Ocular Reflexes Flashcards
Explain in detail, the vestibule-ocular reflex
Head Rotation Excites and Inhibits Labyrinthine Receptor Organs
We will discuss, step by step, the afferent and efferent limbs of the vestibulo-ocular reflex (VOR), using the horizontal VOR as an example. The first step is the excitation / Inhibition of labyrinthine receptor organs, the lateral semicircular ducts.
In the diagrams we use to illustrate this, the head is outlined in a clinical orientation (imagine yourself standing at the footboard of a hospital bed, while the patient is lying on his/her back, and you are looking up from the patient’s feet towards the brain section), similar to a typical MRI. The patient’s right is on the left side of the image, and the patient’s left is on the right side of the image. Anterior (frontal) is at the top and posterior (occipital) at the bottom. Head rotation to the left (counter-clockwise) induces endolymph flows to the right (clockwise) inside both horizontal semicircular canals (this is a movement relative to the walls of the semicircular canals; relative to the environment outside the body, the endolymph, at least initially, does not move). The endolymph movement pushes the cupula in the left horizontal semicircular canal towards the back of the head, and the cupula in the right horizontal semicircular canal towards the front (anterior part) of the head.
The ampullae of both horizontal semicircular canals are oriented towards the midline; the canals themselves expand more laterally. Within the ampullae, the hair cells are localized in the medial walls, with the kinocilium oriented towards the back of the head.
As a consequence, the cilia of the hair cells in the left horizontal semicircular canal are bent towards the back of the head, towards the kinocilium, and they are getting excited. Cilia of hair cells in the right horizontal semicircular canal are bent towards the anterior of the head, away from the kinocilium, so they are inhibited.
In summary, when the head is at rest, both horizontal semicircular canal receptor organs show an intermediate activity between maxim
What are the components of the vestibulo-ocular reflex?
Vestibulo-Ocular Reflex (VOR)
• Velocity: not as fast as saccadic eye movements, but faster than optokinetic
reflex movements
• Function: compensatory eye movements keep a visual target in the center of the visual field following angular acceleration (rotation) of the head
• Control: vestibular input
• works even in the absence of visual input, such as in complete darkness, or in blind
individuals
• Vision: is maintained during vestibulo-ocular reflex movements of the eyes (images on the retinas remain stable)
How is the vestibulo-ocular reflex excitated?
Excitation of the hair cells in one of the horizontal semicircular canals induces Vestibulo-Ocular Reflex Movements away from the excited labyrinth
The semicircular canals relay the information of head rotation to the vestibular nuclei in the brainstem. The excited labyrinth (or the more active semicircular canal receptor organ) is the crucial element in inducing the vestibulo-ocular reflex.
Fibers originating in the excited labyrinth, which form the afferent limb of the VOR, enter the brainstem at the ponto-medullary junction and synapse in the vestibular nucleus.
Axons of neurons located in the Vestibular nucleus cross the body’s midline and synapse in the contralateral Abducens nucleus.
From there, the pathway of the horizontal vestibulo-ocular reflex follows the common final pathway for horizontal eye movements. Briefly, the abducens nucleus activates the Lateral rectus muscle of the right eye and, via fibers crossing the body’s midline end ascending in the medial longitudinal fasciculus (MLF) the oculomotor nucleus. The oculomotor nerve (CN III) then activates the medial rectus muscle of the left eye.
The lateral rectus muscle abducts the right eye and the medial rectus muscle adducts the left eye, so both eyes turn towards the right.
In summary, head rotation to the left causes an excitation of the left labyrinth. The labyrinthine input to the vestibular nucleus activates the contralateral abducens nucleus, which induces conjugate horizontal eye movements to the right.
This way, the vestibulo-ocular reflex induces eye movements that hold images steady on the fovea
during rapid head movements.
What is nystagmus?
A nystagmus is described as rhythmically alternating movements of the eyes.
Most commonly a nystagmus has two phases, a slow phase into one direction and a fast phase in the opposite direction. The slow phase is usually driven by a reflex; the fast phase is a reset mechanism, driven by the saccadic circuitry. For historical reasons, the direction of a nystagmus is named after the fast phase, which is, in essence, only the reset of the system.
We have to distinguish between a physiological nystagmus and a pathological nystagmus. A physiological nystagmus is a nystagmus which can be included in a healthy individual. Pathological nystagmus, on the contrary, is based on a defect in the mechanisms or structures controlling eye movements.
What is the vestibulo-ocular nystagmus?
The Vestibulo-Ocular Nystagmus
The vestibulo-ocular nystagmus is a physiological nystagmus based on the vestibulo-ocular reflex, which happens during head rotation.
Experimentally, it can be induced by placing a person on a rotating chair.
The start of the rotation induces an endolymph flow in the horizontal semicircular canals opposite to the direction of the head rotation, inducing the reflex, followed by a saccadic reset. At the stop of the rotation, the endolymph continues to flow in the horizontal semicircular canals, while the head had come to a rest.
