Ch 8 Muscle Physiology

Major Functions of Muscle Tissue 1. Movement a. Skeletal muscle (attached to bone) - walk, run, write, play the piano, grab and

manipulate objects b. Cardiac muscle (wall of heart) - pump blood through thousands of miles of blood vessels c. Smooth or visceral muscle (walls of internal organs) – e.g., movement of food through

the gut and the contraction of smooth muscle within the urinary bladder 2. Thermogenesis - Generation of body heat. a. Resting Body Temperature 1. Even at rest, the skeletal muscle cells (around 40% of body mass) of the body

generate up to around 70% percent of the resting body temperature; maintaining one’s body temperature is essential for the proper functioning of cellular enzymes

2. synthesis of ATP by cellular respiration generates heat that can be used to maintain body temperature (37C, 98.6F). Only about 40% of the energy in the broken bonds of glucose and other fuel substrates is used to make ATP; the other 60% is liberated as cellular heat that is picked up by the bloodstream

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b. Shivering - Mechanisms within the body that respond to cold temperatures trigger involuntary contractions of skeletal muscle (increased muscle tone)

c. Exercise - Excess heat generated by active skeletal muscle must be dissipated by sweating.

d. Nonshivering thermogenesis: Thermoregulatory mechanisms also control temperature by regulating the release of epinephrine and thyroxin which affect the basal metabolic rate (nonshivering thermogenesis).

3 Types of Muscles 1. Skeletal - Voluntary, striated muscle attached to the bones of the skeleton. Over 600

skeletal muscles in the body. Skeletal muscle is the meat of animals. a. in most vertebrates, muscle makes up the largest group of tissues in the body b. Human body weight made of skeletal muscle: 40% in males (testo effect) and 32% in

females c. smooth and cardiac muscle make up another 10% of body weight 2. Cardiac - involuntary, striated muscle found only in the heart. 3. Smooth - involuntary, nonstriated muscle associated with visceral organs (e.g. uterus, blood

vessels, urinary bladder, gut wall) Structure of Skeletal Muscle 1. Skeletal muscles are organs that consist of muscle cells, connective tissue, nerves and

blood vessels. Organs are structures in the body that consist of 2 or more tissue types. a. blood vessels and nerves run within the connective tissue b. connective tissues of skeletal muscle 1. Epimysium – dense irregular (fibrous) connective tissue that is continuous with

tendons and aponeuroses; the epimysium surrounds the entire muscle 2. Perimysium a. dense irregular connective tissue that is vascularized; contains nerves b. the perimysium organizes the skeletal muscle cells into fascicles containing 10 to

100 muscle cells 3. endomysium – areolar (loose) connective tissue around individual muscle cells 2. Fiber-like cells a. Cells are parallel to one another and run the length of the muscle. As a result, skeletal

muscle cells are described as long and fiber-like. Typically the term muscle cell is replaced with muscle fiber.

b. Skeletal muscle cells are the largest or bulkiest cells in the body, although some neurons are longer. Diameter of skeletal muscle is up to 10x greater than most cells (usually about 10-100 μm). Skeletal muscle cells may be 10” or so in length and visible to the naked eye. A typical muscle cell is about 100 um in diameter and about 2-4 inches in length.

2. Muscle Attachments a. skeletal muscles attach to bones by way of (1) tendons (cord-like attachement) and (2)

aponeuroses (sheet-like attachment) b. muscle contraction pulls on tendons that move bones, hence body parts

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Muscle Origin Insertion Action Deltoid (shoulder) Scapula and clavicle

(aponeurosis) Humerus (tendon)

Abduct arm (lift to side)

3. Sarcolemma and Sarcoplasm - Special terms used by muscle physiologists (sarco – muscle) a. Sarcolemma - plasma or outer membrane of muscle cells b. Sarcoplasm - cytoplasm of muscle fibers 1. Large numbers of mitochondria (1000's per cell) and rich in glycogen (2% of muscle

is glycogen), a carbohydrate that is broken down to glucose during exercise to make ATP

a. glycogenesis (stimulated by INS)makes glycogen at rest and glycogenolysis (stimulated by Epi) breaks it down to glucose during exercise

b. glycogen is a glucose-storage molecule. Skeletal can store up to 400 gms of glycogen (0.8 lb).

c. Glycogen molecules consists of multibranched chains with up to 30,000 glucose molecules

2. Myofibrils a. contractile fibers made of protein b. 100’s to 1000’s of myofibrils in each cell c. account for up to 90% of cell’s total volume d. meat is very high in protein (it also contains glycogen and fat) 3. myoglobin a. O2-storage protein inside muscle cell (at rest some of the O2 going into muscle

cells binds to myoglobin). Found in sarcoplasm b. reddish in color and gives skeletal muscle its red color (the color of red meat) c. during exercise, myoglobin releases its O2 for use by the cell to make ATP by

cellular respiration d. myoglobin is similar to Hb, but only 1 Fe-containing heme group/myoglobin e. the breakdown of myoglobin when meat is cooked produces a brown color f. myoglobin content varies between animals. White meat of chicken is 0.05%

myoglobin, whereas the red meat of cows (beef) is 1.5-2% myoglobin.

