Blog Post #1: The Physiological Sensation of Pain
William B. Rhodes
Pennsylvania State University
Don’t you hate when you catch your dog chewing on something they aren’t supposed to have? As instinct would have it, you yell for them to “drop it!”, and of course they take off furiously running away from you while trying to swallow it. Those are always the moments in which there are only two outcomes: you successfully get the object back from them, or you miserably fall victim to circumstance and inadvertently injure yourself as they get away with what they’ve stolen. This was the situation I found myself in when my dog stole a piece of pizza crust off a plate on my coffee table. As I ran after him I unfortunately fell victim to the latter fate and discovered the pain of stepping on a Lego while running after him barefoot. As I laid there sprawled out on the tile floor, Lego protruding from the sole of my foot, I couldn’t help but wish that our bodies weren’t so perfectly wired to perceive the sensation of extreme pain. Why couldn’t stepping on a Lego feel the same as stepping on a grain of rice? The answer to that ultimately lies in the electrical functioning of our nervous system.
As most of us know, our bodies have a complex sensory system comprised of 5 basic senses; sight, smell, hearing, touch and taste. Each sense has a corresponding set of sensory receptors and neural pathways that receive and transport sensory information to key areas of the brain that are responsible for processing these sensory signals. This concept sounds simple at its basic function, but in reality, these systems contain dozens of substructures and complex processes that work in unison to provide our brain with information about the world around us. More importantly, these complex structures and processes have the ability to not only let us know that we are seeing, smelling, hearing, touching or tasting something, they have the ability to relay the intensity of these senses we are perceiving. This is vital to our survival because it gives us the ability to differentiate between potentially harmful stimuli (such as stepping on a Lego), harmless stimuli (such as stepping on a grain of rice) and pleasurable stimuli.
For purposes of this blog post I will only be discussing the structure and function of nerve cells related to the skin and our perception of pain, temperature, touch, etc. According to Zimmerman, Bai, & Ginty (2014):
Mammalian skin comprises both hairy and nonhairy, or glabrous, skin. Glabrous skin is predominantly found on the hands and feet of most mammals. In this context, glabrous skin is specialized for discriminative touch, determining texture and shape to accurately recognize objects and providing feedback to the central nervous system to mediate proper grip control, reaching, and locomotion. Hairy skin covers more than 90% of the body surface. It also serves a discriminative touch role, albeit with considerably lower spatial acuity as compared with nonhairy skin. (p.950)
So, although hairy skin covers the majority of our bodies, it is our nonhairy skin that provides most of the sensory feedback required for safe interaction with the environment around us.
The first advances in understanding the complex communication system of our bodies were made by 19th century anatomists’ as they discovered what they called a “nerve net” by applying stains to brain tissue (Goldstein, 2015, pp. 28-29). Their early understanding of our nervous system was that it was a continuous network with no sort of mechanisms for starting or stopping signals. This was later disproved by Spanish physiologist Ramon y Cajal with the help of a new type of cell stain that had been invented by an Italian anatomist named Camillo Golgi (Goldstein, 2015, pp. 28-29). Golgi’s stain was unique in that only certain cells would absorb the stain, allowing the stained cells to show up in stark contrast to the other non-stained cells around it (Goldstein, 2015, pp. 28-29). Cajal used Golgi’s stain to observe that the nerve net was actually made up of individual cell structures that were connected together and not a continuous network of nerves. This discovery was integral to our modern day understanding of how neural networks function because it gave way to the discovery of different parts of a neuron and their function in signal transmission to the brain.
The structure of a basic nerve cell, called a neuron, is fundamental to understanding how our sensory systems are able to relay the intensity of a sensation we are experiencing. A basic nerve cell contains an axon and numerous dendrites that function to receive and pass along signals from one nerve to another (Goldstein, 2015, pp. 29-30). Dendrites look much like the roots of a tree and function to receive signals from other neurons close by. The axon is like the tail of the neuron, much longer than a dendrite, and is responsible for sending the signal to the next neuron. Neurons responsible for receiving stimuli from outside sources, such as in the skin, also have a structure called a touch receptor that detect stimulus from the outside environment and transmit this information along the neural circuit (Goldstein, 2015, pp. 29-30).
The key factor in the whether or not a stimulus is perceived by the brain lies in a function of the neuron called the action potential. An action potential can best be described as the threshold level of stimuli received by a neuron in order for it to pass the electrical signal to the next neuron. If a neuron’s receptor is stimulated with a strong enough signal, then it will pass that signal down its axon where it will be sent to the next neuron. If the incoming signal is not strong enough then it will not activate the neuron’s action potential and the signal will not be relayed down the neural circuit to the brain.
So, what does all of this have to do with why stepping on a Lego hurts more than stepping on a grain of rice? It all comes down to a discovery by Edgar Adrian in his study of the relationship between nerve firing and sensory experiences. Adrian started by observing the relationship between the amount of pressure being applied to the skin and the resulting nerve impulses (Goldstein, 2015, p. 32). He found that the strength of the incoming signal required to activate the action potential remained the same as he increased pressure on the skin, but, the rate at which the nerve impulses were fired increased as the pressure on the skin increased (Goldstein, 2015, p. 32). This meant that the intensity of the sensation (pushing harder) did not result in a stronger impulse being fired as many had thought, rather the intensity of the sensation resulted in nerve impulses being transmitted faster.
Because of this discovery we now understand more about how the nerves in our bodies interact with one another and transmit signals to the brain. Since stepping on a Lego hurts more than stepping on a grain of rice, our bodies perceive that pain differently by sending nerve impulses faster down the neural network so that our brain will receive the message as soon as possible. This allows the brain to process the painful stimuli and send signals to our muscles, telling them to stop running and dig the Lego out of our foot. What seems like a simple process has actually taken scientists a very long time to fully understand. With the building blocks these early scientists have provided, our modern day scientists have been able to continue the pursuit of knowledge about the nervous system and its complex interaction with and within the brain.
References
Goldstein, E. B. (2015). Cognitive psychology: connecting mind, research, and everyday experience (4th ed.). Stamford, CT: Cengage Learning.
Zimmerman, A., Bai, L., & Ginty, D. D. (2014). The gentle touch receptors of mammalian skin. Science, 346(6212), 950-954. doi:10.1126/science.1254229