"Ear" - what is it, definition of the term
The auditory structure in mammals consists of three functional regions—external, middle, and inner components—each contributing to sound detection and processing. The external portion collects acoustic waves and funnels them toward a tympanic membrane, which vibrates in response to pressure variations. These vibrations are transmitted through a chain of ossicles within the middle chamber, amplifying the mechanical signal before it reaches the fluid‑filled cochlea of the inner chamber. Inside the cochlea, specialized hair cells convert mechanical energy into neural impulses that travel via the auditory nerve to the brain, where they are interpreted as sound. In rodents such as rats and mice, this organ exhibits a compact morphology adapted for high‑frequency hearing, with proportionally larger cochlear basilar membranes and more densely packed hair cells, enabling precise detection of ultrasonic vocalizations and environmental cues.
Detailed information
The auditory apparatus of rodents such as rats and mice consists of three distinct regions: the external collector, the middle transmission system, and the inner sensory organ. The external collector is formed by a small, mobile flap that directs sound waves toward the auditory canal. Its size and shape differ between species, with the mouse’s flap being relatively shorter and broader than that of the rat.
The middle transmission system contains three ossicles that amplify and convey vibrations to the sensory organ. These bones are:
- Malleus (hammer)
- Incus (anvil)
- Stapes (stirrup)
The malleus attaches to the tympanic membrane, the incus connects the malleus to the stapes, and the stapes interfaces with the oval window of the inner sensory organ.
The inner sensory organ includes the cochlea, the vestibular apparatus, and the associated neural pathways. The cochlea is a coiled structure housing hair cells that transduce mechanical vibrations into neural signals. In rodents, the cochlear length is approximately 5 mm in mice and 7 mm in rats, providing a high density of hair cells that support a broad frequency detection range. The vestibular system, comprising the semicircular canals and otolithic organs, contributes to balance and spatial orientation.
Functional characteristics of the rodent hearing apparatus include:
- Frequency detection from about 1 kHz up to 80–100 kHz, with peak sensitivity near 15–20 kHz.
- Thresholds as low as 10 dB SPL for frequencies in the ultrasonic range.
- Rapid auditory brainstem responses, enabling precise temporal resolution of acoustic stimuli.
These anatomical and physiological features make the rodent auditory system a valuable model for studying auditory processing, genetic influences on hearing, and the effects of ototoxic agents.