LM 1.14: Sympathetic Nervous System

Question 1

what happens when the entire SNS is activated?

Answer 1

there is a cascade of reactions from all the organ systems of the body, which prepare the individual to deal with an emergency

this includes an increase in heart rate, bronchial dilation, increase in cardiac output, and dilation of pupils, all of which are directed towards heightened awareness and preparation to combat danger

blood circulation is preferentially targeted towards skeletal muscle, with a reduction in blood flow towards non-essential organs –> therefore, there is vasoconstriction in the gatrointestinal tract and skin, and compensatory piloerection to allow the body to remain warm

Question 2

what are the short term and long term functions of the SNS?

Answer 2

short term = activated in response to physical danger to deal with an emergency

long term = activated in response to physchological and emotional stress

Question 3

what is another name for the SNS?

Answer 3

thoracoculmbar outflow

this is because of the location of the pre-ganglionic sympathetic nerve cell bodies

Question 4

what is the location of the preganglionic sympathetic cell bodies?

Answer 4

preganglionic sympathetic cell bodies = GVE

the GVEs lie in the lateral horn of the gray matter in a “silo” known as the intermediolateral cell column (IML)

the IML of the lateral gray horn extends from T1-L2/L3 spinal cord segments

the IMLs in T1-L2 lateral horns are somatotopically arranged –> this means that a given body region is ‘mapped’ to a specific CNS region such as the IMLs or the somatosensory cortex

analogous, in principle, to how somatic sensations (e.g. touch) is mapped to the somatosensory cortex, the sympathetic motor pathways from T1-L2 have a somatotopic organization such that visceral structures in the superior ventral body cavity, upper trunk wall or upper limbs receive innervation from more superior IMLs and structures in the inferior ventral body cavity, lower trunk wall or lower limbs receive innervation from more inferior IMLs

so like the the head and thoracic viscera of the foregut are mapped in T1-6 of the IML, abdominal viscera of the midgut are mapped in T7-11 of the IML, and the pelvic viscera of the hindgut are mapped in T12-L2

Question 5

what are the rami communicantes?

Answer 5

a communicating branch between a spinal nerve and the sympathetic trunk

the grey and white rami communicantes are responsible for conveying autonomic signals - specifically for the sympathetic nervous system

their difference in “coloration” is caused by differences in myelination of the nerve fibers contained within. Actually, there are simply more myelinated than unmyelinated fibers in the white rami communicantes while the opposite is true for the grey rami communicantes

Question 6

where are white vs. grey rami communicantes found?

Answer 6

ONLY the T1-L2 spinal cord segments have white rami communicantes through which myelinated T1-L2 preganglionic GVEs gain entry to the sympathetic trunk

ALL spinal cord segments (C1-Co) have grey rami communicantes through which unmyelinated sympathetic postganglionic GVEs exit for parietal distribution to the neck, viscera, body wall, and limbs

Question 7

where are white rami communicantes located?

Answer 7

white rami communicantes exist only at the levels of the spinal cord where the intermediolateral cell column or lateral horn is present which is T1-L2

Question 8

what is the function of the white rami communicantes?

Answer 8

they are are responsible for carrying preganglionic nerve fibers from the spinal cord to the paravertebral ganglia

a single myelinated preganglionic sympathetic GVE makes synaptic contact with usually >30 postganglionic sympathetic GVEs in a single autonomic ganglia

white rami contain mostly myelinated fibers, they’re found only at the T1-L2 spinal cord segments, they are the entrance ramps of GVEs and preganglionic GVEs enter chain via WRC and travel 1 of 5 different pathways

Question 9

what is the function of the grey rami communicantes?

Answer 9

grey rami communicantes exist at every level of the spinal cord from the paravertebral ganglia

they are responsible for carrying postganglionic nerve fibers from the paravertebral ganglia to their destination, and for carrying those preganglionic nerve fibers which enter the paravertebral ganglia but do not synapse

cell bodies of the unmyelinated postganglionic sympathetic GVEs are located in paravertebral or prevertebral autonomic ganglia OR as part of the adrenal medulla

so grey rami communicantes are exit ramps for GVEs, contain mostly unmyelinated fibers, are found at all spinal cord segments and the GVEs destined for parietal distribution to the neck, body wall and limbs exit GRCs

Question 10

what are the two types of sympathetic ganglia?

Answer 10

1. paravertebral

2. prevertebral

Question 11

where are the paravertebral ganglion located?

Answer 11

they’re one of the two types of ganglia associated with the sympathetic nervous system

paravertebral ganglion run on both sides of the spine from the upper cervical spine, superior cervical ganglion at C2-3 level, inferiorly to the ganglion impar (also known and the azygous ganglion or ganglion of Walther)

embryologically, they develop as 31 L/R pairs of interconnected paravertebral ganglia alongside of the 31 segmental spinal cord levels and related somites however, by birth several adjacent ganglia on either side fuse and form larger ganglia such that there are actually ≈20-25 ganglia on each side –> the two chains have the appearance of “beads-on-a-string” and are found bilateral to the vertebral column from C1-Co…hence the name “paravertebral”

the ganglia are distinguished as cervical, thoracic, lumbar, and sacral and, except in the neck, they closely correspond in number to the vertebrae

Question 12

how many paravertebral ganglia are there?

