Acute Pain
What is a acute pain?
Think back to the last time you experienced pain. What are some words that can describe that feeling? Maybe sharp, stinging, dull, annoying, frustrating, miserable, unbearable, or terrifying?
You might notice that the list above contains some words that relate to physical characteristics (e.g., sharp, stinging) and others that relate to emotional characteristics (e.g., terrifying, frustrating).
It's no surprise, then, that the term "pain" itself is defined as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage" [1]. Pain is a perception that is always subjective, meaning that only the person experiencing pain will really know what it feels like.
Pain can be acute or chronic. Acute pain is pain that often happens after an injury, disease process, or medical procedure that heals within 6 months. Acute pain can actually be a helpful signal that we should not do something that would cause tissue damage in the future (e.g., if you put your hand on a stove when the burner is on and feel pain related to a burn, you will learn not to put your hand on the active burner again).
Chronic pain is much more complex and is not considered a helpful phenomenon.
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Nociception vs. Pain
Imagine turning on your sink’s hot water nob. You stick your hand underneath the faucet to feel the temperature of the water. Although it starts out as cold, the water eventually warms and then gets hot. You keep your hand under the water just until the sensation becomes painful and then retract it quickly. How can you sense the moment that “hot” becomes “pain”?
The detailed answer is complicated, but the broad answer involves an exchange of neural activity between your peripheral and central nervous systems (PNS
and CNS, respectively). The CNS includes all of the neurons in your brain and spinal cord, and the PNS are all of the neurons that lie outside of that.
We have specialized neuron receptors in the PNS for tactile, or touch, sensations. These “cutaneous receptors” include thermoreceptors (help sense temperature), mechanoreceptors (help sense pressure), chemoreceptors (help sense chemicals, like capsaicin in spicy peppers), and nociceptors (involved in painful sensations). In our example of the sink above, both thermoreceptors and nociceptors are key.
When the cold water hits our skin, thermoreceptors are activated. This helps us sense the temperature. As the water transitions to hot, our thermoreceptors keep working to let us know that we can continue to keep our hand under the sink without expecting damage to our skin.
When the water gets just hot enough for what your neurons would sense as potentially harmful, nociceptors join the party and start to activate to send your CNS a warning that you should remove your hand to avoid potential damage to your skin. Nociceptors are “pseudounipolar” neurons, meaning they have one long axon that ends in the periphery (e.g., skin, muscle) and extends to its cell body in the spinal cord. Not all nociceptors are created equal; they are characterized based on the axon’s (1) diameter width and (2) presence of myelination. These axons are often referred to as “primary afferent fibers.”
Myelinated axons (called A-delta and A-beta fibers) transmit sharp or pricking localized pain sensations, like after getting a papercut. They send the signal quickly through the PNS because of their myelin. In contrast, unmyelinated axons (called C-fibers) transmit more widespread pain sensations, especially coming from heat and pressure. It’s the pain you might describe as burning or aching.
At the onset of a harmful sensory stimulus, there is a rush of inflammatory and other chemical molecules released from neurons and non-neurons at the site of injury. Once a sensory input activates a nociceptor, the signal gets transmitted from the nociceptor soma in the spinal cord (called the first-order neuron) to a neuron in the spinal cord that extends to the brainstem (called a second order neuron). Depending on the amount of molecules released, there might be a more persistent or greater signal at the level of the first-order neuron, From the second-order neuron, the signal is further passed from the brainstem to other brain structures. This process is shown in the video above.
A variety of brain regions involved in sensation, emotion, memory/learning, and cognition work together to decide how painful the nociceptive signal is, where it's coming from in the body, and what kind of emotional response or action should happen as a result of the pain (e.g., pulling your hand away from the hot water stream).