Lesson Material - Muscle Physiology

In Lesson 5 we will learn about muscle physiology. The focus is on the structure and function of skeletal muscle tissue, how it is stimulated, and how it produces force (contraction). In lab, you will identify basic muscles of the body, forming a foundation to learn about actions later in the course (Unit 4).

Characteristics of muscle tissue

Excitable, elastic, extensible, and contractile

Excitability

Ability to spread electrical charge along the cell membrane

Elasticity

Ability to recoil and return to normal shape after being stretched

Extensibility

Ability to be stretched

Contractility

Ability to shorten (contract) and produce force

Skeletal muscle

Smooth muscle

Cardiac muscle

Skeletal muscle

○Long, cylindrical cells, multinucleated

○Striated, voluntary

Smooth muscle

○Short, fusiform cells

○Non-striated, involuntary

○Lines hollow organs

Cardiac muscle

○Striated, involuntary

○Found in the heart

Functions of Skeletal Muscle

●Movement

●Stabilizes joints

●Control sphincters and reflexes for swallowing, urination, and defecation

●Protection (e.g. abdomen)

●Generates heat

Connective tissue coverings of skeletal muscle

Epimysium, perimysium, and endomysium

Muscle fiber

Another name for a skeletal muscle cell

Epimysium

Covers the entire muscle; blends with the tendon

Perimysium

Surrounds groups of muscle fibers called fascicles

Fascicle

A bundle of muscle fibers

Perimysium function

Provides passageways for blood vessels and nerves to reach muscle fibers

Endomysium

Surrounds each individual muscle fiber (muscle cell)

Tendon

Attaches muscle to bone

Muscle cell organelles

Sarcolemma, sarcoplasmic reticulum, sarcoplasm, myofibrils, and sarcomeres

Sarcolemma

Plasma membrane of a muscle cell

Sarcoplasmic reticulum

Extensive smooth endoplasmic reticulum that stores calcium

Sarcoplasm

Cytoplasm of a muscle cell

Myofibrils

Cylindrical structures made of actin and myosin that contract

Sarcomere

Smallest segment of a myofibril that contracts

Sarcomere

Smallest functional unit of muscle contraction; extends from one Z line to the next

Z lines

Mark the boundaries of each sarcomere

Thick filaments

Made of myosin

Thin filaments

Made of actin

Sliding filament theory

Muscle contraction occurs when actin and myosin slide past one another

Actin and myosin

Do not shorten during contraction

Sarcomere shortening

Occurs as actin and myosin slide over each other, bringing the Z lines closer together

Sarcomere

Sarcomere

Extends from one Z line to the next

Thick filaments

Formed by the protein myosin

Thin filaments

Made of actin, tropomyosin, and troponin

Actin

Globular protein arranged like beads on a string

Tropomyosin

Rope-like protein that covers myosin-binding sites on actin

Troponin

Binds calcium, moves tropomyosin, and exposes myosin-binding sites

A band

Dark band where thick (myosin) filaments are located

I band

Light band containing only thin (actin) filaments

M line

Center of the sarcomere; anchors thick (myosin) filaments

Myosin heads

Globular heads that reach up and attach to actin during contraction

Actin movement

Actin is pulled toward the M line (center of the sarcomere)

Z disc

Anchors thin (actin) filaments

Z disc movement

As actin slides toward the M line, the Z discs are pulled closer together, shortening the sarcomere

Relaxed

Contracted

Relaxed sarcomere

Myosin and actin are not actively interacting; I band and H zone are visible

Contracted sarcomere

Myosin heads contact actin and pull it toward the M line

I band

Becomes smaller during contraction

H zone

Becomes smaller during contraction

Sarcomere shortening

Z discs move closer together as actin slides toward the M line

Muscle contraction

Sarcomere shortens, but actin and myosin filaments do not become shorter

ATP role in muscle contraction

Provides energy for the repeating four-step cross-bridge cycle

Cocking

Myosin ATPase uses energy from ATP to cock (energize) the myosin head

Cross-bridge formation

Calcium binds troponin, moving tropomyosin and exposing myosin-binding sites on actin; myosin attaches to actin

Power stroke

Myosin pulls actin toward the center of the sarcomere (M line)

Detachment

ATP binds to myosin, causing it to release actin

Cross-bridge cycle

Repeats as long as ATP and calcium are present

Neuromuscular junction (NMJ)

The nervous system stimulates specific muscle fibers to contract

The end of the motor neuron

Expands to form the axon terminal.

Axon terminal

Contains vesicles with acetylcholine

Synaptic cleft

A small space between the motor neuron and the skeletal muscle fiber

Acetylcholine receptors

Bind acetylcholine and initiate an action potential (electrical current) in the skeletal muscle fiber

Step 1

Action potential arrives at axon terminal

Step 2

Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal

Step 3

Synaptic vesicles containing acetylcholine merge with the plasma membrane

Step 4

Acetylcholine is released into the synaptic cleft

Step 5

Acetylcholine diffuses across the synaptic cleft

Step 6

Acetylcholine binds to acetylcholine receptors on the muscle fiber

Step 7

The muscle fiber initiates an action potential

Steps of Excitation-Contraction Coupling

An action potential spreads from the NMJ down the sarcolemma. It dives down transverse-tubules(t-

tubule) into the center of the skeletal muscle fiber. Along the t-tubules, the sarcoplasmic reticulum releases calcium

Calcium

Binds to troponin

Troponin

Moves tropomyosin, revealing myosin binding sites on actin. Myosin can now form cross-bridges and move actin

Acetylcholinesterase

Degrades acetylcholine in the synapse

Relaxation

Motor neuron is not active, Calcium is pumped back into sarcoplasmic reticulum, tropomyosin moves to cover the myosin-binding sites

ATP

Also necessary for muscle to relax

ATP role in relaxation

ATP binds to myosin and allows myosin to detach from actin

Rigor mortis

ATP is depleted and myosin gets “stuck” to actin in a contracted state

ATP powers

Ca2+ pumps that move Ca2+ back into the sarcoplasmic reticulum

Sources of ATP

Creatine kinase, Glycolysis, Aerobic respiration

Creatine kinase

One of the first sources of ATP, transfers a phosphate group to ADP, forming ATP

Glycolysis

Second source, produces a small amount of ATP for about 1 minute of contraction

Aerobic respiration

Using mitochondria and oxygen is necessary for long-lasting contraction

Fast glycolytic (FG)

Rely on glycolysis, Produce a lot of energy, but short-term only. Fatigue quickly, White in color; do not have a large blood supply

Fast oxidative (FO)

More fatigue resistant than FG fibers, Intermediate color

Slow oxidative (SO)

Rely on aerobic metabolism, Fatigue resistant, Red in color; large blood supply and lots of myoglobin

Tension

The force produced by a contracting muscle

Isotonic contraction

Amount of force generated remains the same, length of the muscle changes

Concentric contraction

Produces force while shortening

Eccentric contraction

Produces force while lengthening

Isometric contraction

Amount of force generated increases, muscle length remains the same

Motor unit

One motor neuron and all of the muscle fibers it innervates

Motor unit recruitment

One way to increase the amount of force produced

Optimal overlap

Skeletal muscle produces maximal force

Passive length

There is already maximal overlap and no contraction is possible

Muscle is passively stretched

There is not enough overlap and cross-bridges can’t form; this also leads to low force generation

Optimal resting length

Muscles are under tension in our bodies