In contrast to chemical senses (taste and smell), mechanical senses are more like levers and gates. Something must be pushed or moved in order to create a sensation. Mechanical senses detect changes 9n pressure, acceleration, vibrations of tissue damage.
Pressure is recorded by touch receptors. The inner ear signals acceleration. Itch is vibrations detection. And pain is the result of tissue damage.
Touch
Touch is our primary tactile sensation. It is our system for the detection of pressure and vibrations. Like many things in the body, touch receptors are not evenly spread out. Your lips and finger tips are much more sensitive to touch than your elbow and big toe.
Touch can be passive or active. Both include a change in pressure on the skin. Both are mechanical. A pressure plate is depressed, and the change is transducer into a neural signal and sent to the brain.
Passive touch is when something touches you. Active touch, also called haptic, is the exploration of an object or surface. Haptic touch feels multi-sensory because it often included the use of sight, hearing and other senses too. We rarely explore with limited inputs. We try to use whatever resources we have available.
Touch is the detection of pressure. Pressure on receptoropens causes sodium channels in the axon to open. If enough sodium enters, an action potential occurs, sending signals to the brain.
Touch comes in two types: light and deep. Light touch comes from two receptor types which lie close to the surface of the skin. These receptors, Meissner’s and Merle’s, work together.
Meissner’s corpuscles are close to the skin’s surface, and are plentiful on the lips, finger tips, palms, and foreskin. They report light touch, slow vibrations and texture changes. Meissner’s signal onset-offset. They fire once when a coin enters your hand, and again when the coin leaves your grasp. These are the equivalent of a store ‘s articulator which beeps when you enter the shop, and beeps again when you leave.
Merkel’s discs report sustained pressure. They fire for as long as you hold a coin. They only stop when the coin is no longer in your hand. They are close to the surface, are slow adapting, and cover your fingertips.
Deep in the skin are receptors for deep touch and heavy pressure.
Ruffini’s end organs are sustained pressure, and slow adapting detectors of deep pressure. They report skin stretch, are tell you where in the hand your coin is. They stop firing when you drop the coin.
Pacinian corpusles report fast vibrations; and deep pressure. They are fast adapting, and be found in the hand, under the skin, and in joints (reporting the location of your limbs). They are onset-offset receptors, indicating when a coin enters and leaves the hand.
Skin mapping (looking for what is where) shows four things. First, sensations are not consistent across the skin. the lips and tongue are sensitive to touch and temperature but the thigh isn’t. When the thigh is cold, it’s the whole thigh. The fingertips have much smaller zones.
Ernest Weber (1795-1878 explored the just noticeable difference thresholds. When can we tell and not tell the difference between changes in stimulation. With touch, Weber used two pins which touched the skin at the same time. The question is when do they feel like one prick instead of two. The answer varies on location. The zones of detection on the tongue and fingertips is 2-3 mm. Wider than this, it is two separate sentences. Within this zone, it feels like one pin prick. The forehead zone is about 20 mm wide. The 2-point discrimination zone on the back of the hand is 60 mm.
Second, locations shift over time. Ask any diabetic who tests glucose levels with finger sticks; stabbing yourself in the same spot that didn’t hurt last time doesn’t mean it won’t hurt this time. Things under the skin aren’t nailed down; they move and shift. Receptors float.
Third, specific sensations do not correspond with specific receptors, all the time. Touch receptors, for example, respond to pressure but they also report some level of vibration and pain.
Fourth, there is a hierarchy of sensations. There are more spots that report pain than pressure. And there are more spots that detect pressure than temperatures. Touch is more important than temperature but pain is the highest priority.
Temperature
We can detect changes in temperature but the zones are often quite large. The thigh is a single zone. Feet are a combination of spot and zone detection. You might feel a piece of ice on your toe but you usually say that your foot is cold. We cover the whole foot when cold. Or we cover our heads.
Heat rises, and in humans that means it flows out the top of your head. So, when your feet are cold, it does actually make some sense to put a hat on to reduce the amount of heat loss. It is an indirect cure that often works.
We have two independent systems for temperature. Instead on one thermometer measuring absolute temperature, we have two relative systems. One reports warm and the other cold. As the temperature rises, relative to our internal temperature, we feel warmer. The firing rate of warm receptors increases. When the the warm firing rate decreases, we feel less warm but not cool.
Our cold system works in a similar fashion. As our skin is detected as becoming colder, the firing rate of cold receptors increases. As we warm, the cold firing rate decreases.
We feel the coolest when the warm firing rate decreases, and the cold firing rate increases. Touching something cold and something warm at the same time produces hot.
Extreme hot and extreme cold cannot be distinguished. This paradoxical cold can be experienced when you tough a hot stove. There is a moment’s pause while the brain works out what it is feeling. Extreme cold works the same way.warm and cold sensors are both triggered, and the brain has to figure out what is happening.