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4. Transverse or T-tubules (transverse to the long axis of the cell) a. Invaginations that extend from the sarcolemma down into the interior of the cell and

are filled with interstitial fluid b. the terminal cisterns (lateral sacs) of the sarcoplasmic reticulum contact the T-tubules c. Function: Help to conduct the muscle impulse deep into the interior of the muscle cell

to release calcium ions from the cisterns 5. Multinucleate – each skeletal muscle cell arises during embryonic development from the

fusion of a 100 or more small mesodermal cells called myoblasts. a. The many nuclei of skeletal muscle cells are found just beneath the sarcolemma. There

may be as many as 35 nuclei per millimeter of muscle fiber. Some muscle fibers are over 30 cm or 1 foot in length. Thus some skeletal muscle cells have 1000's of nuclei.

b. Some embryonic cells persist within the belly of an adult muscle as satellite cells. These cells retain the ability to divide by mitosis to form myoblasts that fuse together to form functional muscle fibers to repair minor damage to muscle. When a muscle is injured, the satellite stem cells produce a limited number of new myoblasts. This mechanism is not adequate to repair extensive damage

6. Sarcoplasmic reticulum (SR) a. special type of smooth endoplasmic reticulum (SER) that contains sacs called terminal

cisterns or lateral sacs that serve as calcium-storage areas. Web-like network of interconnected tubes that wrap around the myofibrils.

b. Separate segments of SR run between the T-tubules. The ends of each segment expand to form sac-like regions called terminal cisterns (lateral sacs) that butt against the T-Tubules.

c. Calcium ion active transport pumps (Ca2+-ATPase pumps) that move calcium ions from the sarcoplasm into the cisterns. The cistern concentration of calcium in a resting muscle cell is 10,000 times higher than the sarcoplasm. Active transport pumps move

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substances against their concentration gradients. Normally, the sarcoplasm contains very low amounts of calcium.

d. Nerve impulses traveling down T tubules trigger proteins in membrane of T tubule to undergo a shape change that opens voltage-gated calcium channels in lateral sacs. As a result calcium ions are released from the cisterns into the sarcoplasm.

e. Essential Role of Calcium Ions - Release of calcium ions into the sarcoplasm triggers or initiates all the intracellular events of muscle contraction

f. if calcium ions accumulate inside the sarcoplasm of a resting muscle cell they bind to phosphate ions released by ATP hydrolysis to form crystals of hydroxyapatite. This would eventually kill the cell. Thus, they must be stored inside the cisterns of the sarcoplasmic reticulum.

g. triads – lateral sacs on either side of T-tubules plus the T-tubule

Nerve impulse coming down T tubule triggers Ca2+ release from cisterns.

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Motor Unit Concept 1. Motor Unit - One somatic motor neuron and all the skeletal muscle fibers it innervates. a. One motor neuron may have many axonal branches at its distal end b. Each axonal branch stimulates one muscle fiber c. On average, there are about 200 muscle fibers per motor unit (some muscles may have

as few as 3-6 fibers - fine movement of hand - or as many as 2000 fibers per motor unit - gross motor movement such as the biceps brachii). The gastrocnemius, for example, has about 1000 muscle fibers per somatic motor neuron.

2. A given skeletal muscle generally consists of thousands of muscle cells and is typically stimulated by many motor units. In fact, there may be hundreds to thousands of motor units innervating a skeletal muscle. A typical muscle has 100-1000 motor units

3. A thought within a cerebral motor center is conducted as a nerve impulse to stimulate one or more motor neurons. The motor neurons, in turn, send impulses out their axon branches to stimulate some or all of the muscle fibers in a muscle.

a. primary motor cortex in precentral gyrus of frontal lobe of cerebrum contains cell bodies of pyramidal cells in gray matter

b. pyramidal cell axons extend into corticospinal (pyramidal) tracts that decussate in the medulla of the brainstem and travel down the white matter of the spinal cord

c. axons stimulate somatic motor neurons within the spinal cord ventral horns (gray matter)

d. axons of somatic motor neurons go out to stimulate skeletal muscle cells

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Neuromuscular (myoneural) Junction (NMJ) 1. NMJ - point of contact between an axonal ending of a motor neuron and the motor end

plate of a muscle fiber. Special type of synapse between neuron and effector cell 2. Axon terminal (axonal ending) - at the end of one of the many branches of a single motor

neuron; axon terminus is divided into a cluster of synaptic end bulbs a. contains synaptic vesicles with a neurotransmitter inside called acetylcholine (ACh) b. ACh is secreted from the vesicles to the outside of the cell by exocytosis when a nerve

impulse arrives at the axonal ending c. each vesicle contains about 10,000 ACh molecules 3. Motor End Plate (MEP) - part of the sarcolemma below the axonal ending a. contains about 50 million ACh receptors (integral membrane protein) per motor end

plate that bind to ACh b. the binding of ACh to the receptors triggers a muscle impulse (series of self-propagating

action potentials) that spreads along the sarcolemma and down into the T-tubules. This causes the release of calcium ions from the terminal cisterns of the sarcoplasmic reticulum. This ultimately stimulates a muscle to contract.

c. Acetylcholinesterase (AChE) – A hydrolytic enzyme in the MEP that degrades ACh to acetate and choline.

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4. Synaptic Cleft - Interstitial-fluid filled region between the axonal ending and the motor end plate. ACh diffuses across the cleft after its release from the axonal ending before binding to end-plate receptors.

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Myofibrils 1. Contractile rods made of protein that lie parallel to one another in the sarcoplasm and

extend the entire length of the cell. Myofibrils are anchored to each end of the cell. 2. There are 100's to 1000's of myofibrils within a single cell. They account for up to 90

percent of cellular volume and because protein molecules are heavy, they are responsible for the heavy weight of skeletal muscle

3. Made of three types of myofilaments (also called filaments): thick (myosin), thin (actin), and elastic (titin)

Structure of thick filament. 2 myosin heads/myosin. Myosin heads also known as cross bridges

a. Thick or myosin myofilaments 1. Made of the protein myosin. a. Each thick filament has a core of an elastic protein called titin. b. Titin continues across the I band and attaches the thick filament at either end to

the Z discs 2. They are twice as thick (16 nm) as the thin filaments (8 nm) and each consists of

about 300 myosin molecules; 3. each myosin is shaped like a golf club in that it has a long “shaft” and a club-like

head called a myosin head. There are 2 myosin heads/myosin. a. the 2 myosin heads at the end of each myosin molecule are also known as cross

bridges. The myosin heads become cross bridges as they attach to the actin filaments by rigor bonds during muscle contraction

b. myosin heads of the thick filaments are angled away from the H-zone of the A band in a resting muscle cell

c. myosin heads bind to the thin filaments to form a rigor bond when a muscle contracts

d. each thick filament contains about 200 myosin molecules

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4. 2 sites on myosin head a. Actin binding site (binds to actin myofilaments) b. Myosin ATPase site (splits ATP by hydrolysis) 5. Myosin ATPase – is an enzyme that is embedded in the myosin head (it is part of the

myosin head); Myosin ATPase splits ATP (hydrolysis) to ADP and a phosphate group a. ATP hydrolysis during muscle contraction causes the following to occur 1. breaks rigor bond (bond between the myosin head and thin filament that

forms when a muscle contracts and the sarcomeres shorten) 2. recharges the myosin heads so they swivel back into their resting position

