Il TESSUTO MUSCOLARE — Transcript

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1. 0:01Since lesson 84, we have delved into
2. 0:04tissues which, let's remember, are
3. 0:06aggregates of multiple cells sharing
4. 0:09similar morphofunctional
5. 0:10characteristics. We have already
6. 0:15analyzed epithelial tissue, connective
7. 0:17tissue, and nervous tissue. We have yet
8. 0:20to discuss muscle tissue, and we begin
9. 0:22to do so right here. Muscle tissue, as
10. 0:35the name suggests, is the predominant
11. 0:38tissue in muscles, the organs
12. 0:40responsible for movement. You certainly
13. 0:45imagine the movement of parts or the
14. 0:48entire body, but also of solids,
15. 0:51liquids, and gases through hollow
16. 0:54organs. Movement derives from the
17. 0:59ability of muscle fibers—elongated,
18. 1:02highly specialized cells that make up
19. 1:05muscle tissue—to contract. Their task
20. 1:10, to look deeper, is to transform
21. 1:13chemical energy into mechanical energy
22. 1:16for movement and into heat for
23. 1:18maintaining a constant body temperature
24. 1:21. Let us then analyze muscle cells and
25. 1:25their functional properties. Muscle
26. 1:31cells are called muscle fibers or
27. 1:33fibrocells because they are fibers,
28. 1:35that is, cells with an elongated shape.
29. 1:39We have already mentioned the first
30. 1:42property of muscle fibers:
31. 1:44contractility. The other functional
32. 1:49properties that characterize muscle
33. 1:52cells are excitability, a property
34. 1:55already seen for nerve cells in lesson
35. 1:5895, extensibility, and elasticity.
36. 2:03Contractility, the ability of muscle
37. 2:06fibers to contract, lies in the
38. 2:09interaction between certain contractile
39. 2:12proteins of the cytoskeleton. The
40. 2:16proteins actin and myosin. Although
41. 2:21these contractile proteins are present
42. 2:23in all cells to allow for small
43. 2:26movements and shape changes necessary
44. 2:28for, for example, mitosis, phagocytosis
45. 2:31, exocytosis, and amoeboid movement,
46. 2:34muscle cells are specialized for the
47. 2:36contractile function. This is because
48. 2:42muscle fibers contain very high
49. 2:44quantities of contractile proteins and
50. 2:46because, in muscle fibers, contractile
51. 2:49proteins reach a level of organization
52. 2:51that allows for large-scale movements.
53. 2:56Excitability, the ability to react to
54. 2:59stimuli, is due, as happens in nerve
55. 3:02cells, to the ability of muscle fibers
56. 3:05to change their membrane potential in
57. 3:08response to an electrical stimulus.
58. 3:12Muscle cells therefore contract in
59. 3:14response to an electrical stimulus.
60. 3:19Extensibility is the ability of muscle
61. 3:22cells to stretch when pulled; to be
62. 3:24clear, it is the property that allows
63. 3:27muscles to lengthen during stretching.
64. 3:32Finally, elasticity is the property
65. 3:35that returns muscle cells to their
66. 3:37original shape and length at the end of
67. 3:40the stretching stress. Let us now move
68. 3:48from muscle tissue, composed of the
69. 3:50muscle cells whose characteristic
70. 3:52properties we have just analyzed, to
71. 3:54muscles, that is, the organs
72. 3:56responsible for movement and
73. 3:58constituted primarily of muscle tissue.
74. 4:03I added the adverb "primarily" because,
75. 4:06as stated in lesson 84, organs are
76. 4:09aggregates of tissues with different
77. 4:11morphological and functional
78. 4:14characteristics. A muscle, therefore,
79. 4:18is not made only of muscle tissue, but
80. 4:21is primarily composed of it. To go
81. 4:26further, we distinguish three different
82. 4:28types of muscle, primarily constituted
83. 4:30by three different types of muscle
84. 4:32tissue. Skeletal muscle is
85. 4:36predominantly composed of striated
86. 4:39skeletal muscle tissue. Cardiac muscle
87. 4:44is predominantly composed of striated
88. 4:47cardiac muscle tissue, and smooth
89. 4:50muscle is predominantly composed of
90. 4:53smooth muscle tissue. Skeletal muscles
91. 5:01are voluntary muscles, organs whose
92. 5:04contraction is consciously controlled
93. 5:06by the central nervous system. They
94. 5:11attach via tendons to the bones of the
95. 5:14skeleton and determine the movement of
96. 5:17entire body segments, locomotion—that
97. 5:20is, the movement of the whole body—
98. 5:22and the maintenance of posture and body
99. 5:25temperature. The muscle fibers of
100. 5:30skeletal muscles, known as striated
101. 5:32cells because they appear as such under
102. 5:35a microscope, are the most voluminous
103. 5:37and are roughly cylindrical. They are
104. 5:40multinucleated cells, meaning each
105. 5:42contains various nuclei, and the nuclei
106. 5:45are positioned at the cell periphery. A
107. 5:49skeletal muscle fiber is a
108. 5:51morphological syncytium, which means
109. 5:55that several progenitor cells called
110. 5:58myoblasts have fused together into a
111. 6:02multinucleated cell, the skeletal
112. 6:05muscle cell. Syncytium comes from the
113. 6:09Greek "syn," meaning together, and "
114. 6:11kytos," meaning cell. Now,
115. 6:15differentiated skeletal muscle fibers
116. 6:18lose the ability to divide by mitosis,
117. 6:21but outside the sarcolemma of these
118. 6:23cells remain adult, quiescent,
119. 6:26mononucleated stem cells, called
120. 6:28satellite cells. Satellite cells, being
121. 6:33stem cells, retain the ability to
122. 6:36divide by mitosis and can form new
123. 6:39fibers. Muscle damage that is not too
124. 6:43extensive can therefore be repaired
125. 6:45thanks to the intervention of satellite
126. 6:48cells. However, the proliferative
127. 6:52potential of satellite cells is not
128. 6:54infinite, and therefore, in the case of
129. 6:57very extensive or repeated muscle
130. 6:59damage over time, skeletal muscle
131. 7:01tissue is progressively replaced by
132. 7:04fibrous connective tissue, which is
133. 7:06non-contractile tissue. This results in
134. 7:09an irreversible loss of function.
