Physiological Modeling - Introduction to Biomedical Engineering - page 907

TABLE 13.2 List of Abbreviations

Neural Site Abbreviation

Excitatory Burst Neuron EBN

Fastigial Nucleus FN

Inhibitory Burst Neuron IBN

Long Lead Burst Neuron LLBN

Medium Lead Burst Neuron MLBN

Nucleus Reticularis Tegmenti Pontis NRTP

Paramedian Pontine Reticular Formation PPRF

Omnipause Neuron OPN

Substantia Nigra SN

Superior Colliculus SC

Tonic Neuron TN

Vestibular Nucleus VN

Also within the brain stem is another type of saccade neuron called the omnipause
neuron (OPN). The OPN fires tonically at approximately 200 Hz during periods of fixation
and is silent during saccades. The OPN stops firing approximately 10-12 ms before a
saccade and resumes tonic firing approximately 10 ms before the end of the saccade. The
OPNs are known to inhibit the MLBNs and are inhibited by the LLBNs. The OPN activity
is responsible for the precise timing between groups of neurons that causes a saccade.
The SC contains two major functional divisions: a superficial division and an intermedi-
ate or deep. Inputs to the superficial division are almost exclusively visual and originate
from the retina and the visual cortex. The deep layers provide a site of convergence for
sensory signals from several modalities and a source of efferent commands for initiating
saccades. The SC is the initiator of the saccade and is thought to translate visual information
into motor commands.
The deep layers of the SC initiate a saccade based on the distance between the current
position of the eye and the desired target. The neural activity in the SC is organized into
movement fields that are associated with the direction and saccade amplitude and does
not involve the initial position of the eyeball whatsoever. The movement field is shown in
Figure 13.58 for a 20 and 2 saccade. Neurons that are active during a particular saccade
are shown as the dark circle. Notice that the movement field for the 2 saccade is smaller
than that of the 20 saccade.
Active neurons in the deep layers of the SC generate a high-frequency burst of activity
beginning 18-20 ms before a saccade and ending sometime toward the end of the saccade;
the exact timing for the end of the burst firing is quite random and can occur slightly before
or slightly after the saccade ends. Neurons discharging for small saccades have smaller
movement fields, and those for larger saccades have larger movement fields. All of the
movement fields are connected to the same set of LLBNs.