Summarize nystagmus
• Definition: rhythmically alternating eye movements
• Two phases:
• Slow phase
• Driven by a reflex (for example the vestibulo-ocular reflex) • Fast phase
• Resets the eyes through saccadic movements • Direction of a nystagmus:
• Defined by the fast phase
• Example: a right-beating nystagmus (nystagmus to the right) has a slow phase to the left
(driven by a reflex) and a fast phase to the right (saccadic movements).
What phase of nystagmus defines the direction of the nystagmus?
Fast
Contrast fast and pathological nystagmus
- Physiological Nystagmus: in healthy individuals when the visual field changes relative to the person (for example when we look out of the window of a train)
- Pathological Nystagmus: indicating lesions of the vestibular labyrinth, CN VIII, brainstem or cerebellum
What are the neurological examination of brain stem functions?
Two Neurological Examination procedures are used to get insight into functions of the brainstem and the structures and circuitries involved in the vestibulo-ocular reflex, in order to estimate the extent of brainstem damage, specifically in comatose patients. You will learn more about coma and levels of consciousness after midterm in the Complex Brain Functions Module. Oculocephalic Maneuver (Dolls Eyes Maneuver) This examination procedure of a passive (passive for the patient, not for the examiner) head movement maneuver can be applied to comatose patients. It is used to determine, whether the vestibulo-ocular reflex pathway from the medulla to the midbrain is intact. The examiner turns the head of the patient in the horizontal (or vertical) plane and notes whether the ocular excursions in the opposite directions occur.
Caloric Testing of the Vestibulo-Ocular Reflex
In this testing procedure the outer ear canal of a patient is irrigated with cold or warm water, and the examiner observes conjugate eye movements, driven by the vestibulo-ocular reflex (VOR), if the brainstem is intact. It allows testing of brainstem function in comatose patients.
You may have come across the mnemonic “COWS”, which stands for “Cold - Opposite, Warm - Same”. Please note that the mnemonic does not apply to caloric testing of a comatose patient, where only the VOR component can be initiated. Instead, the mnemonic refers to the “reset” component, which defines direction of the nystagmus, and which only occurs in a conscious individual
What is the oculocephalic (dolls eyes) maneuver?
- Oculocephalic (Doll’s Eyes) Maneuver: during passive rotational movements of the head of a comatose patient the eyes perform compensatory conjugate movements in the opposite direction (VOR circuitry intact), or they stay fixed relative to the head (lesion affecting the VOR circuitry in the brainstem)
- Head rotation to the left: both eyes turn to the right, if the brainstem including the underlying circuitry is intact
- Head rotation to the right: both eyes turn to the left, if the brainstem including the underlying circuitry is intact
What is caloric testing?
• Caloric Testing: injection of warm or cold water into the outer ear of a comatose patient induces conjugate eye movements (VOR circuitry intact), or they stay fixed relative to the head (lesion affecting the VOR circuitry in the brainstem)
- Warm water: both eyes move away from the irrigated ear, if the brainstem including the underlying circuitry is intact
- Cold water: both eyes move towards the irrigated ear, if the brainstem including the underlying circuitry is intact
- There is no nystagmus in comatose patients
What is the effect of caloric testing?
Caloric Testing: Warm Water Injection into the Outer Ear causes Vestibulo-Ocular Reflex (VOR) to the Opposite Side
Right labyrinth more active than the left labyrinth
Abducens on the left
Afferent fiber activity:
Warm water injection increases the baseline activity of hair calls
Vestibulo-ocular reflex (VOR) movements of both eyes to the left
Caloric Testing: Cold Water Injection into the Outer Ear causes Vestibulo-Ocular Reflex (VOR) to the Same Side
Vestibulo-ocular reflex (VOR) movements of both eyes to the right
Cold water injection reduces the baseline activity of hair calls
Left labyrinth more active than the right labyrinth
What are the symptoms of vestibular lesions?
- Vertigo
- Subjective vertigo: sensation of movement of self
- Objective vertigo: sensation of movement of environment
- Nausea and vomiting
- Unsteady (ataxic) gait
- Tendency to fall to the side of the lesion
- Pathological nystagmus away from the lesion
What is what is motion sickness?
Motion Sickness (Kinetosis) Motion sickness occurs when susceptible individuals experience a discrepancy between vestibular and visual inputs. The symptoms are similar to those produced by any of the diseases that affect the vestibular apparatus: dizziness, vomiting and sweating. These usually subside over time.
What is alcohol intoxication?
Alcohol Intoxication
Acute alcohol intoxication can also produce vestibular symptoms, such as vertigo. These are usually manifest when lying down (the “spinning bed phenomenon”). It is caused by the interactions of blood alcohol with the endolymph, which can cause convection endolymph flows within the semicircular canals.
