Parasympathetic Nervous System: Animated Scheme (Craniosacral Outflow) — Transcript

Full transcript

1. 0:06What’s up, Taim talks med here. In this video  we’re gonna talk about the parasympathetic nervous
2. 0:10system. As you see from this brief diagram, the  sympathetic and the parasympathetic parts of our
3. 0:16nervous system controls more or less all our  internal organs. Sympathetic being the fight
4. 0:21or flight response, and parasympathetic being  the rest and digest response. And they’re both
5. 0:27as you see here a part of the autonomic  nervous system, which again is the motor
6. 0:32division of our peripheral nervous system. I did  make an introductory video about the peripheral
7. 0:37nervous system, so if you guys have absolutely  no clue what the peripheral nervous system is,
8. 0:42I urge you to watch that one first. But all in  all I’ll try to simplify the parasympathetic
9. 0:48nervous system as much as I can so that it’ll  make sense at a detailed level, within the
10. 0:54aspects of anatomy and physiology at least. So, in this video, we’re going detailed into
11. 0:59the parasympathetic nervous system. And we’re gonna do that by first going
12. 1:04through the general structure and terms associated  with the parasympathetic aspect of the autonomic
13. 1:09nervous system. Basically talk a little bit  about ganglia, the pre and post synaptic
14. 1:14neurons and their neurotransmitters,  and basically how the parasympathetic
15. 1:18nervous system is built in general. Then we’re gonna talk about the cranial outflow,
16. 1:23and go quickly through the pathway of the  cranial nerves involved and what structures
17. 1:27they innervate. And then run through the  sacral outflow, where it originates from
18. 1:32and basically what it innervates and its function. Let’s go ahead and begin with some terms. Now the
19. 1:39autonomic nervous system - so both the sympathetic  and parasympathetic nervous system is made up of a
20. 1:45relay that includes two neurons. And when there’s  a group of nerve cell bodies that are next to
21. 1:51each other within the actual central nervous  system, the whole thing is called a nucleus,
22. 1:57while a group of nerve cell bodies that are  located outside of the central nervous system
23. 2:02is called a ganglion. This is a very  very important thing to remember. Now.
24. 2:08The autonomic nervous system has an affect on  all areas of the body. The Sympathetic outflow
25. 2:14is primarily from the thoracolumbar area, right?  Those are preganglionic cholinergic fibers that
26. 2:20go from the spinal cord towards either the  paravertebral ganglia, or the prevertebral
27. 2:26ganglia, from where postganglionic primarily  adrenergic neurons are gonna go out from.
28. 2:32The parasympathetic nervous system has their  preganglionic fibers coming from the brainstem,
29. 2:38which travels towards a peripherally located  ganglion. As well as from the sacral region,
30. 2:44going towards a parasympathetic  ganglion that lie either near the organ,
31. 2:49or within the organs they innervate, to then  give off postganglionic cholinergic neurons.
32. 2:56Now there are three main categories that we  can see a clear difference between these two
33. 3:01systems. And we already know a little bit about  the sympathetic nervous system since we covered it
34. 3:06in our last video, but in terms of territory. You  know that the sympathetic nervous system is going
35. 3:12to innervate all areas of the body, primarily  because the suprarenal gland is going to spit
36. 3:18a lot of epinephrine and norepinephrine within  the blood. Parasympathetic nervous system is
37. 3:24primarily localized to the innervated areas,  so the distribution is focused on the head,
38. 3:30the body cavities, and the external  genitalia. So the limbs don’t receive
39. 3:35parasympathetic innervation for example,  neither the body walls. In terms of activity,
40. 3:41when you activate the sympathetic nervous system,  you’ll get a more generalized and indirect effect.
41. 3:48This is because for one thing you’ll have a large  amount of catecolamines circulating in the blood,
42. 3:54and another reason si because the ratio  between the pre- and post-ganglionic fibers
43. 3:59are approximately 1:15 or more. So 15 or more  postganglionic fibers are activated just from
44. 4:08one preganglionic fiber. If the parasympathetic  is stimulated, the ratio here is approximately
45. 4:151 preganglilnic fiber to 2 postganlgionic fibers.  So you’ll get a more specific and direct response.
46. 4:22And functions, again. Sympathetic is  more associated with increased level
47. 4:27of activity and assisting in coping  with stress and physical exertion.
48. 4:32Parasympathetic is associated with things like  relaxation, homeostatis, restoration and so on.
