Lecture 8 Muscle Tissue COVID (3) PDF

Support & Movement Muscle Tissue Muscle Tissue Skeletal muscle tissue Cardiac muscle tissue Smooth muscle tissue Skeletal Muscles Skeletal Muscles Attached to the skeletal system Allow body movement Muscular system includes only skeletal muscles Functions of Skeletal Muscles Produce sk...

Support & Movement Muscle Tissue Muscle Tissue Skeletal muscle tissue Cardiac muscle tissue Smooth muscle tissue Skeletal Muscles Skeletal Muscles Attached to the skeletal system Allow body movement Muscular system includes only skeletal muscles Functions of Skeletal Muscles Produce skeletal movement Maintain body position Support soft tissues Guard body openings Maintain body temperature Structure of Skeletal Muscle Muscle cells (fibers) Connective tissues Blood vessels Lymphatics Nerves Levels of Organization of Muscle Tissue Level 1: skeletal muscle Level 2: muscle fascicle Level 3: muscle fiber Level 4: myofibril Level 5: sarcomere Connective Tissue Covering of Skeletal Muscles 3 layers: Skeletal Muscle (organ) Epimysium Perimysium Endomysium Nerve 1. epimysium 2. perimysium 3. endomysium Muscle Muscle Blood fascicle fibers vessels Epimysium Surrounds the entire muscle Skeletal Muscle (organ) Connected to deep fascia Epimysium Perimysium Endomysium Nerve Separates muscle from surrounding tissues Muscle Muscle Blood fascicle fibers vessels Perimysium Surrounds muscle fascicles Skeletal Muscle (organ) Contains blood vessel and nerve Epimysium Perimysium Endomysium Nerve supply to fascicles Muscle Muscle Blood fascicle fibers vessels Endomysium Surrounds individual muscle fibers Skeletal Muscle (organ) Contains capillaries and nerve fibers Epimysium Perimysium Endomysium Nerve Contains satellite (stem) cells for repair Muscle Muscle Blood fascicle fibers vessels Muscle Attachments Endomysium, perimysium and epimysium meet together at muscle ends: ̶to form connective tissue attachment to bone matrix:  tendon (bundle) or  aponeurosis (sheet) Innervations & Blood Supply Skeletal muscles are voluntary muscles, controlled by somatic nerves Muscles have extensive vascular systems that: ̶supply:  large amounts of oxygen  nutrients ̶carry away wastes  Myocyte Skeletal Muscle Cells (fibers) Very long s Multinucleated (hundreds of nuclei) Develop through fusion of myoblasts (mesodermal cells) Structure of Skeletal Muscle Fibers Sacrolemma (= cell membrane) Sarcoplasm (= cytoplasm) Sarcosome (= mitochondrion) Sarcoplasmic reticulum (= SER) Transverse (T) tubule Myofibril Sarcolemma Cell membrane of muscle cell Surrounds the sarcoplasm Change in transmembrane potential begins muscle contraction Myofibril Muscle fiber Sarcolemma Nuclei Sarcoplasm Transverse (T) Tubules Deep invaginations of the sarcolemma Run transversely across myofibrils A momentary Transmit action change cell potential through in electrical potential Allow entire on the muscle fiber to surface of a nerve or muscle cell when stimulated resulting in contract simultaneously transmission of an electrical impulse Sarcoplasmic Reticulum (SR) Smooth endoplasmic reticulum (SER) Surrounds each myofibril Helps transmit action potential to myofibril Forms cisternae: ̶chambers that store Ca2+ ̶release Ca2+ into sarcomeres to begin muscle contraction Triad Formed by 2 terminal cisternae + 1 T tubule Required for excitation-contraction coupling Myofibrils Lengthwise threads within muscle fiber Divided into a number of sarcomeres Made up of bundles of protein filaments (myofilaments) Myofilaments Part of the cytoskeleton Responsible for muscle contraction Composed of proteins Types (based on type of protein): ̶thin filaments ̶thick filaments Thin Filaments Sarcomere H band 4 proteins: Myofibril ̶actin Z line M line ̶nebulin ̶tropomyosin ̶troponin Troponin Active site Nebulin Tropomyosin G-actin molecules F-actin strand Thick Filaments Sarcomere H band 2 proteins: Myofibril ̶myosin Z line M line ̶titin Titin c The structure of thick filaments, showing M line Head Tail the