Vision: Sense of Sight (Nursing)

00:00 The next sense is vision.

00:03 Vision uses visible light which is part of the electromagnetic spectrum with wavelengths from about 400 to 700 nanometers.

00:14 This is referred to as the visible light spectrum and corresponds to the colors of the rainbow ROYGBIV or red, orange, yellow, green, blue, indigo, and violet.

00:28 Taking a step back, wavelength is defined as the distance between two consecutive peaks of an electromagnetic wave.

00:37 The shorter the wavelength, the higher the electromagnetic energy therefore violet light which has the shortest of the wavelengths of our visible light spectrum, admits more energy than red light which has the longer wavelength.

00:56 So how does light actually pass through the eye and send messages to our brain? Light passes through the cornea, the anterior chamber, the pupil, then to the posterior chamber, then to the lens, then to the vitreous humor and then is projected onto the retina.

01:21 In order to determine how the light produces an image, we must first understand how light travels through different media.

01:30 Light refracts or bends when it passes through a transparent substance such as the different humors found in the eye.

01:40 When it passes through a substance with one density and to a second substance with a different density, bending occurs at the junction of these two substances.

01:54 Due to the refraction of light through the eyes, images focused on the retina are inverted and then right to left reverse.

02:04 The brain is then responsible for correcting the image.

02:09 Also, the lens must accomodate to properly focus the objects.

02:15 Accommodation involves the changing of the shape of the lens by the ciliary muscles attached to it.

02:24 An image is projected onto the fovea centralis, the site where our vision is the sharpest.

02:33 In a normal emmetropic eye, light will refract correctly and focus a clear image onto the retina.

02:43 However in some instances, the shape of the eye leads to imperfect vision for example, in myopia or nearsightedness, the eyeball is shaped longer than it should be.

02:58 This causes an image to converge on the front of the retina instead of onto the retina.

03:06 This allows for close objects to be seen sharply but as you move away, distant objects become blurry.

03:15 We correct this with concave lenses which elongate the pathway so that it reaches to the retina.

03:25 The opposite of myopia is hyperopia.

03:29 In hyperopia or farsightedness, the eyeball is shaped shorter than it should be.

03:35 This causes an image to converge behind the retina.

03:40 This allows for distant objects to be seen more sharply than close objects which appear blurry.

03:48 We correct this with convex lenses which shorten the pathway of light so that it hits the retina.

03:58 Next we have astigmatism.

04:00 Astigmatism is a condition where either the cornea of the eye or the lens of the eye or sometimes both have an irregular curve.

04:10 An astigmatism is going to cause blurred or distorted vision.

04:18 So now let's take a look at the process of light transduction.

04:23 Recall that the retina is the structure onto which light is projected.

04:29 The retina contain sensors known as photoreceptors and these photoreceptors are known as rods and cones.

04:39 The rods are necessary for us to see in dim light and our cones are necessary to produce color vision.

04:49 Once light hits the rods or cones, information is going to flow through the outer synaptic layer to the bipolar cells through the intersynaptic layer to the ganglion cells.

05:02 From there, axons of the ganglion cells bundle together and exit out of the back of the eye as the optic nerve.

05:11 Notice that the nerve impulse goes in the opposite direction of the direction of light.

05:22 Rods and cones, the photoreceptor of the retina are going to convert light energy into neural impulses Rods and cones get their name because of their appearance of their outer segments.

05:38 Rods and cones contains photopigments which are necessary for the absorption of light and that it will initiate events that leads to the production of a receptor potential.

05:50 Rods contain only the photo pigment rhodopsin.

05:55 Cones contain three different photopigments, one for each of the three types of cones.

06:02 We have red cones, green cones and blue cones and each has a different photopigment.

06:10 The photopigments are going to respond to light in a cyclical process.

06:16 So let's take a look at the process in our rods.

06:20 First, a photon of light is going to bind to the rhodopsin in the rod.

06:27 This causes the conversion of the molecules cis-retinal to be transformed to trans-retinal.

06:34 This is referred to as the isomerization step.

06:38 Next, the trans-retinal will detach from the the opsin molecule and lead to photo bleaching.

06:47 During this time, the enzyme retinoisomerase in the retina is going to convert the dissociated trans-retinal back into cis-retinal.

06:59 From here, the cis-retinal will attach to the bleached opsin in a regeneration step and the cycle can start all over again.

07:12 So with our vision, we have to sometimes adapt to changes in the amount of light.

07:18 Light adaptation takes place when an individual moves from a dark surrounding to light ones such as exiting out of a movie theater.

07:28 This process occurs in seconds as sensitivity decreases.

07:35 Dark adaptation is the opposite.

07:38 In this process, we're going to be moving from a lighted area into a darker one such as driving along the highway and then entering a tunnel.

07:48 The process of dark adaptation takes minutes to complete as it takes a moment to increase the sensitivity to light.

07:57 This is because rhodopsin in rods generates much slower than the photo pigments found in our cones.

08:06 So how does photo transduction occur at the photoreceptor? This is best explained by starting with the events that happened in the darkness.

08:17 In darkness, cis-retinal is associated with the photopigment.

08:23 Also, cyclic GMP, which is a molecule very similar to cyclic AMP but contains guanine instead of adenine, is present in high concentrations.

08:35 Cyclic-GMP binds to and opens cation channels allowing sodium to flow into these cells.

08:45 Sodium flowing in causes the depolarization and this depolarization causes the opening of voltage-gated calcium channels.

08:56 This then leads to the release of the neurotransmitter glutamate.

09:01 Glutamate inhibits the bipolar cells from transmitting signals to the ganglion cells which provide output from the retina to the brain.

09:13 So in the darkness, this signal is being blocked.

09:19 In the light, this inhibitory effect is blocked when the isomerization of cis-retinal also causes the breakdown of cyclic GMP.

09:31 This now leads to a hyperpolarization that stops the release of the inhibitory neurotransmitter glutamate and now allows for the bipolar cells to be activated and form an action potential.

09:46 From there, it sends signals to the ganglion cells which then sends signals to the brain.

09:54 The neural pathway for vision begins when the rods and cones convert light energy into neural signals that are diorected to the optic nerves.

10:07 From the optic nerve, exiting out of the optic disc of the eyeball at the back of the retina, we find the optic chiasm which is like an x-shape.

10:19 From there, the pathway or the stimulus is going to travel down the optic tract to the lateral geniculate nuclei of the thalamus.

10:30 Here, optic radiations are going to allow information to arrive at the primary visual cortex area of the occipital lobe of the cerebral cortex for perception of what you're viewing.

10:46 Because of where our eyes are located which is on the interior surface of our bodies or in the front of our bodies, our visual fields are going to overlap with each other.

10:58 This gives us binocular vision.

11:01 To give you some contexts, horses' eyes are on the lateral portions of their body.

11:08 Therefore, what they see out of one eye can not be seen out of the other eye.

11:15 The two visual fields of each eyes are the nasal or medial field, and the temporal or lateral fields.

11:24 Visual information from the right half of each visual field will travel to the left side of the brain.

11:33 Visual information from the left half of each visual field will travel to the right side of the brain.