Hi there! Welcome back to NeuroTransmissions.
So we’ve learned about all of our other main senses: sight, sound, smell, and taste.
Now we’re back to talk about the final sensory system - your sense of touch.
Somatosensation is enormously important; how else would you tell the difference between
the gentle touch of a feather and the scratch of sandpaper?
How could you tell if you were wet, or hot, or too cold? Most of all, how could you tell
if you were in pain? Don’t touch that dial...until you find out.
All somatosensory pathways have the same basic structure, so let’s focus on one and then
talk about some variations.
Let’s say I reach out and touch your arm. How do you feel it?
This sensation begins with receptors located in your skin, called mechanoreceptors. Remember
in our episode about the auditory system, we talked about the way that hair cells are
activated by movement?
The mechanoreceptors in your skin are similar - they respond to pressure, or distortion.
We have four major types of mechanoreceptors scattered across our hairless skin, such as
the skin on your fingers and lips, and they all have pretty funny names.
Meissner’s corpuscles, or tactile corpuscles, respond to light touch and vibrations. These
are very sensitive receptors, for fine touch discrimination.
Ruffini endings, or bulbous corpuscles, are located deeper in your skin and in the connective
tissues of the body, and respond to skin stretch and angle change in your joints.
Merkel nerve endings, or Merkel discs, help us detect ongoing pressure, like a hand holding
yours.
Lamellar corpuscles, or Pascinian corpuscles, are highly sensitive to vibration.
These receptors respond to sudden changes or disturbances, and are probably useful for
distinguishing changes in texture.
No matter which kind of receptor they are, they’re located at the ends of afferent
nerve fibers - that is, nerve cells located in the periphery of the body.
The nerves transmitting the sensation have their cell bodies in structures called the
dorsal root ganglia, or DRGs.
The DRGs are located along the outside of your spinal column, and are essentially “bundles”
of cells, collecting all of the nerve fibers into one structure before projecting the signal
on to the spinal cord.
The cells in the spinal cord receive the signal and then send projections across the midline
(the middle of the body), and the signal actually gets sent to the opposite side of your brain
from the side where you felt it - so if I touch your right hand, the sensation gets
passed to the left side of your brain for processing.
The next stop is in the thalamus, acting as a relay for the sensory signals coming from
your skin; finally, projections to the postcentral gyrus in the parietal lobe of your cortex
create a map of your body.
By mapping all of the signals coming in from your skin to dedicated spots on your cortex,
your brain is able to tell you exactly where you felt a given sensation.
Of course, pressure and vibration aren’t the only senses you detect with touch, right?
Our skin is also able to sense temperature and, obviously, pain. So how can we tell if
something is hot or cold, or if a pinch feels uncomfortable?
The trick is in the receptor.
Our ability to sense temperature is still something of a mystery. Scientists think that
different types of nerve fibers are responsible for hot and cold sensation.
There’s also some evidence that a particular family of proteins, called Transient receptor
potential or TRP channels, might play a role - and they’re connected to other types of
“hot” and “cold” we experience too
For example, TRPV1 (V for “vanilloid”) channels respond to capsacin - the chemical
that makes jalapeños spicy.
Another type of TRP channel, TRPM8, responds to menthol, a chemical that is found naturally
in peppermint that produces a “cooling” sensation.
Pain is a different animal.
Here, I’m talking just about “nociceptive”, or peripheral pain, which you sense in response
to harmful stimuli.
To detect pain, your nerves use specialized cells called nociceptors.
These nerve fibers use temperature, pressure, and chemical stimuli to determine if something
is painful or not.
These receptors can be found in your skin, joints, and on some of your internal organs.
Nociceptors are essentially just nerve endings embedded in your skin, like the roots of a plant
Let’s say you’ve just gotten a paper cut (ouch!).
There are three sorts of pain sensation associated with that - first, without thinking, you jerk
your finger away.
This is the result of a reflex arc, wherein a sensory neuron registers the painful stimuli
and passes it along to your spinal cord.
In the reflex response, the information doesn’t get processed by your brain; instead, it’s
passed right back out of your spinal cord via motor neurons that control your movement,
causing your hand to jerk back.
Next, you feel the first conscious “sharp” sensation of pain, as fast signaling nerves
called A-delta fibers, respond, passing the signal to synapses with neurons in your spinal cord.
Like mechanical stimuli, this signal gets passed up to the somatosensory cortex, allowing
you to consciously pinpoint the location of the pain (Ah! My finger!).
These fibers only react to mechanical and temperature stimuli, but not chemical, which
might explain why you don’t always feel the burn of lemon juice in a cut right away!
Other nerve fibers, called C-fibers, react more slowly, passing along the deeper, aching pain
These cells project to the spinal cord as well, though they synapse with a slightly
different set of spinal cord neurons, which project to the brainstem and the thalamus.
Pain, temperature, and touch are all important for helping us to navigate in the world.
We’re still learning a great deal every day about how the body processes these sensations.
One thing we know for certain - your senses are all important for helping you develop
a rich and complete perception of the world around you.
Our sensory systems are vital to letting us move through and interact with our environment,
and better understanding our senses will allow scientists and doctors to develop new treatments
for patients struggling with incomplete or damaged sensory systems.
In a future video, we might talk about some advances being made in this arena - let me
know in the comments or on Twitter if this is a topic you’re interested in hearing more about
So what does our brain do next with all of this sensory information coming in? How do
we use that information? Now that we’ve finished covering the senses, we’ll start
to explore more topics to answer these questions.
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I’m Alie Astrocyte, and until our next transmission, over and out!