A&P1, Unit 3: Muscle Tissue

Duchenne Muscular Dystrophy

Most common is Duchenne Muscular Dystrophy (X-linked); strikes boys almost exclusively (1 in every 3,500 born each year). Carried by females and expressed in males as lack of dystrophin. Victims become clumsy and fall frequently; usually die of respiratory failure in 20s.

State the name for individual muscle cells.

Muscle fibers *Skeletal muscles are composed of hundreds to thousands of these cells

Muscle tension

Muscle tension is the force that a muscle fiber is capable of producing; controlled by stimulation frequency and motor unit recruitment. Total tension depends on the number of muscle fibers that are contracting in unison.

Name the 2 contractile proteins

Myosin and Actin

Name the main protein component of thick filaments and thin filaments

Myosin in thick filaments; Actin in thin filaments *Within myofibrils are smaller protein structures, called filaments or myofilaments. There are thin filaments (about 8nm in diameter) and thick filaments (about 16nm in diameter).

Rigor mortis & Decomposition (Homeostatic Imbalance)

*Cross bridge detachment requires ATP Calcium ions leak out of the SR into sarcoplasm and allow myosin heads to bind to actin (crossbridge); ATP synthesis cases so the myosin heads cannot detach from actin. *3-4 hours after death muscles begin to stiffen with weak rigidity at 12 hours post mortem; last about 24 hours. • Dying cells take in calcium cross bridge formation • No ATP generated to break cross bridges Decomposition: Protelytic enzymes from lysosomes digest the muscle proteins.

Describe the arrangement of muscle fibers and connective tissue in a skeletal muscle

*KEY: A skeletal muscle consists of individual muscle fibers bundled into fascicles and surrounded by three connective tissue layers that are extensions of fascia. Connective tissues surround skeletal muscle fibers and whole muscles, and blood vessels and nerves penetrate skeletal muscles; each muscle is served by one artery, one nerve, and one or more veins. Connective tissue surrounds and protects muscle tissue. The hypodermis (made up of areolar and adipose tissue) separates muscle from skin, insulated and protects. *Skeletal muscle is well supplied with nerves and blood vessels; an artery and one or two veins accompany each nerve that penetrates a skeletal muscle.

BONUS: Name 3 mechanisms that affect the NMJ

1) Alters Release of Acetylcholine (ACh) 2) Blocks Acetylcholine (ACh) 3) Prevents inactivation of Acetylcholine (ACh)

Skeletal muscles attach in at least two places, what are they?

1) Insertion: movable bone 2) Origin: Immovable (less movable) bone Attachment are direct or indirect: -Direct: Epimysium fused to periosteum of bone or perichondrium of cartilage -Indirect: Connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis.

Describe how an impulse (nerve action potential) excites a skeletal muscle fiber

1) Release of acetylcholine *Vesicles undergo exocytosis in response to CA2+ ions. An impulse travels from the brain or spinal cord along a motor neuron to the muscle fiber. Arrival of the impulse at the synaptic end bulb stimulates voltage-gated channels to open; calcium ions (CA2+) flow inward through the open channels. This stimulates the synaptic vesicles to undergo exocytosis; during this the synaptic vesicles fuse with the motor neuron's plasma membrane, releasing ACh into the synaptic cleft. The ACh diffuses across the synaptic cleft between the motor neuron and the motor end plate. 2) Activation of ACh receptors *Bind to nicotinic receptors to open Na+ ion channels Binding of 2 molecules of ACh to a nicotinic receptor on the motor end plate opens an ion channel in the ACh receptor. Once the channel is open, small cations, most importantly Na+ (sodium ions), can flow across the membrane. 3) Generation of muscle action potential *Change in membrane potential triggers contraction The influx of Na+ makes the inside of the muscle fiber more positively charged; this triggers a muscle action potential. The muscle action potential then travels along the sarcolemma and into the T tubules, stimulating the contraction process. 4) Termination of ACh activity *Broken down by acetylcholinesterase enzyme (AChE) The effect of ACh binding is brief as ACh is rapidly broken down by AChE in the synaptic cleft. AChE breaks down ACh into acetyl and choline, products that cannot activate the ACh receptor.

Compare and contrast the three types of muscle tissue with regard to location and nervous system control.