135. 7:15Cardiac muscle is the muscle of the
136. 7:17myocardium, the muscular wall of the
137. 7:20heart, and throughout the body, it is
138. 7:23found only there, in the heart. Now,
139. 7:27the heart is a hollow muscular organ
140. 7:29that, as you all know, serves to pump
141. 7:31blood into the circulatory system.
142. 7:34Cardiac muscle is an involuntary muscle
143. 7:37; we do not control it with the central
144. 7:39nervous system as we do with skeletal
145. 7:42muscles. The rhythmic contraction of
146. 7:45cardiac muscle is triggered by modified
147. 7:48muscle cells and regulated by the
148. 7:50autonomic nervous system. The muscle
149. 7:55fibers of cardiac muscle, or
150. 7:57cardiocytes or cardiomyocytes, appear
151. 8:00striated under a microscope, like those
152. 8:03of skeletal muscle. They are
153. 8:06intermediate in size compared to the
154. 8:08cells of the other two types of muscle
155. 8:11tissue and are cylindrical in shape,
156. 8:13with the characteristic of having
157. 8:15branched ends—Y-shaped terminals—
158. 8:18that allow them to form a
159. 8:19three-dimensional network. Cardiocytes
160. 8:23are mono-or at most binucleated; that
161. 8:26is, they can have one or two nuclei
162. 8:28that are positioned centrally, and they
163. 8:31do not form morphological syncytia like
164. 8:34skeletal muscle cells. The myocardium
165. 8:39as a whole is defined as a functional
166. 8:41syncytium because the various cardiac
167. 8:44muscle cells are joined by junctional
168. 8:47devices and work functionally together
169. 8:49in a coordinated manner. Cardiac muscle
170. 8:55tissue possesses a limited regenerative
171. 8:58potential due to cardiac stem cells.
172. 9:03These stem cells are believed to
173. 9:05intervene in the repair of minor damage
174. 9:08and in the renewal of adult cardiocytes
175. 9:11. As with skeletal muscle tissue, if
176. 9:15the damage is more extensive, the
177. 9:17modest regenerative capacities of these
178. 9:19cells are insufficient and the damaged
179. 9:22functional tissue is replaced by a
180. 9:24fibrous scar of non-contractile and
181. 9:26therefore non-functional connective
182. 9:28tissue. Smooth muscle, like cardiac
183. 9:35muscle, is an involuntary muscle and
184. 9:38its contraction is controlled by the
185. 9:40autonomic nervous system. Unlike the
186. 9:44other two types of muscle, smooth
187. 9:46muscle does not appear striated under a
188. 9:48microscope, which is the reason it is
189. 9:50called smooth. It is also called
190. 9:54visceral muscle because it is found in
191. 9:56the muscular wall of the viscera, i.e.,
192. 9:59the hollow organs that are part of the
193. 10:01respiratory, digestive, urogenital, and
194. 10:03circulatory systems. Smooth muscle
195. 10:08fibers are the smallest, have a
196. 10:10spindle-like appearance, and each
197. 10:12contains a single nucleus placed in a
198. 10:14central position. Smooth muscle tissue,
199. 10:20unlike the other two types of muscle
200. 10:22tissue, has a high regenerative
201. 10:24potential in case of damage, thanks to
202. 10:27the ability of smooth muscle fibers to
203. 10:30undergo cell division. It is also
204. 10:35interesting to point out that smooth
205. 10:37muscle cells can undergo significant
206. 10:40increases in number, a phenomenon
207. 10:42called hyperplasia, and in volume, a
208. 10:44phenomenon called hypertrophy. As
209. 10:48happens, for example, in the uterus
210. 10:49during pregnancy. In our overview, we
211. 10:53only need to mention myoepithelial
212. 10:56cells, which are cells with
213. 10:58characteristics intermediate between
214. 11:00those of smooth muscle (myo prefix) and
215. 11:03epithelial cells (epithelial suffix).
216. 11:08Myoepithelial cells are associated with
217. 11:11the secretory cells of numerous
218. 11:13exocrine glands. We conclude the lesson
219. 11:19here with the plan for the next ones to
220. 11:21go into more detail on the three types
221. 11:23of muscle just seen. So stay tuned with
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