Utricle and saccule

Anatomy

The utricle and saccule are housed in the vestibular labyrinth of the inner ear, a bony cavity within the petrous portion of the temporal bone. Along with the three semicircular canals, they constitute the primary organs responsible for maintaining equilibrium. The utricle lies in the elliptical recess (dorsocaudal region of the vestibule) and is oriented horizontally in the ear. The saccule is smaller in size, lies in the spherical recess (ventrorostral region of the vestibule), and is oriented vertically. This perpendicular orientation allows the utricle and the saccule to detect movement in different planes, effectively complementing each other to detect both horizontal and vertical linear acceleration. The two chambers are connected by the utriculosaccular duct. On the other end the utricle communicates with the semicircular canals and the saccule with the cochlea by ductus reuniens.

Both the utricle and the saccule are organized in three layers; The innermost first layer corresponds to the sensory epithelium, known as the macula, containing specialized hair cells. Through the middle of the macula runs a narrow, curved strip called the striola, which separates two distinct orientations of hair cells within each macula. The surrounding areas are referred to as extrastriolar regions. Each hair cell in the macula is equipped with 50 to 70 stereocilia that progressively increase in height toward a single, taller kinocilium located on one side of the cell. These cilia are interconnected and move together within a gelatinous mass called the otolithic membrane (second layer), which is embedded with small calcium carbonate crystals, known as otoliths or statoconia, forming the outermost third layer. Because the otoliths are heavier than the surrounding endolymph, they respond to gravitational forces and linear acceleration. As the otoliths shift, they cause sliding movement of the top-heavy otolithic membrane over the macula. This, in turn, leads to the deflection of the hair cell cilia, causing some hair cells to depolarize as others hyperpolarize.

Hair cell types

The macula contains two cell types depending on their morphology and innervation:

Type I hair cells are characterized by calyx synapses, where the cell is surrounded by a cup-like structure formed by the nerve endings, known as the calyx.

Type II hair cells are characterized by bouton synapses, where the cell connects with the afferent nerve fibers through small, thickened axon terminal endings.

Both hair cell types synapse with afferent nerve fibers of bipolar neurons, whose cell bodies reside in the vestibular ganglion.

Function

The hair cells of the macula are the primary sensory cells involved in the sensation of equilibrium. They are specialized sensory receptors that form synapses with afferent nerve endings of the vestibulocochlear nerve, transmitting information to the vestibular nuclei within the hindbrain. The utricle is primarily responsible for detecting linear acceleration in the horizontal plane, e.g. when moving forward in a car, while the saccule is sensitive to vertical acceleration, e.g. when jumping or nodding.

Under resting conditions, the afferent nerve fibers send continuous, low-frequency nerve impulses tothe central nervous system (CNS). When the head tilts, the difference in inertia between the hair cell cilia and the otolithic membrane in which they are embedded leads to a shearing force that causes the stereocilia to bend in the direction of the linear acceleration. As a result, ion channels in the cell membranes open, allowing positive ions from the endolymph to flow into the hair cells. This influx leads to a change in the cell’s membrane potential, which is called receptor membrane depolarization and increases the rate of nerve impulses to the CNS. Conversely, when the cilia are bent in the opposite direction, the cell membrane becomes hyperpolarized, leading to a decrease in nerve impulses. The hair cells are oriented in different directions, allowing them to respond to various head positions and movements. The exact position of the head is interpreted by the brain based on the pattern of hair-cell depolarization.

Take a deeper look at the inner ear and vestibular sensations with these study units.

Head-righting reflex

The head-righting reflex is mediated in the cortex and is triggered by sensory input from the otoliths in the utricle and saccule. Its main purpose is to maintain the vertical orientation of the head, thereby providing stability. When the body or head begins to tilt in one direction, the head-righting reflex initiates compensatory movements in the opposite direction. This reflex aids in maintaining the head’s upright position while ensuring the eyes remain horizontally aligned.