Special Senses: Hearing and Balance

Special Senses: Hearing and Balance

• The course objective focuses on identifying micro- and macroscopic structures involved in the special senses using virtual microscopy, 2-D, and 3-D images to connect these processes to the wider nervous system.

Lecture Learning Objectives

• Recall the three regions of the ear and their primary functions.
• List the components of the external ear.
• List the components of the middle ear, including their functions.
• Explain the anatomical reason why juveniles are more prone to middle ear pathologies.
• Visually identify the components of the external, middle and inner ear (using images and histology).
• List the components of the bony and membranous labyrinths of the inner ear and explain the differences between them (e.g. structure, fluid).
• Describe the transmission of sound, including the steps, structures involved and the role of sound frequency.
• Recall the auditory pathways.
• Explain the roles of inner ear structures and associated pathways involved in equilibrium and balance.

External Ear

• The external ear includes the auricle (pinna) and the external acoustic meatus.
• The external acoustic meatus contains ceruminous and sebaceous glands and produces cerumen (earwax).
• The tympanic membrane is also part of the external ear.

Middle Ear

• The tympanic cavity is lined with a continuous mucous membrane.
• Mastoid air cells and the pharyngotympanic tubes (Eustachian tubes) are openings of the middle ear.
• The pharyngotympanic tube connects to the nasopharynx and equalizes pressure in the middle ear.
• The middle ear contains auditory ossicles: malleus, incus and stapes.
• These amplify sound, moving from the tympanic membrane across to the oval window.

Inner Ear

• The inner ear contains a bony labyrinth, filled with perilymph.
• The membranous labyrinth, suspended within the bony labyrinth, contains endolymph.
• It includes the cochlea and semicircular canals.
• Cochlea receives sound waves, using its different sections such as Scala Vestibuli and Scala Tympani, creating vibrations to convert into electrical impulses interpreted as sound by the brain in the auditory pathway.
• The semicircular canals detect rotational acceleration and deceleration of the head by monitoring changes in endolymph movement.

Cochlea

• Cochlea contains three distinct parts which spiral around a bony core called the modiolus.
• Cochlear duct, Scala tympani and Scala vestibuli are parts of this spiral.
• Modiolus carries cochlear nerve (CN VIII) and blood vessels.
• The cochlea communicates with the round window and subarachnoid space.
• Cochlear fluid, perilymph, inside the bony labyrinth stimulates hearing cells on the cochlear duct.
• Organ of Corti, filled with endolymph, is located on the basilar membrane.
• The basilar membrane vibrates, causing the organ of Corti to move causing nerve impulses that reach the hearing center in the brain.
• Hair cells in the organ of Corti detect vibrations which trigger nerve impulses.

Organ of Corti

• Hair cells are located in the organ of Corti which detect vibrations.
• Each hair cell has stereocilia that are moved by vibrations in the cochlear duct, triggering an electrical impulse.
• Sensory end organs are important for auditory transduction and transmission.

Sound Transmission

• Sound waves enter the middle ear via the external ear canal.
• They cause the tympanic membrane to vibrate.
• The vibrations are transferred to the auditory ossicles.
• The vibrations pass into the cochlear duct via the oval window affecting perilymph.
• This creates pressure waves to cause the basilar membrane and spiral organ to vibrate.
• The vibrations cause the hair cells in the organ of Corti to transduce the physical vibrations into electrical signals which travel to the brain.

Auditory Pathways

• Hair cells, spiral ganglion neurons, cochlear nerve, cochlear nuclei, inferior colliculi and thalamus are crucial in auditory pathways.
• The pathway also involves superior olivary nuclei, to aid in localizing sounds in space.
• The brain receives auditory signals. Information is analyzed for pitch, loudness and location.

Equilibrium and Balance

• Vestibule and vestibular labyrinth are important parts of the inner ear for equilibrium and balance.
• Parts like the utricle and saccule have maculae, specialized receptors detecting movement.
• Hair cells have otolith on the maculae which detect gravity and linear acceleration.
• Semicircular canals and ampullae are used for rotational movement of the body, via endolymph movement.

Vestibular Sensation Pathways

• Hair cells in the macula detect and transmit info regarding linear acceleration and gravity through the vestibular nerve(branch of CN VIII) to the vestibular nuclei where secondary neurons synapse.
• Information is relayed to the brainstem and spinal cord to maintain balance through vestibulospinal pathways.
• This pathway also coordinates with visual and neck input affecting equilibrium, balance and posture.

Gross Anatomical Structures (Ear)

• Various components of the ear can be visually identified from images like, pinna, external acoustic meatus, tympanic membrane, malleus, incus, stapes, and the pharyngotympanic tube.

Inner Ear Histology

• Different sections of the inner ear, viewed under microscope, reveal histological components like scala vestibuli, vestibular membrane, cochlear duct, basilar membrane and spiral organ.

This quiz focuses on the anatomy and function of the ear, exploring its role in hearing and balance. Students will identify the structures of the external, middle, and inner ear, as well as discussing their complete physiological pathways. Prepare to engage with histological images and understand the implications of ear structures on health, particularly in juveniles.