The Sense of Touch

Generalities.

    Touch is probably the most stimulated of our sense and the one that we pay least importance. Whether it is an handshake, the caress of the wind, the weight of your clothes or the pressure points under your feet when you walk, the sense of touch is constantly activated. There are several aspects to that sense, it allows us to know the texture and the temperature of an object, and adjust our gait if we walk on a pebble. We can classify the touch (or somatic sensations) with respect to the type of receptor involved: 1) the mechanical sensations to sense tissue motion, 2) thermal sensations to detect hot and cold, and 3) the painful sensations associated fatigue or tissue destruction. Mechanical sensations can further be divided into exteroceptive sensations (which is perceived at the body surface) and proprioceptive sensations (which are more or less unconscious and tell us about the position of our body in space, balance and visceral and deep tissue sensations).

    Although I will describe separately these different types of touch, most often these senses are solicited in concert and it is the integration of this information, in the central nervous system (brain and spinal cord), which really tells us about the nature of the sensation.

    I close my eyes and I feel something in my hand. My skin receptors distinguish the edges and corners of a cube. Then, I can appreciate its texture and its temperature. The tension I need to put in my fingers, my hand, and my arm muscles, to counter gravity, tells me more about the size and the weight of the cube. Then, if I hold it long enough, the pain in my muscles will remind me their level of fatigue and their need for nutrients and oxygen. And, of course, for as long as I keep my eyes closed, I can not know its color is.

Somesthetic Sensations.

    In the skin, there is a host of receivers that we use to feel things. Each receptor type has its own function and, together, they often contribute to a unique sensation.

    The free endings are unmyelinated (lacking insulation) and infiltrate the dermis. Their functions are to transmit heat and pain information. Other endings can curl around hair roots and participate in mechanoreception (sensation of touch).

    Some free endings could be found in the deeper layers of the epidermis, particularly on the palmar surfaces of our hands and the soles of our feet. These endings are attached to modified epithelial cells called Merkel cells and participate in the mechanoreception.

    Pacinian corpuscles are located in the dermis. They are found especially in the fingers, genitals and breasts. They have from 1 to 4 mm long and 0.5 to 1.0 mm in diameter. At its center there is a unmyelinated nervous terminal, surrounded by flattened cells and concentrically arranged. These mechanoreceptors provide the subtle sensation left by a small pressure. Because they adapt quickly, they are particularly useful to detect movement.They also respond to vibrations. We can also found them in other locations such as the joint capsules and the bladder wall where they convey proprioceptive information (unconscious).

    The Ruffini corpuscles are small and elongated, they are about 1 mm long by 0.1 mm in diameter. They are located deep in the dermis and are quite similar to receptors located in the tendons. They provide information about the stretch state of the skin. They provide information on the likely state of skin stretching. Their signal, rather persistent, convey static information about steady state of things.

    The bulbs of Krause are small bodies located in the dermis of mucosal tissues such as the eye, the eyelids, the tongue and the external genital mucosa. They are also mechanoreceptors.

    Finally, Meissner corpuscles are small mechanoreceptors located mainly in regions of high sensitivity such as the palmar surface of the hand and fingers, the soles of our feet, our eyelids, lips, the external genital areas and nipples of the breasts. These receptors are located near the surface of the skin, at the junction of the dermis and the epidermis. They are very sensitive and could be involved in the detection of textures.

    Pacinian, Ruffini and Meissner corpuscles send their information at speeds of 30 to 60m/sec, via large myelinated fibers. Most of the free endings and those associated with hair root, use smaller myelinated fibers to transmit their information at a speed of 6-15m/sec. Altogether, these receptors also transmit information about vibrations. The fastest, such as the Pacinian corpuscles, distinguishes vibrations up to 400-500Hz. The others, such as the Meissner corpuscles, which adapt more slowly, are responsive to the vibrations bellow 80Hz.

Proprioception.

    Proprioception is the perception of our body in space, and most of the time unconsciously. Our brain knows the state of contraction of our muscles and the flexion state of our joints. It can use this proprioceptive information to coordinate movements and reflexes, as well as to maintain balance. While essential, proprioception cannot alone maintain balance and coordinate our body movements. We also need the visual information and the sense of equilibrium (provided be the semicircular canals of the inner ear) which are integrated in the same brain areas and the cerebellum, to fully perceive and control our body.

