Muscular System Anatomy & Physiology
Muscles • From the Latin mus meaning little mouse (Flexing muscles looked like mice scurrying under the skin) • Have ability to transform ATP into mechanical energy • M uscles can only pull, never push, which allows them to exert a force
ATP > ADP + P + Energy ADP + P + Energy > ATP
Actin M yosin Muscle Types • Skeletal Slow to Fast twitch• Cardiac Fast twitch• Smooth Slow twitch • Twitch = contraction • Skeletal & smooth muscle cells are elongated & called fibers • All have contractile myofilaments actin & myosin
Muscle Fxn’s Produce movement• Skeletal – locomotion & manipulation in response to the environment • Cardiac – moves blood• Smooth – propels (squeezes) stuff through the digestive, urinary, circulatory, and reproductive systems M aintaining posture Stabilizing joints Generating heat (40% of your body heat)
Functional Characteristics of Muscle • Excitability (Irritability) – the ability to respond to a stimulus • Contractility – the ability to shorten forcibly when adequately stimulated • Extensibility – the ability to be stretched or extended • Elasticity – the ability of a muscle fiber to recoil & resume its resting length after being stretched
Skeletal Muscle • Striated• M ultinucleate• Voluntary muscles• Can generate great power but fatigue quickly• Nonrhythmic contraction
Gross Anatomy of Skeletal Muscle • Epimysium – outermost layer of dense irregular connective tissue (Surrounds the whole muscle) • Fascicle – bundle of muscle fibers • Perimysium – fibrous C.T. which surrounds the fascicle • Endomysium – each muscle fiber is surrounded by reticular C.T.
Sarcoplasmic Reticulum Sarcolemma Sarcoplasm = (Cytoplasm with lots of glycogen stored) (Plasma membrane)
Sarcomere Functional unit of muscle Protein Titan
Nerve & Blood Supply Each muscle is served by:• 1 nerve• 1 artery• 1 or more veins
Skeletal Muscle At achment Skeletal muscles attach to bones in at least 2 places • W hen the muscle contracts, the moveable bone (Insertion), moves toward the immovable or lessmoveable bone (Origin) • Direct muscle attachment – epimysium fused to periosteum • Indirect muscle attachment – tendon or aponeurosis (flat, sheetlike tendon)
Skeletal Muscle Contraction • Sarcomere contractile unit (zline to zline) • M yosin thick filaments (contain ATPase which is used to split ATP to power muscle contraction) found in the dark Aband. M yosin heads form crossbridges when attached to actin • Actin thin filaments found in the light Iband which are anchored to the zline
Sliding Filament Theory • Hugh Huxley 1954 proposed that during contraction actin will slide past myosin which result in overlapping filaments 1. Cross bridge attachment 2. Power stroke (M yosin head pivots pulling actin) 3. Cross bridge detachment (ATP binds to myosin head loosening the bond to actin) 4. “Cocking” the myosin head – ATPase hydrolyzes ATP to ADP & P i returning the myosin head to it’s cocked position
Tropomyosin & Troponin • Tropomyosin – stiffen the actin protein & block myosin binding sites in relaxed muscle fibers, preventing myosin & actin from forming a crossbridge • Troponin – regulates crossbridge formation. In the presence of Ca2+ troponin moves tropomyosin, thereby exposing the myosin binding sites
Z Line Defines each end of the sacromere. Thin filaments of adjacent sarcomeres are linked together here. A band Consists of overlapping thin and thick filaments. I band Only thin filaments. H zone Only thick filaments. This also shortens during contraction.
Thin Fibers Each thin filament is made of three different proteins. Actin Actin filaments are made of subunits called Gactin; these are globular proteins which are linked together to form a filament. M yosin binding site Each Gactin contains a binding site for myosin head groups (part of the thick filament) Troponin This protein is associated with actin and it binds calcium. Tropomyosin The third protein of the group; when the muscle fiber is not contracting, tropomyosin covers the myosin binding site, preventing the myosin head groups from binding to actin.
