Marah karablieh. Osama khader. Muhammad khatatbeh. 0 P a g e

Transcription

1 15 Marah karablieh Osama khader 0 P a g e Muhammad khatatbeh

2 Cardiac Muscle Physiology Introduction The heart has two ventricles and two atriums. The heart wall is composed primarily of spirally arranged cardiac muscle fibers and has three distinct layers: o Endocardium: the innermost thin layer of epithelial tissue that lines the chambers of the heart. (Endothelium, which also lines the entire circulatory system). o Myocardium: A middle layer, which is composed of cardiac muscle and constitutes the bulk of the heart wall (myo means muscle ; cardia means heart ) o Pericardium: A thin external layer, and it consists of two layers: Visceral (which is closer to the heart) and Parietal with a space separating them. This space is called Pericardial Cavity and contains serous pericardial fluid which gives protection and absorbs shocks. 1 P a g e

3 Comparison between Skeletal and Cardiac Muscles Skeletal Muscle are cylindrical in shape, they are long since they have origin and insertion, and under microscope they appear striated. They are voluntary and needs to be innervated in order to contract, they are not connected to each other; motor nerves supply a number of fibers to cause their contraction (a motor unit). Remember: Happens first: Electrical response: Action potential 2 P a g e Then: Mechanical response: Contraction Excitation contraction coupling is the physiological process of converting an electrical stimulus to a mechanical response. It is the link between the action potential generated in the sarcolemma and the start of a muscle contraction. Note: In addition to gap junctions, cardiac cells have desmosomes in between.

4 Cardiac muscle cells are rectangular in shape, connected to each other by intercalated discs that enable the rapid transmission of electrical impulses through the network. At each intercalated disk the cell membranes fuse with one another to form gap junctions that allow rapid diffusion of ions, that s why when one cell becomes excited, the action potential rapidly spreads to all of them. Gap junctions are hexagonal proteins with open and closed conformations. They re voltage-gated channels, when voltage changes due to depolarization the channels are open and ions diffuse so fast that all cells are depolarized at the same time, so they contract at the same time. Thus, cardiac cells are electrically coupled. Keep in mind, In cardiac muscles: -Plasma membrane: Sarcolemma SL -Endoplasmic Reticulum: Sarcoplasmic Reticulum SR -Cytoplasm: Sarcoplasm If each cell contract by itself, this will cause ventricular fibrillation (may also happen in atrium, atrial fibrillation). Here, the heart pumps little or no blood. Involuntary, although it is supplied by the autonomic nervous system (sympathetic and parasympathetic). This innervation of the cardiac muscle is not important for the initiation of cardiac muscle contraction (during cardiac transplants the autonomic 3 P a g e

5 supply is cut but the heart is still running). The contraction of cardiac muscle follows (law All or None). It s striated, because of the contractile proteins inside the cells The Endoplasmic Reticulum in skeletal cells is well-developed and stores Ca++ in high amounts, but Sarcoplasmic Reticulum is less developed and doesn t store enough Ca++. So how do cardiac muscles contract if they have low amounts of Ca++ inside? Simply, they take Ca++ from the extracellular matrix. That s why during heart transplants, the heart is put into a Calcium solution. The cardiac muscle contracts all the time. This means it needs a lot of energy, that s why it has lots of mitochondria to supply ATP compared to skeletal muscles that, on the other hand, have much more nuclei. The sarcolemma of cardiac and skeletal muscles has deep invaginations called the T-tubules or transverse tubules. The T- tubules of skeletal muscles are found in the I band, they are slender and long; so, each sarcomere has two T-tubules. 4 P a g e

