Chapter 10: Muscle Tissue

Function of muscular tissue

1. Producing body movements 2. Stabilizing body positions 3. Storing and moving substances within the body 4. Generating heat (thermogenesis)

Skeletal muscle tissue

Muscle cell= muscle fiber (elonged shape)

Hypertrophy

Muscle fiber (cell) -Increase size of cell -Increase muscle mass inside each cell by increase # of myofibrils Human growth hormone (HGH) -Stimulates muscle fiber (cell) hypertrophy -HGH increase protein synthesis Testosterone (anabolic steroid) -Increase protein synthesis Muscle atrophy -Wasting away of muscle -Muscles decrease in size due to a decrease in the # of myofibril Disuse atrophy: occurs with paralysis, bedridden, cast, sedentary life style. Denervation atrophy: Nerve supply is damaged or cut. Aging: -40% loss of muscle mass between 50-80 years. -weight bearing exercise can minimize effect of muscle loss.

Nerve and Blood Supply

Muscle: -Innervated (nerve supply) -Vascularized (blood supply) Somatic motor neurons: -Target skeletal muscle cells. -Neurons make and release a chemical called a neurotransmitter (Acetylcholine (Ach)) -Ach needs to be released to have a muscle contraction. -are neurons that stimulate skeletal muscle to contract. -each somatic motor neuron has a threadlike axon that extends form the brain or spinal cord to a group of skeletal muscle fibers. -The axon of a somatic motor neuron typically branches many times, each branch extending to a different skeletal muscle fiber. *Muscle may be fine but it cannot function. Why? -If nerves innervating muscle fibers do not work then the muscle will not function effectively. -If muscles have no blood supply they will not function effectively.

Connective tissue components

-Connective tissue surrounds and protects muscular tissue. -Subcutaneous layer or hypodermis, separates muscle from skin. -composed of areolar CT and adipose tissue. -Fascia (bandage) -holds muscle with similar muscle functions together. -dense sheet or broad band of irregular CT that lines the body wall and limbs and supports and surrounds muscles and other organs of the body. -free movement of muscles: carries nerves, blood vessels, and lymphatic vessels; and fills spaces between muscles. 3 layers of connective tissue: 1. Epimysium: is the outer layer, encircling the entire muscle. It consists of dense irregular connective tissue. 2. Perimysium: is also a layer of dense irregular connective tissue, but it surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called: -Fasicles (little bundles) 3. Endomysium: penetrates the interior of each fascicle and separates individual muscle fibers form one another. The endomysium is mostly reticular fibers. *All three layers form the tendon.

Overview of muscle tissue

3 types of muscle tissue: 1. Skeletal muscle tissue- -move the bones of the skeleton. -striated (Alternating light and dark protein bands) -most muscles voluntary and consciously controlled by neurons (nerve cells) to move. Somatic (voluntary) division of nervous system. -some muscles subconsciously move such as the diaphragm. Contracts and relax to continuously without consciousness so that you don't stop breathing. -subconsciously maintain posture or stabilize body positions. 2. Cardiac muscle tissue- -only contained mostly of the heart wall. -striated, but involuntary, autonomic (involuntary) division of nervous system. -not consciously controlled, natural pacemaker that's built-in rhythm called autorhythmicity. 3. Smooth muscle- -located in the walls of hollow internal structures, such as blood vessels, airways, and most organs in the abdominopelvic cavity. Also found in the skin, attached to hair follicles. -hormones and neurotransmitters can adjust heart rate by speeding or slowing the pacemaker. -nonstriated, involuntary, autorhythmicity of gastrointestinal tract to propel food, autonomic (involuntary) division of nervous system. -regulated by hormones released by endocrine glands.

Contraction and relaxation of skeletal muscle fibers

Contraction cycle 1. ATP hydrolysis(ADP & Phosphate still attached to myosin) 2. Attachment of myosin to actin to form cross-bridges (Phosphate is released) 3. Power stroke(ADP is released) 4. Detachment of myosin from actin(requires ATP) 4 functions of ATP in muscle contraction 1. Step 1 -ATP hydrolysis -ATP->ADP+P+E2(ATPase is an enzyme that breaks down ATP) -E2 help myosin form a cross bridge with actin to move actin toward center of sarcomere 2. Step 4 -Detachment of myosin + actin -Requires ATP 3. Use ATP to drive or run the CA2+ Active Transport Pump 4. ATP used to drive or run the Na+/K+ ATPase pump -Maintains the RMP (resting membrane potential) *Clinical Connection-Rigor Mortis -Begins~3-4 hours after death last ~24 -After death cell membranes of SR become leaky. -Ca2+ starts to leak out of the SR -ATP synthesis stop, No ATP so cross-bridges do not detach; causes muscles to stay contracted for 24 hours -Enzymes in body that destroy protein; destroys actin + myosin + the cross-bridge (connection between the two + muscle "relax") Neuromuscular Junction 1. Release a acetylcholine. 2. Activation of ACh receptors. 3. Production of muscle action potential 4. Termination of ACh activity. -Acetylchoinesterase (AChE) No contraction= Relaxation -Breakdown ACh in the synaptic cleft -AChE=Acetylcholinesterase -Black Ach realease form somatic motor neuron -Botox (Botalism toxin) -Block ACh receptor on motor and plate of sarcolemma -Curare -Atropine -Block nerve AP=anesthetics Contraction -Nerve AP -ACh bind to ACh Receptor -muscle AP -AChE inhibitor= anticholinesterase

