The Autonomic Nervous System (ANS) manages involuntary body functions, regulating processes like heart rate, digestion, and body temperature. Sweating, a mechanism to dissipate heat, is controlled by this system. Sweating is primarily a function of the sympathetic nervous system (SNS), the branch of the ANS commonly associated with “fight-or-flight” responses.
The Dominant Role of the Sympathetic Nervous System
The sympathetic nervous system (SNS) governs the body’s rapid, systemic adjustments, including the widespread cooling mechanism of sweating. This activation is part of the body’s effort to maintain a stable internal temperature. The command center for this process is the hypothalamus.
The hypothalamus functions as the body’s thermostat, sensing internal core temperature and signaling the need for heat loss. When the internal temperature rises, the hypothalamus activates the sudomotor pathway for thermoregulation. This signal travels down the spinal cord to the intermediolateral cell column.
From the spinal cord, preganglionic sympathetic neurons exit and synapse onto postganglionic neurons in the paravertebral ganglia. These postganglionic fibers then travel out to innervate the eccrine sweat glands distributed across the skin surface. This pathway ensures a rapid and generalized sweat response across the body.
The Cholinergic Exception in Sweat Gland Signaling
The sympathetic nervous system typically uses the neurotransmitter norepinephrine for communication, a process known as adrenergic signaling. However, the sympathetic postganglionic fibers that innervate the eccrine sweat glands are an exception. These fibers release acetylcholine (ACh) at the neuro-effector junction instead of norepinephrine.
These nerve fibers are classified as “cholinergic sympathetic fibers.” The acetylcholine released binds to muscarinic receptors (specifically the M3 subtype) on the eccrine sweat gland cells, which triggers the secretion of sweat.
The exception holds pharmacological significance, as the sweating response can be effectively blocked by anticholinergic drugs like atropine. These drugs compete with acetylcholine for binding to the muscarinic receptors on the sweat gland. This blockade demonstrates the reliance of sweat secretion on cholinergic signaling, despite the pathway originating in the sympathetic nervous system.
The development of this signaling system involves a switch during early life. Sympathetic fibers initially exhibit an adrenergic nature but undergo a developmental change to become cholinergic before reaching maturity.
Thermoregulatory vs. Stress-Induced Sweating
Sweating can be triggered by two distinct stimuli, both engaging the sympathetic nervous system through different central pathways. Thermoregulatory sweating is the most common form, activated primarily by elevated core body temperature or warm environmental conditions. This widespread, watery sweat is produced by eccrine glands and is controlled by the thermosensitive preoptic center of the hypothalamus.
The neural signal for thermoregulatory sweating is generalized, leading to sweat production across the body surface to maximize heat dissipation. This response is an automatic adjustment to the body’s thermal state.
Stress-induced or emotional sweating is activated by psychological factors such as anxiety, fear, or pain. While still a sympathetic response, this type of sweating is regulated by different brain regions, including the cerebral cortex and the limbic system. These areas respond to emotional stimuli and signal the sympathetic outflow.
Emotional sweating is often localized to specific areas of the body, such as the palms of the hands, soles of the feet, and the armpits. It involves both eccrine glands in these locations and apocrine glands, particularly in the axillae. This localized response is thought to be an evolutionary holdover, potentially enhancing grip during a flight response.