Pain serves an important role in providing essential protective mechanisms against injury. The International Association for the Study of Pain defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” This definition infers that pain has both sensory as well as affective and cognitive consequences.
Nociceptors are specialized sensory receptors with the ability to detect noxious stimuli and transform the stimuli into electrical signals that the central nervous system interprets. The free nerve endings of primary afferent A delta and C fibers are responsible for nociception (Table 127-1). Nociceptors respond to intense heat, cold, mechanical, and chemical stimuli. The axons of the A delta are thinly myelinated with a faster conduction speed of 5-15 m/s in contrast to the unmyelinated C fibers with a conduction speed of 1-2 m/s. A delta fibers respond to mechanical and thermal stimuli. They carry rapid, sharp pain and are responsible for the initial reflex response to acute pain. C fibers are polymodal, responding to chemical, mechanical, and thermal stimuli. The C fiber activation leads to slow, dull pain.
Comparison of Pain Fibers
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TABLE 127-1 Comparison of Pain Fibers
| ||A delta Fibers ||C Fibers |
|Diameter ||2-5 μm ||<2 μm |
|Myelination ||Thinly ||None |
|Conduction velocity ||5-15 m/s ||<2 m/s |
|Receptor activation threshold ||High and low ||High |
|Sensation on stimulation ||Rapid, sharp, localized pain ||Slow, diffuse, dull pain |
Nociception is defined as the neural occurrences of encoding and processing noxious stimuli. It is the afferent activity produced in the peripheral and central nervous systems by stimuli that have the potential to damage tissue. The succession by which a stimulus is perceived as noxious comprises the following four processes:
Transduction is the process where nociceptors convert different forms of noxious energy to electrical activity or action potentials that can be recognized by the central nervous system.
Transmission is the process by which the electrical activity created by nociceptors is conducted to the central nervous system. There are three components to this process: (1) the peripheral sensory cells in the dorsal root ganglia transmit signals from the site of transduction to the spinal cord; (2) spinal neurons send projections to the brain stem; and (3) neurons of the brainstem project to various cortical sites.
Modulation involves changing or inhibiting transmission of pain impulses in the spinal cord. The multiple, complex pathways involved in the modulation of pain are referred to as the descending modulatory pain pathways and these can lead to either an increase in the transmission of pain impulses (excitatory) or a decrease in transmission (inhibition).