visual rodes and cones, retina

visual the eye

Max Planck Institute study

Study of relative importance of hearing, touch, taste or smell depends on the culture, so hierarchy of senses is shaped by both biological predispositions and cultural influences.

Cross-cultural study

Shows that sight is the most universal sense of all

possible reason:

1. Brain architecture

2. Constancy

3. Coordination

The visual stimulus

• is given by an external factor capable of provoking a reaction in the organism.

What is light?

Radiation that propagates in the form of waves.

Electromagnetic waves

Waves that can propagate in a vacuum.

What does light consist of?

of small blocks of energy called photons or quanta, which propagate in a wave-like fashion.

What are photons or quanta?

it is what light consists of and which propagate in a wave-like fashion.

What does light correspond to?

A narrow band of the electromagnetic spectrum.

eye detection of waves

The human eye detects waves between 380- 760 nm (above ultraviolet and below infrared).

Perceived colour is determined by 3 physical dimensions

1. Tonality (length)
2. Brightness (intensity)
3. Saturation (purity)

Amplitude

• Amplitude refers to the height or strength of a light wave.

• How bright or how dim something is.

Wavelength

Wavelength in vision refers to the distance between two consecutive peaks of a light wave.

Determines the color we can see.

• long and short wavelenghts

Shorter wavelengths

Shorter wavelengths (like blue or violet light) are at the violet end of the spectrum.

Longer wavelength

Longer wavelengths (like red light) are at the red end of the spectrum.

Human eye layers

Screla

Choroid

Retina

Screla

Forms the whites of the eyes, protective and supportive function.

Choroid

Provides the blood supply, with a large number of vessels.

Retina

contains the photoreceptors and nerve cells (cones and rods)

Vision process of light

Light is reflected from objects in the environment and enters the eyes through the pupil, and then the light is focused by the cornea and lens to form sharp images of objects on the retina (where the sensory receptors for vision are located)

Cornea

• Transparent covering at the front of the eye.

• Is responsible for 80% of the image's focus on the eye.

• But it's a fixed element and you can't make adjustments to focus

Lens

Provides the remaining 20% of the eye's adjustment, as it can adapt its shape and adjust the focus to capture stimuli at different distances.

Components of the eye

Iris

Crystalline line

Retina

Bipolar cell layers

Photoreceptors

Iris

Gives colour to the eyes.

Iris is a muscular formation with a central opening called the pupil, which varies in diameter depending on the amount of light entering the eye.

Crystalline lens

• Behind the iris

• lens that change according to the tension exerted on it by the ciliary muscle.
  o Relaxed: flatter glass
  o Contracted: rounder glass

• The lens are used to focus on objects closer or further away, a process known as accommodation.

Retina

The layer of the eye where photoreceptors and nerve cells are located.

Bipolar cell layer

Transmit impulses from the receptors to the next layer.

Ganglion cell layer

Its axons leave the eye through the blind spot, forming the optic nerve.

• retina, ganglion cells receive signals

• In the retina, ganglion cells receive signals from receptors.

• There are 126 million receptors but only 1 million glaglion cells.

• Each ganglion cell receives signals from 126 receptors.

Photoreceptor layer

Cones: allow color vision. More abundant in the center, especially in the fovea.

Rods: more sensitive to light, they allow night vision. More abundant in the periphery.

Rods & cones outer segment

composed of membranous discs that contain a light-sensitive photopigment

Rods & cones inner segment

contains the cell nucleus and gives rise to synaptic endings that make contact with bipolar or horizontal cells.

What do Rods provide?

• A greater sensitivity than cones.

• Increased sensitivity because less light is required to produce a response in the receiver of a rod than in the receiver of a cone.

Cones provide

• provide better insight into details than rodes.
  

• Gives us the ability to see different thing when we move our eyes

• We are scanning what we see in the environment with the cone-rich area of the fovea.

• There is greater visual acuity in the fovea, while objects projected on the peripheral part of the retina are not seen as clearly.

Cones material

• Cell count 6.000.000/in the retina.

• Light sensitivity = Low

• Visual acuity = High

• Adaptation to darkness = Fast

Rodes material

• Cells in the retina 120.000.000/in the retina.

• Light sensitivity = High

• Visual acuity = Low

• Adaptation to darkness = Slow

Ganligion receive and amount

• In the retina, ganglion cells receive signals from receptors.
  There are 126 million receptors but only 1 million glaglion cells.
  Each ganglion cell receives signals from 126 receptors.

Scotopic system

• Rods

• Location = Periphery

• Visual acuity = Little

• Vision = Nocturnal and grey

• Light Sensitivity = Much

• Pigments = Rhodopsin

• Connections = Strong convergence

Photopic system

• Cones

• Location = Fovea

• Visual acuity = Much

• Vision = Diurnal and colour

• Light Sensitivity = Little

• Pigments = 3 types of opsin

• Connections = Little convergence

Purinkje effect

Passage from one system to another; In low light conditions, stimuli in the blue spectrum (short waves) are better perceived.

Two systems for seeing colors

• Trichromatic system

• Opponent-process system

• The perception of colors changes under different lighting conditions e.g. Flower color with different amounts of light

Isomerization

Visual receptors contain visual pigment molecules, which modify their structure when exposed to light called isomerization, which leads to generating a complex cascade of enzymatic and biochemical reactions.

RHODOPSIN

Rods have a substance called RHODOPSIN, which is a photosensitive pigment that is stable in the dark and IS SENSITIVE TO PHOTONS OF LIGHT.

Visual transduction

Light transduction occurs through a chemical process.

Rods have a substance called RHODOPSIN, which is a photosensitive pigment that is stable in the dark and IS SENSITIVE TO PHOTONS OF LIGHT.

Transduction begins when the retinal absorbs a photon of light and isomerization occurs.

• When it receives light, its components are separated into Opsin and Retinal (derived from vitamin A).

CONES TYPES OF 3 different types of photopigments

• iodopsin - PART OF BOTH

• cyanopsin - DETECT BLUE

• erythropsin - DETECT RED

Layers and cells of the eye

1. Outer layer, involved in light-sensitive photoreceptors, which are in contact with the pigmented epithelium.

2. Middle layer, contains bipolar cells.

3. Inner layer, ganglion cells (optic nerve).

4. Horizontal cells, which connect photoreceptors.

5. Amacrine cells, which connect bipolar cells to ganglion cells.

The transformation of light into electricity

• Photoreceptors are activated by light which produces a graduated change in membrane potential and a corresponding alteration in the rate of transmitter release in postsynaptic neurons.

• Light shining on a photoreceptor (rod or a cone) leads the membrane to hyperpolarization (more negative).

• In other words: the membrane is at its resting state first (-65mv) but becomes more negative inside.

Neural convergence

• It occurs when a neuron receives signals from many other neurons.

Why are rods more sensitive than cones?

due to their high convergence