Seismic sensitivity and bone conduction mechanisms enable extratympanic hearing in salamanders

Fish hearing revealed: Do we understand hearing in critical fishes and marine tetrapods

Hearing evolved in lampreys with a frequency range of 50-200 Hz. This hearing range is comparable to that of elasmobranchs, most non-teleosts, and lungfish. Elasmobranchs most likely use the saccule and the papilla neglecta (PN) for hearing. In non-teleosts and teleosts, lungfish, and certain tetrapods the saccule is the likely sensor for sound reception while the lagena and the PN are important for gravistatic sensing. Coelacanth and most tetrapods have a basilar papilla (BP) for hearing. In coelacanth and tetrapods, the hair cells of the BP are in contact with a basilar and a tectorial membrane. These membranes transmit mechanical vibrations. A cochlear aqueduct (CA) provides a connection between the cerebrospinal fluid that has a sodium rich space in coelacanth and tetrapods while the potassium rich endolymph is known in vertebrates. A unique feature is known in basic sarcopterygians, the intracranial joint, that never developed in actinopterygians and has been lost in lungfish and tetrapods. The BP in coelacanths is thought to generate pressure with the intracranial joint that will be transmitted to the CA. Lungs or a swim bladder are not forming in Chondrichthyes, structures that have a major impact on hearing in teleosts and tetrapods.

Central projections of auditory nerve fibers in the western rat snake (Pantherophis obsoletus)

Despite the absence of tympanic middle ears, snakes can hear. They are thought to primarily detect substrate vibration via connections between the lower jaw and the inner ear. We used the western rat snake (Pantherophis obsoletus) to determine how vibration is processed in the brain. We measured vibration-evoked potential recordings to reveal sensitivity to low-frequency vibrations. We then used tract tracing combined with immunohistochemistry and Nissl staining to describe the central projections of the papillar branch of the VIIIth nerve. Applications of biotinylated dextran amine to the basilar papilla (homologous to the organ of Corti of mammals) labeled bouton-like terminals in two first-order cochlear nuclei, a rostrolateral nucleus angularis (NA) and a caudomedial nucleus magnocellularis (NM). NA formed a distinct dorsal eminence, consisted of heterogenous cell types, and was parvalbumin positive. NM was smaller and poorly separated from the surrounding vestibular nuclei. NM was distinguished by positive calbindin label and included fusiform and round cells. Thus, the atympanate western rat snake shares similar first-order projections to tympanate reptiles. Auditory pathways may be used for detecting vibration, not only in snakes but also potentially in atympanate early tetrapods.

Hearing without a tympanic ear

The ability to sense and localize sound is so advantageous for survival that it is difficult to understand the almost 100 million year gap separating the appearance of early tetrapods and the emergence of an impedance-matching tympanic middle ear - which we normally regard as a prerequisite for sensitive hearing on land - in their descendants. Recent studies of hearing in extant atympanate vertebrates have provided significant insights into the ancestral state(s) and the early evolution of the terrestrial tetrapod auditory system. These reveal a mechanism for sound pressure detection and directional hearing in 'earless' atympanate vertebrates that may be generalizable to all tetrapods, including the earliest terrestrial species. Here, we review the structure and function of vertebrate tympanic middle ears and highlight the multiple acquisition and loss events that characterize the complex evolutionary history of this important sensory structure. We describe extratympanic pathways for sound transmission to the inner ear and synthesize findings from recent studies to propose a general mechanism for hearing in 'earless' atympanate vertebrates. Finally, we integrate these studies with research on tympanate species that may also rely on extratympanic mechanisms for acoustic reception of infrasound (<20 Hz) and with studies on human bone conduction mechanisms of hearing.

Bone conduction pathways confer directional cues to salamanders

Sound and vibration are generated by mechanical disturbances within the environment, and the ability to detect and localize these acoustic cues is generally important for survival, as suggested by the early emergence of inherently directional otolithic ears in vertebrate evolutionary history. However, fossil evidence indicates that the water-adapted ear of early terrestrial tetrapods lacked specialized peripheral structures to transduce sound pressure (e.g. tympana). Therefore, early terrestrial hearing should have required nontympanic (or extratympanic) mechanisms for sound detection and localization. Here, we used atympanate salamanders to investigate the efficacy of extratympanic pathways to support directional hearing in air. We assessed peripheral encoding of directional acoustic information using directionally masked auditory brainstem response recordings. We used laser Doppler vibrometry to measure the velocity of sound pressure-induced head vibrations as a key extratympanic mechanism for aerial sound reception in atympanate species. We found that sound generates head vibrations that vary with the angle of the incident sound. This extratympanic pathway for hearing supports a figure-eight pattern of directional auditory sensitivity to airborne sound in the absence of a pressure-transducing tympanic ear.