    Indeed, there are receptors specialized in detecting the degree of stretching of the muscles and tendons. In the joint capsule there are many Ruffini corpuscles which are stimulated by movement of the joint. In tendons, there are specialized receptors, Golgi tendon organs (or Golgi body), which consists of multiple nerve branches interlaced with elastic collagen fibrils. Stretching of the tendon stimulates afferent nerve fibers that transmit information to the spinal cord. If we measure the electrical activity of some of these receptors, we find that they do not respond all at the same angles of rotation of the joint. For example, some fibers have pronounced activity when the joint sit at 60 degrees angle and would be silent otherwise. Others would respond better when the joint rest at 150 degrees angle.

    In the muscles, some muscle fibers are modified to receive a coil of nerve endings, thus forming a network of muscle spindles. The state of stretch, or contraction of the muscle is relayed to the spinal cord and then the brain. The proprioception parameters will be integrated and a motor adjustment will be performed, if correction is needed.

Muscle spindle.

    Proprioception is important for the reflexes which could be evaluated by simple test. What is going on when the doctor taps on your knee with his little hammer? He asks you to put your leg in half-bend position. Then he strike on your knee tendon and make it stretch. This stretching, is interpreted as if the knee is bent more than supposed, and the information is immediately send to the dorsal spinal cord. This signal passes, through a short inter-neuron, or directly to a motor neuron that emerges at the ventral horn of the spinal cord and innervate the muscles of the thigh (quadriceps). The reflex contraction of the thigh muscles will cause a leg extension.

    The sum of these reflexes serves to maintain physical homeostasis (aiming at steady state) of the body. They are happening so fast that the brain is informed of the new situation only some times after these reflexes occurred. For example, if you walk on a small rock your gait will be readjusted before you actually fell the rock. Failing to achieve proper gait modifications will make you trip and hurt yourself.

Thermo receptors.

    Three types of receptor are involved in the sensations of temperature: heat, cold and pain receptors. Pain receptors, however, are activated only by extreme heat or cold.

    Below 10°C, the pain receptors are activated. From 10°C to 15°C, pain sensation stops and the cold receptors become activated. Between 25°C and 35°C, which is within the comfort zone, the cold receptors decrease their activity and the hot receptors start to increase theirs. Above 45°C, hot receptors' activity ceases and this heat strats to activate the pain receptors.

    The electrical activity transmitted by these receptors depends on the rate (speed) of many biochemical reactions and those rates are influenced by the temperature. For each 10°C, the average rate for those biochemical reactions varies by ± 2 to 3 times (refered as the Q10). Furthermore, the frequency of the temperature-dependant neuronal activity is not simply linear, it is complex. In adition, these receptors also have the ability to adapt, at least partially, in about thirty seconds. Thus, when the skin temperature is kept at a stable, hot or cold temperature, the perception of warming or cooling is obliterated. This tempering of the signal allows us to better cope with environmental changes.

    The number of receptors affected by a change of temperature is also very important to define the degree of precision with which we can detect these changes. This is because of the spatial summation phenomenon: the more cells stimulated, the better we could achieve precision (i.e.: the average firing of 10 cells is less accurate than for a 10000 cells). If we are completely immersed in water, we can detect variations as small as 0.01°C whereas if these changes affect only 1 cm square of skin, the minimum variation that could be detected is about 1.0°C.

Pain Sensations.

    The pain is first and foremost a defense mechanism. It triggers a reflex; when your hand touches an heated surface and you take it away so fast that you don't even feel the pain, yet! There are different types of pain: acute and localized like a pinch or cut, burns' pain, and pains which are not localized such as headaches, muscle pain or stomach cramp. People also use other term like throbbing, stabbing, cramping, nausea, electrical pain and many other descriptive to define pain. It is difficult to describe pain because there are several mechanisms involved in its perception. Therefore it is difficult to measure and understand. We can only learn from the results of specific tests using specific methods to induce pain and measure its intensity.

    One can measure pain by pricking the skin with a needle or by exposing it to a heat source. One can define a threshold for pain, it would be the intensity of a stimulus that will trigger, only half of the time, a pain response. We can also measure the minimum change in intensity required to be perceived as a change in intensity of pain. And, we can measure the duration of the perception, its location and its extent. Even if we want to be rigorous, we need to keep in mind that these measures maybe subjective: they are performed on subjects who may have different resistance to pain (it is well known that guys are tougher than girls ;-)).

