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 • Muscles can only pull, never push, which allows them to exert a force
ATP --> ADP + P + Energy ADP + P + Energy ---> ATP
Actin Myosin 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 -Maintaining 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 • Multinucleate • Voluntary muscles • Can generate great power but fatigue quickly • Non-rhythmic 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 (Plasma membrane) Sarcoplasm = (Cytoplasm with lots of glycogen stored)
Functional unit of muscle Sarcomere Protein Titan
Nerve & Blood Supply Each muscle is served by: • 1 nerve • 1 artery • 1 or more veins
Skeletal Muscle Attachment Skeletal muscles attach to bones in at least 2 places • When the muscle contracts, the moveable bone (Insertion), moves toward the immovable or less-moveable bone (Origin) • Direct muscle attachment – epimysium fused to periosteum • Indirect muscle attachment – tendon or aponeurosis (flat, sheet-like tendon)
Skeletal Muscle Contraction • Sarcomere - contractile unit (z-line to z-line) • Myosin - thick filaments (contain ATPase which is used to split ATP to power muscle contraction) found in the dark A-band. Myosin heads form cross-bridges when attached to actin • Actin - thin filaments found in the light I-band which are anchored to the z-line
Sliding Filament Theory • Hugh Huxley 1954 proposed that during contraction actin will slide past myosin which result in overlapping filaments • Cross bridge attachment • Power stroke (Myosin head pivots pulling actin) • Cross bridge detachment (ATP binds to myosin head loosening the bond to actin) • “Cocking” the myosin head – ATPase hydrolyzes ATP to ADP & Pi 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 cross-bridge • Troponin – regulates cross-bridge 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 G-actin; these are globular proteins which are linked together to form a filament. Myosin binding site - Each G-actin 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 Myosin 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. ATP-binding 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
Regulation of Contraction • Muscle 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.
ACH released • AP propagated along membrane and at T-tubules • 3. Ca released from SR voltage gated Ca channel opens • 4. Ca binds to Troponin-C conformation changes favor tropomyosin opens actin sites • 5. myosin cross-bridges attach-detach from actin...pulls filament toward M-line • 6. Ca removed (uptake by SR) • 7. tropomyosin blocks actin sites relaxation
a. b. c.
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 • Myasthenia gravis – autoimmune disease where ACh receptors are broken down by ACh antibodies resulting in drooping eyelids and general muscle fatigue • Curare – arrowhead poison used in South America which blocks ACh receptors resulting in respiratory arrest & death • Cobra venom – same as curare • Botulinum toxin prevents ACh release • Black widow spider venom releases all Ach • Nerve gasses inhibit AChase which keeps cleft flooded with ACh