Ch. 9 - Skeletal Muscle

types of muscle tissue

• skeletal

• cardiac

• smooth

muscle tissue functions

• body movement

• stabilize position

• move substances in body

• guard body entrances/exits

• support soft tissue

• produce heat

• store nutrients

cardiac muscle tissue

• involuntary

• in heart wall

• pumps blood

• striations visible microscopically

• intercalated discs

smooth muscle tissue

• Involuntary

• walls of hollow internal structures

• no striations

skeletal muscle tissue

• Voluntary

• movement - pulling on bones

• organ made of mostly skeletal tissue + CT, nerves, blood vessels

• muscle cell = muscle fiber

muscle tissue properties

excitability, contractility, extensibility, elasticity

Excitability

can produce electrical signals (action potentials)

Contractility

ability to shorten (contract) when stimulated

Extensibility

ability to stretch without being damaged

Elasticity

ability to return to original shape after being stretched

Skeletal muscle structure

epimysium, perimysium, endomysium

Epimysium

• dense sheath of collagen fibers surrounding muscle

• connected to deep fascia

• Separates muscle from other tissues/organs

muscle fascicle

• bundle of muscle fibers

Perimysium

• fibrous layer surrounding fascicle

• contain collagen, elastic fibers w/ blood vessels & nerves to supply them

Endomysium

• surrounds a muscle fiber

  • areolar CT

  • collagen and elastic fibers

  • blood vessels

  • nerves

Myofibrils

• cylindrical structures extending the entire length of the muscle fiber

• Mitochondria along myofibrils

Myofilaments

bundles of protein filaments within myofibrils

Thick and thin filament

thick filaments

primarily composed of myosin

thin filaments

primarily composed of actin

myosatellite cells

stem cells that help repair damaged muscle tissue

endomysium, perimysium, epimysium

are each continuous with tendons and aponeurosis

tendons

Connect muscle to bone

Aponeurosis

strong sheet of tissue that acts as a tendon to attach muscles to bone

skeletal muscle development

myoblasts fuse to form muscle fibers, myoblasts that do not fuse remain in the endomysium and turn into myosatellite cells

special terms for skeletal muscle fibers

sarcolemma and sarcoplasm

Sarcolemma

• plasma membrane of a skeletal muscle fiber

• selective permeability = uneven +/- charges

• reversal of charge is step 1 in muscle contractions

• motor neuron initiates change in charge

Transverse tubules (T-tubules)

• continuous with sarcolemma, extend into sarcoplasm

• form passageways through muscle fiber and encircle sarcomere

(The yellow structure)

sarcoplasmic reticulum

• similar to smooth ER

• stores Ca ions that are actively pumped from the cytosol

• have terminal cisternae on either side of the T tubule

terminal cisternae

enlarged areas of the sarcoplasmic reticulum surrounding the T tubules.

Triad

pairs of terminal cisternae and one T tubule

sarcoplasm

cytoplasm of a muscle cell found between myofibrils

Sarcomere

functional unit of myofibril (region between z lines)

contains Z lines, I band, A band, zone of overlap, M line, and H band

H band

The area around the M line

Has thick filaments but no thin filaments

M line

middle of sarcomere

zone of overlap

where thick and thin filaments overlap

A band

dark area; extends length of the thick filaments

I band

thin filaments only (actin)

Z lines

The ends of a sarcomere.

contractile proteins

generate force during contraction (actin and myosin)

regulatory proteins

switch the contraction process on and off (tropomyosin and troponin)

structural proteins

keep thick and thin filaments in proper alignment (titin and nebulin)

Structure of thin filaments

F-actin, Nebulin, tropomyosin, troponin

F-actin (filamentous actin)

• two twisted rows of globular G-actin

• active sites for binding myosin

Nebulin

Holds F-actin strands together

Tropomyosin

• double stranded protein wrapped around F-actin

• blocks myosin binding site on G-actin molecules in relaxed muscle

Troponin

• holds the tropomyosin in place

• has binding site for Ca2+

Thin filament in the presence of calcium

• when Ca2+ is present it binds appropriate site on troponin

• pulls the tropomyosin off of the F-actin, revealing myosin binding sites

• allowing the muscle to contract

myosin molecule

• 2 twisted myosin subunits

• free heads point towards thin filaments

• forms crossbridges

Titin

• connect thick filament to M & Z lines

• elastic and extensive

Thick + Thin filament together

Myosin head attaches to myosin binding site on F-actin and pulls to contract

sliding filament theory

• sliding occurs in all sarcomeres in each myofibril

• Myofibrils shorten=muscle fibers shorten=muscle contraction

• A band length stays the same

Plasma membranes

positive charges: contain more potassium (K+) inside the cell, and more sodium (Na+) outside the cell

