Chapter 10: Muscle Tissue

(Muscle tissue) Chapter 10 Muscle tissue: made up of cells specialized for contraction Properties of Muscle: · excitability: receive and respond to a stimulus · contractility: muscle can shorten when stimulated · extensibility: ability to stretch 3 Types of Muscle Tissue: ① Skeletal Muscle ② Cardiac Muscle ③ Smooth Muscle · elasticity: ability to recoil to original shape Structure of Skeletal Muscle · skeletal muscles are organs composed mainly of skeletal muscle tissue, but also contain connective tissues, blood vessels, and nerves ↳ each muscle is covered by dense connective tissue · 3 layers of connective tissue: ↳ epimysium: connected to the deep fascia ↳ Perimysium: surrounds the muscle fasicles ↳ endomysium: within the fasicles, they surround the individual muscle cells (muscle fibers) · a singular muscle cell is known as a muscle fiber or myocyte ↳ long, multinucleated, cylindrical shaped cells Bundles within Bundles · a muscle is composed of bundles of fasicles · a fascicle is composed of bundles of muscle fibers · a muscle fiber contains bundles of myofibrils · each myofibril is made of bundles of myofilaments Functions of Skeletal Muscle: · movement · support · protection · heat generation · nutrient storage · blood circulation Structure of Skeletal Muscle Cont'd · each muscle cell is made of myofibrils · myofibrils are made of myofilaments ↳ thick filaments (myosin) ↳ thin filaments (actin) Sarcomere: the functional unit of skeletal muscle Sarcolemma: the plasma membrane of a muscle fiber ↳ has a membrane potential that leads to contractions Sarcoplasm: the cytoplasm of the muscle fiber Transverse Tubules/T Tubules: narrow passageways that are continuous with the Sarcolemma and extend deep into the sarcoplasm ↳ can propagate an electrical impulse (action potential) Sarcoplasmic Reticulum (SR): a tubular network around each myofibril that helps with storage and release of calcium ions Triad: a pair of terminal cisternal plus a T tubule Myofibril: a bundle that carries bundles of filaments called myofilaments Sarcomere · the functional unit of skeletal muscle is a sarcomere · found between two Z-lines (where thin filaments of adjoining sarcomeres connect) · Regions to know: ↳ A Band - where thin and thick filaments overlap ↳ H Band - thin filaments don't quite reach &(no myosin heads) ↳ I band - contains thin filaments but no thick filaments ↳ Z line - marks the beginning and end of each sarcomere ↳ M line - the center line of the H Band Thin Filaments (Actin) · contains F-actin, nebulin, tropomyosin, and troponin proteins ↳ F-actin: a twisted strand made up of two rows of globular G-actin molecules ↳ G-actin: contain active sites where myosin can bind ↳ Nebulin: holds the F-actin strand together · Troponin and tropomyosin are special regulatory proteins ↳ tropomyosin strands cover the active sites on G-actin ↳ troponin is made of 3 subunits: -one binds to tropomyosin -one binds to G-actin -one binds to 2 calcium ions · a contraction can only occur when the troponin-tropomyosin complex changes position, exposing the active sites on actin ↳ this happens when calcium ions bind Thick Filaments (Myosin) · made of a pair of myosin subunits twisted around each other ↳ the long tail binds to other myosin ↳ the globular heads project towards thin filaments · Heads form cross-bridges between thick and thin filaments · Cross-bridge has 2 sites critical to contraction ↳ actin binding site ↳ myosin ATPase (ATP-splitting) site Titin · each thick filament has a core of titin · "Molecular spring" with elastic properties · 2 functions: ↳ stabilizes position of thick filaments along M-line ↳ acts like a spring to help sarcomere return to its resting length Sliding Filament Theory · Theory that during skeletal muscle contraction, thin filaments are sliding toward the center of each sarcomere ↳ A bands and I bands of sarcomere narrow ↳ zones of overlap widen (more overlap) ↳ Z lines more closer together while width of A bands remain unchanged · sliding causes the myofibril to get shorter, which in turn contracts the muscle fiber as a whole (reach, grab, pull) Skeletal Muscle Contraction · nerve cells and skeletal muscle fibers have electrically excitable membranes ↳ review electrical impulses and action potentials (membrane potential) · Muscle contraction involves the following: - communication between nerve and muscle tissue - excitation-contraction coupling - contraction ↳ cross bridge activity - relaxation (contraction of a muscle begins when calcium is released from SR and Ca+ enters the cytosol of the muscle fiber) Neuromuscular Junction · communication between a neuron and a skeletal muscle ↳ refers to synapse where axon terminal of neuron meets specialized section of sarcolemma known as the motor end plate · When a motor neuron stimulates a muscle fiber Acetylcholine (Ach) is released (neurotransmitter) · Ach binds to Ach receptors in motor end plate, which opens up voltage-gated sodium channels, allowing for depolarization and initiation of action potential Excitation-Contraction Coupling · When an action potential reaches the triad, calcium is released from the terminal cisternal (calcium concentration increases 100x) · calcium