orientation of the myosin molecules Hinge d The structure of a myosin molecule Sarcomere Contractile unit of the muscle Structural unit of myofibrils Consists of thin and thick myofilaments Contains alternating dark and light bands (striations) Lines & Zones A bands: myosin filaments (with actin on both ends) M line: center of the A band I bands: actin filaments ONLY (no myosin) Z line: center of the I bands Sarcomere extends between 2 Z lines A band contains 2 zones: ̶ H zone (band):  area around the M line  myosin filaments ONLY (no actin) ̶ Zone of overlap:  thick and thin filaments overlap  darkest area seen by light microscope  Sarcomere I band A band Z line Zone of M line Z line overlap H band Each sarcomere is composed of: ̶One A band in the middle ̶½ of an I band on each side Neuromuscular Junction & Skeletal Muscle Contraction Neuromuscular Junction (NMJ) Specialized intercellular connection between: ̶synaptic terminal of a neuron & ̶motor end plate of a skeletal muscle fiber Neuromuscular Junction (NMJ) Synaptic vesicle Nerve Action Potential Electrical signal: ̶travels along nerve axon ̶ends at synaptic terminal Synaptic terminal: ̶releases neurotransmitter acetylcholine (ACh) from synaptic vesicles into the synaptic cleft Synaptic cleft: ̶gap between synaptic terminal and motor end plate Acetylcholine (ACh) Neurotransmitter required for muscle contraction Stored in synaptic vesicles Travels across synaptic cleft Binds to membrane receptors on sarcolemma at the motor end plate Causes Na+ to rush into sarcoplasm Initiates muscle action potential Quickly breaks down by enzyme acetylcholinesterase (AChE) Muscle Action Potential Electrical signal Generated by sodium influx  [Na+] in the muscle fiber Travels along the T tubules Leads to excitation–contraction coupling Excitation–Contraction Coupling Action potential reaches a triad: ̶Ca2+ release from cisternae of SR ̶triggering contraction Requires myosin heads to be Axon terminal Excitation Excitation Sarcolemma loaded by ATP T tubule Cytosol Sarcoplasmic reticulum Calcium ion release ATP Ca2+ Thick-thin Ca2+ filament interaction Muscle Contraction Caused by interactions of thick and thin filaments of sarcomeres Triggered by Ca2+ ions Follows the sliding filament theory Sliding Filament Theory During muscle contraction: ̶thin filaments slide between thick filaments toward M line ̶Z lines move closer ̶zone of overlap gets thicker ̶H zone gets narrower ̶A band does not change A Review of Muscle Contraction (1) Steps That Initiate a Muscle Contraction 1 ACh released Axon terminal Sarcolemma ACh is released at the neuromuscular junction and binds to ACh receptors on the sarcolemma. Cytosol 2 Action potential reaches T tubule T tubule An action potential is generated and spreads across the membrane surface of the muscle fiber and along the T Sarcoplasmic reticulum tubules. 3 Sarcoplasmic reticulum releases Ca2+ Ca2+ The sarcoplasmic reticulum releases stored calcium ions. Actin 4 Myosin Active site exposure and cross-bridge formation Calcium ions bind to troponin, exposing the active sites on the thin filaments. Cross-bridges form when myosin heads bind to those active sites. 5 Contraction cycle begins The contraction cycle begins as repeated cycles of cross-bridge binding, pivoting, and detachment occur—all powered by ATP. A Review of Muscle Contraction (2) Steps That End a Muscle Contraction Axon 6 ACh is broken down terminal Sarcolemma ACh is broken down by acetylcholinesterase (AChE), ending action potential generation Cytosol T tubule 7 Sarcoplasmic reticulum reabsorbs Ca2+ As the calcium ions are reabsorbed, their concentration in Sarcoplasmic reticulum the cytosol decreases. 