The ear is one of the most complicated anatomical structures in the human body. It functions in two different body systems, the auditory (hearing) and vestibular (balance) systems. The ear is divided into three sections – outer, middle, and inner. While the outer ear is visible, and the middle ear contains parts that can be recognized (hammer, anvil, stirrup, ear drum), the inner ear is a lesser understood fluid-filled sac within the cranium that supports specialized hair cells connected to cranial nerve VIII (vestibulocochlear nerve).
General components of the inner ear
The inner ear interacts with the middle ear via a membrane called the fenestra ovalis (i.e. oval window). Behind this membrane is the vestibule, cochlea, and semicircular canals (see picture of relative placement). The cochlea is part of the auditory system and the semicircular canals are part of the vestibular system, but two sacs in the vestibule (the utricle and saccule) are also involved in sensing position and maintaining balance.
Fluids of the inner ear
The fluid in the inner ear consists of two types, endolymph and perilymph. Endolymph is similar to intracellular fluid but has a high potassium concentration. The vascularized epithelium (called the stria vascularis) of one part of the cochlea (the scala media) secretes endolymph. Traditionally, it was thought that the fluid then flows through the vestibule and saccule to be absorbed for recycling in the endolymphatic sac, but some research is questioning this view.
Perilymph is comparable to cerebrospinal fluid and has different ion concentrations than endolymph – it is high in sodium. The source of this fluid has been debated for decades, it is thought to be secreted by the epithelium of the bony canal of the cochlea and/or sourced from cerebrospinal fluid entering the inner ear through the cochlear aqueduct.
Parts of the auditory system in the inner ear
The main structure of the auditory system is the spiral-shaped cochlea, which consists of three chambers. These chambers, or scalae, consist of two bony labyrinths and one membranous labyrinth. The bony labyrinths are the scala vestibuli and scala tympani and contain perilymph. Within the bony labyrinth is the membranous labyrinth, the scala media. A membrane called the round window is found in the scala tympani, allowing pressure release when sound waves push on the cochlear fluid. The scala media, also known as the cochlear duct, contains the organ of Corti, the organ involved in perceiving sound. The organ of Corti is attached to the basilar membrane of the scala media epithelium. See these illustrations to better understand the structure.
The part of the organ of Corti exposed in the scala media is the tectoral membrane. Protruding from this membrane are the stereocilia of hair cells. Movement of the fluid causes the hair cells to produce action potentials that signal nerve endings in the basilar membrane. Two types of hair cells are known to exist in the cochlea – outer hair cells and inner hair cells. The nerve endings are part of the spiral ganglion in the center of the cochlea.
Parts of the vestibular system in the inner ear
The vestibular system is divided into the otolith organs and the semicircular canals. The system has a membranous labyrinth continuous with that of the cochlea, so the system is filled with endolymph.
The otolith organs are the utricle and saccule. These components of the vestibule contain sensory areas called maculae that have sensory hairs projecting from a gelatinous layer. In this solid-like fluid are small mineral-like objects called otoliths. When these mineralizations move due to a shift in the head’s position (gravity or linear acceleration), they trigger movement of the hairs, telling the brain about the body’s movement.
Each ear has three semicircular canals, each consisting of a bony labyrinth containing a membranous labyrinth. An enlargement at the end of the membrane of each canal, called an ampulla, contains a ridge called the crista, which contains hair cells. This is covered by a gelatinous substance, called the cupula. These hair cells detect angular acceleration. Each canal is responsible for a particular direction – horizontal, posterior, or superior – allowing any rotation of the head to be recognized via the crista ampullaris for the maintenance of balance.
Complex anatomy
The hair cells in both systems communicate via nerves to the sensory parts of the brain, the parts working side by side or together to achieve the sense of hearing and the sense of balance.
Though the anatomy is complex, three-dimensional models like those at HealthLine can help you make sense of it.
