A&P Chapter 09

Muscular System

Anatomy & Physiology

ivyanatomy.com

Chapter 9

Muscle is derived from Musculus, for “Mouse”

Functions of Muscles:1. Body movement 2. Maintain posture3. Produces heat4. Propel substances

through body5. Heartbeat6. Breathing

Types of muscles include:1. Smooth muscle2. Cardiac muscle3. Skeletal muscle

Imagine a mouse running beneath the skin.

Characteristics of smooth muscles• Involuntary control• Tapered cells with a single, central nucleus• Lack striations

Smooth Muscle

• Visceral (single-unit) Smooth Muscle• Form sheets of muscle• Cells are connected by gap junctions• Muscle fibers contract as a group• Rhythmic contractions• Within walls of most hollow organs

(viscera)

• Multi-unit Smooth Muscle• unorganized cells that contract

as individual cells

• Located within the iris of eye and the walls of blood vessels

There are two types of smooth muscles

Smooth Muscle

Autonomic neuron varicosity (swelling)

Autonomic neuron varicosity (swelling)

Gap junction

• Located only in the heart• Striated cells• Intercalated discs contain:

• gap junctions and desmosomes

• Branching muscle fibers, with a single central nucleus• Muscle fibers contract as a unit (syncytium)• Self-exciting and rhythmic

Cardiac Muscle• Cardiac Muscle

• Usually attached to bone or

other connective tissue

• Voluntary control

• Striated (light & dark bands)

• Muscle fibers form bundles

• Several peripheral nuclei

Skeletal Muscle• Skeletal Muscle

Fascia• Dense connective tissue surrounding skeletal muscles

• Superficial fascia – beneath skin• Deep fascia – covers muscles• Serous fascia – surrounds serous membranes

Tendons• Dense connective tissue that attaches muscle to bones• Continuation of muscle fascia and bone periosteum

Aponeurosis• Broad sheet of connective tissue attaching muscles to

bone, or to other muscles.

Skeletal Muscle Coverings

Skeletal Muscle Coverings

epicranial aponeurosis

Epimysium• Connective tissue that covers the entire muscle

• Lies deep to fascia

Perimysium• Surrounds organized bundles of muscle fibers, called fascicles

Endomysium• Connective tissue that covers individual muscle fibers (cells)

Skeletal Muscle Coverings

FascicleOrganized bundle of muscle fibers

Muscle FiberSingle muscle cellCollection of myofibrils

MyofibrilsCollection of myofilaments

MyofilamentsActin filamentMyosin filament

Organization of Skeletal Muscle

Muscle FiberSingle muscle cellCollection of myofibrils

MyofibrilsCollection of myofilaments

MyofilamentsActin (thin) filamentMyosin (thick) filament

Organization of Skeletal Muscle

Sarcolemma• Cell membrane of muscle fibers

Sarcoplasm• Cytoplasm of muscle fibers

Sarcoplasmic Reticulum• Modified Endoplasmic Reticulum• Stores large deposits of Calcium

Organization of Skeletal Muscle

(Transverse)T-tubules: • invaginations of sarcolemma, extending into the sarcoplasm. Cisternae:• enlarged region of sarcoplasmic reticulum, adjacent to the t-tubules Triad • T-tubule + adjacent cisternae

Organization of Skeletal Muscle

Myofibrils are bundles of actin and myosin filaments.

• Actin – thin filament• Myosin – thick filament

Striations appear from the organization of actin and myosin filaments

Organization of Skeletal Muscle

A sarcomere is the functional unit of skeletal muscle

• A sarcomere is the area between adjacent Z-lines.

• During a muscle contraction the z-lines move together and the sarcomere shortens.

Sarcomere

Sarcomere

Z Line (disc) is the attachment site of actin filaments (center of I bands)

Striations appear from alternate light and dark banding patterns.

I Bands (light band): consists of only actin filaments

A Bands (dark band) : consists of myosin filaments and the overlapping portion of actin filaments

Striations

Thick filaments composed of myosin proteins

During muscle contraction the heads on myosin filaments bind to actin filaments forming a Cross-bridge

Thin filamentscomposed of actin proteins

Thin filaments are associated with troponin and tropomyosin proteins

Filaments

When a muscle is at rest, myosin heads are extended in the “cocked” position.

During a contraction, myosin heads bind to actin, forming a cross-bridge and the myosin head pivot forward (Power Stroke) and back (Recovery stroke)

Cross Bridges

Tropomyosin• Blocks binding sites on actin when the

muscle is at rest• Contraction (cross-bridge-cycling) begins

when tropomyosin is repositioned.

