Atkinson Pain Research Laboratory

Barrow Neurological Institute
of St. Joseph's Hospital and Medical Center
Phoenix, Arizona

Supplementary photomicrographs of VMpo

Laboratory Focus

In the Atkinson Pain Research Laboratory, we study connections from the spinal cord to the brain that are involved in "feelings" from the body. The main connection originates in a spinal region called lamina I. Quantitative analyses of lamina I spinothalamic neurons indicate that they serve as "labeled lines" that generate sharp pain, burning pain, warm, cool, itch, muscle ache, sensual touch and other sensations related to the body's physiological condition. 

Anatomic work in the laboratory shows that lamina I neurons project their axons first to autonomic spinal and homeostatic brainstem regions, then to a specific thalamocortical relay nucleus called VMpo, found only in primates and greatly enlarged in humans, which projects in turn to posterior insular cortex. Our work also shows that stimulation of the vagus nerve causes activity within a parallel homeostatic afferent pathway. This finding supports the idea that pain is a reflection of the homeostatic processes in the brain that evolve to maintain the body's health.

These studies have led to the novel concept that these feelings constitute a sense of the body's physiologic condition, and that this interoceptive sensory pathway provides the neuroanatomical foundation for subjective human awareness of the material (sentient) self. This concept converges with the neurological concept that conscious human emotions are based on "somatic markers" that represent the homeostatic condition of the body. These neuroanatomical findings provide a substantive foundation for human awareness and compel a novel conceptual view of pain as a homeostatic emotion. This perspective provides a new paradigm for explaining interactions of pain with emotion and homeostatic processing that leads to novel ideas for therapeutic intervention.
 
Current work in the laboratory addresses the integration of pain, temperature, itch and visceral representations within the insular cortex of the primate; the role of the medial thalamus and anterior cingulate cortex in the inhibition of pain by cooling; the association of deep dorsal horn cells with sensorimotor integration (considered for more than 30 years to be pain cells by others); the role of lamina I spinothalamic neurons in injury-induced sensitization (hyperalgesia); and the characterization of lamina I spinobulbar neurons involved in brainstem homeostatic mechanisms. 

Laboratory Accomplishments

• Physiological demonstrations that lamina I spinothalamic neurons constitute distinct "labeled lines" for the sensations referred to as first and second pain, itch, warm, cool, and muscle ache
• Anatomical demonstrations that lamina I spinothalamic axons can be identified in human spinal cord as the "lateral spinothalamic tract," where cordotomy lesions produce analgesia and thermanesthesia in patients
• Physiological explanation of the thermal grill illusion of pain, which demonstrates that nonpainful cooling inhibits burning pain within the brain
• Use of the thermal grill in a multiple sclerosis patient to obtain evidence supporting our theory that central pain is a disinhibitory thermoregulatory syndrome
• Anatomic, ultrastructural, and physiologic demonstrations that VMpo is a specific relay nucleus for pain and temperature sensations to the insular cortex
• Publication of the novel concept that pain is a homeostatic emotion
• Publication of the fundamental neuroanatomical hypothesis that human subjective awareness is based on a cortical re-representation of the interoceptive state of the body

Selected References

Craig, A.D. (2002) How do you feel?  Interoception: the sense of the physiological condition of the body. Nat Rev Neurosci 3: 655-666.

Craig, A.D. (2003) A new view of pain as a homeostatic emotion. Trends Neurosci 26: 303-307.

Craig, A. D. (2005) Forebrain emotional asymmetry: a neuroanatomical basis? Trends in Cognitive Sciences 9:566-571.

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