Why do we have rods and cones?
Why do we have rods and cones?
Light falling on photoreceptors is transformed by retinal circuitry into a pattern of action potentials that ganglion cell axons convey to the visual centres in the brain. Two systems of photoreceptors exist – rods and cones – allowing the visual system to meet the conflicting demands of sensitivity and acuity, respectively. Rods and cones are distinguished by shape, type of photopigment they contain, distribution across the retina, and pattern of synaptic connections. These properties reflect the fact that the rod and cone systems are specialised for different aspects of vision. The rod system has very low spatial resolution but is extremely sensitive to light; it is therefore specialised for sensitivity at the expense of resolution. Conversely, the cone system has very high spatial resolution but is relatively insensitive to light; it is specialised for acuity at the expense of sensitivity. The properties of the cone system also allow humans and many other animals to see colour.
General function of rods and cones
The retina contains two types of photoreceptors in its outermost layers, rods and cones. Both types have an outer segment (adjacent to the pigment epithelium) composed of membranous disks containing light-sensitive photopigment, and an inner segment that contains the cell nucleus and gives rise to synaptic terminals that contact bipolar or horizontal cells. In a process known as phototransduction, absorption of light by the photopigment in the outer segment of the photoreceptors initiates a cascade of events that changes the membrane potential of the receptor, and therefore the amount of neurotransmitter released by the photoreceptor terminals.
Difference in sensitivity due to differences in transduction mechanisms
Differences in the transduction mechanisms utilised by rods and cones are a major factor in the ability of rods and cones to respond to different ranges of light intensity. For example, rods produce a
Light falling on photoreceptors is transformed by retinal circuitry into a pattern of action potentials that ganglion cell axons convey to the visual centres in the brain. Two systems of photoreceptors exist – rods and cones – allowing the visual system to meet the conflicting demands of sensitivity and acuity, respectively. Rods and cones are distinguished by shape, type of photopigment they contain, distribution across the retina, and pattern of synaptic connections. These properties reflect the fact that the rod and cone systems are specialised for different aspects of vision. The rod system has very low spatial resolution but is extremely sensitive to light; it is therefore specialised for sensitivity at the expense of resolution. Conversely, the cone system has very high spatial resolution but is relatively insensitive to light; it is specialised for acuity at the expense of sensitivity. The properties of the cone system also allow humans and many other animals to see colour.
General function of rods and cones
The retina contains two types of photoreceptors in its outermost layers, rods and cones. Both types have an outer segment (adjacent to the pigment epithelium) composed of membranous disks containing light-sensitive photopigment, and an inner segment that contains the cell nucleus and gives rise to synaptic terminals that contact bipolar or horizontal cells. In a process known as phototransduction, absorption of light by the photopigment in the outer segment of the photoreceptors initiates a cascade of events that changes the membrane potential of the receptor, and therefore the amount of neurotransmitter released by the photoreceptor terminals.
Difference in sensitivity due to differences in transduction mechanisms
Differences in the transduction mechanisms utilised by rods and cones are a major factor in the ability of rods and cones to respond to different ranges of light intensity. For example, rods produce a