The Eye Cornea : Transparent tissue covering the front of the eye. Does not have any blood vessels; does have nerves. Iris: Circular band of muscles that controls the size of the pupil. The pigmentation of the iris gives "color" to the eye. Lens: Transparent tissue that bends light passing through the eye. To focus light, the lens can change shape by bending. Pupil: Hole in the center of the eye where light passes through. Sclera : Protect coating around the posterior five-sixths of the eyeball Retina: Layer of tissue on the back portion of the eye that contains cells responsive to light (photoreceptors)
There are two types of photoreceptors: "rods" and "cones."
Rods are sensitive enough to respond to a single photon, but functioning together they are optimized for seeing in poor light.
Cones are optimized for responding to fine detail and color; they need a lot more light and work best in broad daylight.
Inside the human eye, there are eighteen times more rods than cones. These are arranged in such a way as to produce the best possible combination of night and day vision.
The Photoreceptors: located in the back of the retina When light hits the retina it first processed in the photoreceptors located in the back of the retina. PHOTORECEPTORS BIPOLAR CELLS GANGION CELLS The ganglion cells’ axons join together to form the optic nerve, which exits through the back of the eye at the optic disk.
Rods and cones contain photopigments- chemicals that release energy when struck by light.
Photopigments consist of 11- cis- retinal (derivative of Vitamin A) which is bound to opsin (a protein).
Light converts 11- cis- retinal to all- trans -retinal, which ultimately activates 2 nd messenger systems that work to close NA+ channels, hyperpolarizing the receptor.
More light = hyperpolarization .
Photoreceptors input to retinal bipolar cells, which input to retinal ganglion cells. Photoreceptors and bipolar cells do not produce action potentials . They produce graded potentials. Retinal ganglion cells produce action potentials. light The Photoreceptors: Phototransduction
Glutamate hyperpolarizes some bipolar cells, and depolarizes other bipolar cells. At rest, (i.e. in the dark) photoreceptors continuously release neurotransmitter (glutamate). in the dark Some bipolar cells provide hyperpolarizing input to ganglion cells, and some bipolar cells provide depolarizing input. The Photoreceptors: Phototransduction . . . . . . . . .
close your right eye. With your left eye, look at the red circle. Slowly move your head closer to the image. At a certain distance, the blue line will not look broken!! THE OPTIC DISK: YOUR BLIND SPOT