Saccule, utricle, and the three semicircularducts emanating from the utricle form theorgan of balance, the vestibular apparatus. It contains several sensory fields, namely, the two acoustic maculae, macula of saccule and macula of utricle, and the three ampullarycrests. They all register acceleration andpositional changes and, therefore, serve spatial orientation. The maculae react to linear acceleration in different directions, the crests react to rotational acceleration. The maculae occupy specific positions in space; the macula of the utricle lies roughly horizontally on the floor of the utricle, and the macula of the saccule lies vertically on the anterior wall of the sac-cule. Thus, both are arranged at right angles to each other. (For positions of the semi-circular ducts, )
Acoustic maculae (A).The epithelium lining the endolymphatic space increases in height in the oval areas of the maculae and differentiates into supporting cells and sensory cells. The supporting cells (A1) carry and surround the sensory cells (A2). Each sensory cell is shaped like a flask or am-poule and bears 70 – 80 stereovilli on its api-cal surface (A3). The sensory epithelium is surmounted by a gelatinous membrane, the statoconic (otolithic) membrane (A4), whichcarries crystalline particles of calcium car-bonate, the ear crystals, or statocones(otoliths) (A5). The stereovilli of the sensorycells do not directly project into the stato-conic membrane but are surrounded by a narrow space containing endolymph.
Function of the maculae. The properstimulus for the stereovilli is a shearing force affecting the macula; with increasing acceleration there is a tangential shift be-tween sensory epithelium and statolithic membrane. The resulting deflection of the stereovilli leads to stimulation of the sensory cell and to induction of a nerve im-pulse.
Ampullary crest (B, C). The crest (BC6) isformed by a protrusion in the ampulla and is oriented transversely to the course of the semicircular duct (C). Its surface is covered by supporting cells (B7) and sensory cells(B8). Each sensory cell bears approximately 50 stereovilli (B9) that are considerably longer than those of the macular cells. The ampullary crest occupies about one third of the height of the ampulla. It is surmounted by a gelatinous cap, the ampullarycupula (B – D10), which reaches to the roof of the ampulla. The cupula is traversed by long channels into which the hair bundles of the sensory cells protrude. The bases of the sensory cells are innervated by nerve endings (A – C11).
Function of the semicircular canals (D).
The semicircular canals respond to ro-tational acceleration which sets the en-dolymph in motion. The resulting deflection of the ampullarycupula bends the stereovilli of the sensory cells and acts as the triggering stimulus. For example, if the head is turned to the right (red arrows), the endolymph of the lateral semicircular duct initially remains in place due to its inertia; this results in a relative movement in the opposite direction (hydrodynamic inertia, black arrows) so that both cupulae are de-flected toward the left (D12). The en-dolymph then slowly follows the rotation of the head. However, once the rotation has stopped (broken, arrested arrows), it con-tinues to flow for a certain distance in the same direction so that the cupulae are de-flected to the right (D13). The function of the semicircular ducts serves primarily the reflex eye movements. Rapid eye move-ments caused by rotation of the head (ro-tatorynystagmus) depend on cupular de-flection. The slow component of nystagmus always follows the direction of the cupular deflection.