Exteroception

The senses can be broadly divided into three categories. This classification was introduced by British physiologist Charles Sherrington in the early 20th century and is still used today.

• Exteroception — Senses stimuli from outside the body. Skin sensation is a typical example; more broadly, it also includes sight, hearing, smell, and taste.
• Proprioception — Senses the position, movement, and force of joints and muscles. It is the sense that lets you know where your arm is even with your eyes closed.
• Interoception — Senses the body's internal state, such as hunger, a full bladder, or the heartbeat.

Within the skin, different sensory receptors handle different types of stimuli. Meissner corpuscles and Merkel cells, which are abundant in the fingertips, mainly detect light touch, while Pacinian corpuscles and Ruffini endings deeper in the skin mainly detect firm pressure and vibration. There are also separate receptors for temperature changes and pain receptors that respond to potentially tissue-damaging stimuli.

Signals received by these receptors travel along peripheral nerves into the spinal cord, then pass through the brainstem and thalamus to reach the somatosensory areas of the cerebral cortex. Only there are they interpreted as “something touched the back of my hand” or “this is warm.”

Exteroceptive sensitivity is not uniform throughout the body. The fingertips, lips, and tip of the tongue have densely packed receptors that can distinguish very small differences, while receptors are much more sparsely distributed on the back and thighs. If the two points of a compass are placed on the skin at the same time, the fingertips can detect them as two separate points even when they are only a few millimeters apart. On the back, they need to be several centimeters apart to be felt as two points. This is also why the hands and mouth occupy unusually large areas in the brain's cerebral cortex.

Practitioners of hands-on manual therapy therefore use their fingertips to read tissue texture, temperature, and tension in a patient's body. In turn, the person receiving treatment takes in messages of “safety and comfort” through the pressure and warmth of the touch.

• You quickly get used to the feel of clothing. A clothing tag or the weight of glasses that initially bothered you can eventually become unnoticeable because the brain filters out exteroceptive signals that remain unchanged. This adaptation is a normal function.
• Warm hands help ease tension. When you gently rest a palm or place a hot pack on a stiff neck, a feeling of comfort comes before any change in the muscles themselves. This is because skin sensation affects the level of tension throughout the body.
• The same pressure can feel different depending on the situation. On days when you feel sensitive, or in ticklish areas, light contact can actually feel unpleasant, while a certain degree of steady pressure provides a sense of security. These differences are why people say “a little firmer” or “a little gentler” during a massage.
• Sensation may become dulled or change after an injury. Skin sensation around a surgical scar or in an area where a nerve was compressed may become dulled and then gradually return.

Some sensory receptors in the skin are responsible for pain. These receptors send warnings to protect the body when the skin is cut or exposed to excessive heat or cold. Interestingly, light-touch sensations and pain signals influence one another in the spinal cord. The relief you feel when you instinctively rub a spot you have bumped is explained by touch signals from rubbing meeting pain signals at a shared pathway. This is also related to why gentle massage strokes can feel soothing.

Integrative Manual Therapy (IMT) textbooks discuss proprioception, exteroception, and coordination together in relation to muscle strengthening. In this approach, sensory information entering through the skin is considered one of the inputs to the nervous system that controls movement, so skin sensation is viewed as something that should also be addressed when restoring movement ability. For example, when addressing function around the mouth, sensation in the lips and tongue is examined as well.

• Experience different textures and temperatures. Walking barefoot on grass or sand, or alternating between warm and lukewarm water when washing, enriches sensory input through the skin. Excessively cold or hot stimuli can stress the skin, so stay within a comfortable range.
• Gently stroke your body with your hands. Slowly stroking your arms and legs while applying lotion after a shower can help you notice which areas feel stiff or have dulled sensation.
• Try moving slowly with your eyes closed. Removing visual input lets you focus more on sensations from the skin and muscles. Trying this during yoga or stretching can make your sense of body position much clearer.
• Pay attention if sensation changes suddenly. If tingling or reduced sensation in an arm or leg on one side persists, it is a signal from your body, so consult a professional.