sympathetic nervous system
ABBR: SNS
In the autonomic nervous system, those efferent (motor) circuits in which the preganglionic neurons are located in the thoracic and lumbar spinal cord and the main postganglionic neurotransmitter is norepinephrine.
Anatomy: The primary motor cells of the SNS are found in the intermediolateral column of the thoracic and lumbar segments of the spinal cord. The axons (preganglionic axons) of these neurons exit the spinal ventral roots and synapse on neurons in one or more of the peripheral sympathetic ganglia. Twenty-two sympathetic ganglia (paravertebral ganglia) lie in two chains, one on each side of the vertebral column, with approximately one ganglion per spinal cord segment. Other sympathetic ganglion cells are found in clusters more peripherally, usually entwined in nerve plexuses. Four large unpaired plexuses, the celiac, superior mesenteric, aorticorenal, and inferior mesenteric, are often called the “prevertebral sympathetic ganglia.” Extending from these plexuses are many smaller plexuses that are named for the structures they surround and innervate; e.g., cardiac plexuses, the esophageal plexus, the gastric plexus, and the prostatic plexus.
Physiology and Pharmacology: Activation of the SNS readies an organism to interact with the outside world. Sympathetic activity dilates pupils, increases heart rate, widens airways, increases circulation to skeletal muscles, decreases circulation to the gastrointestinal tract, and increases the availability of glucose, a direct source of energy. Most of these effects are produced by norepinephrine interacting with adrenergic receptors on effector cells (smooth muscle, cardiac muscle, and secretory cells). The various subtypes of adrenergic receptors are associated with characteristic effects. In general, alpha-1 and alpha-2 adrenergic receptors produce smooth muscle contraction, notably constriction of blood vessels; alpha-2 receptors also produce cardiac muscle relaxation. Beta-1 adrenergic receptors produce cardiac muscle contraction, while beta-2 receptors produce smooth muscle contraction, notably in the airways of the lung. Beta-3 receptors, which are found in adipose tissue, stimulate lipolysis. Drugs other than norepinephrine and epinephrine (adrenalin) can act on adrenergic receptors, and receptor subtypes differ in their sensitivity to specific drugs. For example, phenylephrine selectively activates and phentolamine selectively blocks alpha adrenergic receptors, while isoproterenol (isoprenaline) selectively activates and propranolol selectively blocks beta adrenergic receptors. The availability of selective drugs allows some autonomic medical symptoms to be targeted selectively.
SEE: autonomic nervous system for illus. and table; parasympathetic nervous system.