Sweat Glands Are Innervated by the Sympathetic Nervous System
Sweat glands are innervated by postganglionic sympathetic nerve fibers, specifically cholinergic sympathetic C-fibers, making them sympathetically controlled despite using acetylcholine as their neurotransmitter 1.
Key Anatomical and Physiological Features
The innervation of sweat glands represents a unique exception in autonomic physiology:
Sympathetic origin with cholinergic transmission: Sweat glands receive sudomotor innervation from postganglionic, thin, unmyelinated cholinergic sympathetic C-fibers 1. This is anatomically unusual because most sympathetic postganglionic fibers are adrenergic (using norepinephrine), but sweat glands use acetylcholine as their neurotransmitter while remaining part of the sympathetic nervous system.
Spinal cord origin: The sympathetic sudomotor structures originate from the intermediolateral cell columns extending from T1-L2 spinal segments and are under hypothalamic control 2.
No parasympathetic involvement in normal function: Under physiological conditions, the parasympathetic nervous system does not control sweat glands 2, 3.
Clinical Implications
Diagnostic Testing
The sympathetic nature of sweat gland innervation forms the basis for several autonomic function tests:
Thermoregulatory Sweat Testing (TST) evaluates the integrity of central and peripheral sympathetic sudomotor pathways with 91-96% sensitivity in detecting small fiber neuropathy 1, 4.
Quantitative Sudomotor Axon Reflex Test (QSART) assesses postganglionic sympathetic sudomotor function with 75-90% sensitivity 4.
Pathological Conditions
Understanding the sympathetic control is critical for recognizing:
Horner's syndrome: Disruption of sympathetic pathways causes anhidrosis (absent sweating) on the affected side 5.
Spinal cord injury: Transection abolishes supraspinal sympathetic control of sweating. Lesions below T8-T10 typically do not produce reflex sweating, while cervical or high thoracic lesions may cause nonthermoregulatory reflex sweating as a manifestation of autonomic dysreflexia 2.
Diabetic neuropathy: Sudomotor dysfunction from sympathetic C-fiber damage leads to dry skin, impaired quality of life, and increased risk of foot ulceration 1.
Important Caveat: Pathological Parasympathetic Innervation
After injury to postganglionic sympathetic fibers, parasympathetic fibers can sometimes make aberrant functional connections with sweat glands, resulting in pathological sweating triggered by parasympathetic reflexes (such as gustatory sweating) 3. This is an abnormal phenomenon, not the normal physiological state.
Species Variation
In primates, including humans, sweat gland innervation may coexpress both cholinergic and noradrenergic phenotypes, with full expression of proteins required for noradrenergic function 6. However, the primary functional neurotransmitter remains acetylcholine, and the anatomical pathway remains sympathetic.