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Physiology Of Pain

Project by: Jad El Hawly, Alice Taube, Oscar Joseph Ullomi

Supervisor Dr. Zoltan Balazs Barany

Physiology Department, University of Veterinary Medicine, Budapest

What is pain?

Pain is defined by the International Association for the Study of Pain (IASP) as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage".Pain is a subjective experience made up of two complementary aspects, the first one is localized sensation in a particular body part, while the other is an unpleasant quality of varying severity commonly associated with behaviors directed at relieving or terminating the experience, the pain has much in common with other sensory modalities according to National Academy of Sciences (1985). There are specific pain receptors as these are nerve endings, present in most body tissues, that only respond to damaging or potentially damaging stimuli. Which the messages initiated by these noxious stimuli are transmitted by specific, identified nerves to the spinal cord. The perception of pain results from the brain’s processing of new sensory input with existing memories and emotions, is the same way that other perceptions are produced. Pain become painful is when it reaches out the threshold. However, most studies have found that the pain tolerance threshold, the point at which pain becomes unbearable, varies significantly among different people and species.

Two types of pain, the fast pain which is acute, well localized, short duration, and in thin myelinated fiber, while the slow pain is chronic throbbing, localized long, normal duration, and in unmyelinated fiber.

Pain process;

The pain process is made up of four steps (1) transduction, (2) transmission, (3) modulation, and (4) perception

1-Transduction:

Transduction is a process of converting noxious stimulus to action potential. As it refers to the processes by which tissue-damaging stimuli activate nerve endings. The nociceptors equal to pain receptors, they are capable of transducing and encoding noxious stimuli, free nerve ending, and they can respond to one stimulus, many stimulus (called poly modal), or not responding for any stimulus (called silent). So Transduction begins when peripheral terminals of nociceptive C fibers and A-delta(Aδ) fibers are depolarized. Normally, nociceptor terminals have a high activation threshold. They requiring intense stimulation to generate an action potential. Four types of stimuli can activate pain receptors in peripheral tissues: mechanical, heat, cold, and chemical. Mechanical and heat stimuli are usually short, whereas chemical stimuli are usually long lasting. Nothing yet is known about how these stimuli activate nociceptors.

As each of these stimuli have different receptors:

2-Transmissiom:

Pain impulses are transmitted by two fiber systems. The presence of two pain pathways explains the existence of two components of pain: fast by Aδ fibers; and a duller slower (second pain) which is conducted by C fibers. Aδ fibers are myelinated, 2 – 5 µm in diameter and conduct at rates of 12 – 30 m/s, whereas C fibers are unmyelinated, 0.4 – 1.2 µm in diameter and conduct at rates of 0.5 to 2 m/s. As the nociceptive message is transmitted from the periphery to the central nervous system by the axon of the primary afferent nociceptor. This neuron has its cell body in the dorsal root ganglion and a long process, the axon, that divides and sends one branch out to the periphery and one into the spinal cord. There are four physiological mechanisms have been proposed to explain referred pain:

  1. Activity in sympathetic nerves,
  2. Peripheral branching of primary afferent nociceptors,
  3. Convergence projection,
  4. Convergence facilitation,

Transmission of the pain signal effect of voltage gated ion channels and the pharmacological relevance:

3-Modulation:

Modulation is one of the most important discoveries in pain research was that the brain contains substances that have the same pharmacological properties as plant-derived opiates and synthetic opioid drugs. These substances, called endogenous opioid peptides, are present within nerve cells of the peripheral and central nervous systems (Palkovits, 1984), three classes of opioid receptors have been identified: μ-mu, δ-delta and κ-kappa. Modulation of pain occurs peripherally at the nociceptor, in the spinal cord, or in supra spinal structures. This modulation can either inhibit or facilitate pain.

It can be Peripheral or Central modulation