17.3 The Parasympathetic Division

• • The parasympathetic division of the autonomic nervous system operates through a series of interconnected neurons. Efferent parasympathetic neurons originate from cranial nerves III, VII, IX, and X and sacral spinal nerves S2-S4 and synapse with neurons within or near the innervated organ. 
• • Parasympathetic division of the ANS consists of: 
  • Preganglionic neurons located in brainstem and sacral segments of spinal cord 
  • Mesencephalon (midbrain), pons, medulla oblongata contain nuclei associated with cranial nerves III, VII, IX, and X 
  • In sacral segments of spinal cord, autonomic nuclei lie in spinal segments S2-S4 
  • Ganglionic neurons located in peripheral ganglia within or adjacent to the target organs
• Organization and Anatomy of the Parasympathetic Division. Parasympathetic preganglionic fibers leave the brain in cranial nerves III, VII, IX, and X
• Organization and Anatomy of the Parasympathetic Division. Fibers in III, VII, IX control visceral structures in head
• Organization and Anatomy of the Parasympathetic Division. These preganglionic fibers synapse in ciliary, pterygopalatine, submandibular, and otic ganglia
• Organization and Anatomy of the Parasympathetic Division. Sacral parasympathetic outflow doesn’t join ventral rami of spinal nerves
• Organization and Anatomy of the Parasympathetic Division. Instead, preganglionic fibers form distinct pelvic nerves that innervate intramural ganglia in kidney and urinary bladder, terminal portions of large intestine, sex organs
• General Functions of the Parasympathetic Division. Major effects produced by parasympathetic division (only some here):

  • Constriction of pupils, restricts amount of light entering eyes
  • Secretion by digestive/salivary/intestinal glands, pancreas, liver 
  • Secretion of hormones for absorption
  • Increased smooth muscle activity along digestive
• Parasympathetic Activation and Neurotransmitter Release. Parasympathetic neuroeffector junctions small with narrow synaptic clefts
• Parasympathetic Activation and Neurotransmitter Release. Effects of stimulation short because most of ACh released is inactivated by AChE
• Parasympathetic Activation and Neurotransmitter Release. Plasma Membrane Receptors and Responses. Two types of ACh receptors found on postsynaptic plasma membranes: 

  • (1) Nicotinic receptors on surfaces of all ganglionic neurons of parasympathetic and sympathetic divisions and neuromuscular synapses of somatic nervous system 
  • Exposure to ACh causes excitation of ganglionic neuron of muscle fiber through opening of chemically gated sodium channels in postsynaptic membrane
• Parasympathetic Activation and Neurotransmitter Release. Plasma Membrane Receptors and Responses. Two types of ACh receptors found on postsynaptic plasma membranes: 

  • (2) Muscarinic receptors found at cholinergic neuromuscular/neuroglandular junctions in parasympathetic division and few cholinergic neuroeffector junctions in sympathetic division 
  • Stimulation of muscarinic receptors produces longer effects than nicotinic receptors 
  • Response reflects activation/inactivation of enzymes, excitatory or inhibitory 
• Parasympathetic Activation and Neurotransmitter Release. Plasma Membrane Receptors and Responses. Nicotinic receptors bind nicotine. Muscarinic receptors stimulated by muscarine
• A Summary of the Parasympathetic Division. Parasympathetic division includes visceral motor nuclei in brainstem with nerves III, VII, IX, X. Autonomic nuclei lie in lateral portions of anterior horns in sacral segments S2-S4
• A Summary of the Parasympathetic Division. Ganglionic neurons located in terminal or intramural ganglia
• A Summary of the Parasympathetic Division. Parasympathetic division innervates structures in head/organs in thoracic/abdominopelvic cavities
• A Summary of the Parasympathetic Division. All parasympathetic neurons are cholinergic 

  • Release of ACh by preganglionic neurons stimulates nicotinic receptors on ganglionic neurons, effect excitatory 
  • Release of ACh at neuroeffector junctions stimulates muscarinic receptors, effects excitatory or inhibitory depending on enzymes 
• A Summary of the Parasympathetic Division. Effects of parasympathetic stimulation brief and restricted to organs/sites
• Where are intramural ganglia located?

  Intramural ganglia are located in the tissues of their target organs.
• Why does sympathetic stimulation have such widespread effects?

  Sympathetic stimulation has such widespread effects because (1) the extensive divergence of preganglionic fibers in sympathetic division distributes sympathetic output to many different visceral organs and tissues simultaneously, and (2) the release of E and NE by the adrenal medullae affects tissues and organs throughout the body.