1) Skeletal muscle tissue: Attached to bones and skin and functions to move bones of the skeleton. Striated, under voluntary control, contracts rapidly (tires easily, powerful), requires nervous system stimulation. 2) Cardiac muscle tissue: Found only in the heart; forms most of the heart wall. Striated, involuntary, can contract without nervous system stimulation. 3) Smooth muscle tissue: Located in the walls of hollow organs. Not striated, involuntary, can contract without nervous system stimulation.

Describe muscle tension control including discussions of: Motor units, Twitch contractions, Wave summation, Motor unit recruitment.

1. A single impulse in a motor neuron elicits a single muscle twitch contraction in all muscle fibers that it innervates. The frequency of stimulation governs the total tension that can be produced by a single muscle fiber. The total tension produced in a whole muscle depends on the number of fibers contracting in unison. 2. A motor unit is one motor neuron and all the skeletal muscle fibers the motor neuron stimulates. 3. Muscles controlling small, precise movements are innervated by many small motor units; muscles controlling large, powerful movements have fewer, large motor units. The size of a muscle's motor units and the number of motor units activated contribute to the contraction strength. 4. The response of a motor unit to a single impulse in its motor neuron is a twitch contraction. The three phases of a twitch are the latent period of cell events leading up to contraction; the contraction period of power strokes generating tension; and the relaxation period, during which the muscle is allowed to resume its original length. 5. Multiple stimuli that arrive before the muscle fiber has fully relaxed lead to wave summation. When the frequency of stimulation allows partial relaxation it is called unfused tetanus; rapid frequency of stimulation and sustained contraction is called fused tetanus. 6. Motor unit recruitment is the process of increasing the number of contracting motor units; the force of a muscle contraction becomes greater as more motor units are activated. Some motor units contract while others relax; they alternate to relieve one another and create smooth movement.

Explain the energy involved in skeletal muscle fiber contraction including: Three ways ATP is produced, The sources of energy, The order in which the energy sources are utilized as muscle contraction continues.

1. ATP is the only direct source of energy for muscle contraction. Muscle fibers have three ways to produce ATP: from creatine phosphate, by anaerobic cellular respiration, and by aerobic cellular respiration. 2. Muscle fibers break down excess ATP and transfer a phosphate group to creatine, forming creatine phosphate and ADP. During contraction, muscle fibers transfer the phosphate group from creatine phosphate to ADP, forming ATP. Provide enough energy for about 15 seconds of maximized muscle contraction. 3. A muscle at peak activity quickly depletes available ATP and creatine phosphate, and it will then catabolize glucose molecules from glycogen. Glycolysis is the initial pathway in glucose breakdown and yields two ATP molecules and two pyruvic acid molecules. When oxygen is unavailable, anaerobic reactions convert pyruvic acid to lactic acid. Blood removes lactic acid from skeletal muscle and carries much of it to the liver for reconversion to glucose. Provides enough energy for about 30-40 seconds. 4. When oxygen is available, the pyruvic acid molecules from glycolysis enter the mitochondria, where aerobic cellular respiration completely oxidizes each molecule of glucose to generate 36 ATP molecules. Provides energy for activities that last more than 10 minutes.

Name the alternating bands of sarcomeres that form striations

A band: Darker middle with thick and thin overlapping (doesn't shrink); extends the entire length of the thick filaments. H zone: In the center of each A band; Narrow center with only thick filaments (non-overlapping) I band: Near Z discs with only thin filaments; Lighter and less dense. A Z disc passes through the center of each I band. M line: Supporting protein in middle of sarcomere; Center of the H zone.

Neurotransmitter

A chemical released by a motor neuron to communicate with a muscle fiber indirectly. A small gap, called a synaptic cleft, separates a motor neuron from the skeletal muscle fiber; the motor neuron cannot jump the gap to excite the muscle fiber.

Muscle atrophy

A decrease in the size and, therefore, strength of a muscle; a result of progressive loss of myofibrils when muscles are not used or the nerve supply to a muscle is disrupted

Sarcoplasmic reticulum

A fluid-filled system of membranous sacs; encircles each myofibril (similar to smooth ER of non-muscle cells). Dilated end sacs are terminal cisterns, associated with either side of a T-tubule to form a triad. Stores and releases calcium ions (Ca2+) that trigger muscle contraction.

Titin

A key structural protein; the third most plentiful (after actin and myosin). Each titin filament connects a Z disc to an M line, thereby helping stabilize the position of the thick filament. *Has high elasticity which accounts for the elasticity and extensibility of myofibrils.

Synapse

A region where communication occurs between a neuron and another cell.

