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Muscular System FunctionsBody movement (Locomotion)Maintenance of postureRespirationDiaphragm and intercostal contractionsCommunication (Verbal and Facial)Constriction of organs and vesselsPeristalsis of intestinal tractVasoconstriction of b.v. and other structures (pupils)Heart beat Production of body heat (Thermogenesis)

Properties of MuscleExcitability: capacity of muscle to respond to a stimulusContractility: ability of a muscle to shorten and generate pulling forceExtensibility: muscle can be stretched back to its original lengthElasticity: ability of muscle to recoil to original resting length after stretched

Types of MuscleSkeletalAttached to bonesMakes up 40% of body weightResponsible for locomotion, facial expressions, posture, respiratory movements, other types of body movementVoluntary in action; controlled by somatic motor neuronsSmoothIn the walls of hollow organs, blood vessels, eye, glands, uterus, skinSome functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow, In some locations, autorhythmicControlled involuntarily by endocrine and autonomic nervous systemsCardiacHeart: major source of movement of bloodAutorhythmicControlled involuntarily by endocrine and autonomic nervous systems

Connective Tissue Sheaths Connective Tissue of a MuscleEpimysium. Dense regular c.t. surrounding entire muscleSeparates muscle from surrounding tissues and organsConnected to the deep fasciaPerimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicleContains b.v and nervesEndomysium. Loose connective tissue that surrounds individual muscle fibersAlso contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissueCollagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

Nerve and Blood Vessel SupplyMotor neuronsstimulate muscle fibers to contractNeuron axons branch so that each muscle fiber (muscle cell) is innervatedForm a neuromuscular junction (= myoneural junction)Capillary beds surround muscle fibersMuscles require large amts of energyExtensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

Muscle Tissue Types

Skeletal MuscleLong cylindrical cellsMany nuclei per cellStriatedVoluntaryRapid contractions

Basic Features of a Skeletal MuscleMuscle attachmentsMost skeletal muscles run from one bone to anotherOne bone will move other bone remains fixedOrigin less movable attach- mentInsertion more movable attach- ment

Basic Features of a Skeletal MuscleMuscle attachments (continued)Muscles attach to origins and insertions by connective tissueFleshy attachments connective tissue fibers are shortIndirect attachments connective tissue forms a tendon or aponeurosis Bone markings present where tendons meet bonesTubercles, trochanters, and crests

Skeletal Muscle StructureComposed of muscle cells (fibers), connective tissue, blood vessels, nervesFibers are long, cylindrical, and multinucleatedTend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in lengthDevelop from myoblasts; numbers remain constantStriated appearanceNuclei are peripherally located

Muscle Attachments

Antagonistic Muscles

Microanatomy of Skeletal Muscle

Muscle Fiber AnatomySarcolemma - cell membraneSurrounds the sarcoplasm (cytoplasm of fiber)Contains many of the same organelles seen in other cellsAn abundance of the oxygen-binding protein myoglobinPunctuated by openings called the transverse tubules (T-tubules)Narrow tubes that extend into the sarcoplasm at right angles to the surfaceFilled with extracellular fluidMyofibrils -cylindrical structures within muscle fiberAre bundles of protein filaments (=myofilaments)Two types of myofilamentsActin filaments (thin filaments)Myosin filaments (thick filaments)At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemmaWhen myofibril shortens, muscle shortens (contracts)

Sarcoplasmic Reticulum (SR)SR is an elaborate, smooth endoplasmic reticulum runs longitudinally and surrounds each myofibrilForm chambers called terminal cisternae on either side of the T-tubulesA single T-tubule and the 2 terminal cisternae form a triadSR stores Ca++ when muscle not contractingWhen stimulated, calcium released into sarcoplasm SR membrane has Ca++ pumps that function to pump Ca++ out of the sarcoplasm back into the SR after contraction

Sarcoplasmic Reticulum (SR)

Parts of a Muscle

Sarcomeres: Z Disk to Z DiskSarcomere - repeating functional units of a myofibrilAbout 10,000 sarcomeres per myofibril, end to endEach is about 2 m longDifferences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance.A bands: a dark band; full length of thick (myosin) filamentM line - protein to which myosins attachH zone - thick but NO thin filamentsI bands: a light band; from Z disks to ends of thick filamentsThin but NO thick filamentsExtends from A band of one sarcomere to A band of the next sarcomereZ disk: filamentous network of protein. Serves as attachment for actin myofilamentsTitin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment

