Muscle Tissue 11/14 Nearly half of body's mass Three types – Skeletal – Cardiac – Smooth Differ in structure, location, function and activation Muscle terminology: – Sarcolemma: muscle plasma membrane – Sarcoplasm: cytoplasm of a muscle cell – Prefixes for muscle: Myo, mys, and sarco BIO lab 105--Lab 8A-muscle histol1 1/46 Lab 8A, BIO 105

Skeletal muscles – muscles attached to skin and bones and covers bones – Responsible for locomotion and manipulation – Elongated cells called muscle fibers – Striated (striped) – Voluntary (i.e., conscious control) – Contract rapidly; tire easily; powerful – Require nervous system stimulation BIO lab 105--Lab 8-muscle histol2

Cardiac muscle – Only in heart; bulk of heart walls – Striated, but involuntary we can’t control rate and pace of contraction however neural controls allow heart to speed up for short periods – Can contract without nervous system stimulation; rate set by the pacemaker of the heart (group of special cells) – Intercalated discs are gap junctions and allow innervation to spread in coordinated fashion – Muscle is highly resistant to fatigue BIO lab 105--Lab 8-muscle histol3

Smooth muscle – – spindle shaped (fusiform) cells; 1 nucleus – Not striated – Involuntary – Can contract without nervous system stimulation—hormones, local chemical changes and Autonomic Nervous System can stimulate contraction – Muscle contractions are slow and sustained – Role is to force substances through body channels – In walls of hollow organs, e.g., stomach, urinary bladder, and respiratory passageways BIO lab 105--Lab 8-muscle histol4

Skeletal Muscle Each muscle is an organ composed of muscle tissue, blood vessels, nerves and connective tissue – Every skeletal muscle fiber supplied by nerve ending that controls its activity – Huge nutrient and oxygen need; generates large amount of waste BIO lab 105--Lab 8-muscle histol5

Connective tissue sheaths of skeletal muscle – Support cells; reinforce whole muscle – Internal to external Endomysium: areolar connective tissue surrounding each muscle fiber; located immediately superior to sarcolemma Perimysium: fibrous connective tissue surrounding groups of muscle fibers called fascicles Epimysium: dense irregular connective tissue surrounding entire muscle; blends with tendons BIO lab 105--Lab 8-muscle histol6

Muscle Attachments Attach in at least two places – Insertion – movable bone – Origin – immovable (less movable) bone Attachments – fused to periosteum of bone or cartilage – connective tissue wrappings extend beyond muscle as tendon or aponeurosis (sheetlike connective tissue layer where muscle attaches) Don’t need to know terms direct or indirect BIO lab 105--Lab 8-muscle histol7

Microscopic Anatomy of A Skeletal Muscle Fiber Long, cylindrical cell – long because embryonic cells fuse to produce each fiber Multiple nuclei just below the sarcolemma Contain Myofibrils – Densely packed, rodlike, contractile elements Contain sarcomeres – smallest contractile units – Sarcomeres contain myofilaments Exhibit striations - perfectly aligned repeating series of dark A bands and light I bands When myofibrils move, they move as group, not as individual units BIO lab 105--Lab 8-muscle histol8

Sarcomere Smallest contractile or functional and structural unit of muscle fiber Region of myofibril between 2 consecutive Z discs Composed of thick (myosin) and thin (actin) myofilaments made of contractile proteins BIO lab 105--Lab 8-muscle histol9

Myofibril Banding Pattern Orderly arrangement of actin and myosin myofilaments within sarcomere – Actin myofilaments = thin filaments Anchored to Z discs Extend across I band and partway in A band – Myosin myofilaments = thick filaments Extend length of A band Connected at M line BIO lab 105--Lab 8-muscle histol10

Thick Filament--Myosin – Myosin tails-2 twisted, interwoven, rodlike chains; – Myosin heads act as cross bridges during contraction Binding sites for actin filaments Binding sites for ATP ATPase enzymes BIO lab 105--Lab 8-muscle histol11

