Lecture 7 Muscle Tissue Notes PDF

Muscle Tissue Dr. Elita Partosoedarso Links to the segment recordings: Part A, Part B 1  Muscle Tissue overview Properties...

Muscle Tissue Dr. Elita Partosoedarso Links to the segment recordings: Part A, Part B 1  Muscle Tissue overview Properties Basics Types of muscles Basics of smooth and cardiac muscle Cells Structure of the Myofilament Muscle Tissue skeletal muscle Sarcomere Neuromuscular Junction Function of the Contraction of skeletal muscle skeletal muscle Relaxation of skeletal muscle Sources of ATP Others Muscle tension Motor unit 2  Characteristics of ALL muscles Functions of skeletal muscles General Properties of 1. Contractility: ability to contract 1. Posture: Continuous partial contraction of (shorten) and relax to produce some skeletal muscles lead to sitting, standing Muscle Tissue movement and staying still 2. Extensibility: ability to extend, or 2. Heat production: Catabolic process which stretch, to allow muscles to return to produces body heat and maintains their resting length homeostasis 3. Excitability (irritability): ability to be 3. Movement: pulls on bones (other muscles) to stimulated and respond to move the body as a whole or its parts regulatory signals from nerves, 4. Protection: covers internal organs, supports hormones & local stimuli weight of organs, keeps joints and bones from being over stressed All muscles are also richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal 3 Types of Muscle Tissue 1. Skeletal muscle ○ Structure (anatomy): multinucleated, Regular arrangement of actin and myosin fibers into bands of light and dark (striated) ○ Function (physiology): move the skeleton, especially the limbs ○ Location: usually connected to bones or fascia 2. Cardiac muscle ○ Structure (anatomy): 1-2 nuclei, branching cells with intercalated disks organized as a syncytium to allow for coordinated contraction ○ Function (physiology): pump blood through the circulatory system ○ Location: Heart 3. Smooth muscle ○ Structure (anatomy): 1 nucleus, no regular arrangement of actin and myosin proteins in cytoplasm (non-striated/smooth) ○ Function (physiology): Goosebumps, moves food through digestive tract, blood through circulatory system ○ Location: parts of viscera (organs, ducts) throughout the body 4 Characteristics of Cardiac Muscle 1. Highly coordinated contractions of cardiac muscle pump blood into blood vessels of the circulatory system. Pacemaker cells control rate of cardiac contractions 2. Similarity with skeletal muscle: striated, organized into sarcomeres 3. Differences with skeletal muscle: only 1-2 nuclei, multiple mitochondria and myoglobin, extensively branched fibers cells, intercalated discs. 4. Intercalated discs consist of sarcolemma with gap junctions & desmosomes which allow heart to work as a pump by coordinating cardiac contraction Gap junctions: channels between adjacent cells that allow ions to flow from one cell to another quickly. Depolarization spreads quickly between cells to allow for coordinated contraction of entire heart. This electric coupling creates a syncytium (functional unit of contraction). Desmosomes anchor the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting 5  Smooth Muscle 1. Similarity with skeletal muscle: actin & myosin contractile proteins, thick & thin filaments. 2. Differences with skeletal muscle: 1 nucleus, spindle-shaped , no striations, sarcomere, troponin, tropomyosin 3. Thin filaments are anchored by dense bodies (similar to Z-discs) attached to sarcolemma. 4. Ca++ enters sarcoplasm from SR and ECF and binds to regulatory protein calmodulin 6 Structure of a skeletal muscle Three layers of connective tissue enclose a muscle to provides structure while compartmentalizing fibers within it 1.1 Epimysium 1 sheath of dense, irregular connective tissue around each muscle organ allows a muscle to contract/move while maintaining structural integrity 2 separates muscle from other regional tissues/organs in the area, allowing independently movement Deeper 3 2.2 Perimysium middle layer of connective tissue allows nervous system to trigger a specific movement of a muscle by activating fascicle 3.3 Endomysium thin layer of collagen and reticular fibers around each muscle fiber organizes muscle fibers into fascicle (individual bundles) contains extracellular fluid and nutrients supplied by blood 7  Skeletal Muscle Fibers (Cells) Skeletal muscle cells are also called muscle fibers as they are long and cylindrical During early development, embryonic myoblasts, each with its own nucleus, fuse with up to hundreds of other myoblasts to form the multinucleated skeletal muscle fibers (myofibrils) with multiple copies of genes to allow bulk production of proteins and enzymes for muscle contraction. 1.1 Sarcolemma: plasma membrane of muscle fibers 2. Sarcoplasm: cytoplasm of muscle fibers 3.3 Sarcoplasmic reticulum (SR): specialized smooth endoplasmic reticulum: stores, releases, and retrieves calcium ions (Ca++) 4.4 Sarcomere: functional unit of skeletal muscle fiber: highly organized arrangement of contractile proteins (actin, myosin myofilaments) and regulatory proteins (troponin, tropomyosin) 8  The Sarcomere (functional unit of skeletal muscles) The sarcomere is the functional unit of skeletal muscles: 3D cylinders with striations (bands of light and dark due to arrangement of actin and myosin myofilaments) Each myofibril can contain 100-1000s sarcomeres connected end to end All sarcomeres within a myofibril contracts (and relaxes) simultaneously, contracting (and relaxing) the entire myofibril & muscle cell 1. Thin filament Starts from Z-discs and projects partway to the center consists of thinner actin strands and its troponin-tropomyosin complex 2. Thick filament Starts from the center and projects partway to the Z-discs consists of thicker strands and their multiple heads 3. Z-discs (Z-lines) Forms the boundary of sarcomeres at both ends Anchored to actin