Lecture 3: Autonomic Innervation Flashcards
List some general functions of the sympathetic and parasympathetic innervation systems.
Sympathetic: “fight or flight”: increase heart rate, strength of contraction, and volume; dilate coronary arteries; dilate pulmonary passages; constrict pulmonary arteries; shunt blood away from digestive/urinary systems.
Parasympathetic: “rest and digest”. Decrease heart rate, strength of contraction, and volume; constrict coronary arteries; constrict pulmonary passages; dilate pulmonary arteries; dilate passages feeding digestive/urinary systems.
Describe the sympathetic pathway to internal organs. How does the organ location change the pathway?
Cell body is in T1-T5 for thoracic organs; cell body in T6-L2 for abdominal organs. Pathway originates in lateral horn, passes through ventral root, then enters the white ramus communicans.
If the target is a thoracic organ, the neuron synapses in the paravertebral ganglion and continues (as a thoracic splanchnic nerve) through the thoracic cavity, to the target organ.
If the target is an abdominal organ, the neuron passes through the paravertebral ganglion without synapsing, continues (as an abdominal splanchnic nerve) through the abdominal cavity to the prevertebral ganglion. Here it synapses, and then the postganglionic neuron travels to the target organ.
Describe the sympathetic pathway to somatic structures. How does the structure’s location change the pathway?
Cell bodies reside in lateral horns between levels T1-L2. The preganglionic neuron passes through the ventral root, to the white ramus communicans, and then enters the sympathetic trunk if needed to reach a different level. The synapse occurs in the paravertebral ganglion at the level of the structure. The postganglionic neuron travels through the gray ramus communicans, to the ventral ramus if hypaxial (the dorsal ramus if epaxial), to the target structure.
Describe the parasympathetic pathway to internal organs. How does the location of the organ change the pathway?
Parasympathetic pathways originate either in the brain or in the lateral horn of S2-S4.
If the target organ is thoracic or upper abdominal: brain > vagus nerve > wall of target organ > synapse in organ wall > innervate organ.
If target organ is lower abdominal or pelvic: lateral horn of S2-S4 > ventral root > pelvic splanchnic nerve > wall of target organ > synapse in organ wall > innervate organ.
Your patient is experiencing a myocardial infarction with associated angina. Describe the pathway that is involved in this sensation.
Ischemia causes a visceral pain pathway to engage. The visceral pain pathway of a thoracic organ follows the sympathetic pathway, so:
Organ > thoracic cavity as thoracic splanchnic nerve > T1-L2 paravertebral ganglion > white ramus communicans > dorsal root ganglion (cell body) > dorsal root > dorsal horn > CNS
Your patient is in labor and experiencing painful contractions. Describe the pathway involved in experiencing this sensation.
Visceral pain pathway of a pelvic organ follows the parasympathetic pathway backwards. Pelvic organ > pelvic splanchnic nerve > dorsal root ganglion > dorsal root > dorsal horn.
Your patient is complaining of nausea. Explain the pathway involved in this sensation.
Nausea is experienced as a result of a visceral non-pain pathway. It follows the parasympathetic pattern.
If the origin was upper abx: organ > vagus nerve > brain.
If the origin was lower abx: organ > pelvic splanchnic nerve > ventral ramus > dorsal root > dorsal root ganglion (cell body) > CNS.
What is the difference between visceral pain and other visceral afferents?
- ) The stimuli: visceral pain is caused by stimuli such as ischemia, cramping, or stretching, while non-pain afferents carry sensations resulting from nausea, hunger, etc.
- ) Pathways: visceral pain pathways follow a backwards sympathetic or parasympathetic pathway, depending on whether they originate in a thoracic/abx organ or a pelvic organ (respectively). Other viscersal afferents follow parasympathetic pathways backwards.
What is referred pain?
Stimulus affects an internal organ. This can cause visceral pain, but also can cause referred somatic pain: pain in the body wall of the same dermatome to which the affected organ belongs, even though there is no direct stimulus initiating that somatosensory pathway.
How is referred pain clinically significant?
Visceral pain is very diffuse and difficult for patients to localize. However, somatic pain will “map” to diagnostically-relevant dermatomes and can help clarify which organ/s may be affected.
What is the sympathetic trunk and what is it used by?
The sympathetic trunk carries sympathetic efferent pathways to somatic structures. It is used by pathways that have targets outside of the T1-L2 levels.
Compare the postganglionic axons of sympathetic vs parasympathetic pathways.
Sympathetic postganglion axons are relatively long, while parasympathetic postganglionic axons are microscopic.
Motor pathways always pass through this structure.
Ventral root (–> ventral horn).
Sensory pathways always pass through this structure.
Dorsal root (–> ventral horn).
Compare and contrast the white ramus communicans and the gray ramus communicans.
The white rami communicantes are found only between T1-L2. Gray rami communicantes are found at all levels. Both carry ONLY sympathetic pathways.
