MUSCLE and MOVEMENT Chapters 20, 8, 21 Stronger Muscle contraction.

MUSCLE and MOVEMENT Chapters 20, 8, 21 Stronger Muscle contraction

Movement in Animals Locomotion Movement from one location to another Animals are the only multicellular organisms that can actively move place to place Repositioning Movement of animal appendages Internal movement Movement of gases, fluids and ingested solids through the animal

Muscle cells (muscle fibers, myocytes) Contractile cells unique to animals
Two main types Striated Skeletal and cardiac Cardiac muscle is striated but is usually treated separately Smooth In mammals, striated skeletal muscle is voluntary and smooth muscle is involuntary

The musculoskeletal system
Provides the basic form and shape of the organism Mechanical function of support Protection for vulnerable organ within body a set of levers for body movement Heat production

Bundles of multinucleate striated muscle cells (muscle fibers)
Skeletal Muscle Bundles of multinucleate striated muscle cells (muscle fibers) Attaches via connective tissue (collagen fascia or tendons) to bone Often work in opposition (Antagonistic muscle groups) biceps triceps

Figure 20.1a The organization of skeletal muscles
Skeletal Muscle is subdivided into Fascicles, which are bundles of Muscle fibers (single muscle cells) Muscular System, Sliding Filament Theory (1) Skeletal Muscle Structure greatpacificmedia anphys-fig jpg

Figure 20.1b The organization of skeletal muscles
Muscle fiber myofibril Muscle fiber a single multinucleate muscle cell up to 20 cm long in humans mm in diameter MUSCLE FIBER MYOFIBRIL

Figure 20.1b The organization of skeletal muscles:
Muscle fiber myofibril MUSCLE FIBER Each muscle fiber contains many myofibrils 1-2 µm diameter Runs the length of the muscle cell More myofibrils in parallel  more force MYOFIBRIL MYOFIBRIL MYOFIBRIL

Myofibrils are subdivided into sarcomeres
Figure 20.1c The organization of skeletal muscles: MYOFYBRILS and SARCOMERES Myofibrils are subdivided into sarcomeres in humans about 2.5 um long, but in some invertebrates up to 15 um SARCOMERE anphys-fig jpg MYOFYBRIL

Figure 20.1d The organization of skeletal muscles (Part 4)
Sarcomeres contain partially overlapping thick and thin filaments Thick: myosin Thin: actin Decorated by troponin and tropomyosin anphys2e-fig r.jpg

Figure 20.2 Thick (myosin) and thin (actin) myofilaments are arranged in parallel in a sarcomere
AnPhys3e-Fig jpg

Skeletal Muscle Organization

Figure 20.3 Muscle contraction produced by sliding filaments
Muscle contracts when thick filaments slide relative to the thin filaments Animation: Sarcomere Contraction anphys2e-fig jpg

Sliding Filament Mechanism
Myosin heads ‘pull’ on thin filaments Thin filaments slides inward Z- discs on sarcomere move towards each other Sarcomere unit shortens Filaments remain constant size

Muscle, How It Works: Contraction
STRUCTURE OF SARCOMERE four proteins are actin, myosin (force makers) and tropomyosin, and troponin (both regulators) two small molecules are ATP (energy source) and calcium ions (regulator)

Figure 20.4a Myosin molecules form the thick filament
thick filament is polymer of 200 molecules of myosin each molecule has Two intertwined strands, each with a projecting head (towards z-disk) that forms cross bridges long tail (extends towards center)

Thick Filament Structure
Globular heads contain actin binding sites ATP-hydrolyzing sites Heads form cross-bridges with actin

Fig 20.5 Molecular interactions that underlie muscle contraction

Rigor Conformation ATP Binding: Breaking the Cross-bridge anphys2e-fig jpg

ATP hydrolysis (ADP + unreleased Pi) causes the angle of the head to change (cocking) And rebind to the actin (but closer to the z line)

