Types of Pain

• Pain is defined as an unpleasant sensory and emotional experience linked to actual or potential tissue damage.

Types of Pain

• Fast pain includes sharp, pricking, acute, and electric pain sensations.
• Fast pain is felt about 0.1 seconds after a stimulus.
• This pain type occurs when a needle punctures the skin, when the skin is cut, burned, or subjected to electric shock.
• Fast-sharp pain isn't felt in deeper body tissues.
• Mechanical and thermal stimuli elicit fast pain.
• Small type Aδ fibers transmit fast pain in the peripheral nerves at 6-30m/sec to the spinal cord.
• Slow pain involves slow burning, aching, throbbing, nauseous, and chronic pain.
• Tissue destruction usually causes slow pain.
• It can lead to prolonged suffering.
• Slow pain occurs in the skin and deep tissues or organs.
• Mechanical, chemical, and thermal stimuli elicit slow pain.
• Chemicals such as bradykinin, serotonin, histamine, potassium ions, acids, acetylcholine, and proteolytic enzymes excite pain.
• Prostaglandins and substance P enhance the sensitivity of pain endings.
• Type C fibers transmit slow-chronic pain to the spinal cord at 0.5-2m/sec.

Pain Receptors

• Pain receptors are free nerve endings in the skin and tissues.
• They are widespread in the superficial layers of the skin, certain internal tissues, the periosteum, arterial walls, joint surfaces, falx, and tentorium in the cranial vault.
• Deep tissues have sparse pain endings, widespread tissue damage leads to slow-chronic-aching pain.

Adaptation of Pain Receptors

• Pain receptors adapt very little, unlike other sensory receptors.
• Excitation of pain fibers becomes greater when a pain stimulus continues, especially for slow, aching, and nauseous pain.
• Increased sensitivity of the pain receptors is called hyperalgesia.
• Pain receptors' failure to adapt keeps a person aware of tissue damage.

Pathways

• Pain receptors use two pathways to send their signals to the central nervous system, despite only being free nerve endings .
• These pathways correspond to fast-sharp and slow-chronic pain.
• A sudden pain stimulus creates a double pain sensation because of these systems.
• The Aδ fiber pathway transmits fast-sharp pain to the brain, soon followed by slow pain transmitted by the C fiber pathway.
• Sharp pain allows an immediate reaction to remove one's self from the stimulus. Slow pain increases and can become intolerable; this will cause the person to keep trying to relieve the source.
• Upon entering the spinal cord via dorsal spinal roots, pain fibers connect to relay neurons in the dorsal horns.
• There are two systems for processing pain signals on their route to the brain.

Neospinothalamic Tract

• Type Aδ fibers mainly transmit mechanical and acute thermal pain, using this Tract for fast pain.
• The fibers terminate mainly in lamina I of the dorsal horns.
• It excites second-order neurons of the tract.
• Long fibers originate, cross to the other side of the cord, turn upward, and go to the brain through the anterolateral columns.
• The tract fibers terminate in the reticular areas of the brain stem, as the fibers pass to the thalamus without stopping.
• Fibers terminate along with the dorsal column-medial lemniscal tract, which transmits tactile sensations.
• Signals transmit from thalamic areas to basal brain areas and the somatosensory cortex.
• Glutamate allows the Type Aδ pain fibers neurotransmission.

Tactile receptors and pain

• Localizing fast-sharp pain can be more precise compared to slow-chronic pain
• Poor localization happens when only pain receptors are stimulated and there is no activation of tactile receptors.
• When tactile receptors stimulate the dorsal column-medial lemniscal system, the simultaneous stimulation allows nearly exact signal localization.

Paleospinothalamic Pathway

• The paleospinothalamic pathway transmits slow-chronic type C pain fibers and some type Aδ signals.
• Peripheral fibers terminate in laminae II and III of the dorsal horns.
• Signals pass through short fiber neurons within the dorsal horns before entering Lamina V.
• The last neurons in the series generate long axons that join fibers from the fast pain pathway, passing to the opposite side of the cord and travelling towards the brain in the anterolateral pathway.
• The Type C pain fiber terminals in the spinal cord release glutamate and substance P transmitters.
• The "double" pain sensation may result from glutamate transmitting faster pain signals, as opposed to substance P, which transmits slower ones.
• Glutamate transmits fast pain into the central nervous system, while substance P controls slow chronic pain.

Localization of Pain

• The slow-chronic paleospinothalamic pathway terminates widely in the brain stem.
• 10-25% of fibers pass towards the thalamus, but the majority terminate in the reticular nuclei of the medulla, pons, and mesencephalon, the tectal area of the mesencephalon, or the gray region surrounding the aqueduct of Sylvius.
• Electrical stimulation of pain signals' termination areas in the reticular brain stem and thalamus has a strong arousal effect on nervous activity throughout the brain.
• These two areas make up the brain's "arousal system", which make it almost impossible to to sleep when experiencing severe pain.
• Lower brain regions are vital for feelings of suffering because animals with blocked pain signals still show suffering signs when traumatized.
• Short-fiber neurons relay pain signals upward from the brain stem into the thalamus, hypothalamus, and parts of basal brain regions.
• Slow-chronic pain transmitted by this pathway is imprecise.
• Slow-chronic pain's localization is limited to a major body part, such as an arm or leg, instead of a specific limb point.
• The multisynaptic, diffuse connectivity of this pathway leads patients to struggle with locating chronic pain's source.

Referred Pain

• Referred pain occurs when pain is felt away from the location of the tissue causing it.
• When visceral pain refers to the body surface, the person identifies it with the dermatomal segment where the visceral organ originated in the embryo, not its current location.
• Stimulation of visceral pain fibers conducts pain signals through similar neurons that conduct skin signals, thus giving the sensation the pain originates from the skin.