Distinguishing fine details—feeling the numbers on a debit card or knowing how hard to press on the screen of a smartphone—are examples of activities aided by discriminative touch. “Discriminative” in this context refers to the ability to distinguish between different shapes, textures, vibration, and other fine points of touch. A specific set of neurons and pathways into the brain is dedicated to this fine-tuned sense. Interestingly, the sensations of pain and temperature involve entirely different neurons and spinal cord pathways.
Discriminative Touch Experiment
Touch is the most basic sense for animals and discriminative touch (two-point discrimination) is essential for survival. It is used by animals to identify objects that can be tools to access or pick up food. Discriminative touch employs small receptor fields to increase the intensity of the touch sense. If a body region has an extensive number of small receptor fields, it will be more sensitive to external stimuli.
Two toothpicks or similar items
At least three people willing to be subjects
One person will measure distance in millimeters and record the results, one person (tester) will test the two-point discrimination of the different body parts, and the remaining people will be the subjects. If there are only three people, rotate positions so that each person takes a turn as a subject.
1.To test two-point discrimination, the tester will take two toothpicks and simultaneously place them about eight inches (roughly 200 millimeters) apart on the upper back of the subject. The tester does not need to push hard to cause pain, just enough pressure so that the subject can feel the two points. Ask the subject, “How many points do you feel?” If the answer is “one,” then measure and record the distance in millimeters between the two points. If it is “two,” then move the toothpicks closer together about one inch (25 millimeters) and simultaneously place them on the subject’s back.
2.Repeat step 1 until the subject feels only one point; measure and record the distance.
3.Repeat steps 1 and 2 on the back of the hand (starting distance will be about three inches—about 75 millimeters—apart) and ask the subject to close their eyes and turn their head. This is to ensure that the subject is only using the discriminative touch sense and not visual sense to determine the number of points. Move the toothpicks closer together about 10 millimeters at a time.
4.Repeat steps 1 and 2 on the fingertip (starting distance will be about half an inch—about 13 millimeters—apart) and ask the subject to close their eyes and turn their head. The finger is the most sensitive body part and if the points are too large, the tester will have a difficult time getting to “one point.” Move the toothpicks closer together about 1—2 millimeters at a time.
5.Average the distances for each body part and graph the results.
Jennifer L. Hellier
These special nerves generally fall under the category of “mechanoreceptors.” Physical stimulation depresses the neuron’s plasma membrane and ions rush into the cytoplasm, creating an action potential. This signal is relayed to the spinal cord and eventually to the contralateral parietal cortex in the brain. Here the signal is interpreted. With the addition of many individual mechanoreceptors, the brain is able to create a complete picture of a coin’s edge or the sticky side of a stamp.
Anatomy and Physiology
The first step in initiating a “touch” sensation is an action potential from one of the mechanoreceptors in the skin. The neuron’s membrane is depressed/stretched and ions rush in, creating an action potential. While this concept can be applied broadly to discriminative touch neurons, each behaves slightly differently, creating a more detailed ability to distinguish touch. An understanding of the characteristics of each of the corpuscles helps inform the entire process of touch.
Meissner’s corpuscles and Pacinian corpuscles are more similar than different. Both detect pressure, vibration, and the initiation/termination of touch. Once an initial series of action potentials are generated, these nerves slow the frequency of signaling so as not to overwhelm our sensory pathways. Take the example of putting on a shirt. Initially, the weight and texture of the material are sensed. This initial sensation is quickly ignored until there is a change such as tugging on the shirt, adding a coat, and so on. This characteristic explains why both corpuscles are described as “phasic” or “rapidly adapting.”
These two neuron types differ in location within the skin and specific stimulation characteristics. Meissner’s corpuscles exist on the most superficial aspect of the dermis directly below the epidermis. This allows detection of light touch and minimal vibration (approximately 10—50 hertz). They are found in highest concentration in the fingertips and the lips. Slightly deeper in the skin is where Pacinian corpuscles can be found. These neurons detect slightly greater pressure and are more sensitive to vibration (200 hertz). Interestingly, slicing a Pacinian corpuscle in two reveals an onion-like appearance of the nerve ending. This feature allows for the determination of specific surface textures like smooth or rough.
In contrast with the rapidly adapting mechanoreceptor, Ruffini corpuscles and Merkel nerve endings offer examples of slowly adapting mechanoreceptors. Existing deeper in the dermis, these receptors are sensitive to pressure and stretch. The need for a slowly adapting tactile nerve—where the action potential continues to provide information—is exemplified with a cup of water. Once the cup is initially grabbed, the rapidly adapting mechanoreceptors relay the amount of pressure needed to hold the cup as well as the surface texture. This initial signal decreases as long as the cup is held. If the cup began to slide, a signal generated at the Ruffini corpuscle would be relayed to the brain and adjustments could be made to the grip.
Adding another component to the slowly adjusting mechanoreceptors, Merkel nerve endings provide slightly different information than Ruffini corpuscles. The receptive field—or area of skin dedicated to one neuron—is smaller for Merkel nerve endings. These cells are also found in higher density in areas dedicated to discriminative touch such as fingertips. Additionally, these cells are extremely sensitive to skin stretch (only one micrometer needed, i.e., 10−6 meters). This allows for a fine-tuned, slowly adapting mechanoreceptor signal.