Phosphate release and filament sliding: The myosin head swivels, pulling the attached actin toward the midline (power stroke)

The myosin unbinds the ADP and And remains bound to the actin (rigor) (Step 1) until a new molecule of ATP binds to the myosin head (Step 2)

Figure 20.5 Molecular interactions that underlie muscle contraction
Myosin ATPase hydrolizes ATP to ADP and Pi. Energy from the reaction is transferred to the cross-bridge. ADP and Pi remain bound to myosin. The myosin head unbinds ADP and remains tightly bound to actin (rigor). Myosin attachment to actin triggers rapid Pi release and the power stroke. Actin filament is moved about 10 nm toward the center of the sarcomere. ATP binding dissociates myosin from actin. The cross-bridge can now go through the cycle on a new G-actin molecule. The myosin head moves to the cocked position and binds to a G-actin molecule. Rigor is a transient state. ADP Pi ADP ADP Pi ATP ADP Pi Myosin Myosin-binding sites ATP-binding site Actin-binding site 10nm G-actin

Main component is a THIN FILAMENT made of a helical polymer of ACTIN
Forms the binding site for myosin Two regulatory proteins TROPOMYOSIN TROPONIN.

Figure 20.6 The regulation of contraction
TROPOMYOSIN covers myosin binding site in relaxed muscle TROPONIN holds tropomyosin in position These two proteins interact to control the attachment of crossbridges to actin. Initiation of contraction (next slide) anphys2e-fig jpg No Calcium

Figure 20.6 The regulation of contraction
Initiation of contraction Ca2+ binds to troponin Troponin-Ca2+ complex removes tropomyosin blockage of actin sites Heads of thick filament (containing preexisting myosin-ATP complex) form cross bridges to actin strand Myosin head swivels power stroke) as ATP is hydrolyzed and P is released Contraction occurs. anphys2e-fig jpg Calcium ions present

Muscle, How It Works: Contraction
Troponin binds calcium and changes shape Tropomyosin moves out of groove in helix Myosin heads bind to actin filaments Myosin heads walk along the actin filaments and the sarcomere shortens One ATP is hydrolyzed per step per head Rigor mortis happens when ATP is gone and myosin heads bind permanently to actin filaments

MUSCLE, HOW IT WORKS Sequence of events in stimulation and contraction of muscle Stimulation Contraction Relaxation

The T system of Muscle cells
Sarcolemma: the plasma membrane of a muscle cell. T (transverse) tubule: tube like invaginations of the sarcolemma. Sarcoplasmic reticulum: specialized smooth endoplamic reticulum found in muscle cells.

Sarcoplasmic Reticulum (SR)
Microscopic structure of skeletal muscle cell Source of calcium ions that are released upon the impulse to contract. SR

Portion of the SR abutting the T tubule.
Terminal cisternae Portion of the SR abutting the T tubule. The impulse to contract is conducted from the T tubule to the SR via the terminal cisternae Microscopic structure of skeletal muscle cell T-tubules with sarcomere unit

Figure 20.7 Excitation–contraction coupling
Contraction is initiated when a nerve impulse reaches the motor endplate at the neuromuscular junction anphys2e-fig jpg

Skeletal Muscle Contraction: Nerve stimulation
DHPR dihydrropyridine receptor Acetocholene (Ach) released at the motor endplate diffuses across the synaptic space. Ach attaches to Ach receptors on the sarcolemma, opening Na channels and depolarizing the cell. Action potential is propagated along the plasma membrane and down the T tubules depolarizing it.

DHPR dihydrropyridine receptor Acetocholene (Ach) diffuses across the synaptic space. Ach attaches to Ach receptors on the sarcolemma, opening Na channels and depolarizing the cell. Action potential is propagated along the plasma membrane and down the T tubules depolarizing it.

