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"semicircular canals"[MeSH Terms] AND "vestibulo-ocular reflex"[MeSH Terms]

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This slide is about how the horizontal semicircular canals (SCCs) respond when the head starts rotating, keeps rotating, and then stops. The example is a rotation of the head to the right.

The key idea

At rest, the hair cells in both canals fire at a steady tonic rate. Rotation raises the firing rate on one side and lowers it on the other. The brain reads that difference as rotation. Rotation to the right excites the right canal and inhibits the left, so the brain senses "turning right".

Panel 1: Start of head rotation

  1. The head turns right, so the horizontal canals turn right with it.
  2. The endolymph stays still at first because of inertia.
  3. Relative to the canal wall, the endolymph therefore flows the opposite way, from right to left.
  4. The fluid pushes on the cupula and bends the hair cell stereocilia.
    • Right canal: the stereocilia bend toward the kinocilium (toward the utricle). The hair cell depolarizes and firing increases. This is the "Stimulation (+)" label on the right.
    • Left canal: the stereocilia bend away from the kinocilium. The hair cell hyperpolarizes and firing decreases. This is the "Inhibition (-)" label on the left.
  5. Right canal firing is up and left canal firing is down.
  6. This unbalanced input produces the sensation of rotation to the right.
Ganong gives the same sequence. Endolymph is displaced opposite to the direction of rotation because of inertia, and it pushes on the cupula, which bends the hair cell processes. - Ganong's Review of Medical Physiology, 26th ed.

Panel 2: Continued rotation

  • At constant speed, the endolymph catches up and spins at the same rate as the canal.
  • There is no relative flow, so the cupula swings back to its upright position.
  • Firing returns to the balanced baseline and the sensation of rotation fades, even though you are still spinning.

Panel 3: When rotation stops

  1. The canals stop, but the endolymph keeps flowing in the same direction (left to right) because of momentum.
  2. The cupula is now bent in the opposite direction from the one in panel 1.
    • Left canal: the stereocilia bend toward the kinocilium, so firing increases.
    • Right canal: the stereocilia bend away from the kinocilium, so firing decreases.
  3. The pattern is the mirror image of the start of rotation.
  4. The unbalanced input gives a false sensation of rotation to the left, which is vertigo, even though you are standing still.
  5. Within seconds the endolymph slows and the cupula returns to its resting position. Tonic discharge becomes balanced again and the vertigo disappears. Ganong gives the recovery time as about 25 to 30 seconds.

Quick summary

PhaseEndolymph vs canalRight canalLeft canalSensation
Start of right rotationLags (flows opposite)ExcitedInhibitedTurning right
Constant speedMoves with canalBaselineBaselineNone
Sudden stopKeeps moving (same direction)InhibitedExcitedTurning left (vertigo)

Points to remember

  • Stereocilia toward the kinocilium means depolarization and more firing. Away means hyperpolarization and less firing.
  • The canal on the side of the rotation is excited. Costanzo makes the same point for left rotation: the left horizontal canal is excited and the right is inhibited. - Costanzo Physiology, 7th ed., p. 102
  • Linear acceleration does not displace the cupula. The semicircular canals detect angular acceleration, and the otolith organs detect linear acceleration.
  • This is the basis of post-rotatory vertigo and nystagmus, and of the caloric and rotational tests.
I searched recent systematic reviews and found nothing that changes this basic physiology.
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This slide shows where vestibular information goes after the hair cells fire. The photo is rotated, but the content reads clearly.

The main pathway

Cristae (from the canals) and maculae (from the utricle and saccule) send signals along this route:
Hair cells → afferent vestibular nerve → vestibular ganglion (Scarpa's ganglion) → ipsilateral vestibular nuclei in the medulla/pontomedullary junction
The vestibular ganglion holds the cell bodies of the bipolar primary afferent neurons. Their central processes end in the vestibular nuclei. From there the signal branches to five targets.

The five targets

TargetWhat it doesFunction
Cranial nerve III, IV, VI motor nucleiFibers form the medial longitudinal fasciculus (MLF) and go to the extraocular musclesCorrects eye movements to keep objects fixed during head rotation (vestibulo-ocular reflex)
Cerebellum (flocculonodular lobe)Some fibers go directly to the cerebellum without synapsing in the vestibular nucleus. Others go after relaying in the nucleusCoordinates balance and motor output. The cerebellum feeds back to the vestibular nuclei
Brain stem reticular formationVestibulospinal and reticulospinal pathways to the spinal cordAdjusts posture and muscle tone
Spinal cordVestibulospinal tractAdjusts antigravity (extensor) muscle tone to keep the body upright and balanced
Thalamus, then cortexThalamus (VPM nucleus in the diagram) to the sensory cortex, the superior temporal gyrusConscious perception of motion and position

Reading the diagram

  • The MLF has an ascending component, which goes up to nuclei III, IV and VI, and a descending component, which goes down to the spinal cord.
  • Oculomotor (III), trochlear (IV) and abducent (VI) are the eye-movement nuclei on the right side of the diagram.
  • The vestibular nuclei send a branch to the cerebellum through the inferior cerebellar peduncle.
Guyton and Hall describe the same arrangement. Most vestibular nerve fibers end in the vestibular nuclei at the junction of the medulla and pons. Some fibers go directly to the reticular nuclei and to the cerebellar fastigial, uvular and flocculonodular nuclei without synapsing. The second-order neurons send fibers to the cerebellum, the vestibulospinal tracts, the MLF and the reticular nuclei. - Guyton and Hall Textbook of Medical Physiology, Ch. 56. Adams and Victor also state that nuclei III, IV and VI come under vestibular control mainly through the MLF.

