Lab-grown skin encoded touch and heat as different patterns of nerve impulses

Lab-grown skin encoded touch and heat as different patterns of nerve impulses

The tissue model combined human epidermis and dermis, the outer and deeper layers of the skin, with sensory neurons from newborn rats. Electrodes beneath the tissue recorded neuronal activity.

Skin converts pressure and temperature into impulses that peripheral nerves use to inform the brain about touch and heat. In an article published on August 15, the authors studied this conversion in cultured tissue by indenting and heating its surface, then recording the response of neurons beneath the skin layer.

In an earlier study, the same group applied capsaicin, the compound that makes chili peppers hot, to the epidermis and recorded the electrical response. In the new study, the authors tested whether physical stimuli applied to the surface, specifically pressure and temperature, reached the nerve endings.

The authors indented the surface by 100 micrometers three times. During the first contact, the number of impulses increased by an average of 14.68-fold relative to baseline, while the number of electrodes recording activity increased by 8.70-fold (five samples). Indentation produced no impulses in a sample without neurons. The number of impulses also did not increase in a culture assembled 24 hours before the experiment, because the nerve endings had not yet had enough time to grow toward the epidermis. These controls indicate that the recorded signal appears when mature nerve endings reach the skin layer.

Heat-transfer calculations indicated that, during the first five seconds, a probe at 45 °C heated the upper layer of the epidermis to approximately 38 °C and the deeper layer containing the neuronal cell bodies to 28 °C. Over the same five seconds, the number of impulses increased by 61%, while the number of active electrodes increased by 40% (three samples). Comparing impulse shapes and the intervals between impulses allowed the researchers to distinguish the mechanical response from the thermal responses to some extent.

The work received partial support from the European SOMA program, which developed sensory feedback for prosthetic hands. At the other end of the sensory pathway, implants in the sensory cortex produced sensations of touch in the hand for up to ten years in five people with spinal cord injuries. The new model makes it possible to study an earlier step in this pathway: how physical contact is converted into a nerve signal.