DHPR dihydrropyridine receptor Acetocholene (Ach) released at the motor endplate diffuses across the synaptic space. Ach attaches to Ach receptors on the sarcolemma, opening Na channels and depolarizing the cell. Action potential is propagated along the plasma membrane and down the T tubules depolarizing it.

Calcium Storage Ca is stored in the SR
Active transport pumps move Ca from the sarcoplasm into the SR continuously

DHPR dihydrropyridine receptor RyR ryanodine receptor Depolarization of T-tubules causes the SR to depolarize and release Ca++ to the sarcoplasm. Ca2+ diffuses to thin filament Ca2+ binds to troponin Troponin-Ca2+ complex removes tropomyosin blockage of actin sites anphys2e-fig jpg

In the meanwhile, AChE (Acetylcholinesterase) hydrolyzes ACH and the membrane repolarizes, terminating the action potential

Figure 20.7-7 Calcium is bound to the troponin 7: Cross-bridge cycling
Heads of myosin-ATP complex form cross bridges to actin strand Cross bridges swivel as ATP is hydrolyzed and ADP is released Contraction continues as long as Ca is present. contraction anphys2e-fig jpg DHPR dihydrropyridine receptor RyR ryanodine receptor

Skeletal Muscle Contraction: Relaxation
Ca channels close, after AP stops Cytosolic Ca is removed by active transport into the SR after the action potential ends. Tropomyosin blockage is restored; contraction ends and the muscle relaxes.

Biochemistry of muscle contraction A brief history
1864: Myosin (viscous protein) isolated from skeletal muscle 1939: Myosin reactive to ATP 1942-3: Myosin A and myosin B delineated. Myosin B is really actin. 1943: Two forms of actin discovered, globular and fibrous. 1946: Tropomyosin discovered 1949: Interaction of ATP with actin causes the contractile event 1952: A.F. Huxley, PhD thesis sets basis for sliding filament model 1954: Sliding filament model published, mechanism not completely understood 1957: Crossbridges seen with electron microscope 1962: SR as Ca+2 pump discovered 1963: Basic structure of actin, 2 stranded helix worked out. 1963: Troponin discovered 1969: Basics of myosin structure worked out, straight chain with clubs 1971: Cross bridge cycle proposed 1972: Basic principles of muscle contraction regulation understood. 1973: Steric role of troponin and role of Ca+2 figured out, confirmed by electron microscopy in 1997. Ref: Szent-Gyorgyi, A The early history of the biochemistry of muscle contraction. Journal of General Physiology 123:

Whole Muscle Mechanics
In isometric contraction muscle does not shorten Increased force (tension), constant length In isotonic contraction, the muscle shortens as it moves a constant load Constant force, decreased length Most muscle contractions are a combination of isometric and isotonic contractions at different stages

Whole Muscle Mechanics
For experimental purposes the two are separated

Myogram: graph of a muscle contraction.
Latent period is the time it takes for AP to reach Voltage Gated Ca2+ channels and for Ca2+ to increase in the sarcoplasm Contraction is the sliding of filaments Relaxation is the sequestering of Ca2+ in the sarcoplasmic reticulum

Figure 20.9a Recording isometric contractions
In an isometric contraction, tension produced is less than force required to move the load. Measured as changes in tension produced by the muscle. Increased tension during contraction phase, no change in length. Contraction generates force on attachment points

Figure 20.9b Recording isotonic contractions
Muscle shortens upon contraction Measured as changes in length of the muscle. Decreased length, no change in tension (force) The heavier the load the longer the latent period, the less the muscle is able to shorten, and the slower it contracts

Muscle Contractions Twitch Summation Tetanus

Single all-or-none contraction a response to a single, rapid stimulus
Muscle Twitch Single all-or-none contraction a response to a single, rapid stimulus twitch contraction lasts from 20 to 200 msec anphys2e-fig jpg Figure 20.10

Wave Summation Application of stimuli in rapid succession Muscle responds to a second stimulus before fully relaxed from first Increased tension is generated the second contraction is stronger than first

Tetanus With repeated stimulation at high frequency twitches fuse to form steady tension Unfused/incomplete tetanus if stimulate at times/second, there will be partial relaxation between stimuli Fused/complete tetanus (next slide)

Tetany Smooth, sustained muscle contraction due to rapid and repeated stimuli; no relaxation between stimuli stimulate at times/second Ca+2 pump cannot sequester all the Ca+2 before the next stimulus Refractory period of skeletal muscle approx 1-5 msec. Contraction length msec.

