Cellular composition: supporting apparatus
The spiral organ contains two main types of epitheliocytes. Supporting cells form the structural framework and provide nutrition to the receptor elements. The classification of this group is based on their location:
- Pillar cells (inner and outer). They rest on the basilar lamina in two rows. Their apical tips are in tight contact, while their basal bodies diverge. Due to this architecture, an inner tunnel filled with endolymph is formed between the rows. This structure divides the spiral organ into inner and outer parts.
- Phalangeal cells (Deiters' cells). Located on the sides of the pillar cells. The inner ones form a single row, and the outer ones form 3–5 rows. At the apex of these cells, a depression ("bed") is formed, which houses the base of the sensory cell, while a special finger-like process (phalanx) securely fixes the receptor in space.
- Border cells. The inner group lines the spiral sulcus of the limbus (medial to the phalangeal cells). The outer ones (Hensen's cells) cover the lateral part of the basilar lamina, after which the epithelium transitions into the stria vascularis.
Sensory hair epitheliocytes
These are specialized cells responsible for perceiving acoustic signals. They lie in the beds of the phalangeal cells, with the number of sensory and supporting elements strictly correlated.
There are two pools of hair cells: inner (1 row, approximately 3,500 units) and outer (3–5 rows, total number ranging from 12,000 to 20,000). The ultrastructure of these cells exhibits pronounced polarity:
- Apical pole. The surface is covered with a dense glycoprotein layer — the cuticle. Bundles of specialized microvilli, called stereocilia, pierce through it. Their tips come into contact with the overlying tectorial membrane (membrana tectoria).
- Basal pole. Rests on the phalangeal cell. The nucleus is displaced basally, and the cytoplasm is rich in mitochondria, which is associated with the high energy demands of these processes. This same pole forms synaptic contacts with afferent and efferent nerve endings.
Mechanics and physiology of sound perception
The acoustic signal undergoes a multi-stage mechanical pathway. The sound wave causes the tympanic membrane to vibrate. Next, the vibrations are transmitted via the chain of auditory ossicles (malleus $\rightarrow$ incus $\rightarrow$ stapes) to the oval window of the vestibule, initiating the movement of perilymph in the scala vestibuli.
The wave can propagate via a throughput (compensatory) pathway through the helicotrema into the perilymph of the scala tympani, where it is dampened at the secondary tympanic membrane of the round window. However, the transmembrane pathway is critical for hearing: through the vestibular membrane, vibrations are transmitted to the endolymph of the cochlear duct, causing resonant vibration of the tectorial membrane and the basilar lamina.
According to the resonance theory, the basilar lamina contains collagen fibers of varying lengths (they shorten from the apex to the base). At a specific sound frequency, a strictly defined segment of the lamina enters resonance. In this zone, the tectorial membrane shifts relative to the hair cells. Mechanical deflection (bending) of the stereocilia in one direction causes tension and opening of ion channels (excitation/depolarization), while deflection in the opposite direction causes their closure (inhibition/hyperpolarization).
Innervation and signal conduction
Various neurotransmitters, such as acetylcholine, GABA, and glycine, are involved in synaptic signaling. Innervation is divided into afferent and efferent:
- Afferent pathway. The signal from sensory cells is transmitted to the dendrites of sensory neurons of the spiral ganglion (ganglion spirale), located at the base of the osseous spiral lamina. This ganglion contains pseudounipolar and specific bipolar neurons (the first neurons of the auditory pathway). As their dendrites approach the basilar membrane, they lose their myelin sheath. Some of them innervate inner cells, while others pass through the inner tunnel to outer cells. The axons of these neurons form the vestibulocochlear nerve and proceed to the pons.
- Efferent pathway. Formed by fibers of the olivocochlear bundle (Rasmussen's bundle) originating from the superior olivary nuclei of the medulla oblongata. They form chemical inhibitory synapses directly on the hair cells or on the afferent dendrites. The main role of this innervation is an inhibitory action that limits the flow of impulses, protecting the organ of Corti from overexcitation and ensuring the focusing of auditory attention.
To memorize the main types of supporting cells: B-P-P (Border cells, Phalangeal cells, Pillar cells, which form the space—the inner tunnel).
Class and exam questions
How is the inner tunnel formed, and what is it filled with?
The inner tunnel is formed by the diverging bodies of the inner and outer pillar cells, whose apices are in tight contact with each other. The space inside the tunnel is filled with endolymph.
What is the structure of the apical surfaces of sensory cells?
On their apical pole, there is a dense glycoprotein cuticle, through which specialized microvilli—stereocilia—pass, contacting the tectorial membrane.
Where are the cell bodies of the first neurons of the auditory pathway located?
The cell bodies of the primary sensory neurons (including bipolar and pseudounipolar) are located in the spiral ganglion, which is situated in the modiolus of the cochlea.
What else to study on this topic
- Morphology and functions of pillar cells
- Resonance theory of frequency perception by the basilar membrane
- Cochlear hydrodynamics: scala vestibuli and scala tympani
- Structure and cellular composition of the spiral ganglion
- Efferent innervation and Rasmussen's olivocochlear bundle
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