Motor innervation of the toad iris (Bufo marinus)

Area of Science:

• Ophthalmology
• Neuroscience
• Comparative Physiology

Background:

• The sphincter pupillae muscle controls pupil size.
• Understanding iris innervation is crucial for visual system research.

Purpose of the Study:

• To investigate the innervation and control mechanisms of the Bufo marinus (toad) iris.
• To determine how pupillary diameter is regulated in toads.

Main Methods:

• Studied autonomous contraction of sphincter pupillae muscle cells in response to light.
• Investigated sympathetic beta-adrenergic inhibition.
• Examined excitatory innervation via transmural stimulation of isolated irises.
• Assessed pupilloconstriction from cranial nerve stimulation.

Main Results:

• Sphincter pupillae muscles contract autonomously in bright light, causing rapid pupil constriction.
• Strong sympathetic beta-adrenergic inhibition of the sphincter pupillae was observed.
• Inhibitory fibers originate from spinal roots (2nd, 3rd, or 4th ventral).
• No consistent excitatory innervation of the toad iris was detected.
• Cranial nerve stimulation (3rd or 5th) caused inconsistent, minor pupilloconstriction.

Conclusions:

• Toads regulate pupillary diameter by balancing light-induced myogenic contraction with neurogenic relaxation.
• Sympathetic nervous system plays a key inhibitory role in toad pupil control.