Chapter 10 -Muscle Tissue

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1 Chapter 10 -Muscle Tissue Muscles: 1. Overview of Muscle Tissue A. Review 5 functions of muscle tissue. B. Review the 5 properties of muscle tissue. WHICH do they share with nervous tissue? (2, plus the all or nothing principle). C. Skeletal muscle cells are long, striated and multinucleated, and they are subject to voluntary control. Connective tissue coverings (endomysium, perimysium, and epimysium) enclose and protect the muscle fibers and increases strength of skeletal muscle. Skeletal muscles make up most of the muscular system. What makes a tendon? Deep fascia? Aponeuosis? Figure D. Smooth muscle cells are uninucleated, spindle shape, and arranged in opposing layers in the walls of hollow organs. When they contract, substances (food urine, a baby) are moved to a long internal pathways. Smooth muscle control is involuntary. E. Cardiac muscle cells are striated, breaking cells that fit closely together, and are ranged in spiral bundles in the heart. Their contraction pumps blood through the blood vessels. Control is involuntary. The gap junctions in these cells are call intercalated discs. Review Table page 4 and Microscopic Anatomy of a Muscle Cell : ( review Figure 10.3 and Figure 10.8) A. The multinucleate cylindrical skeletal muscle fibers are packed with unique organelles called myofibrils. The banding pattern (striations*) of the myofibrils and the cell is a whole reflects the regular arrangement of thin (actin) and thick (myosin) filaments within the sarcomeres, the contractile units comprising the myofibrils. * A Bands are the dark stripes, and I bands the light stripes. WHAT binding sites are on myosin? actin? B. Each myofibril has a cell membrane called a sarcolemma. Invaginations of the sarcolemma form traverse tubules (T-tubules) to carry the action potential to a specialized ER, called the sarcoplasmic reticulum (SR) which plays an important role in storing and releasing calcium ions. Calcium ions are the final trigger for muscle fiber contraction. C. Each sarcomere is made up of several important components. These include thin actin filaments, thick myosin filaments. The thin actin filaments are attached to a structure called a Z disc, located on each end of the sarcomere. The myosin filaments are held in place by the M line and proteins that also attach them to the Z disc. There is an empty space between opposing actin filaments is called the H zone. (Please review Figure 10.4 and 10.5). PAGE 1 OF 6

2 D. In muscle striations, the A bands are made up of the thick myosin filaments and the I bands are made of the then actin filaments. When the sarcomere contracts, the actin filaments are pulled very closely together and the distance of the H zone shrinks to 0. This is known as the sliding filament theory. E. A motor unit is one motor neuron and all the myofibrils (muscle cells) it controls. Muscles that produce very precise movements like the hands have small motor units- small number of myofibrils to control; but many of them!. Muscles that produce large and forceful movements have large motor units and fewer of them. Remember- Hands region on motor homunculus chapter Skeletal Muscle Activity (See the movies I sent) A. All skeletal muscle cells are stimulated by motor neurons. When the neuron releases a neurotransmitter (acetylcholine), the permeability of the sarcolemma changes, allowing sodium ions to enter the cell. This produces an electric current (action potential), which flows across the entire sarcolemma to the T-tubules and eventually to the SR resulting in instant release of calcium ions into the sarcomeres. B. Calcium binds to regulatory proteins (troponin) attached to tropomyosin fibers on the actin filaments and exposes myosin binding sides, allowing the myosin heads on the thick filament to attach. The attached heads pivot sliding the thin filaments toward the center of the sarcomere (M line), and contraction occurs. ATP provides the energy for the sliding process, which continues as long as the calcium ion is present. C. So to summarize, the scheme of events that cause muscle contraction: I. First start with a motor neuron releasing acetylcholine at the synapse. II. The acetylcholine binds to chemical gated Na+ channels (receptors) on the muscle cell sarcolemma which causes a rapid flow of sodium ions into the muscle cell. This AP flows down the sarcolemma to T-tubules. The T-tubules take the AP to the sarcoplasmic reticulum (SR). III. The influx of sodium ions causes the SR to release calcium ions within the muscle cell which flood the sarcomeres. IV. Calcium binds to troponin on the actin filament and the tropinintropomyosin complex shifts, exposing the myosin binding sites on actin. V. Myosin heads bind to the actin filaments (crossbridge formation) and the head swivels pulling them toward each other (power stroke). This is an ATP dependent process. VI. The myosin heads continue to bind and swivel until the two actin filaments are pulled together the H Zone shrinks to nothing. D. Although individual muscle cells contract completely ( all or nothing principle ) went adequately stimulated, a muscle (an organ) response to stimuli to different PAGE 2 OF 6

3 degrees, that is, it exhibits graded responses due to recruitment of various motor units. So your leg muscles can be slow and delicate like dancing or fast forceful like kicking a ball. E. ATP, the immediate source of energy for muscle contraction, is stored in muscle fibers in small amounts that are quickly used up. ATP is regenerated via three routes. From the fastest to the slowest, these are: I. via a couple reaction of creatine phosphate with ADP, Phosphagen system. Also includes the action of myokinase which takes 2 ADP and makes 1 ATP + AMP. (slide 94) II. via anaerobic glycolysis (cytoplasm) with lactic acid formation, and III. via aerobic respiration in mitochondria. Only aerobic respiration requires O2. IV. muscle cells have their own stored O2 reserves in myoglobin, and their own stored fuel reserves in glycogen. Both are used in # III above. F. Phosphagen system lasts about 10 sec; then anaerobic respiration kicks in for about 50sec, and after ~1 minute aerobic respiration kicks in. G. There are 4 types of muscle fibers. Oxidative are fatigue resistant, glycolytic are susceptible to fatigue (fatigable). DO NOT memorize the 4 types. Remember any muscle contains mixtures of each type. You can modify the number by exercise, but most is determined by heredity (genes). H. If muscle activity is strenuous and prolonged, muscular fatigue occurs because lactic acid accumulates in the muscle and energy (ATP) supply decreases. After exercise, the oxygen deficit is repaid by rapid deep breathing. 4. Cardiac Muscle (see 1. above) Contraction very similar to skeletal muscle. Has pacemaker to set rhythm. AP flows through intercalated discs (electrical synapse) so whole heart muscle contracts instantly. ANS controls the speed and force of contraction. 5. Smooth Muscle- SM (see 1. above) Contraction is different. SM has dense bodies and intermediate filament that form a netting around the cell. They lack T-tubules and have little SR. See Figure As the myosin pulls the actin filaments together (latchbridge mechanism), it causes the netting to shrink and thus the whole cell shrinks. Contraction is slower and lasts longer than skeletal muscle. Because of the latchbridge mechanism, the SM can remain contracted with no addition energy required (fatigue resistant). The ANS (via varicosities) controls SM contraction. ACh and other chemicals (hormones, NE, drugs, etc) can be neurotransmitters. 6. Muscle movements, types, and names. Chapter 11. A. Do not memorize any muscle names for lecture but do so for LAB. PAGE 3 OF 6

4 Figure Figure PAGE 4 OF 6

5 Reference Table PAGE 5 OF 6

6 SUBJECT: ANATOMY & PHYSIOLOGY BIOL 2401 Sarcomere FIGURE 10.5 Z disc Thick filament Thin filament Connectin M line H zone A band I band Z disc Thin filament I band Myosin molecule FIGURE 10.4 Heads Actin binding site ATP and ATPase binding site Tail Troponin Tropomyosin Ca2+ binding site Myosin binding site (green dots) PAGE 6 OF 6