Thin Filament--Actin Twisted double strand of fibrous protein Contains sites for myosin head attachment during contraction Tropomyosin and troponin - proteins bound to actin – Tropomyosin in a relaxed fiber blocks the myosin binding sites Twists around actin fibers and help stiffen and stabilize it – Troponin binds calcium ions BIO lab 105--Lab 8-muscle histol12

Sarcoplasmic Reticulum (SR) Network of smooth endoplasmic reticulum surrounding each myofibril Pairs of terminal cisternae form perpendicular cross channels throughout SR Functions in regulation of intracellular calcium levels – Stores and releases Ca 2+ BIO lab 105--Lab 8-muscle histol13 26/46

T Tubules Continuations of sarcolemma--pushes deeply into interior of cell Increase muscle fiber's surface area Conduct nerve impulses to deepest areas of muscle fiber T tubules run between cross channels called terminal cisternae (cisterns) of the sarcoplasmic reticulum (smooth endoplasmic reticulum) Impulses signal calcium to be released from adjacent terminal cisternae Are associated with paired terminal cisterns to form triads that circle each sarcomere; – triads are where the terminal cisterns border a T tubule BIO lab 105--Lab 8-muscle histol14

Physiology of Skeletal Muscle Fibers For skeletal muscle to contract – Activation (at neuromuscular junction) Requires nervous system stimulation AND an electric current or action potential along sarcolemma Intracellular Ca 2+ levels must rise (final requirement for contraction to begin) BIO lab 105--Lab 8-muscle histol15

Nerve Stimulus and the Neuromuscular Junction Skeletal muscles stimulated by motor neurons of voluntary nervous system Axons of motor neurons travel via nerves to skeletal muscle cells Each axon forms several branches as it enters muscle Each axon ending or branch, forms a neuromuscular junction with a single muscle fiber BIO lab 105--Lab 8-muscle histol16

Neuromuscular Junction (NMJ) is where axon and muscle fiber meet. Axon terminal and muscle fiber separated by space called synaptic cleft Synaptic vesicles from axon terminal contain neurotransmitter acetylcholine (ACh) Sarcolemma in NMJ contain ACh receptors ACh diffuses across synaptic cleft and attaches to ACh receptors in sarcolemma ACh binding triggers electrical events to generate an action potential BIO lab 105--Lab 8-muscle histol17

Action potential causes changes in properties of cell membrane channels Ca 2+ channels open  Ca 2+ moves into nerve axon  causing release of ACh into synaptic cleft ACh diffuses across cleft  sarcolemma  initiates action potential in muscle BIO lab 105--Lab 8-muscle histol18

Contraction of Skeletal Muscles Contraction produces muscle tension, force exerted on load or object to be moved – Refers to activation of sliding filaments and forming of cross bridges between actin and myosin Contraction ends when cross bridges are deactivated because of lack of nerve stimulation or not enough calcium present BIO lab 105--Lab 8-muscle histol19

Sliding Filament Model of Contraction In relaxed state, thin and thick filaments overlap only at ends of A band Sliding filament model of contraction – Upon muscle stimulation, thin filaments slide past thick filaments  actin and myosin overlap to a greater degree When myosin heads bind to actin  cross bridges form and sliding begins – Cross bridges form and break several times, moving thin filaments toward center of sarcomere Causes shortening of muscle fiber BIO lab 105--Lab 8-muscle histol20

Motor Unit: The Nerve-Muscle Functional Unit Each muscle is served by at least one motor nerve – A motor nerve contains axons of many motor neurons – Axons branch into terminals, each of which form a NMJ with single muscle fiber Motor unit = motor neuron and all muscle fibers it supplies – For fine control—each motor neuron supplies a smaller number of fibers – For large, weight bearing muscles, each motor neuron supplies a lot of muscle fibers Muscle’s response to single threshold stimulus called a Muscle Twitch BIO lab 105--Lab 8-muscle histol21

LABWORK 1. Identify and describe the three kinds of muscle tissue (3 microscope slides). 2. Identify and describe all components of a muscle, including connective tissue wrappings around each part (models and muscle cross section slide). 3. Identify and describe microstructure of skeletal muscle cells, and basics involved in contraction mechanism (models and neuromuscular junction slide). 4. Explain the concepts covered about contraction and muscle physiology BIO lab 105--Lab 8-muscle histol22