Sensation and perception HC2
The First steps in Vision: From Light to Neural Signals
The ganglion cells make up the optic nerve
Sensation is the input
Retinal Structure and Function
The retina is a multilayered structure at the back of the eye responsible for
converting light into neural signals. Here are the major components:
* Photoreceptors are specialized nerve cells in the retina that detect light
and convert it into electrical signals, which are then sent to the brain for
visual processing. They are essential for vision and allow us to see in both
bright and dark conditions.
1. Photoreceptors
We can't see colors with rods but with cones – bc rods has one type and it is
difficult to different, and we can see colors with cones from the combined output
of the three types
• Rods:
o Approximately 120 million in the retina.
o Extremely sensitive to light, making them crucial for low-light
(scotopic) vision.
o Insensitive to long wavelengths (red light) daytime.
o Contribute to the Purkinje shift, where perception shifts toward
blue/green under dim conditions.
1
, o One type of rods
• Cones:
o Around 6–7 million, primarily concentrated near in the fovea.
o Responsible for color vision (photopic vision).
o Three types:
▪ S-cones (Short wavelength/Blue): ~2% of cones.
▪ M-cones (Medium wavelength/Green): ~32% of cones.
▪ L-cones (Long wavelength/Red): ~64% of cones, are often
bigger (larger receptic field).
o Cones are less sensitive to light than rods but provide higher
acuity and color perception.
Visual Convergence
o Photoreceptors connect to bipolar cells and then to ganglion cells.
2
, o Rods exhibit greater convergence (many rods synapse onto a single
ganglion cell), enhancing sensitivity but reducing spatial acuity.
o Cones, especially in the fovea, have low convergence (one-to-one
connection with ganglion cells), enhancing acuity.
Horizontal and Amacrine Cells
o Horizontal cells: Facilitate lateral inhibition, enhancing contrast and edge
detection.
o Amacrine cells: Modulate signals between bipolar and ganglion cells,
influencing motion detection and temporal sensitivity.
Ganglion Cells
o The output neurons of the retina.
o Their axons form the optic nerve, transmitting visual information to the
brain.
Retinal Layers and Light Absorption
Light travels through several retinal layers before being absorbed by the
photoreceptors in the outer segments. The process is as follows:
Rhodopsin (in rods) or photopigments (in cones) absorb photons.
This triggers the breakdown of rhodopsin, leading to hyperpolarization of
the photoreceptor cell.
Signal transmission begins, moving through bipolar cells to ganglion cells.
3
, Regeneration of Rhodopsin
After light absorption, rhodopsin is broken down and must be remade for
the photoreceptor to respond to new light stimuli.
Such cells come with synaptic endings at the bottom, then the inner segment but
the magic happens at the outer segment.
Outer segment = many membranes and those membranes make sure we can
capture these photons. In those membranes are cross membranes protein which
catch the photons.
* Photons are the basic particles of light and other forms of electromagnetic
radiation.
WHAT HEPPENS WHEN A PHOTORECEPTOR CATCHES A PHOTON?
4
The First steps in Vision: From Light to Neural Signals
The ganglion cells make up the optic nerve
Sensation is the input
Retinal Structure and Function
The retina is a multilayered structure at the back of the eye responsible for
converting light into neural signals. Here are the major components:
* Photoreceptors are specialized nerve cells in the retina that detect light
and convert it into electrical signals, which are then sent to the brain for
visual processing. They are essential for vision and allow us to see in both
bright and dark conditions.
1. Photoreceptors
We can't see colors with rods but with cones – bc rods has one type and it is
difficult to different, and we can see colors with cones from the combined output
of the three types
• Rods:
o Approximately 120 million in the retina.
o Extremely sensitive to light, making them crucial for low-light
(scotopic) vision.
o Insensitive to long wavelengths (red light) daytime.
o Contribute to the Purkinje shift, where perception shifts toward
blue/green under dim conditions.
1
, o One type of rods
• Cones:
o Around 6–7 million, primarily concentrated near in the fovea.
o Responsible for color vision (photopic vision).
o Three types:
▪ S-cones (Short wavelength/Blue): ~2% of cones.
▪ M-cones (Medium wavelength/Green): ~32% of cones.
▪ L-cones (Long wavelength/Red): ~64% of cones, are often
bigger (larger receptic field).
o Cones are less sensitive to light than rods but provide higher
acuity and color perception.
Visual Convergence
o Photoreceptors connect to bipolar cells and then to ganglion cells.
2
, o Rods exhibit greater convergence (many rods synapse onto a single
ganglion cell), enhancing sensitivity but reducing spatial acuity.
o Cones, especially in the fovea, have low convergence (one-to-one
connection with ganglion cells), enhancing acuity.
Horizontal and Amacrine Cells
o Horizontal cells: Facilitate lateral inhibition, enhancing contrast and edge
detection.
o Amacrine cells: Modulate signals between bipolar and ganglion cells,
influencing motion detection and temporal sensitivity.
Ganglion Cells
o The output neurons of the retina.
o Their axons form the optic nerve, transmitting visual information to the
brain.
Retinal Layers and Light Absorption
Light travels through several retinal layers before being absorbed by the
photoreceptors in the outer segments. The process is as follows:
Rhodopsin (in rods) or photopigments (in cones) absorb photons.
This triggers the breakdown of rhodopsin, leading to hyperpolarization of
the photoreceptor cell.
Signal transmission begins, moving through bipolar cells to ganglion cells.
3
, Regeneration of Rhodopsin
After light absorption, rhodopsin is broken down and must be remade for
the photoreceptor to respond to new light stimuli.
Such cells come with synaptic endings at the bottom, then the inner segment but
the magic happens at the outer segment.
Outer segment = many membranes and those membranes make sure we can
capture these photons. In those membranes are cross membranes protein which
catch the photons.
* Photons are the basic particles of light and other forms of electromagnetic
radiation.
WHAT HEPPENS WHEN A PHOTORECEPTOR CATCHES A PHOTON?
4
