Characteristics of the infant arousal response

Arousal is considered to be an important response to a life-threatening sti mulus. Recently, it has been shown that the infant arousal response to an e levated inspired CO2 level occurs as a sequence of events involving presump tive brain stem responses before awakening (A. Lijowska, N. Reed, B. Chiodi ni, and B. T. Thach. Am. J. Respir. Grit. Care Med. 151: A151, 1995; A. S. Lijowska, N. W. Reed, B. A. Mertins Chiodini, and B. T. Thach. J. Appl. Phy siol. 83: 219-228, 1997). We wanted to further evaluate the relationship of subcortical reflexes to cortical arousal in infants. We used a nonrespirat ory (tactile) stimulus to elicit arousal in infants during non-rapid-eye-mo vement (NREM) and rapid-eye-movement (REM) sleep. We found that a tactile s timulus elicited an arousal sequence that commenced with a spinal withdrawa l reflex, was followed by brain stem responses (respiratory and startle res ponses), and ended in a cortical arousal. The entire pathway or part of it in the order of spinal to cortical responses could be elicited. REM and NRE M responses were similar except for significant differences in the latencie s of spinal and subcortical reflexes. These observations suggest that the i nfant arousal response to a tactile stimulus involves a progression of cent ral nervous system activation from the spinal to cortical levels. The diffe rent components of the arousal pathway may be important for an infant to re spond appropriately to stimuli during sleep without necessarily disturbing sleep.