binds with troponin, changing its shape and opening up the active sites on the actin strand Need to know Figure 10-10 in the textbook! Know the steps! Cross Bridges · Myosin head binds to exposed active site on actin · Myosin head is able to pivot and pull toward the M-line (power stroke) key step in muscle contraction ↳ ADP and phosphate are released from myosin head after power stroke · Action causes sarcomere to shorten, generating muscle tension · A new ATP molecule is needed to release myosin from actin and return it to resting (cocked) position Relaxation · If action potential at triad stops, release of calcium will also stop · troponin and tropomyosin return to resting position, covering active sites on actin · Refer to summary in figure 10-13 Motor Units · motor unit: one motor neuron and all the muscle fibers it controls · Different sizes of motor units ↳ small motor units: produce precise, delicate movements ↳ large motor units: produce larger, more powerful movements · Recruitment: the smooth but steady increase in muscle tension produced by increasing the number of active motor units ↳ You don't need full force/strength to do different tasks Muscle Tone · muscle tone: resting tension or resistance to movement · Some motor contract continuously, producing the resting tension of skeletal muscles referred to as muscle tone · muscle tone maintains posture · Muscle tone vs muscle strength: ↳ muscle tone is the tension present when your muscles are at rest ↳ muscle strength is when you actively work and contract muscles to resist a force Tension · muscle tension is produced internally within the sarcomere · tension must be transmitted to bone (by connective tissue and tendons) before bone can be moved · Muscle is typically attached to at least 2 different bones across a joint ↳ origin: end of muscle attached to stationary part of skeleton ↳ insertion: end of muscle attached to skeletal part that moves · length-tension relationship Treppe: If a muscle fiber is stimulated a second time after a twitch, the contraction will develop slightly higher maximum tension than the first contraction ↳ staircase effect ↳ common in cardiac muscle Wave Summation: If a second stimulus arrives before the relaxation phase has ended, a second, more powerful contraction occurs ↳ also called twitch summation Incomplete tetanus: a muscle producing almost maximum tension during rapid cycles of contraction and relaxation Complete tetanus: when a higher stimulation frequency eliminates the relaxation phase (full contraction with no break) Types of Muscle Contraction ① Isotonic: same tension, changing length ↳ concentric: muscle shortens ↳ eccentric: muscle lengthens ② Isometric: same length, changing tension ↳ muscle is prevented from shortening Muscle Metabolism · muscle metabolism: sum of all physical and chemical changes that occur within a muscle tissue · only energy source for muscle contraction is ATP ↳ available stores of ATP only allow 4-6 seconds of activity Muscle Contraction Requires: ① At rest: direct phosphorylation of ADP by creatine phosphate · circulatory supply ③ Peak activity: aerobic respiration (citric acid cycle& electron · stable pH · additional ATP must be generated: ② Moderate activity: anaerobic respiration (glycolysis) transport chain) · intracellular energy reserve · adequate oxygen Muscle Fatigue and Recovery · fatigue occurs when muscle can no longer perform the required amount of activity Recovery ↳ lactate diffuses into bloodstream (response to stimulus is diminished) ↳ depletion of metabolic reserves ↳ damage to sarcolemma or sarcoplasmic reticulum ↳ decrease in PH (impacts binding of calcium) ↳ heat generated in metabolism is released to maintain normal body temp ↳ oxygen is restored to normal, preexertion levels ↳ pain or sense of weariness Cardiac Muscle Cells · found only in the heart · intercalculated disks, gap junctions, and desmosomes ↳ gap junctions allow for direct electrical connection between cardiac muscle cells · functional syncytium: the cells all work as one · cardiac muscle contracts without neural stimulation (automaticity) ↳ Pacemaker cells determine rate ↳ nervous system can adjust rate, tension, force of contraction · contractions last 10x longer · more sensitive to changes in calcium (since it cannot store it) Structural differences: ↳ small ↳ single central nucleus ↳ no terminal cisternal on SR (less calcium storage) ↳ mainly aerobic metabolism Smooth Muscle Cells · plays a role in multiple body systems · coordinate movement of substances through internal passageways · involuntary control · 2 categories ① Multi-unit smooth muscle cells ↳ each muscle cell may be connected to more than one motor unit ↳ contracts slowly ② Visceral Smooth muscle cells ↳ most lack direct contact with a motor neuron ↳ arranged in sheets or layers with gap junctions/desmosomes ↳ pacesetter cells · Plasticity: ability for smooth muscle cells to function over a range of lengths · Tone: can be impacted by neural, hormonal, and chemical factors Structural differences: · nonstriated · long and slender, spindle-shaped, single nucleus · thin filaments are attached to dense bodies · adjacent smooth muscle cells connect at dense bodies · no t-tubules