Adrenal Medulla: Structure, Function & Diseases

The adrenal gland is divided functionally and topographically into the adrenal cortex (cortex glandulae suprarenalis) and the adrenal medulla (medulla glandulae suprarenalis). The adrenal medulla forms the smaller part of the adrenal gland. Adrenaline and noradrenaline are produced in the medulla of the adrenal gland.

What is the adrenal medulla?

The adrenal gland is a hormonal gland that sits on top of the kidney poles. Two organs are combined in the adrenal gland, which weighs about five grams. One is the adrenal cortex, where hormones are produced, and the other is the adrenal medulla, which is part of the sympathetic nervous system. Thus, the adrenal medulla is not a hormonal gland in the true sense, but an extension of the autonomic nervous system. From a developmental point of view, the adrenal medulla is a sympathetic ganglion, that is, an accumulation of nerve cells belonging to the sphere of action of the sympathetic nerve. The sympathetic nerve is also known as the fight-or-flight nerve. It puts the body on alert and ready to perform. For example, with increased sympathetic nerve activity, the heart beats faster and the bronchial tubes become dilated.

Anatomy and structure

The adrenal medulla is located inside the adrenal gland, surrounded by the adrenal cortex. Embryologically, the adrenal medulla originates from the so-called neural crest. During embryonic development, this structure gives rise primarily to structures of the peripheral nervous system. Thus, the adrenal medulla arises from a part of the nervous system. Therefore, highly specialized nerve cells, the chromaffin cells, of the sympathetic nervous system are found in the adrenal medulla. A distinction is made between the chromaffin A cells (type I) and the chromaffin N cells (type II). The cells are called chromaffin because they stain well with chromium salts. 80% of the cells of the adrenal medulla are A cells, 20% are N cells. The cells are arranged in groups or strands around minute blood vessels (capillaries and venules).

Function and tasks

Looking at the function of the adrenal medulla and specifically the chromaffin cells, it quickly becomes clear why the cells are titled A cells and N cells. The A cells of the adrenal medulla produce the catecholamine epinephrine, and the N cells produce the hormone or neurotransmitter norepinephrine. Adrenaline, also called epinephrine, is known as a stress hormone and is synthesized from the amino acids L-phenylalanine and L-tyrosine. Adrenaline increases heart rate, raises blood pressure, and dilates the bronchial tubes, allowing for deeper breathing. In addition, energy is provided through the breakdown of fat (lipolysis) and the release and production of glucose. Blood flow is centralized, increasing blood flow to vital organs and to the muscles of the legs and arms. Gastrointestinal activity, on the other hand, is inhibited. The release of adrenaline is triggered by nerve stimuli or by other hormones, for example by increased cortisol levels. Triggers can be stress, injuries, inflammation or a too low blood sugar level. If the concentration of adrenaline in the blood is too high, production is physiologically inhibited again by a negative feedback mechanism. Norepinephrine, also known as noradrenaline, is produced from dopamine by the enzyme dopamine hydroxylase. Vitamin C assumes an important function here as a cofactor. Norepinephrine is related to epinephrine, but shows partially different effects than epinephrine due to a missing methyl group in its chemical structure. The main site of action of norepinephrine is the arterioles, i.e. the small arteries in the bloodstream. Norepinephrine leads to a constriction (vasoconstriction) of these vessels. This results in an increase in blood pressure. More important than this hormonal effect, however, is the function of norepinephrine as a neurotransmitter. In the sympathetic nervous system, norepinephrine acts as a transmitter at the synapses. With the help of a neurotransmitter, excitations can be transmitted from one nerve cell to other (nerve) cells. Along with acetylcholine, norepinephrine is the most important neurotransmitter in the autonomic nervous system. Norepinephrine is secreted from the adrenal medulla primarily during stress.

Diseases

Pheochromocytoma is a tumor found predominantly in the adrenal medulla and is also the most common disease of the adrenal medulla.Most often, pheochromocytoma is hormonally active, that is, it produces adrenaline, noradrenaline and, in rare cases, dopamine. The leading symptom of pheochromocytoma is high blood pressure, since both adrenaline and noradrenaline cause an increase in blood pressure by constricting the blood vessels. The tumor does not always produce the hormones evenly. If it repeatedly releases adrenaline or noradrenaline into the blood in large quantities, seizure-like blood pressure crises occur. These are accompanied by feelings of anxiety, sweating and palpitations. The attacks can be triggered, for example, by activation of the abdominal press during bowel movements or by nicotine consumption. In both forms of pheochromocytoma, headaches, nausea, vomiting and an accelerated pulse (tachycardia) also occur. Diagnosis of pheochromocytoma is made by determining the breakdown products of the hormones in the urine. Therapy consists of surgical removal of the tumor tissue. Various diseases such as diabetes mellitus, amyloidosis, porphyria or permanent alcohol abuse can result in an underactivity of the adrenal medulla. Constant stress over a long period of time is also discussed as a triggering factor for adrenal fatigue. Underfunction of the adrenal medulla manifests itself in symptoms such as chronic fatigue, lack of energy and depression. Those affected suffer from recurrent infections, allergies and immune system disorders. They have very poor concentration and a rather short attention span. Digestion is irregular and dizziness occurs especially when standing up quickly. Characteristic of adrenal insufficiency is that the symptoms improve when the stress subsides. For example, symptoms are much milder on vacation.

Typical and common kidney diseases