Il TESSUTO e il MUSCOLO CARDIACO — Transcript

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1. 0:00When we introduced muscle tissue in
2. 0:03lesson 98, we distinguished three types
3. 0:07: skeletal striated muscle tissue,
4. 0:09cardiac striated muscle tissue, and
5. 0:12smooth muscle tissue. We then analyzed
6. 0:17skeletal striated muscle tissue,
7. 0:20skeletal muscles, myofibrils, thick
8. 0:23myofilaments composed mainly of myosin,
9. 0:26and thin myofilaments composed mainly
10. 0:29of actin. We observed the organization
11. 0:33of the contractile proteins actin and
12. 0:35myosin into sarcomeres and discussed
13. 0:38muscle contraction in skeletal muscle.
14. 0:42Regarding muscle tissues, the final
15. 0:44topic of the histology playlist that
16. 0:47began in lesson 84, only cardiac
17. 0:49striated muscle tissue, which makes up
18. 0:52the cardiac muscle, and smooth muscle
19. 0:54tissue, which makes up smooth muscle,
20. 0:57remain. Let's begin with cardiac
21. 1:02striated muscle tissue and the cardiac
22. 1:04muscle. We will start from the heart
23. 1:16and move from the large to the small.
24. 1:22The heart, the hollow organ that pushes
25. 1:24blood through the circulatory system,
26. 1:27consists of a cardiac wall that lines
27. 1:29four cavities. The cardiac wall is in
28. 1:34turn divided into three parts: a very
29. 1:37thin outer layer called the epicardium,
30. 1:40a central contractile muscular part
31. 1:43called the myocardium, and a very thin
32. 1:46inner layer called the endocardium. In
33. 1:50this case as well, Greek can help us
34. 1:53out. In all three names, the suffix "
35. 1:56cardio" comes from the Greek "cardia,"
36. 1:59which means heart. In "epicardium," the
37. 2:03prefix "epi-" comes from the Greek "epi
38. 2:06," which means upon or above. In "
39. 2:10myocardium," the prefix "myo-" comes
40. 2:12from the Greek "myus," which means
41. 2:14muscle. In "endocardium," the prefix "
42. 2:18endo-" comes from the Greek "endon,"
43. 2:20which means inside. The four cavities
44. 2:25are two atria located in the upper part
45. 2:28, the left atrium and the right atrium,
46. 2:30and two ventricles located in the lower
47. 2:33part, the left ventricle and the right
48. 2:36ventricle. The atria receive blood from
49. 2:40the veins, while the ventricles pump
50. 2:42blood into the arteries. Let us now
51. 2:46focus on the muscular part of the heart
52. 2:49, the myocardium, which unlike skeletal
53. 2:52muscle—and like smooth muscle—is
54. 2:54involuntary. The myocardium is the
55. 2:58thickest layer of the cardiac wall. Now
56. 3:02, the heart's four cavities must
57. 3:04withstand different workloads, so the
58. 3:07thickness of the myocardium
59. 3:09corresponding to each of them differs.
60. 3:14Both atria have a very thin myocardium
61. 3:17composed of muscle cells with a small
62. 3:20diameter. The right ventricle has a
63. 3:25medium-thickness myocardium composed of
64. 3:27muscle cells of intermediate diameter.
65. 3:33The left ventricle has the thickest
66. 3:35myocardium, composed of muscle cells
67. 3:38with a large diameter. What has just
68. 3:42been said is explained by considering
69. 3:45that each atrium pushes blood into its
70. 3:48respective empty ventricle. The right
71. 3:53ventricle pushes blood toward the lungs
72. 3:56at a relatively short distance. The
73. 4:01left ventricle pushes blood through the
74. 4:03systemic arterial circuit, that is,
75. 4:06toward the entire body over long
76. 4:08distances. The myocardial muscle fibers
77. 4:12of the left ventricle are, therefore,
78. 4:15the ones subject to the greatest
79. 4:17workload. The myocardium consists
80. 4:24mainly of striated cardiac muscle
81. 4:27tissue, made up of myocardial muscle
82. 4:30cells called myocytes or cardiomyocytes
83. 4:34. Cardiomyocytes are mono-or
84. 4:37binucleated muscle cells, meaning they
85. 4:40contain one or at most two nuclei and
86. 4:42have an elongated shape. A
87. 4:46characteristic feature of
88. 4:47cardiomyocytes is their branched ends,
89. 4:50which allow them to interlock and form
90. 4:53a three-dimensional network. Striated
91. 4:58cardiac muscle tissue, as the name
92. 5:00suggests, has a striated appearance
93. 5:03because, just like in skeletal muscle
94. 5:06tissue, myofibrils made of sarcomeres
95. 5:08in series are present within the
96. 5:11sarcoplasm of the cardiomyocytes. In
97. 5:16addition to myofibrils, the sarcoplasm
98. 5:19of cardiomyocytes contains many
99. 5:22mitochondria and the sarcoplasmic
100. 5:25reticulum; however, the sarcoplasmic
101. 5:28reticulum of cardiomyocytes is
102. 5:31organized more simply than that of
103. 5:34skeletal muscle fibers. It contains no
104. 5:40fenestrated or terminal cisternae, but
105. 5:43rather small transverse expansions of
106. 5:45sarcotubules that form diads, rather
107. 5:48than triads, with the T-tubules.
