From the largest tigers to the tiniest mice, most furry creatures share the common problem of how to dry off without a bath towel. Many have developed a similar strategy: a vigorous, full-body shaking motion that researchers have dubbed “wet dog shakes.” While somewhat silly-looking, this behavior may be important for survival, helping the animal dry off quickly so that it can conserve body heat and precious calories.1
Now, a research team led by Harvard University neurobiologist David Ginty has identified the somatosensory machinery that mediates this behavior. In a study published today (November 7) in Science, Ginty and his team found that a type of mechanoreceptor called a C fiber low-threshold mechanoreceptor (C-LTMR) helps trigger this behavior in response to various stimuli.2 The findings reveal a novel function for C-LTMRs, which are associated with affective touch and pain modulation in both mice and humans.3,4
The researchers found that many kinds of stimuli evoked wet dog shakes in mice: They performed the behavior when they were wet, but also in response to minor irritants—like an oil droplet or a puff of air—applied to the back of the neck. To confirm their suspicion that wet dog shakes were mediated by mechanosensation, as opposed to the sensation of mild cold that is also elicited by wetness or air puffs, researchers deleted the Piezo2 gene, which encodes an ion channel that is crucial for sensing touch. As they predicted, lack of Piezo2 essentially eliminated the shaking behavior in response to water or oil droplets.
Mice—and humans—have various types of mechanosensory neurons that each respond to different kinds of touch. Researchers measured how these neuronal populations responded to the application of an oil droplet and found that three types of low-threshold mechanoreceptors were most responsive. Of these, only the C-LTMRs consistently elicited wet dog shakes when optogenetically stimulated. Conversely, when researchers ablated most of the C-LTMRs, oil droplet-induced shaking behavior was substantially reduced, while the mice otherwise maintained normal locomotor behavior.
Finally, researchers traced the pathway that conducted these sensory signals from the skin to the brain. Previous anatomical studies demonstrated that C-LTMRs gather touch information from the skin and transmit it to the other end of the cell, which is located in the dorsal horn of the spinal cord (in a cross section of the spinal cord, there is a butterfly-shaped region of grey matter comprised largely of cell bodies; the dorsal horn is the butterfly’s upper wing). From there, the researchers traced the signal across the synapse to spinoparabrachial neurons, which carried the signal up the spinal cord to the parabrachial nucleus (PBN), a brainstem region responsible for relaying sensory information to the rest of the brain. When researchers suppressed the ability of the spinal cord neurons to communicate with the PBN, or when they silenced the PBN itself, shaking behavior was decreased, confirming the importance of the spinoparabrachial pathway in mediating wet dog shakes.
While humans have access to bath towels and therefore do not need to shake dry, people do possess similar types of mechanoreceptors. In humans, these C-mechanoreceptors are thought to encode pleasurable touch; they may help to modulate pain and likely play a role in the evaluation of social touch.5–7 Thus, understanding the functions of these mechanoreceptors may lead to new insights into both pleasure and pain.
