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MUSCLE HISTOLOGY: CONTRACTION How do your muscles work? Neuron (nerve cell) Muscle fiber (cell) Gross anatomy of skeletal muscle: Connective tissue coverings • Epimysium -wrap of the muscle -connective tissue -connections with tendons • Endomysium -surrounds each muscle cell -thin connective tissue Microanatomy of muscle Skeletal muscle cell (fiber) -Sarcolemma -membrane (surrounds sarcoplasm) -Sarcoplasmic reticulum -muscle’s endoplasmic reticulum (holds calcium) - T-tubes -runs transversely through sarcoplasm -perpendicular to S. R. - myofibrils - myofilaments Myofilaments -2 types of myofilaments 1) Thick myofilaments -protein molecules: - myosin 2) Thin myofilaments -protein molecules (3): - actin - tropomyosin -troponin Muscle Anatomy Parts of the Neuron 1 Perikaryon -cell body of a neuron (soma) -complex endoplasmic reticulum & ribosomes for protein synthesis 2 Cytoplasmic Processes 1) peripheral process -extends from dendrites to soma 2) central process -extends from soma to terminal branches (axon) Dendrites -connect directly to soma (can have many branches), receives nerve impulses Axon -long thin process extending from the some (only one branch), sends nerve impulses Neuron Physiology Resting Potential of neuron membrane -7OmV inside membrane -higher [K+) potassium ions; lower [Na+] sodium ions +35mV outside membrane -higher [Na+) lower [K+] Na+/K+ Concentration -extracellular [Na+] is about 2Ox greater than intracellular [Na+] -intracellular [K+] is about 25x greater than extracellular [K+) -large # of negatively charged protein molecules within intracellular fluid -Polarized membrane- negative inside & positive outside Nucleus Dendrites Cell body In saltatory conduction, the nerve impulses jump from one node of Ranvier to the next. Nodes of Ranvier Node of Ranvier Schwann cell Myelin sheath -numerous small constrictions of the myelin sheath of axon -impulses jump from one node to another -ALWAYS ONE-WAY TRANSMISSION Action Potential (excitability): -impulse nerve conduction very fast -0.5 meters - 120 meters/second -wave of polarization (wave of negativity) -rapid sequence of depolarization & repolarization Depolarization -extracellular sodium ions [Na+] flood in creating a +3OmV intracellular charge Repolarization: -intracellular potassium ions [K+] go out of the cell creating a negative intracellular charge -after wave of polarization sodium/potassium pump kicks in to restore the original resting potential [Na+ & K+] Action Potential Physiology of skeletal muscle Step 1: A nerve impulse reaches the neuromuscular junction. Motor neuron Axon terminal Synaptic vesicle Synaptic cleft Step 2: Acetylcholine is released at the neuromuscular junction. Step 3: When acetylcholine binds to receptors on the plasma membrane of the muscle cell, an electrochemical message is generated. Acetylcholine Electrical impulse neuromuscular junction Plasma membrane T tubule Myofibril -simulated to contract by nerve impulses -electrical gradients through neurons to muscles (across the neuromuscular junction Neuromuscular junction Sacroplasmic reticulum Exciting of muscle cells (Innervation) Step 4: The electrochemical message spreads through T tubules, causing the release of calcium ions from the sarcoplasmic reticulum. -as action potential reaches sarcolemma it spreads through T-tubes -sarcoplasmic reticulum releases calcium ions (Ca2+) - Ca2+ act as mediators between excitation & contraction excitation in muscle fiber Myofilaments -2 types of myofilaments 1) Thick myofilaments -protein molecules: - myosin 2) Thin myofilaments -protein molecules (3): - actin - tropomyosin -troponin Sarcomere (functional aspect of muscle tissue) -individual myofilament -actual contraction pieces -Z Line to Z line -separations of sarcomeres -A Band -myosin & actin -middle broad section of sarcomere -I Band -near the Z Lines -overlapping sarcomeres along the myofibril -only thin filaments -H Zone -only thick (myosin) filaments -middle of A Band The Sliding Filament Theory Contraction of muscle cells •Sliding filament theory -muscle shortening occurs when thin filaments slide over thick filaments -actin over myosin -actin is a globular protein: arranged as two strings of beads wound in a spiral to form an actin filament -myosin molecule is much larger with globular head & a long tail piece •Sarcomere shortens Cross Bridge Cycle Return to Rest • When the action potential passing down the motor neuron stops, Ach release also stops. However, the stimulus does not end until all remaining molecules of Ach on the motor end plate (sarcolemma) are inactivated • The inactivation is done by an enzyme in the sarcolemma called Acetylcholinesterase (AchE). • Immediately following Ach inactivation, Ca++ ions are returned to the SR by active transport, which requires more energy in the form of ATP. • With the absence of Ca++ ions, the original shape of the thin filaments is restored. • The muscle fiber is ready for the next stimulus because ATP is quickly regenerated by the mitochondria. Let’s review… Major Events of Muscle Contraction – Stimulation occurs when Ach is released from the end of a motor neuron. – Ach diffuses across gap at neuromuscular junction. – Sarcolemma is changed and an impulse travels deep into the fiber through the T-‐ tubules – Ca++ ions are released from SR and bind to troponin molecules. – Tropomyosin molecules move and expose specific sites on actin filaments. – Cross bridges form between actin and myosin filaments. – Actin filaments slide inward along myosin filaments. – Muscle fiber shortens as contraction occurs. Let’s review… Muscle Fiber Relaxation – Achesterase causes Ach to decompose, and sarcolemma is no longer stimulated. – Ca++ are actively transported into SR. – Cross bridges between actin and myosin filaments are broken. – Actin and myosin filaments slide apart. – Muscle fiber lengthens as it relaxes and its resting state is reestablished. – Troponin and tropomyosin molecules inhibit the interaction between actin and myosin filaments.