Physiology-II PHL-226 Physiology of vision By Majid Ahmad Ganaie M. Pharm., Ph.D. Associate Professor Department of Pharmacology E mail: majidsays@gmail.com 1

Vision

The eye can only perceive a small portion of the spectrum of electromagnetic waves

Three layers 1. Outer fibrous tunic Sclera, cornea, limbus 2. Middle vascular tunic Iris, ciliary body, choroid 3. Inner nervous tunic Retina

In order to see an object: 1- The pattern of the object must fall on the vision receptors (rods and cones in the retina)  accommodation 2- The amount of light entering the eye must be regulated (too much light will “bleach out” the signals) 3- The energy from the waves of photons must be transduced into electrical signals 4- The brain must receive and interpret the signals

Accommodation It is the process of adjusting the shape of the lens so that the external image fall exactly on the retina

Accommodation Object is far  the lens flattens Object is near  the lens rounds Figure 10.25

Accommodation abnormalities Myopia Hyperopia Astigmatism: the cornea is irregular  irregular pattern of vision Presbyopia: stiffening of the lens occurring with aging  increased difficulty with near vision

Figure 10.27a–b

Figure 10.27c

Regulation of the amount of light entering the eye The iris controls the amount of light entering the eye cavities The contraction of radial or circular smooth muscles located within the iris permit changes in the pupil diameter

Figure 10.28a

The Pupillary Muscles

Phototransduction - General Rods – respond to almost any photon and are sensitive to light and black and white – can’t distinguish colors. Cones – allow for colour vision. cell body cone cell rod cell nucleus inner segment outer segment synaptic endings membrane of disk retinal opsin synaptic vesicles ion channels in plasma membrane light rays of disk Rhodopsin molecule (opsin + retinal) 3 types of cones – red, blue, green – colour blindness usually results from a deficiency in one or more of the cones (red/green colour blindness is common)

The Visual Pathway: Photons hit the pigment of a photoreceptor    enzymes are activated in the cell which modify its state of polarization  the signals are sent to visual area of the occipital lobe of the brain through the optic nerve

Neural processing The bipolar neurons and ganglion cells process the signal In the fovea where the acuity is the highest: 1 cone  1 bipolar cell  1 ganglion cell At the periphery: many rods  1 bipolar cell … acuity is much decreased Other cells in the retina participate in signal processing

The Visual Pathways Visual Circadian Rhythm Input to suprachiasmic nucleus affects the function of the brainstem Circadian rhythm ties to day-night cycle, and affects metabolic rates

Colour blindness Colour blindness results when there is a deficiency in one or more of the cone colours (i.e red/green deficiency). The patient will have difficulty differentiating between red and green colours and tend to see them all as something in between. Colour blindness is a genetic condition and affects boys much more frequently than girls, because it is a disorder carried on the “X” chromosome. Females are XX and therefore need 2 defective Xs to have colour blindness. If a female has 1 defective X chromosome, they are a carrier. Males have XY, therefore only 1 X chromosome needs to be affected to have colour blindness. Try this simple colour blindness test to see whether or not you have this deficiency.

This is the CONTROL

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