulse travels from vestibular nuclei to cerebellum and thalamus for the adjustment of posture and positioning of the body, respectively. The third nerve nuclei help to fix the eyes during head movement. Animal species like squirrel, hawk and mountain goat and birds have well-developed vestibular system for their survival.
12.3