Fascia

A sheet or broad band of dense connective tissue that supports and surrounds muscles and other organs of the body. Unites muscles with similar functions, carries nerves and vessels, and fills spaces between muscles.

Muscle tone

A small amount of tautness or tension in the muscle due to weak, involuntary contractions of its motor units; to sustain muscle tone, small groups of motor units are activated in a constantly shifting, alternating patter. Muscle tone keeps skeletal muscles firm, not flaccid. A muscle at rest exhibits muscle tone, a small amount of tension due to involuntary alternating contractions of a small number of its motor units that do not produce movement. Damaged motor neurons that cannot maintain muscle tone cause a muscle to become flaccid.

Muscle action potential

A stimulating impulse that propagates along the sarcolemma and transverse tubules; in skeletal muscles, it is generated by acetylcholine, which increases the permeability of the sarcolemma to cations, especially sodium ions (Na+).

Dystrophin

A structural protein that links thin filaments of the sarcomere to integral membrane proteins of the sarcolemma, which are attached in turn to proteins in the connective tissue extracellular matrix that surrounds muscle fibers.

Axon

A threadlike process that extends from the brain or spinal cord to a group of skeletal muscle fibers; each motor neuron has one

ESSAY: Action Potential, Contraction, Relaxation

ACTION POTENTIAL: Begins when the nervous system generates a signal. A motor neuron conducts an action potential (impulse) to its link, or Axon Terminal, with a muscle fiber, at the Neuromuscular Junction (NMJ). At this point, since the Motor Neuron has reached its Axon Terminal, it is has reached the synaptic cleft (gap between it and the muscle fiber). The motor neuron divides into a cluster of synaptic end bulbs/vesicles that stimulate the influx of Calcium Ions (Ca2+). This stimulates the vesicles to undergo exocytosis and fuse with the motor neuron's plasma membrane, which releases the neurotransmitter, Acetylcholine (ACh); Ach diffuses across the synaptic cleft between the motor neuron and the motor end plate. Acetylcholine (ACh) binds to nicotinic receptors (chemically gated ion channels) on the motor end plate to open an ion channel for the influx of Sodium Ions (Na+) across the membrane and into the muscle fiber. Na+ makes the muscle fiber more positively charged which stimulates another action potential (impulse) to travel along the Sarcolemma and into the T tubules where it reaches Myofibrils within the muscle fiber. The arrival of the action potential stimulates the Sarcoplasmic Reticulum and triggers the release of Calcium Ions (Ca2+), stored in the Sarcoplasmic Reticulum, to diffuse into the muscle fiber. [ACh is broken down by Acetylcholinesterase Enzyme (AChE) in the synaptic cleft; effect of ACh is very brief and it is rapidly broken down into Acetyl and Choline] CONTRACTION: Sarcomeres contain thick (myosin) and thin (actin) filaments. Thin filaments contain smaller regulatory proteins called Tropomyosin and Troponin, which help regulate/switch the contraction process on and off. Calcium ions (Ca2+) from the Sarcoplasmic Reticulum bind to the Troponin-Tropomyosin protein complex on thin filament which causes Tropomyosin to uncover the myosin-binding site so that the Myosin head, from the thick filament, can bind to myosin-binding site on Actin, which forms the cross bridge. [In a relaxed muscle, Myosin is blocked from the myosin-binding site of Actin because Tropomyosin covers the site and Troponin holds it in place] Myosin head contains an ATP binding site; ATP is broken down (ATP hydrolysis) into ADP and Pi (Phosphate) by ATPase (Enzyme) to provide the energy for the power stroke (Myosin head pulling the thin filament) [rotates/swivels to release Phosphate group and ADP] - this is how the filaments glide across each other (the gliding mechanism); slides thin filament (Actin) past the thick filament toward M Line. ATP then binds to the Myosin head and it detaches from Actin (doesn't detach until ATP binds to it). ATP is broken down. As sarcomere shortens, the muscle fibers (and muscle) shorten and pull on the bones creating movement. RELAXATION: Cessation of motor neuron impulses stops the release of Acetylcholine (ACh). ACh is broken down in the synaptic cleft to acetyl and choline. Ion channels close so action potential (impulses) stop. Calcium Ion (Ca2+) move back into Sarcoplasmic Reticulum. Troponin-Tropomyozin protein complex slides back to cover the myosin-binding site. Thin filaments (Actin) return to relaxed position.

