Visual excitation

Physiological response glycogen breakdown visual excitation histamine secretion in all allergic reactions slowing of pacemaker activity that controls the rate of the heartbeat... 

The effect of receptor stimulation is thus to catalyze a reaction cycle. This leads to considerable amplification of the initial signal. For example, in the process of visual excitation, the photoisomerization of one rhodopsin molecule leads to the activation of approximately 500 to 1000 transdudn (Gt) molecules, each of which in turn catalyzes the hydrolysis of many hundreds of cyclic guanosine monophosphate (cGMP) molecules by phosphodiesterase. Amplification in the adenylate cyclase cascade is less but still substantial each ligand-bound P-adrenoceptor activates approximately 10 to 20 Gs molecules, each of which in turn catalyzes the production of hundreds of cyclic adenosine monophosphate (cAMP) molecules by adenylate cyclase. 

Wald G Molecular basis of visual excitation. Nature 219 800-807,1968. 

Post-ingestion precursor for RA-R agonist RA 11-cis-Retinal chromophore linked to protein opsin, in visual excitation isomerizes to all-fram-Retinal -George Wald (USA, retinal isomerization), Ragnar Granit (Finland/ Sweden) Haldan Hartline (USA) (Nobel Prize, Physiology/Medicine, 1967, vision)... 

Visual excitation (1) in both vertebrates and invertebrates is initiated via light absorption by visual pigments consisting of a chromophore covalently bound to an apo-protein, opsin. Biochemical extraction studies have shown that in all pigments the chromophore is a Schiff base derivative of 11-cis retinal (Fig. 

Cooper A. Energy uptake in the first step of visual excitation. Nature 1979 282 531-533. 

L. Stryer. 1988. Molecular basis of visual excitation Cold Spring Harbor Symp. Quant. Biol. 53 283-294. (PubMed)... 

Benolken RM, Anderson RE, Wheeler TG. Membrane fatty acids associated with the electrical response in visual excitation. Science 1973 182 1253-1254. 

Liebman PA, Parker KR, Dratz EA. The molecular mechanism of visual excitation and its relation to the structure and composition of the rod outer segment. Ann Rev Physiol 1987 49 765-791. 

I give presentations from mind maps and sometimes hand them out. They re more visually exciting than linear notes. 

Would you expect a rod cell preparation that has been treated with borohydride to be active in the cycle of visual excitation Why or why not ... 

The photoexcitation of rhodopsin leads initially to the isomerization of 11-ds-retinal to all-trans-retinal and ultimately to the cleavage of all-trans-retinal from the protein. In a rod cell preparation that has been treated with borohydride, the Schiff base would be stabilized by reduction, so the removal of the all-trans-retinal would be impaired. As a result, the cycle of visual excitation could not occur. (See Eigures 32.22 and 32.23 and Section 32.3.2 of the text for a discussion of these events.)... 

In the primary event of visual excitation, light iso-merizes the 11-cis-retinal of the opsin to all-tronj-ret-inal. Since all-tranr-retinal does not fit the binding site for 11-cu-retinal, the opsin molecule beeomes unstable and undergoes a series of conformational changes, followed by hydrolysis of the Schiff s base linkage between all-rranr-retinal and opsin (Fig.). Further events in the process are known with some confi-... 

The mechanism through which Ca2+ ions exert their action is not apparent however, there are several intracellular events affected by the divalent cation that might bear a relationship with the phospholipid effect here described. Ca causes neurotransmitter release from nerve terminals (Del Castillo Katz, 1954 Pressman 1972), plays an important role in visual excitation (Hagins, 1972 Hagins Yoshikami, 1975), and influences several other cellular reactions (Carafoli et al., 1976). In addition a direct influence of the divalent cation on the biosynthetic pathway might take place. 

Stryer, L. (1988). Molecular-basis of visual excitation. Cold Spring Harbor Symposia on Quantitative Biology, 53, 283-294. 

Stryer, L. (1991). Visual excitation and recovery. Journal of Biological Chemistry, 266, 10711-14. 

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