Troponin Ca2+ binds to troponin during a muscle contraction.

Troponin moves repositions the tropomyosin filaments, so the myosin and actin filaments can interact, triggers cross-bridge cycling

The troponin-tropomyosin complex controls the activation of cross-bridge cycling (contraction)

Troponin-Tropomyosin Complex

Synapse: Functional (not physical) junction between an axon of a neuron and another cell

The two cells are separated by a physical space, called the synaptic cleft.

Neurotransmitters are stored within synaptic vesicles of the presynaptic cell and they’re released into the synapse.

Synapse

Neuromuscular JunctionNeuromuscular Junction (NMJ) refers to the synapse between an axon and a muscle fiber.

Motor End Plate is a highly folded region of muscle fiber at NMJ that contain abundant mitochondria

Motor neurons innervate effectors (muscles or glands)

A motor unit includes a motor neuron and all of the muscle fibers it controls

Motor Unit

1 motor unit may control between 1 and 1000 muscle fibers

motor neuron

muscle fibers

Stimulus for ContractionAcetylcholine (ACh) is the only neurotransmitter that initiates skeletal muscle contraction

1. Decide to move (voluntary control)

2. An action potential (nerve impulse) travels down axon to axon terminal.

3. Calcium channels open at the axon terminal, Calcium diffuse into axon

4. Exocytosis of Ach from secretory vesicles into synaptic cleft.

Sequence of Actions

Sequence of Actions…Continued

5. ACh binds to receptors on motor endplate, opening Na+ channels

6. Na+ diffuses into the muscle, triggering a muscle impulse (action potential).

Stimulus for Contraction

7. The muscle impulse diffuses across sarcolemma and down the t-tubules into the cisternae of sarcoplasmic reticula.

8. The sarcoplasmic reticula release their calcium supplies into the sarcoplasm.

9. Calcium binds to troponin and the troponin repositions the tropomyosin, exposing actin filaments to the myosin heads.

10. Cross-bridge cycling causes contraction of the muscle.

Stimulus for ContractionSequence of Actions…Continued

7. The muscle impulse diffuses across sarcolemma and down the t-tubules into the cisternae of sarcoplasmic reticula.

8. The sarcoplasmic reticula release their calcium supplies into the sarcoplasm.

During a muscle contractionThick (myosin) filaments and thin (actin) filaments slide across one another

The filaments do not change lengths

Z-bands move closer together causing the sarcomere to shorten.

I bands appear narrow

Sliding Filament Theory of Contraction

During a contraction, Calcium binds to troponin.

Tropomyosin is repositioned, exposing the myosin binding sites on actin filaments

Cross Bridge Cycling

1. Cross-Bridge Formation • Myosin heads bind to actin filaments.• The phosphate is released.

Cross Bridge Cycling

2. Power Stroke• Myosin heads spring forward pulling the

actin filaments.• ADP is released from Myosin

3. Cross-Bridge Release • New molecule of ATP binds to myosin• Myosin head is released from actin.

4. Recovery Stroke• ATP is hydrolyzed into ADP + P• Energy is used to return myosin to

cocked position

Cross Bridge Cycling continues until Calcium is removed from cytosol and tropomyosin covers binding sites on actin filaments.

Cross Bridge Cycling

End of Chapter 9, Section 3

Major Events of Muscle Fiber Contraction1. Decision to move2. Action Potential on Motor Neuron3. Calcium Diffuses into axon terminal4. Exocytosis of ACh into synaptic cleft5. ACh binds to receptors on motor end plate6. Na+ diffuses into muscle fiber, initiating a muscle impulse7. Muscle Impulse (action potential) reaches Sarcoplasmic Reticulum

8. Sarcoplasmic Reticulum releases Calcium into sarcoplasm9. Ca2+ binds to troponin10. Troponin repositions tropomyosin11. Cross-Bridge Cycling

• Cross-bridge Formation• Power Stroke• Cross-bridge Release• Recovery Stroke

Relaxation

When a nerve impulse ceases, two events relax muscle fibers.

1. Acetylcholinesterase breaks down Ach in the synapse.• Prevents continuous stimulation of a muscle fiber.

2. Calcium Pumps (Ca2+ATPase) remove Ca2+ from the sarcoplasm and returns it to the SR.• Without calcium, tropomyosin covers the binding sites on actin

filaments.

Relaxation

Rigor Mortis is a partial contraction of skeletal muscles that occurs a few hours after death.• After death calcium leaks into sarcoplasm, triggering the muscle contractions.• But ATP supplies are diminished after death, so ATP is not available to remove

the cross-bridge linkages between actin and myosin.• muscles do not relax*.• Contraction is sustained until muscles begin to decompose.

