The Peripheral Receptor System

The sensation of pain starts with a physical event such as a cut, burn, inflammation that excites sensory nerve fiber terminal endings including:

N. Vadivelu et al. (eds.), Essentials of Pain Management,

DOI 10.1007/978-0-387-87579-8_3, © Springer Science+Business Media, LLC 2011

Figure 3.1 Pain pathways. Primary afferent neuron in spinal ganglion. Second-order neuron in dorsal horn. Third-order neuron in thalamic nuclei.

• unmyelinated C and A-delta fibers with bipolar cell bodies in the dorsal root ganglion (mentioned above) with proximal endings in the dorsal horn and distal endings in peripheral tissues

• autonomic preganglionic neurones with cell bodies located in the motor nuclei of the brainstem or in the anterolateral horn ofthe spinal cord

These receptors and associated fibers are called nociceptors.

Nociceptors

The nociceptors are divided into several types, based on the stimuli they perceive. Nociceptor types include:

1. Mechanical (pressure, swelling, incision, tumor growth)

2. Chemical (excitatory neurotransmitter, toxic substance, ischemia, infection)

3. Thermal (burn)

4. Polymodal (i.e. the capability to respond to different stimuli: a combination of stimuli, respond to excessive pressure, extremes of temperature and halogens)

The nociceptors are distributed in the somatic structures and visceral structures.

Somatic Structures

Somatic structures (skin and deep tissues: muscles, tendons, bones, joints) respond to a variety of mechanical, chemical, and thermal stimuli leading to a well-perceived and well-localized sensation. Deep somatic nociceptors in tissue are less sensitive to noxious stimuli than cutaneous nociceptors, but are easily sensitized by inflammation. Specific nociceptors may exist in muscles andjoint capsules; they respond to mechanical, thermal, and chemical stimuli, this would explain the presentation of most sports injuries. The cornea and tooth pulp are unique in that they are almost exclusively innervated by nociceptive A-delta and C fibers (cornea) and A-delta, A-beta, and C fibers (teeth).

Visceral Structures

Visceral structures (visceral organs such as liver, gastro-intestinal tract) respond to pain induced by ischemia, spasm, or inflammation of smooth muscle as well as mechanical stimulation such as distension of the mesentery. These fibers run in sympathetic and parasympathetic nerves, and the pain induced is poorly localized.

Visceral organs are generally insensitive and mostly contain silent nociceptors. Some organs appear to have specific nociceptors, such as the heart, lung, testis, and bile ducts. Most other organs, such as the intestines, are innervated by polymodal nociceptors that respond to smooth muscle spasm, ischemia, and inflammation. These receptors generally do not respond to the cutting, burning, or crushing that occurs during surgery. A few organs, such as the brain, lack nociceptors altogether; however, the brain's meningeal coverings do contain nociceptors. This phenomenon explains the need for adequate anesthesia and analgesia only during the beginning of neurosurgical procedures for the dissection and exposure of brain tissue.

Like somatic nociceptors, those in the viscera are the free nerve endings of primary afferent neurons whose cell bodies lie in the dorsal horn. These afferent nerve fibers, however, frequently travel with efferent sympathetic nerve fibers to reach the viscera. Afferent activity from these neurons enters the spinal cord between T1 and L2.

Nociceptive C fibers from the esophagus, larynx, and trachea travel with the vagus nerve to enter the nucleus solitarius in the brainstem. Afferent pain fibers from the bladder, prostate, rectum, cervix and urethra, and genitalia are transmitted into the spinal cord via parasympa-thetic nerves at the level of the S2-S4 nerve roots. Though relatively few compared to somatic pain fibers, fibers from primary visceral afferent neurons enter the cord and synapse more diffusely with single fibers, often synapsing with multiple dermatomal levels and often crossing to the contralateral dorsal horn. This nonspecific synapsing of visceral afferents explains the reason why somatic musculoskeletal pain is arranged in dermatomes but the visceral pain are usually nonspecific and variable in nature.

Somatic nociceptive pain has a dermatomal pattern (Fig. 3.2) and is sharp, crushing, or tearing in character. Somatic nociceptive pain is very well localized, whereas visceral noci-ceptive pain is nondermatomal and cramping or colicky and poorly is localized. Sometimes

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