basic physiology of vision

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Basic Physiology of Vision

The Nature of the Task

As a transducer, the eye must detect and respond to the physical phenomenon of electromagnetic radiation in the wavelengths of the visible spectrum; and it must then transform that electromagneticenergy into a signal the nervous system can perceive and transmit.Detection and conversion of the light into chemically-mediatednervous impulses takes place in the highly-specialized outersegments of the rods and cones. The rods and cones are thefundamental and indispensible cells, because without their ability to

trap photons, no signal would be generated in the first place. The restof the eye has "support" functions. Focusing and light control arechiefly mechanical matters. Once the signal has been generated,initial integration and processing take place in the neural elements of the retina's middle layers: but signal generation is the crucial step.

Rods and cones contain visual pigments, substances capable of absorbing the energy of the visible spectrum. Within the cells a cyclicmetabolic pathway regenerates the pigment, produces more or less of it as needed to adapt to light conditions, and permits constantresponse. The first response to light is, then, a physiochemical

phenomenon, which ultimately leads to an alteration in the surfacecharge of the light sensitive cell. This in turn is translated intonervous signals and action potentials.

Rods are by far the most common receptor in any vertebrate eye,

much more numerous in all species than the cones. Rods are adaptedto respond to all wavelengths of light, while the cones are more or less

wave-length specific and respond to narrow bands of color.Nevertheless, mechanism of detection and transduction is the samein rods and cones; what differs is the specific spectral ranges andsensitivities of the two types of receptors.

Physiology of Vision http://www.vetmed.vt.edu/education/Curriculum/vm8054/eye/PHY

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The rod itself is an elongated cell, bipolar in structure. The body of the rod containing the nucleus is located in the outer nuclear layer of the retina, and it contains all the usual organelles of any cell; all thenormal machinery for making proteins, etc. The rod's outer segmentis convoluted and folded into large numbers of "shelves" or

platforms, constituting a vast increase in the area of cellular plasmamembrane in this region.

The amplification of membrane surface area is a common phenomenon seen in other situations, and in keeping with theengineering analogy of the eye as an optical device, it is a logicalsolution to the problem. Whenever the rate at which some chemicalreaction can be carried out is limited, and when that function is

assigned to membrane-bound sites (e.g.e.g.e.g.e.g., proteins that undergo aconformation change and are part of the plasma membrane) thesimplest way to enhance the total rate of the reaction is to have moremembrane and hence more sites of activity. Put another way,membrane will produced, and then folded and re-folded to increasesurface area, packing far more active sites into a small volume, andgreatly increasing efficiency.

Mechanism of Transduction

Consider the rod inthe diagram at left:membrane-bound

pumps in the innerrod segment (below the "waist") are

actively pumpingsodium ions out. Asfast as they are

pumped out, theouter rod segmentbrings them back in,completing the


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circuit, provided therod is "dark."

In the presence of light, however, thetransport of sodium

back into the outersegment is disruptedand the outersegment becomeshyperpolarized. Theinterference withsodium transport

into the rod outersegment is mediatedby the cyclicdecomposition andreconstitution of the

visual pigment,rhodopsin.

Hyperpolarization of a neuron in response

to a signal is rather unusual; most neurons depolarize, instead. Butthe rod does not, as other neurons do, generate an action potential.Nor does it release neurotransmitters. The hyperpolarizationresponse to the impingement of light is proportional to light intensity;and thus the brighter the illumination the greater thehyperpolarization. The net change in overall membrane charge is

perceived by the integrating neurons of the retina, specifically thehorizontal and bipolar cells. They in turn pass the information (withsuitable inhibitory and/or excitatory signals of their own) to theganglion cells. Ganglion cells, the last intra-ocular neuronal element,send their axons out via the optic nerves and into the visual

processing centers of the central nervous system.

The light level striking the rod is thus translated first into a cascade of


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chemical changes via the rhodopsin cycle, and ultimately into anelectrical one. Thus transduction of the information fromelectromagnetic energy to neuronal signal is complete. The processof color vision involves cones, but the mechanism (absorption of energy, hyperpolarization of the cone surface membrane, and

detection by bipolar and horiszontal cells) is identical.

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basic physiology of vision

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