BIO254:DarkNoise: Difference between revisions

Revision as of 14:35, 30 October 2006

WIKIPEDIA BIO154/254: Molecular and Cellular Neurobiology

[Course Home] Wiki Home People Materials Schedule Help

Introduction:

"Noise is an unwanted signal or a disturbance in an electronic device or instrumeny; broadly, it is a disturbance interfering with the operation of a usually mechanical device or system". (Merriam-Webster Online Dictionary)

Our ability to detect dim lights is limited by noise in the rod photoreceptors. These electrical events prduced in the dark which is indistinguishable from the real signal produced by light especially limit visual sensitivity at low levels of illumination. In 1942 psychophysical experiments of Hecht, Shlaer, and Pirenne shows that human dark-adapted viusal system can successfully detec the absorption of 5-7 photons. More recent reserach indicates that the ultimate limit on the accuracy of photon counting is imposed by dark noise in the retinal rods.

Both rods and cones photoreceptors respond to light by hyperpolarizing the membrane potential. This dark noisesignal affects the synapse of photoreceptors only when it produced a change in membrane current that exceed a certain thresohhold. This threshold is a fundamental characteristic of the phototransducion process, defined as the amplitude of the spontaneous membrane current in the dark, is referred as the dark noise.

It has been been demonstrated that cones have more noise than rods in the absence of any light. (Cones is noiser than rods in the dark.) For example, in primate, the amplitude of the noise is about 0.12pA in cones and is 0.03 pA in rods. Therefore, rods seems to be more sensitive than cones.

- Two components of electrical dark noise

Reserachers (Simon, Lamb & Hodgkin, 1975; Schwartz 1977) first reported a small random fluctuations of the membrane potential of vertebrate photoreceptors in the darkness, this so-called "dakr noise" lessens during the response to bright steady light. Later, Baylor DA et al study this physiological noise in the visual transduction mechanism by recording membrane current from single rod outer segments in pieces of isolated toad retina, they used the suction pipette technique (see below) to examine the components of the dark noie. They found that the dark noise has two components: " a continuous fluctuation of rms amplitude about 0.2pA and occational dicrete events about 1pA in size.

Major reference: J. Physiol. (1980), 309, pp. 591-621) (Please refer to Fig 1 on this paper because I can't insert the figure here.)

- Discrete noise:

The amplitude and power spectrum of the discrete events resembled those of single photon effects in the same rod, which suggets that discrete noise may arise from spontaneous activation of single rhodopsin molecules. Besides, the intervals between these occational discrete events followed the exponential distribution expected of a Poisson process with a mean rate of about one event per 50 sec (20 oC). The temperature dependence of the mean frequency of occurrence of discrete events gave an activation energy of 22kcal mole-1, probably characteristic of thermal isomerization of rhodotin. In fact, further experiments proves that this discrete photon-like events is caused by spontaneous activation of rhodopsin. Because the spontaneous isomerization of 11-cis retinal in rhodopsing is so rare (ie. on average, an isomerization time for a rhodopsin molecule is 3000 years, it doesn't happen often), rods could actually detect single phtotons.

- Continuous noise:

The variance of the continuous event rose linearly with the length of the outer segment drawn into the suction electord, indicating that this component is generated in the outer segment. Also, the power spectrum of a rod's continuous noise was usually fitted by the ... This continuous current fluctuations is caused by spontaneous activation of phosphodiesterase (PDE). In 1996, F. Rieke and D.A. Baylor investigate the molecular mechanism of the continuous component of the eletrical dark noise in toad rods. Selectively disablzing different elements of the phototransduction cascade allow them to examine the contributions to the continuous noise. Their experiments indicate that this noise is generated by spontaneous activation of cGMP phosphodiesterase through a process that does not involve transducin.

Major technique used in dark noise:

Before the suction pipette techniques was developd in 1977: the light response of photoreceptors can only be studied by intracellular recording or by measuring extracelular voltage gradients. The disadvantage of both models is that both models can only provide information averaged over many photoreceptors.

Suction pipette techniques: To fix the problem mentioned above, and to record the elementary events of one photoreceptor, Baylor et al developed a new technique to record the membrane current of a single rod outer segment.

