Autonomic Plexuses
- In the thorax, abdomen, and pelvis, axons of both sympathetic and parasympathetic neurons form tangled networks called autonomic plexuses,
- many of which lie along major arteries.
- The autonomic plexuses also may contain sympathetic ganglia and axons of autonomic sensory neurons.
- The major plexuses in the thorax are the
- cardiac plexus, which supplies the heart,and
- the pulmonary plexus, which supplies the bronchial tree.
- The abdomen and pelvis also contain major autonomic plexuses
- and often the plexuses are named after the artery along which they are distributed.
- is the largest autonomic plexus
- and surrounds the celiac trunk.
- two large celiac ganglia,
- two aorticorenal ganglia,
- and a dense network of autonomic axons
- and is distributed to the stomach, spleen, pancreas, liver, gallbladder, and adrenal medullae.
- contains the superior mesenteric ganglion and
- supplies the small intestine and proximal colon.
- contains the inferior mesenteric ganglion,
- which innervates the distal colon and rectum.
- Axons of some sympathetic postganglionic neurons from the inferior mesenteric ganglion also extend through the hypogastric plexus,
- which is anterior to the fifth lumbar vertebra,
- to supply the urinary bladder and genital organs.
- contains the renal ganglion
- and supplies the renal arteries within the kidneys and ureters.
Pathway from Spinal Cord to Sympathetic Trunk Ganglia
- Cell bodies of sympathetic preganglionic neurons are part of the lateral gray horns of all thoracic segments and of the first two lumbar segments of the spinal cord.
- The preganglionic axons leave the spinal cord along with the somatic motor neurons at the same segmental level.
- the myelinated preganglionic sympathetic axons pass into the anterior root of a spinal nerve and
- enter a short pathway called a white ramus before passing to the nearest sympathetic trunk ganglion on the same side .
- Collectively, the white rami are called the white rami communicantes (ko - -mu--ni-KAN-te-z; singular is ramus communicans).
- Thus, white rami communicantes are structures containing sympathetic preganglionic axons that connect the anterior ramus of the spinal nerve with the ganglia of the sympa-thetic trunk.
- The “white” in their name indicates that they contain myelinated axons.
- Only the thoracic and first two or three lumbar nerves have white rami communicantes.
- The paired sympathetic trunk ganglia are arranged anterior and lateral to the vertebral column, one on either side.
- 3 cervical,
- 11 or 12 thoracic,
- 4 or 5 lumbar,
- 4 or 5 sacral sympathetic trunk ganglia,
- and 1 coccygeal ganglion.
- The right and left coccygeal ganglia are fused together and usually lie at the midline.
- Although the sympathetic trunk ganglia extend inferiorly from the neck, chest, and abdomen to the coccyx, they receive preganglionic axons only from the thoracic and lumbar segments of the spinal cord.
The cervical portion of each sympathetic trunk
- is located in the neck
- and is subdivided into
- superior,
- middle,
- and inferior ganglia.
- serve the head and heart.
- sweat glands,
- smooth muscle of the eye,
- blood vessels of the face,
- lacrimal glands,
- pineal gland,
- nasal mucosa,
- salivary glands (which include the submandibular, sublingual, and parotid glands),
- and the heart.
- innervate the heart.
The thoracic portion of each sympathetic trunk
- lies anterior to the necks of the corresponding ribs.
- This region of the sympathetic trunk receives most of the sympathetic preganglionic axons.
- innervate the heart,
- lungs,
- bronchi,
- and other thoracic viscera.
- these neurons also innervate sweat glands,
- blood vessels,
- and arrector pili muscles of hair follicles.
- lies lateral to the corresponding lumbar vertebrae.
- lies in the pelvic cavity on the medial side of the anterior sacral foramina.
to Visceral Effectors
Axons leave the sympathetic trunk in four possible ways:
(1) They can enter spinal nerves;
(2) they can form cephalic periarterial nerves;
(3) they can form sympathetic nerves; and
(4) they can form splanchnic nerves.
SPINAL NERVES
some of the incoming sympathetic preganglionic neurons synapse with postganglionic neurons in the sympathetic trunk,
- either in the ganglion at the level of entry
- or in a ganglion farther up or down the sympathetic trunk.
