Ch 10: Muscular System

Cells are specialized for contraction

Muscle tissue

\-Smooth

\-Cardiac

\-Skeletal

Types of muscle tissue

Move the body by pulling on bones

Skeletal muscles

Control movements inside the body

Cardiac & Smooth muscle

•Producing movement

•Maintaining posture and body position

•Supporting soft tissues

Skeletal Muscle: Function

•Guarding body entrances and exits

•Maintaining body temperature

•Storing nutrients

Skeletal Muscle: Function

Skeletal muscles have…….

•Skeletal muscle tissue (primarily)



•Connective tissues



•Blood vessels

•Nerves

•**Epimysium**

•**Perimysium**

•**Endomysium**

Skeletal muscle: Layers of connective tissue

Collagen fibers of epimysium, perimysium, and endomysium come together………

\-At the ends of muscles to form

* A **tendon** (bundle)


* Or **aponeurosis** (sheet)

\-To attach skeletal muscles to bones

•Are enormous compared to other cells

•Contain hundreds of nuclei __(multinucleate)__

Skeletal muscle fibers

•Develop by fusion of embryonic cells **(myoblasts)**

•Also known as **striated muscle** cells due to striations

Skeletal muscle fibers

•Plasma membrane of a muscle fiber

•Surrounds the **sarcoplasm**

•A sudden change in membrane potential initiates a contraction

**Sarcolemma**

Cytoplasm of a muscle fiber

Sarcoplasm

\-Tubes that extend from the surface of muscle fiber deep into the sarcoplasm

\-Transmit action potentials from sarcolemma into the cell interior

* Action potentials trigger contraction

**Transverse tubules (T tubules)**

•A tubular network surrounding each myofibril

•Similar to smooth endoplasmic reticulum

**Sarcoplasmic reticulum (SR)**

\-Forms chambers **(terminal cisternae)** that attach to T tubules

* Two terminal cisternae plus a T tubule form a **triad**

\-**Specialized for storage and release of calcium ions**

* Ions are actively transported from the cytosol into terminal cisternae

**Sarcoplasmic reticulum (SR)**

•Lengthwise subdivisions within a muscle fiber

•Responsible for muscle contraction

•Made of bundles of protein filaments **(myofilaments)**

**Myofibrils**

•**Thin filaments**

•**Thick filaments**

Types of myofilaments

\-Type of myofilament

\-Composed primarily of actin

Thin filaments

\-Type of myofilament

\-Composed primarily of myosin

**Thick filaments**

•Smallest functional units of a muscle fiber

•Interactions between filaments produce contraction

**Sarcomeres**

\-Arrangement of filaments accounts for the striated pattern of myofibrils

* Dark bands **(A bands)**
* Light bands **(I bands)**

**Sarcomeres**

\-M line

\-H band

\-Zone of overlap

**A band**

•In center of A band

•Proteins stabilize positions of thick filaments

M line

•On either side of M line

•Has thick filaments but no thin filaments

H band

•Dark region

•Where thick and thin filaments overlap

Zone of overlap

\-Contains thin filaments but no thick filaments

\-Z lines

\-Titin

**I band**

•Bisect I bands



•Mark boundaries between adjacent sarcomeres

Z lines

•Elastic protein

•Extends from tips of thick filaments to the Z line

•Keeps filaments in proper alignment

•Aids in restoring resting sarcomere length

Titin

\-During a contraction


1. H bands and I bands narrow
2. Zones of overlap widen
3. Z lines move closer together
4. The width of A band remains constant

Sliding-filament theory

•Are found in skeletal muscle fibers and neurons

•Depolarization and repolarization events produce **action potentials** (electrical impulses)

**Excitable membranes**

Skeletal muscle fibers contract due to….

Stimulation by motor neurons

\-Synapse between a neuron and a skeletal muscle fiber

\-Axon terminal of the motor neuron releases a **neurotransmitter** into the synaptic cleft, acetylcholine (ACh)

\-ACh binds to and opens a chemically gated Na+ channel on the muscle fiber

* Na+ enters the cell and depolarizes motor end plate
* An action potential is generated

Neuromuscular junction (NMJ)

1. **Neural Control**
2. **Excitation**
3. **Release of Calcium Ions**

Neuromuscular junction (NMJ)

\-A skeletal muscle fiber contracts when stimulated by a motor neuron at a neuromuscular junction

\-The stimulus arrives in the form of an action potential at the axon terminal

Neural Control

The action potential causes the release of ACh into the synaptic cleft, which leads to excitation—the **production of an action potential in the sarcolemma**.