Negative charges: Mostly proteins inside the cell, can't cross plasma membrane, more Cl- outside in extracellular fluid

resting membrane potential

• inside of cell slightly more negative than outside

• Neurons resting potential=-70 mv

• skeletal muscle fibers resting potential=-85 mv

leak channels

channels that are always open and allow ions to move along their gradient

Sodium potassium ion pumps

• constantly work against concentration gradients

• 3 Na+ out of cell, bring 2 K+ ions in

• maintains resting membrane potential

action potential (AP)

A: Na+ into the cell (increases mv)

B: Na+ channels close and K+ channels open (at peak)

C: K+ leaves cell (decreases mv)

D: K+ channel closes but overshoots

E: Na+ K+ pump brings back to resting membrane potential

Neuromuscular junction (NMJ)

• where motor neuron controls skeletal muscle fiber

• 1 NMJ per muscle fiber

• 1 NMJ may branch out and control multiple muscle fibers

NMJ components

axon terminal, synaptic cleft, motor end plate

axon terminal (synaptic terminal)

has vesicles with acetylcholine (neurotransmitter)

synaptic cleft

space between axon terminal and motor end plate

motor end plate of muscle fiber

• junctional folds that increase number of acetylcholine receptors

• contains acetylcholinesterase, breaks down acetylcholine

Activities at the neuromuscular junction

1. electrical impulse arrives at axon terminal, acetylcholine (ACh) is released via exocytosis

2. ACh diffuses across synaptic cleft, binds to ACh receptor membrane channels at motor end plate, changes sarcolemma Na+ permeability, Na+ enters muscle fiber sarcoplasm

3. Na+ influx generates action potential

4. AP generated at motor end plate immediately spreads across entire sarcolemma

5. AP moves down T-tubules between terminal cisternae of sarcoplasmic reticulum (SR)

6. SR releases stored Ca2+ into sarcomeres beginning contraction (excitation contraction coupling)

Steps of Excitation-Contraction Coupling

1. Neural control: AP in motor neuron starts process at neuromuscular junction (NMJ)

2. Excitation: AP causes ACh release from motor neuron which leads to AP in sarcolemma

3. Calcium ion release: muscle fiber action potential travels through T-tubules and triggers release of Ca2+

4. Contraction cycle begins

-4a: Myosin heads are energized and cocked

-4b: Contraction cycle begins, Ca2+ ions arrive from SR

-4c: Active sites exposed, calcium binds to troponin, troponin changes position and moves tropomyosin which exposes the active site on actin

-4d: cross bridges form; myosin heads bind to exposed active sites on actin

-4e: myosin heads pivot towards M-line (center)=power stroke, and adp + p are released

-4f: cross bridges detach, a new ATP attaches to each myosin head, myosin releases from actin

-4g: Free myosin head splits ATP into ADP+P, and that released energy is used to recock the myosin head

5. Sarcomere shortens: thick + thin filaments interact (Sliding filaments) which shortens sarcomeres by pulling the ends of the muscle fiber closer together

6. Muscle tension produced: shortening of muscle fibers causes entire muscle to shorten. This muscle contraction produces a pull, or tension on tendons

motor unit

A single motor neuron and all the muscle fibers it controls

motor unit recruitment

• activation of more motor units for more tension

muscle tone

• resting tension in skeletal muscle

• some motor units are always active to produce low level tension (not movement),

  • done subconsciously

concentric contraction

• muscle tension rises until it exceeds load

• as muscle shortens, tension is constant

eccentric contraction

• peak tension produced is less than load

• muscle lengthens/elongates

isometric contraction

• Muscle length does not change

• tension never exceeds load

Ex: planks

muscle fatigue and recovery

• fatigue: muscle can no longer perform at required level

• recovery period: time needed to return conditions in muscle fibers to pre-exertion levels

fast fibers

• pale color

• large in diameter

• large glycogen reserves

• few mitochondria

• powerful contractions

• fatigue rapidly

Ex: weight lifting, anaerobic exercise

slow fibers

• red color

• smaller diameter

• longer sustained contraction

• resist fatigue

  • large amount of O2, myoglobin, capillaries

Ex: marathon running

intermediate fibers

• little myoglobin

• pale

• more capillaries & fatigue resistant than fast fibers

• Can be aerobic or anaerobic exercise

hypertrophy

• muscle enlargement from repeated, exhaustive stimulation

  • more mitochondria

  • more glycogen

  • more/wider myofibrils

  • more myofilaments

  • steroid hormones

atrophy

• decrease in muscle size, tone, and power

• decreased stimulation or inherited diseases such as Duchenne muscular dystrophy

muscular dystrophy

• group of hereditary diseases characterized by degeneration of muscle and weakness (sex linked, males only)