MUSCLE TISSUE

Joint: a location where two or more bones articulate

Skeletal Muscles

Structure of Skeletal Muscle Connective tissue components Skeletal muscles are surrounded by a fibrous epimysium Connective tissue called perimysium subdivides the muscle into fascicles Each fascicle is subdivided into muscle fibers surrounded by endomysium

Muscle Fiber Structure Have many of the organelles found in other cells Have plasma membranes called sarcolemma Are multinucleated; form a syncytium Are striated I bands: light bands A bands: dark bands Z-lines (discs): dark lines in the middle of the I bands

Skeletal Muscle Fibers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nuclei Muscle fiber © Ed Reschke

Muscles and Bones Generally, both ends of a muscle are attached to bone by tough tendons When a muscle contracts, it shortens. This places tension on tendons connecting it to a bone. This moves the bone at a joint. The bone that moves is attached at the muscle insertion. The bone that does not move is attached at the muscle origin. Movement is toward the insertion

Skeletal Muscle Actions

Mechanisms of Contraction

Neuromuscular Junction

Motor Unit A motor unit is a single motor neuron and all the muscle fibers it innervates; all the muscle fibers in a motor unit contract at once Graded contractions – varied contraction strength due to different numbers of motor units being stimulated Neuromuscular junction: site where a motor neuron stimulates a muscle fiber Motor end plate: area of the muscle fiber sarcolemma where a motor neuron stimulates it using the neurotransmitter, acetylcholine

Motor Unit Somatic motor neuron Motor unit Spinal cord Motor unit Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Somatic motor neuron Motor unit Spinal cord Motor unit Somatic motor neuron (a) Motor unit Neuromuscular junctions Somatic motor axon Skeletal muscle fibers (b)

So how does this contraction work?

Muscle Fiber Binding Each fiber has densely packed subunits called myofibrils that run the length of the muscle fiber Stacked in register so that the dark and light bands align Composed of thick and thin myofilaments

Z line: partitions the sarcomeres A Band: dark band where thick and thin filaments overlap I band: composed of thin filaments – these shorten during contraction H zone: center of A band where only thick filaments are present

Sarcomere Unit of contraction From one Z line to the next Z line Contains overlapping of thick and thin filaments

Arrangement of Thick and Thin Filaments Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sarcomere A band I band H band I band Thin filament Thick filament (a) Z disc Z disc Myofibril Sarcomere Copyright by Dr. R.G. Kessel and R.G. Kardon, Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, W.H. Freeman, 1979

Arrangement of Thick and Thin Filaments Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Z disc Z disc (b) M SR Myofibril M Copyright by Dr. R.G. Kessel and R.G. Kardon, Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, W.H. Freeman, 1979

Cross Bridges More about myofilaments Thick: composed of the protein myosin Each protein has two globular heads with actin-binding sites and ATP-binding sites. Thin: composed of the protein actin Have proteins called tropomyosin and troponin that prevent myosin binding at rest.

Action of Sliding Sliding is produced by several cross bridges that form between myosin and actin. The myosin head serves as a myosin ATPase enzyme, splitting ATP into ADP + Pi. This allows the head to bind to actin when the muscle is stimulated.

Cont’d Release of Pi upon binding cocks the myosin head, producing a power stroke that pulls the thin filament toward the center. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Actin 1 ADP Pi 2 Pi Power stroke

Cont’d After the power stroke, ADP is released and a new ATP binds. This makes myosin release actin. ATP is split. The myosin head straightens out and rebinds to actin farther back. Continues until the sarcomere has shortened

Activation of the Myosin Head Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Troponin Actin Tropomyosin Myosin binding site Thin filament Actin-binding sites ATP-binding site ADP 2 1 Pi Myosin head ATP Myosin tail Thick filament

Cross Bridges 1 Resting fiber; cross bridge is not attached to actin 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 Resting fiber; cross bridge is not attached to actin 6 ATP is hydrolyzed and phosphate binds to myosin, causing cross bridge to return to its original orientation Thin filament ADP Pi Myosin head Cross bridge Thick filament 2 Cross bridge binds to actin ATP 5 A new ATP binds to myosin head, allowing it to release from actin 3 Pi is released from myosin head, causing conformational change in myosin 4 Power stroke causes filaments to slide; ADP is released

So what do we need for skeletal muscle contraction?

Role of Calcium When muscle cells are stimulated, Ca2+ is released inside the muscle fiber. Some attaches to troponin C, causing a conformational change in troponin and tropomyosin. Myosin is allowed access to form cross bridges with actin.

Regulation of Contraction F-actin is made of 300-400 G-actin subunits, arranged in a double row and twisted to form a helix Tropomyosin physically blocks cross bridges

Role of Calcium Actin Tropomyosin Relaxed muscle: ADP tropomyosin Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Actin Tropomyosin Relaxed muscle: tropomyosin blocks the binding site ADP Troponin Pi Binding site Cross bridge Myosin Ca2+ Contracting muscle: myosin head binds to actin Ca2+ ADP Ca2+ Pi

Excitation-Coupling Contraction Sarcoplasmic reticulum (SR) SR is modified endoplasmic reticulum that stores Ca2+ when muscle is at rest. Most is stored in terminal cisternae. When a muscle fiber is stimulated, Ca2+ diffuses out of calcium release channels (ryanodine receptors). At the end of a contraction, Ca2+ is actively pumped back into the SR.

Sarcoplasmic Reticulum

Stimulating a Muscle Fiber Acetylcholine is released from the motor neuron End plate potentials are produced Action potentials are generated (All-or-none event) Voltage-gated calcium channels in transverse tubules change shape and cause calcium channels in SR to open Calcium is released and can bind to troponin C

Stimulating a Muscle Fiber Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axon terminal Sarcolemma 2 1 Ca2+ 3 Transverse tubule 4 Sarcoplasmic reticulum 1 Nicotinic acetylcholine receptor 3 Transverse tubule voltage-gated calcium channel 2 Skeletal muscle voltage-gated sodium channel 4 Sarcoplasmic reticulum calcium release channel

Muscle Relaxation Action potentials cease Calcium release channels close Ca2+-ATPase pumps move Ca2+ back into SR (active transport) No more Ca2+ is available to bind to troponin C Tropomyosin moves to block the myosin heads from binding to actin

Energy Requirements of Skeletal Muscles

Where do muscles get energy? At rest and for mild exercise: from the aerobic respiration of fatty acids For moderate exercise: from glycogen stores For heavy exercise: from blood glucose As exercise intensity and duration increase, GLUT4 channels are inserted into the sarcolemma to allow more glucose into cells.

Aerobic Respiration Provides 95% of the ATP Mitochondria absorbs oxygen, ADP, phosphate ions from cytoplasm Molecules in the Krebs Cycle  oxidation of acetly coA into C02 and ATP

But what happens if you need a quick burst of energy?

Metabolism of Skeletal Muscle Anaerobic for the first 45-90 seconds of moderate to heavy exercise Does not require oxygen Breaks down glycogen reserves Allows time to increase oxygen supply

So we can function without oxygen? Yes, and no

Oxygen Debt When a person exercises, oxygen is withdrawn from reserves in hemoglobin and myoglobin. To create cross bridges in muscle contraction and pump calcium back into SR at rest To metabolize lactic acid Breathing rate continues to be elevated after exercise to repay this debt.

Labeling