Contrast the benefits for your body of aerobic training (aerobic exercise) and strength training (anaerobic training)

Aerobic exercise (aerobic training) increase the supply of oxygen-rich blood available to skeletal muscles for aerobic cellular respirtation; builds endurance for prolonged activities. *Aerobic endurance is the length of time muscles contract using aerobic pathways. Strength training (anaerobic training) stimulates synthesis of muscle proteins and result, over time, in increased muscle size (muscle hypertrophy); builds muscle strength for short-term feats. *Anaerobic threshold is the point at which metabolism converts to anaerobic.

BONUS: Name 2 toxins that affect the NMJ

Alters Release of Acetylcholine (ACh) 1) Black widow spider venom (causes explosive release of ACh) 2) Clostridium Botulinum Toxin (Botox) (blocks release of ACh) 3) Lambert-Eaton syndrome (Self-produced Antibodies inactivate voltage-gates Ca2+ Channels) Blocks Acetylcholine (ACh) 4) Curare (Reversibly binds with ACh receptor sites) 5) Snake venom protein, α-Bungarotoxin (Irreversibly binds with ACh receptor sites); won't ever function again. 6) Myasthenia gravis (Self-produced Antibodies inactivate ACh receptor sites) Prevents inactivation of Acetylcholine (ACh) 7) Organophosphates (certain pesticides and military nerve gas) (Irreversibly inhibits Acetylcholinesterase)

Contraction Cycle

At the onset of contraction, the sarcoplasmic reticulum release Ca2+ into the sarcoplasm which bind to troponin, which moves the troponin-tropomyosin complexes away from the myosin-binding sites on actin. Once uncovered, the contraction cycle begins. The Contraction Cycle - the repeating sequence of events that causes the filaments to slide - consists of four steps: 1) ATP splits *Reorients and energizes myosin head Myosin head includes an ATP-binding site and ATPase, and enzyme that splits ATP into ADP (adenosine diphosphate) and P (a phosphate group); this reorients and energizes the myosin head. 2) Myosin attaches to actin *Head attaches to myosin-binding site The energized myosin head attaches to the myosin-binding site on actin and release the phosphate group. 3) Power stroke occurs *Releases phosphate group, triggering ADP release, and myosin rotation slides actin toward M line Binding of the myosin head to actin triggers the power stroke of contraction. During the power stroke, the myosin head rotates or swivels and releases the ADP. The myosin head generates force as it rotates toward the center of the sarcomere, sliding the thin filament past the thick filament toward the M line. 4) Myosin detaches from actin ATP molecule binds; ATPase split ATP and reorients head At the end of the power stroke, the myosin head remains firmly attached to actin until it binds another molecule of ATP. As ATP binds to the ATP-binding site on the myosin head, the myosin head detaches from actin. As the Z discs come closer together the sarcomere shortens, which causes the whole muscle fiber to shorten, which leads to the shortening of the muscle - which results movement as the muscle pulls on bones.

Sarcomere

Basic functional units of a myofibril. *Extends from one Z disc to the next; the functional units that actin and myosin molecules within the myofibrils are organized into.

Describe skeletal muscle fiber relaxation and the resulting relaxation of an entire skeletal muscle

Cessation of motor neuron impulses stops ACh release. AChE breaks down ACh in synaptic cleft. Ion channels close, as action potential stops. Ca2+ active transport pumps use ATP to move Ca2+ ions back into SR cisterns. Troponin-tropomyosin complex slides back to cover myosin-binding sites. Actin filaments slide, returning to relaxed position.

Motor unit

Consists of one motor neuron plus all the skeletal muscle fibers it stimulates; a single motor neuron makes contact with an average of 150 muscle fibers.

Contract the function of contractile and regulatory proteins

Contractile proteins (myosin and actin) generate force during contraction; regulatory proteins (troponin and tropomyosin) help switch contraction on and off.

Myofibril

Contractile units. Long contractile organelles inside sarcoplasm; extend length of muscle fiber. Arrangement of filaments gives striated appearance. *Contain overlapping thick and thin filaments.

Sarcoplasm

Cytoplasm of muscle fiber; contains organelles. Contains glycogen (glucose storage molecule) and myoglobin (oxygen-binding red protein)

Muscle hypertrophy

Dramatic muscle growth that occurs by enlargement of existing muscle fibers; occurs after birth.

Synaptic end bulb

End of the motor neuron (Axon Terminal) divides into a cluster of synaptic end bulbs; Synaptic vesicles contain acetylcholine (ACh), the neurotransmitter released at the NMJ.

