Most tissues regulated by the autonomic nervous system recieve both sympathetic and parasympathetic input from postganglionic neurons. The responses are typically local, in contrast the adrenal medulla recieves input only from the sympathetic division, yet responses are observed throughout the whole body. Why is this?
The reason for this is rooted in the anatomical and physiological differences between the autonomic nervous system (ANS) and the adrenal medulla. To understand why the responses of the adrenal medulla are observed throughout the whole body, we need to explore the key features of both systems.
The ANS consists of two branches: the sympathetic division and the parasympathetic division. These divisions work in opposition to regulate the functions of various tissues and organs in the body. Both sympathetic and parasympathetic fibers arise from preganglionic neurons in the central nervous system and synapse on postganglionic neurons located outside the central nervous system.
In most tissues regulated by the ANS, such as smooth muscles, glands, and viscera, both sympathetic and parasympathetic fibers converge and innervate the same target tissue. However, the effects of sympathetic and parasympathetic stimulation generally have opposite actions. Sympathetic stimulation tends to produce a "fight or flight" response, increasing heart rate, dilating blood vessels, and promoting energy mobilization, while parasympathetic stimulation is associated with a "rest and digest" response, lowering heart rate, constricting blood vessels, and aiding digestion.
On the other hand, the adrenal medulla is part of the sympathetic division of the ANS but functions slightly differently. It is a specialized neuroendocrine gland located on top of each kidney. The preganglionic sympathetic fibers originating from the spinal cord synapse directly on cells within the adrenal medulla, rather than on postganglionic neurons.
Upon stimulation, the adrenal medulla releases hormones, epinephrine (adrenaline), and norepinephrine (noradrenaline), into the bloodstream. These hormones are then carried throughout the body, affecting target tissues and organs in a widespread manner. The release of epinephrine and norepinephrine leads to numerous physiological responses that prepare the body for quick and intense action, such as increased heart rate, dilation of airways, and heightened alertness.
Thus, unlike most tissues regulated by the ANS, which receive both sympathetic and parasympathetic input and produce local responses, the adrenal medulla only receives sympathetic input but can trigger responses throughout the whole body by releasing hormones into the bloodstream. This allows for a rapid and coordinated physiological response in situations that require immediate action or heightened arousal.