How it fits with your previous slide

When the head rotates, the hair cells change their firing rate (excitation on one side, inhibition on the other). This pathway carries that signal to the targets above, which produce three reflexes:
  • Vestibulo-ocular reflex (via the MLF): the eyes move opposite to the head so the image stays steady. After rotation stops, this is what produces post-rotatory nystagmus.
  • Vestibulospinal and reticulospinal reflexes: posture and tone adjust to keep you balanced.
  • Cortical perception: you feel the rotation (and the vertigo when the input is unbalanced).

Exam notes

  • The first synapse is in the vestibular nuclei, not the thalamus. The vestibular ganglion is the cell-body station only.
  • Direct fibers to the flocculonodular lobe (the vestibulocerebellum, or archicerebellum) bypass the vestibular nuclei.
  • The MLF links the vestibular nuclei to cranial nerves III, IV and VI. A lesion here causes internuclear ophthalmoplegia.
  • Your slide gives the cortical site as the superior temporal gyrus. Textbooks differ on the exact location. Some place the vestibular cortex in the parietoinsular area near the posterior insula and the temporoparietal junction. I'd follow your lecturer's wording for your exam.
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Quick answer

This slide lists the six effects of stimulating the semicircular canals (SCCs). Use the right-rotation example from your first slide for each one.

1. Sensation of rotation

During rotation you feel turning in the same direction as the real rotation. The canal on the side you are turning toward is excited, and the signal reaches the cortex through the vestibular nuclei and thalamus.

2. Nystagmus (details are on the next page)

This is a rhythmic eye movement with two phases:
  • Slow phase: the eyes drift opposite to the rotation. This is the vestibulo-ocular reflex, which works through the vestibular nuclei and the MLF.
  • Fast phase: the eyes jerk back in the direction of rotation. This is a brainstem corrective movement.
Nystagmus is named by its fast phase. After rotation stops, it reverses direction (post-rotatory nystagmus).

3. Maintenance of balance

  • Effect: muscle tone increases on the side of stimulation (ipsilateral) and decreases on the opposite side.
  • Aim: a reflex to support the body on the side that is being stimulated, so you don't fall.
  • Pathway: SCCs → vestibular nuclei → reticular nuclei → vestibulospinal and reticulospinal tracts → spinal motor neurons → muscles on the side of stimulation.

4. Vertigo (details are on the next page)

This is a false sensation of rotation. It happens when the vestibular input from the two sides is unbalanced. It occurs after sudden stopping, and also in vestibular disease.

5. Autonomic changes

  • Effects: nausea, vomiting, bradycardia, hypotension and sweating.
  • Cause: spread of vestibular stimulation to the autonomic centers in the brain stem. This is the basis of motion sickness.

6. Post-rotatory reactions (past pointing test of Barany)

  • Definition: after rotation stops, there are cortical voluntary movements of the limbs and body that try to correct the false sensation of vertigo.
  • Aim: to prevent the body from falling in the direction of the false sensation of rotation.
  • Result: the body falls (or a pointing finger deviates) toward the opposite side. That is the direction of the original, true rotation.

Worked example (rotation to the right, then sudden stop)

  1. After the stop, you get a false sensation of rotation to the left.
  2. Your cortex tries to correct this by moving the body to the right, which is the opposite way.
  3. You therefore fall, or your finger points, toward the right, the direction of the original rotation.
Normal pointing is straight ahead. Past pointing is a sign of vestibular stimulation or disease.

Memory aid

  • Sensation: same direction as the rotation.
  • Nystagmus: the slow phase is opposite and the fast phase is the same direction.
  • Muscle tone: up on the stimulated side.
  • Falling and past pointing: toward the direction of the original rotation, opposite to the false vertigo.
I didn't search the textbooks for this answer. It comes from your slides and standard physiology.
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This is the same slide as your previous photo (page 24, "Effects of stimulation of the SCCs"), so the explanation hasn't changed. Here is a short recap, plus a check against the textbooks.

Recap (rotation to the right)

EffectKey point
1. Sensation of rotationFelt in the same direction as the true rotation
2. NystagmusSlow phase opposite to the rotation (vestibulo-ocular reflex). Fast phase in the same direction as the rotation
3. Maintenance of balanceTone up on the stimulated side and down on the opposite side. Pathway: SCCs → vestibular nuclei → reticular nuclei → vestibulospinal and reticulospinal tracts → motor neurons
4. VertigoFalse sensation of rotation, caused by unbalanced input from the two sides
5. Autonomic changesNausea, vomiting, bradycardia, hypotension and sweating, from spread to brain stem autonomic centers
6. Post-rotatory reactions (past pointing, Barany)After rotation stops, the body and limbs deviate toward the original rotation, opposite to the false vertigo

Textbook check for item 6

The textbooks agree with your slide. Adams and Victor state that in nystagmus of labyrinthine origin, past pointing and falling are in the direction of the slow phase. - Adams and Victor's Principles of Neurology, 12th ed.
This fits the slide's example:
  1. Rotate to the right, then stop suddenly.
  2. The post-rotatory nystagmus has its slow phase to the right and its fast phase to the left. You feel false rotation to the left.
  3. The cortex tries to correct the false sensation of leftward rotation by pushing the body and limbs to the right.
  4. You fall and past point to the right, the same direction as the slow phase and the original rotation.

Common exam trap

Don't confuse the direction of the false sensation with the direction of falling. The false rotation is to the left and the fall is to the right. They are always opposite.
If you want, send the next page (the one covering nystagmus and vertigo) and I'll go through it the same way.
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