19.5:
Sympathetic Signaling
Sympathetic signaling activates the sympathetic nervous system through the interactions between neurotransmitters and their receptors on target cells.
The preganglionic fibers of the sympathetic division release the neurotransmitter acetylcholine, which binds to nicotinic receptors on postganglionic neurons, activating them.
All activated sympathetic postganglionic fibers, with the exception of those that innervate sweat glands, release norepinephrine.
The released norepinephrine binds to specific adrenergic receptors present on effector cells.
There are two kinds of adrenergic receptors: alpha and beta.
Alpha receptors can further be categorized into alpha-1 and alpha-2 receptors.
Alpha-1 receptors, found in smooth muscles, generally exert an excitatory effect on the host cell, while alpha-2 receptors, located on preganglionic fibers, exert an inhibitory effect.
Beta receptors are divided into beta-1, beta-2, and beta-3 receptors.
Beta-1 receptors, found in cardiac muscles, pituitary gland, and adipose cells, increase heart rate and boost metabolism.
Beta-2 receptors, located in smooth muscles lining the airways, relax respiratory muscles, widening airways for easier breathing.
Beta-3 receptors, found in the brown adipose tissues, cause lipolysis.
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.