What Zebrafish Teach Us About Touch
Unlike the sense of vision, which is perceived only by light-sensitive photoreceptors in our eyes, the mechanoreceptors that respond to light touch are located in sensory neurons all over the body. Our sense of touch starts in the skin, where sensory neurons with elaborate dendrites just below the skin’s surface provide dense coverage over the entire area of the body. When we touch something, the mechanical pressure created by the contact between an object and our skin opens mechanoreceptors that cause the sensory neuron to fire an action potential and activate downstream neurons. We are constantly coming into physical contact with objects and people in our environment, and as a result a large number sensory neurons are being activated over many different areas of our body at any given moment! How does the nervous system handle all of this incoming tactile information?
Animals, including humans, have developed fast, involuntary movements to certain kinds of touch known as reflexes. Reflexes are an important evolutionary advancement that allows animals to respond quickly to touch arriving suddenly from an external source because these signals could represent danger, such as a predator or an unsafe environment, or rewards such as food or water. For example, most animals, including humans, show a strong startle reaction in response to sudden or threatening stimuli, like when something unexpectedly touches them on the back of the neck. However, as you may have noticed, it is impossible to us to startle (or tickle!) ourselves by touching our own skin. How does the nervous system ignore touch inputs coming from our own movements (e.g. when we touch something) while still robustly responding to ones that arise from external sources (e.g. when something touches us).
A recent study by Knogler and Drapeau shows a simple mechanism by which circuits in the spinal cord of the embryonic zebrafish distinguishes between these two types of input (2014). When the fish produces coiling or swimming movements, a synchronized signal about the movements, known as corollary discharge (Crapse and Sommer, 2008), is automatically sent to a population of sensory interneurons in the pathway responsible for the sense of touch. Zebrafish normally show a robust startle reflex when their skin is touched at rest, but the corollary discharge signal temporarily silences the sensory interneurons so that they are unable to activate this reflex pathway during ongoing movements. By doing this, the nervous system of the zebrafish ignores “unimportant” sensory feedback produced by their own movements and saves energy for when it really matters – like escaping from a predator!
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Unexpectedly, the researchers showed that at least at these early embryonic stages, all spinal neurons received the same corollary discharge signal regardless of their role in the sensory or motor circuit. However, only the sensory interneurons that mediate touch responses were silenced because they expressed a rare type of glycine receptor not found in other spinal neurons that made them uniquely susceptible to the signal. The finding that even the simple, one-day old embryonic nervous system is organized to filter out self-generated sensory information suggests that this is an important task. Indeed, it is now believed that some of the major symptoms of schizophrenia such as delusions and hallucinations may be due to a defect in this type of mechanism such that the nervous system fails to properly classify and filter out self-generated actions (Lisman, 2012). Understanding simple neural circuits in model organisms such as zebrafish is an important first step towards unravelling the complexity of the human brain.
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Written by Laura D. Knogler.
Image by Jooyeun Lee.
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Knogler L.D. (2014). Sensory gating of an embryonic zebrafish interneuron during spontaneous motor behaviors, Frontiers in Neural Circuits, 8 DOI: http://dx.doi.org/10.3389/fncir.2014.00121
Crapse T.B. (2008). Corollary discharge across the animal kingdom, Nature Reviews Neuroscience, 9 (8) 587-600. DOI: http://dx.doi.org/10.1038/nrn2457
Lisman J. (2012). Excitation, inhibition, local oscillations, or large-scale loops: what causes the symptoms of schizophrenia?, Current Opinion in Neurobiology, 22 (3) 537-544. DOI: http://dx.doi.org/10.1016/j.conb.2011.10.018
Videos courtesy of Joel Ryan.
