Cellular Origin and Nature
Histogenetically, the cells of the medulla originate from the neural crest, specifically the ganglionic plates. Morphologically and functionally, medullary endocrinocytes are direct analogs of effector sympathetic neurons. Like classical nerve cells, they specialize in the active synthesis of catecholamines.
However, there is one critical difference: medullary endocrinocytes completely lack any processes—they have neither axons nor dendrites. This structural simplification has a profound physiological consequence. The catecholamines they produce cannot be released into a synaptic cleft as local neurotransmitters. Instead, they are secreted directly into the bloodstream, where they function as systemic hormones.
Neural Regulation of Secretory Activity
Unlike the adrenal cortex, which is controlled primarily by humoral factors, the medulla is subject to strict and rapid neural regulation. This control is exerted by the sympathetic nervous system.
Every individual endocrinocyte in the medullary zone is approached by a preganglionic sympathetic nerve fiber. This histological architecture is strikingly reminiscent of the innervation of neurons inside peripheral sympathetic ganglia. Due to this direct synaptic contact, an incoming nerve impulse immediately and directly stimulates the cell, triggering a rapid release of synthesized hormones.
Role in Acute Stress Response
The medulla is a key participant in the body's emergency response to acute stress situations. This protective mechanism is implemented simultaneously through two parallel pathways:
- Direct neural stimulation. Immediate activation of effector target organs via sympathetic nerve fibers.
- Humoral component. A massive release of catecholamines by medullary cells into the systemic circulation, globally restructuring body function.
Unique Adrenal Hemodynamics
The blood supply system of the adrenal gland is entirely unique and designed to ensure the closest possible functional relationship between the cortex and the medulla.
Blood inflow occurs via two pathways:
- Through the cortical venous system. Sinusoidal capillaries permeating the cortex continue into the medulla. The physiological significance of this phenomenon is critical: the blood reaching the central zone is already richly saturated with cortical hormones (glucocorticoids). The presence of these hormones is essential for the proper synthesis of catecholamines.
- Direct arterial supply. Specialized medullary arterioles are present in the organ. They pass transit through the entire thickness of the cortex without branching and break up into a dense capillary network exclusively within the medulla.
The capillaries of the medullary part, like those in the cortex, have fenestrated endothelium, which maximally facilitates the transport of large molecules into the bloodstream.
Blood outflow: From the capillaries, blood collects into venules, then forms a medullary venous plexus and drains into the central vein. Next, the blood flow distributes in two directions: part goes into the portal venous system, delivering hormones straight to the liver to influence its metabolism, and part goes into the inferior vena cava for immediate distribution of hormones to all other body organs.