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Neurophysiology I. AUTONOMIC NERVOUS SYSTEM (ANS) is a set of pathways to and from the central nervous system (CNS) that innervates and regulates smooth muscle, cardiac muscle, and glands. is distinct from the somatic nervous system, which innervates skeletal muscle. has three divisions: sympathetic...
Neurophysiology I. AUTONOMIC NERVOUS SYSTEM (ANS) is a set of pathways to and from the central nervous system (CNS) that innervates and regulates smooth muscle, cardiac muscle, and glands. is distinct from the somatic nervous system, which innervates skeletal muscle. has three divisions: sympathetic, parasympathetic, and enteric (the enteric division is discussed in Chapter 6). A. Organization of the ANS (Table 2.1 and Figure 2.1) 1. Synapses between neurons are made in the autonomic ganglia. a. Parasympathetic ganglia are located in or near the effector organs. b. Sympathetic ganglia are located in the paravertebral chain. 2. Preganglionic neurons have their cell bodies in the CNS and synapse in autonomic ganglia. Preganglionic neurons of the sympathetic nervous system originate in spinal cord segments Tl-L3 or the thoracolumbar region. Preganglionic neurons of the parasympathetic nervous system originate in the nuclei of cranial nerves and in spinal cord segments S2-S4 or the craniosacral region. 3. Postganglionic neurons of both divisions have their cell bodies in the autonomic ganglia and synapse on effector organs (e.g., heart, blood vessels, sweat glands). 4. Adrenal medulla is a specialized ganglion of the sympathetic nervous system. Preganglionic fibers synapse directly on chromaffin cells in the adrenal medulla. The chromaffin cells secrete epinephrine (80%) and norepinephrine (20%) into the circulation (see Figure 2.1). Pheochromocytoma is a tumor of the adrenal medulla that secretes excessive amounts of catecholamines and is associated with increased excretion of 3-methoxy-4-hydroxymandelic acid (VMA). B. Neurotransmitters of the ANS Adrenergic neurons release norepinephrine as the neurotransmitter. Cholinergic neurons, whether in the sympathetic or parasympathetic nervous system, release acetylcholine (ACh) as the neurotransmitter. Nonadrenergic, noncholinergic neurons include some postganglionic parasympathetic neurons of the gastrointestinal (GI) tract, which release substance P, vasoactive intestinal peptide (VIP), or nitric oxide (NO). 33 34 BRS Physiology t a b I a 2.1 Organization of the Autonomic Nervous System Characteristic Sympathetic Parasympathetic Somatic* Origin of preganglionic nerve Nuclei of spinal cord segments T1-T12; l1-L.3 (thoracolumbar) Nuclei of cranial nerves Ill, VII, IX, and X; spinal cord segments S2-S4 (craniosacral) length of preganglionic nerve axon Short Long Neurotransmitter in ganglion ACh ACh Receptor type in ganglion Nicotinic Nicotinic length of postganglionic nerve axon long Short Effector organs Smooth and cardiac muscle; glands Smooth and cardiac muscle; glands Skeletal muscle Neurotransmitter in effector organs Norepinephrine (except sweat glands, which useACh) ACh ACh (synapse is neuromuscular junction) Receptor types in effector organs a,, ~ ~,, and ~z Muscarinic Nicotinic "Somatic nervous system has been included for comparison. ACh = acetylcholine. Parasympathetic Preganglionic (....,__ _ _ _ _.......;;~;;....-------< Postganglionic ( ACh'J Sympathetic e Preganglionic ( ACh Adrenal 0 Effector organ CNS J e l Postgangl~·~ \.Nicotinic receptor (NN)
