Muscular System Overview of Muscle Tissues Types of Muscle Tissue o Skeletal and smooth muscles which are elongated are called muscle fibers o Myo- and Mys- = muscle o Sarco = flesh refers to muscle; i.e., sarcolemma ( muscle + husk ) plasma membrane of muscle cells o Skeletal muscle tissue are the longest muscle cells and have obvious stripes called striations. Voluntary muscles = only type subject to conscious control. o Cardiac muscle tissue occurs only in the heart where it constitutes most of the heart walls. Cardiac muscles are striated, but not voluntary (i.e., most of us have no conscious control over how fast our heart beats). o Smooth muscle tissue is found in the walls of hollow visceral organs (e.g., stomach, urinary bladder, respiratory passages). Role is to force fluids and other substances through internal body channels Smooth muscle cells are elongated fibers (like skeletal muscle fibers), but have no striations. Contractions of smooth muscle fibers are slow, sustained, and involuntary. Special Characteristics of Muscle Tissue o Excitability (i.e., responsiveness or irritability) the ability to receive and respond to a stimulus (i.e., any change in the environment both inside and outside the body). o Contractility ability to shorten forcibly when adequately stimulated. o Extensibility ability to be stretched or extended o Elasticity ability of a muscle cell to recoil and resume its resting length after being stretched Muscle Functions o Producing Movement Skeletal Muscle Just about all movements of the human body and its parts result from muscle contraction (i.e., skeletal, cardiac and smooth muscle). o Maintaining Posture and Body Position Skeletal muscle functions continuously to adjust our posture and positioning in relation to gravity o Stabilizing Joints Skeletal muscles stabilize and strengthen the joints of the skeleton o Generating Heat Muscles generate heat as they contract This heat is vitally important in maintaining normal body temperature. Skeletal muscle accounts for at least 40% of body mass (most responsible for generating heat). o Miscellaneous Skeletal muscles protect internal organs by enclosing them Smooth muscles regulate passage of internal substances, dilate and constrict pupils of eyes, and form arrector pili muscles attached to hair follicles (i.e., goose bumps).
Gross Anatomy of a Skeletal Muscle o Nerve & Blood Supply Generally, each muscle is served by one nerve, an artery and one or two veins Skeletal muscles have a nerve ending that controls its activity Cardiac and smooth muscles can contract with the absence of nerve stimulation Skeletal muscles require huge amounts of oxygen via blood supply and give off large amounts of metabolic waste removed via veins. o Connective Tissue Sheaths Individual muscle fibers are wrapped and held together by several different connective tissue sheaths They give support to each muscle cell and reinforce the whole muscle preventing the bulging muscles from bursting during exceptionally strong contractions. All connective tissue sheaths are continuous with one another as well as with the tendons that join muscles to bones. Connective tissue sheaths transmit force from contracting muscles to the bones to be moved The sheaths contribute somewhat to the natural elasticity of muscle tissue, and also provide entry and exit routs for the blood vessles and nerve fibers. From external to Internal in orientation: Epimysium o Meaning outside the muscle ; an overcoat of dense irregular connective tissue that surrounds the whole muscle. o Sometimes it blends with the deep fascia that lies between neighboring muscles or the superficial fascia deep to the skin Perimysium o Meaning around the muscle [fascicles] ; o Within each skeletal muscle, the muscle fibers are grouped into fascicles that resemble bundles of sticks that are wrapped around with perimysium, a layer of fibrous connective tissue. Endomysium o Meaning within the muscle ; whispy sheath of connective tissue that surrounds each individual muscle fiber o It consists of fine areolar connective tissue. Microscopic Anatomy of a Skeletal Muscle o Sarcolemma = plasma membranes surface of a muscle fiber o Sarcoplasm = cytoplasm of a muscle cell similar to the cytoplasm of other cells, but contains unusually large amounts of glycosomes (granules of stored glycogen that provide glucose during periods of muscle cell activity) and myoglobin, a red pigment that stores oxygen (similar to hemoglobin). o Myofibrils Rodlike bundle of contractile filaments found in muscle fibers Hundreds to thousands of myofibrils are in a single muscle fiber; they account for about 80% of cellular volume. Striations a repeating series of dark and light bands along the length of the myofibril
