Muscle Tissue Nearly half of body's mass Three types – Skeletal – Cardiac – Smooth Differ in structure, location, function and activation BIO lab 105--Lab 8-muscle histol1

Similarities: Skeletal and smooth muscle cells are elongated and are called muscle fibers Muscle contraction depends upon 2 types of myofilaments: actin and myosin Muscle terminology is similar: – Prefixes for muscle: Myo, mys, and sarco - – Sarcolemma: muscle plasma membrane – Sarcoplasm: cytoplasm of a muscle cell BIO lab 105--Lab 8-muscle histol2

Types of Muscle Tissue Skeletal muscles – muscles that are attached to skin and bones and also cover the bones – Elongated cells called muscle fibers – Striated (striped) – Voluntary (i.e., conscious control) – Contract rapidly; tire easily; powerful – Responsible for locomotion and manipulation – Require nervous system stimulation BIO lab 105--Lab 8-muscle histol3

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) – Muscle is highly resistant to fatigue BIO lab 105--Lab 8-muscle histol4

Smooth muscle- spindle shaped (fusiform) cells; 1 nucleus – Not striated – Involuntary – Requires Autonomic Nervous System stimulation to contract – Role is to force substances through body channels – In walls of hollow organs, e.g., stomach, urinary bladder, and respiratory passageways Muscle contractions are slow and sustained BIO lab 105--Lab 8-muscle histol5

Special Characteristics of Muscle Tissue Excitability or irritability: ability to receive and respond to stimuli Contractility: ability to shorten forcibly when stimulated Extensibility: ability to be stretched Elasticity: ability to recoil and resume original resting length BIO lab 105--Lab 8-muscle histol6

Muscle Functions 1.Skeletal muscle responsible for movement of bones and manipulation of objects; locomotion 2.Cardiac muscle responsible for sending blood throughout the body 3.Smooth muscle helps maintain blood pressure and squeezes or propels substances through organs (i.e. food, feces) 4.Maintaining posture and body position 5.Stabilize and strengthen joints 6.Heat generation (especially skeletal muscle) Additional functions – Protects organs, forms valves, controls pupil size, causes "goosebumps" BIO lab 105--Lab 8-muscle histol7

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 histol8

Connective tissue sheaths of skeletal muscle – Support cells; reinforce whole muscle – External to internal Epimysium: dense irregular connective tissue surrounding entire muscle; may blend with fascia (tendons) Perimysium: fibrous connective tissue surrounding groups of muscle fibers called fascicles Endomysium: fine areolar connective tissue surrounding each muscle fiber; is located immediately superior to the sarcolemma BIO lab 105--Lab 8-muscle histol9

Nerve and Blood Supply – Each muscle cell is served by one nerve, an artery and one or more veins which enter at the central part of the muscle and then branch – Every skeletal muscle fiber has a nerve ending which controls each contraction – Contracting fibers require a continuous supply of oxygen and nutrients (via arteries) and – Waste removal (via veins) BIO lab 105--Lab 8-muscle histol10

Attachments Attach in at least two places – Insertion – movable bone – Origin – immovable (less movable) bone Attachments direct or indirect – Direct—epimysium fused to periosteum of bone or to cartilage – Indirect—connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis BIO lab 105--Lab 8-muscle histol11

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 Sarcoplasm has many glycosomes for glycogen storage, and myoglobin for O 2 storage Each fiber is supplied with a nerve ending to control contraction BIO lab 105--Lab 8-muscle histol12

Myofibrils Densely packed, rodlike, contractile elements ~80% of cell volume 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 a group, not as individual units BIO lab 105--Lab 8-muscle histol13

© 2013 Pearson Education, Inc. H zone: lighter region in midsection of dark A band where filaments do not overlap M line: line of protein myomesin bisects H zone Z disc (line): coin-shaped sheet of proteins on midline of light I band that anchors actin filaments and connects myofibrils to one another Thick filaments: run entire length of an A band Thin filaments: run length of I band and partway into A band Sarcomere: region between two successive Z discs Small part of one myofibril enlarged to show the myofilaments responsible fr the banding pattern. Each sarcomere extends from one Z disc to the next. Thin (actin) filament Z discH zoneZ disc Thick (myosin) filament I band A band I bandM line Sarcomere M-lines are where the myosin filaments are held in alignment

Sarcomere Smallest contractile or functional and structural unit of muscle fiber Composed of thick and thin myofilaments made of contractile proteins Region of myofibril between 2 consecutive Z discs BIO lab 105--Lab 8-muscle histol15

Myofibril Banding Pattern Orderly arrangement of actin and myosin myofilaments within sarcomere – Actin myofilaments = thin filaments Extend across I band and partway in A band Anchored to Z discs – Myosin myofilaments = thick filaments Extend length of A band Connected at M line BIO lab 105--Lab 8-muscle histol16

Thick Filament Composed of protein myosin – Myosin tails-2 twisted, interwoven, rodlike chains; made up of polypeptides – Myosin heads, also polypeptide chains, act as cross bridges during contraction Binding sites for actin (thin) filaments Binding sites for ATP ATPase enzymes BIO lab 105--Lab 8-muscle histol17

Thin Filament 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 histol18

Sarcoplasmic Reticulum (SR) Network of smooth endoplasmic reticulum surrounding each myofibril – Most run longitudinally 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 histol19

T Tubules Continuations of sarcolemma--protrudes deeply into the interior of the cell Increase muscle fiber's surface area Conduct nerve impulses to the deepest areas of the muscle fiber Impulses signal calcium to be released from the adjacent terminal cistermae Associated with paired terminal cisterns to form triads that circle each sarcomere BIO lab 105--Lab 8-muscle histol20

