Pain
-
In rats anesthetized with sodium pentobarbital, we quantitatively analyzed descending modulation from the midbrain of a nociceptive flexion withdrawal reflex and responses of associated spinal neurons. We monitored the isometric force of hind limb withdrawal elicited by noxious heat stimuli (42-54 degrees C, 10 sec) on the hind paw. In one series of experiments, single-fiber EMG electrodes recorded responses of single muscle fibers (i.e., motor units) in biceps femoris during the hind limb withdrawal, without and during electrical stimulation in the midbrain periaqueductal gray (PAG) or lateral midbrain reticular formation (LRF). ⋯ Following supplemental administration of pentobarbital (10-30 mg/kg i.v.), withdrawals and motor unit responses to heat were suppressed while dorsal horn unit responses were unchanged or enhanced. Also, in 12/42 cases, withdrawals and motor unit responses decremented markedly during the initial 3 trials of heat, while simultaneously recorded dorsal horn unit responses remained stable. These results indicate that the withdrawal reflex and associated motor units can be markedly suppressed in the absence of concomitant changes in responsiveness of dorsal horn neurons, and are discussed in terms of the neurocircuitry of spinal flexor reflexes and their descending modulation.
-
Pain responsiveness was investigated experimentally as a function of age and childbirth pain experience. Sensitivity to cold pressor-induced pain was assessed through threshold, tolerance, and visual analog pain ratings. It was hypothesized that childbirth pain experience would mostly modify experimental pain judgment, in accordance with the adaptation-levels model. ⋯ Thus, painful childbirth experience is sufficient to raise cold pressor pain threshold. This finding has never before been reported in the pain literature. It is consistent with anecdotal reports from parous women who, when providing cold pressor pain judgments, say that "nothing compares to labor pain."