(only charged myosin heads are capable of interacting with actin filaments)(analogous to cocking the hammer on a pistol)

b. enzyme is only active when the myosin head is attached to actin filament c. After ATP hydrolysis, the ADP and phosphate group remain attached to the

charged myosin head 6. myosin heads at rest are “charged” with ADP and Pi attached to them. a. myosin heads are “cocked” and contain potential energy b. the potential energy is released when the myosin heads bind to thin filament

and swivel forward into a power stroke c. the energy released from ATP hydrolysis is transferred to and stored in the

“cocked” myosin heads (cross bridges).

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Structure of thin filament

b. Thin or actin myofilaments 1. Made primarily of the protein actin. Actin is the most abundant protein in muscle

followed by myosin and titin. There are more thin filaments than thick filaments. 2. G-actin molecules are spherical (globular) and they polymerize into long bead-like

strands of F-actin (filamentous). Each actin myofilament consists of 2 F-actin strands twisted together

3. each G actin molecule within the F actin chain has a binding site for a myosin cross bridge

4. Two other proteins associate with the actin filaments: Troponin and tropomyosin; these regulatory proteins function to prevent contraction until a muscle is stimulated

a. tropomyosin is a long, thread-like molecule that covers the actin binding sites that bind to myosin cross bridges

1. Spirals around the thin filaments and covers the sites that bind to the myosin heads or cross bridges of the thick filaments. Active sites are called myosin binding site (MBS)

2. The charged myosin heads cannot bind to the thin filaments when the sites are covered by tropomyosin

3. each tropomyosin covers about 10 binding sites on the actin filament b. troponin 1. Attached to the tropomyosin molecules

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2. When troponin binds to calcium ions, it undergoes a conformational change that shifts the tropomyosin off the binding sites

3. Once the binding sites are available, charged myosin heads bind to them 4. the calcium ions that bond to troponin are released from the cisterns of the

sarcoplasmic reticulum when a muscle cell is stimulated to contract c. Elastic myofilament 1. Made of the protein titin (the largest protein known in any organism with 34,350

aa’s in humans). Also known as connectin. Titin is encoded by the TTN gene on the long arm of c’some 2.

2. This third type of filament anchors the thick filaments to the Z-discs and helps to stabilize the position of the thick filaments (keeps them centered in the middle of the sarcomere).

3. The elastic filament runs the length of the thick filament (forming its core) to attach to the M line which is down the middle of the H-zone of the A band. Myomesin is the primary protein of the M line.

4. The elastic filament holds the thick filaments in place and helps the muscle spring back into shape after being stretched (by contraction of antagonist).

5. Titin undergoes elastic recoil like a spring or rubber band when stretched d. actin and myosin are not unique to muscle cells. They occur in all cells where they

function in cell movements, mitosis, and intracellular transport. Dystrophin is another important structural protein that attaches the thin filaments to proteins embedded in the sarcolemma.

4. Sarcomeres a. myofibrils are organized into a series of repeating units called sarcomeres. b. The arrangement of sarcomeres within myofibrils gives striated muscle (skeletal and

cardiac) its striped appearance. In smooth muscle, the myofibrils are not arranged in an orderly parallel manner, hence no striations.

c. there are thousands of sarcomeres per myofibril (average is about 10,000 sarcomeres per myfibril)

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Sarcomere - smallest contractile unit 1. a sarcomere is the region between two neighboring Z-lines (Z-discs) that form the borders

at the end of each sarcomere; Z-discs are made of a protein called alpha-actinin. Titin (elastic filament) and actin (thin filament) bind to α-actinin of the Z disc

2. Z-discs are made of α-actinin and hold sarcomeres together in a long chain 3. Bands and Zones - as they appear in a relaxed sarcomere a. A (dark or anisotropic) Band - anisotropic means it can polarize visible light 1. contains both thick and thin filaments 2. thick filaments overlap the thin filaments 3. H-zone (fully visible only when muscle in relaxed state) a. zone in the middle of the A band; M line in middle of H zone b. contains only myosin at the center of the A band of a resting sarcomere c. shrinks or disappears when a sarcomere contracts 4. M line (M-middle) is a set of supporting proteins down the middle of the H zone that

holds the thick filaments together into a vertical stack; the M line stabilizes the myosin filaments in the middle of the sarcomere

b. I (light or isotropic) Band - will not polarize light 1. contains thin filaments and a Z-disc 2. Z-disc passes through the center of each I band. (Z discs are made of alpha-actinin).