Motor end plate

Folded region of sarcolemma opposite synaptic bulb increases surface area; abundant acetylcholine receptors (bind to ACh).

Distinguish between isotonic and isometric contractions

Isotonic contraction: Isotonic contractions involve a change in muscle length without a change in its tension. There are two types: 1) Concentric isotonic contractions occur when the muscle shortens 2) Eccentric isotonic contractions occur when the muscle lengthens Isometric contraction: Occurs when the load equals or exceeds the muscle tension, and the muscle does not lengthen or shorten.

Muscular dystrophy

Refers to a group of inherited muscle-destroying diseases that cause progressive degeneration of skeletal muscle fibers. Muscle fibers atrophy and degenerate; muscles enlarge due to fat and connective tissue deposits. No cure. Prednisone improves muscle strength and function. Myoblast transfer therapy disappointing. Coaxing dystrophic muscles to produce more utrophin (protein similar to dystrophin) successful in mice. Viral gene therapy and infusion of stem cells with correct dystrophin genes show promise.

Oxygen debt

Refers to added oxygen, over and above the oxygen consumed at rest, that is taken into the body after exercise. Heavy breathing after prolonged muscle activity helps to repay the oxygen debt, more accurately referred to as recovery oxygen uptake. This increased oxygen intake helps to restore metabolic conditions to the resting level by (1) conversion of lactic acid back to glycogen, (2) resynthesis of creatine phosphate and ATP, and (3) replacement of oxygen removed from myoglobin in muscle fibers.

The sliding filament mechanism

Refers to the myosin heads that attach to and "walk" along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line. As a result, the thin filaments slide inward and meet at the center of a sarcomere; the Z discs come closer together, and the sarcomere shortens.

Excitation-contraction coupling

Refers to the steps that connect excitation (a muscle action potential propagating along the sarcolemma and into the T tubules) to contraction (sliding of the filaments). An increase in Ca2+ concentration in the sarcoplasm starts muscle contraction; a decrease stops it. -Action Potential (AP) propagated along sarcomere to T tubules -Voltage-sensitive proteins stimulate Ca2+ release from sarcoplasmic reticulum (SR); Relase Ca2+ into sarcoplasm; bind to troponin, moving tropomyosin, and exposing myosin-binding sites on actin.

Action Potential as metaphor for "Flushing Toilet"

Resting sarcolemma is polarized by the voltage across membrane; action potential cause by changes in electrical charges Occurs in three steps: 1) End plate potential (local depolarization) Ach binding open chemically gated ion channels. Simultaneous diffusion of Na+ (inward) and K+ (outward); Sodium-Potassium Pump. More Na+ diffuses in, so sarcolemma is less negative. Local depolarization = end plate potential. 2) Depolarization (generation and propagation of an action potential [AP]) End plate potential spreads to adjacent membrane areas. Voltage-gated Na+ channels open. Na+ influx decreases membrane voltage toward critical voltage called threshold (the point at which the toilet flushes). Once initiated, is unstoppable - leads to muscle fiber contraction. AP spread across sarcolemma - leads to voltage-gated Na+ channels open in adjacent patch, causing it to depolarize to threshold. 3) Repolarization (restoring electrical conditions to RMP) Na+ channels close and voltage-gated K+ channels open. K+ efflux rapidly restores resting polarity. Fiber cannot be stimulated - in refractory period until repolarization complete (like how you can't flush the toilet right after doing it). Ionic conditions of resting state restored by Na+-K+ pump (Sodium-Potassium Pump).

Myosin-binding site

Site on an actin molecule where a myosin head can attach.

Define the two abnormal contractions of skeletal muscles, cramp and spasm

Spasm: A sudden involuntary contraction of a single muscle in a large group of muscles. Cramp: A painful spasmodic contraction. * Cramps may be caused by inadequate blood flow to muscles, overuse of a muscle, dehydration, injury, holding a position for prolonged periods, and low blood levels of electrolytes, such as potassium.

Twitch contraction

The brief contraction of all the muscle fibers in a motor unit in response to a single impulse in its motor neuron

Aponeurosis

The connective tissue layers (epimysium, perimysium, and endomysium) may extend together beyond the muscle fibers to form a broad, flat sheet called an aponeurosis that attaches muscle to another muscle or bone. Ex: Epicranial aponeurosis between frontal and occipital bellies of occipitofrontalis muscle.

