Sympathetic Nervous System

Overview
  • The Sympathetic Nervous System (SNS) is a division of the autonomic nervous system that exerts subconscious control on a variety of visceral functions. Here we discuss specific features of the SNS; however, general themes are discussed on the Autonomic Nervous System page.
Physiological Anatomy
  • Overview
    • Like all autonomic pathways those of the SNS are composed of two-neuron systems composed of pre- and post-ganglionic neurons. The cell bodies of preganglionic neurons reside within the CNS and their axon extends to specialized sympathetic ganglia outside of the CNS where they synapse on postganglionic neuron cell bodies. The axons of these postganglionic neurons then extend to the target organ.
  • Preganglionic Neurons
    • SNS preganglionic neuron cell bodies reside within the spinal cord, between segments T1 and L2. Upon exiting at these levels, the nerves enter a chain of ganglia that lies adjacent and parallel to the spinal cord, known as the "Sympathetic Chain". Most preganglionic nerves synapse on postganglionic neurons within the sympathetic chain at or near the level from which they originally exited the spinal cord. As a consequence, the vast majority of preganglionic sympathetic fibers are extremely short, only traversing the distance from the spinal cord to the adjacent sympathetic chain. However, a number of preganglionic neurons exit the sympathetic chain and travel to a handful of peripheral sympathetic ganglia not located within the sympathetic chain. Finally, a third set of preganglionic neurons travel to and synapse directly on cells of the adrenal medulla.
  • Postganglionic Neurons
    • Sympathetic Postganglionic exit and travel to their target organs. As most postganglionic neurons originate within the sympathetic chain, the average sympathetic postganglionic neuron has a long axon that traverses a lengthy distance to the target organ.
  • Overall Organization
    • The distribution of sympathetic fibers is not as cleanly organized as that of somatic nerves which display a dermatomal distribution. Roughy speaking, nerves that originate at higher segments along the sympathetic chain tend to innervate more caudal organs and vessels; however, there is signifiant geographic overlap.
The Adrenal Medulla
  • The adrenal medulla represents a special case within the SNS. It is directly innervated by preganglionic neurons that secrete acetylcholine on to nicotinic receptors of the medullary chromaffin cells. This stimulates synthesis and release of circulating catecholamines by the chromaffin cells. As such, the adrenal medulla can be thought of as a specialized ganglia within the SNS. However, rather than sending out postganglionic fibers, the adrenal medulla simply secretes circulating catecholamines.
  • The medullary catecholamines mostly consist of soluble norepinephrine (80%) and a smaller proportion of soluble epinephrine. These physiological function of these circulating catecholamines will be discussed in further detail in an adrenal medullary physiology page, to be written.
Neurotransmitters and Receptors
  • Sympathetic Ganglia
    • The neurotransmitter/receptor combination at all ganglia is similar and is identical to that of the parasympathetic nervous system. Preganglionic neurons secrete acetylcholine which binds the nicotinic acetylcholine receptor on the postganglionic cell body. This interaction is excitatory and thus acts to transmit stimulate the postganglionic cell to fire, transmitting the stimulus through the ganglion.
  • Target Organs
    • The situation at the target organ is much more complicated. The vast majority of postganglionic neurons secrete norepinephrine at their synapse with the target organ. However, a large variety of adrenergic receptors can be expressed by the target organ at the synapse. These include Alpha1, Alpha2, Beta1, and Beta2 Receptors. Binding of these receptors can have a variety of effects on the target organs depending on the downstream signaling cascades that are activated in particular tissues.
    • One exception to this rule is that a handful of sympathetic postganglionic neurons secrete acetylcholine at their receptors. These cholinergic postganglionic neurons only innervate sweat glands and bind muscarinic receptors on the eccrine cells.
Function
  • Overview
    • Below we detail the diverse effects that stimulation of SNS fibers exert on target organs. As mentioned, these effects are mediated by a number of different adrenergic receptors on target organs. It is important to know which adrenergic receptors are associated with which organs and the specific effects of their stimulation. This will allow for a better understanding of the effects of drugs that specifically stimulate particular adrenergic receptors.
    • Sadly, there is no clear-cut common theme to capture the effects of the SNS; however, SNS outflow generally appears to render the body more capable of withstanding stressful situations. This involves increasing the systemic arterial pressure, increasing heart rate and contractility, diverting blood flow from the intestines and kidneys toward muscles, and increasing blood glucose concentrations. These are all physiological response that would be expected in an individual encountering a highly stressful situation that may require a "Fight or Flight" response.
    • In such stressful situations, the entire SNS appears to be activated with outflow occurring to all target organs simultaneously. However, the SNS is capable of selective stimulation of specific target organs as well
  • Eyes
    • Pupillary Dilation: Via stimulation of Alpha1 Receptors on the pupillary dilator muscle
  • Glands
    • Sweating: Via stimulation of muscarinic receptors on eccrine glands and apocrine glands. This is the only context in which sympathetic postganglionic fibers release acetylcholine.
  • GI Motility
    • Although GI motility is directly controlled by the enteric nervous system, sympathetic fibers can modulate its activity. In general, sympathetic outflow via a variety of adrenergic receptors inhibits peristalisis and constricts GI sphincters. These effects are discussed in more detail under autonomic GI neural control.
  • Heart
    • Increased heart rate and contractility: All of the effects of the SNS on the heart are mediated via beta1 receptors and act to increase the heart rate and contractility. Together, these effects increase cardiac output and are discussed in further detail under autonomic cardiac regulation.
  • Vasculature
    • Increased Systemic Vascular Resistance (SVR): Via alpha1 receptors on arterioles, stimulation of the SNS is a powerful systemic vasoconstrictor, yielding dramatically increased SVR. This effect, together with the enhanced cardiac output, can yield impressive increases in the systemic arterial pressure.
    • Increased venous return: SNS stimulation of alpha1 receptors in the venous system results in venoconstriction, thus pushing blood back to the heart, enhancing venous return and consequently cardiac preload. An in-depth physiological explanation of this phenomenon can be found in cardiocirculatory integration
  • Kidneys
    • Reduce GFR: Via alpha1 receptors stimulates vasoconstriction of renal afferent and efferent arterioles, yielding reduced renal blood flow and GFR (Discussed in more detail in SNS - Renal Effects)
    • Increase Salt and Water Resorption: Via beta1 receptors stimulates salt and water resorption from the proximal tubule.
  • Genitourinary System
    • Urinary Retention: The SNS increases internal urethral sphincter tone via alpha1 receptors and also relaxes the bladder detrusor muscle via beta2 receptor stimulation. Together, these effects promote urinary retention.
    • Ejaculation: Although the parasympathetic nervous system is required for erection, the SNS triggers ejaculation.
  • Metabolism
    • Gluconeogenesis and Glycogenolysis: Via a variety of adrenergic receptors, the SNS stimulates gluconeogenesis and glycogenolysis, yielding increased blood glucose concentrations and thus shunting energy to active muscles.