Z discs define the edges of sarcomeres 3. the thin filaments are found in both the A and the I bands, whereas the thick

filaments are found in only the A band 4. during muscle contraction, the I bands decrease in width as the thin filaments slide

past the thick filaments and the sarcomeres shorten 4. One muscle cell may contain 1000’s of myofibrils and each myofibril contains 1000’s of

sarcomeres

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Changes in banding pattern during shortening

Sliding Filament Mechanism of Muscle Contraction 1. When a muscle contracts, the thin myofilaments within a sarcomere slide past the

stationary thick filaments 2. The sliding of the thin myofilaments causes the sarcomeres to shorten which ultimately

causes the myofibrils of a muscle fiber to shorten or contract. 3. Since the myofibrils are attached to opposite ends of the muscle fiber, the cell contracts or

shortens as does the whole muscle. 4. muscles usually contract (or shorten) by 35% or more of their resting length 5. the direction of contraction is from the muscle insertion towards the fixed origin where the

muscle is anchored 6. the H-zone and the I band shrink when sarcomeres shorten as a muscle contracts. The H

zone may disappear if the thin filaments meet in the middle of the A band 7. the thick and thin filaments do not change length during sarcomere shortening 8. the width of the A band stays the same during contraction as the sarcomeres shorten

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Excitation-Contraction Coupling

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Excitation-Contraction Coupling Phrase used to describe the series of events that begins with a nerve impulse on a motor neuron arriving at the neuromuscular junction and ends with the contraction of the muscle fiber. 1. Excitation of Motor Unit: Nerve impulse moves along all the axonal branches of a somatic

motor neuron and arrives at the neuromuscular junctions (NMJ). 2. Acetylcholine Release: In response to the arrival of the nerve impulse at the neuromuscular

junction, calcium ions flux into the axon ending and trigger the release of the neurotransmitter ACh from synaptic vesicles in the axon terminus into the synaptic cleft by exocytosis through the axolemma;

3. ACh-Receptor Binding on the Motor End Plate: ACh diffuses across the fluid-filled cleft and binds to ACh receptors embedded within the motor end plate of the sarcolemma

a. muscle impulses occur on the sarcolemma outside of the MEP 1. voltage-gated Na+ channels and voltage-gated K+ channels are found along the

entire length of the sarcolemma, but not at the MEP 3. the Motor End Plate (MEP) contains the Neurotransmitter-Receptor gated ion

channels 4. the muscle impulse (like a nerve impulse) is a series of self-propagating AP’s that

begins at the edges of the motor end plate 5. Na+/K+ Active Transport Pumps are found at the MEP and along the entire length of

the sarcolemma b. Ach binding to a receptor opens non-specific gated ion channels in the MEP that allow

both Na+ and K+ to move through along their df gradients (Na+ in, K+ out) c. more Na+ move through the channels then K+ ions, hence the major effect of ion

movement is a Na+ influx that depolarizes the membrane (graded or localized transmembrane potential change at the MEP)

d. Na+ coming in diffuse away from the MEP in both directions to adjacent sections of the sarcolemma just outside the MEP and bring those sections of the sarcolemma to Threshold Potential (TP) (-55 mV).

e. TP causes the voltage-gated Na+ channels on the sarcolemma outside the MEP to open causing Na+ to rapidly df in, thus triggering the first AP that self-propagates as a muscle impulse away from the MEP to both ends of the muscle cell where it dies out

4. Muscle Impulse: The binding of ACh to its receptor initiates a muscle impulse (similar to a nerve impulse) that spreads (self-propagates as a series of action potentials) across the sarcolemma and down into the transverse tubules to the interior of the muscle fiber.

5. Calcium Ion Release from the Cisterns of the Sarcoplasmic Reticulum a. Once in the T-tubules, the muscle impulse stimulates the terminal cisterns (lateral sacs)

of the sarcoplasmic reticulum to open voltage-gated Ca2+ channels that allow the release of calcium ions into the sarcoplasm around the myofibrils (calcium ions diffuse through the channels as they open).

b. Massive movement of calcium ions out of cisterns within 1 msec. 6. Calcium Ions Bind to Troponin: Calcium ions bind to troponin on the thin filaments. The

binding of calcium to troponin causes the troponin to change shape. As troponin changes

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shape, it pulls the long, thread-like tropomyosin molecules off the binding sites on the actin filaments. Calcium ions are the intracellular trigger for muscle contraction.

7. Rigor bond Formation and Power Stroke a. Once the binding sites on the actin filaments are exposed, the charged myosin heads

(cross bridges with ADP and Pi attached) bind to the thin filaments to form a rigor bond. b. The rigor bond is important to prevent the back slippage of the myofilaments so that the

muscle cell can increase the intensity of the contraction and maintain it long enough to do muscular work.

8. Power Stroke a. Once the rigor bond forms, the charged myosin heads swivel forward by 45o in what is

known as the power stroke. The myosin heads swivel as if hinged sending the actin filaments towards the middle of the H zone (center of the sarcomere)

b. Once the rigor bond forms and the myosin head swivels, ADP and inorganic phosphate (Pi) fall off the myosin head after the power stroke

c. a single power stroke moves the thin filament by one micron (micrometer, 10-6 m) which is only a small fraction of the distance that the thin filaments need to travel to contract a muscle

d. repeated power strokes occur at cross bridges that swivel asynchronously. e. ATP breaks the rigor bond by attaching to the myosin head. ATPase activity in the

myosin head splits the ATP to ADP and Pi which remain attached to the cross bridge f. The myosin head is now energized or charged (has potential energy) and returns to its

cocked position. g. Power strokes, rigor bonds and recharging of the myosin heads continues so long as

calcium ions are available in the sarcoplasm 9. Sliding Filament Mechanism: As the myosin heads bind they form cross bridges and swivel

forward, the thin filaments slide past the stationary thick filaments and all of the sarcomeres of a myofibril shorten, hence the muscle fiber contracts

a. Z-lines brought closer together b. H-zone (in middle of A Band) disappears, but the A band stays the same length c. I band decreases in width d. shortening of all the sarcomeres within a myofibril causes the muscle to contract which

pulls the insertion end of the fiber towards the immoveable origin e. A skeletal muscle may shorten up to 30-35 percent of its total length (even up to 60%) f. the myofibrils are attached to the ends of the muscle cell and to the sarcolemma by a

cytoplasmic protein called dystrophin; thus shortening of the myofibrils causes the muscle cell to shorten from the insertion towards the origin.

g. at any given moment during contraction about half of the myosin heads are bound to the thin filaments and the other half are extending forward to grasp the thin filament further down. The myosin heads power stroke sequentially rather than all at once.