6 While in the cardiac muscle, they are wider and shorter and occur at the Z line (disk); so, each sarcomere has one T-tubule. The sarcomere is the distance between two Z lines. Skeletal Muscles -Voluntary -Attached to bones or skin -Very long, cylindrical, multinucleate, cells -Striated: packed with orderly arrangement of contractive proteins -Not self-stimulating: each fiber innervated by branch of motor neuron as part of motor unit -Under control of nervous system -High number of nuclei mitochondria -Fast Contracting -No rhythmic contractions -Fatigues Easily Cardiac muscles -Involuntary -Found only in the Heart -Branching chains of cells connected by intercalated discs, with single nucleus and striations -Striated: many contractive proteins in orderly arrangement -Self-stimulating: impulse spreads from cell to cell -Under control of nervous and endocrine systems and various chemicals -Intermediate energy requirement: higher number of mitochondria than the skeletal muscle cells. -Intermediate speed of contraction lets contraction spreads quickly through tissue due to intercalated discs (gap junction) -Rhythmic contractions -Doesn t fatigue 5 P a g e

7 Action Potential Skeletal Muscles -Rest Membrane Potential is -70 mv -When stimulated, Na+ channels are opened and slow depolarization begins until reaching the threshold -After that, fast depolarization takes place due to opening of Na+ voltage-gated channels (firing stage), trying to reach a potential of +61 (Eq. potential of Na+) but that s impossible because cells contain ions other that Sodium -Then, Na+ channels close and K+ channels open causing repolarization (falling stage) -This whole process takes 1millisecond 10 milliseconds maximum 6 P a g e

8 Cardiac Muscle -Resting membrane potential is -90 mv (Phase 4) o Phase 0 (Depolarization): When the cardiac cell is stimulated, fast voltage-gated Na+ channels open and permit Na+ to flow into the cell and depolarize it. The membrane potential reaches about mv. In this phase, we have an increase in the permeability of Na+ and a decrease in the permeability of K+. Important note: The decrease of the K+ level to less than what it was during the resting stage is important, but why? Because if that didn t happen, K+ will diffuse to the outside, and Ca++ will take its place. Therefore, that way, a positive went out & a positive went in, there s no benefit in that! As a result, there will be no Plateau. 7 P a g e

9 o Phase 1 (Partial or Initial Repolarization): Na+ channels close, the cell begins to repolarize, and transient K+ and Cl- specialized channels open. Again, the decrease in K+ permeability during Phase 0 and 1 is very important in order to maintain the Plateau. o Phase 2 (Plateau): this phase plays the main role in contraction as it maintains depolarization. Slow voltagegated Ca++ channels open, which induces the release of Ca++ from SR, and K+ channels close. o Phase 3 (Rapid Repolarization): Ca ++ channels close and slow voltage-gated K+ channels open, this permits K+ to rapidly exit the cell, ends the plateau and returns the cell membrane potential to its resting level. o Phase 4 (Resting membrane potential): rearrangement of ions by Na - K pump. -The whole process takes milliseconds, thanks to the long refractory period (an absolute refractory period from the beginning to half of the repolarization stage where the muscle cannot contract, and a relative refractory after it where the muscle may contract to a stronger stimulus). 8 P a g e

10 Contraction and Tetanisation 9 P a g e First, let us know what exactly Tetanisation means, To tetanize: to stimulate a muscle at progressively higher frequencies until successive contractions fuse and cannot be distinguished from one another, and tetanus: a state of muscular contractions without periods of relaxations. In the skeletal action potential: Because of the short refractory period, if there's another stimulus, a new action

11 potential and another contraction will start in the relative refractory period of first action potential. This is called Tetanus or Spasm. (Relative refractory period: the last half of the repolarization phase until the action potential has ended). In the cardiac action potential: When the heart is given an action potential, it will NOT give the muscle Tetanus no matter how strong the action potential is, but the question is why? That is because it has a VERY long Absolute Refractory Period. 10 P a g e

12 ك ن اهل م ة ال ت ال ت ف ت والش م س ال ت ال ت غ ر ب واألم ل األب د ي ف م ن ال أ م ل ف ي ه ال أ م ل م ن ه P a g e