General Anatomy and Receptor Localization
The vestibular part of the membranous labyrinth is anatomically divided into two functional zones:
- Vestibule (vestibulum). Includes two membranous sacs: the spherical sac (sacculus) and the elliptical sac (utriculus). Receptor zones here are represented by specialized epithelial patches called maculae (maculae).
- Three semicircular ducts. Each duct features an ampulla. The receptor areas inside the ampulla are called cristae ampullares and consist of connective tissue elevations covered by sensory epithelium.
Areas of the labyrinth devoid of receptors are lined with simple squamous epithelium, while secretory epithelium is located in certain zones of the sacs and ducts. Its function is to produce endolymph and maintain the metabolism of supporting structures.
Histophysiology of the Maculae and Otolithic Membrane
The epithelial patches (maculae) are responsible for the perception of gravity. The utriculus detects linear acceleration and head position, whereas the sacculus responds to gravity as well as vibration (membrane oscillations).
The structure of the maculae includes three main elements:
- Sensory hair cells — located within the epithelium but never contacting the basement membrane.
- Supporting cells — lie between the sensory cells, resting on the basement membrane with their nuclei forming a lower cell layer.
- Otolithic membrane — a gelatinous substance covering the epithelium, containing otoliths (statoconia), which are calcium carbonate crystals. The mass of these crystals causes the membrane to shift under the influence of gravity, stimulating the receptors.
Morphology of Sensory Hair Cells
Vestibular receptor cells are divided into two types based on their shape and innervation pattern:
- Type I (pear-shaped cells). Have a broad, rounded base and a narrowed neck. The dendrite terminal of the afferent neuron forms a large cup-shaped synapse (calyx) around the cell. Efferent (inhibitory) fibers do not contact the cell directly; instead, they form axodendritic synapses directly on the nerve calyx.
- Type II (cylindrical cells). Have a prismatic shape. Nerve endings approach them punctiformly, forming synapses exclusively at the basal base of the cell.
The apical surface of both types is structurally identical. The top features a glycoprotein cuticular plate, through which 60–80 non-motile stereocilia and one motile cilium — the kinocilium — project into the otolithic membrane (or cupula).
Cristae Ampullares and the Mechanism of Excitation
The receptor apparatus of the semicircular ducts (cristae ampullares) detects angular accelerations (body rotation). Their epithelium is similar to the maculae, but instead of an otolithic membrane, a transparent gelatinous cupula rises above the cells.
Signal Transduction (Perception Mechanism):
- During head rotation, the endolymph in the semicircular duct shifts.
- The endolymph flow displaces the cupula (similarly to how the otolithic membrane shifts due to gravity in the sacs).
- Mechanical displacement causes the bundle of stereocilia to bend toward the kinocilium.
- This stretches the apical plasmalemma and opens ion channels.
- Excitation of the sensory cell occurs.
The signal is transmitted to the dendrites of bipolar neurons in the vestibular ganglion (located in the internal acoustic meatus). Their axons form the vestibular division of the vestibulocochlear nerve (CN VIII) and project to the pontine nuclei.