49. 4:39Now I will go through the effect of the  parasympathetic nervous system to each
50. 4:44specific organ later in this video, but as  you see from just the functional area. The
51. 4:49sympathetic and parasympathetic divisions often  function as antagonistic systems, that is, they
52. 4:55produce activities in opposition to one another.  So for example, sympathetic activity increases
53. 5:02heart rate, causes bronchodilation, decreased  peristalsis in the gut tube, closing of the
54. 5:08sphincters, relaxation of the general bladder  wall, and dilation of the pupils. While the
55. 5:14parasympathetic activity results in decelerated  heart rate, bronchoconstriction, increased gut
56. 5:20peristalsis, opening of sphincters, contraction of  the bladder wall, and constriction of the pupils.
57. 5:27However, not always are they antagonistic. The  two divisions may also be complementary to one
58. 5:37another, they can also work as a synergistic  system. For example, in normal sexual function,
59. 5:44parasympathetic activity produces erection, and  sympathetic activity results in ejaculation.
60. 5:51So here these two systems complement each  other. Another thing is that, one division
61. 5:57may function independently of the other, for  example sympathetic stimulation activates sweat
62. 6:03gland secretion, but parasympathetics play  absolutely no role in sweat gland activity.
63. 6:10Now, lastly before we go on an talk about the  actual outflow of this system. Let’s quickly
64. 6:16just go through how all of this function. First  we have a preganglionic neuron located within
65. 6:22the brainstem or the sacral spinal cord. These  preganglionic neuron release the neurotransmitter
66. 6:28acetylcholine which binds to binds to nicotinic  receptors on the cell membrane of postganglionic
67. 6:34neuron cell bodies. Nicotinic receptors are  ion channels that open when acetylcholine
68. 6:41binds to them, and they allow positive ions like  sodium and potassium to cross the cell membrane,
69. 6:46activating the postganglionic neurons. Postganglionic neurons are also called
70. 6:52cholinergic neurons because just  like the preganglionic neurons,
71. 6:56they also release acetylcholine. This time,  however, the acetylcholine binds to muscarinic
72. 7:02receptors on the cells of target organs. Muscarinic receptors are G-protein-coupled
73. 7:08receptors, meaning that when acetylcholine binds,  they activate the G proteins to ultimately enable
74. 7:16cells to change in a number of ways, and  that’s how the parasympathetic nervous
75. 7:20system creates change at the cellular level. So, sympathetic has adrenergic postganglionic
76. 7:27neurons primarily. Parasympathetic has  cholinergic postganglionic neurons. Awesome.
77. 7:34So. Here we see the mesencephalon, Pons, Midbrain  and the spinal cord. Now the parasympathetic
78. 7:41nervous system is primarily going to be  something called craniosacral outflow.
79. 7:46Those in the brain, the cranial part, primarily  come from specific parasympathetic nuclei located
80. 7:53within the brainsteim of certain cranial nerves.  Those are the Oculomotor nerve, Facial nerve,
81. 7:59Glossopharyngeal nerve, and the vagus nerve. The sacral outflow, or the sacral part has
82. 8:05their preganglionic neurons originate in the  lateral gray matter of the second, third,
83. 8:10and fourth sacral segments of the spinal cord.  However, because the number of cells here is
84. 8:16insufficient to form a distinct bulge such as seen  in the thoracolumbar region, you’ll rarely see a
85. 8:22lateral gray horn. It may be absent sometimes.  Myelinated axons leave the spinal cord in the
86. 8:36anterior nerve roots of the corresponding spinal  nerves, then leave the S2 to S4 spinal nerves,
87. 8:42and form the pelvic splanchnic nerves. Because of  these levels of origin and departure from the CNS,
88. 8:50we call the parasympathetic division a what?  We refer to it as craniosacral outflow.
89. 8:57Now let’s start the most proximal nerve of  the cranial outflow. The oculomotor nerve’s
90. 9:02parasympathetic nucleus is located within the  mesencephalon, specifically at the level of the
91. 9:08superior colliculus. Just to refresh your memory  I’m not going to talk about all this in detail,
92. 9:13cuz we already covered all the cranial  nerves already in the previous videos.
93. 9:17But here you see the posterior view of the  mesencephalon. If we cut the mesencephalon
94. 9:21at the level of the superior colliculus and  look at the cross section, you’ll see this.
95. 9:27So, we can see the Superior colliculi, the  Cerebral peduncles, the Interpeduncular space,
96. 9:33and the aqueduct of the midbrain, which connects  the fourth ventricle to the third ventricle.
97. 9:38Within the midbrain, we can find the substantia  nigra, we can find the superior colluculi. We can
98. 9:41see the periaqueductal gray matter, the reticular  formation, the red nuclei which take in impulses
99. 9:47from the brain and the cerebellum, and give  off rubrospinal tracts for muscle coordination.
