Sympathetic Nervous System

Perhaps one of the most widely documented of the stress responsive systems is the sympathetic nervous system. The impact of stress-induced catecholamine secretion for the maintenance of homeostatic processes was initially recognized by Walter Cannon in his seminal work during the first third of the 20th century (Cannon, 1935). In his original review, Cannon described the role of catecholamine secretion from the adrenal medulla as an essential element for (a) the mobilization of glucose to feed-heightened cellular activity during times of stress, and (b) effective physiological coping in the face of (diverse) challenges to homeostasis. Even at this early juncture, Cannon recognized that in the absence of a properly functioning sympathetic response to stress, the ability of the organism to survive the impending challenge was monumentally impaired.

Activation of the sympathetic nervous system produces an immediate and sustained increase in catecholamine secretion (i.e., epinephrine and norepinephrine; EPI/NE). Sympathetic nerve terminals secrete EPI/NE directly onto target tissues, which elicits an immediate postsynaptic response that clears and subsides within a very short time frame (i.e., within a few seconds). For instance, secretion of EPI/NE from sympathetic nerve terminals directly onto cardiac muscle potently increases heart rate and strengthens the force of contractions. Meanwhile, EPI/NE release within the eye dilates the pupil to boost visual acuity and responsivity. These are just two common examples of how direct sympathetic innervation can promote coordinated activity within different effector organs, and thus promote survival in a threatening context.

In addition to direct sympathetic input to target organs, the adrenal medulla also secretes EPI/NE into the general circulation during times of stress. Thus, EPI/NE release from the adrenal medulla has the ability to affect numerous target organs and cells distal to the site of origin, with a duration of action that persists 2 to 10 times as long as direct EPI/NE release from sympathetic nerve terminals (since the clearance rate in blood is much slower than that at synaptic clefts). Secretion of EPI/NE in this endocrine fashion augments the effects produced by EPI/NE released from sympathetic nerve terminals and serves as an avenue by which the functions of target cells that are not under direct sympathetic influence (such as immune cells) can also be modulated during times of stress. Irrespective of the source of catecholamine secretion, the importance of this response is clearly underwritten by the redundancy inherent in the system.

Catecholamines within the central nervous system also play a prominent role in coordinating an organism's response to stress. For instance, the locus coeruleus is a major catecholamine center in the brainstem that is responsible for coordinating stress responses via interactions with higher brain structures such as the hypothalamus, the amygdala, and the cortex (Svensson, 1987). Specifically, environmental stimuli that require perceptual organization, cognitive appraisal, or affective evaluation in order to be deemed "stressful" eventually activate peripheral sympathetics via descending autonomic output through the locus coeruleus. In this role, the locus coeruleus also serves as a final site of integration for the propagation of certain peripheral autonomic responses to stress.

On the other end of the spectrum would be physiological threats to homeostasis such as hypoxia, hypoglycemia, or hemorrhagic shock that are detected in brainstem structures such as the pons, medulla, and reticular formation. In cases such as these where threat is not necessarily detected by the cognitive or perceptual apparatus of the organism, but rather by alarm systems that continuously monitor peripheral physiological status, the locus coeruleus sends ascending catecholaminergic input to higher brain centers. Such information is then processed by higher cognitive structures in order to "encourage" behavioral strategies that will alleviate the threat to homeostasis. Thus, brainstem autonomic nuclei such as the locus coeruleus are critical sites of integration for threatening stimuli irrespective of whether the threat originates from higher brain centers or peripheral challenges to homeostasis [see Harro and Oreland (2001) for an excellent review].

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