All temperature measurements are relative to physiological zero, the temperature of your body. If your body is cold, cool will feel warm. If you have a fever, a warm cloth will feel cool. We are very good at making relative judgments. We cannot make absolute measures.
Pain
Pain is the detection of tissue damage. It does so using two systems. There is a system for immediate damage, and another system for chronic damage. Sharp and dull pain are independent systems They travel in different areas of the spinal chord, and treatment for one is not usually effective for the other.
Sharp pain is often “good pain.” It tells you to pi-utter down the hot frying pan. It is good because it delivers its message, and after you put down the pan the pain goes away. A serve burn takes longer to go away but it does disappear.
Sharp pain travels quickly to the brain so something can be done about it. There is an immediacy about it. It interrupts other thoughts and actions. Sharp pain travels on thinly myelinated A-Delta nerves.
Nerves come in varying thicknesses and wrappings. A-Alpha are the thickest nerves. Think of them are thick, heavy duty wires, or large diameter hoses. They are also covered with a thick layer of insulation. These large, heavily myelinated nerves carry instructions to the muscles. They are for quick reactions. Signals can travel at 265 mph.
A-Beta nerves report tough information. They are largish, myelinated nerves that travel at 165 m-h.
Sharp pain, onset touch and heat information travels of A-Delta nerves. These are small, thin nerves. They are thinly myelinated. They travel at 75 mph.
Dull pain, sustained touch and itch information travel on unmyelinated, thin nerves called C fibers. They travel at 2 mph. Dull pain is meaner. Even after you remove the heat, dull pain continues. Arthritis hurts for hours, cancer pain hurts for days. Dull pain lingers. It signals tissue damage.
When you move your foot, A-Alphas send proprioceptive information to the brain, indicating where your foot is. When you stub your toe on an object you forgot to put away, the collision signal travels to the brain on A-Beta neurons. Sharp pain arrives on A-Deltas, and dull pain arrives last via C fibers. One event produces a cascade of responses.
All tactile senses except pain adapt quickly. Touch and pressure reset quickly. Pain lasts. Sharp pain lasts as long as the thorn is in your paw. Dull pain lasts longer. The survival function of sharp pain is to remove the offend object. The function of dull pain is to notify you of damage, and the need to rest and recover.
Detecting Pain
Nociceptors, pain receptors, are bipolar neurons whose cells are in the dorsal root of spinal cord Damage to the skin, muscles, joints and organs is sent up the back of the spine to the brain. Damage to the back causes a decrease in sensation but still allows movement and counterattacks because the motor neurons travel down the front of spinal cord. This design separates sensation and movement into independent systems.
The degree of pain depends on the level of stimulation. Lots of stimulation equates to lots of pain. The other factor is the sensitivity of the receptors. There are several types of nociceptors1 to choose from. But none of them exclusively report pain.
Pain is not the result of stimulating pain receptors. There are none. That is, there are none which only report pain. Instead, the body uses other receptors already present to achieve the same result.
First, pain can be triggered by thermal nociceptors. When there are extreme temperatures, the sensation can be perceived as pain.
Second, all touch receptor, except Pacinian corpuscles, can report pain. When the pressure is to great, we experience it as pain.
Third, some nerve endings act as silent nociceptors. They respond to inflammation and chemical changes. Once activated, they are sensitive to thermal and mechanical stresses too
Fourth, the body has some polymodal nociceptors. They respond to everything. They report thermal, mechanical, chemical and pain signals.
Relieving Pain
Anesthesia prevents nerves from firing. Novocain, for example, blocks the sodium channels; they can’t open, regardless of how much stimulation they receive. This shut down of neural impulses impacts all mechanical receptors in the region. Temperature is the first to be blocked, followed by pressure and then pain. They go out in this order, and return in this order.
Diabetic neuropathy often produces shooting pains. Nerves close to the surface get triggered spontaneously. For some people, this discomfort can be treated with a localized application of capsaicin. Capsaicin is found in peppers, and causes its own kind of tingling, which you either like of wish to avoid. But as a treatment for pain, it works by providing a streaky stream of stimulant to pain receptors, overwhelming them and blocking their signals. Prescription versions of capsaicin are available in lotion and patches.
Steroids reduce the inflammation in a joint, so there is less rubbing of bones. Synthetic steroids try to mimic the natural ones released by the adrenal gland. They tell the immune system to temporarily ignore this area; don’t send healing cells, just leave it alone.
Tissue damage causes inflammation. So not sending histamine and other agents is one approach to pain relief . It doesn’t take away the damage, just distracts from it temporarily.
Another way to distract ourselves from pain is self-hypnosis. The best known system of attentional distraction is the Lamaze method, used in child birth to distract from the pain. The theory is that you can’t feel the pain is you are focusing on your breathing. You might get the same effect by shouting out your multiplication tables but breathing is more elemental.
