Structure of Rods & Cones
RODS:
-Slender cells - 100 to 120 micron long
-Outer segment contains the photo- sensitive pigment rhodopsin
-Electron microscopic examination shows that the outer segment contains 600 to 1000 transversely arranged membrane-bound lamellae, or discs
-The rhodopsin molecules are located within the membrane of the discs
-Each disc measures approximately 211 micron in diameter and about 14 nm thick
-When the discs reach the tip of the segment, the free end and the contained discs are phagocytosed by the cells of the pigment epithelium
-This sloughing of the discs of the rods occurs in a 24-hour cycle
-The connecting stalk, which is eccentrically placed, contains a modified cilium which possess 9 doublet microtubules
2 areas in inner segment:
-Ellipsoid - located next to connecting stalk
-Myoid - Located towards the vitreous
-Ellipsoid contains the basal body, with numerous mitochondria
-Myoid contains the granular and agranular endoplasmic reticulum, free ribosomes, and Golgi apparatus
Rod outer fibre - 1 micron wide - joins inner segment to cell body
Rod inner fibre - has microtubules and connects cell body to spherule
Spherule - has many presynaptic vesicles and synapses with dendrites of bipolar cells
CONES:
-Long, slender cells, measuring about 65 to 75 microns in length
-Outer segment is conical, has wider base
-The membranes of the transversely arranged discs are continuous with the outer plasma membrane, thus the laminae of the discs are continuous with the extracellular space
-The tips of the cones are not phagocytosed by the pigment cells
-Cones contain photochemical iodopsins
-The cone outer segment is connected to the inner segment by an eccentrically placed modified cilium
-The structure of the inner cone segment resembles that of the inner segment of the rod
-The inner cone segment merges with the body, which contains a large, pale-staining nucleus
-The body of the cone is connected by inner fibre to the cone pedicle which synapses with dendrites of bipolar cells
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