Adrenal medulla

Typically occurring during the "fight-or-flight" response or during exercise, a mass sympathetic discharge involves simultaneous stimulation of organs and tissues throughout the body. Included among these tissues are the adrenal medullae, which release epinephrine and norepinephrine into the blood. In large part, the indirect effects of these catecholamines are similar to, and therefore reinforce, those of direct sympathetic stimulation. However, some important differences in effects of the circulating catecholamines and those of norepinephrine released from sympathetic nerves include:

• Duration of activity

• Breadth of activity

• Affinity for b2-receptors

Because duration of activity of the catecholamines is significantly longer than that of neuronally released norepinephrine, the effects on tissues are more prolonged. This difference has to do with the mechanism of inactiva-tion of these substances. Norepinephrine is immediately removed from the neuroeffector synapse by way of reuptake into the postganglionic neuron. This rapid removal limits duration of the effect of this neurotransmitter. In contrast, no enzymes are in the blood to degrade the catecholamines; instead, they are inactivated by COMT in the liver. As one might expect, hepatic clearance of these hormones from the blood would require several passes through the circulation. Therefore, the catecholamines are available to cause their effects for a comparatively longer period of time (up to 1 to 2 minutes as opposed to milliseconds).

Because catecholamines travel in the blood, organs and tissues throughout the body are exposed to them. Therefore, they are capable of stimulating tissues that are not directly innervated by sympathetic nerve fibers, hepato-cytes, and adipose tissue, in particular. As a result, the catecholamines have a much wider breadth of activity compared to norepinephrine released from sympathetic nerves.

The third important feature distinguishing catecholamines from neu-ronally released norepinephrine involves epinephrine's affinity for P2-recep-tors. Norepinephrine has a very limited affinity for these receptors. Therefore, circulating epinephrine causes effects that differ from those of direct sympathetic innervation, including:

• Greater stimulatory effect on the heart

• Relaxation of smooth muscle

• Gastrointestinal

• Genitourinary

Epinephrine and norepinephrine have equal affinity for P1-receptors, the predominant adrenergic receptors on the heart. However, the human heart also contains a small percentage of P2-receptors that, like P1-receptors, are excitatory. Therefore, epinephrine is capable of stimulating a greater number of receptors and causing a greater stimulatory effect on the myocardium.

Beta-two adrenergic receptors are also found on smooth muscle in several organ systems. These receptors tend to be inhibitory and cause relaxation of the smooth muscle. Vascular smooth muscle in skeletal muscle contains ar and P2-receptors. Norepinephrine, which stimulates only the excitatory aj-receptors, causes strong vasoconstriction; however, epinephrine, which stimulates both types of receptors, causes only weak vasoconstriction. The vasodilation resulting from P2-receptor stimulation opposes and therefore weakens vasoconstriction resulting from a1-receptor stimulation. Given that skeletal muscle may account for 40% of an adult's body weight, the potential difference in vasoconstriction, blood pressure, and distribution of blood flow could be quite significant.

Another noteworthy example of the relaxation of smooth muscle by way of P2-receptor stimulation involves airways. Bronchodilation, or opening of the airways, facilitates airflow in the lungs. Any direct sympathetic innervation to the lungs is irrelevant in this respect because only circulating epinephrine is capable of stimulating these receptors on airway smooth muscle.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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