19.9:
Parasympathetic Signaling
Parasympathetic signaling relays information from the nervous system to target organs, promoting rest, digestion, and relaxation responses. This pathway uses the neurotransmitter acetylcholine and its specific receptors, called cholinergic receptors.
There are two types of cholinergic receptors: nicotinic receptors in postganglionic fibers and muscarinic receptors in target tissues such as smooth muscles, cardiac muscles, and glands.
The parasympathetic signaling pathway begins when preganglionic neurons release acetylcholine. The released acetylcholine then binds and activates the nicotinic receptors on the postganglionic fibers.
The activated cells in postganglionic fibers release additional acetylcholine, subsequently stimulating the muscarinic receptors on the target tissue. Muscarinic receptors can either promote excitatory or inhibitory effects.
For instance, the activation of muscarinic receptors leads to the relaxation of the sphincter muscles in the gastrointestinal tract. This promotes the flow of food particles through the digestive system, aiding digestion.
In contrast, activation of muscarinic receptors in the smooth muscle lining of the bladder causes their activation and subsequent contraction. This contraction promotes urination.
19.9:
Parasympathetic Signaling
Parasympathetic signaling plays a crucial role in regulating various physiological processes. It involves the release of acetylcholine (ACh) by parasympathetic neurons, which can have localized and short-lived effects. The majority of ACh released is rapidly inactivated at the synapse by the enzyme acetylcholinesterase (AChE), which hydrolyzes Ach into choline and acetate. Additionally, the tissue cholinesterase deactivates any ACh diffusing into the surrounding tissues.
The effects of parasympathetic stimulation are tightly controlled and depend on the specific receptors involved, allowing for precise modulation of target organs and tissues. There are two main types of cholinergic receptors: nicotinic and muscarinic receptors.
Nicotinic Receptors
Nicotinic receptors are found on postganglionic cells of both the parasympathetic and sympathetic divisions and at neuromuscular junctions in the somatic nervous system. Activation of these receptors by ACh leads to the opening of chemically gated sodium (Na+) channels in the postsynaptic membrane of the postganglionic neuron or motor end plate of the muscle fiber. This excitation results in the activation of the respective cell.
Muscarinic Receptors
Muscarinic receptors are found at cholinergic neuromuscular or neuroglandular junctions in the parasympathetic division and a few cholinergic junctions in the sympathetic division. Muscarinic receptors are G protein-coupled receptors, and their stimulation and subsequent activation of G proteins produce longer-lasting effects than nicotinic receptors. The response elicited can be either excitatory or inhibitory, depending on the activation or inactivation of specific enzymes.