10. Sustain a Contraction a. If AP’s continue to arrive at the NMJ, then high Ca2+ levels remain in the sarcoplasm. b. As a result, the myosin heads will continue to asynchronously bind and release from the

thin filaments to sustain a contraction

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11. Relax the muscle a. If the AP’s stop arriving at the NMJ, then the Ca2+ are taken up by the cisterns of the SR

which decreases the Ca2+ in the sarcoplasm. b. With no Ca2+ on troponin, the tropomyosin moves back to its original position and

blocks the myosin cross bridge binding sites on actin c. The contraction stops and the thin filaments passively slide back to their original relaxed

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Cross bridge cycle

Three Events that Lead to Relaxation 1. Acetylcholinesterase (AChE) a. Hydrolytic enzyme embedded within motor end plate of the neuromuscular junction b. Function 1. Hydrolyze acetylcholine in the synaptic cleft to acetate and choline 2. This removes the neurotransmitter from the neuromuscular junction, thus removing

the stimulus for the muscle impulse

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3. One AChE destroys about 25,000 ACh molecules per second. The products are then taken up by the axonal ending for resynthesis to ACh (recycling)

2. ATP Breaks the Rigor Bond: ATP binds to the myosin head which a. breaks the rigor bond as the ATP is hydrolyzed to ADP + Pi by ATPase which allows the

filaments to slide back to their resting positions; ATPase is part of the myosin head. b. recharges the myosin head as it returns to its resting position ("cocking a trigger") - this

adds potential energy to the system to allow for another contraction. ADP and P remain attached to the charged myosin head.

c. the ADP and Pi stay attached to the charged myosin head. ADP and Pi are released from the myosin head as it forms a cross bridge and undergoes the power stroke. ADP and Pi block the active site of ATPase in a charged myosin head

d. ATP is essential for muscle contraction (indirectly) and relaxation (directly) 3. Active Transport of Calcium Ions Out of Sarcoplasm a. As calcium ions fall off troponin they drift back towards the cisterns where they are are

actively transported out of the sarcoplasm and resequestered into the terminal cisterns of the sarcoplasmic reticulum.

b There are calcium ion active transport pumps in the membranes of the cisterns that are powered by ATP hydrolysis (Ca2+ ATPase pumps).

c. This causes calcium ions to fall off the troponin which allows tropomyosin to slide back over the binding sites on the thin filaments to prevent another rigor bond from forming. Once covered, the binding sites are unavailable to the charged myosin heads.

d. relaxation occurs as the Ca2+ is returned to the terminal cisterns of the SR

Hrs after Death Stages of Rigor Mortis 3-4 hrs Muscles begin to stiffen 12 hrs Peak rigidity, then muscles start to soften 48-60 Rigidity gone

Rigor Mortis 1. Muscles stiffen after death. 2. This shows that ATP is necessary for the breaking of rigor bonds (cross-bridge or myosin

head detachment from the actin filaments requires ATP hydrolysis). It is also necessary for pumping Ca2+ into the terminal cisterns

3. when die, no blood circulates to skeletal muscle. Without oxygen and nutrients, cells of the body cannot make ATP (enough ATP in cells at death to relax any muscles that were contracting at time of death)

a. without ATP, the calcium ion active transport pumps stop working b. within a few hours of death, the calcium ions leak out of the cisterns into the

sarcoplasm and can’t be pumped back in c. as calcium ions increase in the sarcoplasm and bind to troponin, tropomyosin shifts off

of the binding sites on the actin d. ATP charged myosin heads (charged with ATP before death) bind to actin to form rigor

bonds and undergo power strokes as they swivel forward e. muscle contracts and stiffens (this occurs 3 to 4 hours after death)

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f. since ATP is not made in a dead person’s cells, the rigor bonds can’t be broken g. once rigor mortis sets in, the muscles stiffen like a board. The effect lasts for 15 or so

hours h. about 24 hours after death, proteolytic enzymes leak out of lysosomes and they break

down the myofilaments – the rigor then subsides in a day or so 4. Muscles begin to stiffen 3 to 4 hours after death. Peak rigidity occurs at 12 hours, then

dissipates over the next 48 to 60 hours. Eventually the muscle softens as the proteins and other components of muscle tissue begin to breakdown.

5. Without a renewable supply of ATP within cells, the rigor bonds are maintained as they form, hence muscles remain in a contracted state

1. ATP is necessary to break the rigor bonds that form between the myosin heads and the thin filaments

2. ATP is necessary to operate the calcium ion active transport pumps. Without ATP, calcium ions leak out of the cisterns and remain in the sarcoplasm, hence the troponin-tropomyosin complex stays shifted off the thin filament binding sites

6. Forensic biologists investigate biological evidence associated with a crime (e.g., semen, DNA from hair, blood samples, seeds stuck to tires). They can determine the time of death by stage of rigor mortis and other factors.

Duration of ATP production

15 seconds 1 minute (30-60 seconds) Several hours

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Skeletal Muscle Metabolism Skeletal muscle cells make large amounts of ATP in order to sustain muscle contractions during exercise. ATP is generated by (1) cellular respiration and from (2) creatine phosphate (CP) Why is ATP needed for muscle contraction? 1. Myosin ATPase splits ATP to break rigor bonds after the power stroke and recharge the

myosin heads 2. Active transport (Ca2+ ATPase) of Ca2+ from sarcoplasm into the terminal cisterns so that a

muscle can relax involves the hydrolysis of ATP 3. very little ATP is stored in muscle cells. There is only enough ATP stored in a cell for a few

seconds of activity Oxidation-Reduction Reactions (Redox Rcns) 1. coupled set of reactions that exchange e’s between 2 substances a. oxidation – atom or molecule loses 1 or more e’s b. reduction – atom or molecule gains 1 or more e’s