A Bands (dark) and I Bands (light) nearly aligned with one another Each A band has a lighter region in its midsection called the H Zone (H for helle = bright ) Each H zone is bisected vertically by a dark line called the M Line (M for middle) formed by molecules of protein myomesin. The light I bands also have a midline interruption, a dark area called the Z Disc (or Z line) o Sarcomere Smallest contractile unit of a muscle fiber (i.e., functional unit of skeletal muscle). Region of the myofibril between two successive Z discs (i.e., has an A band flanked by half an I band at each end). Within each myofibril, the sarcomers are aligned end-to-end like boxcars in a train o Myofilaments or filaments Muscle contraction depends on the myosin- and actin-containing myofilaments. On a molecular level, each sarcomere is made up of small structures called myofilaments Thick filaments containing myosin that extend the entire length of the A band o Each thick filament contains about 300 myosin molecules bundled together Each myosin molecule is shaped like a golf club with two heads (i.e., it has a long rod-like tail and two golf-club heads (cross-bridges)) Cross-bridge (aka, myosin head) has two important binding sites: 1) ATP (i.e., adenosine triphosphate; an organic molecule that stores and releases chemical energy within the cell composed of adenine, ribose sugar and three phosphate groups), 2) Actin The linking of thick and thin filaments together by the heads of the myosin protein creating a power stroke (movement creating a muscle contraction) Thin filaments containing actin that extend across the I band and partway into the A band. o Each thin filament is composed of three molecules: Actin Main component of the thin filament structured into a double helical chain Each actin subunit contains specific sites to which the myosin heads bind Tropomyosin A regulatory protein entwined around the actin It covers the binding sites on the actin subunits preventing myosin head binding in an unstimulated muscle Troponin
o Sarcoplasmic Reticulum A molecule attached to and periodically spaced along the tropomyosin strand Functions to move the tropomyosin strand uncovering the myosin binding sites on the actin subunits By themselves, troponin molecules are unable to move tropomyosin o After an action potential, calcium ions are released and bind with the troponin which cause the tropomyosin strands to move. An elaborate smooth endoplasmic reticulum surrounding each myofibril communicating at the H zone and forming terminal cisternae ( end sacs ) at the A band-i band junctions Endoplasmic reticulum ( network within the cytoplasm ) is an extensive system of interconnected tubes and parallel membranes enclosing fluid-filled cavities (cisternae). They play an important role in calcium ion storage and release during muscle contraction. Calcium provides the final go signal for contraction. o T Tubules An elongated tube at each A band-i band junction where the sarcolemma of the muscle cell protrudes deep into the cell interior T tubules are continuations of the sarcolemma; they conduct nerve impulses to the deepest regions of the muscle cell and to every sarcomere; these impulses signal the release of calcium ions from the adjacent terminal cisternae. Sliding Filament Model of Contraction o Six Steps of Cross Bridge Cycling 1. The influx of calcium released from the terminal cisternae trigger the exposure of binding sites on actin Presence of an action potential in the muscle cell membrane. Release of calcium ions from the terminal cisternae (i.e., end sacs of the sarcoplasmic reticulum containing calcium ions). Calcium ions rush into the cytosol (i.e., a viscous, semitransparent fluid substance of cytoplasm in which other elements are suspended) and bind to the troponin. There is a change in the conformation of the troponin-tropomyosin complex. This tropomyosin slides over, exposing the binding sites on actin. 2. The binding of myosin to actin When a binding site on actin is exposed, an energized myosin head can bind to it forming a cross bridge. 3. The power stroke of the myosin head causes the sliding of the thin filaments The binding of myosin to actin brings about a change in the conformation of the myosin head, resulting in the release of ADP and inorganic phosphate.
