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Muscle Physiology

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Muscle Physiology Chapter 11 • Characteristics of Muscle Tissue • Types of Muscle • Skeletal Muscle • Motor Units • Skeletal Muscle Contraction • Skeletal Muscle Metabolism • Cardiac Muscle • Smooth Muscle Characteristics of Muscle • Responsive (excitable) – capable of response to chemical signals, electrical signals, and stretch • Conductive – local electrical change in a cell triggers a wave of excitation that travels along the cell • Contractile – cells shorten when stimulated by converting the chemical energy of ATP into mechanical energy • Extensible – cells are capable of being stretched • Elastic – cells return to original resting length after being stretched • General Characteristics: muscle tissue is composed of connective tissue, blood vessels, nerves, lymphatics and muscle cells. Types of Muscle • Striated Muscle (cytoplasm has cross striations) – Striated Skeletal Muscle (voluntary muscle) – Striated Cardiac Muscle • Smooth Muscle (cytoplasm without cross striations) Skeletal Muscle • Skeletal Muscle is voluntary striated muscle attached to bones • Voluntary means under conscious control • Cells of skeletal muscle tissue are called muscle fibers = muscle cells = myofibers • Skeletal muscle cells are as long as the whole muscle • Skeletal muscle is a striated muscle. – striated muscle exhibits microscopic alternating light and dark transverse bands or cross striations in the cytoplasm that are from the highly organized contractile proteins of the cytoskeleton Muscle – one of the 600 named units of the human muscular system Fascicle – bundles of muscle fibers that form the visible grain of a muscle Muscle Fiber – individual skeletal muscle cells Myofibril – long, thin cords of contractile Myofibrils proteins Multinucleated Skeletal Muscle Fibers Sarcolemma Syncytial Skeletal Muscle Development Each muscle fiber has multiple nuclei flattened against the inside of the sarcolemma. Multiple nuclei are from the fusion of multiple myoblasts (derived from a condensation of mesenchymal cells) during development forming a syncytium. A syncytium is a multinucleated mass of cytoplasm surrounded by a single plasma membrane. Satellite Cells outside of the sarcolemma between muscle fibers can multiply to produce a small number of new myofibers or they can add their nuclei to existing muscle fibers. Skeletal Muscle Fibers • Sarcolemma is the specialized plasma membrane of muscle cells. – sarcolemma is polarized at rest and can be depolarized by acetylcholine released by motor neurons – tubular infoldings of the plasma membrane are called transverse tubules (T-tubules) that penetrate into the cell and carry the electrochemical current into the cell • Sarcoplasm is the specialized cytoplasm of muscle cells. – sarcoplasm is filled with highly organized myofibrils (bundles of parallel protein microfilaments of actin and myosin) and glycogen for stored energy and myoglobin for storing oxygen • Sarcoplasmic Reticulum is the specialized endoplasmic reticulum of muscle cells. – the sarcoplasmic reticulum is a series of interconnected tubules connected to dilated, storage sacs called terminal cisternae that store calcium ions (Ca++) Sarcomeres: functional units Muscle Filaments of the Sarcomere • Sarcomeres are the functional units of muscle • Sarcomeres extend from one Z line to the next Z line. • Thin filaments are actin • Thick filaments are myosin M • Elastic filaments are titin • Ahm, I Zee! • A band extends from myosin tip to myosin tip. Regions of – A stands for anisotropic which is a term for the way polarized light passes through the the thick filaments giving it a dark appearance. Sarcomere • H band is the central region of the A band and is a region of myosin without actin. – H stands for Helle (German for bright). I A I • M line is a disk of protein that anchors the myosin filaments. Z H Z – M stands for Mittel (German for middle) • I band is the thin filament region – I stands for Isotropic: polarized light passes easily through it giving it a light appearance • Z line is a disc of alpha actinin protein that anchors titin and actin filaments – Z stands for Zwischen (German for M between) Regions of the Sarcomere Electron Micrograph of a Sarcomere M line Nucleus Contractile Proteins and Regulatory Proteins • Actin and Myosin are contractile proteins – movements of actin and myosin contract the cell • Troponin and Tropomyosin are regulatory proteins that act like a switch that starts and stops contraction of muscle cells – The regulatory proteins are dependent upon Ca++ Thick Filaments • Thick filaments are made of hundreds of myosin molecules • Myosin is arranged in bundles with the heads directed outward in a spiral array around the bundled tails Myosin • Myosin is composed of