Excitation

\-This action potential travels along the sarcolemma and down T tubules to the triads

\-This triggers the release of calcium ions from the terminal cisternae of the sarcoplasmic reticulum

Release of Calcium Ions

1. (ACh) is released
2. Action potential reaches T Tubule
3. Sarcoplasmic reticulum releases Ca+2
4. Active sites are exposed, and cross-bridges form
5. Contraction cycle begins

Steps that Initiate a Muscle Contraction

\-1st Step that Initiates a Muscle Contraction

(ACh) is released at the neuromuscular junction and binds to (ACh) receptors on the sarcolemma

(ACh) is released

\-2nd Step that Initiates a Muscle Contraction

\-An action potential is generated and spreads across the membrane surface of the muscle fiber and along the T tubules

Action potential reaches T Tubule

\-3rd Step that Initiates a Muscle Contraction

\-The sarcoplasmic reticulum releases stored calcium ions

Sarcoplasmic reticulum releases Ca+2

\-4th Step that Initiates a Muscle Contraction

\-Calcium ions bind to troponin, exposing the active sites on the thin filaments

\-Cross-bridges form when myosin heads bind to those active sites

Active sites are exposed, and cross-bridges form

\-5th Step that Initiates a Muscle Contraction

\-The contraction cycle begins as repeated cycles of cross-bridge binding, pivoting, and detachment occur- all powered by ATP.

Contraction cycle begins

1. ACh is broken down
2. Sarcoplasmic reticulum reabsorbs Ca+2
3. Active sites covered, and cross-bridge formation ends
4. Contraction ends
5. Muscle relaxation occurs

Steps that End a Muscle Contraction

\-1st Step that Ends a Muscle Contraction

\-ACh is broken down by acetylcholinesterase (ACHE), ending action potential generation

ACh is broken down

\-2nd Step that Ends a Muscle Contraction

\-As the calcium ions are reabsorbed, their concentration in the cytosol decreases

Sarcoplasmic reticulum reabsorbs Ca+2

\-3rd Step that Ends a Muscle Contraction

\-Without calcium ions, the tropomyosin returns to its normal position and the active sites are covered again.

Active sites covered, and cross-bridge formation ends

\-4th Step that Ends a Muscle Contraction

\-Without cross-bridge formation, contraction ends

Contraction ends

\-5th Step that Ends a Muscle Contraction

\-The muscle returns passively to its resting length

Muscle relaxation occurs

\-When stimulated, it develops enough tension to lift the weight

\-The tension remains constant, but the muscle shortens

Isotonic Contractions

When stimulated, the tension rises, but the muscle length stays the same

__**Isometric**__ Contractions

•Contracting muscles use a lot of ATP

•Muscles store enough ATP to start a contraction

•More ATP must be generated to sustain a contraction

ATP (adenosine triphosphate) is the only energy source used directly for muscle contraction

\-ATP transfers energy to **creatine**

\-Creating **creatine phosphate (CP)**

* Used to store energy and convert ADP back to ATP

At rest, skeletal muscle fibers produce more ATP than needed

\-Enzyme

\-Catalyzes the conversion of ADP to ATP using the energy stored in CP

Creatine kinase (CK)

When CP is used up…..

Other mechanisms are used to generate ATP

•Direct phosphorylation of ADP by creatine phosphate (CP)

•**Anaerobic metabolism (glycolysis)**

•**Aerobic metabolism (citric acid cycle and electron transport chain)**

Generate ATP

•The demand for ATP is low, and sufficient oxygen is

available for mitochondria to meet that demand

•Fatty acids are absorbed and broken down in the

mitochondria creating a surplus of ATP

•Some mitochondrial ATP is used to convert absorbed

glucose to glycogen

•Mitochondrial ATP is also used to convert creatine to

creatine phosphate (CP)

•This results in the muscle's buildup of energy reserves (glycogen and CP)

Muscle Metabolism in a Resting Muscle Fiber

•The demand for ATP increases

•There is still enough oxygen for the mitochondria to

meet the increased demand, but no excess ATP is

produced

•ATP is generated primarily by the aerobic metabolism of

glucose from stored glycogen

•If the glycogen reserves are low, the muscle fiber can

also break down other substrates, such as fatty acids

•All of the ATP being generated is used to power muscle contraction

Muscle Metabolism during Moderate Activity

•The demand for ATP is enormous; oxygen cannot

diffuse into the fiber fast enough for the mitochondria

to meet that demand. Only a third of the cell's ATP

needs can be met by the mitochondria (not shown).