Once a signal is generated in either a rapidly or slowly adjusting mechanoreceptor, it then makes a rapid journey through the dorsal column—medial lemniscal system. The corpuscles previously mentioned make up the first-order neurons with cell bodies outside of the spinal cord. These first-order neurons are covered in insulation known as myelin and are known as fast-conducting fibers. The first-order neurons enter the posterior spinal cord and ascend toward a component in the brainstem known as the medulla.
At this point, the first-order and second-order neurons synapse at either the nucleus gracilis (sensation from the legs) or the nucleus cuneatus (sensation from the arms). The second-order neurons then cross to the contralateral medulla via the medial lemniscus pathway. These neurons continue to the thalamus where a synapse is made with third-order neurons. A signal from an initial touch sensation finally climbs to the postcentral gyrus or primary somatosensory cortex of the parietal lobe via the internal capsule.
Tactile stimulation from the skin is precisely ordered in the brain. This pattern or map of the human body within the postcentral gyrus is known as the homunculus. Imagine a small human image applied to the lateral aspect of the brain, with legs dangling into the sagittal fissure and the remainder of the body along the outside of the parietal lobe. A tactile stimulation from the hands is processed in a unique order and then compared to a similar stimulation from the feet. Though the journey is complicated—from corpuscle to postcentral gyrus—the total time elapsed is milliseconds, which is 10−3 seconds.
Disease and Disability
Taking a deeper look at the homunculus in the primary sensory cortex shows that not all parts are created equal. A larger portion of space is reserved for areas with more discriminative touch sensors. This dedicated space, however, is not stationary. If there is constantly more or less information from a certain peripheral location, the homunculus will restructure. Plasticity is the formal name for this restructuring.
Reading braille is an example of an increase in the amount of discriminative touch information relayed to the cortex. Persons with visual impairment have benefited from this form of written communication since the mid-1800s. Braille was initially a French military invention, a way to read without light. The series of small, palpable bumps are interpreted when fingertips gently pass over them. Utilizing the Meissner’s corpuscles, Merkel nerve endings, and the dorsal column—medial lemniscal system, a signal is interpreted in the primary sensory cortex.
Over time, a greater area in the homunculus is dedicated to the processing of tactile stimulation. The primary sensory cortex remodels, allowing for physically more space and numerically more neuronal connections to be made. This neurological plasticity explains why a person with visual impairments becomes advanced in “seeing” the world through physical touch.
An extremity amputation is the converse example to visual impairment in the primary visual cortex. The space devoted to a hand, for example, in the homunculus is enormous. If there is no tactile stimulation, this area eventually diminishes in size and other sensory areas will increase in size. An ipsilateral forearm might become more receptive to discriminative touch, taking over a larger area in the primary sensory cortex.
When considering the process of neurologic plasticity, however, a simplified explanation can be given. If the area of the homunculus slowly expands with more tactile stimulation such as reading braille, so too should the primary cortex area slowly shrink if tactile stimulation is absent. Unfortunately, the process of rearranging neurons can be imprecise. The new connections in the sensory cortex might confuse a person into thinking he or she is experiencing sensation—ranging from mild tingling to intense pain—in the missing limb.
A clinical application of discriminative touch relates to the disease of diabetes. Every six months to a year, persons with this affliction are asked to visit a physician’s office to have the sensation in their feet evaluated. This task is accomplished by gently touching the foot with a thin, flexible piece of plastic called a “monofilament.” The longer people live with diabetes, the more likely they are to lose discriminative sensation in their extremities. This process typically starts with the toes and slowly works up the foot.
The process of diminished sensation is known as “diabetic neuropathy.” How this takes place is a story of sugar. Diabetic people tend to have difficulty reducing the level of sugar in their circulation. If not well controlled, these sugar levels remain elevated and work into tissues such as the kidney, retina, and eventually small neurons. The excess sugar inside the neuron slowly builds up. Water molecules follow these additional sugar molecules and steadily cause damage to the neurons. Smallest, weakest-walled neurons are the first to be damaged. The various corpuscles and nerve endings dedicated to discriminative touch happen to be some of the smaller nerves in the extremities.
Why is this process of slow sensory loss important? Two concepts must be considered. First, damage to the tactile nerves in areas such as the foot can lead to wounds. Imagine a rock inside a shoe. A person without diabetic neuropathy would immediately sense the uncomfortable feeling and remove the offending agent. If this small irritant were not removed, it could slowly cause a blister or open wound on the foot. This becomes a problem with diabetics because of the second important concept: diabetes slows the speed and effectiveness of wound healing.
See also: Homunculus; Meissner’s Corpuscles; Phantom Pain; Sensory Receptors; Somatosensory Cortex; Somatosensory System; Touch
McGlone, Francis, Ake B. Vallbo, Hakan Olausson, Line Loken, & Johan Wessberg. (2007). Discriminative touch and emotional touch. Canadian Journal of Experimental Psychology, 61(3), 173—183.