* Notice that ATP is required for muscle relaxation!

ATP provides the energy to power the interaction between actin & myosin filaments.

• However, ATP is quickly spent and must be replenished

New ATP molecules are synthesized by 1. Creatine Phosphate2. Glycolysis (anaerobic respiration)3. Aerobic Respiration

Energy Sources for Contraction

The energy from creatine phosphate hydrolysis cannot be used to directly power muscles. Instead, it’s used to produce new ATP.

Creatine Phosphate

Creatine Phosphate can be hydrolyzed into Creatine + Phosphate, releasing energy that is used to make new ATP.

Energy Sources for Contraction

When cellular ATP is abundant, creatine phosphate can be replenished by phosphorylating creatine.

Creatine Phosphate provides energy for only about 10 seconds of a high intensity muscle contraction.

Creatine Phosphate Continued

Energy Sources for Contraction

Anaerobic respiration (glycolysis) occurs in the cytosol of the cell and does not require oxygen.

Glucose molecules are partially broken down producing just 2 ATP for each glucose.

If there isn’t sufficient oxygen available, glycolysis produces lactic acid as a byproduct.

Energy Sources for ContractionGlycolysis

Aerobic respiration (uses oxygen) occurs in the mitochondria and it includes the citric acid cycle & electron transport chain.

Aerobic respiration is a slower reaction than glycolysis, but it produces the most ATP.

Energy Sources for ContractionAerobic Respiration

MyoglobinOxygen binding protein (similar to hemoglobin) within muscles

-Provides additional oxygen supply to muscles

During rest or moderate exercise, respiratory & cardiovascular systems supply enough O2 to support aerobic respiration

Anaerobic (Lactic Acid) Threshold: Shift in metabolism from aerobic to anaerobic, during strenuous muscle activity, when the above systems cannot supply the necessary O2. Lactic acid is produced.

Oxygen debt: Amount of oxygen needed by liver cells to convert the accumulated lactic acid to glucose, and to restore muscle ATP and creatine phosphate concentrations.

Energy Sources for Contraction

Aerobic RespirationAerobic respiration is used primarily at rest or during light exercise.

Muscles that rely on aerobic respiration have plenty of mitochondria and a good blood supply.

Oxygen debt of glycolysis

Exercise and strenuous activity depends on anaerobic respiration for ATP supplies.

Oxygen debt is the amount of oxygen needed by liver cells to convert accumulated lactic acid back to glucose.

During exercise anaerobic respiration causes lactic acid to accumulate in the cells.

After exercise, when oxygen is available the O2 is used to convert lactic acid back to glucose in the liver.

Muscle Fatigue

• Muscle Fatigue = Inability for the muscle to contract

• Several factors can cause muscle fatigue:• Decreased blood flow• Ion imbalances across the sarcolemma• Lactic acid accumulation – (greatest cause of fatigue)

• Cramp: • A cramp is a sustained, involuntary, and painful muscle

contraction• It’s due to electrolyte imbalance surrounding muscle

Heat Production• Heat is produced as a by-product of cellular

respiration

• Muscle cells are major source of body heat

• Blood transports heat throughout body core

A muscle contraction can be observed by removing a single skeletal muscle and connecting it to a device (myograph) that senses and records changes in the overall length of the muscle fiber.

A threshold stimulus is the minimum stimulus that elicits a muscle fiber contraction

Muscle Response

The muscle fiber will not contract at all if the stimulus is less than threshold.

all-or-none responseA threshold stimulus will cause the muscle fiber to contract fully and completely.

A stronger stimulus does not produce a stronger contraction!

subthreshold stimulus

Recording of a Muscle Contraction

2. Period of contraction

3. Period of relaxation

A twitch is a single contractile response to a stimulus

A twitch can be divided into three periods. 1. Latent period

brief delay between the stimulus and the muscle contraction

The latent period is less than 2 milliseconds in humans

Summation

If the muscle is allowed to relax completely before each stimulus than the muscle will contract with the same force.

If the muscle is stimulated again before it has completely relaxed, then the force of the next contraction increases.

i.e. stimulating the muscle at a rapid frequency increases the force of contraction. This is called summation

Tetanic Contraction (c) If the muscle is stimulated at a high frequency the contractions fuse together and cannot be distinguished.

A tetanic contraction results in a maximal sustained contraction without relaxation

Summation

all-or-none response

A muscle that is stimulated with threshold potential contracts completely and fully.

A stronger stimulus does not produce a stronger contraction!