108. 5:52T-tubules, which we recall are
109. 5:55invaginations of the sarcolemma, are
110. 5:58fewer and wider than those in skeletal
111. 6:01muscle. Now, the contraction of cardiac
112. 6:06muscle, like that of skeletal muscle,
113. 6:08requires the presence of calcium ions,
114. 6:10but cardiomyocytes, lacking terminal
115. 6:12cisternae, have no intracellular
116. 6:14calcium reserves and thus depend on an
117. 6:17exogenous supply of calcium ions.
118. 6:20Calcium ions that are transported by
119. 6:22the aforementioned few but wide
120. 6:25T-tubules. The heart pumps blood into
121. 6:31circulation, and this occurs through
122. 6:34the myocardium, the muscular wall of
123. 6:36the heart tasked with rhythmically
124. 6:38contracting this hollow organ. Zooming
125. 6:42into the heart, we can say that the
126. 6:44entire contraction of this fundamental
127. 6:46organ occurs at the level of the
128. 6:48myofibrils present in the
129. 6:50cardiomyocytes. Can you imagine the
130. 6:53level of organizational complexity that
131. 6:56this entails? Well, the various
132. 7:00cardiomyocytes behave as if they were a
133. 7:02single large cell, even though they do
134. 7:05not form morphological syncytia as
135. 7:07occurs in skeletal muscles. It is said
136. 7:11that the cardiac muscle, the myocardium
137. 7:14, is a functional syncytium.
138. 7:19Cardiomyocytes, the cells constituting
139. 7:21striated cardiac muscle tissue, are in
140. 7:24fact tightly connected to each other
141. 7:27via specialized junctional complexes
142. 7:30and interdigitated areas of adjacent
143. 7:32cardiomyocyte membranes called
144. 7:35intercalated discs or scalariform discs
145. 7:38. The junctional devices that make up
146. 7:43the intercalated discs are of three
147. 7:45types: desmosomes, fasciae adherentes,
148. 7:48and gap junctions. Desmosomes and
149. 7:52fasciae adherentes serve to
150. 7:54mechanically anchor and adhere the
151. 7:56cardiomyocytes to one another. Gap
152. 8:00junctions, on the other hand, connect
153. 8:02adjacent cardiomyocytes, facilitating
154. 8:05the rapid passage of small molecules
155. 8:07and ions between them, thereby allowing
156. 8:10the propagation of the electrical
157. 8:12signal and the coordination of
158. 8:15activities. Having said all that, I
159. 8:19would say it's time to distinguish
160. 8:21between my working myocardium and my
161. 8:24specific or conduction myocardium. My
162. 8:31working myocardium is the working part
163. 8:33of the heart muscle; it is made up of
164. 8:35the majority of cardiocytes and is
165. 8:37responsible for cardiac contraction. My
166. 8:41specific or conduction myocardium is
167. 8:43the leading part of the heart muscle.
168. 8:47It consists of modified cardiocytes
169. 8:50specialized in the properties of
170. 8:52self-excitability and electrical
171. 8:54impulse conductivity, and it is
172. 8:56responsible for autonomously generating
173. 8:59the electrical impulse. My specific or
174. 9:03conduction myocardium generates the
175. 9:05electrical impulse without the
176. 9:07intervention of the nervous system. The
177. 9:11intervention of the autonomic nervous
178. 9:13system is solely aimed at regulating
179. 9:16the heart rate and serves to adapt the
180. 9:19power and work of the myocardium to the
181. 9:21body's changing demands. The set of
182. 9:27structures tasked with generating and
183. 9:29conducting the electrical stimulus is
184. 9:32called the heart's conduction system,
185. 9:35and within it, we distinguish the
186. 9:38sinoatrial node, the atrioventricular
187. 9:41node, the common atrioventricular
188. 9:43bundle of His, and the Purkinje fibers.
189. 9:48The sinoatrial node, located in the
190. 9:51upper part of the right atrium, is the
191. 9:54rhythm generator. The pac rhythmically
192. 10:00generates electrical impulses that
193. 10:02spread through the musculature of the
194. 10:05atria to the atrioventricular located
195. 10:11in the lower part of the right atrium.
196. 10:14In the atrioventricular node, the
197. 10:17electrical signal undergoes a slight
198. 10:19delay, necessary to give the ventricles
199. 10:21time to fill completely after the
200. 10:24atrial contraction. Starting from the
201. 10:28atrioventricular node, the
202. 10:30atrioventricular bundle of His
203. 10:32originates, which divides into two
204. 10:34trunks in the interventricular septum
205. 10:37area: a right branch and a left branch.
206. 10:41To simplify a bit, both branches of the
207. 10:43atrioventricular bundle subdivide and
208. 10:45form the Purkinje fibers, a network of
209. 10:48fibers in direct contact with the
210. 10:50fibers of the common working myocardium
211. 10:52. When the electrical impulse, having
212. 10:57started from the sinoatrial node and
213. 10:59passed through the atrioventricular
214. 11:01node, reaches the ventricles, the heart
215. 11:04beats and blood is pushed out of it.
216. 11:08That's all for this lesson. Stay
217. 11:11connected with me and biology to finish
218. 11:13our journey through histology with
219. 11:16smooth muscle tissue and then move on
220. 11:18together toward other destinations and
221. 11:21other topics. If I have been helpful,
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226. 11:34happy studying everyone.