Answer 12

3 cervical ganglia

12 thoracic ganglia

4 lumbar ganglia

4-5 sacral ganglia

only the cervical ganglia have specific names

Question 13

where do the cervical ganglia project into their targets?

Answer 13

1. the superior cervical ganglion is the largest and is adjacent to C2 & C3 and the postganglionic axons project to the targets = heart and neck or the head via “hitchhiking” on the carotid arteries
2. the middle cervical ganglion is the smallest and is adjacent to C6 and projects into the heart and neck
3. the inferior cervical ganglion may be fused with the first thoracic ganglion to form a single structure, the stellate ganglion, adjacent to C7 and project to the targets = heart, lower neck, arm, and posterior cranial arteries

nerves emerging from cervical sympathetic ganglia contribute to the cardiac plexus and the ansa sublclavius, among other things

Question 14

what is the ganglion impair?

Answer 14

in the coccygeal region the 1 paravertebral ganglion called the ganglion impar is where the R & L sympathetic chains unite in the midline

Question 15

what are prevertebral ganglia?

Answer 15

the second type of sympathetic ganglia

they lie between the paravertebral ganglia and the target organ

Question 16

what is the function of the prevertebral ganglia?

Answer 16

the pre-vertebral ganglia are the nodules where preganglionic neurons synapse with their postganglionic counterparts

the nerves that synapse in the prevertebral ganglia innervate the pelvic viscera = enteric nervous system, renal system, bladder, etc.

Question 17

what are the 4 major prevertebral ganglia?

Answer 17

1. celiac ganglia - at the celiac trunk
2. aortico-rental ganglion = at the junction of the renal artery and the abdominal aorta
3. superior mesenteric ganglia and plexus = at the superior mesenteric artery
4. inferior mesenteric ganglia and plexus = at the inferior mesenteric artery

Question 18

what is the function of the splanchnic nerves?

Answer 18

they convey sympathetic fibers between the paravertebral and prevertebral ganglia

the splanchnic nerves are made up of Sympathetic GVEs (motor) and their accompanying GVAs (sensory) associated with the viscera

the splanchnic nerves to the Pre-vertebral Ganglia & Plexuses are divided into Cardiopulmonary and Abdominoplevic divisions

Question 19

why are cardiac splanchnics not considered true splanchnics?

Answer 19

because they are actually made up of post-ganglionic sympathetic fibers

they convey POSTganglionic sympathetic GVEs and, as such they do not classify them as splanchnic nerves

Question 20

what do the cardiopulmonary splanchnic nerves consist of?

Answer 20

1. superior Cardiac Branch –> Superior cervical ganglion
2. middle Cardiac Branch –-> Middle cervical ganglion
3. Inferior Cardiac Branch –-> Inferior cervical or Stellate ganglion

Question 21

what do the abdominopelvis splanchnic nerves consist of?

Answer 21

1. celiac ganglion and plexus –> greater splanchnic nerve T5-T9
2. lesser splanchnic nerve = T10-11
3. aortico-rental ganglion and plexus – least splanchnic nerve = T12
4. superior mesenteric ganglion and plexus – upper lumbar splanchnic nerves = L1-L3
5. inferior mesenteric ganglion plexus – lower lumbar splanchnic nerves = L3-L5
6. inferior hypogastric plexus = sacral splanchnic nerves = S1-S5

Question 22

what are the sacral splanchnic nerves?

Answer 22

splanchnic nerves that connect the inferior hypogastric plexus to the sympathetic trunk in the pelvis

the sacral sympathetic nerves arise from the sacral part of the sympathetic trunk, emerging anteriorly from the lower lumbar and sacral ganglia

they travel to the ipsilateral inferior hypogastric plexus, where the preganglionic nerve fibers synapse with the postganglionic sympathetic neurons

the post-ganglionic fibers ascend to the superior hypogastric plexus, the aortic plexus, and the inferior mesenteric plexus, where they are distributed to the anal canal

from the inferior hypogastric plexus, they also innervate pelvic organs and vessels

the sacral sympathetic nerves contain a mix of preganglionic and postganglionic sympathetic fibers, but mostly preganglionic. They also contain general visceral afferent fibers

Question 23

what are the pelvic splanchnic nerves?

Answer 23

they arise from the sacral spinal nerves (S2-4) to provide parasympathetic fibers to the inferior hypogastric plexus

Question 24

what neurotransmitter is released from both the sympathetic and parasympathetic divisions of the nervous system?

Answer 24

ACh

ACh is released from the preganglionic nerve fibers and binds to Nicotinic receptors post-synaptically on the cell bodies of post-ganglionic fibers