Tendon

The connective tissue layers (epimysium, perimysium, and endomysium) may extend together beyond the muscle fibers to form a ropelike tendon that attaches a muscle to the periosteum of a bone. Ex: Achilles tendon attaches gastrocnemius muscle of calf to calceneus of tarsus.

Axon Terminal

The end of a motor neuron axon (at the NMJ)

Muscle fatigue

The inability of muscle to contract forcefully after prolonged activity. Factors that contribute are depletion of creatine phosphate, insufficient oxygen, depletion of glycogen and other nutrients, buildup of lactic acid and ADP, and failure of impulses in the motor neuron to release enough acetylcholine (ACh). Before muscle fatigue occurs, a person may experience tiredlness and desire to cease activity (central fatigue).

Myosin

The main protein component of thick filaments; a contractile protein that pushes or pulls various cellular structures to achieve movement by converting the chemical energy in ATP to mechanical energy of motion, that is, the production of force. *Has a distinct head and tail end. Head end is also called a cross bridge and plays and important role in muscle contraction.

Actin

The main protein component of thin filaments; individual actin molecules join to form a thin filament that is twisted into a helix; each has a myosin-binding site, where a myosin head can attach.

Motor Neurons

The neurons that stimulate muscle fibers to contract

Acetylcholine (Ach)

The neurotransmitter released at the Neuromuscular Junction (NMJ).

Sarcolemma

The plasma membrane of a muscle fiber; allows for travel of muscle action potentials

Neuromuscular Junction (NMJ)

The synapse between a motor neuron and a skeletal muscle fiber.

Name the 2 Regulatory proteins

Thin filaments also contain smaller amounts of two regulatory proteins: Tropomyosin and Troponin. Tropomyosin and Troponin; help switch the contraction process on and off. In relaxed muscle, myosin is blocked from binding to actin because strands of tropomyosin cover the myosin-binding sites on actin. The tropomyosin strand, in turn, is held in place by troponin.

Latent Period

Time between AP initiation and beginning of contraction; time when E-C coupling events occur.

Name the 2 structural proteins

Titin and Dystrophin

T tubule

Transverse tubule; Tiny tunnel-like invaginations of the sarcolemma extend in toward the center of each muscle fiber. Open the cell's exterior, thus filled with interstitial fluid; also allow for travel of muscle action potentials

Z disc

Zigzag shaped regions of dense protein material that separate one sarcomere from the next.

Role of Calcium (Ca2+) in Contraction

• At low intracellular Ca2+ concentration -Tropomyosin blocks active sites on actin -Myosin heads cannot attach to actin' -Muscle fiber relaxed • At higher intracellular Ca2+ concentrations -Ca2+ binds to troponin; Troponin changes shape and moves tropomyosin away from myosin-binding sites; Myosin heads bind to actin, causing sarcomere shortening and muscle contraction -When nervous stimulation ceases, Ca2+ pumped back in SR and contraction ends

Cross Bridge Cycle

• Continues as long as Ca2+ signal and adequate ATP present • Cross bridge formation - high-energy myosin head attaches to thin filament • Working (power) stroke - myosin head pivots and pulls thin filament toward M line • Cross bridge detachment - ATP attached to myosin head and cross bridge detaches • "Cocking" of myosin head - energy from hydrolysis of ATP cocks myosin head into high-energy state

Describe the properties of muscle tissue

• Electrical excitability Produce electrical signals called action potentials (impulses) in response to certain stimuli; a property of both muscle cells and neurons. • Contractility Ablility to shorten forcefully when stimulated, generating tension. • Extensibility Ability to stretch within limits without being damaged • Elasticity Ability to return to original length after contraction or extension

Three layers of connective tissue extend from the fascia to further protect and strengthen skeletal muscle

• Epimysium: The outermost layer of dense connective tissue, encircles the entire muscle. • Permysium: Dense irregular connective tissue surrounds groups of 10 to 100 or more muscle fibers (cells), separating them into bundles called fascicles. • Endomysium: Penetrates the interior of each fascicle and separates individual muscle fibers from one another; a thin sheath of areolar connective tissue.

Describe the functions of muscle tissue

• Produces body movements Integrated function of skeletal muscles with bones and joints. • Stabilizes body positions Skeletal muscle contractions without movement (stabilize joints and maintain body posture) • Moves substances within the body All three kinds of muscles as part of different organ systems • Generates heat Involuntary shivering of skeletal muscles helps maintain temperature homeostasis; as muscles contract they give off heat.