Microscopic anatomy of skeletal muscle fiber

Myofiber=muscle fiber= muscle cell 1. Sarcolemma= plasma membrane of a skeletal muscle fiber (cell) 2. Transverse tubule = T-tubule= part of the plasma membrane (sarcolemma) that extends into the interior of the cells. -T-tubules surround the myofibrils. 3. Sarcoplasm= cytoplasm inside muscle fiber(cell) -contains glycogen= stored form of glucose -myoglobin= protein binds oxygen & releases oxygen to mitochondria for ATP production. 4. Filaments & Sarcomeres -Myofibril (inside the cell) contain contractile element called filaments. 5. Sliding filament mechanism & sarcoplasmic reticulum (include terminal cisterns) Function of sarcoplasmic reticulum (SR): 1. Store Ca2+(ions) 2. Release Ca2+ into the sarcoplasm for muscle contraction to occur. *What can cause the release of Ca2+ from the SR? -A muscle Action Potential: trigger the release of Ca2+ from the SR by opening Calcium Release Channels (CRC) -Ca2+ Active Transport Pump- uses ATP to move Ca2+ constantly from the sarcoplasm into the SR. 2 types of filaments: 1. Thick filament -contains the protein myosin (head & tail) -motor protein move structure in skeletal muscle cell -Myosin moves the thin filament toward the center of the sarcomere -ATP is required by myosin to do work -Myosin protein convert chemical energy in (movement) ATP molecule (E2) to get mechanical energy of motion. 2. Thin filament -contains the protein actin & regulatory protein: -Troponin (has CA2+ binding site) -Tropomyosin Structural proteins: 1. Titan: -like a spring contributes to elasticity in the myofibril 2. Dystrophin: -links to sarcolemma to strengthen it. -transmit tension generated by sarcomeres to the tendon (tendon of insertion to move the skeletal framework) *Duchenne Muscular Dystrophy -Inherited disease that destroys muscle fibers(cells) -DNA=RNA=Protein -Mutation (changed) in gene that codes for protein dystrophin, results in very little of the protein being made. -Without this structural protein dystrophin the muscle cell is vulnerable to tearing when the muscle contracts. Sarcomere- unit in the myofibril. Extends from one Z disc to another Z disc.

Muscle Metabolism

Production of ATP in muscle fibers: 1. Creatine phosphate: -Energy-rich molecule that is found in muscle fibers. -Enzyme creatine kinase (CK) catalyzes the transfer of one of the high-energy phosphate groups from ATP to creatine, forming creatine phosphate and ADP. -Creatine is a small, amino acid-like molecule that is synthesized in the liver, kidneys, and pancreas and then transported to muscle fibers. -Creatine phosphate is 3-6 times more plentiful than ATP in the sarcoplasm of a relaxed muscle fiber. -ADP increases when contraction occurs CK catalyzes the transfer of high-energy phosphate group from creatine phosphate back to ADP, which generate new ATP molecules. -Creatine phosphate is the first source of energy when muscle contraction begins because of the rapid formation of ATP. -Creatine phosphate and ATP provide enough energy for muscles to contract maximally for about 15 seconds. 2. Anaerobic Glycolysis -When creatine phosphate within the muscle fiber is depleted by continued muscle activity, glucose is catabolized to generate ATP. -Glyogen breaks down and produce glucose within the muscle fibers. -A reaction called glycolysis quickly breaks down each glucose molecule into 2 pyruvic acid molecules. -2 ATP is produced by glycolysis process. -Glycolysis does not require oxygen whether oxygen is present or not. -During heavy exercise, the skeletal muscle fiber does not have enough oxygen. The pyruvic acid generated from glycolysis is converted to lactic acid. -Anaerobic glycolysis- the process by which the breakdown of glucose gives rise to lactic acid when oxygen is absent or at a low concentration. -2 min of maximal muscle activity. Aerobic respiration: -Pyruvic acid formed by glycolysis enters the mitochondria when oxygen is present. -Aerobic respiration requires oxygen for reactions (the Krebs cycle and the electron transport chain) that produce ATP, carbon dioxide, water, and heat. -Each molecule of glucose catabolized under aerobic conditions yields about 30 or 32 molecules of ATP. -Hemoglobin(found only in RBC) and Myoglobin(found only in muscle cells) are oxygen-binding proteins. -ATP and nutrients such as pyruvic acid obtained form the glycolysis of glucose, fatty acids from the acid obtained form the glycolysis of glucose, fatty acids form the breakdown of protein, are available during resting periods and light to moderate exercise. -Aerobic respiration provides nearly all of the needed ATP for activities that last form several minutes to hours.