Structure of Actin and Myosin

Myosin (Thick) MyofilamentMany elongated myosin molecules shaped like golf clubs. Single filament contains roughly 300 myosin moleculesMolecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally. Myosin headsCan bind to active sites on the actin molecules to form cross-bridges. (Actin binding site)Attached to the rod portion by a hinge region that can bend and straighten during contraction. Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

Actin (Thin) MyofilamentsThin Filament: composed of 3 major proteinsF (fibrous) actinTropomyosinTroponinTwo strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere.Composed of G actin monomers each of which has a myosin-binding site (see yellow dot)Actin site can bind myosin during muscle contraction. Tropomyosin: an elongated protein winds along the groove of the F actin double helix. Troponin is composed of three subunits: Tn-A : binds to actinTn-T :binds to tropomyosin,Tn-C :binds to calcium ions.

Now, putting it all together to perform the function of muscle: Contraction

H Band

Ca2+

Excitation-Contraction CouplingMuscle contraction

Alpha motor neurons release AchACh produces large EPSP in muscle fibers (via nicotinic Ach receptorsEPSP evokes action potentialAction potential (excitation) triggers Ca2+ release, leads to fiber contractionRelaxation, Ca2+ levels lowered by organelle reuptake

Excitation-Contraction Coupling

Excitation-Contraction Coupling

Sliding Filament Model of ContractionThin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degreeIn the relaxed state, thin and thick filaments overlap only slightlyUpon stimulation, myosin heads bind to actin and sliding begins

The lever movement drives displacement of the actin filament relative to the myosin head (~5 nm), and by deforming internal elastic structures, produces force (~5 pN).

Thick and thin filaments interdigitate and slide relative to each other.How striated muscle works: The Sliding Filament Model

Neuromuscular Junction

Neuromuscular JunctionRegion where the motor neuron stimulates the muscle fiberThe neuromuscular junction is formed by :1. End of motor neuron axon (axon terminal)Terminals have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)2. The motor end plate of a muscleA specific part of the sarcolemma that contains ACh receptorsThough exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

Neuromuscular Junction

Motor Unit: The Nerve-Muscle Functional UnitA motor unit is a motor neuron and all the muscle fibers it suppliesThe number of muscle fibers per motor unit can vary from a few (4-6) to hundreds (1200-1500)Muscles that control fine movements (fingers, eyes) have small motor unitsLarge weight-bearing muscles (thighs, hips) have large motor units

Motor Unit: The Nerve-Muscle Functional Unit

Muscle fibers from a motor unit are spread throughout the muscleNot confined to one fascicleTherefore, contraction of a single motor unit causes weak contraction of the entire muscleStronger and stronger contractions of a muscle require more and more motor units being stimulated (recruited)

Motor UnitAll the muscle cells controlled by one nerve cell

Acetylcholine Opens Na+ Channel

Muscle Contraction SummaryNerve impulse reaches myoneural junctionAcetylcholine is released from motor neuronAch binds with receptors in the muscle membrane to allow sodium to enterSodium influx will generate an action potential in the sarcolemma

Muscle Contraction (Contd)Action potential travels down T tubuleSarcoplamic reticulum releases calciumCalcium binds with troponin to move the troponin, tropomyosin complexBinding sites in the actin filament are exposed

Muscle Contraction (contd)Myosin head attach to binding sites and create a power strokeATP detaches myosin heads and energizes them for another contactionWhen action potentials cease the muscle stop contracting

Contraction Speed

IV- Muscle metabolismMuscle fibers use ATP (only first few seconds) for contractionATP must then be generated by the muscle cell: - from creatine phosphate, first - from glucose and glycogen - from fatty-acids

ATP formation from the above compound is possible if oxygen is present (oxidative phosphorylation: 36 ATP per glucose)Oxygen is delivered to the muscle by myoglobin, a molecule with high affinity to oxygen and related to hemoglobin

Figure 12.11

If the effort is strong and sustained, the muscle might not have enough oxygen delivered to it by myoglobin anaerobic glycolysis with only 2 ATP formed per glucose and synthesis of lactic acid

Consequence of anaerobic metabolism?

