67: Phylogeny and development of the organ of balance and hearing Flashcards
What are the key features of the inner ear in cyclostomes (lampreys and hagfish)?
o Inner ear features a single cavity with vertical semicircular canals.
o Lampreys possess two semicircular canals; hagfish have only one.
o The simplicity of the cyclostomes’ inner ear may result from secondary degeneration.
o This basic structure correlates with their limited swimming capabilities.
Describe the vestibular apparatus in cartilaginous and bony fish.
o Consists of three semicircular canals, a utricle, and a saccule.
o The saccule includes an enlargement called the lagena, akin to the cochlea.
o The saccule and lagena’s maculae, together with lateral line neuromasts, detect low-frequency, high-intensity sounds.
o In Chondrichthyes (sharks and rays), the vestibular apparatus reaches biomechanical excellence.
How do the ear cavity and auditory tube evolve in amphibians?
o The middle ear cavity and auditory tube evolve from the canal connecting the fish spiracle to the pharynx.
o The tympanic membrane develops from the thinned skin formerly covering the spiracle.
o The hyomandibula transforms into the middle ear’s auditory ossicle (columella or stapes).
o The basilar papilla, analogous to the organ of Corti, emerges in the saccule.
What are the key features of the basilar papilla in reptiles and birds?
o The basilar papilla locates within the elongated lagena, broadening the range of detectable frequencies.
o Some reptiles develop an outer ear canal; others do not.
o Crocodilians and birds feature a more extended ear canal.
Describe the evolution and structure of the mammalian ear.
o The cochlea, evolving from the elongated and curved lagena, becomes the primary hearing structure within the petrous part of the temporal bone.
o The middle ear contains three auditory ossicles: malleus, incus, and stapes.
The malleus adjoins the tympanic membrane, and the stapes connects to the oval window.
o The outer ear, including the auricle or pinna, channels sound waves toward the tympanic membrane.
o Most mammals, excluding humans, have developed auricular muscles for pinpointing sound origins.
What is the path of sound in the mammalian ear?
o The auricle gathers and funnels sound waves into the ear canal.
o The tympanic membrane resonates with these sound waves.
o Ossicle vibrations transfer from the tympanic membrane to the oval window.
o Vibrations move through the oval window into the fluid of the cochlea.
o Cochlear hair cells detect these vibrations and convert them into electrical signals.
o These signals are relayed to the brain by the vestibulocochlear nerve (cranial nerve VIII) for interpretation.
What is the first step in the path of sound in the mammalian ear?
Sound waves enter through the auricle, travel down the ear canal (external acoustic meatus), and vibrate the eardrum (tympanic membrane).
What happens after the eardrum vibrates in the path of sound?
These vibrations are amplified by the auditory ossicles (malleus, incus, and stapes) in the middle ear.
How are pressure waves created in the inner ear?
The stapes at the oval window creates pressure waves in the inner ear’s perilymph within the scala vestibuli.
What happens in the cochlear duct’s endolymph?
Waves move the vestibular membrane, causing waves in the cochlear duct’s endolymph, which displaces the basilar membrane and distorts hair cells in the spiral organ, initiating nerve signals.
How are nerve signals processed after being initiated in the cochlear duct?
Nerve signals are sent to the brain via the cochlear nerve (part of CN VIII) for sound processing.
What happens to the remaining pressure waves in the inner ear?
Remaining pressure waves transfer to the scala tympani’s perilymph and are absorbed as the round window bulges.
What initiates the development of the inner ear?
Development initiates with the formation of otic placodes, thickened regions of surface ectoderm located at the level of the future rhombencephalon on the embryo’s head.
What are Otic Pits?
These placodes invaginate, forming inwardly curved depressions known as otic pits.
What are Otic Vesicles?
Otic pits then detach from the surface ectoderm, transforming into fluid-filled sacs called otic vesicles. Each placode develops into one otic vesicle.
