Neuroscience: The Function of Pause Cells

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Questions and Answers

What is the primary function of pause cells in the brain?

To inhibit the antagonist muscles of the intended eye movement

To control vertical eye movements

To synchronize the activity of the premotor burst neurons

To tonically inhibit burst cells and prevent unwanted saccades (correct)

What is the effect of dysfunctional pause cells?

Opsoclonus (correct)

Difficulty with vertical eye movements

Unwanted saccades during fixation

Inability to control antagonist muscles

What type of cells inhibit the antagonist muscles of the intended eye movement?

Pause cells

Inhibitory burst cells (correct)

Premotor burst neurons

Tonic cells

What is the relationship between the discharge rates of excitatory burst cells and inhibitory burst cells?

They are inversely proportional Signup and view all the answers

What type of cells discharge in relation to eye position?

Tonic cells Signup and view all the answers

Which structure sends information to the contralateral pons for horizontal saccadic control?

Frontal eye field Signup and view all the answers

What nerve is responsible for activating the ipsilateral medial rectus during horizontal saccadic control?

CN III Signup and view all the answers

Which structure is involved in vertical saccadic control?

Midbrain Signup and view all the answers

Which system can move the eyes without interaction from the FEF?

The vestibular system Signup and view all the answers

A patient with a problem in higher cortical areas of control would likely have difficulty with what type of saccade?

Voluntary saccade Signup and view all the answers

What is characteristic of patients with Huntington's disease?

Abnormal voluntary saccade initiation Signup and view all the answers

What is the term for saccades that are too large or too small?

Dysmetric saccades Signup and view all the answers

What is the term for slow saccades of restricted amplitude?

Slowed dynamics Signup and view all the answers

What is the term for saccades that are followed by a slow drift back to the original position?

Glissades Signup and view all the answers

What is the term for saccades that are too fast or too slow?

Abnormal peak velocity Signup and view all the answers

What is the term for corrective saccades that are made to compensate for an initial saccade that undershot the target?

Refixation saccades Signup and view all the answers

What happens to the innervation of the paretic lateral rectus after patching the normal eye for a few days?

It is increased Signup and view all the answers

What region is responsible for receiving sensory information from the parieto-occipital temporal region?

Frontal Eye Fields Signup and view all the answers

Lesions of the frontal eye fields result in which of the following?

All of the above Signup and view all the answers

What is the function of the frontal eye fields in generating saccades?

Generating vertical and horizontal saccades Signup and view all the answers

What is the role of the cerebellum in saccadic control?

Saccade adaptation Signup and view all the answers

What is the neural map of visual space found in?

Frontal eye fields Signup and view all the answers

What do efferent projections from the frontal eye fields go to?

Unspecified Signup and view all the answers

What is the purpose of the frontal eye fields in determining the occurrence of a future saccade?

To determine the initial information on size and direction of the saccade Signup and view all the answers

What is the purpose of checking if a patient can perform saccades in the absence of a visual task?

To check if the patient can make saccades without relying on visual feedback Signup and view all the answers

What is the velocity above which catch-up saccades are needed during pursuit?

30 degrees/sec Signup and view all the answers

What is the primary function of smooth pursuits?

To stabilize the image of a slowly moving small object on the fovea Signup and view all the answers

What is the role of smooth pursuit during combined eye-head tracking?

To cancel the vestibulo-ocular reflex Signup and view all the answers

What is the latency of smooth pursuit?

130 msec Signup and view all the answers

What determines the accuracy of smooth pursuit?

The gain of smooth pursuit Signup and view all the answers

What is the maximum target velocity that can be tracked by smooth pursuit?

90 degrees/sec Signup and view all the answers

What type of eye movement is smooth pursuit?

Not a reflexive eye movement Signup and view all the answers

Patients with lesions in the frontal eye fields (FEF) exhibit difficulty with which type of pursuit?

Horizontal pursuit toward the side of the lesion Signup and view all the answers

Which structure is responsible for a deficit in horizontal pursuit toward the side of the lesion?

Dorsolateral pontine nucleus (DLPN) Signup and view all the answers

What is the effect of extensive lesions in the cerebellum on pursuit?

Abolish pursuit Signup and view all the answers

What is a characteristic of abnormal pursuits in patients with multiple sclerosis and cerebellar ataxia?

Jerky pursuits in both horizontal directions Signup and view all the answers

What is the term used to describe abnormal pursuits characterized by a jerky or ratchety motion?

