MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Y.I. Fedoniuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

SECTION 3. ANATOMICAL AND PHYSIOLOGICAL ASPECTS OF BODY FUNCTION SELF-REGULATION

GENERAL ISSUES OF THE ANATOMY AND PHYSIOLOGY OF SENSORY SYSTEMS

THE ORGAN OF VISION

2. PHYSIOLOGY OF THE VISUAL SENSORY SYSTEM

2.2. PUPIL REACTIONS

Light rays enter the eye through the pupil. The pupil facilitates a clear image of objects on the retina by letting through only the central rays, thereby reducing spherical aberration. In young adults, the pupil diameter can range from 1.5 to 8 mm, resulting in a 30-fold change in retinal illumination levels. When external illumination decreases, the pupil reflexively dilates, and when it increases, it constricts, aiding in eye adaptation. Illuminating one eye causes its pupil to constrict, known as the direct pupillary light reflex. Simultaneously, the pupil of the unilluminated eye also constricts, which is referred to as the consensual pupillary light reflex.

Pupillary reactions are reflexive processes. Parasympathetic nerves innervate the sphincter pupillae, which constricts the pupil, whereas sympathetic nerves innervate the dilator pupillae Muscle, which dilates it.

Activation of the sympathetic Nervous system (such as during emotions or pain) causes pupillary dilation. Pupil dilation is a critical symptom in The Development of pain Shock. Pupils also dilate during cerebral Hypoxia; therefore, pupillary dilation during deep anesthesia serves as a warning sign of life-threatening hypoxia.

The visual analyzer consists of peripheral, conductive, and central divisions, serving to perceive, transmit, and analyze visual information. The peripheral division is represented by the receptor apparatus of the retina, which in humans comprises 120 million rods and 6 million cones. The density of cones is maximal in the center of the retina—the fovea centralis—where rods are entirely absent.

Cones are the receptors for daylight and Color Vision. They exhibit low light sensitivity, yet higher spatial resolution and faster response times compared to rods. Rods function as the receptors for scotopic (dim-light) vision.

Photoreceptors contain visual pigments: rhodopsin in rods and iodopsin in cones. These are pigmented Proteins composed of a protein moiety—opsin—and a light-absorbing substance, the chromophore. For instance, rods contain the protein opsin combined with the chromophore retinol, which is an aldehyde of vitamin A.

When a photoreceptor pigment absorbs a light quantum, an electrical phenomenon occurs—the receptor potential—which is subsequently converted into a Nerve Impulse within The Cell Biomembranes.