Instead, the strength of a muscle is increased by recruitment of additional motor units.

Recruitment of Motor Units

Recruitment of Motor Units

Recall that a motor unit is a motor neuron plus all of the fibers it controls.

• Muscles are composed of many motor units.

• As a general rule, motor units are recruited in order of their size• Small motor units are stimulated with light activities, but additional

motor units are recruited with higher intensity activity.

Recruitment – progressive activation of motor units to increase the force of a muscle contraction.

As the intensity of stimulation increases, recruitment of motor units continues until all motor units are activated.

Sustained Contractions

The central nervous system can increase thestrength of contractions in 2 ways:

1. Recruitment• Smaller motor units are recruited first, followed by larger motor units.• The result is a sustained contraction of increasing strength.

2. Increased firing rate• A high frequency of action potentials results in summation of the muscle

contractions. • If the frequency is too high, however, it may produce tetanic contractions, in which

case the muscle does not relax.

Muscle tone is produced because some muscles are in a continuous state of partial contraction in response to repeated nerve impulses from the spinal cord.

Types of Contractions

Isotonic – muscle contracts and changes length Concentric – shortening of muscle (a) Eccentric – lengthening of muscle (b)

Isometric – muscle contracts but does not change length (c)

Isometric contractions stabilizes posture and holds the body upright

Fast twitch and slow twitch muscle fibers

Fast & Slow twitch refers to the contraction speed, and to whether muscle fibers produce ATP oxidatively (by aerobic respiration) or glycolytically (by glycolysis)

Slow-twitch fibers (Type I)• Always oxidative and resistant to fatigue

• Contain myoglobin for oxygen storage “red fibers”

• Also have many mitochondria for aerobic respiration

• Good blood supply

Slow-twitch fibers (Type I)

Slow-twitch fibers rely on aerobic respiration for energy (ATP) and are resistant to fatigue.

Slow-Twitch fibers contain abundant myoglobin for oxygen storage “red fibers” and mitochondria to carry out aerobic respiration.

Because of their oxygen demands, slow-twitch fibers have a good blood supply.

Slow-twitch fibers are best suited for endurance exercise over a long period with little force.

Geese, known for their sustained flights have predominately red (Slow-Twitch) pectoralis muscle fibers. The reddish appearance is from the large amount of myoglobin.

Slow Twitch Fibers

Fast-twitch glycolytic fibers contract rapidly and with great force, but they fatigue quickly.

They are best suited for rapid contractions over a short duration.

Fast-twitch glycolytic fibers (type IIa) contain very little mitochondria and myoglobin and are “white fibers”

Fast-twitch glycolytic fibers (Type IIb)

Chickens can only fly in short bursts, so the chicken breast is composed of primarily fast-twitch fibers for the power of liftoff. These muscle can act more powerfully but they fatigue quickly. They have smaller blood supply, so the breast meat is light.

Fast Twitch Fibers

Fast-twitch intermediate or fast oxidative fibers contain intermediate amounts of myoglobin.

They contract rapidly but also have the capacity to generate energy through aerobic respiration.

Fast-twitch Intermediate fibers (Type IIa)

Fast Twitch Fibers

Attribution• Muscular System Anterior View By Termininja (Pectoralis major.png Tibial anterior.png) [CC BY-SA 3.0

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• Wood Mouse Rasbak [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/b/bd/Apodemus_sylvaticus_bosmuis.jpg

• Flexed Arm Supinate By EncycloPetey assumed (based on copyright claims). [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/4/41/Arm_flex_supinate.jpg

• Smooth Muscle Contraction By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/5/54/1028_Smooth_Muscle_Contraction.jpg

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• Skeletal, Smooth, Cardiac Muscle Fibers By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/e/e5/414_Skeletal_Smooth_Cardiac.jpg

• Lateral Head Muscles By Patrick J. Lynch, medical illustrator (Patrick J. Lynch, medical illustrator) [CC BY 2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/d/d5/Lateral_head_anatomy_detail.jpg

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• Sarcomere By Slashme at English Wikipedia When using this image in external works, it may be cited as follows: Richfield, David. "Medical gallery of David Richfield 2014". Wikiversity Journal of Medicine 1 (2). DOI:10.15347/wjm/2014.009. ISSN 2001-8762. (http://en.wikipedia.org/wiki/Sarcomere) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/6/6e/Sarcomere.svg

• Thin and Thick Filaments By Raul654 CCBY SA3.0 https://upload.wikimedia.org/wikipedia/commons/6/66/Thin_filament.jpg• Sliding Filament By Gal gavriel (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia

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