The Dorsal Horn

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The dorsal horn is the rostral projection of C and A-delta fiber afferents in Lissauers tract (LT) which enter the spinal column, ascend or descend one or two spinal segments in this tract before penetrating the gray matter of the dorsal horn where they synapse on second-order neurons. The synapse is an important checkpoint in modulation of the nociceptive information and is affected by various biochemical excitatory or inhibitory substances (Zeilhofer 2005).

For A-delta fiber the neurotransmitter in the dorsal horn is glutamate acting on AMPA receptors. For C fiber, the neurotransmitter in the dorsal horn is glutamate along with certain peptides such as substance P and the receptors for glutamate are AMPA and N-methyl-D-aspartate (NMDA). NMDA receptors are stimulated by prolonged depolarization. Continual stimulation of C fibers cause excitation in the post synaptic neurons in the dorsal horn which is intensified by concurrent NMDA activity (Rygh et al. 2005).

Algesic or pain-producing substances include serotonin, histamine, prostaglandins, bradykinin, substance P, substance K, the amino acids glutamate and aspartate, calcitonin gene-related peptide, vasoactive intestinal peptide, cholecystokinin, adenosine triphosphate, and acetylcholine.

Analgesic or pain-inhibiting substances are inhibitory neuromediators and include the endogenous opioids (enkephalins, dynorphins, and beta-endorphins), somatostatin, serotonin, norepinephrine, gamma-aminobutyric acid, and neurotensin. Endogenous analgesics activate opioid, alpha-adrenergic, and other receptors that either inhibit release of Glu from primary nociceptive afferents or diminish postsynaptic responses of second-order neurons.

Histologically the gray matter of the spinal cord is divided into ten "laminae" (Fig. 3.5). The dorsal horn is divided into (I-V), components of which deal with most incoming pain fibres: Lamina I: posterior marginal nucleus, Lamina II/III: substantia gelatinosa, Lamina III/IV/V: nucleus proprius, Lamina VI: nucleus dorsalis. Lamina VII is in between these laminae and the more ventral Laminae VIII (motor interneurons) and IX (motor interneurons), and X refers to the gray matter around the central canal of the spinal cord.

Axons in LT once within the dorsal horn give off branches that contact neurons located in several of Rexed's laminae. The A-delta and the C fibers give branches to innervate neurons in Rexed's Laminae I and II. From Rexed's Lamina II the information is transmitted to second-order projection neurons in Laminae IV, V, and VI. The axons of these second-order neurons in Laminae IV-VI cross the midline and ascend into the brainstem and thalamus in the anterolateral quadrant of the contralateral half of the spinal cord. These fibers, together with axons from second-order Lamina I neurons, form the spinothalamic tract. This pathway is referred to as the anterolateral system.

VIII

VIII

Laminae

Nuclei

Figure 3.5 Histologically, the gray matter of the spinal cord is divided into ten "Laminae." The dorsal horn is divided into five Laminae (I—V), components of which deal with most incoming pain fibres: Lamina I: posterior marginal nucleus, Lamina II/III: substantia gelatinosa, Lamina III/IV/V: nucleus proprius, Lamina VI: nucleus dorsalis. Laminae VII is in between these laminae and the more ventral Laminae VIII (motor interneurons) and IX (motor interneurons), and X refers to the gray matter around the central canal of the spinal cord.

Nucleus proprius (posterior thoracic nucleus or column of Clarke)

Intermediolateral nucleus

Substantia gelatinosa

Motor neurons of the anterior horn

Laminae

Nuclei

Figure 3.5 Histologically, the gray matter of the spinal cord is divided into ten "Laminae." The dorsal horn is divided into five Laminae (I—V), components of which deal with most incoming pain fibres: Lamina I: posterior marginal nucleus, Lamina II/III: substantia gelatinosa, Lamina III/IV/V: nucleus proprius, Lamina VI: nucleus dorsalis. Laminae VII is in between these laminae and the more ventral Laminae VIII (motor interneurons) and IX (motor interneurons), and X refers to the gray matter around the central canal of the spinal cord.

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