Exam 4 Review:  Chapter 15:  Retina

retina - Neural/sensory tunic of the eyeball; contains photoreceptors (rods, cones) in the deepest layer, bipolar neurons, amacrine cells and horizontal cells in the middle layer, and ganglion cells in the superficial layer.

photoreceptors - Specialized receptor cells that respond to light energy by depolarizing to generate nerve impulses; e.g., the rods and cones within the retina of the eye; transduction occurs when a visual pigment is broken down by light energy and the reaction products trigger a biochemical reaction pathway which leads to an impulse generation and the transmission of a graded potential to the postsynaptic cell, the bipolar neuron.

optic disc = blind spot - the point where the optic nerve enters the retina (neural/sensory tunic of the eye); it is not sensitive to light due to the absence of photoreceptor cells; there are various experiments that may be performed to demonstrate the presence of the blind spot in one's field of vision.  See The Blind Spot.

rods - One of the two types of photosensitive cells in the retina (neural/sensory tunic of the eye), these cells predominate away from the center of the field of vision (fovea centralis) and are most responsive in low levels of illumination; they are not color sensitive.

cones - One of the two types of photoreceptor cells in the retina (neural/sensory tunic of the eye); they provide for color vision;  these cells predominate near the center of the field of vision (fovea centralis) and are most responsive in bright levels of illumination; there are three subtypes, red, blue, and green, which are sensitive to certain portions of the spectrum of visible light.  Click here for a fascinating optical illusion involving the performance of the cones in your retina.

macula densa - The small and highly sensitive part of the retina (neural/sensory tunic of the eye), roughly in the center of the retina, lateral to the optic nerve, responsible for detailed central vision; it contains the fovea centralis where cones are concentrated in a region which is a slight depression where the bipolar neurons and ganglion cells and the capillaries are pushed to the side so that light falls on the photoreceptors without as much interference; the macula densa allows one to appreciate detail and perform tasks that require central vision such reading. 

fovea centralis - A small depression near the center of the retina (neural/sensory tunic of the eye), constituting the area of most acute vision where the bipolar neurons and ganglion cells and the capillaries are pushed to the side so that light falls on the photoreceptors (predominantly cones) without as much interference.

Micrograph of the Fovea Centralis

 

Identify and describe:

 

1. the location and specific sites, including cellular components, for reception and transduction for the sensations of:

 

(c) visionphotoreceptor cells, the rods (one type), and the cone cells (three different types), located in the deep, nervous layer of the retina of the eye.

List and describe:

3. the sequence of components and physiological events in transduction of  vision.

 

1.  photons of light enter the eye, and after passing through many other structures, strike a particular rod or cone cell, the photoreceptor cell.
2.  photons of light are absorbed by visual pigments = photopigments within the rod or cone.  [Note:  a particular molecule, retinal, binds with one of four types of opsin proteins, to form the different types visual pigments = photopigments found within a rod or one of the three types of cones, red, green and blue.  
3.  as the photons of light are absorbed by visual pigments = photopigments within the rod or cone, they break down, releasing retinal and an opsin.
4.  the breakdown of visual pigments = photopigments triggers a complex internal cytoplasmic enzyme cascade (chemistry details not completely understood).
5.  the resulting metabolic events create second messengers which alter the membrane resting potential.
6.  this generates graded membrane potential which are transmitted to the bipolar neurons in the retina.
7.  additonal graded membrane potentialsare transmitted from the bipolar neurons to the ganglion cells in the retina
8.  if the threshold level is reached for a ganglion cell, an action potential = nerve impulse is generated which is transmitted to the CNS.

 

The diagram below summarizes light transduction graphically.

 

Sketch and label:

1. a section through the retina describing the main cell layers, identifying the cell types present, indicating the direction of light transmission and the direction of visual data transmission and processing.

 

A)  Light transmission, as noted by the yellow arrow at the bottom right of the figure, is through the ganglion cell and bipolar cell layers before reaching the rods and cones in the deepest layer of the retina.  Light does not pass beyond the pigmented layer.

B)  Visual data processing uses the reverse direction as rods and cones transmit graded potentials to bipolar neurons, which are also influenced by horizontal cells and amacrine cells.  Then, bipolar neurons transmit graded potentials to the dendrites of ganglion cells.  If the threshold is reached, then a ganglion cell will generate an action potential = nerve impulse which is transmitted to the CNS.


2. the parts of a typical or generalized photoreceptor cell. Explain the function of each labeled cell part.

 

nucleus - contains hereditary material

mitochondria - ATP energy production

stacked pigment membranes - contain photosensitive photopigments

cytoplasm - where enzyme cascade that ultimately generates graded potentials occurs

synaptic terminal - where graded potential transmitted to bipolar neuron

 

 


 

Recall that humans lack a reflective tapetum lucidum layer in the retina where it joins the vascular tunic underneath.  Many vertebrates do have a tapetum lucidum layer which improves their eyes' efficiency in forming images under conditions of dim light such as dawn and dusk.  Here is a photo of light reflected back from the tapetum lucidum of a cat.  (Not just any cat, but Stripes, Dr. Schiller's cat!)  And to the right, the "red eye" from a flash photo of a human eye.