Somatosensation (Biophysics of touch)

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✋ The Biophysics of Touch & Texture Sensing

Sometimes when you’re going shopping, you pick 2 shirts, but one of them seems more soft and comfortable. How can you know which one is softer just by touching them? Now we’re going to explore how we can sense different textures, and how quickly we react to such sensations.

When we touch something, we detect mechanical forces (pressure, vibration, and stretch for instance). These forces are then turned into electrical signals that travel through our nerves to the brain where it can translate and understand it.

Let’s dive deep into this phenomenon. How does our skin turn the sense into an electrical signal? There are several specialized Mechanoreceptors in the skin responsible for this, these are:

  1. Merkel cells: responsible for steady pressure and texture.
  2. Meissner cells: for light touch.
  3. Pacinian corpuscles: for deep pressure and vibration.
  4. Ruffini endings: for skin stretch.

Each one of these contains mechanosensitive ion channels (like Piezo1/Piezo2) that respond to physical deformation. When these channels open, ions like sodium (Na⁺) or calcium (Ca²⁺) flow into the cell, which changes its electrical charge, creating an electrical signal (action potential) that travels to the brain, then the brain translates the signals and interprets them as texture, pressure, or movement. By this process, a new mechanism is introduced to help us in interacting with the world, and the study of it has been employed in many fields like pain research, and robotics and sensory devices.

🧪 Fun Experiment: Test Your Touch!

Materials:

  • Blindfold or scarf
  • Stopwatch or phone timer
  • 4–5 textured materials (e.g., smooth plastic, cotton fabric, sandpaper, rubber, aluminum foil)
  • Small consistent pressing tool (e.g., pencil eraser or cotton swab)
  • A partner to apply stimuli
  • A notebook or data sheet
  • Optional: small weights or coins (to apply consistent pressure)

Part 1: Texture Identification (Sensory Discrimination)

  1. Blindfold the participant to remove visual clues.
  2. Randomly touch their fingertip with different textures for 2–3 seconds each.
  3. Ask them to identify or describe the texture.
  4. Repeat 2–3 times per texture and record accuracy.

Part 2: Pressure vs. Reaction Time

  1. Choose one texture (e.g., soft fabric or rubber).
  2. Have the participant rest their hand on a surface, eyes closed.
  3. The partner gently taps their fingertip with the eraser/tool using different pressure levels:
    • Light (just touching)
    • Medium (slightly pressing)
    • Hard (firm press — you can use coins stacked for consistency)
  4. As soon as the participant feels the touch, they say “now!”
  5. Use a stopwatch to measure reaction time from touch to response.
  6. Do at least 5 trials per pressure level, and calculate the average reaction time for each.

OPTIONAL: Later procedures to follow:

  • Compare how accurately the textures were identified.
  • Plot a simple graph: Pressure level (x-axis) vs Average reaction time (y-axis).
  • Discuss: Did stronger pressure lead to faster reaction times? Why might that happen based on mechanoreceptor activation?

📊 Final Analysis

  • Texture sensitivity depends on how well Merkel and Meissner receptors detect fine details.
  • Reaction time changes with stimulus strength due to the quicker and more reliable opening of mechanosensitive ion channels.

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