Thick Fibers Consists of a bundle of proteins called myosin M yosin Tails Each tail has two head groups at the same end and each has two binding sites. heads form cross bridges Actin binding site This binds to the myosin binding site on actin, when it is exposed as tropomyosin moves. ATPbinding site Binds to ATP; splits the molecule and the released energy is used to drive the movement of the myosin head groups.
Calcium • Sarcoplasmic reticulum – regulates intracellular Ca2+ by storing & releasing Ca2+ when a stimulus causes the muscle to contract • T (Transverse) Tubules – extensions of the sarcolemma which allow for rapid impulse transmission through the muscle which ensures a single muscle contraction
Step 1. The infux of calcium, Step 6. The transport of calcium ions triggering the exposure of back into the sarcoplasmic reticulum. binding sites on actin. Step 5. The hydrolysis of ATP, whichleads to the reenergizing and repositioning of the cross bridge. Step 2. The binding of myosin to actin. Step 4. The binding of ATP to the cross bridge, which results in the cross bridge disconnecting from actin. Step 3. The power stroke of the cross bridge that causes the sliding of the thin filaments.
Regulation of Contraction • M uscle contraction is stimulated by an action potential from a nerve • The neuromuscular junction (motor end plate) in skeletal muscle is regulated by acetylcholine (ACh) • Ach needs to be broken down as soon as it is used; the enzyme acetlycholinesterase serves this function.
1. AC H released 2. AP propagated along m em brane and at Ttubules 3. C a released from SR voltage gated C a channel opens 4. C a binds to TroponinC conform ation changes favor tropom yosin opens actin sites 5. m yosin crossbridges attachdetach from actin…pulls filam ent tow ard M line 6.C a rem oved (uptake by SR) 7. tropom yosin blocks actin sites relaxation
c a. b. .
Generating an Action Potential • Depolarization (Na channels open) • Repolarization (Na channels close K channels open) • Refractory Period (K channels close) • Na/K pump
Homeostatic Imbalance • M yasthenia gravis – autoim m une disease w here ACh receptors are broken dow n by ACh antibodies resulting in drooping eyelids and general m uscle fatigue • Curare – arrow head poison used in South Am erica w hich blocks ACh receptors resulting in respiratory arrest & death • Cobra venom – sam e as curare • Botulinum toxin prevents ACh release • Black w idow spider venom releases all Ach • Nerve gasses inhibit AChase w hich keeps cleft flooded w ith ACh
ACh destruction • After ACh initiates the action potential the ACh is broken down by acetylcholinesterase • This prevents continued muscle contraction in the absence of additional nerve stimuli
M ysostatins control muscle growth Absence of results in enlarged muscle development
Energy for muscle contraction ATP is needed for…1) contraction Cocking and detachment of the myosin head. 2) calsequestrin Pumping calcium into the SR of the sarcoplasm. 3) Na+/K+ATPase Needed for impulse conduction.
ATP Production • ATP lasts Only a few seconds during active muscle contraction as ATP stores are used up. • ATP is quickly reconstituted There are several mechanisms that replenish the ATP stores. • Sources of energy for ATP production 1) phosphocreatine (creatine phosphate, CP) ATP is produced from another high energy molecule called phosphocreatine. • Creatine kinase• Breaks down phosphocreatine, releasing a phosphate and energy. The energy is used to make new ATP. • This store of energy lasts about 815 seconds.
ATP Production 2) Anaerobic Respiration & Glycolysis Glycogen A polymer of glucose. For the muscle to recover the lactic acid needs to be removed quickly; a well vascularized muscle serves this purpose. lasts about 2 minutes. Lactic acid/pyruvic acidThe product of anaerobic respiration is pyruvate; as no oxygen is available the pyruvate is converted to lactic acid. As lactic acid builds up in the muscles it changes the pH of the tissues in the muscle, which causes a decrease in the efficiency of proteins and enzymes. This leads to soreness and fatigue.
ATP Production • Oxidative metabolism a.k.a. Aerobic Respiration Oxygen used by mitochondria to produce 36 net ATPs (2 from glycolysis and 34 from Krebs and ETC) • Can produce ATP indefinitely as long as you have oxygen and energy stores (fat, proteins or glucose.)