saccule

The saccule is a small membranous sac located in the vestibule of the inner ear. It is one of the otolith organs, along with the utricle, and plays a role in sensing linear acceleration and head position relative to gravity.

Anatomy and structure: The saccule lies near the cochlea and is connected to the utricle by the utriculosaccular duct. Its sensory epithelium, the macula sacculi, contains hair cells with stereocilia and a single kinocilium, supported by supporting cells. The hair cells are embedded in a gelatinous otolithic membrane that rests atop the macula and contains otoconia, small calcium carbonate crystals. The orientation of the macula provides directional sensitivity to vertical movements.

Function: When the head moves or tilts, gravity causes the otoconia to shift the otolithic membrane, bending the hair cells. This alters the rate of neurotransmitter release to the vestibular nerve fibers, encoding the sensation of linear acceleration, particularly in the vertical plane, and contributing to equilibrioception.

Neural pathways and role: Afferent signals from the saccule travel via the vestibular portion of the eighth cranial nerve to the vestibular nuclei in the brainstem and onward to cerebellar and ocular motor circuits, supporting balance, posture, and gaze stabilization through reflexes such as the vestibulo-ocular reflex.

Clinical relevance: Saccular dysfunction can contribute to vertigo and balance disorders. Age-related or pathological changes affecting the otolithic membrane or otoconia can impair vertical motion perception. Menière’s disease and other inner-ear conditions may involve otolithic structures, including the saccule, though assessment is typically part of broader vestibular testing.