The Retina

Within the bulbus oculi (eyeball) is an inner layer called the retina. See Figure 13-1 for the location of the retina within the bulbus oculi. See Figure 13-2 for the types of cells found within the retina.

Bulbus Oculi
Figure 13-1. A focal-axis section of the bulbus oculi.
Figure 13-2. Cellular detail of the retina.

a. Visual Fields (Figure 13-3). When a human looks at an object, light from the right half of the visual field goes to the left half of each eye. Likewise, light from the left half of the visual field goes to the right half of each eye. Later, in paragraph 13-4, we will see how the information from both eyes about a given half of the visual field is brought together by the nervous system.

b. Photoreception and Signal Transmission. The cells of the retina include special photoreceptor cells in the form of cones and rods. The light ray stimulus chemically changes the visual chemical of the cones and rods. This produces a receptor potential which passes through the bodies of the rods and cones and which acts at the synapses to induce a signal in the bipolar cells. This signal is then transmitted to the ganglion cells.

Retina And Visual Potentials
Figure 13-3. Scheme of visual input.

(1) Cones. The cones of the retina are for acute vision and also receive color information. The cones tend to be concentrated at the rear of the eyeball. The greatest concentration is within the macula lutea at the inner end of the focal axis (Figure 13-1).

(2) Rods. Light received by the rods is perceived in terms of black and white. The rods are sensitive to less intensive light than the cones. The rods are concentrated to the sides of the eyeball.

(3) Signal transmission. The stimulus from the photoreceptors (cones and rods) is transferred to the bipolar cells. In turn, the stimulus is transferred to the ganglion cells, the cells of the innermost layer of the retina. The axons of the ganglion cells converge to the back side of the eyeball. The axons leave the eyeball to become the optic nerve, surrounded by a dense FCT sheath. There are no photoreceptors in the circular area where the axons of the ganglion cells exit the eyeball; thus, this area is called the blind spot.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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