Clostridium tetani Tetanus or lockjaw is spastic paralysis
Bacterial toxin blocks the neurotransmitters GABA and glycine in the spinal cord, which is required to check nervous impulses. This produces generalized muscular spasms by preventing the inhibition of contraction of opposing sets of muscles – hence, tetanic or spastic paralysis. A soldier dying from tetanus. Painting by Charles Bell in the Royal College of Surgeons, Edinburgh 1809

How does this differ from the effects of the bacterium Clostridium botulinum
botulinum toxin blocks what? the release of Ach from presynaptic terminal. Causes What? muscle weakness / flaccid paralysis Botox paralyzes muscles in face so that wrinkles will not form

Fig. 20.11: Length-Tension Relationship
Tension depends on the length of the muscle when stimulated. Force generated is associated with starting length of muscle Muscle tension is greatest when muscle is at its ideal starting length. Developed by sarcomere Added by experimenter

strength of tetanus depends on length of sarcomere
Figure The relationship between the length and the tension produced by skeletal muscle strength of tetanus depends on length of sarcomere force strongest at resting length (3) and less at both longer and shorter lengths at long lengths, there is not much filament overlap at short lengths the myosin heads interfere

Box 20.1 The electric eel Electrophorus electricus possesses both strong and weak electric organs
Electric organs – modified skeletal muscle Electrocytes – stacked in columns Respond to signals from motor neurons All electrocytes in column depolarize spontaneously Up to 600 V discharge anphys2e-box jpg

Energy Sources for Contraction
Glucose Glycogen and fat are also kept in muscle cells for longer work Even longer work gets energy from metabolism of food and fat stores in the liver

Anaerobic Glycolysis Net production of 2 ATP per glucose
Lactic acid is a toxic byproduct of the regeneration of NAD needed for glycolysis

Figure 20.13 The production and use of ATP
ATP: produced by cellular respiration provides energy for muscle contraction Aerobic catabolism: efficient, slow ATP generation only enough free ATP for 2-4 seconds of work (about 10 contractions) anphys2e-fig jpg

Energy Sources for Contraction
Creatine Phosphate: high energy phosphate bond that can quickly regenerate ATP from ADP only enough of this for about a 100 yard dash (10-20 seconds) When ATP is plentiful, creatine phosphokinase in the mitochondria stores excess energy as creatine phosphate

Production of ATP Anaerobic glycolysis: not enough oxygen Rapid but inefficient (2 ATP/glucose) lactic acid builds up until muscle can’t work

Fig 8.6 Major paths by which lactic acid is metabolized when O2 is available
Lactic acid is converted back to pyruvic acid. Pyruvic acid can enter the Krebs cycle and be oxidized or liver cells convert pyruvic acid to glucose using ATP (also requires Oxygen). Oxygen debt is the amount of oxygen needed to remove the accumulated lactic acid.

The cause of fatigue During intense exercise, vertebrate skeletal muscles produce lactic acid, which results in cellular acidification. This appears related to fatigue, but the actual mechanism remains unclear (build up of Ca2+ and depletion of phosphocreatine may play a role).

Figure 8.8 The fueling of intense, sustained muscular work in humans
During less intense exercise, muscles run out of glycogen and become dependant upon blood glucose. The efficiency of this transfer are associated with fatigue.