2. in biological systems, e’s often move with H+ as H atoms

3. Cellular respiration involves a series of redox reactions

4. Electron carrier Coenzymes – non-protein organic chemicals that bind to and activate

enzymes (oxidoreductases). Derived from B vitamins (energy-boost vitamins) a. NAD+ (derived from niacin, Vit B3) is in oxidized form. It gains 2e’ and H+ to become

NADH (reduced form)

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b. FAD (derived from riboflavin, Vit B2) is in oxidized form. It gains 2e- and 2H+ to become FADH2 (reduced form)

c. Reduced conenzymes are e carriers that carry their electrons to the Electron Transport Chain where they are used to make ATP by oxidative phosphorylation

5. ADP/ATP movement into and out of mitochondrion a. ADP and ATP diffuse through open channels in the outer membrane b. ATP-ADP translocase (antiport) is in the inner membrane. It moves ADP into the matrix

and ATP into the inner membrane space by facilitated diffusion Cellular Respiration of Glucose Overview

Glucose + 6O2 → 6CO2 + 6H2O + around 32 ATP 1. Glycolysis occurs in the cytoplasm a. anaerobic process (rxns do not require O2) that occurs in the cytoplasm b. glucose is broken down to 2 pyruvate c. results in a net gain of 2 ATP and forms 2 NADH (reduced coenzymes which carry e’s to

the ETC’s in the mitochondria to make ATP). Coenzymes are non-protein organic chemicals that bind to and activate enzymes (NAD+ and FAD)

d. if there is enough O2 in the cell, then all of the pyruvate goes into the mitochondria 1. pyruvate freely diffuses across the outer mitochondrial membrane through channels

made of transmembrane proteins 2. pyruvate translocase in the inner membrane is a symport pump that moves both pyruvate

and H+ from the inner membrane space to the matrix

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2. Mitochondrial Stages a. all of the mitochondrial stages of cellular respiration require O2 b. if O2 is available, then pyruvate enters the mitochondrion and is broken down to CO2

and H2O as about 30 ATP form 1. most of the ATP gained by cellular respiration (about 95%) is formed in mitochondria 2. the CO2 that forms is a gaseous waste product of cellular respiration. 3. CO2 diffuses out of the cell, then into the blood. The blood takes it to the lung where

it is exhaled. c. if O2 is completely unavailable, then the mitochondrial stages of cellular respiration shut

down. This only happens in the cells of a dead person. There is always some O2 entering muscle cells at all times in living people although O2 can be limiting to a cell during exercise

d. CO2 is generated in the mitochondria by the oxidation of pyruvate and the Kreb’s Cycle e. most of the ATP (about 95%) that forms from the breakdown of glucose comes from the

mitochondrial stages of cellular respiration 1. glycolysis produces a net gain of 2 ATP per glucose 2. the mitochondria produce about 30 ATP per glucose f. Pathways in the Mitochondrion 1. oxidation of pyruvate (pyruvate to Acetyl CoA) 2. Kreb’s Cycle 3. Electron Transport Chain and chemiosmosis g. if insufficient O2 is available such as occurs during exercise, then some of the pyruvate

does not enter the mitochondrion and gets converted to lactic acid within the sarcoplasm

.

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Glucose + O2 -> 6 CO2 + 6 H2O + 32 ATP NADH and FADH2 are coenzymes. Coenzymes are organic chemicals that must bind to the active site of an enzyme before it can function. Some coenzymes are derived from vitamins. NAD is derived from Vitamin B3 (niacin) and FAD is derived from vitamin B2 (riboflavin). Chemiosmosis is the diffusion of ions across a semipermeable membrane (e.g., df of H+ across inner mitochondrial membrane to activate ATPase.

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Cellular Respiration ATP Synthesis Stage

Site

O2-requiring

CO2

ATP by SLP

Reduced coenzymes

(ETC) ATP by Ox P

Glycolysis Cyto Anaerobic 2 2 NADH 5 Ox of Pyr Mito Aerobic 2 2 NADH 5 Krebs Cycle Mito Aerobic 4 2 6 NADH

2 FADH2 15 3

Totals 6 4 28 SLP – substrate-level phosphorylation; Ox P – oxidative phosphorylation * Assumes 2.5 ATP/NADH and 1.5 ATP/FADH2; in some cells, the electrons from the NADH produced by glycolysis are passed to FAD inside the mitochondria which reduces total ATP yield Oxidative Phosphorylation (formation of ATP by the ETC and ATP synthase) Aerobic phase that occurs in the mitochondrion NADH and FADH2 pass their electrons to the molecules of the ETC As e’s go down the ETC they generate a proton gradient that activates an enzyme called

ATP Synthase ATP synthase catalyzes the reaction: ADP + Pi → ATP On average the electrons from every

o NADH result in the formation of around 2.5 ATP’s o FADH2 result in the formation of around 1.5 2 ATP

10 NADH e’s yield 25 ATP 2 FADH2 e’s yield 3 ATP Around 1 ATP/4.3 protons Chemiosmosis during cellular respiration is the diffusion of H+ through an ion channel

that activates ATP synthase to make ATP by ox P Total Gain of 32 ATP from Cellular Respiration of One Glucose Net gain of 4 ATP by substrate-level phosphorylation (2 ATP/glycolysis, 2 ATP/Krebs

cycle) 28 ATP by oxidative phosphorylation

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Anaerobic and aerobic ATP production

Three Pathways supply ATP in an exercising skeletal muscle CP and glycolysis can rapidly generate ATP for up to 2 minutes of strenuous exercise. Beyond two minutes, the intensity of exercise must decrease to some extent as oxidative phosphorylation supplies most of the ATP. Oxidative phosphorylation can generate ATP as rapidly as the other two pathways and requires breathing and cardiovascular changes 1. Phosphogen System (supplies ATP for 8-12 seconds)(Fast Energy System) – makes sure that

a person can start to exercise immediately a. 8-12 seconds: the Phosphogen system is a Fast Energy System that rapidly provides ATP

to cells for around 8-12 seconds. It consists of 2 energy-rich molecules with high-energy phosphate bonds: ATP and CP