Muscle fatigueMuscle fatigue: a decline in the ability of the muscle to sustain the strength of contractionCauses: - rapid build-up of lactic acid - decrease in oxygen supply - decrease in energy supply (glucose, glycogen, fatty-acids)Decreased neurotransmitter at the synapse - psychological causes

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******Skeletal muscle attaches to our skeleton. *The muscle cells a long and cylindrical. *Each muscle cell has many nuclei. *Skeletal muscle tissue is striated. It has tiny bands that run across the muscle cells. *Skeletal muscle is voluntary. We can move them when we want to. *Skeletal muscle is capable of rapid contractions. It is the most rapid of the muscle types.

****In this unit we will primarily study skeletal muscle. Each muscle cell is called a muscle fiber. Within each muscle fiber are many myofibrils.

*********Dark and light bands can be seen in the muscle fiber and also in the smaller myofibrils. An enlargement of the myofibril reveals that they are made of smaller filaments or myofilaments. *There is a thick filament called myosin and *a thin filament called actin. Note the I band, A band H zone or band and Z disc or line. These will be discussed shortly.

*A small section of a myofibril is illustrated here. Note the thick myosin filaments are arranged between overlapping actin filaments. *The two Z lines mark the boundary of a sarcomere. The sarcomere is the functional unit of a muscle cell .We will examine how sarcomeres function to help us better understand how muscles work.

*A myosin molecule is elongated with an enlarged head at the end.

*Many myosin molecules form the thick myosin filament. It has many heads projecting away from the main molecule.

*The thinner actin filament is composed of three parts: actin, tropomyosin and troponin. *Here is a sarcomere illustrating the thin actin and thick myosin filaments. The area of the sarcomere has only myosin is called the H band.

*Which filament has moved as the sarcomere contracted? Note the thick myosin filaments have not changed, but the thin actin filaments have moved closer together.

*The actin filaments are moved by the heads of the myosin filaments. In step one the myosin head attaches to an actin filament to create a cross bridge. Step two shows that the attached myosin head bends to move the actin filament. The myosin head as expended energy to create this movement. This is a power stroke or working stroke. Step three shows that energy in the form of ATP will unhook the myosin head. In step 4 the myosin head is cocked and ready to attach to an actin filament to start another power stroke.

*The string of green circles represents an actin filament. There are binding sites in the filament for the attachment of myosin heads. *In a relaxed muscle the binding sites are covered by tropomyosin. The tropomyosin has molecules of troponin attached to it. *Calcium, shown in yellow, will attach to troponin. *Calcium will change the position of the troponin, tropomyosin complex. *The troponin, tropomyosin complex has now moved so that the binding sites are longer covered by the troponin, tropomyosin complex.

*The binding sites are now exposed and myosin heads are able to attach to form cross bridges.*

*This diagram shows the microanatomy of skeletal muscle tissue again. *The blue sarcoplasmic reticulum is actually the endoplasmic reticulum. It stores calcium. *The mitochondria are illustrated in orange. They generate ATP, which provides the energy for muscle contractions.

**The next few slides will summarize the events of a muscle contraction.The nerve impulse reaches the neuromuscular junction (myoneural junction).

*****A motor unit is all the muscle cells controlled by one nerve cell. This diagram represents two motor units. Motor unit one illustrates two muscle cells controlled by one nerve cell. When the nerve sends a message it will cause both muscle cells to contract. Motor unit two has three muscle cells innervated by one nerve cell.

*Acetylcholine is released from the motor neuron.

*Acetylcholine binds with receptors in the muscle membrane to allow sodium ions to enter the muscle. *The influx of sodium will create an action potential in the sarcolemma. Note: This is the same mechanism for generating action potentials for the nerve impulse. The action potential travels down a T tubule. As the action potential passes through the sarcoplamic reticulum it stimulates the release of calcium ions. Calcium binds with troponin to move tropomyosin and expose the binding sites. Myosin heads attach to the binding sites of the actin filament and create a power stroke. ATP detaches the myosin heads and energizes them for another contraction. The process will continue until the action potentials cease. Without action potentials the calcium ions will return to the sarcoplasmic reticulum.

Refresher Course in Muscle Physiology4/11/2003EB2003--Susan Brooks*Catalytic domain responsible for binding & hydrolysis of ATP, and binding actin.Neck region responsible for transport of the load.Converter responsible for energy transduction.