NEUROBIOLOGY OF A SIMPLE OCULOMOTOR SYSTEM

Neural mechanisms underlying fundamental vertebrate motor
activities can be usefully studies in model systems from other
phyla which offer the advantage of a limited number of neurons
accessible to intracellular techniques and modern structural
analysis. One such activity, generically known as the oculomotor
response, comprises complex patterns of behaviour mediated by
the visual and vestibular systems: gaze, object scanning and
fixation, and the stabilization of the retinal image compensating
for complex spatial displacements of the head and body.
Oculomotor abnormalities in humans can indicate the onset of
central nervous trauma and disease. A greater understanding of
the cellular organization of a complex multisensory oculomotor
pathway could provide useful leads and suggestions for research
on the physiologically less accessible vertebrate counterpart. One such model is provided by the dipteran Calliphora
erythrocephala, which has a wide range of sophisticated
oculomotor behaviours. In both sexes panoramic motion elicits
compensatory head and body movements. Objects is visual space
elicit fixation and orientation behaviour. In Calliphora there is a
profound sexual dimorphism of the eye: in males there are more
receptors, a zone of high visual acuity, and unique sex-specific
neurons in the visual centers. Only males are able to sustain
fixation, tracking and interception of small rapidly moving
objects. This is distinct from behaviour shared by both sexes in
which panoramic flow fields are computed by elemental motion-
detectors and relayed to giant tectal neurons. Ultimately,
mechanosensory information, derived from strategically placed
organs for balance tactile perception is integrated with visual
information in discrete brain centers from which originate 1)
descending pathways to thoracic motor centers controlling head
and body musculature and 2) interneurons supplying cerebellarlike
higher centers in the midbrain. This research will employ intracellular recordings and stainings,
and sophisticated light and electron microscopical strategies to
dissect the cellular organization of identified nerve cells, from
the receptors to the motor neurons and musculature. The
research will focus on the following: 1) retinotopic organization
and multimodal response characteristics of visual interneurons; 2)
intracellular recordings and synaptic relationships of descending
motor neurons; 3) innervation, physiology, and dynamics of
effector muscles; 4) skeletal attachment, arthrology, and
behaviour. This research proposes to broaden our understanding
of neural mechanisms involved in visuo-mechanosensory control of
eye movements. A complete description of motor control in this
model will contribute a major step towards understanding
circuitry for complex oculomotor behaviour and the role of
uniquely identified neurons in visual pursuit and interception.