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Traditional science considers the skin as our one and only sensory organ of touch. Our relationship with our skin is typically superficial, but it is really one of the miracles of evolution. Our skin is an intricate organ of consummate sophistication that has developed over millions of years. It comprises three distinct layers:
- The epidermis is the outermost layer of skin that provides a barrier against foreign substances and trauma. It also creates our skin color and tone and contains endocrine cells called Langerhans, which act as a frontline for the immune system of the skin and other parts of the body.
- The dermis is the skin’s thick middle layer of fibrous, elastic tissue made up of collagen and elastin. The dermis gives skin its suppleness and strength and contains sweat and sebaceous glands, nerve endings, blood vessels, connective tissue, and hair follicles.
- The hypodermis is the deeper subcutaneous tissue made of fat, connective tissue, nerve fibers, and blood vessels that helps insulate the body from heat and cold, provides protective padding for bones and organs, keeps the skin attached to the muscles and tendons underneath, and stores energy.
As the sensory organ for touch, skin is one of the main gateways into the subconscious feeling intelligence we possess. It is also the largest organ of the body, with an area of eighteen to twenty square feet and a weight of seven to nine pounds. Sensory neurons densely enervate the skin, which has one of the highest concentrations of neurotransmitter/receptor cells outside of the brain, especially in the hands (palms, fingertips) and feet (soles, toes).
Author Peter Sterios. Photo by Jason and Laura Photography.
The sensory nerves of the hands and feet each have a specialized capsule on the peripheral end called a mechanoreceptor that physically links the nerve ending to the surrounding skin tissue. The physical sense of touch works like this: Mechanical deformations of the skin and soft tissues of the body cause a change in the shape of the capsule surrounding these nerve endings, which in turn detect this change in shape and produce a signal that is propagated to the rest of the nervous system, noting the touch’s location on the skin, the amount of force, and its velocity. Other touch receptors in the skin produce additional signals in response to the object’s temperature and shape, as well as the presence of chemical agents on the skin.
Sensory neurons transmit signals to the thalamus and areas of the cerebral cortex in the brain. The specific location of sensory neuron synapses in the brain determines how the touch signal is interpreted. All of our brains are similar in the broad arrangement of these sensory neurons, but research shows that “the details of the somatotopic map characterize each individual and are determined largely by experience [emphasis mine].”
The types of touch we experience throughout our lives affect the architecture of our brains, which in turn affects our interpretation and response to different types of touch. In general, “repetitive activation of a pathway strengthens those synapses, making it easier to pass information forward.” Thus, the more often we experience a type of touch, the better able our brains are to interpret that information. Conversely, if there is a lack of touch, the sensory neurons will not be activated, and the synapses in that neuronal pathway will never strengthen.
For more than a century, scientists have been intrigued by the electrodermal activity of the skin and its relationship to both the sympathetic and parasympathetic nervous systems. Even more compelling is current research showing a relationship between the skin and the central nervous, immune, and endocrine systems — especially the hypothalamic-pituitary-adrenal stress axis — acting in concert to control our body’s homeostasis. This communication between the skin and the body’s central biological and neurological systems occurs through local production and systemic release of classical hormones, neuropeptides, neurotransmitters, and biological regulators. The unique connection the skin has with the nervous and immune systems through touch is known as the neuro-immuno-cutaneous system, which displays effects through stimulation of sweat, capillary constriction or dilation, and even change in skin tone/color. This common phenomenon is one that many law enforcement and military agencies use in determining if a subject is telling the truth; they measure a change in the electrical properties of a body experiencing an emotional reaction, known as the psychogalvanic reflex or galvanic skin response.
Photo from Adobe Stock/Julia
Five Somatic Senses
Of our five physical senses, touch is unique, in that it’s actually five separate somatic senses controlled by an extensive network of nerve endings and touch receptors, mostly in the skin and connective tissues. There are four main types of senses, plus one anomalous sense (not skin related, called the vestibular system, which is responsible for balance and is centered in the inner ear):
- Mechanoreception perceives pressure (deep, gentle, and sustained), texture, vibration, stretching of skin, and the rotational movement of limbs. Its receptor cells are generally found in non-hairy skin such as the palms, lips, tongue, soles of the feet, fingertips, eyelids, and the face, as well as deeper in connective tissue along muscle, tendons, and joints.
- Thermoreception perceives the sensations of an object’s temperature through two types of receptor cells (heat and cold) located all over the body, with the highest concentration found in the face and ears.
- Nocireception detects pain or stimuli that can cause damage to the body: mechanical (scrape, cut, tear, break), thermal (burn or freeze), or chemical (toxins from plants, animals, or synthetic). It’s no surprise that nocireceptor cells number in the millions; are found throughout the body, not just in the skin; and perceive different types of pain, including sharp, piercing, dull, throbbing, pins and needles, tickle, and itch, to name a few.
- Proprioception senses the body’s position and motion in space and relative to other parts of the body. There are two types of receptors: one measures stretch, which is located in muscles and tendons, indicating limb location; the other, which is located in cartilage, measures stress load and slippage in joints, indicating limb speed and direction.
Information gathered through all the somatic senses is channeled through the spinal cord and is processed in parts of the brain that produce neural maps specialized for touch and movement. These maps allow you to rapidly identify one body part from another and determine their unique locations and the physical action required to generate and coordinate optimum motion efficiently, whether you’re consciously fine-tuning movement in a yoga pose or letting the autopilot of your subconscious handle low-level repetitive tasks like scratching an itch. These maps are updated constantly with new information generated by new experiences inputted through the somatic senses, developing a constantly changing sense of self.
Apart from these maps for touch and movement, even more remarkable is a third map for the visceral terrain of your inner body — your internal organs and glands. According to Sandra and Matthew Blakeslee, “This map is uniquely super-developed in the human species, and it gives us a level of access to the ebb and flow of our internal sensations unequaled anywhere else in the animal kingdom.” Although each of us has the ability to feel at this level, for many it remains part of the subconscious. For those of us who can sense our visceral terrain, our emotions are often the facilitators that bring this subtle awareness into the conscious realm. This is especially true around the heart — think heavyhearted (sadness) or lighthearted (joy) — and the belly — butterflies (anxiety/fear) and fire in the belly (passion/courage).
Excerpted from Gravity and Grace: How to Awaken Your Subtle Body and the Healing Power of Yoga by Peter Sterios. Sounds True, October 2019. Reprinted with permission.