1. 2 seconds: ATP within the cell (supports 2 seconds of activity) – muscle cells store very little ATP

2. 10 seconds: Creatine Phosphate (CP) inside the cell that provides ATP for around 10 seconds

b. Like ATP, CP contains a high-energy phosphate bond that can be given to ADP c. CP builds up in resting muscle cells by the reversible reaction below. Creatine in a

resting muscle cell reacts with ATP to form CP and ATP Creatine kinase

CP + ADP <-------------> Creatine + ATP d. creatine is made in the liver and kidneys from amino acids (glycine and arginine), then

transported by the blood to muscle cells 1. 95% of the creatine in the body is found in skeletal muscle cells (it is found in other

cells as well to include neurons) 2. there is no dietary requirement for creatine. The body makes what it needs. e. resting muscle cells can store 5x more CP than ATP f. thus, most of the energy within a muscle cell at rest is stored in the high energy

phosphate bonds of CP g. during exercise, ATP is broken down to ADP + inorganic phosphate (Pi)

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1. the ADP is rapidly converted to ATP by CP 2. this pathway provides a source of ATP for short bursts of activity lasting for about

8-12 seconds (weight lifting exercise, quick sprint) 3. during a short burst of activity, ATP levels stay constant while CP stores decline until

they are depleted 4. CP is used to replenish ATP in order to exercise immediately and intensely for

several seconds h. some people take creatine as a dietary supplement, but there is a limit to how much CP

muscle cells can hold 2. After 8-12 seconds for another 2 minutes: Glycogen – Lactic Acid System - Anaerobic

Glycolysis (up to 2 minutes)(Fast Energy System) a. when CP stores are gone (within 10 seconds of starting an exercise), the muscles turn

to glycolysis to make ATP rapidly b. glycolysis doesn’t make ATP as fast as the CP system, but it is still faster than the third

method (oxidative phosphorylation) c. as the phosphagen system is used up within the first 5-10 seconds of heavy exercise, the

muscles shift to anaerobic fermentation (glycogen-lactic acid system) until the cardiopulmonary system can catch up with oxygen demand of exercising muscle.

d. glycolysis can rapidly generate ATP anaerobically so that one can continue to exercise hard for up to 2 minutes

e. Anaerobic glycolysis depends on the rapid breakdown of muscle glycogen to glucose 1. muscle glycogen can supply glucose by glycogenolysis for around 40 minutes (maybe

several hours) before it is depleted Glycogen -> G6P (only 1 enzyme-mediated rcn so very fast)

G6P enters glycolysis as it forms 2. during the first hour of exercise most glucose comes from muscle glycogen. After

that more and more glucose comes from the blood. Muscles also use fatty acids as a fuel substrate

3. blood glucose levels are maintained by hepatic glycogenolysis (breakdown of glycogen in liver)

f. muscle cells are never fully anaerobic (lack O2 completely) unless someone is dead. 1. There is always some O2 supplied to muscle cells, but not always enough for all of

the pyruvate generated at the end of glycolysis to go into the mitochondria 2. glycolysis can form pyruvate faster than can be matched by the O2 supply needed

for all of it to enter mitochondria 3. there are limits to how much O2 can be delivered to muscle cells (breathing and

cardiovascular) 4. as a result, lactic acid builds up during anaerobic glycolysis (normal pH in cell is

around 7.1. It can drop to 6.4 during exercise) 5. lactic acid is formed by the reaction below in order to regenerate NAD+ for glycolysis

to keep running lactate dehydrogenase

pyruvic acid + NADH ---------------------- lactic acid + NAD+

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6. lactic acid build up in muscle cells leads to acidosis (decrease pH), fatigue and

soreness Lactic acid -> lactate + H+

7. during exercise, lactic acid leaves muscle cells and travels by way of the blood to the liver

8. after exercise, the liver converts lactic acid to glucose. This is an ATP-requiring process that contributes to the O2 debt (heavy breathing after exercise to provide extra O2 to repay the O2 debt that one accrues during exercise)

7. for short periods, glycolysis is so rapid that it can produce more ATP per unit time than the slower formation of ATP by oxidative phosphorylation

a. this forms pyruvic acid faster than it can be absorbed into the mitochondria if O2 is limiting leaving some pyruvic acid in the cytoplasm.

b. the pyruvic acid left in the cytoplasm is converted to lactic acid 8. the pH in the sarcoplasm of an exercising muscle can drop from around 7.1 to as low

as 6.4. This can distrupt enzyme activity in muscle cell 3. After 2 minutes to several hours: Mitochondrial or Aerobic Stages - Oxidative

phosphorylation (depends on glycolysis to form pyruvate)(sustained exercise for 2 minutes to several hours)(endurance activities). Mito production of ATP by aerobic stages of cellular respiration

a. glycolysis and the oxidative pathways in the mitochondria generate ATP when CP stores are gone

1. during light to moderate exercise (walking) the O2 supply matches the rate at which pyruvate forms, hence glycolysis doesn’t operate anaerobically and all of the pyruvate goes into the mitochondrion

2. if all of the pyruvate that forms goes into the mitochondria then lactic acid doesn’t form

3. if more pyruvate forms than the supply of O2 can handle, then some lactic acid forms

4. well-conditioned athletes deliver more oxygen to muscle cells than those that are not conditioned and form less lactic acid than an untrained individual

5. stored glycogen can supply glucose for cellular respiration for around 40 minutes b. oxidative phosphorylation supplies most of the ATP for activities lasting more than a

few minutes to several hours; this pathway makes ATP as pairs of e’s go down the electron transport chain in the inner mitochondrial membrane (cristae)