myofilaments 9  Myofilaments ▪ Each myofibril contains 1000s of thick and thin myofilaments ▪ Four different kinds of protein molecules make up myofilaments Protein molecules 1.1 Actin (thin filaments): contains active sites (myosin binding sites) which bind to myosin heads 2.2 Myosin (thick filament): Contains myosin heads that are chemically attracted to actin and forms cross bridges with actin 3.3 Tropomyosin (regulatory protein): at rest, it blocks the myosin binding sites on actin molecules when 4.4 Troponin (regulatory protein): at rest, it holds tropomyosin in place, can bind to calcium (Ca2+) ions 10 10 The Neuromuscular Junction Ion channel 1. Location: site where nerve ending meets the muscle fiber 2. All living cells have membrane potentials (electrical gradients across their membranes): -60 to -90 mV 3. When the membrane potential becomes LESS negative, depolarization occurs and an action potential can start 4. Membrane potentials change when ions either enter or leave the cell through ion channels which can open and close depending on the stimuli. This change generates electrical signals (action potential) which travel quickly over long distances. An action potential in a nerve at the NMJ releases a neurotransmitter which leads to the start of an action potential in the muscle. This action potential in the muscle causes muscle contraction (Excitation-contraction coupling) 5. Every skeletal muscle fiber is innervated by a motor neuron at the NMJ A signal from the motor neuron can cause the contraction of skeletal muscle fibers Each motor neuron can innervate from 10s to 1000s skeletal muscle fibers 11  Contraction of a skeletal muscle Part 1 Action potential (AP) reaches the end of the motor neuron 1 Neurotransmitter (acetylcholine or ACh) is released into the NMJ ACh binds to specific receptors on ligand gated ion channels for 2 sodium on the skeletal muscle fiber 1 3 Sodium channels open: sodium enters sarcoplasm of muscle fiber Membrane potential of muscle fiber changes AP starts along the sarcolemma of muscle fiber: AP travels into the 4 interior of the cell via T-tubules (extensions of the sarcolemma) Continued on the next slide 4 2 3 4 12  Contraction of a skeletal muscle Part 2 4 AP starts along the sarcolemma of muscle fiber: AP travels into the interior of 4 the skeletal muscle cell via T-tubules (extensions of the sarcolemma) SR: Sarcoplasmic Reticulum Ca++ : Calcium ions Action potential depolarizes the cell membrane 5 Voltage-gated Ca++ channels in SR Ca++ diffuses out of SR into sarcoplasm 5 Ca++ binds to troponin on thin filament 6 6 Troponin-tropomyosin complex moves to expose myosin-binding sites 7 Myosin binds actin at its myosin-binding site to form cross-bridge 7 8 Adenosine diphosphate (ADP) and inorganic phosphate (Pi) generated in the previous contraction cycle are released 8 Myosin head pivots toward M-line at center of the sarcomere- power stroke New ATP attaches to the myosin head 9 9 Cross-bridge is detached ATPase in myosin head Angle of myosin head moves into a cocked 10 hydrolyzes ATP to ADP and position (re-cock), ready to form another Pi, releasing energy crossbridge with next myosin -binding site 10 Cross-bridge cycling: power stroke, detach, re-cock, power stroke, detach, re-cock, etc  Sliding Filament Model of Contraction 1. Overview: contraction of skeletal muscle fiber contracts as the thin filaments are pulled and then slide past the thick filaments within the fiber’s sarcomeres 2. Requires Ca++ and ATP Ca++ initiates contraction by exposing actin-binding site to form myosin crossbridges ATP sustains contraction: Each cycle in cross-bridge cycling requires energy provided by hydrolysis of ATP Without ATP, the myosin head remains attached to actin: rigor mortis Myosin is in a high-energy configuration when myosin head is cocked: this energy is used during the power stroke 14  Relaxation of a Muscle Fiber Muscle contraction usually stops when 1 Nerve signal stops Muscle runs out of ATP and becomes fatigued 2 4 5 3 Nerve signal stops 1 Release of ACh stops 2 6 Ligand gated Na+ channels close 3 7 Sarcolemma and T-tubules repolarizes 4 Voltage-gated Ca++ channels in the SR close 5 8 Ca++ ions are pumped back into SR using ATP 6 Tropomyosin moves to cover myosin-binding sites 7 Thick and thin filament interaction relaxes 8 15  Skeletal muscle only has a small amount of ATP stored In order to sustain contraction, ATP must be replaced quickly 1 Sources of ATP 1.1 Creatine phosphate: Excess ATP transfers energy by Sources of ATP producing ADP and creatine phosphate. When energy is needed, creatine phosphate transfers its phosphate back to ADP to form ATP and creatine. Can only provide 15 seconds worth of energy 2.2 Glycolysis anaerobic breakdown of glucose to produce ATP, at a slower rate than creatinine phosphate. Provides 1 2 minute burst of energy 3.3 Aerobic respiration aerobic breakdown of glucose or other nutrients in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. More efficient, produces 95% of ATP 3 16  Motor Units 1 Each skeletal muscle fiber is innervated by only one motor neuron. Each motor neuron innervates more than one muscle fiber, the number depends on the nature of the muscle 1.1 Motor unit: group of muscle fibers innervated by a single motor neuron skeletal muscle fiber Small motor units can innervate less than 10 muscle fibers and permit very fine motor control of the muscle, eg eyeball movements. have smaller, lower-threshold motor neurons that are more excitable Larger motor units can supply 1000s of muscle fibers in a muscle are concerned with simple, or “gross,” movements, eg thigh muscles. bigger, higher-threshold motor neurons 2.2 Recruitment process where smaller motor units tend to be recruited first before 2 larger ones, increasing the muscle contraction. Recruitment of more motor units will increase the strength of muscle contraction: allows for variation in picking up a feather vs a heavy weight. 17