Semicircular Canal Of Internal Ear

INTRODUCTION:
  • The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. 
  • Semicircular canal perceives Angular acceleration
ANATOMY:
  • The angle formed by three semicircular canals is the Solid angle.
  • Three semicircular canals
  • The three canals lie at right angles to each other but the one which lies at right angles to the axis of rotation is stimulated the most.

  1. Lateral (Horizontal)
  2. Superior ( Anterior vertical)
  3. Posterior ( Posterior vertical)
  • Horizontal semicircular canal
  1. Shortest of the three canals. It will respond maximum to rotation on the vertical axis and so on
  2. Movement of fluid within this canal corresponds to rotation of the head around a vertical axis (i.e. the neck).
  3. Projects as rounded bulge in middle ear, aditus & antrum
  4. Makes an angle of 30 with horizontal plane.
  5. It measures from 12 to 15 mm., and its arch is directed horizontally backward and laterally; thus each semicircular canal stands at right angles to the other two. 

  • Superior semicircular canal

  1. Detects rotation of the head around a rostral-caudal (anterior-posterior) axis, or in other words rotation in the coronal plane.Example, when you move your head to touch your shoulders.
  2. 15 to 20 mm in length, is vertical in direction.Arcuate eminence of the petrous temporal bone is formed by it
  3. Its lateral extremity is ampullated, and opens into the upper part of the vestibule; the opposite end joins with the upper part of the posterior canal to form the crus commune, which opens into the upper and medial part of the vestibule.

  • Posterior semicircular canal

  1. The posterior semicircular canal detects rotations of the head in around the lateral axis. This occurs, for example, when nodding your head.
  2. It detects rotations of the head in the sagittal plane.
  3. Nodding of head.
  4. 18 to 22 mm.
FUNCTION:
Canal have endolymph with motion sensors
                    ↓ 
At the base canal is enlarged
                    ↓
opens into the utricle and has a dilated sac at one end called the osseous ampullae
                    ↓
Within ampulla mound of hair cells and crista ampullaris (with cytoplasmic projections on the apical surface called stereocilia Embedded in cupula)
                    ↓
Head rotates the duct moves but the endolymph lags behind owing to inertia.
                    ↓ 
Deflects the cupula and bends the stereocilia within.
                    ↓
Alters an electric signal
                    ↓
Transmitted to the brain (10sec)
                    ↓
Endolymph catches up with the movement of the duct and the cupula                    ↓

  • Stopping the sensation of acceleration
  • Cupula is not displaced by gravity, unlike the otolithic membranes of the utricle and saccule(specific gravity of the cupula is comparable to endolymph).
  • As with macular hair cells, hair cells of the crista ampullaris will depolarise when the stereocilia deflect towards the kinocilium.
  • Deflection in the opposite direction results in hyperpolarisation and inhibition.
  • In the horizontal canal, ampullopetal flow is necessary for hair-cell stimulation, whereas ampullofugal flow is necessary in the anterior and posterior canals
  • Semicircular canal is responsible for detecting rotational acceleration
Exam Question
  • Semicircular canal perceives Angular acceleration

  • Semicircular canal is responsible for detecting rotational acceleration
  • Arcuate eminence of the petrous temporal bone is formed by Superior semicircular canal
  • Horizontal semicircular canal is lateral
  • Crus commune is a part of Semi circular canal
  • Semicircular canals are stimulated by Rotation
  • Horizontal semicircular canal responds to Rotational acceleration
  • Angular movements are sensed by semicircular canal
  • Semicircular canals are arranged at right angles to each other

Don't Forget to Solve all the previous Year Question asked on Semicircular Canal Of Internal Ear