Sweat is a colorless and odorless liquid, consisting of water and a varying content of salts, which is secreted by the so-called eccrine sweat glands. These glands are tiny tubular structures (about 0.6 mm diam.), located under the epidermis (the superficial horny layer) are embedded in the underlying dermis and connected by a small excretory duct with the skin surface. They are distributed all over the body, most densely on palms and soles (200/mm 2).
Like the muscle tissue, the sweat gland needs a nervous signal to function. Therefore, on each gland a nerve fiber ends with its terminal. Once a nervous signal (an electrical impulse) reaches the nerve ending, a transmitter substance neurotransmitter is released from the terminal and reaches and stimulates the gland to produce and expel sweat. The neurotransmitter that activates the sweat glands is a substance called acetylcholine. All nerve fibers that end on and control the sweat glands belong to the Autonomic Nervous System, specifically to Sympathetic Nervous System.
The main function of the sweat glands to cool the body, as soon as the body temperature rises. This is achieved by secretion of sweat, which evaporates on the skin surface. Evaporation is a process that requires a large amount of energy. The energy required for this purpose (thermal energy) is removed from the body. Centrally located in the brain, the body temperature is constantly monitored and fine-tuned by the Sympathetic Nervous System (SNS). When the body temperature increases and temperature receptors provide this information to the control mechanisms in the brain stem, the SNS starts transmitting nervous signals to the sweat glands. Conversely, the signals may be reduced or interrupted as soon as the temperature approaches a normal level. For some people, depending on genetic and constitutional factors, this mechanism is not optimally calibrated, leading to an exaggerated reaction (excessive sweating) compared to the actual requirements.