Anatomy Notes - Muscular System

Chapter 7 Muscular System I) Five Primary Functions 1) Movement of skeleton 2) Maintain posture and body position 3) Support soft tissues 4) Guard entrances and exits 5) Maintains body temperature II) Composition of a Skeletal Muscle 1) Connective tissue organization a) Epimysium: surrounds entire muscle − separates it from other tissues/organs. b) Perimysium: divides muscles into fascicles (bundles of fibers) − contains blood vessels and nerves. c) Endomysium: surrounds individual muscle fibers and binds them together. d) Tendons: unite each layer of connective tissue into dense connective tissue at the ends of the muscles − attaches muscles to bones (intertwined with the periosteum) 2) Blood Vessels and Nerves a) Muscles require lots of energy − Need to bring in nutrients and carry waste away b) Only contracting under stimulation of the central nervous system (CNS) − Skeletal muscles are voluntary III) Features of Skeletal Muscle 1) Sarcolemma and Transverse Tubules a) Sarcolemma: Cell membrane of muscle cells, contains sarcoplasm b) Transverse Tubules (T tubules): tunnels through a muscle fiver filled with extracellular fluid − Help large groups of muscle fibers contract simultaneously 2) Myofibrils: bundles of thick and thin filaments (myofilaments); surrounded by T tubules a) Thin filaments = actin b) Thick filaments = myosin * These are responsible for muscle contraction 3) Sarcoplasmic Reticulum (SR) − Specialized type of endoplasmic reticulum − Contains a high concentration of calcium ions (Ca2+) − Ca2+ is actively pumped into SR and released just prior to muscle contraction 4) Sarcomeres: functional unit of myofilaments; smallest unit of a

muscle fiber; give skeletal muscles a banded (striated) apperance a) Z-lines: proteins that form boundaries between sarcomeres (thin filaments) b) M-lines: connect thick filaments in the center of the sarcomere c) A-band: contains thick filaments, does not change in length when muscles contract d) I-band: contains only thing filaments, decreases in length when muscles contract 5) Thick and Thin Filaments a) Thin Filaments: have actin molecules with active sites (where myosin heads may bind) − At rest, active sites are covered with troponin-tropomyosin complex b) Thick Filaments: are made of myosin which have head and tail regions − Head attaches to active site during contraction c) Ca2+ is the key that unlocks the active site; allows binding of myosin and muscle contraction 6) Sliding filaments and cross-bridges a) When muscles contract: − H-band and I-band decrease in length − Zone of overlap increases − A-band doesn’t change in length → This means that the filaments must be sliding towards the Z-lines, thick filaments stay stationary, although heads move/rotate → Called the Sliding Filament Theory b) When myosin heads connect with the active sites, they form cross-bridges IV) The Neuromuscular Junction (NMJ) 1) Beginning a contraction at the NMJ a) Synaptic Terminal: one of many branched endings of a motor neuron b) Acetylcholine (ACh): a neurotransmitter (NT) contained within vesicles at the synaptic terminal c) Synaptic Cleft: space between the synaptic terminal and the sarcolemma d) Motor Endplate: the region of the sarcolemma with ACh receptors e) Acetylcholinesterase (AChE): enzyme that breaks down ACh f) Action Potential (AP): electrical impulse 2) Muscle Stimulation Process Step 1: AP arrives at synaptic terminal

Step 2: ACh is released into synaptic cleft Step 3: ACh binds to receptors on the motor endplate - Changes permeability of motor endplate to sodium ions (Na+) - Na+ rushes into sarcolemma, generating an AP Step 4: AP spreads over sarcolemma and into T tubules, triggering a sudden, massive release of Ca2+ Step 5: ACh is broken down by AChE 3) Contraction cycle − At rest, myosin head has split ATP into ADP and a phosphate group and stores the energy for later Step 1: Ca2+ binds to troponin and exposes active site (on actin) Step 2: Cross-bridge forms between myosin head and active site Step 3: Cross-bridge pivots, releases ADP and a phosphate group from myosin head; uses energy stored from breaking down ATP Step 4: Cross-bridge detaches and another ATP molecule binds to myosin Step 5: ATP is hydrolyzed (split with water) - This recharges the myosin head - Can form another cross bridge → Go back to step 2 again until there is not enough ATP or Ca2+ V) Muscle Tension Tension: active force of muscle cells pulling on collagen fibers Compression: a push applied to an object - Muscles can only contract (generate tension) - The amount of tension depends on the number of pivoting crossbridges - Fibers are either “on” or “off” - Generation of tension varies because: 1) Fiber resting length at the time of stimulation - Zone of overlap 2) Frequency of stimulation - Tension of entire muscle depends on: 1) Frequency of stimulation 2) Number of fibers that are activated A) Frequency of Stimulation 1) Twitch: a single stimulus-contraction-relaxation sequence of a muscle fiber 2) Myogram: a graph of tension development in a muscle during a twitch a) Latent Period: an action potential sweeps the sarcolemma − Ca2+ are released − No tension is produced yet

b) Contraction Phase: tension rises/increases because crossbridges are interaction with active sites c) Relaxation Phase: tension falls as Ca2+ levels drop and cross-bridges separate 3) Summation and Incomplete Tetanus a) Summation: addition of one twitch to another before the relaxation phase ends − Results in a more powerful contraction b) Incomplete Tetanus: almost peak tension − Happens during rapid cycles of contraction and relaxation − Nearly all muscles do this 4) Complete Tetanus: rate of stimulation increases until relaxation is eliminated − Maximum tension B) Number of Muscle Fibers Activated - Smooth movements are achieved by controlling the number of fibers activated. 1) Motor Unit: All the muscle fibers controlled by a single motor neuron a) Smaller motor units allow for finer, more precise movements b) Fibers of different motor units are intermingled so that contractions occur in the same direction, regardless of the number of motor units stimulated 2) Recruitment: activating more and more motor units over time; eventually results in complete tetanus. 3) Muscle Tone: Resting tension in a skeletal muscle - Does not produce movement, Example: maintaining posture - Atrophy: weakening and shrinking of muscles due to lack of use - Dying fibers are not replaced, functional losses can be permanent C) Isotonic and Isometric Contractions 1) Isotonic: equal tension − Length of the muscle changes − Example: walking, lifting something, running 2) Isometric: equal length − Increasing tension cannot be greater than resistance − Example: pulling or pushing an immoveable object D) Muscle Elongation - Combination of three things: 1) Elastic forces – generated when muscles pull on connective tissues

− Tissues will recoil 2) Opposing muscle groups – one contracts while the other relaxes − Example: biceps and triceps 3) Gravity − Example: letting biceps relax, forearm will fall and muscle will relax VI) ATP (Adenosine TriPhosphate) A) ATP and CP (Creatine Phosphate) Reserves 1) ATP simply transfers energy, it doesn’t store it a) Extra ATP gives a phosphate group to creatine and forms CP b) CP recharges the ADP (from when myosin split ATP into ADP and a phosphate group) c) Creatine Phosphate Kinase (CPK): enzyme that facilitates the storage of phosphate groups with creatine 2)