two entwined polypeptides (each shaped like a golf club with a spiral handle) • The H Zone of a sarcomere is a region with no heads that contains the M line Thin Filaments • Thin filaments are composed of two strands of fibrous actin composed of 6 or 7 globular actin (G actin) subunits each with an active site. • Tropomyosin molecules cover and block the active sites of 6 or 7 G actin subunits. • One calcium-binding troponin molecule is attached to each tropomyosin molecule Overlap of Thick and Thin Filaments H band M line Elastic Filaments • Huge springy protein called Titin is an elastic filament that connects the Z disc to the M line • Titin passes through the bundles of thick filaments • Functions of Titin – keeps thick and thin filaments aligned M with each other – resist overstretching – help the cell recoil to its resting length (provides elasticity) Relaxed versus Contracted Sarcomere • Muscle cells shorten because individual sarcomeres shorten by pulling Z discs closer together. • Notice that filament overlap changes, but neither thick nor thick filaments change length during shortening. • During contraction: – A band length stays the same – H band shrinks – I band shrinks • During relaxation, compressed titin rebounds and pushes Z disks apart to the resting length The Sarcomere in Action http://www.fbs.leeds.ac.uk/research/contractility/titin.htm The Sarcomere in Action http://www.siumed.edu/~dking2/ssb/muscle.htm#1a Skeletal Muscle Innervation • Skeletal muscles are activated by motor neurons • Motor Neurons branch out of the central nervous system from the skull (cranial nerves) and the spine (spinal nerves) • Somatic motor neurons innervate skeletal muscles • Autonomic motor neurons innervate cardiac muscle, smooth muscle or glands Muscle Innervation Afferent Efferent Efferent Innervation of Skeletal Muscle • Skeletal muscle cell will not contract unless it is stimulated by a nerve cell – nerve cell = neuron – paralysis is a loss of functional innervation and results in the loss of voluntary control of the muscle and eventually atrophy of the muscle • Axons of motor neurons are branched. – each axon can branch a few times (3-6) or many times (over 200) – Each axon branch contacts one muscle fiber – axons = nerve fibers • A motor unit is a motor neuron and all the muscle fibers it innervates Motor Units • Muscle cells of a Motor Unit are dispersed throughout a muscle – provides ability to sustain long- term contraction as motor units take turns resting • Small Motor Units provide Fine Control – small motor units contain as few as 3-6 muscle fibers per nerve fiber – example: eye muscles • Large Motor Units are for Strength – large motor units have as many as 1000 muscle fibers per nerve fiber – example: gastrocnemius muscle Neuromuscular Junction • Neuromuscular Junction (NMJ) is a synapse between a nerve fiber and a muscle cell. • Synapse is the functional connection between a nerve cell and its target cell. • Components of the NMJ – terminal boutton (axon terminal, synaptic knob, terminal button, axonal swelling, synaptic bulb, end bulb) is the swollen end of a nerve fiber and contains vesicles of the neurotransmitter acetylcholine (ACh) – motor end plate is the specialized region of muscle cell membrane under the terminal boutton – motor end plate membrane has ACh receptors on junctional folds which bind ACh released from the nerve – acetylcholinesterase (AChE) is an enzyme in the basal lamina in the synaptic cleft that breaks down ACh and causes relaxation – synaptic cleft is the gap between the nerve and muscle cells – Schwann cells cover the axon and the NMJ The Neuromuscular Junction Neuromuscular Junction Animation http://www.mhhe.com/biosci/esp/200 2_general/Esp/folder_structure/su/m4 /s10/sum4s10_7.htm Muscle and Nerve Electrochemical Communication • At rest, muscle cell and nerve cell membranes are polarized (charged). Changes in charge are relayed from one cell to another. • Membrane polarity or charge is measured in units of volts – car battery = 12 volts – flashlight battery = 1.5 volts – muscle cell membrane = .06 volts or 60 millivolts (mV) • Difference in charge across the membrane is called the membrane potential. – Resting Membrane Potential is established by a Na+/K+ exchange pump that results in high [Na+] outside of cell and high [K+] and anions inside of cell resulting in a slightly negative voltage inside the cell (-60 mV). • Sarcolemma depolarizes in response to motor neuron stimulation from release of ACh. • Voltage change spreads across the membrane as an action potential Action Potential Opening of Na+ channels is triggered by Resting membrane ACh and allows Na+ to rush in depolarizing potential of -60 mV is the membrane. established by the Na+/K+ Membrane depolarization triggers the ATPase pump and non- opening
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