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Pain science #1 – acute pain

We all know that pain is part of human life and everyday routine. For some, pain is present daily for even decades, while for others, it occurs less frequently. The International Association for the Study of Pain (IASP) has defined pain as follows: "Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage." It is particularly essential to note from the definition that pain does not necessarily involve tissue damage or structural injury. Biological, social, and psychological factors all influence the experience of pain. Pain can be categorized as acute (lasting 0-6 weeks), subacute (6-12 weeks), or chronic (lasting more than 12 weeks). Especially in chronic pain, it is typical that a clear cause of pain cannot be identified (even with imaging techniques). (1,2,3,4)

This article addresses acute pain and how it forms. Typical conditions causing acute, or sudden, pain include various injuries and inflammations. In such cases, the role of pain is to act as the body's alarm system. If acute pain appears suddenly and intensely without an apparent cause, it is often advisable to seek medical attention.

How does acute pain start?

  1. Some stimulus (e.g., the nail depicted in the image below) triggers the activation of nociceptors, or free nerve endings. After activation, the nerve endings convey information about the event to the dorsal root of the spinal cord. This information is also referred to as nociceptive information. Free nerve endings are practically found throughout the human body, and their function is to sense various stimuli and transmit information about them to the brain. For example, on the skin, they transmit information about touch.

  2. In the spinal cord, the nerve relays the message to another nerve through a synapse, or nerve junction. Subsequently, the message continues upward toward the brain. In the brain, the message reaches the thalamus, which serves as a kind of sorting center for information.

  3. The thalamus transfers the information to different parts of the brain, after which the brain forms its own understanding of the situation. If it is, for example, an acute accident, this understanding usually leads to the sensation of pain. It is important to remember that nerves themselves do not know what kind of information they are transmitting, and it is only in the brain where the decision is made whether the signal in question is potentially dangerous or not. Thus, pain does not travel as pain through the nerves; rather, information travels as electrical impulses. (1,2,3)

Acute pain

Spinal Cord, ascending Pathways, and descending Pathways

The spinal cord acts like an elevator, transmitting information between the body and the brain. From the spinal cord, spinal nerves also emerge, branching out into peripheral nerves to various parts of the body. Peripheral nerves enable sensations such as touch and muscle function.

Regarding pain, especially the dorsal root of the spinal cord is crucial, as it essentially determines whether the message continues towards the brain or not. If the message remains at the level of the spinal cord and does not reach the brain, pain does not occur. Nerve pathways that carry pain signals upward toward the brain are called ascending pathways. In addition to ascending pathways, humans also have descending pathways. Descending pathways travel from various parts of the brain to the dorsal root of the spinal cord, where their role is to decrease or increase the amount of nerve signaling. Descending pathways play a significant role in pain modulation. For example, the pain-relieving effect of manual therapy, such as massage, is believed to be related to the activation of descending pathways by the pleasant sensation following the massage, preventing pain signals from reaching the brain. (1,2,3)

Mechanisms of pain

Nociceptive stimuli (cold, heat, chemicals, or mechanical force) thus trigger the activation of free nerve endings, often resulting in pain. Acute pain is typically nociceptive in nature. Nociceptive pains can be categorized into inflammatory, ischemic, and mechanical pains.

Mechanical pain usually feels quite pinpoint or sharp and is localized to a relatively small area. Mechanical stimuli, such as compression of different tissues, can cause mechanical pain. For example, osteoarthritis can cause mechanical pain. The pain may also start suddenly and be provoked by a specific movement.

Ischemic pain results from tissue oxygen deprivation. Prolonged sitting in the same position, for example, can lead to this. When tissues do not receive enough oxygen, acidosis occurs, i.e., the tissue becomes acidic. Nociceptors detect this chemical change and signal it to the brain, after which we feel pain. Ischemic pain can be quite diffuse and/or numb. Exercise is usually the best treatment for ischemic pain.

Inflammatory pain often follows injuries, for example. When tissue is damaged, it releases various inflammatory mediators that activate nerves. Nerves, in turn, release more neurotransmitters, which contribute to vasodilation and swelling.

The nerve itself can also cause pain if it is damaged. A nerve injury results in a local inflammatory reaction, and interestingly, regarding nerves, this inflammatory reaction can spread bidirectionally along the nerve. As a result of this phenomenon, nerve-related disorders can cause symptoms over a wide area.


Important things to remember about pain:

  1. Pain does not always indicate that there is an injury or damage in the body – this especially applies to chronic pain situations but is also possible in acute pain.

  2. A person experiencing pain is the only reliable source to describe how the pain feels – professionals should therefore respect this narrative.

  3. Pain does not automatically mean that movement/exercise should be stopped – although often in the acute phase, especially due to injury, it may be evident that the load should be temporarily reduced.

  4. Often, pains result from multiple pain mechanisms, but identifying pain patterns helps in treatment and planning.

  5. Pain experience is influenced by both biological, psychological, and social factors.

Ilari Keckman

Osteopath, sports massage therapist & educator

Joonas Virtanen

Osteopath, sports massage therapist & fitness coach


2. Ammattilaisen Kipukirja 2020: Luomajoki ym., VK-Kustannus


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