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Section 1, Chapter 9 Muscular System . Muscle is derived from Musculus , for “Mouse”. Imagine a mouse running beneath the skin . Functions of Muscles: Body movement Maintain posture Produces heat Propel substances through body Heartbeat. Types of muscles include: Smooth muscle

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Section 1, Chapter 9 Muscular System

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Section 1, Chapter 9

Muscular System


Muscle is derived from Musculus, for “Mouse”

Imagine a mouse running beneath the skin.

Functions of Muscles:

Body movement

Maintain posture

Produces heat

Propel substances through body


Types of muscles include:

Smooth muscle

Cardiac muscle

Skeletal muscle


Smooth Muscle

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

There are two types of smooth muscles

  • Multi-unit Smooth Muscle
    • unorganized cells that contract as individual cells
    • Located within the iris of eye and the walls of blood vessels
  • 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)
cardiac muscle
Cardiac Muscle
  • Located only in the heart
  • Striated cells
  • Intercalated discs
  • Muscle fibers branch
  • Muscle fibers contract
  • as a unit
  • Self-exciting and rhythmic
skeletal muscle
Skeletal Muscle
  • Usually attached to bone
  • Voluntary control
  • Striated (light & dark bands)
  • Muscle fibers form bundles
  • Several peripheral nuclei
coverings of skeletal muscle
Coverings of Skeletal Muscle
  • Fascia
  • Dense connective tissue surrounding skeletal muscles
  • 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.

Coverings of Skeletal Muscle

  • 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)

Figure 9.3 Scanning electron micrograph of a fascicle surrounded by its perimysium. Muscle fibers within the fascicle are surrounded by endomysium.


Organization of Skeletal Muscle

  • Fascicle
    • Organized bundle of muscle fibers
  • Muscle Fiber
    • Single muscle cell
    • Collection of myofibrils
  • Myofibrils
    • Collection of myofilaments
  • Myofilaments
    • Actin filament
    • Myosin filament

Figure 9.2

Skeletal muscle organization


Skeletal Muscle Fibers

  • Sarcolemma
  • Cell membrane of muscle fibers
  • Sarcoplasm
  • Cytoplasm of muscle fibers
  • Sarcoplasmic Reticulum
  • Modified Endoplasmic Reticulum
  • Stores large deposits of Calcium



Skeletal Muscle Fibers

  • (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

Openings into t-tubules



  • Myofibrils are bundles of actin and myosin filaments.
  • Actin – thin filament
  • Myosin – thick filament
  • Striations appear from the organization of actin and myosin filaments

Figure 9.4 Organization of actin and myosin filaments



  • 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.

Striations appear from alternate light and dark banding patterns.

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

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

Figure 9.5 thin and thick filaments in a sarcomere.



  • Thin filaments
    • composed of actin proteins
    • Thin filaments are associated with troponin
    • and tropomyosin proteins
  • Thick filaments
    • composed of myosin proteins
    • During muscle contraction the heads on myosin filaments bind to actin filaments forming a Cross-bridge


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)


Troponin-Tropomyosin Complex

The troponin-tropomyosin complex prevents cross-bridge formation when the muscle is at rest.

  • Tropomyosin
    • Blocks binding sites on
    • actin when a muscle is
    • at rest
  • Troponin
    • Ca2+ binds to troponin
    • during a muscle contraction.
    • Troponin moves repositions the tropomyosin filaments, so the myosin and actin filaments can interact.

End of section 1, chapter 9


Chapter 9, Section 2

Muscle Contractions



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.


Neuromuscular Junction

Neuromuscular 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

Figure 9.8a. General NMJ


Motor Unit

Motor neurons innervate effectors (muscles or glands)

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

1 motor unit may control between 1 and 1000 muscle fibers

Figure 9.9 two motor units. The muscle fibers of a motor unit are innervated (controlled) by a single motor neuron.


Stimulus for Contraction

Acetylcholine (ACh) is the only neurotransmitter that initiates skeletal muscle contraction

Sequence of Actions

  • A nerve impulse (Action Potential) reaches axon terminal
  • The impulse opens calcium channels at the axon terminal
      • Calcium diffuse into axon
  • The calcium triggers the release of ACh from vesicles into synaptic cleft.

Stimulus for Contraction

Sequence of Actions…Continued

ACh diffuses across synaptic cleft & binds to receptors on motor endplate.

ACh opens Na+ channels on muscle

Na+ floods into the muscle, initiating a muscle impulse.

A muscle impulse (action potential) is propagated across the entire muscle.


