Fluorescence light micrograph of nerve fibres (white) in the iris muscle of a rat's eye, showing the envelopment of a blood vessel (running left to right) by nerve fibres. The section is prepared to show the noradrenaline content of the nerve fibres. Noradrenaline & its relative adrenaline (catecholamine hormones) are released by the medulla of the adrenal glands in response to acute physical & psychological stress; the released catecholamines act on adrenergic receptors throughout body, particularly in heart, lungs, blood vessels, and visceral & skeletal muscle. The adrenergic nerve fibres in the iris cause dilation of the pupil. Magnification: X 16 at 35mm size.