Our sense of touch
If one of our intentions in yoga posture practice is to heighten our awareness of our sense of touch, then it's helpful to appreciate what is actually happening when we feel with our skin. Neurology is the branch of biology that deals with the functions of the nervous system. Let's take a light look at the neurology of our sense of touch.
Our skin is loaded with sensory nerve endings. Each ending will only respond to certain types of stimuli. Some respond to movement, others to chemical changes, others to temperature changes. One might detect the movement of hairs, others different kinds of touch, others a pain or an itch.
Each sensory nerve ending has physical properties that allow it to detect certain signals. Some have structures that transmit or amplify movement, like pressure or vibration. Others have receptors in their cell membranes that change shape in response to a temperature change or a chemical.
Ultimately, any type of sensory signal—whether it's temperature, movement, or a chemical—needs to be converted to an electrical impulse that can travel along neurons to the brain. To fire off a nerve impulse, little doors called channels on the sensory cells open and close, and allow ions to pass in and out. Ions are atoms with an electrical charge.
Different types of skin have different arrangements of sensory nerve endings. The skin on the back of the hand, with its embedded hairs, has a different arrangement of sensory endings than the skin on the palm.
Our hair is a part of the sensory system. When a light breeze blows across the skin, the hairs wave, transmitting the air movement to sensory nerve endings in the skin.
The density of sensory endings varies too. They are crowded much more closely on the fingertips than on the skin between the shoulder blades. This difference makes sense when you think how much we use our fingertips to explore the world, and how little we learn through the skin on our backs.
Not surprisingly, these differences are reflected in our brains. The fingertips and mouth have more dedicated brain space than the rest of our bodies.
Skin is a mosaic. Different points on the skin are sensitive to different types of stimuli. In response to stimuli, the sensory nerve endings send signals to the brain. The brain pulls together information from multiple signals, to help us understand what's happening to our skin. Ultimately, it's the brain that gives us a sense of what we're touching. The brain collects sensory information from all over the body, filters out the unimportant bits, and processes the rest to help us understand what's happening around us.
Discover the sensors that allow us to feel
Our sense of touch
Mechanical signals - what we often call 'touch' - stretch, vibrate or push on the skin. Nerve endings that detect mechanical signals are embedded within special structures. Each type of structure has physical characteristics that make it sensitive to a specific type of touch. Touch sensors include Merkel cells, Ruffini corpuscles, Pacinian corpuscles, lanceolate endings and Meissner's corpuscles.
In hairless skin, Merkel cells sit just under the surface. In hairy skin they surround the base of fine hairs. Their small size and relatively hard texture makes Merkel cells sensitive to very light touch. In response to pressure against a fine hair or on nearby skin, they release chemical signals onto the nerve endings they're associated with. Merkel cells are especially good at detecting edges and texture. In the fingertips, they help us read Braille dots. They also give us a sense of form, knowing what's in our hand without having to look.
Ruffini corpuscles are long, thin structures that sit deep under the skin and inside joints. When they are stretched they squeeze the nerve endings inside them, transmitting information about pressure and form over a relatively large area. They help us feel where our body parts are in space and they help us feel and control objects, such as a pencil in your hand.
Pacinian corpuscles are relatively large and squishy, like water balloons that sit deep under the skin. They respond not to sustained pressure, but to rapid changes in pressure. Poking or vibration on the skin deforms the Pacinian corpuscle, warping the nerve ending sitting inside of it. Pacinian corpuscles help us feel texture as we slide our hands across an object.
Most of our skin is covered with fine and not so fine hairs. Under the skin, lanceolate nerve endings reach up and down and around the base of each hair. When something brushes against the hair, it pushes and stretches the endings, transmitting information about the direction and speed of movement across the skin. People and animals have multiple types of hair, all associated with different kinds of sensory nerve endings.
Meissner's corpuscles help us feel small movements across hairless skin, including the lips, the palms of the hands and the soles of the feet. These little structures, which sit just below the skin surface, are made up of stacks of flat, slippery layers. A nerve ending grows up the centre of the stack, sending branches into the space between the layers. Gentle movement across the skin causes the layers to slide across one another and rub against the nerve ending branches.
Sensing temperature and chemical signals
Nerve endings that respond to temperature and chemical signals are covered with temperature and chemical sensing ion channels. These channels are made of proteins that protrude through the nerve-ending surface, forming a pore that can open and close. When the temperature is within the right range, or when the right chemical binds, the proteins change shape, opening the pore and allowing ions to pass through the cell membrane.
Pain is a signal that something is wrong. We have multiple types of pain sensing nerve endings. Some endings are specific to painful heat or painful cold, but most can respond to multiple types of signals. A single nerve ending may respond to extreme heat, extreme cold, extreme pressure and chemicals from damaged cells. Yet regardless of the cause, the nerve fibre sends a single message to the brain: Pain!
Itch sensing nerve endings are sensitive to certain chemical signals, including histamine, an inflammatory chemical released by immune cells. Histamine is the reason we get red and itchy from a mosquito bite.
Temperature & chemical sensors
Humans have at least six types of temperature sensitive TRP (or 'trip') channels, each of which is sensitive to a different temperature range. The type of trip channel a nerve ending has on its surface defines the range of temperatures it will respond to. We have different types of nerve endings that respond specifically to warm, cool, hot or cold temperatures.
Some of these trip channels also respond to chemical signals. One heat-sensing channel is also activated by the chemical capsaicin in chilli peppers - that's why chilli peppers taste hot. Chemicals in mint activate cold-sensing trip channels.
Some animals, including rattle snakes and vampire bats, use trip channels embedded in specialized organs to 'see' the body of their prey.