Major ascending pathways that transmit nociceptive information
The three major pathways of nociceptive information are the spinothalamic, spinoparabrachial and spinoreticular tracts. The cell origin of the spinothalamic tract is in the dorsal horn and intermediate gray matter of the spinal cord. Axons of these neurons cross to the anterolateral quadrant and ascend to the thalamus, where they synapse on neurons in the lateral thalamus and in the intralaminar nuclei, located more medially. An additional ascending pathway, recently described, arises from neurons in the most superficial lamina of the dorsal horn, lamina I. These neurons project to the rostral brainstem, particularly to the parabrachial nuclei of the dorosolateral pons. This pathway has now been strongly implicated in generating the emotional component of the pain experience as parabrachial neurons project to the amygdala, as well as to the insular and anterior cingulate cortex, namely to limbic areas the process emotions.
The spinoreticular pathway parallels the spinothalamic tract. Neurons at the origin of the spinoreticular pathway are abundant in the deeper parts of the dorsal horn and in the ventral horn (laminae VII and VIII). The axons of these neurons project bilaterally to reticular formations at all levels of the brainstem. The output of the reticular neurons is predominantly to intralaminar thalamic nuclei and to the hypothalamus, thus the origin of the term spinoreticulothalamic pathway .
There are other ascending pathways, including one that projects directly from the spinal cord to the hypothalamus. Also, a visceral “pain” pathway that courses in the dorsal columns of the spinal cord has been described.