8 Active sites covered, and cross-bridge formation ends Ca2+ Without calcium ions, the tropomyosin returns to its normal Actin position and the active sites are covered again. Myosin 9 Contraction ends Without cross-bridge formation, contraction ends. 10 Muscle relaxation occurs The muscle returns passively to its resting length. Remember! During contraction, skeletal muscle fibers shorten as thin filaments slide between thick filaments SR releases Ca2+ when a motor neuron stimulates the muscle fiber Free Ca2+ in the sarcoplasm triggers contraction Contraction is an active process requires ATP Relaxation and return to resting length is a passive process however, it still requires ATP Rigor Mortis Fixed muscular contraction after death Caused when: ̶ion pumps cease to function ̶calcium builds up in the sarcoplasm Definitions & Concepts Motor unit: single motor neuron and muscle fiber it innervates Motor end plate: pocket formed around motor neuron by the sarcolemma Muscle tone: normal tension and firmness of a muscle at rest Muscle twitch: fine movements of a small area of muscle Muscle Tension Action exerted on connective tissue fibers when muscle fiber shortens at contraction 2 Types: ̶isotonic contraction:  skeletal muscle changes length resulting in motion ̶isometric contraction:  skeletal muscle develops tension but is prevented from changing length Tetany (Physiological Tetanus) Sustained muscle contraction without periods of relaxation due to repetitive stimulation of muscle fibers Incomplete: ̶less frequent stimulation (one/sec) repetitive contractions and relaxation Complete: ̶high frequency stimulation contractions sum muscle never begins to relax continuous contraction  = Stimulus Maximum tension (in tetanus) Tension Maximum tension (in treppe) Time Time a Treppe. Treppe is an increase in b Wave summation. Wave summation peak tension with each successive stimulus delivered shortly after the occurs when successive stimuli completion of the relaxation phase of arrive before the relaxation phase the preceding twitch. has been completed. Maximum tension (in tetanus) Tension Time Time c Incomplete tetanus. Incomplete d Complete tetanus. During tetanus occurs if the stimulus complete tetanus, the stimulus frequency increases further. Tension frequency is so high that the relaxation production rises to a peak, and the phase is eliminated. Tension plateaus periods of relaxation are very brief. at maximum levels. Tetanus Acute infectious disease Cause: ̶toxin of Clostridium tetani bacteria ̶through contaminated wounds ̶affects the CNS continuous muscle contractions Manifestations: ̶general spasm of skeletal muscles ̶can be fatal Electromyogram (EMG) Measurement of electrical activity of muscle fibers (muscle response to nervous stimulation) Represented as a graph of twitch tension development Energy Storage in Muscle Fibers Substr Role Procedure ate ‒Glycolysis Breaks down Glycoge (anaerobic) glucose generates CK n ‒Krebs cycle ATP (aerobic) ATP: adenosine triphosphate ADP: adenosine diphosphate P: phosphate E: energy ATP Source CP: creatine phosphate of energy C: creatine ATP  ADP + P + E CK: creatine kinase enzyme Restores ATP ADP + CP ATP + C ATP Generation Anaerobic Aerobic Reaction Reaction Site Cytosol Mitochondria Requires O2 No Yes Chemical Glycolysis Krebs cycle process Muscle Peak activity Resting condition 1 glucose Pyruvic acid Reaction 2 pyruvic acid CO2 + H2O ATP / 1 2 34 Energy Use & Muscle Activity At rest & mild exertion: oxygen is available ATP generation starts in cytoplasm (glycolysis) completed in mitochondria (Krebs cycle – aerobic) At peak exertion: muscles lack oxygen to support mitochondria muscles rely on glycolysis for ATP pyruvic acid builds up converted to lactic acid Sustained condition muscle fatigue Muscle Fatigue Inability of a skeletal muscle to perform required activities Results: ̶depletion of metabolic reserves ̶accumulation of lactic acid ̶muscle exhaustion and cramps ̶damage to sarcolemma and sarcoplasmic reticulum  Muscle Metabolism Muscle Metabolism in a Resting Muscle Fiber Muscle Metabolism during Peak Activity Fatty acids O2 G Lactate Blood vessels Muscle Metabolism during Moderate Activity Glucose Glycogen Glucose Glycogen Fatty acids O 2 ADP 2 ADP ADP ADP CP 2 ATP CP Pyruvate ATP Mitochondria ATP