The muscle impulse causes the release of calcium from the SR. Calcium binds to troponin and tropomyosin is repositioned exposing the actin filaments.


Stimulus for contraction continued…

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

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

10. Calcium binds to troponin and the troponin repositions the tropomyosin, so the myosin can bind to actin.

11. Cross-bridge cycling causes the muscle to contract.


Excitation-Contraction Coupling

Calcium released from sarcoplasmic reticulum binds to troponin.

Troponin moves tropomyosin, exposing actin filaments to myosin cross-bridges.

myosin heads bind to actin, forming a cross bridge and cross-bridge cycling causes the muscle to contract.

End of section 2, chapter 9


section 3, chapter 9

Sliding Filament Theory of Contraction


The Sliding Filament Model of Muscle Contraction

  • During a muscle contraction
    • Thick (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

Figure 9.11a. Individual sarcomeres shorten as thick and thin filaments slide past one another.


Cross Bridge Cycling

When a muscle is relaxed, tropmyosincovers the binding sites on actin.

A molecule of ADP and Phosphate remains attached to myosin from the previous contraction.


Cross Bridge Cycling

  • During a contraction, Calcium binds to troponin.
    • Tropomyosin is repositioned, exposing the myosin binding sites on actin filaments

Cross Bridge Cycling

Myosin heads bind to actin filaments.

The phosphate is released.


Cross Bridge Cycling

Myosin heads spring forward “Power Stroke” pulling the actin filaments.

ADP is released from Myosin


Cross Bridge Cycling

  • Myosin is released from actin.
  • A new molecule of ATP binds to myosin, causing it to be released from the actin filament.
    • ATP is not yet broken down, but it is essential to release the cross-bridges.

Cross Bridge Cycling

ATP is broken down, providing the energy to “cock” the myosin filaments (recovery stroke).

Steps 1-6 are repeated several times.


Figure 9.10. The cross-bridge cycle. The cycle continues as long as ATP is present, and nerve impulses release Acetylcholoine.

Watch the You-Tube video “Sliding Filament” to view cross-bridge cycling in action.



  • When a nerve impulse ceases, two events relax muscle fibers.
  • Acetylcholinesterase breaks down Ach in the synapse.
    • Prevents continuous stimulation of a muscle fiber.
  • 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.


* Notice that ATP is required for muscle relaxation!


section 4, chapter 9

Energy Sources for Contraction


Energy Sources for Contraction

  • 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

Hydrolysis of Creatine Phosphate

Glycolysis (anaerobic respiration)

Aerobic Respiration


Creatine Phosphate

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

The energy from creatine phosphate hydrolysis cannot be used to directly power muscles.

Instead, it’s used to produce new ATP.


Creatine Phosphate…continued

    • 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.


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.


Oxygen debt of glycolysis

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

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.

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


Aerobic Respiration

  • 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.
  • Myoglobin
    • Oxygen binding protein (similar to hemoglobin) within muscles
    • -Provides additional oxygen supply to muscles

Aerobic Respiration

Aerobic 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.


Energy Sources for Contraction

Figure 9.13. The oxygen required for aerobic respiration is carried in the blood and stored in myoglobin. In the absence of oxygen, anaerobic respiration uses pyruvic acid to produce lactic acid.


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

End of Chapter 9, Section 4


section 5, chapter 9

muscular responses


Muscle Response

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

all-or-none response

A threshold stimulus will cause the muscle fiber to contract fully and completely.

A stronger stimulus does not produce a stronger contraction!


subthreshold stimulus


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


Recording of a Muscle Contraction

A twitch is a single contractile response to a stimulus

A twitch can be divided into three periods.

  • Latent period
  • brief delay between the stimulus
  • and the muscle contraction
    • The latent period is less than 2 milliseconds in humans
  • 2. Period of contraction
  • 3. Period of relaxation


series of twitches

Figure 9.17a

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


Figure 9.17b

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

Figure 9.17c


Recruitment of Motor Units

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

  • 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.

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 the
  • strength of contractions in 2 ways:
  • Recruitment
    • Smaller motor units are recruited first, followed by larger motor units.
    • The result is a sustained contraction of increasing strength.
  • 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
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

Figure 9.18. muscle contractions


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 are best suited for endurance exercise over a long period with little force.

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.


Fast twitch muscle fibers – two types

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 muscle fibers – two types

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 and slow twitch muscle fibers

Migrating birds have abundant slow-twitch fibers for flying long distances, which is why their flesh is dark.

Chickens that can only flap around the barnyard have abundant fast-twitch muscles and mostly white flesh.

End of Chapter 9