Here is how it works: first, small pieces of retina were isolated from dark-adapted toad and kept in oxygenated toad Ringer. By using an inverted microscope and infrared image converter, they draw a single rod outer segment and fit it into a close-fitting suction electrode containing Ringer. A current sensor between the pipette and a reference electrode in a bathing solution recorded the bulk of membrane current flowing through the region of the outer segment of the rod within the pipette. Finally, a transverse slit of light (or no light=dark) is applied to the outer segment from an optical stimulator.

Recording membrane current from single rod outer segments:

YAU, K.-W., LAMB, T.D. & BAYLOR, D. A. (1977) Lght-induced fluctuations in membrane current of signle toad rod outer segments. Nature, Lond. 269, 78-80 McBURNEY, R.N. & NORMANN, R.A. (1977) Current and voltage responses from single rods in toad retina. J. gen. Physiol. 70, 12a

Notes

- In darkness, primate rods give occasional spontaneous signal resembling reponses to single photo, just like toad. This photon-like dark noise in primate rods may result from thermal isomerization of rhodopsin. Under this assumption, plus the number of rhodopsin in a rod and rate of the noise events, the half-life of the thermal decay is 420 years. This greate stability of rhodopsin in darkness allows primate to have reliable detectin of very dim light.

- As mentioned before, retinal cones have more dark noise than rods. (Lamb and Simon. 1977; Schnapf et al., 1990; Schneeweis and Schnapf, 1999) However, neither the molecular origin of the cone noise nor its functional implications are known until later the Neuron paper of 2000. Origin and Functional Impact of Dakr Noise in Retinal Cones. In this paper, they show the origin of the dark noise in salamander cones varies with cone type. Details could be found on this paper.

Other Reference: Important papers for details

Hescht, Shlaer, and Pirenna, J. Gen. Physiol. 25:819 (1942).

Schwartz, E. A. J. Physiol. 272, 217-246

Simon, E. J., Lamb, T.D. & Hodgkin, A.L. (1975) Nature, Lond. 256, 661-662.

Baylor, D. A. (1987) Investigative ophthalmology & visual Science. Proctor Lecture Vol 28. 34-50.

Schnapf, J. L., Baylor, D. A. (1984) J. Physiol 357. pp. 575-602

Rieke, F., Baylor, D. A. (2000) Neuron Vol 26, 181-186

Rieke, F., Baylor, D. A. (1996) Biophysical Journal (1996) Vol 71, 2553-2572

Useful links:

http://webvision.med.utah.edu/ link title

Recent updates to the site:

List of abbreviations:

N: This edit created a new page (also see list of new pages)
m: This is a minor edit
b: This edit was performed by a bot
(±123): The page size changed by this number of bytes

13 May 2024

16:54

Upload log David Altman talk contribs uploaded File:Hornbeck WUbites Spring 2024.pdf ‎

16:54

Altman:WUbites‎ diffhist +137‎ David Altman talk contribs (→‎WUbites)

			16:16		User:Massih Forootan‎‎ 2 changes history −73‎ [Massih Forootan‎ (2×)]
					16:16 (cur \| prev) +55‎ Massih Forootan talk contribs (→‎Professional Experience)
					16:10 (cur \| prev) −128‎ Massih Forootan talk contribs (→‎Link to Publications)

			13:39		Beauchamp:Publications‎‎ 3 changes history +123‎ [Michael S Beauchamp‎ (3×)]
					13:39 (cur \| prev) 0‎ Michael S Beauchamp talk contribs
					13:39 (cur \| prev) −6‎ Michael S Beauchamp talk contribs
					13:33 (cur \| prev) +129‎ Michael S Beauchamp talk contribs

			08:33		RAVE‎‎ 3 changes history +860‎ [Zhengjia‎ (3×)]
					08:33 (cur \| prev) +830‎ Zhengjia talk contribs
					08:27 (cur \| prev) +29‎ Zhengjia talk contribs
					08:14 (cur \| prev) +1‎ Zhengjia talk contribs

			06:12		Xu lab:Publications‎‎ 2 changes history +525‎ [Peisheng Xu‎ (2×)]
					06:12 (cur \| prev) +288‎ Peisheng Xu talk contribs
					06:02 (cur \| prev) +237‎ Peisheng Xu talk contribs