5.9: Autonomic Tone

The concept of autonomic tone showcases the intricate interplay between the sympathetic and parasympathetic divisions, underscoring their distinct roles in orchestrating the body's physiological responses. Within each organ system, there often exists a predominant tendency toward either sympathetic or parasympathetic influence in its resting state. This inherent tendency is termed the autonomic tone of the system. Without parasympathetic input, the intrinsic rate of the heart would be around 100 beats per minute (bpm). However, under its unique influence, the parasympathetic system reduces this rate to the typical resting heart rate of 60–80 bpm, suggesting the intricacies of autonomic regulation within the body. This phenomenon designates the heart as under parasympathetic tone, highlighting the dominance of the parasympathetic division in regulating heart rate. In contrast, blood pressure is heavily influenced by the autonomic tone, with the sympathetic division playing a pivotal role. Smooth muscle within the walls of blood vessels contains specialized adrenergic receptors, highly responsive to the neurotransmitter NE. NE binds to these receptors upon release from postganglionic sympathetic fibers, provoking vessel constriction and consequent elevation of blood pressure. E and NE, released from the adrenal medulla, also interact with these receptors as they traverse the bloodstream, further contributing to blood pressure regulation. Thus, the sympathetic division exerts predominant control over the tone of systemic blood vessels. However, the sympathetic division exhibits versatility in certain situations. Blood vessels within skeletal muscle, particularly those in the lower limbs, tend to dilate rather than constrict when subjected to sympathetic input. This divergence from the usual vasoconstriction allows for increased blood flow to the skeletal muscles actively engaged in the "fight-or-flight" response, ensuring they receive adequate oxygen and nutrients. Parasympathetic projections to blood vessels are a rarity, primarily limited to those within the erectile tissue of reproductive organs where postganglionic parasympathetic fibers release ACh, inducing vessel dilation and promoting the engorgement of erectile tissue.¹⁸

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