Neurology | Sympathetic Nervous System

This video delves into the sympathetic nervous system, building upon the introduction to the autonomic nervous system. It focuses on specific splanchnic nerves, target organs, and their functions. The thoracolumbar outflow (T1 to L2/L3) is discussed as the origin of preganglionic motor neurons located in the intermediolateral column or lateral gray horn. Ganglia, which are groups of cell bodies outside the central nervous system, are categorized into chain ganglia and prevertebral (collateral) ganglia. The superior, middle, and inferior cervical ganglia are introduced, with a note on the stellar ganglia formed by a fusion of the inferior cervical ganglion and T1.

Preganglionic fibers from T1-T3 ascend to the superior cervical ganglion, where they synapse. Postganglionic fibers then exit, forming the carotid plexus around the internal carotid artery. These fibers target various head and neck structures. In the eye, they dilate pupils (dilator pupillae) for far vision and flatten the lens (ciliary muscle) for accommodation. For salivary glands, norepinephrine causes vasoconstriction and thicker mucus production. For lacrimal glands, it decreases lacrimation by constricting blood vessels or directly acting on the glands. Some fibers also supply erector pili muscles, vasomotor functions to blood vessels, and sudoriferous (sweat) glands for sudorimotor control. Middle and inferior cervical ganglia primarily innervate the heart, lungs, and esophagus, forming the cardiac, esophageal, and pulmonary plexuses.

Sympathetic fibers from the cervical ganglia and T1-T5 contribute to plexuses targeting the heart, esophagus, and lungs. In the heart, it increases heart rate (positive chronotropic action) by acting on nodal cells and increases contractility (cardiac output) by acting on the myocardium, leading to increased blood pressure. In the esophagus, sympathetic stimulation decreases peristalsis. In the lungs, it dilates bronchioles to improve airflow and decreases secretions, also causing vasoconstriction of bronchial arteries to prevent fluid accumulation, while noting the difference from pulmonary arteries.

Originating from T5-T9, the greater splanchnic nerve passes through chain ganglia without synapsing to reach the celiac ganglion, a prevertebral ganglion located near the aorta. From here, postganglionic fibers innervate organs supplied by the celiac trunk. For the stomach, it inhibits peristalsis and secretions, constricts blood vessels to decrease blood flow, and contracts the pyloric sphincter to slow digestion. For the liver, it promotes glycogenolysis, increasing blood glucose. For the biliary tree, it decreases contractions and bile flow. For the pancreas, it decreases insulin production and increases glucagon production. It can also cause slight splenic contraction.

A branch of the greater splanchnic nerve directly innervates the adrenal medulla. This is a unique exception where preganglionic fibers directly stimulate chromaffin cells, considered modified postganglionic neurons. These cells release norepinephrine and epinephrine directly into the circulation, causing widespread and powerful sympathetic effects. Another branch of the greater splanchnic nerve (or sometimes directly from T10-T11) can go to the superior mesenteric ganglion. This ganglion primarily inhibits digestive processes in the duodenum (distal half), jejunum, ileum, cecum, ascending colon, and proximal two-thirds of the transverse colon by decreasing peristalsis, absorption, and secretions, largely through vasoconstriction.

The lesser splanchnic nerve, originating from T10-T11, passes through chain ganglia and synapses in the aortico-renal ganglion. Postganglionic fibers go to the kidney and ureter. Its actions include decreasing urine production and inducing renin release from juxtaglomerular cells to increase blood pressure, mainly by vasoconstricting renal blood vessels. The least splanchnic nerve, from T12, also contributes to the renal plexus, supporting these functions by decreasing peristalsis of the ureter and supporting decreased urine production.

Lumbar splanchnic nerves (L1-L2, sometimes L3) pass through chain ganglia to reach the inferior mesenteric ganglion. This ganglion innervates the descending colon, sigmoid colon, upper rectum, and distal third of the transverse colon. Its primary function is to decrease peristalsis, inhibiting defecation during a sympathetic response.

Fibers from T12-L3 pass through chain ganglia to the superior hypogastric plexus and intermesenteric plexus. From the superior hypogastric plexus, fibers contribute to the detrusor muscle of the bladder, relaxing it to prevent urination, and to the internal urethral sphincter, causing constriction. The superior hypogastric plexus also gives rise to hypogastric nerves which contribute to the inferior hypogastric plexus. Sacral splanchnic nerves (from T10-L2) and contributions from the hypogastric nerves form the inferior hypogastric plexus, which innervates the gonads. In males, it initiates ejaculation and causes seminal fluid production. In females, it causes uterine contractions in non-pregnant women, but receptors can switch during pregnancy to prevent contractions.

The sympathetic nervous system receives significant input from higher brain centers. The hypothalamus and limbic nuclei are major contributors, influencing sympathetic activity. Cortical control is also possible, as seen in biofeedback techniques where individuals can voluntarily influence physiological responses like heart rate. These higher centers send presynaptic fibers to stimulate the preganglionic motor neurons in the spinal cord, activating the sympathetic 'fight or flight' response.