Human Thermoregulation and Sudomotor Function

Thermoregulation in humans is a complex physiological process that maintains a stable internal body temperature. This is achieved through a variety of mechanisms, including metabolic heat production, peripheral blood flow regulation, and evaporative heat loss primarily via sweat production. The eccrine sweat glands, distributed across the body surface, play a central role in this process.

Eccrine Sweat Glands and Sweat Production

Eccrine sweat glands are coiled tubular structures located in the dermis and connected to the skin surface via a duct. These glands are innervated by the sympathetic nervous system, specifically cholinergic fibers. Activation of these fibers stimulates the secretion of a hypotonic saline solution (sweat) onto the skin surface.

Sweat Composition

Sweat is primarily water, but also contains electrolytes such as sodium, chloride, potassium, and bicarbonate. It also includes small amounts of urea, lactate, and ammonia.

Mechanism of Secretion

The secretory cells of the eccrine gland actively transport chloride ions into the lumen of the gland. This creates an osmotic gradient, drawing water into the lumen. Sodium and other electrolytes follow passively. As the fluid moves through the duct, sodium and chloride are reabsorbed, resulting in a hypotonic solution.

Neural Control of Sudomotor Activity

Sudomotor activity (sweat gland function) is primarily controlled by the sympathetic nervous system. The hypothalamus, acting as a central control center, receives input from thermoreceptors in the skin, internal organs, and the hypothalamus itself. This input is integrated to determine the appropriate response needed to maintain body temperature.

Hypothalamic Pathways

The hypothalamus sends signals to preganglionic sympathetic neurons in the spinal cord. These neurons synapse with postganglionic sympathetic neurons that innervate the eccrine sweat glands. The neurotransmitter released at the sweat gland is acetylcholine, a notable exception to the typical adrenergic neurotransmitter release of the sympathetic nervous system.

Factors Influencing Sudomotor Output

• Ambient Temperature: Increased ambient temperature activates thermoreceptors, triggering increased sudomotor activity.
• Physical Activity: Exercise increases metabolic heat production, leading to elevated body temperature and increased sweat production.
• Emotional Stress: Psychological stress can activate the sympathetic nervous system, leading to increased sweating, particularly in the palms, soles, and axillae.
• Hormonal Influences: Certain hormones, such as epinephrine and norepinephrine, can stimulate sweat production. Menopause in women is often associated with hot flashes, which are characterized by sudden increases in body temperature and sweating.
• Medical Conditions: Certain medical conditions, such as hyperthyroidism, diabetes, and some neurological disorders, can affect sudomotor function, leading to increased or decreased sweating. Certain medications can also alter sweat production.

Types of Sweat Glands

While eccrine glands are primarily responsible for thermoregulation, apocrine sweat glands also contribute to sweat production, although their primary function is not thermoregulation. Apocrine glands are located in the axillae, groin, and areolae and produce a thicker, more viscous sweat that contains lipids and proteins. This sweat is odorless when secreted, but bacteria on the skin break it down, producing body odor.

Disorders of Sweat Production

• Hyperhidrosis: Excessive sweating, which can be localized or generalized.
• Hypohidrosis/Anhidrosis: Reduced or absent sweating, which can be a sign of underlying medical conditions.