Muscle cramps Muscle cramp: Frequent involuntary spasms of voluntary (skeletal) muscle (e.g. tentanus, and contracture). The causes of muscle cramps are not extremely well understood. True cramps: Believed to be caused by hyperexcitability of nerves that control muscles. Causes of the hyperexcitability include injury, fatigue, and dehydration – often mediated through imbalances of electrolytes, esp. high (or maybe low) blood K+ and low blood Ca2+.

Classification of Skeletal Muscle Fibers
Tonic Fibers do not generate action potentials Mainly in postural muscles of lower vertebrates; slow cross-bridge cycling produces long sustained contraction (resistant to fatigue) In mammals, only occur in some eye muscles

2. Twitch muscle fibers Common fibers, which generate muscle contractions (twitches) through Action Potentials Fast glycolytic (FG): Can split ~600 ATPs per second Fast oxidative [glycolytic] (FO[G]): intermediate between FG and SO Slow oxidative (SO): Can split ~300 ATPs per second

Fast glycolytic fibers (FG)
FG fibers for power Fast glycolytic fibers (FG) Contract quickly, generate lots of power. Mainly fueled by glycolysis; high levels of glycolytic enzymes & low mitochondrial volume. Slow oxidative fibers (SO): Contract relatively slowly. primarily fueled by oxidative metabolism, high levels of aerobic enzymes and lots of mitochondria and myoglobin. High levels of myoglobin and mitochondria make SO fibers more resistant to fatigue Figure Two top athletes who differ in the fiber composition of their thigh muscles. FG = light stain; SO = dark stain

SO fibers for endurance
Fast glycolytic fibers (FG) Slow oxidative fibers (SO): Contract relatively slowly; fueled by oxidative metabolism, high levels of aerobic enzymes High levels of myoglobin & mitochondria make SO fibers more resistant to fatigue Figure Two top athletes who differ in the fiber composition of their thigh muscles. FG = light stain; SO = dark stain

Muscle fiber diversity: Muscle fiber diversity is plastic…to a degree.
All vertebrates have a mixture of fibers, but some species are adapted to have significantly more SO fibers (e.g. western toad) or more FG fibers (e.g. leopard frog). Even individuals of a species are born with a certain ratio of fibers. This ratio can be changed by exercise, but not completely reversed.

Most muscles contain a combination of both FG and SO muscles.
Figure Whole muscles typically consist of mixtures of different types of fibers Most muscles contain a combination of both FG and SO muscles. For example, the calf muscle of a cat contains: 45% FG and 25% SO (the other portion are intermediate fibers). anphys-fig jpg Slow oxidative (red): small diameter Fast glycolytic (yellow): large Fast oxidative glycolytic (orange): intermediate

Fish contain both red muscle (SO) and white muscle (FG).
Whole muscles typically consist of mixtures of different types of fibers Fish contain both red muscle (SO) and white muscle (FG). The SO muscle is for steady swimming performance. The FG muscle is for chase/escape performance. anphys-fig jpg

OXIDATIVE TWITCH MUSCLES Slow Twitch and Fast Twitch A
Slow oxidative (slow-twitch) red in color (lots of mitochondria, myoglobin & blood vessels) prolonged, sustained contractions for maintaining posture and endurance activities Fast oxidative glycolytic (fast-twitch A) orange in color (intermediate mitochondria, myoglobin & blood vessels) split ATP at very fast rate; used for walking and sprinting

GLYCOLYTIC Fast Twitch B
Fast glycolytic (fast-twitch B) white in color (few mitochondria & blood vessels, low myoglobin) produce a lot of force for short duration; used for weight-lifting used for quick movements (i.e., jumping) fatigue quickly adapted for anaerobic respiration

anphys2e-table jpg

Table 20.2--Twitch Fiber Diversity
SO fibers for endurance : FG fibers for power

The Motor Unit Each muscle fiber receives synaptic contact by only one Motor Neuron Motor unit = one somatic motor neuron & all the skeletal muscle cells (fibers) it stimulates

