19.5: Sympathetic Signaling
Sympathetic signaling, a vital part of the autonomic nervous system, plays a crucial role in mobilizing the body's resources in response to stress or emergencies. It involves the transmission of nerve impulses from sympathetic preganglionic fibers to postganglionic fibers. This results in the release of specific neurotransmitters and activation of adrenergic receptors.
Sympathetic preganglionic fibers release the neurotransmitter acetylcholine (ACh) onto the ganglionic neurons in the sympathetic ganglia. This ACh binds to nicotinic cholinergic receptors on the postganglionic neurons, transmitting nerve impulses. Once activated, the postganglionic fibers release norepinephrine (NE) as the primary neurotransmitter onto their target tissues.
Adrenergic Receptors
The target tissues contain adrenergic receptors, which are classified into two main types: alpha and beta receptors. These receptors are further subdivided into specific subtypes, each with different functional effects upon activation.
Alpha-Adrenergic Receptors
Alpha-adrenergic receptors are divided into alpha-1 and alpha-2 subtypes. Alpha-1 receptors are typically found in smooth muscle cells of blood vessels, causing vasoconstriction when activated. Alpha-2 receptors are present on preganglionic neurons and exert an inhibitory effect. They are also found on the presynaptic terminal of postganglionic neurons, where they function as autoreceptors, regulating NE release. The alpha-2 receptors help coordinate sympathetic and parasympathetic activities.
Beta-Adrenergic Receptors
Beta-adrenergic receptors consist of beta-1, beta-2, and beta-3 subtypes. Beta-1 receptors are primarily found in the heart, where their activation increases heart rate and the force of contraction. Beta-2 receptors are present in smooth muscles lining the airways and cause bronchodilation. Beta-3 receptors are primarily located in adipose tissue, where their activation increases lipolysis and the breakdown of fat cells.