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 BIO lab 105--Lab 8-muscle histol21

Z discH zone I band A band I bandM line Cross bridges form and break several times, ratcheting thin filaments toward center of sarcomere Causes shortening of muscle fiber Pulls Z discs toward M line I bands shorten; Z discs closer; H zones disappear; A bands move closer (length stays same)

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 briefly (final step for contraction to begin) BIO lab 105--Lab 8-muscle histol23

Nerve Stimulus and the Neuromuscular Junction Skeletal muscles stimulated by motor neurons of the voluntary nervous system Axons of motor neurons travel via nerves to skeletal muscle cells Each axon forms several branches as it enters the muscle Each axon ending or branch, forms a neuromuscular junction with a single muscle fiber BIO lab 105--Lab 8-muscle histol24

Neuromuscular Junction (NMJ) is where the axon and muscle fiber meet. Axon terminal and muscle fiber separated by gel- filled space called synaptic cleft Synaptic vesicles from the axon terminal contain the neurotransmitter acetylcholine (ACh) Sarcolemma in the NMJ contain ACh receptors ACh diffuses across the synaptic cleft and attaches to the ACh receptors in the sarcolemma ACh binding triggers the electrical events to generate an action potential BIO lab 105--Lab 8-muscle histol25

Action potential causes changes in the properties of the cell membrane channels Ca 2+ channels open  Ca 2+ moves into the nerve axon and this causes release of the ACh into the synaptic cleft ACh diffuses across the cleft  sarcolemma  initiates an action potential in the muscle BIO lab 105--Lab 8-muscle histol26

Contraction of Skeletal Muscles Contraction produces muscle tension, force exerted on load or object to be moved – Refers to the activation of the sliding filaments and the forming of cross bridges between the actin and myosin Contraction ends when cross bridges are deactivated because of a lack of nerve stimulation or not enough calcium present BIO lab 105--Lab 8-muscle histol27

Rigor mortis – Cross bridge detachment requires ATP – 3–4 hours after death muscles begin to stiffen with peak rigidity at 12 hours post mortem Dying cells take in calcium  cross bridge formation No ATP generated to break cross bridges – Muscles relax as muscle proteins break down after death BIO lab 105--Lab 8-muscle histol28

2 types of muscle contraction are: – Isometric contraction: no shortening; muscle tension increases – Isotonic contraction: muscle shortens because muscle tension is high enough – Force and duration of contraction vary in response to stimuli of different frequencies and intensities BIO lab 105--Lab 8-muscle histol29

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 the 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 The muscle’s response to a single threshold stimulus is called a Muscle Twitch BIO lab 105--Lab 8-muscle histol30

Muscle Twitch Three phases of muscle twitch – Latent period: sequence of events involved in spreading the action potential along the sarcolemma leads to the sliding of the muscle filaments (excitation-contraction coupling); no muscle tension seen yet – Period of contraction: cross bridge formation; tension increases – Period of relaxation: Ca 2+ reentry into SR; tension declines to zero Muscle contracts faster than it relaxes BIO lab 105--Lab 8-muscle histol31

Graded muscle responses – Varying strength of contraction for different demands Required for proper control of skeletal movement Responses graded by 1.Changing frequency of stimulation 2.Changing strength of stimulation BIO lab 105--Lab 8-muscle histol32

Response to Change in Stimulus Frequency Single stimulus results in single contractile response— muscle twitch – We typically don’t see this in healthy people Wave summation – Increased stimulus frequency (muscle does not completely relax between stimuli) causes a more forceful contraction Additional Ca 2+ release with second stimulus stimulates more shortening Produces smooth, continuous contractions Further increase in stimulus frequency causing a sustained but quivering contraction  incomplete tetanus BIO lab 105--Lab 8-muscle histol33

Response to Change in Stimulus Frequency If stimuli are given quickly enough, muscle reaches maximal tension  fused (complete) tetanus results – Smooth, sustained contraction – Do not confuse with “tetanus”-the bacterial disease – See fused tetanus only in cases of super-human strength—lifting a car off someone – No muscle relaxation  muscle fatigue Muscle cannot contract; zero tension BIO lab 105--Lab 8-muscle histol34

Response to Change in Stimulus Strength Threshold stimulus: stimulus strength causing first observable muscle contraction Recruitment (multiple motor unit summation) controls force of contraction Subthreshold stimuli – no observable contractions Maximal stimulus – strongest stimulus that increases contractile force BIO lab 105--Lab 8-muscle histol35

Muscle Fatigue Physiological inability to contract despite continued stimulation Occurs when – ATP production fails to keep pace with ATP use – Ionic imbalances – Lactic acid accumulates in the muscle – Prolonged exercise damages SR and interferes with Ca 2+ regulation and release BIO lab 105--Lab 8-muscle histol36

Muscle Fiber Type Classified according to two characteristics – Speed of contraction: slow or fast fibers according to Speed at which myosin ATPases split ATP Pattern of electrical activity of motor neurons – Metabolic pathways for ATP synthesis Oxidative fibers—use aerobic pathways Glycolytic fibers—use anaerobic glycolysis BIO lab 105--Lab 8-muscle histol37

LABWORK 1. Identify and describe the three kinds of muscle tissue (3 microscope slides). 2. Identify and describe all the components of a muscle, including the connective tissue wrappings around each part (models and muscle cross section slide). 3. Identify and describe the microstructure of skeletal muscle cells, and the basics involved in the contraction mechanism (models and neuromuscular junction slide). 4. Explain the concepts covered about contraction and muscle physiology (the frog muscle video). BIO lab 105--Lab 8-muscle histol38