The receptors in body tissues which detect injury are called nociceptors. The mechanical, thermal or chemical stimuli which activate nociceptors are called noxious stimuli. Nociceptors are free nerve endings present in skin, subcutaneous tissue, fascia, muscle, periosteum, joints, blood vessels, and viscera. Activation of these receptors results in nerve impulses which travel along the afferent fibers in the nerves and are transmitted to the entral nervous system (CNS)
Afferent nerve fibers which carry the sensation of pain to the central nervous system are A-delta and c fibers. The diameter of A-delta fibers is 2 to 5 u; their conduction velocity is 12 to 30 meters/sec, and they are thinly myelinated. The diameter of afferent C fibers, is 0.1 to 1.3 u; they conduct impulses slowly (0.6 to 2 meters/sec) and are unmyelinated. A-delta fibers transmit bright, well-localized, nonpersistent pain which is conducted fast, and is immediately associated with the injury. The pain arising from afferent C fibers is dull, poorly localized, persistent, and slowly conducted. Some of these receptors are sensitive only to mechanical stimuli, others are sensitive to only thermal stimuli and others are polymodal, e.g. C-polymodal nociceptors which respond to noxious mechanical, thermal, and chemical stimuli.It has been proposed that tissue injury may release histamine, serotonin, and bradykinin. These agents can activate nociceptive nerve endings. Prostaglandins do not activate nociceptive nerve endings. However, in presence of inflammation, prostaglandins appear to sensitize pain receptors to mechanical and chemical stimuli.The cell bodies of these afferents are located in the dorsal root ganglia. Within the dorsal root, the cells are arranged somatotopically. Recent evidence shows that unmyelinated axons arising from the sensory dorsal root ganglion also travel in the ventral roots. These fibers in the ventral root may explain why dorsal rhizotomy is not always successful in relieving pain. In such instances, ganglionectomy has been recommended to relieve pain.The marginal layer of the dorsal horn (lamina 1) receives primarily thinly myelinated (A-delta) fibers and some unmyelinated C afferent fibers. Response latencies of the neurons in this lamina indicate that there is a direct monosynaptic input from the primary afferents. Lamina II and III comprise the substantia gelatinosa in the dorsal horn. Substantia gelatinosa received primarily C fiber afferents and some A-delta fibers in the deeper region. Within the substantia gelatinosa, characteristic arrangement of neuropil called glomeruli have been observed. These glomeruli may be important in integrating and modulating the afferent input of the dorsal horn. The relationship of the primary afferent terminals, the dendrites, and axons of the substantia gelatinosa interneurons has been partially determined in the nucleus caudalis of trigeminal complex. The nucleus caudalis is similar to the spinal dorsal horn anatomically as well as physiologically. One type of excitatory interneuron in the substantia gelatinosa of the nucleus caudalis, the stalked cell, has its dendrites within the substantia gelatinosa but projects its axon into the lamina I. Its dendrites may receive excitatory impulses from the primary afferents in the lamina II and III and transmit them to the cells in lamina I. The other class of interneuron has its dendrites, as well as axon, in the substantia gelatinosa. Its axon probably forms axoaxonic inhabitory synapses on the primary afferent endings of the glomeruli. These neurons also may form axodendritic synapses with the stalked cell dendrites in the glomeruli. These interneurons may reduce the transfer of primary afferent information to lamina I cells by inhibiting either axoaxonic synapses on the primary afferent endings or the axodendritic synapses with the stalked cells which in turn would reduce information to lamina I cells.The nociceptive impulses transmitted to the dorsal horn are carried to different parts of the CNS. Some impulses are transmitted to the somatic motor neurons in the ventral horn, some reach preganglionic autonomic neurons in the ventrolateral horn, and others ascend in the spinal cord, medullary reticular formation, thalamus, and cortex. The axons of most of the neurons in the dorsal horn of the spinal cord cross in the midline and ascend to different parts of the neuraxis; however a small ipsilateral component also exists. The sensation of pain ascends to different parts of neuraxis via the lateral spinothalamic tract. The deeper layers of the laminae in the spinal gray matter send projections, via the lateral spinothalamic tract, to the ipsilateral nucleus reticularis gigantocellularis and lateral reticular nucleus of the medulla. In the nucleus reticularis gigantocellularis, there are neurons which respond only to noxious stimuli while others respond to mechanical as well as noxious stimuli. In the mesencephalon, some of the fibers in the lateral spinothalamis tract project to the periaqueductal (central) gray matter.In the thalamus, two major nuclear groups, the ventrobasal (VBG) and intralaminar (ILG) nuclear groups, may be involved in pain. The ventrobasal complex includes ventroposterolateral (VPLN) and ventroposteromedial (VPMN) nuclei. The intralaminar complex comprises; parafascicular (PFN), paracentral (PCN), central lateral (CLN), and centromedian (CMN) nuclei. At the level of diencephalon, the lateral spinothalamic tract bifurcates into a lateral and medial branch. The lateral branch projects mainly to the ventrobasal complex and the medial branch projects to the intralaminar nuclei.
In the cortex, the primary and secondary somatosensory areas participate in nociception. The primary somatosensory area consists of postcentral gyrus and the secondary somatosensory area includes the superior lip of the lateral sulcus. The ventrobasal and intralaminar complexes of the thalamus project somatotopically to both primary and secondary somatosensory areas. Most cells in the primary somatosensory area are modality specific: they respond to either light touch, deep pressure, warning, cooling, or tooth pulp stimulation. In the secondary somatosensory area, specific and nonspecific polymodal cells are found.
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