Structure
The lateral (horizontal) semicircular duct is a component of the vestibular part of the membranous labyrinth in the inner ear. It is a tube-like structure, which is housed within the lateral semicircular canal of the bony labyrinth. The duct contains a fluid called endolymph and is surrounded by the perilymph within the bony labyrinth.
It is positioned in a horizontal plane and is related almost parallel to the axis of the petrous part of the temporal bone.
Anteriorly, the arch of the lateral semicircular duct is dilated to form a sac-like structure called the lateral membranous ampulla, which opens anteriorly into the utricle, within the bony vestibule. It lies just underneath the opening of the anterior ampulla.
The posterior end of the lateral semicircular duct opens into the posterior vestibule, above the posterior ampulla.
The dilated lateral ampulla contains sensory hair cells. Numerous stereocilia and a single large kinocilium can be seen emerging from the apices of the hair cells. These are embedded in a gelatinous substance called the cupula, which extends to the roof of the ampulla. The longest stereocilium is the one nearest to the kinocilium gradually decreasing in length with accordance to their distance to the kinocilium.
Related parts of the anatomy
Key Features & Anatomical Relations
The lateral semicircular duct is facing at right angle to the anterior (superior) and posterior semicircular ducts. The former is horizontally located while the latter is vertically located.
Function
The lateral semicircular duct detects the angular acceleration/deceleration of the head during its rotatory movements in a transverse/horizontal plane (along a vertical axis), e.g. when turning the neck to the left and right-hand sides prior to crossing a road.
When the head changes position, the endolymph in the lateral semicircular duct lags behind due to inertia and bends the cupula and the stereocilia of the hair cells. The mechanical stimulation of the hair cells (due to deflection of the stereocilia) sends electrical signals to the brain. Movement towards the kinocilium depolarizes the hair cells causing stimulation, whereas movement away from the kinocilium hyperpolarizes the hair cell decreasing firing of the afferent neuron.
Within approximately 10 seconds of achieving constant motion, the endolymph catches up with the movement of the lateral semicircular duct and the cupula is no longer affected, stopping the sensation of acceleration (Khan & Chang, 2013; Tascioglu, 2005).
List of Clinical Correlates
- Motion sickness
- Vertigo
References
Khan, S. & Chang, R. (2013) Anatomy of the vestibular system: a review. NeuroRehabilitation, 32(3), 437-43.
Tascioglu, A. B. (2005) Brief review of vestibular system anatomy and its higher order projections, 4.
