This study investigates the circuitry of neurons in the perioculomotor region, which subserve the components of the near response and ocular orientation, and which may be important in the etiology of strabismus and amblyopia.
The aims are directed at determining the premotor inputs to the preganglionic parasympathetic motor neurons that control the lens and pupil, and the motor neurons that control the multiply innervated muscle fibers (MIFs) in the extraocular muscles. MIFs are highly fatigue resistant and tonically active. They help adjust the pulley system, and are associated with palisade endings that may subserve ocular proprioception. Consequently, they may play a role in controlling normal eye orientation, and eye misalignment in strabismus. The preganglionic and MIF motor neuron groups are found in the perioculomotor region. This region also contains interneurons including those that utilize the neuropeptide urocortin, which is believed to play a role in stress behaviors. In order to define the circuitry that underlies the control of the lens, pupil and MIF fibers in the extraocular muscles, we must identify the premotor inputs to each. The proposed experiments will determine which of these elements is targeted by three of the known inputs to the perioculomotor region: the central mesencephalic reticular formation (cMRF), which may contain both saccade-related and vergence-related neurons, the fastigial nucleus, which is believed to modulate near triad responses in an adaptive manner, and the superior colliculus, which has a well known role in saccadic eye movements, but might play a role in the near response, as well. The proposal contains 4 inter-related neuroanatomical aims carried out in macaque monkeys. They feature experiments that combine anterograde tracers with either retrograde or immunohistochemical cell identification, and feature both LM and EM analysis.
Aim 1 is directed at determining the ultrastructural differences between MIF and SIF motor neurons. Examples of the two groups, retrogradely labeled from the medial rectus muscle, will be examined with the electron microscope in order to determine what differences in their synaptic arrangements underlie their functional differences.
Aim 2 will test whether the cMRF contacts medial and superior rectus MIF and SIF motor neurons and preganglionic motor neurons. The pattern of connections will provide insight into whether the cMRF perioculomotor projection is concerned with vergence or conjugate gaze, or has even wider functions.
Aim 3 will test the hypothesis that the fastigial nucleus is directly manipulating the motoneurons responsible for the near response.
Aim 4 tests whether the tectal projection to the perioculomotor region is part of its conjugate function, with the dendrites of vertical gaze motor neurons being its target, or is a pathway that modulates components of the near response.
Work towards cures for strabismus and amblyopia are among the prime goals of the NEI, but one of the great difficulties we have in addressing these clinical problems is a lack of understanding of their underlying mechanisms, and specifically a poor understanding of the neuronal circuits that control lens focus or that adjust and maintain the proper eye alignment.
The aims of this grant are directed at determining the premotor inputs to the preganglionic parasympathetic motor neurons that control the lens and pupil, and the motor neurons that control the multiply innervated muscle fibers (MIFs) in the extraocular muscles, which may be important for sensing and adjusting the long term orientation of the eyes. The proposed experiments are the first to directly test which brain structures supply input to these motor neurons.
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