Direkt zu

 

Phototransduction

Phototransduction describes the process of converting light to electrical signals in a photoreceptor cell in the eyes of vertebrates. These species rely on retinal rods and cones to form image-like vision. Rods are remarkably sensitive, being able to transduce the absorption of a single photon. Cones are responsible for vision at daylight conditions. The higher temporal resolution of the cones causes a loss of sensitivity towards light. Depending on the number of different types of opsines present in the eye, color vision is more or less distinctive. Each photopsine, always bound to the same chromophore retinal, gives an additional color impulse. Humans have rhodopsin (λmax = 500 nm) for brightness vision in the rods and cyanopsin (blue, λmax = 420 nm), iodopsin (green, λmax = 522 nm) and porphyropsin (red, λmax = 543 nm) in the cones for color vision. The mouse has become a popular animal model for studies of the phototransduction in vertebrates over the last decades due to the similarity to primate photoreceptors.[1,2] Neglecting the cell body itself, rods and cones basically consist of three functional regions (Figure 1):

  1. the outer segment is made of membrane disks with spaces of about 28nm. Besides other transduction components, the disks contain the visual pigments rhodopsin in rods and cone pigments in cones. In rods, the disks are internalized and thus separated from the plasma membrane. The disks of the cones are placed directly on the plasma membrane, offering a larger surface area for e.g. rapid chromophore transfer to regenerate pigments. Both are renewed progressively to maintain a constant outer segment length and with this the sensitivity towards light.
  2. the inner segment consists of the endoplasmatic reticulum and the Golgi apparatus. A high density of mitochondria allows to meet the high energy demands of the phototransduction process.
  3. the synaptic terminal transmits the incoming light signal to the second-order neurons in the retina.

The rhodopsin molecule in the rods turns enzymatically active upon absorbing a photon. This happens by transforming the light-sensitive molecule 11-cis-retinal to all-trans-retinal, leading to the separation of the retinal from the protein component of the rhodopsin. Subsequently, a signal transfer and enzyme activation cascade resulting in a closed ion channel for Na+ and Ca2+ in the plasma membrane follows.[4] In the absence of light, the open channels for both Na+/Ca2+ and K+ ions provide an inward current to depolarize the membrane. Accordingly, closing only the Na+/Ca2+ channel causes a hyperpolarization at the membrane and a decrease in the release of exciatory neurotransmitter as the K+ channels are still open. Until the opsin is deactivated by phosphorylation and chemically bound to arrestin, the closed channel forces a neuronal signal of about 40 mV to be sent to the retina for about 0.4 s.[5]

 

phototransduction

Figure 1: Structural drawing of the photoreceptors and the channel system. The three regions of photoreceptors are shown on the left: the outer segment, the inner segment and the synaptic terminals. The sodium ion channels gated by messenger nucleotides are depicted on the right: In the dark, the channels are open causing a depolarized membrane. Closing the channels under the influence of light generates a distinct membrane potential.[3]

 

After a complicated reactivation process by the retinal isomerase, all-trans-retinal is returned to 11-cis-retinal (see Figure 2) and can be reinserted into opsin.[6] The absorption maximum of 11-cis-retinal lies at about 498 nm in the range of green light.

 rhodopsin

Figure 2: The chromophore 11-cis-retinal can be transferred to all-trans-retinal by the absorption of light.

 

  1. Visual System (sensory System) Part 2, English, http://what-when-how.com/ wp-content/uploads/2012/04/tmp15F56_thumb4_thumb.jpg (visited on 02/16/2017).
  2. ISAYAMA, A. L. ZIMMERMAN, C. L. MAKINO: The Molecular Design of Visual Transduction, Biophysical Journal, 2000, 1–16.
  3. J. DEGRIP, K. J. ROTHSCHILD in:Handbook of Biological Physics, 2000, Elsevier, 1– 54, DOI 10.1016/s1383-8121(00)80004-4.
  4. B. LESKOV, V. A. KLENCHIN, J. W. HANDY, G. G. WHITLOCK, V. I. GOVARDOVSKII, M. D. BOWNDS, T. D. LAMB, E. N. PUGH, V. Y. ARSHAVSKY: The Gain of Rod Phototransduction, Neuron, 2000, 27 (3), 525–537, DOI 10.1016/s0896-6273(00) 00063-5.
  5. ANDREUCCI, P. BISEGNA, G. CARUSO, H. E. HAMM, E. DIBENEDETTO: Mathematical Model of the Spatio-Temporal Dynamics of Second Messengers in Visual Transduction, Biophysical Journal, 2003, 85 (3), 1358–1376, DOI 10.1016/s0006-3495(03) 74570-6.
  6. D. HAMER, S. C. NICHOLAS, D. TRANCHINA, T. LAMB, J. L. P. JARVINEN: Toward a unified model of vertebrate rod phototransduction, Visual Neuroscience, 2005, 22 (04), 417–436, DOI 10.1017/s0952523805224045.

 

 
Found an error or want to submit a new article? Please send your correction or article in by mail in word or pdf to photo-wiki@itc.uni-stuttgart.de.