Saccadic oscillations - membrane, model, and medicine

Abstract

Saccadic oscillations are continuous back-to-back saccades that cause excessive image motion across fovea and threaten clear vision. Acquired processes, related to immune or metabolic mechanisms, are common culprits. Saccadic oscillations are also seen in degenerative cerebellar disease or as a part of a familial syndrome of saccadic oscillations and limb tremor. Some normal individuals have innate ability to voluntarily trigger saccadic oscillations (i.e. voluntary nystagmus). Contemporary theory for the pathogenesis for saccadic oscillations has emphasized hyperexcitable or disinhibited state of the brainstem saccadic burst neuron membrane. This review discusses etiologies and treatment of saccadic oscillations in light of novel cell membrane based theory.

Keywords: computational simulation; genetic; ion channels; opsoclonus; oscillopsia; paraneoplastic; saccade.

Figure 1

This schematic diagram summarizes central saccade generating pathways. NRTP: Nucleus reticularis tegmenti pontis; SC: superior coliculus; OMV: oculomotor vermis; FOR: fastigial oculomotor region, FOR; OPN: omnipause neurons; E: excitatory burst neurons; I: inhibitory burst neurons; III: oculomotor nucleus; VI: abducens nucleus; in: interneuron; mn: motoneuron, Dashed line: midline, green pathway: excitatory projection; red pathway: inhibitory projections. (Modified from Shaikh, et al., 2008 with permission [13])

Figure 2

This is the outline of neuromimetic model of saccadic burst generators (see [2] for deails). The model is based on traditional local-feedback model of the burst neuron [19]. Red is inhibitory and green is excitatory local feedback (A). Such feedback is fundamental to efficiently accomplish accurate yet high velocity saccades. (B) Schematic illustration of the neuromimetic model of the burst neuron and its membrane. The neuromimetic model obeys the classic Hodgkin-Huxley equations for membrane potential, in addition, it also has parameters for hyperpolarization activated cation current (I_h), low-threshold calcium current (I_T), NMDA and AMPA sensitive glutamate receptors and glycine receptor (Modified from Shaikh, et al., 2008 with permission [13]).