Nervous System III Anatomy and Physiology Notes Ch. 11c Nervous System III Anatomy and Physiology

Structure of the Eye The eye has 3 layers – an outer fibrous tunic, a middle vascular tunic, and an inner nervous tunic. Spaces in the eye are filled with fluids that support the eyes wall and internal structure and help it maintain its shape.

The Outer Tunic The anterior portion of the outer tunic bulges and forms the cornea. The cornea helps focus entering light rays. The cornea is transparent – it has no blood vessels. There are nerve fibers for pain receptors and cold receptors in the cornea. The cornea is continuous with the sclera – the white part of the eye. This area protects the eye and provides attachment for muscles. Lastly at the back of the eye is the optic nerve.

The Middle Tunic This area includes the choroid coat – this area has many blood vessels that help nourish surrounding areas. The ciliary body is the thickest part of the middle tunic. It forms a ring within the front of the eye. This also has muscles that hold the eye lens in place. To focus on objects that are close the lens becomes convex.

The iris is a thin diaphragm made of connective tissue and smooth muscle. It is the colored portion of the eye. It is between the lens and cornea. The pupil is a circular opening in the center of the iris. The size of the pupil constantly changes in response to light. A bright light constricts the pupil and little to no light dilates the pupil.

The Inner Tunic The inner tunic is made of the retina – the area that contains visual receptor cells. The retina is very complex – it 5 major groups of retinal neurons. The nerve fibers are photoreceptors, bipolar neurons, and ganglion cells. These nerve fibers provide a pathway for impulses of the photoreceptors to the optic nerve and brain.

In the central area of the retina is a yellowish spot called the macula lutea. There is also a depression in its center called the fovea centralis – this produces the sharpest vision.

Light Refraction When a person sees in object the rays of light enter the eye and the image of what is seen focuses on the retina. This is called refraction. When an image forms in the eye it is upside down and reversed from left to right. The cortex of the cerebrum takes the image and flips to the real position.

Photoreceptors There are 2 types of photoreceptors in the eye – rods (with long, thin projections at their ends) and cones (have short, blunt projections). The retina has 100 million rods and 3 million cones. Visual receptors are stimulated only when light hits them.

Rods and cones function differently Rods and cones function differently. Rods are more sensitive to light so they provide vision in dim light. Rods produce colorless vision, cones detect color. Cones provide sharp images and rods produce an outline of objects. The central part of the eye (called the fovea centralis) does not have rods but is very tightly packed with cones.

Stereoscopic Vision This is simultaneously perceiving distance, depth, height, and width of objects. This is possible because the pupils are 6-7 cm apart. If an object is less than 20 feet away the right eye sees a little more of one side of the object while the left side see a little more of the other object. This gives things a three dimensional images.

Life-Span Changes As a person ages they experience diminished senses. Hearing – by 60 ¼ of the population experiences hearing loss. It may become more difficult to hear high pitches and the sounds like f, g, s, t, sh, th, z and ch. Vision may decline. Dry eyes are common. People may see floaters (moving specks) or flashes of light. Also it becomes harder to read small print up close. Glaucoma can develop, this is where fluid pressure in the eye increases and it may lead to blindness.