交感神経のシグナル伝達

Sympathetic signaling involves two key neurotransmitters released sequentially. Preganglionic fibers release acetylcholine, which binds to nicotinic receptors on postganglionic neurons. Postganglionic fibers then release norepinephrine onto target tissues, except those innervating sweat glands. This dual release mechanism activates the sympathetic response throughout the body.

Alpha-1 receptors, found in smooth muscle cells, produce excitatory effects like vasoconstriction when activated. Alpha-2 receptors, located on preganglionic fibers and presynaptic terminals, exert inhibitory effects and function as autoreceptors regulating norepinephrine release. This dual mechanism coordinates sympathetic and parasympathetic activities effectively.

Beta-1 receptors in cardiac muscle increase heart rate and force of contraction, boosting metabolism and energy availability. Beta-2 receptors in airway smooth muscle cause bronchodilation, widening airways for easier breathing during sympathetic activation. Together, these receptors mobilize the body's resources during stress or emergencies.

Beta-3 receptors are primarily found in brown adipose tissue throughout the body. When activated, they trigger lipolysis, the breakdown of fat cells, increasing energy availability for immediate use. This metabolic response supports the sympathetic nervous system's role in mobilizing resources during stress.

Preganglionic fibers release acetylcholine into sympathetic ganglia, where it binds to nicotinic cholinergic receptors on postganglionic neurons. This binding transmits nerve impulses and activates the postganglionic fibers, enabling them to release norepinephrine onto target tissues. This sequential activation is essential for sympathetic signaling.

Sympathetic signaling mobilizes the body's resources in response to stress or emergencies through neurotransmitter-receptor interactions. Norepinephrine binds to alpha and beta receptors on target tissues, triggering widespread effects including increased heart rate, bronchodilation, and metabolic activation. This coordinated response prepares the body for fight-or-flight situations.

Norepinephrine is released by postganglionic fibers to activate adrenergic receptors on target tissues, producing widespread sympathetic effects throughout the body. This neurotransmitter binds to alpha and beta receptors, triggering responses like increased heart rate, bronchodilation, and metabolic activation. Sweat glands are the notable exception, using acetylcholine instead.