Classification principles
From a histological perspective, the organs of hearing, balance, and taste are classified as secondary sense organs.
The key morphological difference in this group lies in the nature of the receptive element. Unlike primary sense systems, where the stimulus is captured directly by a nerve cell, reception here is not carried out by neurons. This function is performed by specialized epithelial cells known as epitheliosensory (or sensoepithelial) cells.
Mechanism of sensory signal transmission
The process of incoming stimulus processing involves a strictly defined sequence of stages:
- Perception of the stimulus.
- Transduction (conversion of stimulus energy).
- Signal conduction.
- Transmission of the impulse.
At the cellular level, this mechanism unfolds as follows: exposure to a specific stimulus alters the transmembrane potential on the membrane of the sensitive epitheliosensory cell. The resulting signal is then transmitted via a synaptic contact to the dendrites of neurons that form the pathways. Notably, this transmission stage to the dendrites proceeds similarly to processes in primary sense organs.
Organ of hearing and balance: specifics and topography
The apparatus responsible for hearing and spatial orientation possesses a wide range of sensitivity. It is capable of responding to four main types of physical stimuli:
- Sound waves;
- Vibration;
- Gravitational force;
- Angular accelerations (arising, for example, during human body rotation).
Anatomical structure and topography
Structurally, the organ is divided into three interconnected parts: the external, middle, and internal ear.
Topographically, these parts have different supporting framework structures. The external part, which includes the auricle and the initial third of the external acoustic meatus, is cartilaginous. All other deeper and more complex structures of the organ of hearing and balance are securely protected and located within the substance of the temporal bone of the skull.
Briefly: answers to topical questions
What is the embryonic source of development for the epitheliosensory cells of the inner ear?
The embryonic source of development for the epitheliosensory cells of the inner ear is the cranial ectoderm of the embryo at the level of the myelencephalon.
During embryogenesis, the following structures form sequentially:
- Otic placodes — initial ectodermal thickenings.
- Otic pits — the result of placode invagination.
- Otic vesicles — detached structures lined with stratified (multiserial) epithelium.
It is from the stratified epithelium of the otic vesicles, following their division into anterior and posterior parts, that the epitheliocytes of all receptor structures of the inner ear (spiral organ, maculae of the vestibule, and ampullary crests) develop.
What cell types are present in a taste bud?
A taste bud contains four types of cells:
- Receptor (sensory) epitheliocytes — have a dark spindle-shaped nucleus in the upper row and microvilli at the apical pole.
- Supporting epitheliocytes — have a round nucleus in the middle row and participate in synthesizing adsorbent substances to concentrate taste tastants.
- Basal epitheliocytes — cambial (stem) elements at the base of the bud that ensure the renewal of the cell population.
- Perigemma cells — located at the periphery of the taste bud.
What types of epitheliosensory cells are distinguished in the spiral (corti) organ?
Two types of epitheliosensory hair cells are distinguished in the spiral (Corti) organ:
- Inner hair cells — arranged strictly in a single row, with a total count of about 3,500.
- Outer hair cells — arranged in 3–5 rows, with a total count reaching 12,000–20,000.
These sensory cells lie within the recesses of phalangeal cells. Their apical poles feature a dense cuticle and stereocilia (bundles of specialized microvilli) that contact the tectorial membrane.
Where are the receptor cells of the balance organ (vestibular apparatus) localized?
The receptor cells of the balance organ are localized in specific structures of the membranous labyrinth:
- Maculae (maculae) — receptor areas located in the epithelium of the utricle (utriculus) and saccule (sacculus) of the vestibule. The sensory cells here are covered by an otolithic membrane. The macula of the utricle perceives gravitational forces, while the macula of the saccule perceives gravity and vibration.
- Ampullary crests (cristae ampullares) — receptor structures located in the ampullae of the semicircular canals. The sensory cells of the crests are connected to a gelatinous cupula (cupula) and respond to angular accelerations during rotation.
Secondary receptor organs are easy to remember with the mnemonic: "A tasty EAR loses BALANCE — that's SECONDARY."
Class and exam questions
What acts as the receptor in secondary sense organs?
In these organs, reception is performed not by neurons, but by specialized epithelial (epitheliosensory) cells.
What are the similarities in signal transmission with primary sensory organs?
In both cases, the signal is ultimately transmitted via a synapse to the dendrites of pathway neurons.
Where are the structures of the middle and inner ear located?
They are localized inside the temporal bone of the skull.
What else to study on this topic
- Histology of epitheliosensory cells
- Mechanisms of sound wave transduction
- Structure and functions of the external ear
- Anatomical features of the middle ear
- Reception of angular acceleration and gravity in the inner ear
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