VISCEROSENSORY PATHWAYS
I. VISCEROSENSORY RECEPTORS
A. Somatosensory system conveys information from skin, joints and skeletal muscle – related to external environment
B. Viscerosensory (visceral afferent) information is related to internal environment
C. Categorized as:
1. Nociceptors – free nerve endings respond to stimuli related to tissue damage (pathology or benign conditions)
2. Physiologic receptors – continual monitoring – associated with visceral reflexes (cough, visceral stretch, osmoreceptors, thermoreceptors, baroreflexes, chemoreflexes)
a. baroreceptors in aortic arch and carotid sinus
b. chemoreceptors in aortic bodies and carotid bodies
c. osmoreceptors and thermoreceptors in hypothalamus
D. Viscerosensory fibers (GVA) contained in autonomic
1. Predominate in parasympathetic nerves (80% of CN X)
2. Unmyelinated or thinly myelinated (slow conducting)
3. Information from physiologic receptors carried in parasympathetic nerves
4. Information from nociceptors carried in sympathetic nerves
E. Sympathetic afferents – from cardiac and splanchnic nerves
1. Stomach > greater splanchnic n. > sympathetic trunk > WRC > spinal nerve
2. Cell bodies in DRG T1-L2
3. Fibers enter at spinal level that receive GVE – input from same level (i.e. stomach = T5-9)
4. Visceral pain is poorly localized (low receptor density, large receptive fields and converging pathways)
F. Referred pain
– noxious stimuli in viscera are perceived as arising from superficial (somatic) parts of the body
1. Transmitted via sympathetic sensory fibers - referred to somatic structures whose afferents enter the spinal cord via same dorsal roots
2. Mechanism thought to involve convergence of somatic and visceral inputs onto pools of dorsal horn neurons
3. Visceral nociceptor fibers synapse on spinothalamic tract cells or send collaterals to dorsal horn tract cells
4. Angina – pain from heart disease – perceived as pain radiating down arm(s) or sternum
a. Afferents from heart enter sympathetic trunk via cardiac nerves and enter cord at T1-5 and send collaterals to tract cells
b. Afferents from chest wall and arm enter spinal cord also at T1-5
c. Cerebral cortex interprets pain as originating from surface of body (chest and / or arm)
G. Sacral parasympathetic afferents
1. pelvic nerves from S2-4 contain viscerosensory fibers
2. Cell bodies in DRG levels S2-4
3. Central processes enter ventral roots to relay cells in dorsal horn
4. Relay information related to bladder or bowel distention
5. GVE cells in S2-4 receive viscerosensory afferents from same level as pelvic viscera – spinal autonomic reflexes
H. Cranial parasympathetic afferents – cell bodies in cranial nerve ganglia (inferior)
1. Mostly contained in CN IX (see diagram=>) and X (viewed previously)
2. Visceral afferents traveling with CN IX originate from;
a. baroreceptors of carotid sinus and tactile inputs from oropharynx (afferent limb of gag reflex)
b. chemoreceptors in carotid body
3. Visceral afferents traveling with CN X originate from;
a. baroreceptors in aortic arch
b. thoracic and abdominal visceral (to level of splenic flexure)
4. Central processes of visceral afferents in CN IX and X synapse in brainstem nuclei related to visceral function (heart rate, blood pressure)
I. Cardiovascular System
1. Barorecptors-change blood pressure
i. Aortic arch
ii. Carotid Sinus
iii. Reflex Pathways in Brainstem
2. Bainbridge Reflex-regulates cardiac output
J. Respiratory System
1. Carotid Bodies - Chemoreceptors
K. Visceral
1. GI-Distention
2. Pain Pathways-Associated with surgery
i. distention
ii. irritation
iii. inflammation
iv. ischemia