The Muscular System Chapter 8

Types of muscle & function Skeletal- 40-50% of total body weight- voluntary mostly movement of bone & body parts Stabilizing body positions Cardiac- only in heart- involuntary Heart only Develops pressure for arterial blood flow Smooth- grouped in walls of hollow organs Sphincters regulate flow in tubes Maintain diameter of tubes Move material in GI tract and reproductive organs

Muscle Functions Produce body movements Stabilize body positions Regulate organ volume Moving substances internally Producing heat

Skeletal Muscle Tissue Muscle includes: muscle fibers, connective tissue, nerves & blood vessels Wrapped in Epimysium Perimysium surrounds fiber bundles called fascicles Endomysium surrounds each individual fiber

Skeletal Muscle Tissue Well-supplied with blood vessels and nerves Terminal of a neuron on each muscle fiber

Figure 8.1

Muscle Histology elongated cylindrical cells = muscle fibers plasma membrane = sarcolemma Transverse (T- tubules) tunnel from surface to center of each fiber Multiple nuclei lie near surface Cytoplasm = sarcoplasm

Figure 8.2a

Muscle histology (cont.) Throughout sarcoplasm is sarcoplasmic reticulum Stores Calcium ions Sarcoplasm contains myoglobin Red pigmented protein related to Hemoglobin that carries oxygen Along entire length are myofibrils Myofibrils made of protein filaments Come in thick and thin filaments

Figure 8.2b

Sarcomere Filaments overlap in repeating patterns Unit structure is called sarcomere Separated by Z-discs Darker area = A-band associated with thick filaments H-zone has no thin filaments I-band has thin filaments no thick filaments

Figure 8.2c

Figure 8.3a

Figure 8.3b

Functional Structure Thick filament (myosin) has moveable heads Thin filaments (actin) are anchored to Z-discs Contain myosin binding sites for myosin head Also contain tropomyosin & troponin Tropomyosin blocks myosin binding site at rest

Sliding Filament Mechanism During contraction myosin heads bind actin sites Pull and slide actin molecules (and Z-discs) toward H-zone I-bands and H-zones narrow Sliding generates force and shortens sarcomeres and thus fibers.

Figure 8.4

Neuromuscular Interaction Nerve signal triggers muscle action potential Delivered by motor neuron One neuron can trigger 1 or more fibers at the same time Neuron plus triggered fibers = motor unit

Neuromuscular Junction neuronal ending to muscle fiber = Neuromuscular junction Synaptic end bulbs (at neuron terminal) Release neurotransmitter Muscular area = Motor end plate Between is synaptic cleft

Figure 8.5

Action at NMJ Release of acetylcholine (ACh) Diffuses across cleft 2. Activation of ACh receptors 3. Generation of Muscle Action Potential Repeats with each neuronal action potential 4. Breakdown of ACh

Contraction Trigger Muscle action potential=> Ca2+ release from Sacroplasmic Reticulum (SR) Ca2+ binds to troponin => Moves tropomyosin off actin sites => Myosin binds & starts cycle

Contraction Cycle Myosin binds to actin & releases phosphate group (Forming crossbridges) Crossbridge swivels releasing ADP & shortening sarcomere (Power stroke) ATP binds to Myosin => release of myosin from actin ATP broken down to ADP & Pi => activates myosin head to bind and start again Repeats as long as Ca2+ concentration is high

Figure 8.6

Relaxation Breakdown of Ach to stop muscle Action potentials Ca2+ ions transported back into SR lowering concentration=> This takes ATP tropomyosin covers actin binding sites

Figure 8.7

Muscle Tone Even at rest some motor neuron activity occurs = Muscle Tone If nerves are cut fiber becomes flaccid (very limp)

Metabolism Rapid changes from very low ATP consumption to high levels of consumption Creatine phosphate (high energy store) Fast & good for ~ 15 sec

Figure 8.8a

Glycolysis Break down glucose to 2 pyruvates getting 2 ATPs If insufficient mitochondria or oxygen pyruvate => lactic acid Get about 30-40 seconds more at max.

Figure 8.8b

Aerobic Cellular Respiration Production of ATP in mitochondria Requires oxygen and carbon substrate Produces CO2 and H2O and heat.

Fatigue Inability to contract forcefully after prolonged activity Limiting factors can include: Ca2+ Creatine Phosphate Oxygen Build up of acid Neuronal failure

Oxygen Use After Exercise Convert lactic acid back to glucose in liver Resynthesize Creatine Phosphate and ATP Replace oxygen removed from myoglobin

Control of Muscle Contraction Single Action Potential(AP) =>twitch Smaller than maximum muscle force Total tension of fiber depends on frequency of APs (number/second) Require wave summation Maximum = tetanus Total tension of muscle depends on number of fibers contracting in unison Increasing numbers = Motor unit recruitment

Figure 8.9

Figure 8.10

Fiber types Slow oxidative (SO)- small diameter & red large amounts of myoglobin and mitochondria ATP production primarily oxidative Fatigue resistant- Fast oxidative- glycolytic (FOG) Large diameter = many myofibrils Many mitochondria and high glycolytic capacity Fast glycolytic fibers (FG) white, fast & powerful and fast fatiguing For strong, short term use

Recruitment Muscle contractions only use the fibers required for the work Recruited in order: SO=>FOG=>FG if force is constant and the muscle shortens = Isotonic Contraction If length is constant and the force varies = Isometric Contraction The latter is often a postural muscle activity

Effects of Exercise SO/FG fiber ratio genetically determined High FG => sprinters High SO=> marathoners Endurance exercise gives FG=> FOG Increased diameter and numbers of mitochondria Strength exercise increases size & strength of FG fibers

Cardiac Muscle Striated, short fibers and branched Single central nucleus; Cells joined by gap junctions & desmosomes Thickened joint area called intercalated discs Some cardiac muscles generate own AP- autorhythmicity Involuntary

Cardiac muscle No nerve- internal pacemaker Ca2+- from S.R. and extracellular space separate cells with gap junctions -> electrical connections

Figure 15.2b

Smooth muscle Involuntary In internal organs Filaments not regular so not striated Visceral (single unit) type or Form sheets and are autorhythmic Contract as a unit Multi-unit type- each has own nerve and can contract independently

Smooth Muscle Graded contractions and slow responses Often sustain long term tone Often triggered by autonomic nerves modulated chemically, nerves, by mechanical events (stretching)

Figure 8.11

Aging Like bone there is a slow progressive loss of skeletal muscle mass Relative number of SO fibers tends to increase

Movement Move one bone relative to another Origin => most stationary end Location where the tendon attaches Insertion => the most mobile end Location where tendon inserts Action => the motion or function of the muscle

Figure 8.12

Movement (cont.) Generally arranged in opposing pairs Flexors- extensors; abductors- adductors The major actor = Prime mover or agonist The one with opposite effect = antagonist Synergists- help prime mover Fixators- stabilize origin of prime mover Role of muscle varies with motion

Naming Terms-Table 8.2 Direction relative to body axes e.g. Lateralis, medialis (medius), intermedius, rectus Specific regions e.g. abdominus, Brachialis, cleido, oculo-, uro-, Origin e.g. biceps, triceps, quadriceps Shape e.g. deltoid, orbicularis, serratus, trapezius

Names (Cont.) Other features Actions Specific references e.g. alba, brevis, longus, magnus, vastus Actions e.g. abductor, adductor, flexor, extensor Specific references e.g. Buccinator (trumpeter), Sartorius (like a tailor)

Figure 8-13a

Figure 8-13b

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Figure 8.24ab

Figure 8.24cd