At the same time, the myosin head flexes, pulling the thin filament inward toward the center of the sarcomere. This movement is called the power stroke. The chemical energy of ATP has been transformed into the mechanical energy of a contraction. 4. The binding of ATP to the myosin head, which results in the myosin head disconnecting from actin In order to disconnect the myosin head from actin, an ATP molecule must bind to its site on the myosin head. If ATP is depleted in a muscle, the muscle will remain in a state of rigor o The condition called rigor mortis occurs after death due to depletion of ATP. Without ATP, the calcium ion pumps no longer function. The exposed actin binding sites attract myosin cross bridges which attach but are not able to detach. o The body becomes stiff due to these bound myosin cross bridges. o The Muscles will remain in rigor until the cellular proteins begin to break down, usually within 24-48 hours. 5. The hydrolysis of ATP, which leads to the re-energizing and repositioning of the myosin head The release of the myosin head from actin triggers the hydrolysis (i.e., a chemical process in which an enzyme uses water to split one molecule into smaller parts) of the ATP molecule into ADP and P i Energy is transferred from ATP to the myosin head, which returns to its high-energy conformation 6. The transport of calcium ions back into the sarcoplasmic reticulum Calcium is actively transported from the cytosol into the sarcoplasmic reticulum (i.e., endoplasmic reticulum of the muscle cell; its interconnecting tubules surround each myofibril like the sleeve of a loosely knit sweater) by ion pumps (i.e., an integral protein of a cell membrane that actively transports ions from one side of the membrane to the other). o The active transport (i.e., the movement of materials across a cell membrane against a concentration gradient. This process uses a carrier protein molecule and requires cellular energy usually ATP) of calcium involves specialized ion pumps in the membrane of the sarcoplasmic reticulum o These pumps must be energized by ATP o Note that during a regular muscle contraction, all myosin heads would cycle, but not all are bound or disconnected at the same time. Physiology of Skeletal Muscle Fibers o For a skeletal muscle to contract: It must be activated (i.e., stimulated by a nerve ending) causing a change in the membrane potential Next, it must generate and propagate an electrical current (i.e., action potential) along its sarcolemma
A resting sarcolemma (like the plasma membranes of cells) is polarized (i.e., has a potential difference or voltage across the membrane with the inside being negative relative to the outside membrane surface). Finally, a short-lived rise in intracellular calcium ion levels occurs to trigger a contraction Motor neuron = a nerve cell that activates a skeletal muscle fiber o Motor neurons reside in the brain or spinal cord, but their long axons (i.e., threadlike extensions bundled with nerves) travel to a muscle fiber. Neuromuscular junction = region where a motor neuron comes into close contact with a skeletal muscle cell o As a rule, each muscle fiber has only one neuromuscular junction, located approximately midway along its length o Synaptic cleft = the remaining functional space within the neuromuscular junction between the motor neuron and the muscle fiber. o Acetylcholine (ACh) = a neurotransmitter released by the motor neurons within the synaptic cleft; when released, it stimulates an electrical impulse creating an action potential o Acetylcholinesterase = an enzyme located in the synaptic cleft that binds with and breaks down acetylcholine; terminates action potential Excitation-Contraction Coupling sequence of events by which transmission of an action potential along the sarcolemma leads to the sliding of myofilaments. Depolarization loss or reduction of negative membrane potential Repolarization movement of the membrane potential to the initial, negative resting (polarized) state. Refractory Period a period during the repolarization phase when the cell cannot be stimulated again until repolarization is complete. Events at the Neuromuscular Junction Action potential arrives at axon terminal of motor neuron Voltage-gated calcium-ion channels open and calcium-ions enter the axon terminal Calcium-ion entry causes the release of acetylcholine out of the axon terminal Acetylcholine diffuses across the synaptic cleft and binds to receptors in the sarcolemma Acetylcholine binding opens ion channels that allow simultaneous passage of Sodium-ions into the muscle fiber and Potassium-ions out of the muscle fiber. o More sodium-ions enter than potassium-ions leave this produces a local change in the membrane potential called depolarization (i.e., the interior of the sarcolemma becomes slightly less negative. This local electrical event is called the end plate potential).
o The action potential is propagated (moves along the length of the sarcolemma) as the local depolarization wave spreads to adjacent areas of the sarcolemma and opens voltage-gated sodium channels there. Again, Sodium-ions, normally restricted from entering, diffuse into the cell following its electrochemical gradient. Acetylcholine effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase. Muscle Metabolism o ATP is the only source of energy used directly for contractile activities including: Cross bridge movement Cross bridge detachment Operation of the calcium pump in the sarcoplasmic reticulum o Muscles store only 4 to 6 seconds worth of ATP just enough to get going o Fortunately, after ATP is hydrolyzed to ADP and inorganic phosphate in muscle fibers, it is regenerated within a fraction of a second by one or more of three different pathways. o Muscle fatigue = state of physiological inability to contract even though the muscle still may be receiving stimuli o Contractures = a state of continuous contraction because the cross bridges are unable to detach due to a lack of ATP