Creatine Glucose Glycogen CO2 Creatine Lactate 2 ADP To myofibrils to support 2 ATP H+ muscle contraction Pyruvate 34 ADP 34 ATP CO2 To myofibrils to support muscle contraction Recovery Period Time required by muscles after exertion to return to normal Oxygen becomes available Mitochondria resume activity Muscles & Heat Production Muscle contraction produces heat Up to 70% of muscle energy can be spent as heat  body temperature Shivering in cold weather! Hormones & Muscle Metabolism Growth hormone and testosterone: ̶stimulate synthesis of contractile proteins and enlargement of skeletal muscles Thyroid hormones: ̶elevate rate of energy consumption in resting and active skeletal muscles Epinephrine: ̶increases duration and force of contraction Muscle Performance Power: maximum amount of tension produced Endurance: amount of time an activity can be sustained Power and endurance depend on: physical condition types of muscle fibers Types of Skeletal Muscle Fibers Muscle Hypertrophy & Atrophy uscle Hypertrophy: m b er o f m N OT N u OE S r s D ̶due to heavy training fibe se a incre ̶increases diameter of muscle fibers ̶increases number of myofibrils ̶increases mitochondria and glycogen reserves Atrophy: ̶due to lack of muscle activity ̶reduces muscle size, tone and power Use it or lose it! Muscle tone indicates base activity in motor units of skeletal muscles Muscles become flaccid when inactive for days or weeks With prolonged inactivity, fibrous tissue may replace muscle fibers What you don’t use, you lose Cardiac Muscle Cardiac Tissue ONLY in the heart Contains cardiac muscle fibers (cardiomyocytes = cardiocytes) Characteristics of Cardiomyocytes Cardiac muscle tissue LM × 575 Cardiac muscle Short – branched cell (intact) Intercalated disc (sectioned) Involuntary Mononucleated Striated Intercalated discs Cardiac muscle cell (sectioned) Short and wide T tubules Myofibrils SR has no terminal cisternae Entrance to T tubule Sarcolemma Mitochondrion No triads Aerobic (high in myoglobin, mitochondria) Contact of sarcoplasmic Have intercalated discs Sarcoplasmic reticulum with T tubule Myofibrils reticulum Intercalated Discs Specialized contact points between cardiomyocytes: ̶join cell membranes (gap junctions, desmosomes) ̶branching sites Functions: Cardiac muscle cell ̶maintain structure Intercalated ̶enhance molecular and electrical discs connections Nucleus ̶conduct action potentials Coordination of Cardiomyocytes Intercalated discs connect cardiomyocytes mechanically, chemically and electrically Heart works like a single, fused mass of cells  Physiological Characteristics of Cardiac Tissue Contractibility Automaticity Conductivity Excitability No Tetany Smooth Muscles Smooth Muscles in Body Systems Blood vessels: ̶ regulate blood flow and pressure Digestive and urinary systems: ̶ produce wall contractions ̶ form sphincters Reproductive system: ̶ produce movements Respiratory system: ̶ in trachea, bronchi, and bronchioles Other places: ̶ e.g., arrector pili muscles , intrinsic eye muscles Characteristics of Smooth Muscle Cells  Fusiform shaped Relaxed (sectional view) Dense body  Involuntary Circular  Mononucleated muscle layer T Actin Myosin  Nonstriated Relaxed (superficial view)  Controlled by pacesetter cells  No tendons or aponeuroses  No T tubules Longitudinal Intermediate filaments (desmin) Adjacent smooth muscle cells are bound together at dense bodies, muscle layer transmitting the contractile forces  No sarcomeres: L from cell to cell throughout the tissue. ̶ scattered myofilaments Contracted (superficial ̶ thin filaments attached to view)  Dense bodies: Smooth muscle tissue LM × 100 ̶ transmit contractions between cells Comparing Skeletal, Cardiac, & Smooth Muscle Tissues no terminal cisternae, no Triads Skeletal vs. Cardiac vs. Smooth Muscle CellsSkeletal muscle Involuntary Multinucleated Cardiac muscle Terminal cisternae T tubules Smooth muscle Tirads Fusiform Areobic metabolism only Tetany Sarcomere Pacemaker/Pacesetter Dense bodies