- The axons of some of these postganglionic neurons leave the sympathetic trunk by entering a short pathway called a gray ramus
- and then merge with the anterior ramus of a spinal nerve.
- Therefore, gray rami communicantes are structures containing sympathetic postganglionic axons that connect the ganglia of the sympathetic trunk to spinal nerves .
- The “gray” in their name indicates that they contain unmyelinated axons.
- Gray rami communicantes outnumber the white rami because there is a gray ramus leading to each of the 31 pairs of spinal nerves.
- to the visceral effectors in the skin of the neck, trunk, and limbs,
- including sweat glands, smooth muscle in blood vessels, and arrector pili muscles of hair follicles.
- Some sympathetic preganglionic neurons that enter the sympathetic trunk ascend to the superior cervical ganglion, where they synapse with postganglionic neurons.
- The axons of some of these postganglionic neurons leave the sympathetic trunk by forming cephalic periarterial nerves, nerves that extend to the head by wrapping around
- and following the course of various arteries (such as the carotid arteries) that pass from the neck to the head .
provide sympathetic innervation to
- visceral effectors in the skin of the face (sweat glands, smooth muscle of blood vessels, and arrector pili muscles of hair follicles)
- as well as other visceral effectors of the head (smooth muscle of the eye, lacrimal glands, pineal gland, nasal mucosa, and salivary glands).
SYMPATHETIC NERVES
- Some of the incoming sympathetic preganglionic neurons synapse with postganglionic neurons in one or more ganglia of the sympathetic trunk.
- Then, the axons of the postganglionic neurons leave the trunk by forming sympathetic nerves that extend to visceral effectors in the thoracic cavity
- Sympathetic nerves provide sympathetic innervation to the heart and lungs.
Sympathetic innervation of the heart
- consists of axons of preganglionic neurons that enter the sympathetic trunk
- and then form synapses with postganglionic neurons in the superior, middle, and inferior cervical ganglia
- and first through fourth thoracic ganglia (T1–T4).
- From these ganglia, axons of postganglionic neurons exit the sympathetic trunk by forming sympathetic nerves that enter the cardiac plexus to supply the heart .
Sympathetic nerves to the lungs.
Sympathetic innervation of the lungs
- consists of axons of preganglionic neurons that enter the sympathetic trunk
- and then form synapses with postganglionic neurons in the second through fourth thoracic ganglia (T2–T4).
- From these ganglia, axons of sympathetic postganglionic neurons exit the trunk by forming sympathetic nerves that enter the pulmonary plexus to supply the smooth muscle of the bronchi and bronchioles of the lungs
- some sympathetic preganglionic axons pass through the sympathetic trunk without terminating in it.
- Beyond the trunk, they form nerves known as splanchnic nerves (SPLANK-nik;,
- which extend to outlying prevertebral ganglia.
Splanchnic nerves to abdominopelvic organs.
- Most sympathetic preganglionic axons that enter splanchnic nerves are destined to synapse with sympathetic postganglionic neurons in the prevertebral ganglia that supply the organs of the abdominopelvic cavity.
- Preganglionic axons from the fifth through ninth or tenth thoracic ganglia (T5–T9 or T10) form the greater splanchnic nerve.
- It pierces the diaphragm, and
- enters the celiac ganglion of the celiac plexus.
- From there, postganglionic neurons follow and innervate blood vessels to the stomach, spleen, liver, kidneys, and small intestine.
- Preganglionic axons from the tenth and eleventh thoracic ganglia (T10–T11) form the lesser splanchnic nerve.
- It pierces the diaphragm
- and passes through the celiac plexus to enter the aorticorenal ganglion and superior mesenteric ganglion of the superior mesenteric plexus.
- Postganglionic neurons from the superior mesenteric ganglion follow and innervate blood vessels of the small intestine and proximal colon.
- which is not always present,
- is formed by preganglionic axons from the twelfth thoracic ganglia (T12) or a branch of the lesser splanchnic nerve.
- It pierces the diaphragm
- and enters the renal plexus near the kidney.
Postganglionic neurons from the renal plexus
- supply kidney arterioles and the ureters.