100. 9:52At this level, we can also find the nucleus  of the oculomotor nerve, which will give off
101. 9:58fibers travelling towards the anterior side,  and leave through the sulcus of the oculomotor
102. 10:03nerve on the anterior side of the midbrain. The oculomotor nerve is a nerve that consists of
103. 10:09somatic fibers and preganglionic parasympathetic  fibers. The somatic fibers are the fibers you see
104. 10:16here, coming from the nucleus of the oculomotor  nerve, which sipplies the extrinsic muscles of
105. 10:21the eyeball. The parasympathetic portion of  the oculomotor nerve comes from the accessory
106. 10:27nucleus of the oculomotor nerve, which is also  called Edinger Westphal nucleus. They give off
107. 10:33parasympathetic fibres that go together with  the oculomotor nerve, forming the oculomotor
108. 10:38nerve complex. And this is the nucleus I’m  talking about here. This nucleus will give
109. 10:45off preganglionic parasympathetic fibers that’ll  travel through the cavernous sinus, go through the
110. 10:51superior orbital fissure and the common tendinous  ring, to synapse with a postganglionic neuron in
111. 10:58the ciliary ganglion, from where postganglionic  parasympathetic fibers will go as short ciliary
112. 11:05nervs towards the cilliaris muscle. The  cilliaris muscle is then going to contract,
113. 11:10and when it contracts, the zonular fibers  gets relaxed, which allowed the lens to
114. 11:16become more globular like. When the lens becomes  globular, it helps with near vision. So this is an
115. 11:22accommodation response. Helps with near vision. Now, the other muscle that it goes to is called
116. 11:27the sphincter pupullae. When the sphincter  pupillae contracts, it squeezes the pupile hole
117. 11:33and makes it really small. So it causes pupile  constriction. And when you constrict the pupile,
118. 11:38it allowed less rays to come into the eye,  which also has an affect on near vision.
119. 11:44So that’s it. That’s the oculomotor  nerve. Next one is the facial nerve.
120. 11:49The facial nerve has several nuclei that give  off fibers that travel within this actual nerve,
121. 11:55but one of the nuclei is called the superior  salivatory nucleus, which give off preganglionic
122. 12:01parasympathetic fibers. These fibers are gonna go  leave, go through the internal acoustic meatus,
123. 12:08and give off an important nerve called the greater  petrosal nerve. This nerve will synapse with the
124. 12:14pterygopalatine ganglion, and then these fibers  will go innervate several glands. Specifically the
125. 12:21lacrimal glands, can be the glands in the nasal  cavity, sinuses and the palate. So it’s gonna
126. 12:27innervate these glands, release acetocholine which  will stimulate these cells to start increasing the
127. 12:32production of these watery secretions, for  example you’ll have more tear production,
128. 12:37nasal secretion, palatine secretion and so on.  It’s also going to give off the chorda tympani,
129. 12:44which will synpase with postganglionic neurons in  the submandibular ganglion. These postganglionic
130. 12:50fibers will innervate the submandibular and the  sublingual salivary glands. And it will stimulate
131. 12:56them into secreting more saliva along with  digestive enzymes aswell like salivary amylase.
132. 13:02So that is that one. Now let’s do the  glossopharyngeal. The glossopharyngeal
133. 13:06nerve has its parasympathetic origin  within the inferior salivatory nucleus.
134. 13:12Which give off preganglionic parasympathetic  fibers that go through the jugular foramen,
135. 13:17then remember they travel through the  tympanic nerve, through this tympanic plexus,
136. 13:23and ultimately leave as the lesser petrosal  nerve, which will finally end up in the Otic
137. 13:28ganglion. This otic ganglion will then  give off postganglionic parasympathetic
138. 13:33fibers to the last salivary gland, one  of the big ones. Which one is that?
139. 13:39It's the parotid gland. So it’s going to  stimulate the parotid salivary gland to start
140. 13:44increasing its secretions, so lots of watery  secretions and salivary amylase, in order to
141. 13:50lubricate out food and digest it chemically  a little bit aswell through that amylase.
142. 13:56So that was this one. Last one is the biggest one.  The vagus nerve. And this one has the posterior
143. 14:03nucleus of vagus nerve, providing parasympathetic  innervation to the majority of organs within us.
144. 14:09I’m not gonna go in detail into each and  every branch, but I’m gonna keep it very
145. 14:14simple and say that it’s going to leave  the cranium through the jugular foramen,
146. 14:19then give off a few fibers towards the upper  respiratory tract, the larynx and the trachea.
147. 14:24To basically release acetylcholine to increase  the mucus secretion and cause a little bit of
148. 14:29contraction of the smooth muscle to constrict the  airways. Remember airway smooth muscles are extend
149. 14:35from the trachea throughout the bronchial tree,  so it increases in number the further distally
150. 14:41you get. And right now we’re activating the  parasympathetic nervous system, you’re resting.
