Cellular Composition: Supporting Apparatus
The spiral organ features two main types of epithelial cells. Supporting cells create a structural framework and nourish the sensory elements. This group is classified based on their location:
- Pillar cells (inner and outer). These rest on the basilar lamina in two rows. Their apical tips make tight contact, while their basal bodies diverge. This architecture creates an inner tunnel between the rows, filled with endolymph. This structure divides the spiral organ into inner and outer compartments.
- Pharyngeal cells (Deiters' cells). Located lateral to the pillar cells. The inner cells form a single row, while the outer cells align in 3–5 rows. The apex of these cells forms a depression (a "cup") that houses the base of the sensory cell, while a specialized finger-like process (the phalangeal process) securely fixes the receptor in space.
- Border cells. The inner group lines the spiral sulcus of the spiral limbus (medial to the phalangeal cells). Outer cells (Hensen's cells) cover the lateral part of the basilar lamina before the epithelium transitions into the stria vascularis.
Sensory Hair Epitheliocytes
These are specialized cells responsible for acoustic signal perception. They reside in the cups of the supporting phalangeal cells, with a strict numerical correlation between sensory and supporting elements.
There are two pools of hair cells: inner (1 row, about 3,500 units) and outer (3–5 rows, total count ranging from 12,000 to 20,000). Their ultrastructure shows marked polarity:
- Apical pole. The surface is covered by a dense glycoprotein layer—the cuticular plate. Piercing through it are bundles of specialized microvilli known as stereocilia. Their tips contact the overlying tectorial membrane (membrana tectoria).
- Basal pole. Rests on the supporting cell. The nucleus is displaced basally, and the cytoplasm is rich in mitochondria to support high metabolic demands. This pole forms synaptic contacts with afferent and efferent nerve endings.
Mechanics and Physiology of Sound Perception
The acoustic signal undergoes a multi-step mechanical journey. A sound wave vibrates the tympanic membrane. The vibrations are then transmitted via the auditory ossicle chain (malleus $\rightarrow$ incus $\rightarrow$ stapes) to the oval window of the vestibule, initiating perilymph movement in the scala vestibuli.
Waves can travel through the compensatory pathway via the helicotrema into the perilymph of the scala tympani, where they are 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 basilar lamina.
According to the resonance theory, the basilar lamina contains collagen fibers of varying lengths (shortening from the apex to the base). A specific frequency of sound causes a strictly localized region of the lamina to resonate. In this zone, the tectorial membrane shifts relative to the hair cells. Mechanical deflection (bending) of the stereocilia in one direction causes tension and opens ion channels (excitation/depolarization), while deflection in the opposite direction closes them (inhibition/hyperpolarization).
Innervation and Signal Transmission
Various neurotransmitters, such as acetylcholine, GABA, and glycine, participate in synaptic signal transmission. Innervation is divided into afferent and efferent pathways:
- Afferent pathway. Signals from sensory cells travel to the dendrites of sensory neurons in the spiral ganglion (ganglion spirale), located at the base of the osseous spiral lamina. This ganglion contains pseudounipolar and specialized bipolar neurons (the first neurons of the auditory pathway). Their dendrites lose their myelin sheath as they approach the basilar membrane. Some innervate the inner cells, while others pass through the inner tunnel to reach the outer cells. The axons of these neurons form the cochlear nerve and project 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 primary role of this innervation is an inhibitory action that dampens impulse traffic, protecting the spiral organ from overstimulation and sharpening auditory attention.