- from the first through fourth lumbar ganglia (L1–L4)
- enter the inferior mesenteric plexus and
- terminate in the inferior mesenteric ganglion,
- where they synapse with postganglionic neurons.
- extend through the inferior mesenteric plexus
- to supply the distal colon and rectum;
- they also extend through the hypogastric plexus to supply blood vessels of the distal colon, rectum, urinary bladder, and genital organs.
- Postganglionic axons leaving the prevertebral ganglia follow the course of various arteries to abdominal and pelvic visceral effectors.
- Some sympathetic preganglionic axons pass, without synapsing,
- through the sympathetic trunk, greater splanchnic nerves, and celiac ganglion,
- and then extend to chromaffin cells in the adrenal medullae of the adrenal glands
- Developmentally, the adrenal medullae and sympathetic ganglia are derived from the same tissue, the neural crest.
- The adrenal medullae are modified sympathetic ganglia,
- and the chromaffin cells are similar to sympathetic postganglionic neurons except they lack dendrites and axons.
- Rather than extending to another organ, however, these cells release hormones into the blood.
- Upon stimulation by sympathetic preganglionic neurons, the chromaffin cells of the adrenal medullae release a mixture of
- catecholamine hormones—about
- 80% epinephrine,
- 20% norepinephrine,
- and a trace amount of dopamine.
Structure of the Parasympathetic Division
Cell bodies of parasympathetic preganglionic neurons are found
- in nuclei in the brain stem
- and in the lateral gray matter of the second through fourth sacral segments of the spinal cord .
- Their axons emerge as part of a cranial nerve or as part of the anterior root of a spinal nerve.
- consists of preganglionic axons that extend from the brain stem in four cranial nerves.
- consists of preganglionic axons in anterior roots of the second through fourth sacral spinal nerves.
- end in terminal ganglia,
- where they synapse with postganglionic neurons.
- has four pairs of ganglia
- and the ganglia associated with the vagus (X) nerve.
- The four pairs of cranial parasympathetic ganglia innervate structures in the head
- and are located close to the organs they innervate
1. The ciliary ganglia
- lie lateral to each optic (II) nerve near the posterior aspect of the orbit.
- Preganglionic axons pass with the oculomotor (III) nerves to the ciliary ganglia.
- Postganglionic axons from the ganglia innervate smooth muscle fibers in the eyeball.
- are located lateral to the sphenopalatine foramen,
- between the sphenoid and palatine bones.
- They receive preganglionic axons from the facial (VII) nerve
- and send postganglionic axons to the nasal mucosa, palate, pharynx, and lacrimal glands.
3. The submandibular ganglia
- are found near the ducts of the submandibular salivary glands.
- They receive preganglionic axons from the facial nerves
- and send postganglionic axons to the submandibular and sublingual salivary glands.
4. The otic ganglia
- are situated just inferior to each foramen ovale.
- They receive preganglionic axons from the glossopharyngeal (IX) nerves
- and send postganglionic axons to the parotid salivary glands.
- Preganglionic axons that leave the brain as part of the vagus (X) nerves carry nearly 80% of the total craniosacral outflow.
- Vagal axons extend to many terminal ganglia in the thorax and abdomen.
- As the vagus nerve passes through the thorax, it sends axons to the heart and the airways of the lungs.
- In the abdomen, it supplies the liver, gallbladder, stomach, pancreas, small intestine, and part of the large intestine.
- consists of preganglionic axons from the anterior roots of the second through fourth sacral spinal nerves (S2–S4).
- As the preganglionic axons course through the sacral spinal nerves, they branch off these nerves to form
Pelvic splanchnic nerves
- Pelvic splanchnic nerves synapse with parasympathetic postganglionic neurons located in terminal ganglia in the walls of the innervated viscera.
- From the terminal ganglia, parasympathetic postganglionic axons innervate smooth muscle and glands in the walls of the colon, ureters, urinary bladder, and reproductive organs.
CLINICAL CONNECTION
Horner’s Syndrome
In Horner’s syndrome,
- the sympathetic innervation to one side of the face is lost due to
- an inherited mutation,
- an injury,
- or a disease that affects sympathetic outflow through the superior cervical ganglion.
- ptosis (drooping of the upper eyelid),
- miosis (constricted pupil),
- and anhidrosis (lack of sweating).
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