151. 14:47You don’t need all that air in, and you wanna  produce more mucus to humidify the air and help
152. 14:53with protection of certain praticles or foreign  organisms as way to protect the airways. So that’s
153. 15:00what’s primarily is going to happen here. Then  we got some branches for the cardiac plexus.
154. 15:06Now we have to be very careful because the  parasympathetic nervous system doesn’t really
155. 15:10affect the cardiac muscle cells so much like the  sympathetic nervous system do. The parasympathetic
156. 15:15nervous system is primarily going to affect the  conducting system, nodule cells. So the SA node,
157. 15:21and the AV node. Now, you’re resting, you  wanna decrease the heart rate. So it has a
158. 15:26negative chronotropic and dromotropic action. The vagus nerve is going to contribute to the
159. 15:31pulmonary plexus, causing bronchoconstriction and  a little bit of increased mucus secretion aswell,
160. 15:37right? Same as what it did in the  the upper respiratory tract. Then
161. 15:40the nerve is going to continue along the  esophagus and form an esophageal plexus.
162. 15:45Now when you’re resting and digesting, what do you  want the GI tract to do? To increase motility. So
163. 15:52it primarily increases the peristalsis. Now  the vagus nerve is going to continue along
164. 15:59the esophagus, go through the diaphragmatic  hiatus and then split into a left and a right
165. 16:05vagus nerve. Going anterior and posterior to the  stomach. The left one will give a nerve towards
166. 16:11the liver and the biliary tree, going within the  hepatic plexus. Basically helping the liver to
167. 16:17be able to stimulate glycogenesis, storing the  glucose. Remember you’re resting and digesting,
168. 16:23you wanna digest, so it also helps contract your  gallbladder to a little degree, the anterior vagal
169. 16:29trunk primarily stimulate the liver. The stomach  is also going to be innervated to increase in
170. 16:35motility and increase in gastric secretion. Now one of the major branches of this nerve
171. 16:41are the celiac branches, going towards many  different plexuses within the abdominal
172. 16:46cavity. Primarily the celiac plexus, but it can  also go to the splenic plexus, hepatic plexus,
173. 16:53renal plexus, suprarenal plexus and the superior  mesenteric plexus. The inferior mesenteric plexus
174. 16:59is more for the sacral outflow. But it’s going  to basically help innervate a lot of different
175. 17:05organs within us, to help with the rest and digest  state. Like increase in urine production, increase
176. 17:10motility and secretion and absorption from the GI  tract, stimulate the pancreas in the exocrine and
177. 17:16endocrine secretion, like releasing insulin. When  it comes to the large intestine however it really
178. 17:21only innervates the proximal parts up until  the proximal 2/3s of the transverse colon. The
179. 17:27rest of it being innervates by the sacral outflow. Alright, what else. From the actual posterior
180. 17:34vagal trunk, you might have branches that instead  of going through the celiac branches, they may go
181. 17:37to hepatic plexus directly, or branches  that go towards the renal plexus directly.
182. 17:43So that was mainly the cranial outflow. Let’s now quickly do the sacral outflow.
183. 17:49The sacral preganglionic neurons originate from  the segments S2, S3 and S4. These axons leave the
184. 17:57spinal cord in the anterior nerve roots of  the corresponding spinal nerves, then leave
185. 18:02the S2 to S4 spinal nerves, and form the pelvic  splanchnic nerves. The pelvis splanchnic nerves
186. 18:09will run into the inferior hypogastric plexus,  and then innervate the descending colon, remaining
187. 18:15transverse colon, sigmoid colon and the rectum.  Increasing the motility, increase the secretion
188. 18:21and increasing absorption. Also controlling the  internal anal sphincter which relaxes to offer the
189. 18:28faeces to move forward if the rectum is full. If  you don’t have time for that at that moment, you
190. 18:33contract the external anal sphincter consciously  to prevent the poop from exiting at that moment.
191. 18:39It’s also going to innervate the bladder wall,  to cause bladder wall contraction, and internal
192. 18:44urethral sphincter relaxation to basically help  you urinate. And lastly, it’s also going to
193. 18:50innervate the male and female genitals. For the  male reproductive system, remember it’s going
194. 18:55to help engorging the penis with blood, helping  with erection by basically releasing acetylcholine
195. 19:00to stimulate the cavernosal endothelial cells to  produce nitric oxide, which is going to act on
196. 19:06the corpora cavernosa. Causing smooth muscle  cell relaxation, vasodilation and erection.
197. 19:12For female, it’s going to increase  the blood flow to the clitoris,
198. 19:16engorging the clitoris with blood. So that was everything I had for the
199. 19:21parasympathetic nervous system. And we now covered  both the sympathetic and the parasympathetic
200. 19:25nervous system in the last two videos. Thank you so much for watching another
201. 19:29one of my videos. If you enjoyed, learned  something from it, please remember to like,
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