Cogwheel pursuit Signup and view all the answers

What is the effect of a cerebellar infarction on the right side on pursuit?

Asymmetric impairment of pursuit to the right Signup and view all the answers

What is a common classification of abnormal pursuits?

Defects in initiation, gain, and saccadic overlay Signup and view all the answers

What type of lesions are frequent causes of smooth-pursuit dysfunction?

Cerebellar and pontine lesions Signup and view all the answers

Study Notes

Saccadic Eye Movement

Pause cells tonically inhibit burst cells to prevent unwanted saccades during fixation
They inhibit burst neurons within the same paramedian pontine reticular formation
They help synchronize the activity of premotor burst neurons
Dysfunctional pause cells result in opsoclonus
Inhibitory burst cells inhibit antagonist muscles of the intended eye movement
Their discharge rates are inversely proportional to those of excitatory burst cells
Tonic cells discharge in relation to eye position, increasing proportionately with eccentric gaze position

Saccadic Control: Horizontal

Frontal eye field sends information to the contralateral pons (where CN VI and the paramedian pontine reticular formation are)
CN VI sends the signal to ipsilateral lateral rectus to contract and communicates with the contralateral CN III through the medial longitudinal fasciculus
CN III activates the ipsilateral medial rectus

Saccadic Control: Vertical

Frontal eye field sends information to the midbrain (where CN III and CN IV are)
CN III and CN IV together control the vertical recti and the obliques
They activate the corresponding muscles

Saccadic System Adaptation

Patching the normal eye for a few days results in central adaptation within the saccadic system, increasing innervation to the paretic lateral rectus
The cerebellum is important for saccade adaptation

Saccadic Control: Supranuclear

Frontal Eye Fields (FEF) are the most important for generating vertical and horizontal saccades
Efferent projections from frontal eye fields go to the contralateral pons
They receive sensory information from the parieto-occipital temporal region (POT) and determine the occurrence of a future saccade
Lesions of the frontal eye fields result in increased saccadic latencies, slowed saccades, and impaired predictive tracking
The vestibular system can also move the eyes without interaction from the FEF

Anomalies with Saccade Initiation

Delayed initiation: abnormally long latency (>250 msec)
Focal lesions, especially of the FEF, can cause abnormal voluntary saccade initiation
Patients with Huntington's disease have normal amplitude of random saccade and frequency of nystagmus but characteristically abnormal voluntary saccade initiation

Anomalies with Saccade Size

Dysmetric saccades with associated corrective saccades
Static overshoot: a high gain saccade seen in cerebellar disease, not frequent in normals
Hypermetric saccades are made also by patients with VF defects (hemianopia)

Anomalies with Saccade Velocity

Slowed dynamics: peak velocity 2-std below mean value, increased saccadic duration
Slowed saccades of restricted amplitude reflect abnormalities of the ocular motor periphery
They are a sign possibly pointing to: introrbital tumors and contusions, Graves syndrome, hyperthyroidism, Myasthenia gravis

Pursuits Eye Movement

Allow the eyes to closely follow a moving object
Pursuit is modified by ongoing visual feedback
Pursuing a target needs catch-up saccades if moving target velocity is greater than 30 degrees/sec

Functions of Smooth Pursuits

Stabilizes the image of a small moving target on the fovea
Cancels the VOR during combined eye-head tracking
Cancels optokinetic nystagmus during tracking of a small, moving target against a detailed stationary background

Smooth-Pursuit Tracking

Generated in response to retinal image motion
Match eye velocity to target velocity; target velocities can be up to 90 deg/sec
Latency is about 130 msec (compare this to VOR and saccades)
Accuracy of SP is determined by its gain; smooth pursuit gain -> eye velocity/target velocity
Smooth pursuit is not a reflexive eye movement

Abnormal Pursuits Classification

Defects found in: initiation, latency, initial 100 msec open loop response, maximum eye acceleration, gain, and saccadic overlay

Examples of Lesions that Affect Smooth Pursuit

A cerebellar infarction on the right side causes asymmetric impairment of pursuit
Multiple sclerosis and cerebellar ataxia cause jerky pursuits in both horizontal directions
Cerebellar and pontine lesions are frequent causes of smooth-pursuit dysfunction

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Description

Learn about the role of pause cells in regulating saccades during fixation, and their impact on burst neurons and opsoclonus.