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c. this pathway can’t kick in immediately as is the case with CP and glycolysis. It takes a few minutes for the body to change the breathing rate (12 at rest to 20 during exercise) and induce cardiovascular changes (increase HR, SV, CO and BP) to increase O2 supply to exercising muscle cells

1. glycolysis in cytoplasm supplies 2 ATP per glucose 2. the Krebs cycle in mitochondrion makes 2 ATP per glucose 3. oxidative phosphorylation supplies around 28 ATP per glucose 4. oxidative phosphorylation generates ATP more slowly than glycolysis since there

are more steps involved d. this pathway is fueled by glucose and fatty acids. They are broken down to CO2 and

H2O as ATP is generated e. Activities that can be supported by oxidative phosphorylation without generating lactic

acid are called Aerobic Exercises f. Breathing and Cardiovascular changes during exercise to increase O2 delivery to

muscle cells. Exercise is a stressor that will stimulate release of Epi 1. Increase breathing rate and depth – bring more O2 into body 2. increase in heart rate and stroke volume (heart beats more forcefully to pump out

more blood) 3. vasodilation in muscle to increase blood supply (hence more glucose, fatty acids

and O2) 4. hemoglobin in RBC’s deliver more O2 to exercising muscle cells than resting muscle

cells (Bohr Effect) g. at peak exertion, the mitochondria may only provide about 1/3 of the ATP needed by

muscle cells for contraction. Glycolysis produces the rest. 1. When glycolysis makes pyruvate faster than it can go into and be used in the

mitochondria, then pyruvate levels increase in the sarcoplasm. 2. Under these O2-limiting conditions, some of the pyruvate is converted to lactic acid. 3. The production of lactic acid during peak activity lowers the intracellular and

extracellular pH. 4. After only a few seconds of peak activity, the lowering of pH (acidosis) can alter the

function of key enzymes such that the cell can no longer contract efficiently causing muscle fatigue.

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Duration of Exercise System Description of System First 15 seconds Phosphagen System Stored ATP and CP Next 2 minutes Glycogen-Lactic Acid system Blood glucose and glucose

from glycogen are used to make ATP by lactic acid fermentation

After about 2 minutes of exercise until stop exercising

Aerobic Respiration After about 60 seconds or so of exercise the cardiopulmonary system delivers oxygen to exercising skeletal muscle fast enough to sustain aerobic cellular respiration

During exercise that lasts for more than 10 minutes, more than 90% of the ATP is produced aerobically. Oxygen Debt (Oxygen Deficit) or Excess Postexercise O2 Consumption (EPOC) 1. Amount of extra oxygen needed immediately after exercise (beyond resting oxygen

consumption) to restore the body back to its normal resting state. 2. The oxygen debt accumulates during exercise and it is paid back after exercise is over as one

breathes heavily for a few minutes. When one finishes exercising, one might expect that the heavy breathing that occurs during exercise would immediately stop since extra oxygen shouldn’t be needed to maintain a resting metabolism, but that isn’t the case because of the oxygen debt.

3. After prolonged exercise, a person continues to breathe heavily for several minutes to take in extra oxygen to pay back the oxygen debt.

a. the extra O2 allows liver (and muscle) cells to make more ATP than normal for a few minutes in order to convert lactic acid back to pyruvate. Two pyruvate molecules can then be combined into glucose.

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1. Some of the lactic that forms during exercise can be converted back to pyruvate which can then into the mito and be used to make ATP

2. Some of the LA goes into the blood and then to the liver where it can be converted to glucose by the ATP-requiring Cori Cycle

b. extra O2 is used to oxygenate myoglobin in skeletal muscle cells; store oxygen in resting

muscle cells c. extra O2 is used to make the extra ATP in muscle cells to synthesize glycogen and to

build up creatine phosphate reserves that were used up during exercise. Restock glucose as glycogen.

Exercise Pay Back O2 Debt After Exercise Myoglobin lose O2 Oxygenate myoglobin

Lactic Acid is produced Convert LA back to glucose in liver CP depletion to make ATP Reform CP from ATP

Glycogenolysis occurs to make glucose Glycogenesis Muscle Fatigue 1. strength of contraction becomes progressively weaker until the muscle no longer responds 2. Fatigue occurs when the muscle cannot produce enough ATP to satisfy demand and/or

when one is psychologically tired. 3. Contributing factors a. lack of enough O2 to generate sufficient ATP b. glycogen depletion (decrease the rapid ATP-generating system) c. ionic imbalances – K+ accumulate in T-tubules as a result of repeated muscle impulses.

This can interfere with the ability of muscle cells to release Ca2+ from cisterns d. LA production decreases pH of sarcoplasm. This can lead to a decrease in the activity of

enzymes such that muscles don’t contract effectively. Buffers within skeletal muscles help to prevent pH changes. LA contributes indirectly to fatigue by its effect on pH.

e. unknown factors

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Lactic Acid Production 1. during strenuous exercise, muscle cells convert some of the pyruvate that forms during

glycolysis to lactic acid. Lactic acid doesn’t directly contribute to fatigues, but it will decrease the pH of muscle cells (from around 7.1 to as low as 6.4) which contributes to fatigue.

2. if lactic acid builds up it contributes to muscle ache or the “burn” that occurs during exercise. Lactic acid is cleared from the body within 30 – 60 minutes after exercise so it doesn’t contribute to the muscle soreness that occurs after exercise is over.

3. better conditioned athletes have efficient systems to deliver a lot of oxygen to muscle cells during exercise and produce less lactic acid than a non-conditioned individual

a. more capillary beds b. higher amounts of myoglobin in the cells c. stronger heart d. additional mitochondria 5. Some of the lactic acid produced enters the blood stream and most of that is taken up by

liver cells. Liver cells then convert the lactic acid to glucose by reversing the steps of glycolysis. Some of the lactic acid that builds up in muscle cells is eventually converted back to pyruvate as O2 becomes available. The pyruvate can then enter the mito to make ATP.

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Electron Transport Chain and Chemiosmosis

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