The Muscular System

There are four characteristics associated with muscle tissue: Excitability Contractility Extensibility Elasticity - Tissue can receive & respond to stimulation - Tissue can shorten & thicken - Tissue can lengthen - After contracting or lengthening, tissue always wants to return to its resting state

The characteristics of muscle tissue enable it to perform some important functions, including: Movement – both voluntary & involuntary Maintaining posture Supporting soft tissues within body cavities Guarding entrances & exits of the body Maintaining body temperature

Types of muscle tissue: Skeletal Cardiac Smooth (Visceral)

Types of muscle tissue: Skeletal muscle tissue Associated with & attached to the skeleton Under our conscious (voluntary) control Microscopically the tissue appears striated Cells are long, cylindrical & multinucleate

Cardiac muscle tissue Makes up myocardium of heart Unconsciously (involuntarily) controlled Microscopically appears striated Cells are short, branching & have a single nucleus Cells connect to each other at intercalated discs

Makes up walls of organs & blood vessels Smooth (visceral) muscle tissue Makes up walls of organs & blood vessels Tissue is non-striated & involuntary Cells are short, spindle-shaped & have a single nucleus Tissue is extremely extensible, while still retaining ability to contract

Connective Tissue of a Muscle Epimysium. Dense regular connective tissue surrounding the entire muscle Separates muscle from surrounding tissues and organs Connected to the deep fascia Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle Contains b.v and nerves Endomysium. Loose connective tissue that surrounds individual muscle fibers Also contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissue) Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

Anatomy of skeletal muscles epimysium tendon perimysium Muscle Fascicle Surrounded by perimysium endomysium Skeletal muscle Skeletal muscle fiber (cell) Surrounded by epimysium Surrounded by endomysium Play IP Anatomy of Skeletal muscles (IP p. 4-6)

Basic Features of a Skeletal Muscle Muscle attachments Most skeletal muscles run from one bone to another One bone will move – other bone remains fixed Origin – less movable attachment Insertion – more movable attachment

Microanatomy of a Muscle Fiber (cell)

Muscle Fiber Anatomy Sarcolemma - cell membrane Surrounds the sarcoplasm (cytoplasm of fiber) Punctuated by openings called the transverse tubules (T-tubules) - Narrow tubes that extend into the sarcoplasm that are filled with extracellular fluid Contains many of the same organelles seen in other cells An abundance of the oxygen-binding protein myoglobin Myofibrils -cylindrical structures within muscle fiber Are bundles of protein filaments (=myofilaments) Two types of myofilaments Actin filaments (thin filaments) Myosin filaments (thick filaments) When myofibril shortens, muscle shortens (contracts)

Microanatomy of a Muscle Fiber (Cell) transverse (T) tubules sarcoplasmic reticulum sarcolemma terminal cisternae myoglobin mitochondria thick myofilament thin myofilament myofibril triad nuclei

Sarcomere - repeating functional units of a myofibril *Differences 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) filament M line - protein to which myosins attach H zone - thick but NO thin filaments I bands: a light band; from Z disks to ends of thick filaments Thin but NO thick filaments Extends from A band of one sarcomere to A band of the next sarcomere Z disk: filamentous network of protein. Serves as attachment for actin myofilaments Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment

Thin Myofilament (myosin binding site)

Thick myofilament (has ATP & actin binding site) *Play IP sliding filament theory p.5-14 for overview of thin & thick filaments

Sarcomere A band Z line Z line I band Zone of overlap M line H zone I band Zone of overlap Zone of overlap M line Thin myofilaments Thick myofilaments

Sarcoplasmic Reticulum (SR) SR is an elaborate, smooth endoplasmic reticulum runs longitudinally and surrounds each myofibril Form chambers called terminal cisternae on either side of the T-tubules A single T-tubule and the 2 terminal cisternae form a triad SR stores Ca++ when muscle not contracting When 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)

Myosin (Thick) Myofilament Many elongated myosin molecules shaped like golf clubs. Single filament contains roughly 300 myosin molecules Myosin heads Can 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) Myofilaments Thin Filament: composed of 3 major proteins F (fibrous) actin Tropomyosin Troponin

Sliding Filament Theory – Ca2+ Muscle contraction occurs when the muscle receives an impulse from the nerve cell in the form of an action potential. The nerve impulse penetrates the muscle fiber via the transverse tubules This causes calcium (found in the terminal cisternae) to be released. Calcium ions bind to troponin (on actin)

Sliding Filament Theory – Ca2+ During binding, troponin changes shape, causing troponin molecules to move out the way of the myosin binding site Extended myosin heads bind to actin Bound ADP and phosphate are released, causing the myosin heads to return to their original position Returning to their original position allows actin filaments to be pulled into the sarcomere

Sliding Filament Theory – Ca2+ ATP binds to myosin heads causing them to let go Process repeats itself so long as calcium is present Overview of Sliding Filament Theory Animation

Sliding Filament Theory Myosin heads attach to actin molecules (at binding (active) site) Myosin “pulls” on actin, causing thin myofilaments to slide across thick myofilaments, towards the center of the sarcomere Sarcomere shortens, I bands get smaller, H zone gets smaller, & zone of overlap increases

Once a muscle fiber begins to contract, it will contract maximally As sarcomeres shorten, myofibril shortens. As myofibrils shorten, so does muscle fiber Once a muscle fiber begins to contract, it will contract maximally This is known as the “all or none” principle Sliding Filament Theory Animation