•The rest of the ATP comes from glycolysis, and when

this produces pyruvate faster than the mitochondria

can utilize it, the pyruvate builds up in the cytosol

•The pyruvate is converted to lactate. Hydrogen ions

from ATP hydrolysis are not absorbed by the

mitochondria

•The buildup of hydrogen ions increases cytosol

acidity, which inhibits muscle contraction, leading to

rapid fatigue.

Muscle Metabolism during Peak Activity

When muscles can no longer perform at a required level, they are **fatigued**

Muscle fatigue

Muscle fatigue is correlated with….

•Depletion of metabolic reserves

•Damage to the sarcolemma and sarcoplasmic reticulum

•Decline in pH, which affects calcium ion binding and alters enzyme activities

•Weariness due to low blood pH and pain

•Also called **excess post-exercise oxygen consumption (EPOC)**

**Oxygen debt**

\-After exercise or other exertion

* The body needs more oxygen than usual to normalize metabolic activities
* Breathing rate and depth are increased

**Oxygen debt**

Muscle growth from heavy training

Muscle **hypertrophy**

•Diameter of muscle fibers



•Number of myofibrils



•Number of mitochondria



•Glycogen reserves 

Increased by Muscle **hypertrophy**

Reduction of muscle size, tone, and power due to lack of activity

Muscle **atrophy**

Destruction of muscle tissue

Muscular dystrophy

\-Cardiac muscle cells

\-Intercalated discs

Cardiac muscle tissue

•Found only in the heart



•Have excitable membranes



•Striated like skeletal muscle cells

Cardiac muscle cells

•Specialized connections



•Join sarcolemmas of adjacent cardiac muscle cells by gap junctions and desmosomes

Intercalated discs

\-Stabilizing positions of adjacent cells

\-Maintaining the three-dimensional structure of tissue

\-Allowing ions to move from one cell to another

* So cardiac muscle cells beat in rhythm

Intercalated discs: Functions

\-Are small

\-Are typically branched with a single nucleus

\-Have short, wide T tubules

* No triads

Cardiac Muscle Tissue: Structural Characteristics

\-Have SR with no terminal cisternae

\-Are almost dependent on aerobic metabolism

* Contain lots of myoglobin, many mitochondria

\-Contact each other via intercalated discs

Cardiac Muscle Tissue: Structural Characteristics

**•Automaticity**

•Nervous system can alter the pace and tension of contractions

•Contractions last 10 times longer than those in skeletal muscle, and refractory periods are longer

•Wave summation and tetanic contractions are prevented due to special properties of sarcolemma

Cardiac Muscle: Functional Characteristic

•Contraction without neural stimulation



•Controlled by **pacemaker cells**

Automaticity

\-Integumentary system

* Arrector pili muscles erect hairs

\-Cardiovascular and respiratory systems

* Regulates blood pressure and airflow

Smooth muscle tissue

\-Digestive and urinary systems

* Forms sphincters
* Moves materials along and out of the body

\-Reproductive system

* Transports gametes and expels the fetus

Smooth muscle tissue

\-Long, slender, spindle-shaped cells

\-Single, central nucleus

\-No T tubules, myofibrils, or sarcomeres

* **Nonstriated muscle**

Smooth muscle: Structural Characteristics

\-Scattered thick filaments with many myosin heads

\-Thin filaments attached to **dense bodies**

* Dense bodies connect adjacent cells, transmitting contractions

\-No tendons or aponeuroses

Smooth muscle: Structural Characteristics

\-**Relaxed (sectional view)**

**-Relaxed (superficial view)**

**-Contracted (superficial view)**

Smooth Muscle Contraction

•Myosin and actin are scattered throughout the cell •Dense bodies are attached to actin.

Relaxed (sectional view)

\-Intermediate filaments (desmin) network between the dense bodies.

\-Adjacent smooth muscle cells are bound together at dense bodies, transmitting the contractile forces from cell to cell through the tissue.

Relaxed (superficial view)

The cells are shortened due to the tightening of the filaments at the dense bodies attached to the sarcolemma.

Contracted (superficial view)

Epimysium

Perimysium

Endomysium

Nerve

Muscle fascicle

Muscle fibers

Blood vessels

Epimysium

Blood vessels and nerves

Endomysium

Perimysium

Muscle fascicle

Muscle fiber

Myofibril

Storage of calcium

Distribute action potentials throughout the interior of the skeletal muscle cell

Making of energy (ATP); "power house" of the cell