The Motor Unit One nerve cell supplies on average 150 muscle cells that all contract in unison. When a motor neuron fires, all muscle fibers in the motor unit contract. All or none principle

Figure 20.15 Vertebrate skeletal muscles consist of many different, independent motor units
Total strength of a contraction depends on how many motor units are activated & how large the motor units are anphys2e-fig jpg

Why you should never arm wrestle a chimpanzee Alan Walker,"The Strength of Great Apes and the Speed of Humans," Current Anthropology, April 2008. Humans huge surplus of motor neurons allows you to engage smaller portions of your muscles at any given time. Chimpanzee has fewer motor neurons. Each neuron triggers a greater number of muscle fibers, resulting in a greater proportion of muscle activation.

Why you should never arm wrestle a chimpanzee Alan Walker,"The Strength of Great Apes and the Speed of Humans," Current Anthropology, April 2008. our brain limits the degree of your muscle activation in an attempt to prevent damage to the fine motor control components of your muscles; and two, a Chimpanzee has no such limitation. So as the Chimpanzee tears off your arm easily and beats you over the head with it, think to yourself that rather than engaging in an arm wrestle with a Chimp, which has four times your strength, try sitting at home playing your Nintendo Wii instead, the precise motions for which it seems we are supremely evolved.

Graded Contraction Contractions of muscle fibers are all or none How does the nervous system produce graded muscle contractions? Vary the frequency of action potentials Rate of stimulation that is very fast results in tetanus One neuron can cause many muscle fibers to contract at the same time

Muscle Force muscle force is proportional to cross-section, not length though it is proportional to sarcomere length, which some animals take advantage of smaller animals, due to surface volume effects, are relatively much stronger for example, a 1 cm ant is 188 times stronger than I am

MUSCLE, HOW IT IS USED Skeletal Muscle Cardiac muscle Smooth muscle

Skeletal muscle attaches to bone, skin or fascia striated with light & dark bands visible with scope voluntary control of contraction & relaxation

Cardiac Muscle looks like striated muscle but is different in a couple ways
Short, branched fibers joined end-to-end at intercalated disks containing gap junctions. gap junctions pass the electrical signal for spreading contraction All heart cells contract nearly simultaneously Contraction in one part sends wave of contraction throughout has far more mitochondria, because it works non-stop Intercalated disk Sarcomere Mitochondrion

Cardiac Muscle Can only contract rhythmically, no sustained contraction possible our heartbeat is controlled by modified heart cells (myogenic) Autonomic Nervous system can modulate in some invertebrates it is controlled by the brain (neurogenic)

Cardiac Muscle

Smooth Muscle Small, involuntary muscle cells can be stimulated by autonomic nervous system (involuntary), hormones, local chemicals Tapering at ends Single, oval, centrally located nucleus

Smooth Muscle found in organs and blood vessels Only a few muscle fibers innervated in each group Impulse spreads through gap junctions no SR, and no troponin Slow, regular contractions Prolonged contractions (long duration)

Smooth Muscle Lacks sarcomeres
Uses actin and myosin sliding filaments, but the fibers are not in register with each other So you don’t see any conspicuous bands (no striations)

Myosin-linked regulation of smooth muscle contraction requires Ca2+ ions, calmodulin, and myosin light-chain kinase AnPhys3e-Fig jpg

Smooth Muscle Sliding of thick & thin filaments generates tension Transferred to intermediate filaments & dense bodies attached to sarcolemma Contract in all dimensions Muscle fiber contracts and twists into a helix as it shortens -- relaxes by untwisting

Smooth Muscle Grouped into sheets in walls of hollow organs Longitudinal layer – muscle fibers run parallel to organ’s long axis Circular layer – muscle fibers run around circumference of the organ Both layers participate in peristalsis

MUSCLE and MOVEMENT Chapters 20, 8, 21 Stronger Muscle contraction.

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MUSCLE and MOVEMENT Chapters 20, 8, 21 Stronger Muscle contraction.

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