Long distance runners focus on their breathing and running rhythm to ignore the pain of overexertion. They self-hypnotize themselves into a trace, an altered state of consciousness they call a “runner’s high.” Similarly, people with chronic pain learn to focus their attention on something else.
There are, of course, limits to self deception. Sometimes we need help. Opioids would be the first choice, except for the drastic side effects of addiction and death. Hydrocodone is the most used opioid to treat of pain. Morphine is still used to treat cancer pain but only in terminal patients. Instead, we rely on non-steroidal anti-inflammatory drugs (NSAIDs).
The goal of NSAIDs is to block the pain signals before they reach the brain. When tissue is damaged, prostaglandins are releases which trigger pain signals. Stop these proteins from being released or synthesized, no pain is felt.
There are three major enzymes which synthesize these prostaglandin proteins. They are cyclooxyrgenase 1 (Cox-1), cyclooxyrgenase 2 (Cox-2), and cyclooxyrgenase 3 (Cox-3). Some substances, like aspirin, block all three. Acetaminophen (Tylenol) blocks Cox 3 only, but doesn’t irritate the stomach as much. It is also less likely to cause of Reye’s syndrome.
As much as we dislike pain, it is essential to our survival. There is a rare condition called Congenital Insensitivity To Pain. Born without any sense of pain, patients continue activity after injury. They don’t know when to stop. They can’t detect broken bones or even severe gashes. They often get pressure sores and damaged joints.
Once pain signals get to the brain, there is further analysis. Some people like the pain of spicy food. Some like the physical pain from being spanked mixed with the euphoria of orgasm. What is pleasant to one can be unpleasant to another.
Cultural differences are usually in the expression of pain. Not displaying pain in particular, and emotion in general, varies by culture. We feel the same sensations but don’t necessarily share our experiences with others.
We use the same system for real sensations and imagined experiences. We use visual systems for seeing and imagining. We use the same spatial reasoning processes for actual objects and creating mental images. And we use the same system for real pain and emotional pain. When you have a romantic breakup, treat it like a headache: take some Tylenol.
The surgical removal of any body part can result in the perception of pain from the missing structure. Phantom pain is the result of the brain not making sense of the lack of feedback signals. The phenomenon is most common in the amputation of a limb (95-98%) but can occur after the removal of an eye, or organ. The sensations can dissipate quickly or never go away. There is no reliable treatment.
In general, pain can be assessed using the acrostic WILDA, which stand for Words, Intensity, Location, Duration, and Aggrieving factors. How people describe their pain can be a helpful guide to their discomfort and the impact it is having on their lives. In addition to Words, Intensity (usually on a scale of 1 to 10) , Location (exactly where does it hurt) and Duration (how long has this been going on) are important. The consideration of Aggravating events rounds out the assessment because we often forget to mention others things that are going on in our lives.
Itch
Itch is the perfect counterpoint for pain. when one goes up, the other goes down. Opium addicts who are feeling no pain are going crazy with itch. And if you’re scratching, you’re not itching. We scratch because the pain of doing so interrupts our pain. Unfortunately, vigorous scratching causes pain, which causes itching, which causes scratching.
When tissue is damaged, histamine is released. It increases the permeability of capillaries, allowing more white blood cells to be released at the damaged site. it also sends an itch signal to the brain so any irritant can be brushed or scratched off the skin. When you scratch, neuropeptides are released which trigger the release of more histamine, and the cycle continues.itching can also occur from contact with certain plants but the itch-scratch-itch cycle is the same.
Like pain, itch uses slow unmyelinated C fibers to transmit signals to the brain. Along the way, gastin-releasing peptides are released in the spine. These g-protein mediated neuropeptides provide extended effects, keeping itch sensations from quick adaptation.
Like pain, itch is perceived pretty evenly across the skin. Like pain, itch originate in the skin but they are two distinct systems. Itch occurs only in the two top layers of skin, and doesn’t occur in joints or muscles.
Like pain, itch can be caused by localized conditions or by systemic issues. -Robles can occur any way along the neural pathway. Central nervous system diseases, such as multiple sclerosis, can cause itching sensations which are independent of skin conditions. You feel itch even when there is nothing causing it. It is similar to the itching sensations felt by opioid use. Whatever causes the reduction of pain increases the likelihood of itch.
Like pain, people vary in sensitivity. Interestingly, if you are sensitive to pain, you will be insensitive to itch. Unlike pain, itch is contagious. We can begin to feel it by reading about it, talking about it, or by watching others scratch.
Treatments for itching have included mild electric shock, noxious heat and cognitive therapy. But the most common treatment is taking antihistamines.
Hearing
Want to jump ahead?
- What Is Perception?
- Perceptual Efficiency
- Vision
- Taste
- Smell
- Touch, Temperature, Pain & Itch
- Hearing
- Vestibular
- Visceral
- Proprioception
- Time
Photo by Jackson David on Unsplash