    Another way to describe the pain threshold is to test the subject at different intensity and different duration of the stimulus. For example, using a lamp and a lens, we produce a hot spot on the forehead of the subject, we vary the intensity of the lamp and the duration of the stimulus. In such case we obtain an asymptotic relation showing that when the intensity of the stimulus is high, it takes little to induce pain, whereas if the stimulus intensity is milder, it needs to be applied for a longer duration to be perceived as painful. This test considers the threshold of pain being the minimum stimulus intensity necessary to induce pain when applied for an infinite time, the asymptote (parallel to the X axis) of this intensity-duration relation. It is a reliable definition that demonstrated that almost everyone has the same pain threshold to heat of 45°C on the skin. Not everyone, however, react the same way to the various types of pain, and cope with it.

    In a similar experiment, determination of the smallest possible change in the stimulus intensity that could be detected led to the observation that an average person could distinguish 22 pain levels, between a barely perceptible pain and one that we can no longer endure.

    The pain receptors (unmyelinated free nerve endings) do not adjust their electrical activity when stimulated, they do not adapt. This means that the perception of pain does not fade away with time. For example, as long as tissue damage is present, the pain will persist and we will be more careful not to hurt those tissue. Sometimes, as for inflammatory pain, the threshold for receptor activation gets progressively lower as the stimulus persists and the pain becomes more painful. This increased sensitivity is called: hyperalgesia. When cell destruction occurs, all kinds of molecules are released and may be involved in the transmission of pain. Wasted tissue, lack of oxygen, lactic acid molecule like kinins build up and causes pain.

Central Pathways.

    Sensory information from the body, not the head, enters the central nervous system by the dorsal horns of the spinal cord. The cell bodies of the sensory afferent are located in the dorsal root ganglia (DRG), near the dorsal horns of the spinal cord where they send there axons to. In the central nervous system we distinguish two main ways of transmission: the lemniscal and the spinothalamic pathways. Here are some characteristics for those pathway.

    In the lemniscal pathway the first class axon of the mechano- and proprioceptive receptors enters in the dorsal column of the spinal cord. At this level, the axon gives rise to a collateral axon that may participate in spinal reflexes, and main axon continues upward in a dorsal column nuclei to reach the dorsal medulla where it will synapse with a second order neuron. This second-order neuron decussate immediately to the other side of the medulla and reach the thalamus. The face and the head sensory information from the trigeminal afferent also reach the thalamus. The fact that this second order neuron decussate across the nervous system means that the somatosensory information from the left side of the body will be analyzed by our right brain, and vice versa.

    From the thalamus, a third-order neuron emerges and projects to the post central gyrus of the cerebral cortex, the somatosensory cortex. All the way up, and more noticeable at the level of the somatosensory cortex, these fibres are organized in a somatotopic fashion, so that the fibers from the lower body parts are found close to the center of the nervous system and those from the upper body are found more laterally. The resulting organization is called the somatosensory homunculus (illustrated on the left). This representation tells us that the face and the hand are highly represented at the level of the sensory cortex compared to the arms and legs which are less represented.

    This organization suggests a linear relationship between the number of peripheral receptors and their representation at the sensory cortex. In addition, there are three levels of neurons involved in the signal transmission, with relays in the brainstem and thalamus. At each relay, there is a convergence of signals to increase the reliability of the information. And, there is also a system of lateral inhibition between sensory fibers where the excited neuron inhibits its neighbor thus increasing the discrimination between those two points (the strongest signal prevails and is better localized, while surrounding signals are damped).

Convergence and lateral inhibition of sensory activity.

    The spinothalamic pathway carries information that does not require a transmission as fast and precise: it carries pain, hot, cold, and sexual sensations. As for the lemniscal pathway, sensory fibers of the spinothalamic tract enters the spinal cord via the dorsal horn. Then, the fibers travel up or down for a few segments of the spine and connect with a second order neuron. This second order neuron will cross the spinal cord (decussating) to the other side and to the ventral region, and then travel up to the thalamic nuclei. Finally, a third-order neurons relay information to the primary and secondary sensory cortices.