Many receptors adapt to repeated stimulation

Some sensory receptor cells give gradually diminishing responses to maintained or repeated stimulation. This phenomenon, known as sensory adaptation, enables an animal to ignore background or unchanging conditions while remaining sensitive to changes and new information. (Note that this use of the term “adaptation” is different from its application in an evolutionary context.) When you get dressed, you feel each item of clothing touch your skin, but the sensation of clothes touching your skin is not constantly on your mind throughout the day. You are immediately aware of new sensations, however, such as your shoe coming untied or someone touching your back.

Some sensory receptor cells adapt very little or very slowly; examples are some types of pain receptors and the mechanoreceptors that control balance. You do not want to ignore pain, which usually is signaling that something is wrong in your body, and to maintain equilibrium you must continuously know (albeit unconsciously) the tensions and forces on all of your joints and muscles.

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investigating life

How Do Pit Vipers “See” in the Dark?

experiment

Original Paper: Gracheva, E. O. et al. 2010. Molecular basis of infrared detection by snakes. Nature 464: 1006–1012.

Neurons of the pit organ have their cell bodies in a neural ganglion in the head—the trigeminal ganglion (TG). Sensory neurons conducting information from the body of the snake have their cell bodies in neural ganglia associated with the spinal cord—the dorsal root ganglia (DRGs). Any receptor protein that is involved in sensing heat by the pit organ or by temperature sensors in the skin should be expressed in the TG or the DRGs, respectively.

image

work with the data

The data from the experiment comparing TRP channel expression in the TG and the DRGs support the conclusion that the TRPA1 channel could be the infrared (IR) sensor in the pit organ. Another piece of evidence would be finding that the TRPA1 gene is not highly expressed in the TG of snakes without pit organs, or that it is highly expressed in snakes not closely related to rattlesnakes but that have pit organs. Accordingly, the investigators performed RNA-seq analysis of the TG and the DRGs of rat snakes (no pits) and pythons (pits).

RESULTS

(Simulated data, arbitrary units)

Rat snake Python
Channel TG DRGs TG DRGs
TRPA1 105 110 900,000 50
TRPV1 85 90 15 9

QUESTIONS

Question 1

Compare the relative expression of TRPV1 and TRPA1 in the TG and the DRGs of the rat snake (no pits) and the python (pits). What is the major difference?

The expression of TRPA1 in the python is much greater in the TG than in the DRGs. In contrast, the expression of TRPA1 in the rat snake is low and not very different in the TG and the DRGs. There are no remarkable differences in the expression of TRPV1 in the TG versus the DRGs in either the rat snake or the python.

Question 2

Explain how these results do or do not support the conclusion that the TRPA1 channel is the IR sensor in the pit organ.

Since the expression of TRPA1 is dramatically higher in the TG of the pit snake but not in the TG of the non-pit snake, these data support the conclusion that the TRPA1 channel plays a role in the function of the pit organ.

Question 3

What functional data on the TRPA1 channel would further support the conclusion that TRPA1 is the IR sensor in the pit organ?

If expression of the TRPA1 channel confers on a gene expression model system a temperature sensitivity in the range characteristic of the pit organ, that result would be strong evidence that the TRPA1 channel is the IR sensor in the pit organ.

A similar work with the data exercise may be assigned in LaunchPad.